Browsing by Author "Yoshizumi, Terry T"

Purpose:

Our goal was to test a novel concept approximating organ dose measurements using the single mean energy of the two sources in dual-energy (DE) CT environment. Therefore, the purpose of this study was two-fold: (1) To obtain experimental validation of dose equivalency between MOSFET and ion chamber (as gold standard) under a dual-energy environment; (2) To estimate the effective dose (ED) using MOSFET detectors and an anthropomorphic phantom in DE CT scans.

Materials and Methods:

A commercial dual source CT (DSCT) scanner was employed for the study. The scanner was operated at 80kV/140kV (Sn added) using an abdomen/pelvis scanning protocol. A five-phase approach was used. Specific goals for each phase are as follows: (1) Characterize the mean energy from the combined clinical 80kV/Sn140kV beams; (2) Estimate the f-factor for tissues from the mean energy; (3) Calibrate the MOSFET detectors using the mean energy; (4) Validate MOSFET calibration with a CTDI phantom; (5) Measure organ doses for a typical abdomen/pelvis scan using a male anthropomorphic phantom and derive ED using ICRP 103 tissue weighting factors. For validation of dose equivalency, a MOSFET detector and ion chamber measured the dose at the center cavity of a CTDI body phantom. A student t-test was used to determine if the difference between the two was statistically significant.

Results:

The mean energy was calculated to be 67 kVp based on the corresponding spectra for the clinical DE beams. Using the Mean Energy Method, the tissue dose in the center cavity of the CT body phantom was 2.08 ± (2.70%) cGy with an ion chamber and 2.20 ± (4.82%) cGy with MOSFET respectively with a percent difference of 5.91% between the two measurements. The results (p = 0.15) showed no statistically significant difference. ED for DE abdomen/pelvis scan was calculated as 5.01 ± (2.34%) mSv by the MOSFET method and 5.56 mSv by the DLP method respectively.

Conclusion:

There has been no physical method to measure organ doses in DE CT scans. We have developed and validated a novel approach, the Mean Energy Method - for organ dose estimation in DE CT scans. ED from the anthropomorphic phantom compared well (within 11%) between the MOSFET method and DLP method.

Purpose: (1) To benchmark the accuracy of effective dose equivalent (EDE) of the single- and double-badge methods (NRC 2002-06) using the commercially available radiation monitors in clinical settings, (2) to study the transmission properties of various shielding materials, (3) to evaluate the accuracy of film badge readings compared to a calibrated ion chamber, (4) to benchmark the accuracy of effective dose (ED) of the single- and double-badge methods (NRC 2002-06) using the MOSFET method, and (5) to investigate the organ dose and image quality in a thoracic MDCT scan under the following conditions: (a) tube current modulation (TCM) without a Bismuth shield, (b) TCM with a Bismuth shield, and (c) manually reduced tube current (RTC) with no Bismuth shield.

Methods and Materials: (Project 1): Radiation workers in interventional radiology and cardiac catheterization laboratory were provided with two monitors and asked to place one at the collar and the other underneath the lead apron. Two commercial radiation monitor vendors were used for the study; both vendors were accredited by the National Voluntary Laboratory Accreditation Program (NVLAP). Effective dose equivalent (EDE) was computed by single-badge and double-badge methods based on the NRC Publication 2002-06. Data were plotted EDE1 (single badge) vs. collar reading and EDE2 (double badge) vs. collar reading. Data on EDE2 vs. collar reading were fitted by linear regression and a new equation for the EDE2collar was derived for routine clinical EDE estimation.

(Project 2): The transmission properties of lead aprons and a thyroid shield were measured using a 6-cc ion chamber and electrometer. These measurements were taken on a GE VCT (64 slice) scanner at 80, 100, and 120 kVp. The different types of lead aprons studied included lead free, lightweight lead, and fully leaded.

(Project 3): The accuracy of film badges was evaluated by comparing the reported deep dose equivalent of the film badge readings to the ion chamber readings measured during the same exposure. The measurements were made on a Philips Standard Radiography unit (Duke North, Room H1) at 80, 100, and 120 kVp. Two badges were exposed with the ion chamber per energy.

(Project 4): An adult male anthropomorphic phantom was loaded with 20 diagnostic MOSFET detectors and scanned without lead aprons using a whole body computed tomography (CT) protocol. All measurements were taken on a GE VCT (64 slice) scanner at 80, 100, 120 kVp. Two commercial film badges were placed on the phantom at the collar location and waist location. Individual organ doses in the phantom were corrected for lead apron attenuation factor and ED was computed using ICRP 103 tissue weighting factors. The single badge conversion coefficient (CC) was determined for each energy by taking the ratio of the ED to collar badge reading. The reported deep dose equivalent for the collar badge was plotted against the MOSFET effective dose and a new equation for EDcollar was derived.

(Project 5): Organ dose was measured with MOSFETs using an adult female anthropomorphic phantom; the phantom was scanned with pulmonary embolus protocol. All measurements were performed with a 64-slice scanner at 120 kVp. The reference exposure and reduced exposure (with 4-ply Bi shield) was measured with an ion chamber located at the level of the breast. The tube current was reduced by normalizing the reference tube current to the ratio of the reduced exposure to the reference exposure. Image quality was measured using a high contrast insert placed in the lung. Regions of interest (ROIs) were drawn in the breast, lung, and heart to measure HU change and noise. ROIs were drawn in the lung and high contrast insert to measure signal-to-noise ratio (SNR) and percent contrast (%Contrast).

Results: (Project 1): From the data, it can be seen that EDE1 read about a factor of six greater than EDE2. The new equation for EDE2collar yielded a slope of 0.06992, a y-intercept of -1.682, and a r2 value of 0.9081.

(Project 2): The transmission for the fully leaded, lightweight lead aprons, and lead free apron were 3.19%, 3.71%, and 7.06% at 80 kVp; 6.58%, 8.07%, and 13.04% at 100 kVp; 7.61%, 12.05%, and 17.84% at 120 kVp, respectively. The attenuation for the thyroid shield was 3.02%, 6.35%, and 7.74% at 80, 100, and 120 kVp, respectively.

(Project 3): The average badge reading was 3.49 ± 1.01% mSv at 80 kVp; 4.80 ± 7.37% mSv at 100 kVp; 4.90 ± 14.1% mSv at 120 kVp. The dose to soft tissue measured by the ion chamber was 4.53 mSv at 80 kVp; 5.71 mSv at 100 kVp; 6.35 mSv at 120 kVp. The film badge reading differed from the ion chamber measurement by -22.8%, -15.9%, and -22.9% at 80, 100, and 120 kVp, respectively.

(Project 4): The ED and % difference between the single-badge method (NRC 2002-06) and the MOSFET method were as follows: 11.65 mSv vs. 0.50 mSv (2331%) for 80 kVp; 27.85 mSv vs. 2.14mSv (1301%) for 100 kVp; 38.59 mSv vs. 4.98 mSv (775%) for 120 kVp, respectively. The ED and % difference between the double-badge method (NRC 2002-06) and the MOSFET method were as follows: 4.07 mSv vs. 0.50 mSv (808%) for 80 kVp; 16.9 mSv vs. 2.14 mSv (791%) for 100 kVp; 25.4 mSv vs. 4.98 mSv (510%) for 120 kVp, respectively. The single badge conversion factors were 0.01 ± 14.8% (80 kVp), 0.02 ± 9.5% (100 kVp), and 0.04 ± 15.7% (120 kVp). The plot of collar badge reading vs. MOSFET effective dose yielded an equation with a slope of 0.0483, a y-intercept of -1.6517, and a R2 value of 0.92929.

(Project 5): Organ doses (mGy) for the three scans (TCM, TCM with Bi, and RTC with no Bi) were 45.8, 27.1, and 27.8 to the breast; 51.6, 47.0, and 29.1 to the lung; and 42.1, 35.0 and 24.9 to the heart, respectively. HU increase was greatest in the TCM with Bi scan. The SNRs were 77.1, 63.7, and 59.2 and the %Contrast values were 369.5, 347.1, and 362.7 with TCM, TCM with Bi, and RTC, respectively.

Conclusions: (Project 1): A new EDE estimation method has been developed based on the results of two-badge system. The method would enable us to compute new EDE values knowing only the collar badge reading. Since EDE2 reads a factor of six less than EDE1, this provides a realistic advantage in regulatory compliance for interventional and cardiac catheterization personnel. Further, new EDE conversion coefficients should be developed for better assessment of EDE.

(Project 2): The fully leaded shielding materials had the lowest percent transmission. It should be noted that radiation workers are generally exposed to only scattered radiation of lower energy. Although this study did not measure attenuation properties at lower energies, it is expected that the percentage of attenuation will only increase with lower energies.

(Project 3): The reported deep dose equivalent (DDE) underestimated the dose to soft tissue compared to the calibrated ion chamber readings. This may be due to the fact that DDE is the dose equivalent at a depth of 10 mm.

(Project 4): Current regulatory ED conversion coefficient (CC) with single collar badge is 0.3; for double-badge system, they are 0.04 and 1.5 for the collar and under the apron respectively. Based on our findings we recommend the current collar CC be dropped due to the overestimation of ED. Since occupational workers are exposed mainly to scattered x-rays of lower energy, a collar CC of 0.01 (80 kVp data) may be a more viable option. The double badge system seems to provide a better coefficient for the collar as 0.04; however, exposure readings under the apron are usually negligible to zero with lead aprons.

(Project 5): For thoracic CT using RTC will result in similar global reduction in organ dose; the use of Bismuth with TCM will lead to an overall decrease in organ dose and more marked dose reduction for the breast. There was a significant difference in SNR (p = 0.0003) and %Contrast (p < 0.0001) in the TCM with Bismuth scan compared to the reference scan (TCM). The RTC scan also demonstrated a significant decrease in SNR and %Contrast with p < 0.0001 for both. While the TCM scan demonstrated superior image quality, the trade-off is in the increased dose to the breast.

