M Khatonabadi

The feasibility of a regional CTDIvol to estimate organ dose from tube current modulated CT exams

PURPOSE In AAPM Task Group 204, the size-specific dose estimate (SSDE) was developed by providing size adjustment factors which are applied to the Computed Tomography (CT) standardized dose metric, CTDI(vol). However, that work focused on fixed tube current scans and did not specifically address tube current modulation (TCM) scans, which are currently the majority of clinical scans performed. The purpose of this study was to extend the SSDE concept to account for TCM by investigating the feasibility of using anatomic and organ specific regions of scanner output to improve accuracy of dose estimates. METHODS Thirty-nine adult abdomen/pelvis and 32 chest scans from clinically indicated CT exams acquired on a multidetector CT using TCM were obtained with Institutional Review Board approval for generating voxelized models. Along with image data, raw projection data were obtained to extract TCM functions for use in Monte Carlo simulations. Patient size was calculated using the effective diameter described in TG 204. In addition, the scanner-reported CTDI(vo)l (CTDI(vol),global) was obtained for each patient, which is based on the average tube current across the entire scan. For the abdomen/pelvis scans, liver, spleen, and kidneys were manually segmented from the patient datasets; for the chest scans, lungs and for female models only, glandular breast tissue were segmented. For each patient organ doses were estimated using Monte Carlo Methods. To investigate the utility of regional measures of scanner output, regional and organ anatomic boundaries were identified from image data and used to calculate regional and organ-specific average tube current values. From these regional and organ-specific averages, CTDI(vol) values, referred to as regional and organ-specific CTDI(vol), were calculated for each patient. Using an approach similar to TG 204, all CTDI(vol) values were used to normalize simulated organ doses; and the ability of each normalized dose to correlate with patient size was investigated. RESULTS For all five organs, the correlations with patient size increased when organ doses were normalized by regional and organ-specific CTDI(vol) values. For example, when estimating dose to the liver, CTDI(vol),global yielded a R(2) value of 0.26, which improved to 0.77 and 0.86, when using the regional and organ-specific CTDI(vol) for abdomen and liver, respectively. For breast dose, the global CTDI(vol) yielded a R(2) value of 0.08, which improved to 0.58 and 0.83, when using the regional and organ-specific CTDI(vol) for chest and breasts, respectively. The R(2) values also increased once the thoracic models were separated for the analysis into females and males, indicating differences between genders in this region not explained by a simple measure of effective diameter. CONCLUSIONS This work demonstrated the utility of regional and organ-specific CTDI(vol) as normalization factors when using TCM. It was demonstrated that CTDI(vol),global is not an effective normalization factor in TCM exams where attenuation (and therefore tube current) varies considerably throughout the scan, such as abdomen/pelvis and even thorax. These exams can be more accurately assessed for dose using regional CTDI(vol) descriptors that account for local variations in scanner output present when TCM is employed.

A comparison of methods to estimate organ doses in CT when utilizing approximations to the tube current modulation function

