CT breast dose reduction with the use of breast positioning and organ‐based tube current modulation

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136262/1/mp12076.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136262/2/mp12076_am.pd

The Relationship between Organ Dose and Patients Size in Multidetector Computed Tomography (MDCT) Scans Utilizing Tube Current Modulation (TCM)

Computed Tomography (CT) has been one of the leading imaging modalities in today's practice of Radiology. Since its introduction in 1970s, its unique tomographic capability has not only prevented countless number of unnecessary surgeries but also saved lives by early detection of disease. Radiation dose from CT has been estimated to contribute to almost 50% of all medical radiation exposures. Concerns about radiation-induced carcinogenesis have resulted in efforts that encourage monitoring and reporting radiation dose from CT examinations. It has been suggested that the most appropriate quantity for assessing risk of carcinogenesis from x-ray imaging procedures is the radiation dose to individual patients. Currently employed dose metrics used to report patient dose are CTDIvol and DLP, neither of which is patient-specific dose, let alone dose to individual organs.CTDIvol is dose to a homogenous cylindrical phantom, which is defined for fixed tube current CT exams. With the implementation of Tube Current Modulation (TCM) feature in almost all clinical CT protocols as an intended means for dose reduction, while maintaining an appropriate diagnostic image quality, CTDIvol definition was standardized across scanners to reflect dose to CTDI phantom based on the average tube current across the entire scan length. Depending on the type of CT exam, the average tube current used to report a CTDIvol value may or may not represent the actual tube current at a specific table location. In addition to not taking into account variation of the tube current across a single exam, CTDIvol is size-independent, i.e. patients with different sizes have the same CTDIvol value if scanned using the same imaging parameters. To adjust CTDIvol for size, AAPM Task Group 204 was established and subsequently published a report containing conversions as a function of effective diameter which can be applied to scanner-reported CTDIvolto adjust for patient size. However, the generated conversion factors were based on fixe tube current measurements and Monte Carlo simulations and failed to take into account TCM. Additionally, the size metric used in TG 204 was entirely based on patients' physical dimensions and does not take into account variations in composition and density among patients, let alone within a single patient; i.e. differences between chest and abdomen in terms of attenuation properties could not be explained with a simple measure of dimension such as effective diameter. Instead attenuation-based metrics need to be implemented to explain these differences. The overall purpose of this dissertation was to improve organ dose estimation from Computed Tomography exams by: (a) taking into account the commonly used feature in CT protocols, Tube Current Modulation (TCM), (b) employing a more appropriate way of reporting CTDI for TCM exams and (c) using a patient size descriptor capable of describing the attenuation properties of individual patients.For this dissertation a validated Monte Carlo based MDCT model capable of simulating organ dose was utilized to estimate organ dose to voxelized patient models undergoing tube current modulated CT examinations. Both detailed TCM and z-axis-only modulation information were used in the simulations in case raw projection data was not accessible. In addition to simulated organ doses different CTDIvol values based on the type of patient model, abdomen versus chest, were calculated. These CTDIvol values included regional CTDIvol,regional and organ-specific CTDIvol,organ along with scanner-reported CTDIvol, referred to as global CTDIvol,global. Furthermore different size metrics, such as effective diameter and attenuation-based metrics, were calculated for every axial CT image within a series and averaged corresponding to the same regions and images used to calculated the above mentioned regional and organ-specific CTDIvol values. Using an approach similar to previous efforts and AAPM Task Group 204, the estimated organ doses were normalized by CT Dose Index (CTDIvol) values. However, for TCM scans normalized organ doses by CTDIvol,globalwere observed to not have a strong correlation with patient size. This result was quite different from that observed previously for fixed tube current exams. In contrast, when regional descriptors of scanner output (CTDIvol,regional and CTDIvol,organ) were used as a modified normalization factor, the results demonstrated significantly improved correlations with patient size.Additionally, an attenuation-based patient size metric, the water equivalent diameter (WED), was investigated in terms of its ability to describe the effects of patient size on organ dose. WED was compared to the size metric introduced in TG204, effective diameter, which is based only on patient morphology (e.g. perimeter) and not on attenuation. Results of the comparisons demonstrated no statistically significant improvements of correlation between normalized organ doses and size metric once WED was utilized, except for normalized lung dose. Although there were no statistically significant improvements, the correlation of determination, R2, increased for almost all organs once WED was employed. Similarly, there was no statistically significant difference between differently averaged size metrics, i.e. global average of size metrics versus regional average of size metrics, except for normalized lung dose, which showed a statistically significant improvement in R2 once a regional WED was employed as a size metric compared to global WED. Using improved normalization quantity and patient size metric for tube current modulated CT examinations, Generalized Linear Models were used to generate a predictive model capable of estimating dose from TCM exams using regional CTDIvol) and WED. Different models based on scanners and organs were generated to establish the level of accuracy of each model and to determine the level of specification needed to achieve best organ dose estimates. Additionally, models with different response variables, normalized organ dose versus actual organ dose, were explored and compared. When tested using a separate test set, investigated models with regional CTDIvol) either as a predictor or normalization factor resulted in very similar results while models created with global CTDIvol) as a predictor resulted in underestimation of organ dose across all organs. Additionally, it was shown that a model based on pooled data was not significantly different than scanner and organ-specific models since the pooled-data model resulted in employing significant categorical predictors such as scanners and organs. This observation confirms the fact that TCM algorithms are different across scanners and regional CTDIvol) is not capable of eliminating these differences, but it can eliminate differences among TCM functions across a single CT scanner. Predictive organ dose estimates using generated models resulted in a mean percent difference of less than 10% when compared to actual Monte Carlo simulated organ doses. The improvement of the newly generated model was also compared against currently used dose metrics, CTDIvol) , SSDE, and ImPACT. While comparisons with actual Monte Carlo simulated organ doses resulted in statistically significant differences between conventional dose metrics and simulated organ doses, comparisons with organ estimates from the newly developed model resulted in no difference from Monte Carlo simulated organ doses. This work demonstrated the feasibility of estimating organ dose from tube current modulated scans from three major CT manufacturers using an improved descriptor of tube current modulated scans as normalization quantity or predictor and a patient size metric based on patients attenuation properties

