C Cagnon

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.

TH-AB-207A-04: Assessment of Patients’ Cumulative Effective Dose From CT Examinations

PURPOSE The Joint Commission requires institutions to consider patients age and recent imaging exams when deciding on the most appropriate type of imaging exam. Additionally, knowing patients imaging history can help prevent duplicate scans. Radiation dose management software affords new opportunities to identify and utilize patients with high cumulative doses as one proxy for subsequent review of imaging history and opportunities in avoiding redundant exams. METHODS Using dose management software (Radimetrics, Bayer Healthcare) a total of 72073 CT examinations performed from Jan 2015 to Jan 2016 were examined to categorize patients with a cumulative effective dose of 100 mSv and above. This threshold was selected based on epidemiological studies on populations exposed to radiation, which demonstrate a statistical increase of cancer risk at doses above 100 mSv. Histories of patients with highest cumulative dose and highest number of exams were further investigated by a Radiologist for appropriateness of recurrent studies and potential opportunities for reduction. RESULTS Out of 34762 patients, 927 (2.7%) were identified with a cumulative dose of 100 mSv and above. The highest cumulative dose (842 mSv) belonged to an oncology patient who underwent 2 diagnostic exams and 9 interventional ablative CT guided procedures. The patient with highest number of exams (56 counts) and cumulative dose of 170 mSv was a 17 year old trauma patient. An imaging history review of these two patients did not suggest any superfluous scans. CONCLUSION Our limited pilot study suggests that recurrent CT exams for patients with oncologic or severe trauma history may be warranted and appropriate. As a result, for future studies we will be focusing on high dose patient cohorts not associated with oncology or severe trauma. Additionally, the review process itself has suggested areas for potential improvement in patient care, including improved documentation and Radiologist involvement in patient management. Dr. McNitt-Grays disclosures: Institutional research agreement, Siemens Healthcare; Past recipient, research grant support, Siemens Healthcare; Consultant, Toshiba America Medical Systems; Consultant, Samsung Electronics.

TH-EF-BRA-01: Patient-Size Specific Analysis of CT Doses From Lung Cancer Screening

Purpose: The US Centers for Medicare & Medicaid Services (CMS) recently issued its final decision to approve the use of low dose CT for lung cancer screening and described some dose index requirements. The aim of this study is to evaluate CT dose indices from lung cancer screening taking into account patient size. Methods: CT dose index data (CTDIvol, DLP) and patient size (height and weight) to calculate body mass index (BMI) were collected for lung cancer screening exams from Jan 1st to Dec 31th, 2014. All exams used Tube Current Modulation with identical Image Quality Reference parameter (Quality Reference mAs). X-ray dose management software was used to mine the data. CTDIvol and DLP values were analyzed based on patient’s BMI classification. For a subset of patients, the average water equivalent diameter was estimated as well as Size Specific Dose Estimate (SSDE). All results were compared to the Medicare requirements (CTDIvol ≤ 3.0 mGy for a standard sized patient; appropriate reductions for smaller patients and increases for larger patients). Results: The dose indices over 351 patients were (mean ± SD): CTDIvol of 2.1 ± 0.7 mGy (min-max: 1.2 – 6.0 mGy); DLP of 75 ± 24 mGy*cm (min-max: 38 – 202 mGy*cm). The CTDIvol values by patient size (BMI) were 1.4 ± 0.2 mGy for underweight (BMI 3.0 mGy; for each of these cases, the BMI was at least 27.0. Conclusion: The scan protocol used resulted in CTDIvol values well within the Medicare requirements. Patients with larger BMI receive higher CTDIvol, but values > 3.0 mGy only occurred for overweight and obese patients.

MO-E-17A-08: Attenuation-Based Size Adjusted, Scanner-Independent Organ Dose Estimates for Head CT Exams: TG 204 for Head CT.

