A Turner

The feasibility of a scanner-independent technique to estimate organ dose from MDCT scans: Using CTDIvol to account for differences between scanners

PURPOSE Monte Carlo radiation transport techniques have made it possible to accurately estimate the radiation dose to radiosensitive organs in patient models from scans performed with modern multidetector row computed tomography (MDCT) scanners. However, there is considerable variation in organ doses across scanners, even when similar acquisition conditions are used. The purpose of this study was to investigate the feasibility of a technique to estimate organ doses that would be scanner independent. This was accomplished by assessing the ability of CTDIvol measurements to account for differences in MDCT scanners that lead to organ dose differences. METHODS Monte Carlo simulations of 64-slice MDCT scanners from each of the four major manufacturers were performed. An adult female patient model from the GSF family of voxelized phantoms was used in which all ICRP Publication 103 radiosensitive organs were identified. A 120 kVp, full-body helical scan with a pitch of 1 was simulated for each scanner using similar scan protocols across scanners. From each simulated scan, the radiation dose to each organ was obtained on a per mA s basis (mGy/mA s). In addition, CTDIvol values were obtained from each scanner for the selected scan parameters. Then, to demonstrate the feasibility of generating organ dose estimates from scanner-independent coefficients, the simulated organ dose values resulting from each scanner were normalized by the CTDIvol value for those acquisition conditions. RESULTS CTDIvol values across scanners showed considerable variation as the coefficient of variation (CoV) across scanners was 34.1%. The simulated patient scans also demonstrated considerable differences in organ dose values, which varied by up to a factor of approximately 2 between some of the scanners. The CoV across scanners for the simulated organ doses ranged from 26.7% (for the adrenals) to 37.7% (for the thyroid), with a mean CoV of 31.5% across all organs. However, when organ doses are normalized by CTDIvoI values, the differences across scanners become very small. For the CTDIvol, normalized dose values the CoVs across scanners for different organs ranged from a minimum of 2.4% (for skin tissue) to a maximum of 8.5% (for the adrenals) with a mean of 5.2%. CONCLUSIONS This work has revealed that there is considerable variation among modern MDCT scanners in both CTDIvol and organ dose values. Because these variations are similar, CTDIvol can be used as a normalization factor with excellent results. This demonstrates the feasibility of establishing scanner-independent organ dose estimates by using CTDIvol to account for the differences between scanners.

Monte Carlo reference data sets for imaging research: Executive summary of the report of AAPM Research Committee Task Group 195

The use of Monte Carlo simulations in diagnostic medical imaging research is widespread due to its flexibility and ability to estimate quantities that are challenging to measure empirically. However, any new Monte Carlo simulation code needs to be validated before it can be used reliably. The type and degree of validation required depends on the goals of the research project, but, typically, such validation involves either comparison of simulation results to physical measurements or to previously published results obtained with established Monte Carlo codes. The former is complicated due to nuances of experimental conditions and uncertainty, while the latter is challenging due to typical graphical presentation and lack of simulation details in previous publications. In addition, entering the field of Monte Carlo simulations in general involves a steep learning curve. It is not a simple task to learn how to program and interpret a Monte Carlo simulation, even when using one of the publicly available code packages. This Task Group report provides a common reference for benchmarking Monte Carlo simulations across a range of Monte Carlo codes and simulation scenarios. In the report, all simulation conditions are provided for six different Monte Carlo simulation cases that involve common x-ray based imaging research areas. The results obtained for the six cases using four publicly available Monte Carlo software packages are included in tabular form. In addition to a full description of all simulation conditions and results, a discussion and comparison of results among the Monte Carlo packages and the lessons learned during the compilation of these results are included. This abridged version of the report includes only an introductory description of the six cases and a brief example of the results of one of the cases. This work provides an investigator the necessary information to benchmark his/her Monte Carlo simulation software against the reference cases included here before performing his/her own novel research. In addition, an investigator entering the field of Monte Carlo simulations can use these descriptions and results as a self-teaching tool to ensure that he/she is able to perform a specific simulation correctly. Finally, educators can assign these cases as learning projects as part of course objectives or training programs.

