Browsing by Subject "Dose"

Recently, there has been a lot of effort placed on monitoring patient dose from medical procedures. The majority of people's concern has been focused on computed tomography because of the higher amounts of patient dose associated with CT exams. Our institution currently has dose monitoring programs for CT, nuclear medicine, and digital projection radiography. However, there is currently no established way to track patient dose for computed radiography. The current method of tracking computed radiography is to track exposure indicators which are not directly meaningful to patient dose. In order to address this issue, I have expanded on the exposure indicator tracking by adding a conversion for estimated patient effective dose in computed radiography.

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.

Recently, there has been renewed interest in decreasing radiation dose to patients from diagnostic imaging procedures. So far, efforts to decrease radiation dose have focused on the amount of radiation delivered from typical techniques and fail to capture the variation in radiation dose between patients. Despite the feasibility of estimating patient-specific radiation doses and the potential for this practice to aid in protocol optimization, it is not currently standard procedure for hospitals to monitor radiation dose for all patients. To address this shortcoming, we have developed an institution-wide patient-specific radiation dose monitoring program for computed tomography, digital radiography, and nuclear medicine.

Introduction: Stereotactic body radiation therapy (SBRT) is a common treatment techniquethat can be used to treat tumors for multiple cancer sites. Density heterogeneity in the target volume and beam path combined with small treatment fields has made dose calculation in lung SBRT difficult. Dose calculation algorithms used historically have difficulty modelling the extreme density heterogeneity present in lung SBRT and have been shown to overestimate the dose delivered to tumors situated in the lung parenchyma. Recently, more advanced algorithms that directly model heterogeneity have been implemented for clinical treatment planning. The limited accuracy of historically utilized dose calculation algorithms has raised questions about their effects on local control due to the possibility of tumor underdosing. The first part of this work establishes a proper dose normalization technique when implementing these advanced algorithms for treatment planning in order to keep consistent radiation beam settings and to quantify the dosimetric effect of various dose normalizations. The second aim is to quantify the effects dosimetric accuracy has on local control in lung SBRT.

Materials/Methods: 87 lung SBRT plans with doses originally calculated with the AnisotropicAnalytical Algorithm (AAA) had their doses recalculated with the new Acuros XB (AXB) algorithm, which is able to directly model the heterogeneity of the lungs and treatment volume. After recalculation, the plan was normalized to the planning target volume (PTV) D95%, internal target volume (ITV) D99%, and to match the original PTV coverage. The percentage change in total monitor units (MU) between the AXB renormalized plans and the original AAA plans were calculated to quantify how the delivered radiation would change when implementing the AXB algorithm for treatment planning. Percentage changes in relevant PTV and ITV dose metrics as well as absolute changes in relevant organ at risk. (OAR) dose metrics were quantified to compare plan dosimetry. OAR doses were also compared to the current institutional planning objectives to investigate the feasibility of meeting the current objectives with the new algorithm. 162 patients previously treated with SBRT were selected from a retrospective protocol comparing the efficacy of SBRT and surgery for treatment of early-stage non-small cell lung cancer. Plans had their doses originally computed with the Pencil Beam Convolution (PBC, n = 8) algorithm or AAA (n = 156). Each plan was recalculated with AXB with identical beam settings. A subset was also recalculated with Monte Carlo to validate the accuracy of the AXB calculations. Percentage changes in relevant PTV and ITV biologically effective doses (BED) were calculated between the original and AXB plans to quantify the magnitude of the dosimetric differences between the old and new algorithm. A multivariable linear regression was performed to investigate which patient and treatment parameters influenced the magnitude of these dosimetric changes. A competing risk analysis was performed to quantify the association between the magnitude of the dosimetric changes and local failure.