PURPOSE Most methods to estimate patient dose from computed tomography (CT) exams have been developed based on fixed tube current scans. However, in current clinical practice, many CT exams are performed using tube current modulation (TCM). Detailed information about the TCM function is difficult to obtain and therefore not easily integrated into patient dose estimate methods. The purpose of this study was to investigate the accuracy of organ dose estimates obtained using methods that approximate the TCM function using more readily available data compared to estimates obtained using the detailed description of the TCM function. METHODS Twenty adult female models generated from actual patient thoracic CT exams and 20 pediatric female models generated from whole body PET∕CT exams were obtained with IRB (Institutional Review Board) approval. Detailed TCM function for each patient was obtained from projection data. Monte Carlo based models of each scanner and patient model were developed that incorporated the detailed TCM function for each patient model. Lungs and glandular breast tissue were identified in each patient model so that organ doses could be estimated from simulations. Three sets of simulations were performed: one using the original detailed TCM function (x, y, and z modulations), one using an approximation to the TCM function (only the z-axis or longitudinal modulation extracted from the image data), and the third was a fixed tube current simulation using a single tube current value which was equal to the average tube current over the entire exam. Differences from the reference (detailed TCM) method were calculated based on organ dose estimates. Pearsons correlation coefficients were calculated between methods after testing for normality. Equivalence test was performed to compare the equivalence limit between each method (longitudinal approximated TCM and fixed tube current method) and the detailed TCM method. Minimum equivalence limit was reported for each organ. RESULTS Doses estimated using the longitudinal approximated TCM resulted in small differences from doses obtained using the detailed TCM function. The calculated root-mean-square errors (RMSE) for adult female chest simulations were 9% and 3% for breasts and lungs, respectively; for pediatric female chest and whole body simulations RMSE were 9% and 7% for breasts and 3% and 1% for lungs, respectively. Pearsons correlation coefficients were consistently high for the longitudinal approximated TCM method, ranging from 0.947 to 0.999, compared to the fixed tube current value ranging from 0.8099 to 0.9916. In addition, an equivalence test illustrated that across all models the longitudinal approximated TCM is equivalent to the detailed TCM function within up to 3% for lungs and breasts. CONCLUSIONS While the best estimate of organ dose requires the detailed description of the TCM function for each patient, extracting these values can be difficult. The presented results show that an approximation using available data extracted from the DICOM header provides organ dose estimates with RMSE of less than 10%. On the other hand, the use of the overall average tube current as a single tube current value was shown to result in poor and inconsistent estimates of organ doses.

Peak skin and eye lens radiation dose from brain perfusion CT based on Monte Carlo simulation

OBJECTIVE The purpose of our study was to accurately estimate the radiation dose to skin and the eye lens from clinical CT brain perfusion studies, investigate how well scanner output (expressed as volume CT dose index [CTDI(vol)]) matches these estimated doses, and investigate the efficacy of eye lens dose reduction techniques. MATERIALS AND METHODS Peak skin dose and eye lens dose were estimated using Monte Carlo simulation methods on a voxelized patient model and 64-MDCT scanners from four major manufacturers. A range of clinical protocols was evaluated. CTDI(vol) for each scanner was obtained from the scanner console. Dose reduction to the eye lens was evaluated for various gantry tilt angles as well as scan locations. RESULTS Peak skin dose and eye lens dose ranged from 81 mGy to 348 mGy, depending on the scanner and protocol used. Peak skin dose and eye lens dose were observed to be 66-79% and 59-63%, respectively, of the CTDI(vol) values reported by the scanners. The eye lens dose was significantly reduced when the eye lenses were not directly irradiated. CONCLUSION CTDI(vol) should not be interpreted as patient dose; this study has shown it to overestimate dose to the skin or eye lens. These results may be used to provide more accurate estimates of actual dose to ensure that protocols are operated safely below thresholds. Tilting the gantry or moving the scanning region further away from the eyes are effective for reducing lens dose in clinical practice. These actions should be considered when they are consistent with the clinical task and patient anatomy.

Radiation dose reduction in medical x‐ray CT via Fourier‐based iterative reconstruction