Purpose:

The overall purpose of this study was a comparison of radiation exposure for patients and staff during intra-operative imaging for orthopedic lumbar spine surgery. In order to achieve this, we: (1) Characterized each x-ray machine for physics performance, (2) Measured occupational radiation exposure inside the surgical suite for multiple intra-operative imaging devices utilizing currently in place clinical protocols for abdominal/spinal imaging, and (3) Measured specific organ doses for a phantom of three different Body Mass Indices (BMI) for each machine. We also compared the dose changes relative to changes in BMI as well as surgical image quality changes relative to BMI. This served as the majority of the first phase of a two phase project. The purpose of the second phase of the project will be to optimize scan parameters for surgical hardware placement in terms of image quality and organ dose for the devices that allow for modifications of scanner settings.

Materials and Methods:

(1) X-Ray quality control meters were used to verify particular beam characteristics and additional information was calculated from the beam data. Both a small volume ionization chamber as well as Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) dosimeters were used to validate linear response of new design X-Ray tubes. (2) Both handheld ionization chamber survey meters as well as Geiger-Muller based personal dose meters were used to measure stray radiation for room surveys in locations representative of typical radiation worker positions during intra-operative imaging. (3) MOSFET dosimeters were placed in an adult male anthropomorphic phantom representing a normal BMI. 20 MOSFETs were used in nine organs with two small volume ion chambers used for skin surface dosimetry. Two additional layers of adipose equivalent material were progressively added to the phantom to represent BMI values of overweight and obese.

Results:

(1) The maximum tube potential, half value layer (HVL), effective energy, and soft tissue f-factor for each machine is as follows: IMRIS VISIUS iCT: 118.4 kVp, 7.66 mm Al, 53.64 keV, and 0.934 cGy/R; Mobis Airo: 122.3 kVp, 7.21 mm Al, 51.31 keV, and 0.925 cGy/R; Siemens ARCADIS Orbic 3D: 83 kVp, 7.12 mm Al, 32.76 keV, and 0.914 cGy/R; GE OEC 9900 Elite: 75 kVp, 4.25 mm Al, 46.6 keV, and 0.920 cGy/R. (2) The highest exposure rates measured during clinically implemented protocols for each scanner are as follows: IMRIS VISIUS iCT: 800 mR/hr; Mobis Airo: 6.47 R/hr; Siemens ARCADIS Orbic 3D: 26.4 mR/hr. (3) The effective dose per scan of each device for a full lumbar spine scan are as follows, for normal, overweight, and obese BMI, respectively: IMRIS VISIUS iCT: 12.00 ± 0.30 mSv, 15.91 ± 0.75 mSv, and 23.23 ± 0.55 mSv; Mobius Airo: 5.90 ± 0.25 mSv, 4.97 ± 0.12 mSv, and 3.44 ± 0.21 mSv; Siemens ARCADIS Orbic 3D: 0.30 ± 0.03 mSv, 0.39 ± 0.02 mSv, and 0.28 ± 0.03 mSv; GE OEC 9900 Elite: 0.44 mSv, 0.77 mSv, and 1.14 mSv.

Conclusion:

(1) The IMRIS VISIUS iCT i-Fluoro capable CT scanner and Mobius Airo mobile CT scanner have similar beam characteristics with significantly different tube parameter modulation protocols. Siemens ARCADIS Orbic 3D and GE OEC 9900 offer comparable beam characteristics but different imaging methods. All scanners performed within factory specifications. (2) The IMRIS VISIUS iCT should not be used in i-Fluoro mode for surgical procedures active during scanning due to the 1.42 cGy/s point dose rate in the beam field. The high exposure rate from the Mobius Airo is offset by short scan times and can be mitigated by ensuring enforcement of currently established radiation protection regulations and policies. Minimal stray radiation is measured from the Siemens ARCADIS Orbic 3D. (3) The differences in tube modulation of the CT scanners means the Mobius Airo offers a significantly reduced effective dose with increasing patient BMI over the IMRIS VISIUS iCT. Effective dose from the CT scanners varies as much as one to two orders of magnitude higher than the C Arms, but the Siemens ARCADIS Orbic 3D offers unusable image quality for patients with higher than normal BMI. Based off of physician reported usable surgical image quality of Mobius Airo, this device is recommended for continued integration and implementation during routine surgical procedures for patients of all BMI in orthopedic lumbar spine surgery.

Purpose: (1) To verify the accuracy and linearity of the ThermoScientific Radeye G Personal Rate Meter with respect to exposure rate across the full dynamic range of the instrument. (2) Use a combination of empirical data and Monte Carlo methods to estimate dose distribution in a GE Discovery 690 PET/CT scanner room and adjacent hallway. (3) Quantify components of occupational dose to PET technologists.

Materials & Methods: (Project 1) The Radeye unit and a calibrated ion chamber were placed in the beam of a Cesium 137 calibrator. They were exposed from 46 μR/hr to 1 R/hr with the pulse of each beam lasting for 90 seconds. The Radeye made 15 exposure rate measurements during each pulse. The ion chamber was read in the mid-point of each pulse's duration. (Project 2) Six Radeye units were placed at key points within the Discovery 690 scan room and two were placed in the adjacent hallway. 1600 exposure rate measurements were made over eleven hours during each day of operation. Data was collected for seven days. The total integrated data from the detectors inside the room was used to develop a Monte Carlo model of the room using FLUKA software. This model was then able to estimate the contribution from radiation escaping the scan room to the detectors in the hallway. (Project 3) Three PET technologists wore Radeye units while performing their daily tasks. The detectors recorded a mean exposure rate over each 25 second sampling period. The technologists were also asked to maintain a written log of all their interaction with radioactive material as well as their interactions with injected patients. Each day the Radeye unit produced a plot of radiation exposure with respect to time. Each interaction with radioactivity from the logs was highlighted on the plot and integrated to obtain the exposure received while performing that task.

Results: (Project 1) The Radeye deviated from the known value of exposure by up to 9.3% and deviated from the ion chamber measurement by up to 8.6% for exposure rates of 1 mR/hr and greater. The Radeye measured up to 29.6% higher than the known rate and up to 33.6% higher than the ion chamber measurement for exposure rates less than 1 mR/hr. The variance in the Radeye measurements decreased as exposure rate increased. The standard deviation of the Radeye measurements were less than 4% of their respective mean values for exposure rates less than 1 mR/hr. This value increased for lower exposure rates, up to 14% at 0.046 mR/hr. (Project 2) Mean daily exposures to five points in the PET/CT scan room were measured for CT and PET emissions separately. A Monte Carlo model of the scan room was created to model the distribution, including an initial approximation for the scanner gantry. The simulations showed that the virtual scanner should be thinner (i.e. less attenuating), especially for the 511KeV PET photons. (Project 3) The mean exposure received per dose draw and accompanying injection was 0.70±0.23mR for the 113 injections recorded over the course of the study. No correlation was observed between the dosage injected and the exposure received. The percent contributed to the total exposure by each category and participant was as follows. Technologist #1: 68% from Dose Draw, 6% from Patient Positioning, 4% from Patient Transport, 1% from General Patient Care, 21% from nonspecific sources. Technologist #2: 34%, 32%, 14%, 6%, and 14%. Technologist #3: 32%, 32%, 16%, <1%, and 20%. The dose draws and accompanying injections account for between one and two thirds of daily exposure. This indicates it is likely a 30% daily dose reduction could be achieved with use of automated injection equipment.

Project 1: Chest Phantom Development for Chest X-ray Radiation Protection Surveys

Purpose: Develop an acrylic phantom to accurately represent an average adult’s chest for use in radiographic chest unit radiation protection surveys.

Materials and Methods: 6 sheets of 3.81 cm thick acrylic were cut and assembled to form a 30.5 x 30.5 x 20.3 cm hollow box phantom. The acrylic served as tissue equivalent material and the hollow center simulated lungs in a human patient. Six sheets of 1 mm thick aluminum were cut to line the inner walls of the acrylic phantom to potentially boost scatter radiation. Three phantoms underwent posterior-anterior (PA) and lateral chest protocol radiographic scans: the acrylic phantom (with and without the aluminum lining), a 3 gallon water bottle filled with water, and an adult male anthropomorphic phantom. The phantoms were set up as though they were adult patients and scanned with automatic exposure control. Scatter radiation was measured with ion chamber survey meters at 4 points within the room for each phantom and protocol. The scatter data from the acrylic phantom and water bottle were compared to the anthropomorphic phantom to determine which one more accurately represented an adult patient.

Results: For the PA protocol, the average percent difference in measurements between the acrylic phantom and anthropomorphic phantom was 33.3±28.8% with the aluminum lining and 33.0±21.2% without the lining. The percent difference between the water bottle and anthropomorphic phantom was 66.5±42.0%. For the lateral protocol, the average percent difference in measurements between the acrylic phantom and anthropomorphic phantom was 157.6±5.6% with the aluminum lining and 143.0±17.6% without the lining. The percent difference between the water bottle and anthropomorphic phantom was 78.3±22.8%.

Conclusions: The acrylic phantom provided a more accurate comparison to the anthropomorphic phantom than the water bottle for the PA protocol. For the lateral protocol, neither the acrylic phantom nor water bottle provided an adequate comparison to the anthropomorphic phantom.

Project 2: Internal Beta Dosimetry of an Iodine-131 Labelled Elastin-Like Polypeptide

Purpose: Develop a model and simulation to better understand the dosimetry of an I-131 labeled elastin-like polypeptide (ELP) brachytherapy technique.

Materials and Methods: To develop the model, an average scenario based on mouse trials was explored. A 125 mg tumor was approximated as a sphere, with the I-131 ELP injected into its center. The ELP solidifies into a spherical depot – approximately 1/3 the volume of the tumor – and becomes a permanent brachytherapy source. The injected activity of I-131 was 1.25 mCi. I-131 primarily emits β radiation with an average energy of 182 keV, therefore it was determined that all such emissions were confined within the bounds of the tumor. Gamma emissions associated with I-131 were ignored as they were determined to have enough energy to escape the bounds of the tumor without any interaction. This model was implemented into a simulation using the Monte Carlo program FLUKA. From this simulation, the absorbed dose to the tumor and ELP depot, along with the dose profile, was calculated.

Results: The tumor received an absorbed dose of 72.3 Gy while the ELP received 1.14×10^3 Gy. From the dose profile, it was determined that 99% of the absorbed dose to the tumor was highly localized to a 0.3 mm region surrounding the ELP depot.

Conclusions: The model and simulation provided a better understanding of the dosimetry underlying the novel ELP brachytherapy technique. Results obtained demonstrated that the ELP method delivers doses that are comparable to current conventional brachytherapy techniques.