Interpolation Method for Calculation of Computed Tomography Dose from Angular Varying Tube Current

The scope and magnitude of radiation dose from computed tomography (CT) examination has led to increased scrutiny and focus on accurate dose tracking. The use of tube current modulation (TCM) results complicates dose tracking by generating unique scans that are specific to the patient. Three methods of estimating the radiation dose from a CT examination that uses TCM are compared: using the average current for an entire scan, using the average current for each slice in the scan, and using an estimation of the angular variation of the dose contribution. To determine the impact of TCM on the radiation dose received, a set of angular weighting functions for each tissue of the body are derived by fitting a function to the relative dose contributions tabulated for the four principle exposure projections. This weighting function is applied to the angular tube current function to determine the organ dose contributions from a single rotation. Since the angular tube current function is not typically known, a method for estimating that function is also presented. The organ doses calculated using these three methods are compared to simulations that explicitly include the estimated TCM function

Estimating lung, breast, and effective dose from low-dose lung cancer screening CT exams with tube current modulation across a range of patient sizes.

PurposeThe purpose of this study was to estimate the radiation dose to the lung and breast as well as the effective dose from tube current modulated (TCM) lung cancer screening (LCS) scans across a range of patient sizes.MethodsMonte Carlo (MC) methods were used to calculate lung, breast, and effective doses from a low-dose LCS protocol for a 64-slice CT that used TCM. Scanning parameters were from the protocols published by AAPM's Alliance for Quality CT. To determine lung, breast, and effective doses from lung cancer screening, eight GSF/ICRP voxelized phantom models with all radiosensitive organs identified were used to estimate lung, breast, and effective doses. Additionally, to extend the limited size range provided by the GSF/ICRP phantom models, 30 voxelized patient models of thoracic anatomy were generated from LCS patient data. For these patient models, lung and breast were semi-automatically segmented. TCM schemes for each of the GSF/ICRP phantom models were generated using a validated method wherein tissue attenuation and scanner limitations were used to determine the TCM output as a function of table position and source angle. TCM schemes for voxelized patient models were extracted from the raw projection data. The water equivalent diameter, Dw, was used as the patient size descriptor. Dw was estimated for the GSF/ICRP models. For the thoracic patient models, Dw was extracted from the DICOM header of the CT localizer radiograph. MC simulations were performed using the TCM scheme for each model. Absolute organ doses were tallied and effective doses were calculated using ICRP 103 tissue weighting factors for the GSF/ICRP models. Metrics of scanner radiation output were determined based on each model's TCM scheme, including CTDIvol, dose length product (DLP), and CTDIvol, Low Att, a previously described regional metric of scanner output covering most of the lungs and breast. All lung and breast doses values were normalized by scan-specific CTDIvol and CTDIvol, Low Att. Effective doses were normalized by scan-specific CTDIvol and DLP. Absolute and normalized doses were reported as a function of Dw.ResultsLung doses normalized by CTDIvol, Low Att were modeled as an exponential relationship with respect to Dw with coefficients of determination (R-2) of 0.80. Breast dose normalized by CTDIvol, Low Att was modeled with an exponential relationship to Dw with an R-2 of 0.23. For all eight GSF/ICRP phantom models, the effective dose using TCM protocols