PURPOSE A Fourier-based iterative reconstruction technique, termed Equally Sloped Tomography (EST), is developed in conjunction with advanced mathematical regularization to investigate radiation dose reduction in x-ray CT. The method is experimentally implemented on fan-beam CT and evaluated as a function of imaging dose on a series of image quality phantoms and anonymous pediatric patient data sets. Numerical simulation experiments are also performed to explore the extension of EST to helical cone-beam geometry. METHODS EST is a Fourier based iterative algorithm, which iterates back and forth between real and Fourier space utilizing the algebraically exact pseudopolar fast Fourier transform (PPFFT). In each iteration, physical constraints and mathematical regularization are applied in real space, while the measured data are enforced in Fourier space. The algorithm is automatically terminated when a proposed termination criterion is met. Experimentally, fan-beam projections were acquired by the Siemens z-flying focal spot technology, and subsequently interleaved and rebinned to a pseudopolar grid. Image quality phantoms were scanned at systematically varied mAs settings, reconstructed by EST and conventional reconstruction methods such as filtered back projection (FBP), and quantified using metrics including resolution, signal-to-noise ratios (SNRs), and contrast-to-noise ratios (CNRs). Pediatric data sets were reconstructed at their original acquisition settings and additionally simulated to lower dose settings for comparison and evaluation of the potential for radiation dose reduction. Numerical experiments were conducted to quantify EST and other iterative methods in terms of image quality and computation time. The extension of EST to helical cone-beam CT was implemented by using the advanced single-slice rebinning (ASSR) method. RESULTS Based on the phantom and pediatric patient fan-beam CT data, it is demonstrated that EST reconstructions with the lowest scanner flux setting of 39 mAs produce comparable image quality, resolution, and contrast relative to FBP with the 140 mAs flux setting. Compared to the algebraic reconstruction technique and the expectation maximization statistical reconstruction algorithm, a significant reduction in computation time is achieved with EST. Finally, numerical experiments on helical cone-beam CT data suggest that the combination of EST and ASSR produces reconstructions with higher image quality and lower noise than the Feldkamp Davis and Kress (FDK) method and the conventional ASSR approach. CONCLUSIONS A Fourier-based iterative method has been applied to the reconstruction of fan-bean CT data with reduced x-ray fluence. This method incorporates advantageous features in both real and Fourier space iterative schemes: using a fast and algebraically exact method to calculate forward projection, enforcing the measured data in Fourier space, and applying physical constraints and flexible regularization in real space. Our results suggest that EST can be utilized for radiation dose reduction in x-ray CT via the readily implementable technique of lowering mAs settings. Numerical experiments further indicate that EST requires less computation time than several other iterative algorithms and can, in principle, be extended to helical cone-beam geometry in combination with the ASSR method.

Varying kVp as a means of reducing CT breast dose to pediatric patients

We investigated the possibility of reducing radiation dose to the breast tissue of pediatric females by using multiple tube voltages within a single CT examination. The peak kilovoltage (kVp) was adjusted when the x-ray beam was directly exposing the representative breast tissue of a 5-year-old, 10-year-old, and an adult female anthropomorphic phantom; this strategy was called kVp splitting and was emulated by using a different kVp over the anterior and posterior tube angles. Dose savings from kVp splitting were calculated relative to using a fixed kVp over all tube angles and the results indicated savings in all three phantoms when using 80 kVp over the posterior tube angles regardless of the anterior kVp. Monte Carlo (MC) simulations with and without kVp splitting were performed to estimate absorbed breast dose in voxelized models constructed from the CT images of pediatric female patients; 80 kVp was used over the posterior tube angles. The MC simulations revealed breast dose savings of between 9.8% and 33% from using kVp splitting compared to simulations using a fixed kVp protocol with the anterior technique. Before this strategy could be implemented clinically, the development of suitable image reconstruction algorithms and the image quality of scans with kVp splitting would need further study.

SU‐E‐I‐47: A Classification of Validation Tasks for Monte Carlo Simulations of CT Scanners: From Simple to Complex Source Models and Geometries

PURPOSE To develop a classification system of validation tasks for Monte Carlo simulations of CT scanners that reflects the level of complexities of source and object models used in Monte Carlo simulation methods within the medical physics community and specifically within X-ray CT dosimetry simulations. METHODS In order to quantify the relative strength of CT Monte Carlo models developed for CT dosimetry with respect to imaging parameters such as source motion, tube current modulation, phantom type and location of measurement, we have devised a classification system consisting of six levels, each with a number of sublevels that encompass an extensive number of validation scenarios. The complexity of the levels/sublevels ranges from very simple source and object models, such as simulating and validating a CTDI in air measurement at isocenter with a single axial source rotation and fixed tube current, to complex source and patient models, such as simulating and validating surface and depth dose integrated readings from TLDs for a patient model with helical source movement and tube current modulation. 15 unique CT Monte Carlo models found in the literature with some validation of Monte Carlo simulations were examined within the scope of the classification system. RESULTS All 15 models had a primary validation measurement that fell under the first two levels of the classification system, CTDI measurements. Only 5 models demonstrated validation of the model beyond CTDI measurements. Only 2 of these models demonstrated helical scan validation. No models demonstrated tube current modulation validation. CONCLUSION This investigation has revealed that a vast majority of advanced CT dosimetry Monte Carlo simulations lack validation beyond CTDI measurements. This classification system has the potential to be used as a justification metric as to the appropriateness of a particular CT Monte Carlo model for use in a specific CT dosimetry simulation. (a) Dr. McNitt-Gray: Institutional research agreement, Siemens AG; Recipient research support Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski, LLC; (b) K McMillan: Research partially supported by a grant from Siemens AG; Research partially supported by a grant from AACR; Research partially supported by a grant from TRDRP; (c) M Khatonabadi: Research partially supported by a grant from Siemens AG.