Results: Normalizing the AXB plan to the PTV D95% and keeping the original PTVcoverage typically resulted in a total MU increase (average increase of 7.0% and 7.9%, respectively) while normalizing to the ITV D99% resulted in similar total MU (average increase of 0.31%). When normalizing to the PTV D95%, the AXB plans had increased PTV and ITV D1%[Gy] (median increases of 3.4% and 3.2%, respectively) while normalizing to the ITV D99% showed a median 1.9% decrease. Normalizing the AXB plans to the PTV D95% typically resulted in increased OAR dose for all OARs and an inferior ability to meet the OAR planning constraints. Reoptimization of the renormalized plans showed the current OAR objectives to be manageable when using the AXB algorithm. The AXB dose calculations were much more consistent with Monte Carlo than were the original dose calculations. A large range of dosimetric decreases upon recalculation with AXB were observed for both patients who failed locally and those who were controlled. Higher beam energy was found to increase the magnitude of the dosimetric decreases (expected decrease in PTV mean BED of 3.6%, 5.9%, and 9.1% when using 6X, 10X, or 15X, respectively). Total lung volume was also associated with an increased magnitude of dosimetric decrease (expected decerease of 0.8% per 500 cc for the PTV mean BED). The median follow-up time of the cohort was 26 months. 15 patients experienced local failures. Upon univariate analysis, the dosimetric decreases in the PTV and ITV D1% BED were found to be associated with local failure (hazard ratio (HR) of 0.89 (p=0.04) and 0.87 (p=0.02), respectively). Upon multivariate analysis, the dosimetric decrease in the ITV D1% BED remained significant when controlling for PTV volume (HR=0.89 (p=0.04)).

Conclusions: More accurate dose calculation algorithms are beginning to be implementedfor clinical treatment planning. When implementing these new algorithms, issues arise with dose normalization due to the potential for vast differences between the dose distributions calculated with the different algorithms. Normalizing the dose to the PTV D95% in the AXB plan will result in a delivered dose increase relative to a AAA plan while normalizing to the ITV D99% will keep similar delivered doses between the plans. Dose metrics typically increase when normalizing to the PTV D95% (for targets and OARs) while normalizing to the ITV D99% typically decreased the reported dose metrics. The OAR planning objectives are manageable using the AXB algorithm. Many factors are related to the magnitude of the dosimetric decreases observed when recalculating plans with AXB, including but not limited to beam energy and lung volume. Most of the investigated dose metrics were not associated with local failure, but the change in the PTV and ITV D1% BEDs were found to be associated with local failure in the univariate analysis.

X-ray mammography has been the gold standard for breast imaging for decades, despite the significant limitations posed by the two dimensional (2D) image acquisitions. Difficulty in diagnosing lesions close to the chest wall and axilla, high amount of structural overlap and patient discomfort due to compression are only some of these limitations. To overcome these drawbacks, three dimensional (3D) breast imaging modalities have been developed including dual modality single photon emission computed tomography (SPECT) and computed tomography (CT) systems. This thesis focuses on the development and integration of the next generation of such a device for dedicated breast imaging. The goals of this dissertation work are to: [1] understand and characterize any effects of fully 3-D trajectories on reconstructed image scatter correction, absorbed dose and Hounsifeld Unit accuracy, and [2] design, develop and implement the fully flexible, third generation hybrid SPECT-CT system capable of traversing complex 3D orbits about a pendant breast volume, without interference from the other. Such a system would overcome artifacts resulting from incompletely sampled divergent cone beam imaging schemes and allow imaging closer to the chest wall, which other systems currently under research and development elsewhere cannot achieve.

The dependence of x-ray scatter radiation on object shape, size, material composition and the CT acquisition trajectory, was investigated with a well-established beam stop array (BSA) scatter correction method. While the 2D scatter to primary ratio (SPR) was the main metric used to characterize total system scatter, a new metric called ‘normalized scatter contribution’ was developed to compare the results of scatter correction on 3D reconstructed volumes. Scatter estimation studies were undertaken with a sinusoidal saddle (±15° polar tilt) orbit and a traditional circular (AZOR) orbit. Clinical studies to acquire data for scatter correction were used to evaluate the 2D SPR on a small set of patients scanned with the AZOR orbit. Clinical SPR results showed clear dependence of scatter on breast composition and glandular tissue distribution, otherwise consistent with the overall phantom-based size and density measurements. Additionally, SPR dependence was also observed on the acquisition trajectory where 2D scatter increased with an increase in the polar tilt angle of the system.