Project 3: I-131 Beta Detection Using a Scintillating Nanoparticle Detector

Purpose: Determine if a scintillating nanocrystal fiber optic detector (nano-FOD) could detect β emissions from I-131.

Materials and Methods: The nano-FOD’s β response was tested using a source vial containing 101 mCi of I-131 in 2 mL of stabilizing solution. A glass vial containing the I-131 was placed inside a lead pig for shielding. A 1 mm diameter hole was drilled through the tops of the vial and pig to allow insertion of the nano-FOD. Measurements were taken every day over a 17 day period by repeatedly submerging the nano-FOD in the I-131 solution and recording the voltage signal it produced. The activity at the time of measurement was calculated based on the time and date of data acquisition. The net signal and signal-to-noise ratio (SNR) were then calculated and plotted as functions of I-131 concentration.

Results: The nano-FOD produced a measurable response when exposed to the β emissions of I-131. The net signal and SNR both demonstrated a linear correlation with the concentration of I-131.

Conclusions: The nano-FOD was demonstrated to be capable of β detection with a linear correlation to activity. If the signals measured can be calibrated to radiation exposure, then the nano-FOD has promising applications as a novel β detector.

Cone beam computed tomography (CBCT) is a 3D x-ray imaging technique in which the x-ray beam is transmitted to an object with wide beam geometry producing a 2D image per projection. Due to its faster image acquisition time, wide coverage length per scan, and fewer motion artifacts, the CBCT system is rapidly replacing the conventional CT system and becoming popular in diagnostic and therapeutic radiology. However, there are few studies performed in CBCT dosimetry because of the absence of a standard dosimetric protocol for CBCT. Computed tomography dose index (CTDI), a standardized metric in conventional CT dosimetry, or direct organ dose measurements have been limitedly used in the CBCT dosimetry.

This dissertation investigated the CBCT dosimetry from the CTDI method to the organ, effective dose, risk estimations with physical measurements and Monte Carlo (MC) simulations.

An On-Board Imager (OBI, Varian Medical Systems, Palo Alto, CA) was used to perform old and new CBCT scan protocols. The new CBCT protocols introduced both partial and full angle scan modes while the old CBCT protocols only used the full angle mode. A metal-oxide-semiconductor-field-effect transistor (MOSFET) and an ion chamber were employed to measure the cone beam CTDI (CTDI_CB) in CT phantoms and organ dose in a 5-year-old pediatric anthropomorphic phantom. Radiochromic film was also employed to measure the axial dose profiles. A point dose method was used in the CTDI estimation.

The BEAMnrc/EGSnrc MC system was used to simulate the CBCT scans; the MC model of the OBI x-ray tube was built into the system and validated by measurements characterizing the cone beam quality in the aspects of the x-ray spectrum, half value layer (HVL) and dose profiles for both full-fan and half-fan modes. Using the validated MC model, CTDI_CB, dose profile integral (DPI), cone beam dose length product (DLP_CB), and organ doses were calculated with voxelized MC CT phantoms or anthropomorphic phantoms. Effective dose and radiation risks were estimated from the organ dose results.

The CTDI_CB of the old protocols were found to be 84 and 45 mGy for standard dose, head and body protocols. The CTDI_CB of the new protocols were found to be 6.0, 3.2, 29.0, 25.4, 23.8, and 7.7 mGy for the standard dose head, low dose head, high quality head, pelvis, pelvis spotlight, and low dose thorax protocols respectively. The new scan protocols were found to be advantageous in reducing the patient dose while offering acceptable image quality.

The mean effective dose (ED) was found to be 37.8 ±0.7 mSv for the standard head and 8.1±0.2 mSv for the low dose head protocols (old) in the 5-year-old phantom. The lifetime attributable risk (LAR) of cancer incidence ranged from 23 to 144 cases per 100,000 exposed persons for the standard-dose mode and from five to 31 cases per 100,000 exposed persons for the low-dose mode. The relative risk (RR) of cancer incidence ranged from 1.003 to 1.054 for the standard-dose mode and from 1.001 to 1.012 for the low-dose mode.

The MC method successfully estimated the CTDI_CB, organ and effective dose despite the heavy calculation time. The point dose method was found to be capable of estimating the CBCT dose with reasonable accuracy in the clinical environment.

The fields of medical health physics, imaging, and radiotherapy have pushed the development and implementation of numerous radiation monitoring systems. Furthermore, detection and measurement of ionizing radiation is essential for many industrial processes. Various detection systems including ion chambers, thermoluminescent detectors, electronic portal imaging devices, semiconductor detectors, and scintillation-based systems have been developed to suit this need. Diagnostic imaging systems most often make use of large arrays of inorganic scintillation crystals. These crystals must be grown using specialized equipment in a laboratory environment. Furthermore, the crystal geometry is limited to relatively small volumes, and production time is on the order of months. Plastic scintillation materials have also been extensively studied for dosimetry applications. These detectors offer high sensitivity with lower production cost and a production timeline on the order of days. Plastic scintillators are most often created by extrusion, casting, and injection molding. These techniques allow for larger volume detectors, but their geometry is still limited in most cases to regular geometric shapes. In recent years, advancements in 3D printing technology have been proposed as alternative manufacturing methods for radiation detectors. These techniques offer the ability for rapid prototyping and allow for at-will creation of complex detector geometries that would otherwise be prohibitively time consuming and expensive using current scintillator manufacturing methods. Furthermore, the wide availability of affordable off-the-shelf consumer 3D printers allows detector manufacturing outside of laboratory environments. The primary focus of this dissertation is the development and characterization of 3D printed radiation detectors using [Y1.903; Eu0.1, Li0.16] scintillating nanoparticles suspended in a printable glycol-modified polyethylene terephthalate (PETG) filament. We assess this technology for use in two applications: (1) as a real-time x-ray imaging screen, and (2) as an inorganic scintillation detector element in a fiber-optic probe dosimeter. (1) The imaging screen was characterized by investigating the accuracy of the scintillation image vs incident exposure patterns, the radiation stability of the detectors, and their ability to differentiate tissue thickness and material density in biologically relevant samples. Scintillation images were captured using a smartphone camera situated outside of the primary x-ray field. A housing apparatus was designed to hold the detector plane perpendicular to the field, and above an optical grade mirror angled 45° relative to the camera. Accuracy of the scintillation image was investigated using cutout-patterned lead masks to attenuate portions of the incident x-ray field. Localization of photons generated in the detector volume was quantified for 5 printed samples using local contrast between adjacent areas of the scintillation image corresponding to shielded and unshielded regions of the detector surface. We calculated the difference in scintillation intensity between these regions of the scintillation image were 7.97 ± 5.4% times higher than measured for the baseline shielded areas. Radiation damage effects on scintillation light output due to prolonged exposures was assessed using 6 detector samples. One detector was used as a control group, while the remaining 5 accumulated absorbed dose using a Cs-137 Irradiator to provide lifetime doses ranging from 1.3 – 14 kGy. The average surface scintillation intensity for each detector was measured relative to the control detector prior to and post irradiation. Relative scintillation intensity showed no discernable change due to the lifetime accumulated dose values investigated. Performance of the detector screen imaging biologically relevant samples was assessed in three stages. Firstly, the ability of the scintillation image to show increased attenuation due to material thickness was demonstrated by imaging a mouse femur. The image showed clear signal difference in thicker portions of the bone, allowing for a pseudo-topological reconstruction of the femur based on pixel gray values in the smartphone camera image. The second stage was demonstrating signal differentiation from attenuation differences due to material density in the range of biological tissues. Tissue-equivalent phantoms representative of lung, breast, soft tissue, brain, 1 year-old bone, and adult bone were used in this study. The phantoms were imaged in groups at various x-ray fields of tube potential from 40-120 kVp. Minimal differences in tissue differentiation were seen across this energy range. Our results suggest the material density threshold for differentiation lies between 0.08 and 0.15 g/cm3. The third stage of the printed detector screen assessment focused on imaging anatomical features of a complete biological sample using a plasticized mouse. Scintillation images were captured corresponding to 120 kVp x-ray projections of 4 regions of the mouse. Specifically, regions centered on the head, neck, torso, and hindquarters were imaged. Radiochromic film was placed on top of the detector plane to provide a comparison x-ray projection image. These scintillation images demonstrated the presence of prominent skeletal structures, and the torso image showed clearly defined lung volumes, a region of increased attenuation representative of the mouse liver, and hints of a gradient of attenuation for overlapping organs of the digestive track. These investigations provide proof-of-principle for the use of 3D printed real-time imaging screens. (2) A fiber-optic probe detector was developed using an aluminum brace to couple 3D printed detector chips to an acrylic light guide in order to funnel scintillation photons into the terminal end of a 0.6 mm diameter optical fiber. The probe detector was fitted with 1 mm thick and 2mm thick detector chips printed at maximum scintillation nanomaterial concentration. Fluorescence spectrometer measurements of these two configurations showed comparable scintillation intensity under 130 kVp x-ray excitation, suggesting that the observed scintillation photons are primarily generated on the surface of the printed detector chip. The probe detector light output was then measured with 1 mm thick chips printed at scintillator loading concentrations of 1, 5, 10, 25, and 35% by weight. Fluorescence spectrometer measurements showed monotonic increase in scintillation intensity vs detector chip concentration. Dose response curves for probe detector fitted with 35% printed chips under 80, 160, and 240 kVp excitation were plotted using a NIST-traceable ion chamber as a gold standard for dose measurement. The detector signal was shown to have a strongly linear relationship to incident dose rate for all three energy x-ray fields. The lower detection limit for 80, 160, and 240 kVp exposures was calculated to be 3.55 ± 0.16 cGy/min, 4.09 ± 0.18 cGy/min, and 4.93 ± 0.22 cGy/min respectively. We conclude that 3D printed scintillation detectors are viable for use in optical fiber dosimetry systems, In addition to investigations into 3D printed radiation detectors, this dissertation also serves to extend the applications and physical characterization of the novel Nano-FOD detection system. This detector makes use of inorganic scintillating nanomaterials coupled with an optical fiber and photodiode to provide real-time dose rate measurements. This work builds on previous characterization studies by implementing a methodology for determining lower detection limits using signal vs dose rate calibration curves. Lower detection limits for 5 Nano-FOD detectors were calculated for 60, 80, 100, 120, 150, 200, and 250 kVp x-ray fields. We observed roughly 30% standard deviation in detection limits among the five sampled Nano-FODs at each energy level measured. In addition to this measurement, we quantified sensitivity variations using dose rate calibration factors for all fibers at each energy level. We also explored the capacity of the Nano-FOD system for in vivo measurement of I-131 in small animal applications. This proof-of-concept study focused on in vitro measurement of 103 mCi of I-131 mixed with 2ml of stabilizing solution inside of a lead shielded glass vial. Two Nano-FOD detectors were used in the investigation, one of which was shielded from β particles via an acrylic sheath. Measurements for each detector were taken over a period of 20 days in order to observe the decay behavior of the Nano-FOD signals. The signal of the shielded fiber was subtracted from the unshielded fiber signal after accounting for differences in diode sensitivity, detector sensitivity, and γ attenuation due to the acrylic sheath. The first two of these correction factors were calculated using data from lower detection limit investigations. The difference in incident γ dose rate on the two detectors due to attenuation was derived computationally using the FLUKA Monte Carlo simulation package to model our experimental geometry. Nano-FOD signal from β- emissions was isolated using this two-fiber subtraction method and shown to decay with a half-life of 7.73 ± 0.31 days. These results demonstrate the viability of the two-fiber subtraction method for I-131 β- dose measurement using the Nano-FOD system.