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.

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.

DICOM structured report to track patient's radiation dose to organs from abdominal CT exam

The dramatic increase of diagnostic imaging capabilities over the past decade has contributed to increased radiation exposure to patient populations. Several factors have contributed to the increase in imaging procedures: wider availability of imaging modalities, increase in technical capabilities, rise in demand by patients and clinicians, favorable reimbursement, and lack of guidelines to control utilization. The primary focus of this research is to provide in depth information about radiation doses that patients receive as a result of CT exams, with the initial investigation involving abdominal CT exams. Current dose measurement methods (i.e. CTDIvol Computed Tomography Dose Index) do not provide direct information about a patients organ dose. We have developed a method to determine CTDIvol normalized organ doses using a set of organ specific exponential regression equations. These exponential equations along with measured CTDIvol are used to calculate organ dose estimates from abdominal CT scans for eight different patient models. For each patient, organ dose and CTDIvol were estimated for an abdominal CT scan. We then modified the DICOM Radiation Dose Structured Report (RDSR) to store the pertinent patient information on radiation dose to their abdominal organs.

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.

The accuracy of estimated organ doses from Monte Carlo CT simulations using cylindrical regions of interest within organs

The purpose of this study was to investigate the accuracy of Monte Carlo simulated organ doses using cylindrical ROIs within the organs of patient models as an alternative method to full organ segmentations. Full segmentation and placement of circular ROIs at the approximate volumetric centroid of liver, kidneys and spleen were performed for 20 patient models. For liver and spleen, ROIs with 2cm diameter were placed on 5 consecutive slices; for the kidneys 1cm ROIs were used. Voxelized models were generated and both fixed and modulated tube current simulations were performed and organ doses for each method (full segmentation and ROIs) were recorded. For the fixed tube current simulations, doses simulated using circular ROIs differed from those simulated using full segmentations: for liver, these differences ranged from -5.6% to 10.8% with a Root Mean Square (RMS) difference of 5.9%. For spleen these differences ranged from -9.5% to 5.7% with an RMS of 5.17%; and for kidney the differences ranged from -12.9% to 14.4% for left kidney with an RMS of 6.8%, and from -12.3% to 12.8% for right kidney with an RMS of 6.6%. Full body segmentations need expertise and are time consuming. Instead using circular ROIs to approximate the full segmentation would simplify this task and make dose calculations for a larger set of models feasible. It was shown that dose calculations using ROIs are comparable to those using full segmentations. For the fixed current simulations the maximum RMS value was 6.8% and for the TCM it was 6.9%.

TH‐E‐211‐08: The Energy Dependence of Small Volume Ionization Chambers and Solid State Detectors at Diagnostic Energy Ranges for CT Dosimetry — Assessment in Air and In Phantom

Purpose: While several ionization chambers and solid state detectors are commercially available for CT, their energy dependency has yet to be determined. This work evaluates the energy response of a 0.6‐cc ionization chamber and solid state dosimeter against an ionization chamber specifically calibrated at several diagnostic energy levels (reference chamber) as a function of HVL for in‐air and in‐CTDI phantom measurements. Methods: For two CTscanner models of known difference in beam spectra, HVLs for all available kVps were obtained with measurements (in‐air) and from Monte Carlo simulations (in head and body CTDI phantom). The reference ionization chamber was calibrated by the UWADCL using reference beams with HVLs in the CT range. Calibration factors for the reference chamber at each measured/simulated HVL were calculated based on polynomial fit of the UWADCL‐beam factors. For each scanner, exposure conditions were established at each kVp by varying mAs to obtain similar results in the reference chamber. Both test dosimeters were then scanned at these conditions and the exposure (or dose) relative to the reference chamber was obtained; no specific energy corrections were applied for either test device. The same procedure was performed with the head and body CTDI phantoms for both CTscanners. Results: HVLs from the scanners ranged from 3.49 to 9.7mm Al in‐air and from 3.1 to 8.8mm Al in‐phantom. Doses measured by the test chamber matched those of the reference chamber across scanners and kVps to within 3.5% in‐air, 4.9% in head phantom and 11.8% in body phantom. Differences in the solid state device were larger: as high as 16%, 21%, and 37% for in‐air, head, and body phantom, respectively. Conclusions: While ionization chambers have some energy dependence in the diagnostic range, solid state detectors exhibit a more pronounced energy dependence and correction factors should be applied based on HVL.