The dose delivered by any imaging system is of primary importance from the patient’s point of view, and therefore trajectory related differences in the dose distribution in a target volume were evaluated. Monte Carlo simulations as well as physical measurements using radiochromic film were undertaken using saddle and AZOR orbits. Results illustrated that both orbits deliver comparable dose to the target volume, and only slightly differ in distribution within the volume. Simulations and measurements showed similar results, and all measured dose values were within the standard screening mammography-specific, 6 mGy dose limit, which is used as a benchmark for dose comparisons.

Hounsfield Units (HU) are used clinically in differentiating tissue types in a reconstructed CT image, and therefore the HU accuracy of a system is very important, especially when using non-traditional trajectories. Uniform phantoms filled with various uniform density fluids were used to investigate differences in HU accuracy between saddle and AZOR orbits. Results illustrate the considerably better performance of the saddle orbit, especially close to the chest and nipple region of what would clinically be a pedant breast volume. The AZOR orbit causes shading artifacts near the nipple, due to insufficient sampling, rendering a major portion of the scanned phantom unusable, whereas the saddle orbit performs exceptionally well and provides a tighter distribution of HU values in reconstructed volumes.

Finally, the third generation, fully-suspended SPECT-CT system was designed in and developed in our lab. A novel mechanical method using a linear motor was developed for tilting the CT system. A new x-ray source and a custom made 40 x 30 cm2 detector were integrated on to this system. The SPECT system was nested, in the center of the gantry, orthogonal to the CT source-detector pair. The SPECT system tilts on a goniometer, and the newly developed CT tilting mechanism allows ±15° maximum polar tilting of the CT system. The entire gantry is mounted on a rotation stage, allowing complex arbitrary trajectories for each system, without interference from the other, while having a common field of view. This hybrid system shows potential to be used clinically as a diagnostic tool for dedicated breast imaging.

The overarching goal of this project was to investigate organ dose assessment and variability using Monte Carlo methods to study two areas of medical imaging – fluoroscopy and computed tomography. Namely, these studies were intended to (1) provide estimates of the dose incurred by fluoroscopy-guided spinal injection procedures, and (2) investigate dose heterogeneity in chest and abdominopelvic computed tomography (CT) scans for a range of patient sizes. Fluoroscopy dose estimates were calculated using GEANT4, by recreating the patient procedures of six lumbar-sacral epidural injections. Computed tomography dose was estimated with a GPU-accelerated Monte Carlo package, MCGPU. Both simulations used a library of digital human (XCAT) phantoms, which were previously derived from real-patient CT scans. The fluoroscopy simulations suggest that smaller patients have a higher effective dose per dose area product, and the overall results agreed with previous experimental measurements. Variation of absorbed dose within a given organ was calculated for chest and abdominopelvic CT protocols. It was found that the 95th percentile dose can be over 11 times the mean organ dose in pediatric and adult phantoms. Furthermore, if the organ dose is calculated using only voxels within the beam or all the voxels within an organ, the result can change the result by a factor of 8. The change in dose was found to be higher for organs that have smaller fractions within the beam. Several models of tissue-weighted dose were also investigated, following similar methods to those used for effective dose. It was found that these tissue-weighted dose calculations can vary by up to 13% depending on whether the out of field dose is included. We also found that the results were not significantly affected by the pitch or projections per rotation. The results have shown that dose-volume details may be hidden by average dose estimates and suggested the need to consider intra-organ dose heterogeneity in CT dose calculations, particularly in the case of sensitive tissues (e.g., bone marrow) and populations (e.g., pediatric).

Since its inception in the 1970's, computed tomography (CT) has revolutionized the practice of medicine and evolved into an essential tool for diagnosing numerous diseases not only in adults but also in children. The clinical utility of CT examinations has led to a rapid expansion in CT use and a corresponding increase in the radiation burden to patients. CT radiation is of particular concern to children, whose rapidly growing tissues are more susceptible to radiation-induced cancer and who have longer life spans during which cancerous changes might occur. In recent years, the increasing awareness of CT radiation risk to children has brought about growing efforts to reduce CT dose to the pediatric population. The key element of all dose reduction efforts is to reduce radiation dose while maintaining diagnostic accuracy. Substantiating the tradeoff between the two is the motivation behind this dissertation work.