Commercial x-ray irradiator units for small animal irradiation in preliminary cancer studies have become common in radiobiology research. As institutions and researchers acquire new equipment that is simpler to use, x-ray units are typically operated by users without supervision and physics support following initial set-up and training by manufacturers. However, experiments can have widely varying methods of set-up, calibration, and dosimetry. This has led to a documented lack of reproducibility in a variety of small animal studies. A primary contributing factor in this is the lack of standardization of dose delivery in small animals. It has been noted that in some cases the extreme steepness in radiobiology response curves can lead to a change in biological response from 5% to 90% levels with a variance in dose of 10%. Large scale studies that compare dose deliveries at several sites aim to describe a clear picture of the role of inaccurate dosimetry in the documented lack of reproducibility in preclinical studies. Small animal dosimetry is typically simplified into a single look-up table tabulated by device manufacturers or institutional physics groups.

Thermoluminescent dosimeters (TLDs), specifically TLD-100 LiF chips, are generally accepted as the gold standard in kV x-ray dosimetry for small animal studies and these types of large scale projects. However, it is equally well known that these dosimeters require specific calibrations to convert light output to dose. Many comparison studies use half value layer (HVL) measurements to match TLD calibration curves to dose measurements. The dissertation will determine the appropriateness of the use of HVL as a normalizing factor for polychromatic x-ray beams.

In addition to current dosimeter technology, our laboratory developed a novel dosimeter (Nano-FOD) that uses the combination of an organic scintillator pellet and a fiber optic technology to measure dose in real time. The scintillator pellet is composed of Europium-doped yttrium oxide which was demonstrated to have improved stimulated light production in nano-particle form vs. its bulk form, so the material was adapted for our application. To expand to new applications, such as organ-specific in vivo dosimetry for small animals, several physics characteristics have been investigated to inform us of the detector’s expected behavior.

Since TLDs are known to have slight differences in response based on batch and manufacturer date, three TLD batches from our institution that had been routinely used in kV x-ray applications were acquired. Batches were purchased between 2003 and 2011. Each batch was exposed at 5 different kVp values: 135, 150, 200, 250 and 320. At each kVp, the HVLs with matching filtration (2.5 mm Al + 0.1 mm Cu) as well as the necessary filtration to match the HVL at varying kVp values within ±5% was measured. The TLDs were exposed to these beams with matching beam filtrations as well as HVL-matched beams and measured calibration curves in each beam. A linear least-square fit was applied to each calibration curve and all R2 values were greater than 0.97. There was no correlation found between HVL and calibration slope in any of the three batches. With this information, it was determined that calibration curves from HVL matching in broad spectrum beams, such as those used in small animal irradiators, can lead to dose discrepancy of up to 300% at a true dose of 200 cGy. There was less variation between doses at lower energies, such as 135 and 150 kVp. In higher energy beams, there is a larger contribution of photons at characteristic energies. To minimize dose errors, the results of this study lead us to conclude that it is necessary to match both HVL and kVp to achieve an accurate dose calibration curve for TLD-100 chips.

Our institution dosimetry protocol calls for both HVL and kVp matching inherently since calibration and exposure are usually taken in identical beams. To confirm the accuracy of our current methodologies, our x-ray irradiator and filters were recreated in the FLUKA advanced interface (flair). By modeling one of our small animal plexiglass phantoms, dose deposited in TLD dosimeters placed centrally in the phantom was calculated. These doses were compared to measured dose from TLDs and the nano-FOD. Doses agreed to within 1% between Monte Carlo and nano-FOD.

The nano-FOD has current applications in high dose rate (HDR) brachytherapy, micro beam radiation therapy and x-ray dosimetry. Previous studies determined the angular dependence, lifetime radiation effects and linearity of the dosimetry. In this dissertation, data was compiled on temperature dependence and the detector energy response in orthovoltage and megavolt (MV) x-rays. At temperatures between 5 and 46 C, the detector response fell within ±5% of the mean value. An appropriate, distance-based calibration methodology for MV x-rays that address the energy dependence of our detector was determined. These characteristics allowed us to explore other applications of the nano-FOD technology.

A clinical trial in external beam radiation therapy (EBRT) was designed to test the feasibility of using our real-time nano-particle fiber optic detector (nano-FOD) in clinical EBRT treatments. Prior to patient accrual, the detector system was enhanced with improved Cerenkov subtraction via a dual fiber system to complete preliminary calibration. To calibrate our detector, a depth-dependent calibration method using beam data tables for comparison was developed. In phantom studies, overall dose agreement to fell within 5% using this calibration curve.

In patient studies, the nano-FOD was used to measure skin dose during various types of EBRT treatments including intensity modulated radiation therapy (IMRT) and volumetric arc modulated radiation therapy (VMAT). Accrued patients were being treated for a variety of malignancies in a number of areas on the chest and lower abdomen/pelvis. All nano-FOD measurements were compared to calculated values from the clinical treatment planning system (TPS). To date, a total of 15 patients have been accrued for grand total of 56 measurements. Overall percent difference was calculated to be around 10%. In addition, the effects of bolus were investigated in this study. Bolus is used to boost skin dose and in patients were bolus was used improved accuracy to within 6% was observed. The nano-FOD is concluded to provide a viable option for skin dose monitoring in EBRT and our calibration methodology is effective in this application.

Traditional dosimetric devices are inherently point dose dosimeters (PDDs) and can only measure the magnitude of the radiation exposure; hence, they are one-dimensional (1D). To measure the magnitude and spatial location of dose within a volume either several PDDs must be used at one time, or one PDD must be translated from point-to-point. Using PDDs for spatially distributed, two-dimensional (2D), dosimetry is laborious, time consuming, limited in spatial resolution, susceptible to positioning errors, and the currently accepted approach to measuring dose distribution in 2D. This work seeks to expand the current limits of indirectly ionizing radiation dosimetry by using radiochromic film (RCF) for a high-resolution, accurate dosimetry system. Using RCF will extend the current field of radiation dosimetry to spatially quantitative 2D and three-dimensional (3D) measurements.

This work was generalized into two aims. The first aim was the development of the RCF dosimetry system; it was accomplished by characterizing the film and the readout devices and developing a method to calibrate film response for absolute dose measurements. The second aim was to apply the RCF dosimetry system to three areas of dosimetry that were inherently volumetric and could benefit from multiple dimensional (2D or 3D) dose analysis. These areas were representative of a broad range of radiation energy levels and were: low-mammography, intermediate-computed tomography (CT), and high-radiobiologcal small animal irradiation and cancer patient treatment verification. The application of a single dosimeter over a broad range of energy levels is currently unavailable for most traditional dosimeters, and thus, was used to demonstrate the robustness and flexibility of the RCF dosimetry system.

Two types of RCF were characterized for this work: EBT and XRQA film. Both films were investigated for: radiation interaction with film structure; light interaction with film structure for optimal film readout (densitometry) sensitivity; range of absorbed dose measurements; dependence of film dose measurement response as a function of changing radiation energy; fractionation and dose rate effects on film measurement response; film response sensitivity to ambient factors; and stability of measured film response with time. EBT film was shown to have the following properties: near water equivalent atomic weight (Z_eff); dynamic dose range of (10-1-102) Gy; 3% change in optical density (OD) response for a single exposure level when exposed to radiation energies from (75-18,000) kV; and best digitized using transmission densitometry. XRQA film was shown to have: a Zeff of ~25; a 12 fold increase in sensitivity at lower photon energies for a dynamic dose range of 10-3-100 Gy, a difference of 25% in OD response when comparing 120 kV to 320 kV, and best digitized using reflective densitometry. Both XRQA and EBT films were shown to have: a temporal stability (ΔOD) of ~1% for t > 24 hr post film exposure for up to ~20 days; a change in dose response of ~0.03 mGy hr-1 when exposed to fluorescent room lighting at standard room temperature and humidity levels; a negligible dose rate and fractionation effect when operated within the optimal dose ranges; and a light wavelength dependence with dose for film readout.

The flat bed scanner was chosen as the primary film digitizer due to its availability, cost, OD range, functionality (transmission and reflection scanning), and digitization speed. As a cost verses functionality comparison, the intrinsic and operational limitations were determined for two flat bed scanners. The EPSON V700 and 10000XL exhibited equal spatial and OD accuracy. The combined precision of both the scanner light sources and CCD sensors measured < 2% and < 7% deviation in pixel light intensities for 50 consecutive scans, respectively. Both scanner light sources were shown to be uniform in transmission and reflection scan modes along the center axis of light source translation. Additionally, RCFs demonstrated a larger dynamic range in pixel light intensities, and to be less sensitive to off axis light inhomogeneity, when scanned in landscape mode (long axis of film parallel with axis of light source translation). The EPSON 10000XL demonstrated slightly better light source/CCD temporal stability and provided a capacity to scan larger film formats at the center of the scanner in landscape mode. However, the EPSON V700 only measured an overall difference in accuracy and precision by 2%, and though smaller in size, at the time of this work, was one sixth the cost of the 10000XL. A scan protocol was developed to maximize RCF digitization accuracy and precision, and a calibration fitting function was developed for RCF absolute dosimetry. The fitting function demonstrated a superior goodness of fit for both RCF types over a large range of absorbed dose levels as compared to the currently accepted function found in literature.