PURPOSE AAPM Task Group 204 described size specific dose estimates (SSDE) for body scans. The purpose of this work is to use a similar approach to develop patient-specific, scanner-independent organ dose estimates for head CT exams using an attenuation-based size metric. METHODS For eight patient models from the GSF family of voxelized phantoms, dose to brain and lens of the eye was estimated using Monte Carlo simulations of contiguous axial scans for 64-slice MDCT scanners from four major manufacturers. Organ doses were normalized by scannerspecific 16 cm CTDIvol values and averaged across all scanners to obtain scanner-independent CTDIvol-to-organ-dose conversion coefficients for each patient model. Head size was measured at the first slice superior to the eyes; patient perimeter and effective diameter (ED) were measured directly from the GSF data. Because the GSF models use organ identification codes instead of Hounsfield units, water equivalent diameter (WED) was estimated indirectly. Using the image data from 42 patients ranging from 2 weeks old to adult, the perimeter, ED and WED size metrics were obtained and correlations between each metric were established. Applying these correlations to the GSF perimeter and ED measurements, WED was calculated for each model. The relationship between the various patient size metrics and CTDIvol-to-organ-dose conversion coefficients was then described. RESULTS The analysis of patient images demonstrated the correlation between WED and ED across a wide range of patient sizes. When applied to the GSF patient models, an exponential relationship between CTDIvol-to-organ-dose conversion coefficients and the WED size metric was observed with correlation coefficients of 0.93 and 0.77 for the brain and lens of the eye, respectively. CONCLUSION Strong correlation exists between CTDIvol normalized brain dose and WED. For the lens of the eye, a lower correlation is observed, primarily due to surface dose variations. Funding Support: Siemens-UCLA Radiology Master Research Agreement; Disclosures - Michael McNitt-Gray: Institutional Research Agreement, Siemens AG; Research Support, Siemens AG; Consultant, Flaherty Sensabaugh Bonasso PLLC; Consultant, Fulbright and Jaworski.

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.

TU‐G‐103‐04: The Accuracy of Monte Carlo Based Dose Estimates Compared to In‐Vivo Dose Measurements

PURPOSE The purpose of this study was to extend the validation of Monte Carlo (MC) simulation based dose estimates by comparing simulated values with in-vivo measurements from clinical patient scans. METHODS The Institutional Review Board approved the acquisition of in-vivo rectal dose measurements in a pilot study of 9 patients undergoing CT Colonography on an MDCT (LightSpeed VCT, GE Healthcare). Per patient, two scans, prone and supine, were acquired both using a fixed mAs technique. In-vivo dose measurements were obtained using TLD capsules that were affixed to the inner lumen of rectal catheters. Dose from the TLDs were determined taking into account their energy response. Simulations were performed using voxelized models of the patients and the TLDs based on CT image data. A previously developed MC based model was used to simulate the scanner and each patient scan. Dose to the TLD was estimated for each patient scan. The measured and simulated in-vivo TLD dose values were compared for all 9 patients and the Root Mean Square (RMS) error was calculated as well as the correlation. Additionally the simulated results were compared with calculated Size Specific Dose Estimates (SSDEs) for each patient. RESULTS The RMS error between TLD measurements and MC simulations was 12.2% with a maximum of 20.3% and minimum of - 19.7%. Comparison of simulations and SSDE resulted in an RMS of 17.0% with a minimum of - 22.9% and maximum of 22.8%. CONCLUSION The results of this study demonstrated that MC simulations using voxelized patient and equivalent source model Result in reasonably accurate doses compared to actual measured doses. Several possible improvements to the performance of the MC model were identified (including reconstructing images at the biggest field of view for full coverage of the anatomy, etc.) 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 and Kyle McMillan ripient research support Siemens AG.

MO‐D‐134‐04: Calculating Size Specific Dose Estimates (SSDE): The Effect of Using Water Equivalent Diameter (WED) Vs. Effective Diameter (ED) On Organ Dose Estimates When Applying the Conversion Coefficients of TG204