The first part of this dissertation involved the development of an accurate method for estimating patient-specific radiation dose and potential cancer risk from CT examinations. A Monte Carlo program was developed and validated for dose simulation in a state-of-the-art CT system. Combined with realistic computer models of patients created from clinical CT data, the program was applied to estimate patient-specific dose from pediatric chest and abdomen-pelvic CT examinations and to investigate the dose variation across patients due to the variability of patient anatomy and body habitus. The Monte Carlo method was further employed to investigate the effects of patient size and scan parameters on dose and risk for the entire pediatric population.

The second part of this dissertation involved the development of tools needed to study the diagnostic accuracy of small lung nodules on pediatric CT images. A prior method for modeling two-dimensional symmetric liver/lung lesions was extended to create three-dimensional nodules with asymmetric shapes and diffused margins. A method was also developed to estimate quantum noise in the lung region of a CT image based on patient size.

The last part of this dissertation involved assessment of diagnostic accuracy using receiver operating characteristic (ROC) observer experiments. A pilot study of 13 pediatric patients (1-7 years old) was first conducted to evaluate the effect of tube current on diagnostic accuracy, as measured by the area under the ROC curve (Az). A study of 30 pediatric patients (0-15 years old) was then conducted to assess protocol- and scanner-independent relationships between image quality (nodule detectability and noise) and diagnostic accuracy. The relationships between diagnostic accuracy and nodule detectability, between noise and scan parameters, and between dose/risk and scan parameters were lastly combined to yield the relationship between diagnostic accuracy and dose/risk.

For pediatric patients in the same weight/protocol group, organ dose variation across patients was found to be generally small for large organs in the scan coverage (< 10%), larger for small organs in the scan coverage (1-18%), and the largest for organs partially or completely outside the scan coverage (6-77%). Across the entire pediatric population, dose and risk associated with a chest scan protocol decreased exponentially with increasing patient size. The average chest diameter was found to be a stronger predictor of dose and risk than weight and total scan length.

The effects of bowtie filter and beam collimation on dose and risk were small compared to the effects of helical pitch and tube potential. The effects of any scan parameter were patient size-dependent, which could not be reflected by the difference in volume-weighted CT dose index (CTDIvol).

Over a nodule detectability (product of nodule peak contrast and display diameter to noise ratio) range of approximately 52-374 mm with an average of 143 mm, tube current or dose had a weak effect on the diagnostic accuracy of lung nodules. The effect of 75% dose reduction was comparable to inter-observer variability, suggesting a potential for dose reduction.

Diagnostic accuracy increased with increasing nodule detectability over the range of 25-374 mm, but reached a plateau beyond a threshold of ~ 99 mm. The trend was analogous to the relationship between Az and signal-to-noise ratio and suggested that the performance of the radiologists saturates (or increases slowly) beyond a threshold nodule detectability level; further reducing noise or increasing contrast to improve nodule detectability beyond the threshold yields little gain in diagnostic accuracy.

For a typical product of nodule contrast and physical diameter (1400 HU·mm) and a set of most commonly used scan parameters (tube potential of 120 kVp, helical pitch of 1.375, slice thickness of 5 mm, gantry rotation period of 0.4 second, image pixel size of 0.48 mm), diagnostic accuracy increased with effective dose and effective risk for a given patient size, but reached a plateau beyond a threshold dose/risk level. At a given effective dose, Az increased with decreasing patient size, i.e., the dose needed to achieve the same noise and hence diagnostic accuracy increased with patient size. To achieve an Az of 0.90, the dose needed for a 22-cm diameter (male) patient was about quadruple of that for a 10-cm diameter patient. While the effective risk associated with achieving the same diagnostic accuracy also increased with patient size, the risk associated with an Az of 0.90 was only twice as high for a 22-cm diameter (male) patient than for a 10-cm diameter patient due to the older age of the larger patient.

The research in this dissertation has two important clinical implications. First, the quantitative relationships between patient dose/risk and patient size, between patient dose/risk and scan parameters, between diagnostic accuracy and image quality, and between diagnostic accuracy and radiation dose can guide the design of pediatric CT protocols to achieve the desired diagnostic accuracy at the minimum radiation dose. Second, patient-specific dose and risk information, when included in a patient's dosimetry and medical records, can inform healthcare providers of prior radiation exposure and aid in decisions for image utilization, including the situation where multiple examinations are being considered.