The RCF dosimetry system was applied to three novel areas from which a benefit could be derived for 2D or 3D dosimetric information. The first area was for a 3D dosimetry of a pendant breast in 3D-CT mammography. The novel method of developing a volumetric image of the breast from a CT acquisition technique was empirically measured for its dosimetry and compared to standard dual field digital mammography. The second area was dose reduction in CT for pediatric and adult scan protocols. In this application, novel methodologies were developed to measure 3D organ dosimetry and characterize a dose reduction scan protocol for pediatric and adult body habitus. The third area was in the field of small animal irradiation for radiobiology purposes and cancer patient treatment verification. Two methods for small animal irradiation were analyzed for their dosimetry. The first technique was within a gamma irradiator environment using a 137Cs source (663 keV), and the second, a novel approach to mouse irradiation, was developed for fast neutron (10 MeV) irradiated by a Tandem Van de Graff accelerator in a 2H(d,n)3He reaction. For the patient cancer treatment, RCF was used to verify a 3D radiochromic plastic, PRESAGETM, using multi-leaf collimation (MLC) on a medical linear accelerator (LINAC) with 6 MV x-rays. The RCF and PRESAGETM dosimeters were employed to verify a simple respiratory-gated lung treatment for a small nodule; the film was considered the gold standard. In every case, the RCF dosimetry system was verified for accuracy using a traditional PDD as the golden standard. When considering all areas of radiation energy applications, the RCF dosimetry system agreed to better than 7% of the golden standard, and in some cases within better than 1%. In many instances, this work provided vital dosimetric information that otherwise was not captured using the PDD in similar geometry. This work demonstrates the need for RCF to more accurately measure volumetric dose.

Purpose

The objective of our study was to test a new approach to approximating organ dose by using the effective energy of the combined 80kV/140kV beam used in fast kV switch dual-energy (DE) computed tomography (CT). The two primary focuses of the study were to first validate experimentally the dose equivalency between MOSFET and ion chamber (as a gold standard) in a fast kV switch DE environment, and secondly to estimate effective dose (ED) of DECT scans using MOSFET detectors and an anthropomorphic phantom.

Materials and Methods

A GE Discovery 750 CT scanner was employed using a fast-kV switch abdomen/pelvis protocol alternating between 80 kV and 140 kV. The specific aims of our study were to (1) Characterize the effective energy of the dual energy environment; (2) Estimate the f-factor for soft tissue; (3) Calibrate the MOSFET detectors using a beam with effective energy equal to the combined DE environment; (4) Validate our calibration by using MOSFET detectors and ion chamber to measure dose at the center of a CTDI body phantom; (5) Measure ED for an abdomen/pelvis scan using an anthropomorphic phantom and applying ICRP 103 tissue weighting factors; and (6) Estimate ED using AAPM Dose Length Product (DLP) method. The effective energy of the combined beam was calculated by measuring dose with an ion chamber under varying thicknesses of aluminum to determine half-value layer (HVL).

Results

The effective energy of the combined dual-energy beams was found to be 42.8 kV. After calibration, tissue dose in the center of the CTDI body phantom was measured at 1.71 ± 0.01 cGy using an ion chamber, and 1.73±0.04 and 1.69±0.09 using two separate MOSFET detectors. This result showed a -0.93% and 1.40 % difference, respectively, between ion chamber and MOSFET. ED from the dual-energy scan was calculated as 16.49 ± 0.04 mSv by the MOSFET method and 14.62 mSv by the DLP method.

Project 1: Effects of High Volume MOSFET Usage on Dosimetry in Pediatric CT

Purpose

The objective of this study was to determine if using large numbers of Metal-Oxide-Semiconducting-Field-Effect Transistors, MOSFETs, effects the results of dosimetry studies done with pediatric phantoms due to the attenuation properties of the MOSFETs. The two primary focuses of the study were first to experimentally determine the degree to which high numbers of MOSFET detectors attenuate an X-ray beam of Computed Tomography (CT) quality and second, to experimentally verify the effect that the large number of MOSFETs have on dose in a pediatric phantom undergoing a routine CT examination.

Materials and Methods

A Precision X-Ray X-Rad 320 set to 120kVp with an effective half value layer of 7.30mm aluminum was used in concert with a tissue equivalent block phantom and several used MOSFET cables to determine the attenuation properties of the MOSFET cables by measuring the dose (via a 0.18cc ion chamber) given to a point in the center of the phantom in a 0.5 min exposure with a variety of MOSFET arrangements. After the attenuating properties of the cables were known, a GE Discovery 750 CT scanner was employed using a routine chest CT protocol in concert with a 10-year-old Atom Dosimetry Phantom and MOSFET dosimeters in 5 different locations in and on the phantom (upper left lung (ULL), upper right lung (URL), lower left lung (LLL), lower right lung (LRL), and the center of the chest to represent skin dose). Twenty-eight used MOSFET cables were arranged and taped on the chest of the phantom to cover 30% of the circumference of the phantom (19.2 cm). Scans using tube current modulation and not using tube current modulation were taken at 30, 20, 10, and 0% circumference coverage and 28 MOSFETs bundled and laid to the side of the phantom. The dose to the various MOSFET locations in and on the chest were calculated and the image quality was accessed in several of these situations by taking the standard deviation of a large regions of interest in both the lung and the soft tissue of the chest to measure the noise.

Results

The proof of concept experiment found that the main cable of the MOSET, not the ends closest to the reading tip, is the most attenuating part of the cable. The proof of concept also found that increasing the number of MOSFET layers to 1, 2, 3, and 4 layers decreased the dose to the center of the phantom by 17.92, 28.04, 39.98, 42.49% respectively. Increasing the percent of the block phantom covered to 10, 30, and 50% coverage decreased the dose to the center of the phantom by 17.92, 17.80, and 18.17% respectively.

Project 2: Pediatric Lens of the Eye Dose Reduction Using Siemens Care kV

Purpose

The Siemens Care kV is a software that recommends a tube potential (kV) setting for CT scans based on the thickness of the anatomy being scanned in order to reduce dose on a patient to patient basis. Pediatric cranial scans at Duke do not use this software nor do they use tube current modulation. Dose to the lens of the eye in pediatric patients can lead to lens opacity later in life [10]. The goal of this project was to determine if Care kV can be used in pediatric cranial scans to reduce the dose to the lens of the eye while maintaining adequate image quality.

Materials and Methods

A Siemens SOMATOM Force CT scanner performing a routine cranial scan protocol was used in concert with two Atom Dosimetry Phantoms (1-year-old and 5-year-old) and MOSFET dosimeters to determine the effect changing the reference tube potential of the Care kV software would have on dose and image quality (measured with CNR). The settings used with Care kV were off, and semi reference tube potential 120, 110, and 100 kV.

Results

Dose to the lens of the eye was reduced for the 1-year old phantom by 9.601, 17.572, and 19.724% by using Care kV with tube potential set to 120, 110 and 100 kV respectively. Dose to the lens of the eye was reduced for the 5-year old phantom by 1.060, 8.859, and17.854% by using Care kV with tube potential set to 120, 110 and 100 kV respectively. Soft tissue CNR was reduced for the 1-year old phantom by 8.812, 11.001, and 5.018% by using Care kV with tube potential set to 120, 110 and 100 kV respectively. Soft tissue CNR was reduced for the 5-year old phantom by 3.473, 5.517, and 3.248% by using Care kV with tube potential set to 120, 110 and 100 kV respectively. Bone CNR was reduced for the 1-year old phantom by 4.447, 8.175, and 10.046% by using Care kV with tube potential set to 120, 110 and 100 kV respectively. Bone CNR was reduced for the 5-year old phantom by 4.782, 7.966, and 11.715% by using Care kV with tube potential set to 120, 110 and 100 kV respectively.

Project 3: Designing Quality Assurance for Cesium Calibration Source

Purpose

North Caroline regulations state that survey meters must be traceable to NIST. The Cs-137 Calibration source used by Duke was installed in 2005 and has since not been measured except for routine calibration of survey meters. The goal of this project was to measure the geometry and dose rate of the source and make a recommendation as to how and how often quality assurance measurements should be made with a NIST traceable ion chamber.

Materials and Methods

Gafchromic XR QA2 radiochromic film was placed in the source beam to measure the angle of the source collimator. Two 0.18 cc and a 6 cc ion chamber were used in a variety of combinations of distance from source and attenuation to determine the exposure rate of the calibration source and compare it to the current calibration table in use.

Results

The collimator angles for the top, bottom, left, and right were calculated to be 12.13, 9.648, 11.58, and 11.58, respectively. The two 0.18 cc ion chambers deviated from the table values by more than 30% for every measurement. The 6 cc ion chamber deviated from the calibration table in use by 9.55, 8.13, 3.36, and 3.72% for 30 cm no attenuation, 30 cm 2x attenuation, 100 cm no attenuation, and 100 cm 2x attenuation measurements respectively.

Atrial Fibrillation (AF) is an ever increasing health risk in the United States. The most common type of cardiac arrhythmia, AF is associated with increased mortality and ischemic cerebrovascular events. Managing AF can include, among other treatments, an interventional procedure called catheter ablation. The procedure involves the use of biplane fluoroscopy during which a patient can be exposed to radiation for as much as two hours or more. The deleterious effects of radiation become a concern when dealing with long fluoroscopy times, and because the AF ablation procedure is elective, it makes relating the risks of radiation ever more essential.

This study hopes to quantify the risk through the derivation of dose conversion coefficients (DCCs) from the dose-area product (DAP) with the intent that DCCs can be used to provide estimates of effective dose (ED) for typical AF ablation procedures. A bi-plane fluoroscopic and angiographic system was used for the simulated AF ablation procedures. For acquisition of organ dose measurements, 20 diagnostic metal-oxide-silicon field effect transistor (MOSFET) detectors were placed at selected organs in a male anthropomorphic phantom, and these detectors were attached to 4 bias supplies to obtain organ dose readings. The DAP was recorded from the system console and independently validated with an ionization chamber and radiochromic film. Bi-plane fluoroscopy was performed on the phantom for 10 minutes to acquire the dose rate for each organ, and the average clinical procedure time was multiplied by each organ dose rate to obtain individual organ doses. The effective dose (ED) was computed by summing the product of each organ dose and the corresponding tissue weighting factor from the ICRP publication 103. Further risk calculations were done according to the BEIR VII Phase 2 report to obtain relative and lifetime attributable risks of cancer for an average AF ablation procedure.