WE‐A‐218‐10: The Tradeoff between Diagnostic Performance and Radiation Dose for CT Imaging in the Diagnosis of Appendicitis Across Observers with Various Levels of Experience

Purpose: To investigate the tradeoffs between radiationdose and diagnostic performance in CT for a challenging clinical task (diagnosis of appendicitis). Methods: This IRB approved study utilized data for 20 patients undergoing clinical CT exams for indications of appendicitis. Medical records were reviewed to establish true diagnosis and identified 10 positive and 10 negative cases. Original (100%) and simulated reduced dose levels (70%, 50%, 30%, 20% of original) were created with a validated software tool using raw projection data from each scan. An observer study was performed with 6 radiologists (of different training and cross‐sectional reading experience) reviewing each case at each dose level in stratified random order over several sessions. Readers assessed image quality and provided confidence in their diagnosis of appendicitis, each on a 5 point scale. Receiver Operating Characteristics (ROC) curves were generated for each dose level using all rating levels and from the resulting ROC curves, the AUC (Area under curve) was calculated for each dose level. This analysis was repeated for groups of readers with different experience levels. Results: The ROC curves averaged over all 6 observers and corresponding AUC values showed indifferent performances for all the dose levels. For the 2 non‐abdominal trained, occasional CT readers, the performance did not decrease until 30% dose level. For the 2 non‐abdominal trained, routine CT readers, the performance did not decrease until 20% dose level. For the 2 abdominal trained, routine CT readers, the performance is consistent across all the dose levels. Conclusions: This preliminary study demonstrated the tradeoffs between radiationdose and diagnostic performance and indicated that: (a) There is essentially no difference between diagnostic performance of 100%, 70%, and 50% dose level for all 6 observers. (b) For abdominal CT specialists, the diagnostic difference is not substantially compromised even at 20% dose levels. (c) For non‐abdominal trained CT readers, the performance declines at 30% and 20% dose levels. For Michael McNitt‐ Gray: Institutional research agreement, Siemens AG Recipient research support Siemens AG

TH‐A‐214‐08: Change in X‐Ray CT Spectra inside of Dosimetry Phantoms: Beam Hardening or Beam Softening?

Purpose: To account for the energy dependence of film‐ or solid state‐based dosimeters used in CT, they are often calibrated in air. However, they are used in a CTDI phantom, which may produce a significantly different spectrum due to scatter. The purpose of this study is to use Monte Carlo simulations to investigate the change in CT x‐ray energy spectra between exposures in air and in CTDI phantoms. Methods: The x‐ray fluence in air, and inside both 16 and 32 cm phantoms were estimated using Monte Carlo simulations. A Siemens Sensation 64 CTscanner using 24×1.2mm beam collimation, and a Toshiba Aquilion 64 CTscanner using 8×4mm beam collimation were simulated. In addition, simulations were performed using 2mm collimation for the Toshiba Aquilion 64. For all conditions, the spectra were estimated by tallying at within the phantom to estimate x‐ray fluence. Based on these spectra, the average energy and estimated Half Value Layer were obtained and compared. Results: For Sensation 64 scanner, the HVL decreased from 9.8 mm Al in air to 7.9 and 7.2 mm Al in center of head and body phantoms, respectively. For Aquilion 64 scanner the HVL decreased from 6.2 mm Al in air to 5.9 and 5.8 mm Al in center of head and body phantoms, respectively; however results also showed that the HVLs increased at 12:00 position (to 6.4 and 6.8 mm Al). For Aquilion scanner at narrow collimation setting, the HVLs increased at all positions. Conclusions: The spectra inside phantoms are nearly always different from that of air — under some conditions it is harder and under others it is softer. These differences in spectra should be taken into account when calibrating dosimeters that have energy dependence.