PURPOSE AAPM Task Group (TG) 204 described the Size-Specific Dose Estimate (SSDE) using effective diameter (ED) as the size parameter in the conversion tables. The purpose of this investigation was to assess the effect on SSDE when a different size metric, water equivalent diameter (WED), was used with the conversion tables; and to compare these values to results from detailed Monte Carlo (MC) simulations. METHODS For a set of 101 thoracic and 82 abdomen/pelvis scans, including adult and pediatric patients, both ED and WED were calculated at the middle of the scan length. Each size metric was used as the input value to the SSDE conversion tables in TG204 to obtain two sets of SSDE values (SSDEED and SSDEWED). Additionally, voxelized patient models were generated from original scans and used in MC simulations to estimate dose to liver, spleen, and kidneys in the abdomen/pelvis, as well as lung and glandular breast tissue in the thoracic examinations under fixed tube current mode. SSDEED, SSDEWED were compared to organ doses by calculating percent differences and Root Mean Square errors. RESULTS For the abdomen, SSDEED and SSDEWED resulted in different RMSE from organ dose (for kidneys 8.8% and 6.4%, respectively) while SSDEWED demonstrated higher correlation with kidney dose than SSDEED (0.89 and 0.78, respectively). Similarly, for thorax, SSDEED and SSDEWED showed different RMSE (for lung 17.0% and 13.1%, respectively), with higher correlation for SSDEWED than SSDEED (0.92 and 0.78, respectively). CONCLUSION Generated conversion factors based on ED appear to be applicable to WED and as originally assumed using WED not only improves dose estimates in the thorax but it also improves estimates in the abdomen. It is worth mentioning that these conclusions only apply to fixed tube current examinations while for tube current modulated (TCM) exams new set of conversion factors are recommended. 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: ipient research support Siemens AG.

The relationship between organ dose and patient size in tube current modulated adult thoracic CT scans

Recently published AAPM Task Group 204 developed conversion coefficients that use scanner reported CTDIvol to estimate dose to the center of patient undergoing fixed tube current body exam. However, most performed CT exams use TCM to reduce dose to patients. Therefore, the purpose of this study was to investigate the correlation between organ dose and a variety of patient size metrics in adult chest CT scans that use tube current modulation (TCM). Monte Carlo simulations were performed for 32 voxelized models with contoured lungs and glandular breasts tissue, consisting of females and males. These simulations made use of patients actual TCM data to estimate organ dose. Using image data, different size metrics were calculated, these measurements were all performed on one slice, at the level of patients nipple. Estimated doses were normalized by scanner-reported CTDIvol and plotted versus different metrics. CTDIvol values were plotted versus different metrics to look at scanners output versus size. The metrics performed similarly in terms of correlating with organ dose. Looking at each gender separately, for male models normalized lung dose showed a better linear correlation (r2=0.91) with effective diameter, while female models showed higher correlation (r2=0.59) with the anterior-posterior measurement. There was essentially no correlation observed between size and CTDIvol-normalized breast dose. However, a linear relationship was observed between absolute breast dose and size. Dose to lungs and breasts were consistently higher in females with similar size as males which could be due to shape and composition differences between genders in the thoracic region.

TU‐G‐217BCD‐07: Improving Size Specific Dose Estimates (TG 204): Extension to Tube Current Modulated CT Scans with Regional and Local CTDIvol Values

Purpose: The recently published TG 204 report established conversion factors based on measurements and Monte Carlo simulations, which adjust scanner‐reported CTDIvol for patient size, called size specific dose estimates (SSDE). However, those estimates were based on fixed tube current scans, while most performed clinical protocols make use of Tube Current Modulation (TCM). Therefore, the purpose of this study was to assess the performance of these conversion factors compared to Monte Carlo based simulated organ dose from fixed and TCM scans and to introduce region and organ specific CTDIvol to extend the calculated SSDEs to TCM scans. Methods: Using Monte Carlo based methods; liver doses were estimated from both constant tube current and TCM abdomen/pelvis CT exams. The effective diameter was calculated for each model and used to look up conversion factors which were then applied to scanner‐reported CTDIvol (CTDIvol,TCM)and CTDIvol based on constant tube current exam (CTDIvol,cTC) to calculate SSDEs. Calculated SSDEs were compared to liver dose estimates from constant tube current and TCM simulations. Additionally, regional and liver‐specific CTDIvol (CTDIvol,Abdomen and CTDIvol,Liver) were calculated and used to calculate TCM adjusted SSDEs. Results:Liver doses obtained from fixed tube current simulations were compared to SSDEs calculated using CTDIvol,cTC, and doses obtained from TCM simulations were compared to scanner‐reported CTDIvol,TCM. For these two datasets RMSE values of 3.0 and 3.3 mGy were calculated, respectively. RMSE value decreased to 1.5 and 1.3 mGy for SSDEs calculated based CTDIvol,Abdomen and CTDIvol,Liver, respectively. Conclusion: Conversion factors determined in TG 204 are based on fixed tube current measurements and Monte Carlo simulations and need further adjustments if they are being used to assess TCM exams. We have shown that region and organ‐specific CTDIvol can be used to adjust for TCM and to improve the performance of calculated SSDEs to estimate dose from TCM exams.