The ED was computed separately for the biplane fluoroscopic and angiographic system's `low' and `normal fluoro' automated settings, yielding 27.9 mSv and 45.6 mSv respectively for an average procedure time of88.1 minutes. The corresponding DAP was 48.7 Gy cm2 and 79.1 Gy*cm2 for low and normal settings respectively. The independently measured DAP was found to be within 0.1 % of that measured by the fluoroscopy system's onboard flat panel detectors. DCCs were calculated to be 0.573 and 0.577 for the respective low and normal settings. The results proved to be very closely matched, which was to be expected. However, the results are higher than some other published DCCs for the same type of procedure. The calculated cancer risks were fairly low due to the age of most patients (less than 5 excess incidents of cancer per 100,000 exposed for stomach colon liver; incidence of lung cancers estimated at 130-300 per 100,000 exposed), but concern remains that longer procedures could increase the risk of erythema or other serious skin injuries.

The second section of this thesis study involves the quantification and distribution of radiation dose in small animals undergoing irradiation in a orthovoltage x-ray unit. Extensive research is being done with small animals, particularly mice and rats, in fields such as cancer therapy, radiation biology and radiological countermeasures. Results and conclusion are often drawn from research based solely on manufacturer's specifications of the delivered dose rate without independent verification or adequate understanding of the machines' capabilities. Accurate radiation dose information is paramount when conducting research in this arena.

Traditional methods of dosimetry, namely thermoluminescence dosimeters (TLDs) are challenging and often time consuming. This section hopes to show that in place of TLDs, MOSFETs can provide accurate, precise dose information comparable with TLDs and ionization chambers. Measurements of all three dosimeters are compared in a small animal irradiator in phantoms and in vivo. Measurements done with MOSFETs are shown to deviate by 2.5% from that of the ADCL calibrated ionization chamber while TLDs showed a 7% deviation. Dose distributions within a phantom are also measured using radiochromic film to estimate the attenuation and show that dose is not uniform throughout the mouse. A dose decrease of approximately 30% is observed in a water phantom, which was only slightly mitigated by a hardening the beam with additional filtration. A Bland-Altman plot was created to show that the MOSFETs and TLDs used to make the dose measurements are statistically equivalent. The results show that all measurements made over a range of doses fall within 1.96 standard deviations of the mean.

Purpose:

(Project 1) The purpose of this study was three-fold: 1) to estimate the organ doses and effective dose (ED) for patients undergoing neurovascular imaging protocols, 2) to study the effect of beam collimation on ED for 3-D imaging protocols, and 3) to derive protocol-specific DAP-to-ED conversion factors.

(Project 2) The Cs-137 irradiator is one of the most commonly used irradiation device in radiobiological research. The purpose of this study is to develop a simple method to estimate the current source radioactivity of a Cs-137 irradiator (Mark I-68A, JL Shepherd).

Material and Methods:

(Project 1) A cone-beam CT system (Philips Allura Xper FD20/20) was used to measure the organ doses for seven 3-D (cone-beam CT and 3-D Rotational Angiography protocols) and eight 2-D (fluoroscopy and digital subtraction angiography) imaging protocols. Organ dose measurements were performed on an adult male anthropomorphic phantom (CIRS, Norfolk, VA) with 20 MOSFET detectors (Best Medical Canada, Ottawa, Canada) placed in selected organs. The dose area product (DAP) values were recorded from console. The ED values were computed by multiplying measured organ doses to corresponding ICRP 103 tissue weighting factors. The ED of four 3-D imaging protocols were also measured with standardized beam collimation to compare with the ED associated with the same protocols without beam collimation.

(Project 2) Three positions along the peak-dose irradiation direction within the irradiation chamber were picked as the reference dosimetry positions. Individual dose rate at each of these positions was measured by an ion chamber in "Gy/sec", as well as estimated by Monte Carlo simulation in "Gy/primary event". The source activity, "disintegration/sec", was then derived from these two sets of values and corrected by the branching ratio of the main 662 keV emission.

Results:

(Project 1) For the seven 3-D imaging protocols with uncollimated setting, the EDs ranged from 0.16 mSv to 1.6 mSv, and the DAP-to-ED conversion factors range from 0.037 to 0.17 mSv/Gy∙cm2. For four protocols with beam collimation, ED was reduced approximately by a factor of 2, and the DAP-to-ED conversion factors by approximately 30%. For the eight 2-D imaging protocols, the ED rates ranged from 0.02 mSv/sec to 0.04 mSv/sec (for DSA) and from 0.0011 mSv/sec to 0.0027 mSv/sec (for fluoroscopy), and the DAP-to-ED conversion factors range from 0.045 to 0.068 mSv/Gy∙cm2 (for DSA) and factors range from 0.0029 to 0.059 mSv/Gy∙cm2 (for fluoroscopy).

(Project 2) For the irradiator in question, the source activity, as of Nov. 17, 2011, was estimated to be 2770 Curies. The current activity from the manufacturer was calculated to be 5900 Curies.

Conclusion:

(Project 1) We have measured ED for standard adult neuro imaging protocols in a C-arm cone-beam CT system. Our results provide a simple means of ED estimation using DAP values from console in the C-arm cone-beam CT system.

(Project 2) Our method offers a convenient means to estimate the source activity. The result was compared to the value computed from the manufacturer. We have found discrepancies between the two: 41%, 86%, and 97%, assessed at location 1, 2, and 3, respectively.

AbstractProject 1: Measuring the Effects on Operator Dose of Changing Clinical Settings Purpose: This study was initiated as part of a multi-faceted investigation of occupational dose to Interventional Radiologists consequential to their role as operators of fluoroscopy equipment. This project aims to qualitatively evaluate general dose reduction techniques, including clinical protocol settings on different interventional fluoroscopes to determine the specific impact on operator dose at Duke University Hospital. Materials and Methods: For each unit, analogous baseline settings were selected with a general abdominal protocol. The patient table was set to a source-to-object distance (SOD) of 62.23 cm (24.5 in) and a patient phantom was placed in the beam as a scatter medium similar to a typical patient abdomen. An anthropomorphic “operator” phantom was draped with a lead apron and positioned to one side of the patient table with an ion chamber placed at collar level. The ion chamber was placed such that the center of the active volume was 38.1 cm (15 in) lateral to and 63.5 cm (25 in) inferior from the center of the flat-paneled detector. A series of scans was taken on each unit, with each one having a selected variable changed, and the exposure readings from the ion chamber were recorded for comparison. Results: The effects on operator exposure rate of personnel height, contour shield use, cine mode, magnification, low dose mode, and source-to-image distance (SID) were analyzed. Operator height was found to have a larger effect on exposure rate reduction with distance than anticipated. Use of the contour shield reduced the operator exposure rate by over 90% on each unit. Use of cine mode drastically increased the exposure rate to the operator, while magnification, low dose mode, and decreasing SID all resulted in lower exposure rates. Conclusions: Operators can utilize these results to contextualize the effects of their own dose reduction techniques. Knowledge and familiarity of the techniques which offer the best exposure rate reduction can guide radiation protection practices among staff and help to optimize occupational doses. Project 2: Developing a Conceptual Framework for Analyzing the Radiation Dose Structured Report Purpose: When investigating occupational dose to Interventional Radiologists, it is important to be able to accurately compare metrics related to dose from historical procedures. The Radiation Dose Structured Report (RDSR) provides characteristic data from historical procedures. With an appropriate framework for analyzing RDSR data, performance metrics between operators or units can be compared, and identified trends can be used to develop dose reduction techniques specific to the organization. Materials and Methods: RDSR data from five interventional fluoroscopy systems (K1 – K5) was extracted for a three-year period from July 2019 through August 2022, and multiple metrics of comparison were selected for analysis. To determine differences in machine output, air kerma rates of similar procedures were compared, as well as the overall machine utilization for each year. Differences in operator-selectable variable were compared through air kerma rate per procedure, fluoroscopy time per procedure (limited to central line procedures), and operator caseload makeup. Results: Machine comparison of air kerma rates showed a consistently higher median and variability on the Philips Allura systems compared to the other three units. The Philips AlluraClarity unit in suite K2 was noticeably under-utilized by Interventional Radiology staff due to it being the primary fluoroscope used by Neurosurgery staff who were outside the scope of this investigation. Operator air kerma rates were compared from August 2021 through August 2022 and largely showed similar median values and variability. Fluoroscopy time per procedure fit to lognormal distributions and compared through their distribution parameter μ showed a median value which dipped during the second year for most providers. One operater also had a consistently higher median time per procedure for all three years. Conclusions: The analysis described by this framework provides a means of utilizing RDSR data to compare performance of interventional procedures. Continual local analysis of these metrics can be used to guide operator training to ensure that occupational doses are optimized to be as low as reasonably achievable. This is an initial approach that can be expanded through investigation and further characterization of procedure data included in the RDSR.

The outcomes for both (i) radiation therapy and (ii) preclinical small animal radio- biology studies are dependent on the delivery of a known quantity of radiation to a specific and intentional location. Adverse effects can result from these procedures if the dose to the target is too high or low, and can also result from an incorrect spatial distribution in which nearby normal healthy tissue can be undesirably damaged by poor radiation delivery techniques. Thus, in mice and humans alike, the spatial dose distributions from radiation sources should be well characterized in terms of the absolute dose quantity, and with pin-point accuracy. When dealing with the steep spatial dose gradients consequential to either (i) high dose rate (HDR) brachytherapy or (ii) within the small organs and tissue inhomogeneities of mice, obtaining accurate and highly precise dose results can be very challenging, considering commercially available radiation detection tools, such as ion chambers, are often too large for in-vivo use.