was below 1.6 mSv. Effective doses showed some size dependence but when normalized by DLP demonstrated a constant behavior.ConclusionLung, breast, and effective doses from LCS CT exams with TCM were estimated with respect to patient size. Normalized lung dose can be reasonably estimated with a measure of a patient size such as Dw and regional metric of CTDIvol covering the thorax such as CTDIvol, Low Att, while normalized breast dose can also be estimated with a regional metric of CTDIvol but with a larger degree of variability than observed for lung. Effective dose normalized by DLP can be estimated with a constant multiplier

Estimating organ doses from tube current modulated CT examinations using a generalized linear model

PurposeCurrently, available Computed Tomography dose metrics are mostly based on fixed tube current Monte Carlo (MC) simulations and/or physical measurements such as the size specific dose estimate (SSDE). In addition to not being able to account for Tube Current Modulation (TCM), these dose metrics do not represent actual patient dose. The purpose of this study was to generate and evaluate a dose estimation model based on the Generalized Linear Model (GLM), which extends the ability to estimate organ dose from tube current modulated examinations by incorporating regional descriptors of patient size, scanner output, and other scan-specific variables as needed.MethodsThe collection of a total of 332 patient CT scans at four different institutions was approved by each institution's IRB and used to generate and test organ dose estimation models. The patient population consisted of pediatric and adult patients and included thoracic and abdomen/pelvis scans. The scans were performed on three different CT scanner systems. Manual segmentation of organs, depending on the examined anatomy, was performed on each patient's image series. In addition to the collected images, detailed TCM data were collected for all patients scanned on Siemens CT scanners, while for all GE and Toshiba patients, data representing z-axis-only TCM, extracted from the DICOM header of the images, were used for TCM simulations. A validated MC dosimetry package was used to perform detailed simulation of CT examinations on all 332 patient models to estimate dose to each segmented organ (lungs, breasts, liver, spleen, and kidneys), denoted as reference organ dose values. Approximately 60% of the data were used to train a dose estimation model, while the remaining 40% was used to evaluate performance. Two different methodologies were explored using GLM to generate a dose estimation model: (a) using the conventional exponential relationship between normalized organ dose and size with regional water equivalent diameter (WED) and regional CTDIvol as variables and (b) using the same exponential relationship with the addition of categorical variables such as scanner model and organ to provide a more complete estimate of factors that may affect organ dose. Finally, estimates from generated models were compared to those obtained from SSDE and ImPACT.ResultsThe Generalized Linear Model yielded organ dose estimates that were significantly closer to the MC reference organ dose values than were organ doses estimated via SSDE or ImPACT. Moreover, the GLM estimates were better than those of SSDE or ImPACT irrespective of whether or not categorical variables were used in the model. While the improvement associated with a categorical variable was substantial in estimating breast dose, the improvement was minor for other organs.ConclusionsThe GLM approach extends the current CT dose estimation methods by allowing the use of additional variables to more accurately estimate organ dose from TCM scans. Thus, this approach may be able to overcome the limitations of current CT dose metrics to provide more accurate estimates of patient dose, in particular, dose to organs with considerable variability across the population