TH‐E‐211‐01: The Ability of CTDIvol to Estimate Organ Doses from Typical Tube Current Modulated Abdomen/Pelvis CT Scans

Purpose: When tube current modulation (TCM) scanning is performed over multiple anatomic regions (e.g. abdomen/pelvis), typically only the average CTDIvol over all regions is reported. The purpose of this work was to compare simulated organ doses from tube current modulated scans with different CTDIvol values (including a region specific CTDIvol) and to investigate the feasibility of estimating organ doses using CTDIvol and patient size. Methods: Voxelized models with contoured abdominal organs(liver, spleen and kidneys) were created using 40 abdomen/pelvis scans with tube current modulation (Care Dose4D) acquired on a Siemens Sensation 64. Dose to these organs were estimated using Monte Carlo methods that simulate actual scans. Simulated organ doses were compared with three CTDIvol values: CTDIvol reported on the scanner (CTDIvol, Abd/Pel), CTDIvol based on average effective mAs of abdomen region of the scan (CTDIvol,Abd), and CTDIvol based on average effective mAs from images containing the organ (e.g. CTDIvol,Liver). Organ doses were normalized by each CTDIvol and the relationship between each of these ratios and patient perimeter was investigated. These results were also compared to organ doses normalized by CTDIvol for 32cm CTDI phantom for fixed tube current. Results: This study showed that CTDIvol based on the entire exam underestimates organ dose by up to 33%. This percentage increased to 43% and 38% for CTDIvol,Abd and CTDIvol,Liver, respectively. However, organ doses normalized by CTDIvol,Abd and CTDIvol, Liver showed a strong linear correlation with patient perimeter. Similar linear correlation was observed for fixed tube current results. Conclusions: When using TCM, the CTDIvol reported is over the entire scan which does not correlate well with actual patient dose. However, specific region‐CTDIvol‐normalized organ doses showed strong correlation with patient size and therefore can be used to estimate organ doses from a TCM scan using patient perimeter and region specific CTDIvol.

WE‐C‐110‐10: Reducing Dose to Breast Glandular Tissue by Adjusting the Tube Start Angle and Table Height in Helical CT Scans

Purpose: To investigate potential radiation dose reduction to peripheral organs (such as glandular breast tissue) of the patient during helical CT exams by adjusting the tube start angle and table height. Methods: A Radcal 0.6cc ionization chamber was placed on the anterior surface of an anthropomorphic thorax phantom. Helical CT scans were performed on a Siemens Sensation 64 CT scanner using 24×1.2mm collimation, 120kVp, 100mAs with a pitch of 1.5. CareDose4D was turned off. The table was set to a variety of heights (table height of 100mm, 130mm, 160mm, 200mm, and 255mm) to investigate the effect of table height on peripheral organ doses. Since the tube start angle is random and is not under the users control for helical scans, 24 helical scans were performed under each table height to yield a variety of tube start angles. The exposure of the ionization chamber for each scan was recorded. After scanning, the raw projection data were extracted and read in by a Siemens proprietary MATLAB code to obtain the tube start angle for each scan. Results: At a fixed table height, the exposure varies by more than a factor of two depending on the tube start angle. As the table moves higher (the ionization chamber closer to the top of the gantry), the exposure decreases. While keeping all the scanning parameters (kVp, mAs, pitch) the same, by just adjusting the table height and the tube start angle, the lowest measured exposure is 732mR, while the highest measured exposure is 2810mR. Conclusions: The uncontrolled tube start angle in current commercial CT scanners results in great uncertainties in dose to peripheral organs. Moving the table up decreases anterior organ doses. By adjusting both the tube start angle the table height, dose to a point in glandular breast tissue could be reduced by 74%.