In this dissertation two tools are developed and applied for both clinical and preclinical radiation measurement. The first tool is a novel radiation detector for acquiring physical measurements, fabricated from an inorganic nano-crystalline scintillator that has been fixed on an optical fiber terminus. This dosimeter allows for the measurement of point doses to sub-millimeter resolution, and has the ability to be placed in-vivo in humans and small animals. Real-time data is displayed to the user to provide instant quality assurance and dose-rate information. The second tool utilizes an open source Monte Carlo particle transport code, and was applied for small animal dosimetry studies to calculate organ doses and recommend new techniques of dose prescription in mice, as well as to characterize dose to the murine bone marrow compartment with micron-scale resolution.

Hardware design changes were implemented to reduce the overall fiber diameter to <0.9 mm for the nano-crystalline scintillator based fiber optic detector (NanoFOD) system. Lower limits of device sensitivity were found to be approximately 0.05 cGy/s. Herein, this detector was demonstrated to perform quality assurance of clinical 192Ir HDR brachytherapy procedures, providing comparable dose measurements as thermo-luminescent dosimeters and accuracy within 20% of the treatment planning software (TPS) for 27 treatments conducted, with an inter-quartile range ratio to the TPS dose value of (1.02-0.94=0.08). After removing contaminant signals (Cerenkov and diode background), calibration of the detector enabled accurate dose measurements for vaginal applicator brachytherapy procedures. For 192Ir use, energy response changed by a factor of 2.25 over the SDD values of 3 to 9 cm; however a cap made of 0.2 mm thickness silver reduced energy dependence to a factor of 1.25 over the same SDD range, but had the consequence of reducing overall sensitivity by 33%.

For preclinical measurements, dose accuracy of the NanoFOD was within 1.3% of MOSFET measured dose values in a cylindrical mouse phantom at 225 kV for x-ray irradiation at angles of 0, 90, 180, and 270˝. The NanoFOD exhibited small changes in angular sensitivity, with a coefficient of variation (COV) of 3.6% at 120 kV and 1% at 225 kV. When the NanoFOD was placed alongside a MOSFET in the liver of a sacrificed mouse and treatment was delivered at 225 kV with 0.3 mm Cu filter, the dose difference was only 1.09% with use of the 4x4 cm collimator, and -0.03% with no collimation. Additionally, the NanoFOD utilized a scintillator of 11 µm thickness to measure small x-ray fields for microbeam radiation therapy (MRT) applications, and achieved 2.7% dose accuracy of the microbeam peak in comparison to radiochromic film. Modest differences between the full-width at half maximum measured lateral dimension of the MRT system were observed between the NanoFOD (420 µm) and radiochromic film (320 µm), but these differences have been explained mostly as an artifact due to the geometry used and volumetric effects in the scintillator material. Characterization of the energy dependence for the yttrium-oxide based scintillator material was performed in the range of 40-320 kV (2 mm Al filtration), and the maximum device sensitivity was achieved at 100 kV. Tissue maximum ratio data measurements were carried out on a small animal x-ray irradiator system at 320 kV and demonstrated an average difference of 0.9% as compared to a MOSFET dosimeter in the range of 2.5 to 33 cm depth in tissue equivalent plastic blocks. Irradiation of the NanoFOD fiber and scintillator material on a 137Cs gamma irradiator to 1600 Gy did not produce any measurable change in light output, suggesting that the NanoFOD system may be re-used without the need for replacement or recalibration over its lifetime.

For small animal irradiator systems, researchers can deliver a given dose to a target organ by controlling exposure time. Currently, researchers calculate this exposure time by dividing the total dose that they wish to deliver by a single provided dose rate value. This method is independent of the target organ. Studies conducted here used Monte Carlo particle transport codes to justify a new method of dose prescription in mice, that considers organ specific doses. Monte Carlo simulations were performed in the Geant4 Application for Tomographic Emission (GATE) toolkit using a MOBY mouse whole-body phantom. The non-homogeneous phantom was comprised of 256x256x800 voxels of size 0.145x0.145x0.145 mm3. Differences of up to 20-30% in dose to soft-tissue target organs was demonstrated, and methods for alleviating these errors were suggested during whole body radiation of mice by utilizing organ specific and x-ray tube filter specific dose rates for all irradiations.

Monte Carlo analysis was used on 1 µm resolution CT images of a mouse femur and a mouse vertebra to calculate the dose gradients within the bone marrow (BM) compartment of mice based on different radiation beam qualities relevant to x-ray and isotope type irradiators. Results and findings indicated that soft x-ray beams (160 kV at 0.62 mm Cu HVL and 320 kV at 1 mm Cu HVL) lead to substantially higher dose to BM within close proximity to mineral bone (within about 60 µm) as compared to hard x-ray beams (320 kV at 4 mm Cu HVL) and isotope based gamma irradiators (137Cs). The average dose increases to the BM in the vertebra for these four aforementioned radiation beam qualities were found to be 31%, 17%, 8%, and 1%, respectively. Both in-vitro and in-vivo experimental studies confirmed these simulation results, demonstrating that the 320 kV, 1 mm Cu HVL beam caused statistically significant increased killing to the BM cells at 6 Gy dose levels in comparison to both the 320 kV, 4 mm Cu HVL and the 662 keV, 137Cs beams.

Purpose: (1) Evaluate use of portal monitor in a radiological emergency, (2) study the variation of MOSFET sensitivity with integrated dose history and (3) assess the accuracy of AAPM 96 formalism for estimating effective dose from pediatric CT scans.

Methods and Materials: (Project 1): The maximum distance between the portal monitor and a variety of test sources where radiation could be detected was measured as well as the exposure rate at the center of the portal monitor. This information was used in conjunction with queue theory to create a plan of use for the portal monitor as well as improvements to make it more suitable for a radiological emergency.

(Project 2): The variation of MOSFET sensitivity was investigated for two diagnostic and two therapeutic MOSFETs (high and standard bias, respectively). The MOSFETs were exposed to an orthovoltage beam with an ion chamber in order to obtain the calibration factor (mV/cGy) at several points over the MOSFET lifespan. The diagnostic MOSFETs were exposed to a 120 kVp beam and the therapeutic MOSFETs were exposed to a 250 kVp beam.

(Project 3): Organ doses were measured for a variety of pediatric CT protocols using diagnostic MOSFETs loaded into a pediatric anthropomorphic phantom. The protocols investigated were head, chest, abdomen-pelvis and chest-abdomen-pelvis. Organ doses were measured for these protocols using a GE and a Siemens 64 slice CT. The effective dose was obtained using the ICRP 103 formalism. These values of effective dose were compared to the effective dose estimation method described by AAPM report 96, which employs a DLP-to-effective dose conversion factor.

Results: (Project 1): The minimum exposure rate at the center of the portal monitor where radiation could be detected was measured to be 30 μR/hr. In order to prevent false positive readings when using the portal monitor in a radiological emergency, there must be some distance between the portal monitor and the group of people waiting to be scanned, which depends on the collective activity. Assuming a collective activity of 100 mCi of 137Cs, there must be at least 33.1 m between the portal monitor and those waiting to be scanned. Additionally, multiple portal monitors should be at least 10 m apart in order to prevent false positive readings, which assumes an individual contamination of as much as 10 mCi of 137Cs. If portable shielding is available, these distances can be dramatically decreased.

(Project 2): The diagnostic MOSFETs showed a fairly constant sensitivity from 0 mV to 12,000 mV, with a maximum variation of 4%. At this point, the calibration factor decreased by an average of 19% from 12,000 mV to 18,000 mV. Conversely, the therapeutic MOSFETs showed an approximately linear decrease in sensitivity. The calibration factor decreased by an average of 3% every 3,000 mV until 18,000 mV.

(Project 3): The effective dose for the GE scans was underestimated by 133.27%, 55.84%, 30.24% and 19.13% for the head, chest, abdomen-pelvis and chest-abdomen- pelvis scans, respectively. Conversely, the AAPM 96 formalism underestimated the effective dose from the Siemens head scan but overestimated the effective dose from the three other Siemens scans. The effective dose for the Siemens head scan was underestimated by 88.66% but the effective dose was overestimated by 102.81%, 114.52% and 96.19% for the chest, abdomen-pelvis and chest-abdomen-pelvis scans, respectively.

Conclusion: (Project 1): The portal monitor is not suitable for use in a radiological emergency unless an abundance of space is available. In order to improve the portal monitor, the sensitivity should be reduced and shielding should be added around the detectors.

(Project 2): Data suggests that the diagnostic MOSFETs can be used with their initial calibration factor until the age of 15,000 mV. At this time, a new calibration factor should be obtained or the MOSFETs should be discarded. On the other hand, a correction factor can be applied to the initial calibration factor for the therapeutic MOSFETs. This takes advantage of the approximately linear nature of the decrease in sensitivity with integrated dose.

(Project 3): A "one size fits all" conversion factor for estimating CT effective dose is not sufficient. This conversion factor should be expanded to specific CT manufacturers in addition to patient age and scan location. Additionally, these conversion factors should be updated with modern Monte Carlo simulations.

Project 1: Physics characterization of TLD-600 and TLD-700

Purpose:

It is suggested that a pair of TLD-600 and TLD-700 can measure the exposure in neutron-photon mix fields. But the basic information of physics characterization of TLD-600 and 700 are not available. The purpose of this study was study the individual TLD variation and the energy dependence of TLD-600 and TLD-700.

Methods:

The individual calibration factors for 52 TLD-600 chips and 51 TLD-700 chips were determined under x-ray beams of 60 kVp, 80 kVp, 120 kVp, a mono-energetic 662 keV gamma beam of a Cs-137 source, and an Am-Be neutron beam (4.4 MeV). The individual calibration factor was calculated as the ratio of the group average response in uC/mR and the individual response in uC/mR. In addition, energy corrections factors for the individual calibration factors were determined, from each of the x-ray beams (60 kVp, 80 kVp, 120 kVp) to the 662 keV Cs-137 gamma beams.

Results:

For TLD-600, the range and relative standard deviation of the individual calibration factors are: 60 kVp (0.94003-1.0927, 3.5369%), 80 kVp (0.9395-1.0867, 3.0952%), 120 kVp (0.83403-1.0796, 4.5732%), 662 keV (0.80465-1.1926, 9.2515% ), AmBe (0.91740-0.94905, 3.0882% ); and the energy corrections factors relative to the 662 keV Cs-137 beams are: 60 kVp (1.2223), 80 kVp (1.1013), 120 kVp (1.0299).