SU‐GG‐I‐37: Reducing Eye Lens Dose during Brain Perfusion CT Examinations by Moving the Scan Location or Tilting the Gantry Angle.

Purpose: Brain perfusion CT studies may result in radiation doses to the eye lens because of repeated scans that may in some cases be high enough to cause deterministic effects, such as cataracts. The purpose of this study is to investigate the eye lens dose from brain perfusion CT studies, and the dose reduction achieved by clinically practical approaches, such as moving the x‐ray beam away from the eye lens or tilting the gantry angle. Method and Materials: Eye lens doses were estimated using the Monte Carlo method with: (a) a detailed voxelized patient model including a model of the lens of the eye; and (b) detailed CT source models of a Siemens Sensation 64 scanner using the widest collimation (28.8mm) and 120 kVp tube voltage. Simulated brain perfusion axial scans were performed at various scan locations from 5.5cm above the eye lens to 5.5cm below the eye lens with 0.5cm intervals to investigate the scatter contribution to the eye lens dose. For the scan location where the eye lens is completely in the beam, the gantry was tilted at 5, 10, 15, 20, 25 and 30 degrees to study the dose reduction. Results: Eye lens dose drops dramatically as the scan location moves away. When the lenses are just outside the primary x‐ray beam, the dose is 17% of the maximum dose when they are completely in the beam. Tilting the gantry angle by 15 degree reduces the eye lens dose by 87%. Conclusion: The eye lens dose from CT perfusion examinations can be reduced by moving the beam away from the eyes since the scatter component is fairly small. When the examination has to be performed right over the location of the eyes, tilting the gantry angle is another effective method to reduce the eye lens dose

TU‐A‐201B‐04: Estimating Dose to Eye Lens and Skin from Radiation Dose from CT Brain Perfusion Examinations: Comparison to CTDIvol Values.

Purpose: In brain perfusion studies, the patient's head is scanned repeatedly at one location over a short period of time to monitor contrast wash in and wash out. This may result in high radiation doses to the skin and the eye lens and possibly deterministic effects. The purpose of this study is to estimate the radiation dose to skin and eye lens from brain perfusion studies under a variety of scanning conditions and to compare these to CTDIvol. Method and Materials: Skin dose and eye lens dose were estimated using Monte Carlo simulations with a detailed patient model (GSF Model Irene) and CT source models. Brain perfusion scans were simulated with axial scans using the widest available collimation at various scan locations. For each available kVp, the total mAs (mAs/rotation × number of rotations) to reach 2 Gy for eye lens and for skin was determined. Meanwhile, CTDIvol under each condition was obtained to investigate how well it predicts these doses. Results: For all kVps at four different scanners, the total number of rotations that would cause the dose to eye lens and skin reach 2Gy were calculated. For example, for a 300 mAs/rotation scan at 120kVp for scanner B, 58 rotations would result in an eye lens dose of 2Gy, and 47 rotations would result in a maximum skin dose of 2Gy. Depending on different kVp, CT scanners, and scan location, CTDIvol overestimates the eye lens dose by 46% to 18500% and it overestimates the skin dose by 25% to 82%. Conclusion: This study provides detailed information about the radiation dose to eye lens and skin from CT brain perfusion examinations. CTDIvol reported on the scanner console generally overestimates the dose to eye lens and skin. The results could help to improve the design of CT scan protocols