MO‐D‐134‐02: Estimating Organ Dose From CT Scans Performed with Tube Current Modulated Scans

PURPOSE To develop and evaluate a method for estimating organ dose from CT scans performed with tube current modulation (TCM) using a size metric that accounts for patient attenuation as well as a regional descriptor of scanner output. METHODS 100 chest and 81 abdomen/pelvis patient scans acquired using TCM on a Multidetector row CT scanner (CareDose 4D, Sensation 64, Siemens Healthcare) were collected under IRB approval to generate voxelized patient models. The TCM information was extracted from the raw projection data for use in Monte Carlo (MC) simulations which accounted for both scanner and patient details. For each patient, water equivalent diameter (WED) was used as the size metric to account for patient attenuation. WED was calculated on each image based on semiautomatic segmentation. A regional CTDIvol value was used as a normalization quantity to account for local variations in scanner output. MC simulations were used to estimate dose to lungs and glandular breast tissue in the chest models, and liver, spleen, and kidneys in the abdomen/pelvis models. Half of the models in each exam category were used as a training set to develop the organ dose estimation model based on WED and CTDIvol,regional; the remaining cases were used as a test set. Organ doses were estimated using each model and were compared to the detailed MC simulation results and an RMSE across cases was calculated. RESULTS For chest exams, the RMSE between the model and detailed MC simulations was 14.9% for lung and 13.6% for breast. For abdomen/pelvis exams, the RMSE was 15.0% for kidney, 9.2% for liver and 12.2% for spleen. CONCLUSION This method to estimate organ dose from CT scans using TCM demonstrated agreement of within 33% between the model and detailed MC simulations and shows promise for estimating organ doses when TCM is used. Dr. Michael McNitt-Gray: Institutional research agreement, Siemens AG Recipient research support Siemens AG Consultant, Flaherty Sensabaugh Bonasso PLLC Consultant, Fulbright and Jaworski, LLC Maryam Khatonabadi: Recipient research support Siemens AG.

MO‐D‐134‐09: Assessing DNA Damage Repair From CT Studies in Whole Blood

PURPOSE To assess biological damage due to radiation from CT studies using in vivo and ex vivo blood samples processed with a newly developed protocol involving flow cytometry. METHODS This study was carried out under IRB approval. Blood samples were collected from 21 patients undergoing clinically-indicated CT exams. Blood was procured prior to, immediately after and 30minutes following irradiation. For each patient, a sample of pre-scan blood was transferred into a vial and positioned on the patient within the scan region for ex vivo comparative analysis. Whole blood samples were fixed immediately following each collection to arrest cellular metabolism. The cellular response to DNA double strand breaks (DSBs) was analyzed by flow cytometric quantification of gammaH2AX fluorescence. Median fluorescence of treatment samples were compared to non-irradiated control blood samples for each patient. RESULTS Four patient samples were excluded due to technical issues in collection and analysis. The remaining 17 showed two major trends for gammaH2AX signal following irradiation in vivo. Some samples showed an increase immediately following irradiation and a further increase at 30minutes. Others showed an increase immediately following irradiation then a subsequent decrease at 30minutes. However, ex vivo samples showed different kinetics to in vivo counterparts. Interestingly, two patients had high basal gammaH2AX fluorescent signal (p<0.01) and exhibited large increases following irradiation. This Result was apparent for both patients after in vivo analysis, but only one patient after ex vivo analysis. CONCLUSION These results indicate significant variation in damage and repair kinetics across a small sample of patients; furthermore, we demonstrated differences between in vivo and ex vivo samples. Rapid analysis and accurate timing offer the ability to assess a large number of patients to establish baseline population data. Additionally, this method may serve as an indicator to identify patients for further analysis of radiation sensitivity. Research partially supported by a grant from Siemens AG.