For TLD-700 the range and relative standard deviation of the individual calibration factors are: 60 kVp (0.94351-1.0630, 2.6044%), 80 kVp (0.91690-1.0614, 2.6996%), 120 kVp (0.95697-1.0474, 2.3606%), 662 keV (0.91348-1.2270 , 4.2243%), AmBe (0.79330-1.2268 , 9.1577%); and the energy corrections factors relative to the 662 keV Cs-137 beams are: 60 kVp (1.0373), 80 kVp (0.97661), 120 kVp (0.88532).

Conclusion:

We have measured individual calibration factors and the average energy correction factors for photon beams and Am-Be neutron beams. Our results will be used in the future experiments and measurements with TLD-600 and TLD-700.

Project 2: Acceptance testing of new X-RAD 160 Biological X-Ray Irradiator

Purpose:

An X-RAD 160 Biological X-Ray Irradiator was recently installed at Duke University to serve as a key device for cellular radiobiology research. The purpose of this study is to perform acceptance testing on the new irradiator for operator radiation safety and irradiation specifications.

Methods:

The acceptance testing included tests of the following components: (1) Leakage radiation survey, (2) Half-value layer (beam quality), (3) Uniformity, (4) KVp accuracy, (5) Exposure at varying mA (linearity of mA), (6) Exposure at varying kVp, (7) Inverse square measurements, (8) Field size measurement, (9) Exposure constancy.

The irradiation parameters for each components of first round of acceptance testing performed on September 21, 2012 were: Leakage radiation survey (none, 160 kVp, 18 mA, 200s), Beam quality (40cm, 50-140 kVp in 10 kVp incensement, 1 mA, 10s, none), Uniformity (40cm, 160 kVp, 18 mA, 15s, F1), KVp accuracy (40cm, 50-150 kVp in 10 kVp incensement, 10 mA, 15s, none), Linearity of mA (40cm, 160 kVp, 2-18 mA, 15s, none), Inverse square measurements (20-63cm, 160 kVp, 1mA, 30s, none), Field size measurement (40cm, 160 kVp, 10 mA, 15s, none), Exposure constancy (40cm, 160 kVp, 18 mA, 20s, none).

The irradiation parameters for each components for each components of second round of acceptance testing performed on November 18, 2012 were: Beam quality (40cm, 35-150 kVp, 1 mA, 10s, F1) , KVp accuracy (40cm, 35-150 kVp, 1 mA, 10s, F1), Variation of kVp (40cm, 160 kVp, 18 mA, 30s, F1), Linearity of mA (40cm, 160 kVp, 1-18 mA, 30s, F1), Uniformity (40cm, 160 kVp, 18 mA, 30s, F1), Inverse square measurements (20-63cm, 160 kVp, 18 mA, 30s, F1).

Results:

The first round of acceptance testing performed on September 21, 2012 failed due to the fact that the measured exposure along the X-axis was significantly non-uniform; the exposure greatly decreases going in the left direction, which is a clear indication of un-corrected anode heel effect. After the X-ray tube was returned to the manufacturer, the beam was reconfigured by tilting the X-ray tube. Another round of acceptance testing was performed on December 18, 2012.

Conclusion:

The acceptance testing fulfilled the initial purpose. The machine is currently used normally In the following experiments; routine maintenance and quality assurance (QA) are required.

Patients undergoing cardiac catheterization are potentially at risk of radiation-induced health effects from the interventional fluoroscopic X-ray imaging used throughout the clinical procedure. The amount of radiation exposure is highly dependent on the complexity of the procedure and the level of optimization in imaging parameters applied by the clinician. For cardiac catheterization, patient radiation dosimetry, for key organs as well as whole-body effective, is challenging due to the lack of fixed imaging protocols, unlike other common X-ray based imaging modalities.

Pediatric patients are at a greater risk compared to adults due to their greater cellular radio-sensitivities as well as longer remaining life-expectancy following the radiation exposure. In terms of radiation dosimetry, they are often more challenging due to greater variation in body size, which often triggers a wider range of imaging parameters in modern imaging systems with automatic dose rate modulation.

The overall objective of this dissertation was to develop a comprehensive method of radiation dose estimation for pediatric patients undergoing cardiac catheterization. In this dissertation, the research is divided into two main parts: the Physics Component and the Clinical Component. A proof-of-principle study focused on two patient age groups (Newborn and Five-year-old), one popular biplane imaging system, and the clinical practice of two pediatric cardiologists at one large academic medical center.

The Physics Component includes experiments relevant to the physical measurement of patient organ dose using high-sensitivity MOSFET dosimeters placed in anthropomorphic pediatric phantoms.

First, the three-dimensional angular dependence of MOSFET detectors in scatter medium under fluoroscopic irradiation was characterized. A custom-made spherical scatter phantom was used to measure response variations in three-dimensional angular orientations. The results were to be used as angular dependence correction factors for the MOSFET organ dose measurements in the following studies. Minor angular dependence (< ±20% at all angles tested, < ±10% at clinically relevant angles in cardiac catheterization) was observed.

Second, the cardiac dose for common fluoroscopic imaging techniques for pediatric patients in the two age groups was measured. Imaging technique settings with variations of individual key imaging parameters were tested to observe the quantitative effect of imaging optimization or lack thereof. Along with each measurement, the two standard system output indices, the Air Kerma (AK) and Dose-Area Product (DAP), were also recorded and compared to the measured cardiac and skin doses – the lack of correlation between the indices and the organ doses shed light to the substantial limitation of the indices in representing patient radiation dose, at least within the scope of this dissertation.

Third, the effective dose (ED) for Posterior-Anterior and Lateral fluoroscopic imaging techniques for pediatric patients in the two age groups was determined. In addition, the dosimetric effect of removing the anti-scatter grid was studied, for which a factor-of-two ED rate reduction was observed for the imaging techniques.

The Clinical Component involved analytical research to develop a validated retrospective cardiac dose reconstruction formulation and to propose the new Optimization Index which evaluates the level of optimization of the clinician’s imaging usage during a procedure; and small sample group of actual procedures were used to demonstrate applicability of these formulations.

In its entirety, the research represents a first-of-its-kind comprehensive approach in radiation dosimetry for pediatric cardiac catheterization; and separately, it is also modular enough that each individual section can serve as study templates for small-scale dosimetric studies of similar purposes. The data collected and algorithmic formulations developed can be of use in areas of personalized patient dosimetry, clinician training, image quality studies and radiation-associated health effect research.

While it is well known that exposure to radiation can result in cataract formation, questions still remain about the presence of a dose threshold in radiation cataractogenesis. Since the exposure history from diagnostic CT exams is well documented in a patient’s medical record, the population of patients chronically exposed to radiation from head CT exams may be an interesting area to explore for further research in this area. However, there are some challenges in estimating lens dose from head CT exams. An accurate lens dosimetry model would have to account for differences in imaging protocols, differences in head size, and the use of any dose reduction methods.

The overall objective of this dissertation was to develop a comprehensive method to estimate radiation dose to the lens of the eye for patients receiving CT scans of the head. This research is comprised of a physics component, in which a lens dosimetry model was derived for head CT, and a clinical component, which involved the application of that dosimetry model to patient data.

The physics component includes experiments related to the physical measurement of the radiation dose to the lens by various types of dosimeters placed within anthropomorphic phantoms. These dosimeters include high-sensitivity MOSFETs, TLDs, and radiochromic film. The six anthropomorphic phantoms used in these experiments range in age from newborn to adult.

First, the lens dose from five clinically relevant head CT protocols was measured in the anthropomorphic phantoms with MOSFET dosimeters on two state-of-the-art CT scanners. The volume CT dose index (CTDIvol), which is a standard CT output index, was compared to the measured lens doses. Phantom age-specific CTDIvol-to-lens dose conversion factors were derived using linear regression analysis. Since head size can vary among individuals of the same age, a method was derived to estimate the CTDIvol-to-lens dose conversion factor using the effective head diameter. These conversion factors were derived for each scanner individually, but also were derived with the combined data from the two scanners as a means to investigate the feasibility of a scanner-independent method. Using the scanner-independent method to derive the CTDIvol-to-lens dose conversion factor from the effective head diameter, most of the fitted lens dose values fell within 10-15% of the measured values from the phantom study, suggesting that this is a fairly accurate method of estimating lens dose from the CTDIvol with knowledge of the patient’s head size.

Second, the dose reduction potential of organ-based tube current modulation (OB-TCM) and its effect on the CTDIvol-to-lens dose estimation method was investigated. The lens dose was measured with MOSFET dosimeters placed within the same six anthropomorphic phantoms. The phantoms were scanned with the five clinical head CT protocols with OB-TCM enabled on the one scanner model at our institution equipped with this software. The average decrease in lens dose with OB-TCM ranged from 13.5 to 26.0%. Using the size-specific method to derive the CTDIvol-to-lens dose conversion factor from the effective head diameter for protocols with OB-TCM, the majority of the fitted lens dose values fell within 15-18% of the measured values from the phantom study.

Third, the effect of gantry angulation on lens dose was investigated by measuring the lens dose with TLDs placed within the six anthropomorphic phantoms. The 2-dimensional spatial distribution of dose within the areas of the phantoms containing the orbit was measured with radiochromic film. A method was derived to determine the CTDIvol-to-lens dose conversion factor based upon distance from the primary beam scan range to the lens. The average dose to the lens region decreased substantially for almost all the phantoms (ranging from 67 to 92%) when the orbit was exposed to scattered radiation compared to the primary beam. The effectiveness of this method to reduce lens dose is highly dependent upon the shape and size of the head, which influences whether or not the angled scan range coverage can include the entire brain volume and still avoid the orbit.

The clinical component of this dissertation involved performing retrospective patient studies in the pediatric and adult populations, and reconstructing the lens doses from head CT examinations with the methods derived in the physics component. The cumulative lens doses in the patients selected for the retrospective study ranged from 40 to 1020 mGy in the pediatric group, and 53 to 2900 mGy in the adult group.

This dissertation represents a comprehensive approach to lens of the eye dosimetry in CT imaging of the head. The collected data and derived formulas can be used in future studies on radiation-induced cataracts from repeated CT imaging of the head. Additionally, it can be used in the areas of personalized patient dose management, and protocol optimization and clinician training.