Browsing by Subject "MOSFET"

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.

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.