Tube Current Modulation Scans Research Articles

Patient size matters: Effect of tube current modulation on size-specific dose estimates (SSDE) and image quality in low-dose lung cancer screening CT.

PurposeWe compare the effect of tube current modulation (TCM) and fixed tube current (FTC) on size‐specific dose estimates (SSDE) and image quality in lung cancer screening with low‐dose CT (LDCT) for patients of all sizes.MethodsInitially, 107 lung screening examinations were performed using FTC, which satisfied the Centers for Medicare & Medicaid Services' volumetric CT dose index (CTDIvol) limit of 3.0 mGy for standard‐sized patients. Following protocol modification, 287 examinations were performed using TCM. Patient size and examination parameters were collected and water‐equivalent diameter (Dw) and SSDE were determined for each patient. Regression models were used to correlate CTDIvol and SSDE with Dw. Objective and subjective image quality were measured in 20 patients who had consecutive annual screenings with both FTC and TCM.ResultsCTDIvol was 2.3 mGy for all FTC scans and increased exponentially with Dw (range = 0.96–4.50 mGy, R2 = 0.73) for TCM scans. As patient Dw increased, SSDE decreased for FTC examinations (R2 = 1) and increased for TCM examinations (R2 = 0.54). Image quality measurements were superior with FTC for smaller sized patients and with TCM for larger sized patients (R2 > 0.5, P < 0.005). Radiologist graded all images acceptable for diagnostic evaluation of lung cancer screening.ConclusionAlthough FTC protocol offered a consistently low CTDIvol for all patients, it yielded unnecessarily high SSDE for small patients and increased image noise for large patients. Lung cancer screening with LDCT using TCM produces radiation doses that are appropriately reduced for small patients and increased for large patients with diagnostic image quality for all patients.

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

Currently, 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. The 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. The 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. The 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.

Convolution-based estimation of organ dose in tube current modulated CT

Estimating organ dose for clinical patients requires accurate modeling of the patient anatomy and the dose field of the CT exam. The modeling of patient anatomy can be achieved using a library of representative computational phantoms (Samei et al Pediatr. Radiol. 44 460–7). The modeling of the dose field can be challenging for CT exams performed with a tube current modulation (TCM) technique. The purpose of this work was to effectively model the dose field for TCM exams using a convolution-based method. A framework was further proposed for prospective and retrospective organ dose estimation in clinical practice.The study included 60 adult patients (age range: 18–70 years, weight range: 60–180 kg). Patient-specific computational phantoms were generated based on patient CT image datasets. A previously validated Monte Carlo simulation program was used to model a clinical CT scanner (SOMATOM Definition Flash, Siemens Healthcare, Forchheim, Germany). A practical strategy was developed to achieve real-time organ dose estimation for a given clinical patient. CTDIvol-normalized organ dose coefficients () under constant tube current were estimated and modeled as a function of patient size. Each clinical patient in the library was optimally matched to another computational phantom to obtain a representation of organ location/distribution. The patient organ distribution was convolved with a dose distribution profile to generate values that quantified the regional dose field for each organ. The organ dose was estimated by multiplying with the organ dose coefficients (). To validate the accuracy of this dose estimation technique, the organ dose of the original clinical patient was estimated using Monte Carlo program with TCM profiles explicitly modeled. The discrepancy between the estimated organ dose and dose simulated using TCM Monte Carlo program was quantified. We further compared the convolution-based organ dose estimation method with two other strategies with different approaches of quantifying the irradiation field.The proposed convolution-based estimation method showed good accuracy with the organ dose simulated using the TCM Monte Carlo simulation. The average percentage error (normalized by CTDIvol) was generally within 10% across all organs and modulation profiles, except for organs located in the pelvic and shoulder regions.This study developed an improved method that accurately quantifies the irradiation field under TCM scans. The results suggested that organ dose could be estimated in real-time both prospectively (with the localizer information only) and retrospectively (with acquired CT data).

Radiation dose calculations for CT scans with tube current modulation using the approach to equilibrium function.

The approach to equilibrium function has been used previously to calculate the radiation dose to a shift-invariant medium undergoing CT scans with constant tube current [Li, Zhang, and Liu, Med. Phys. 39, 5347-5352 (2012)]. The authors have adapted this method to CT scans with tube current modulation (TCM). For a scan with variable tube current, the scan range was divided into multiple subscan ranges, each with a nearly constant tube current. Then the dose calculation algorithm presented previously was applied. For a clinical CT scan series that presented tube current per slice, the authors adopted an efficient approach that computed the longitudinal dose distribution for one scan length equal to the slice thickness, which center was at z = 0. The cumulative dose at a specific point was a summation of the contributions from all slices and the overscan. The dose calculations performed for a total of four constant and variable tube current distributions agreed with the published results of Dixon and Boone [Med. Phys. 40, 111920 (14pp.) (2013)]. For an abdomen/pelvis scan of an anthropomorphic phantom (model ATOM 701-B, CIRS, Inc., VA) on a GE Lightspeed Pro 16 scanner with 120 kV, N × T = 20 mm, pitch = 1.375, z axis current modulation (auto mA), and angular current modulation (smart mA), dose measurements were performed using two lines of optically stimulated luminescence dosimeters, one of which was placed near the phantom center and the other on the surface. Dose calculations were performed on the central and peripheral axes of a cylinder containing water, whose cross-sectional mass was about equal to that of the ATOM phantom in its abdominal region, and the results agreed with the measurements within 28.4%. The described method provides an effective approach that takes into account subject size, scan length, and constant or variable tube current to evaluate CT dose to a shift-invariant medium. For a clinical CT scan, dose calculations may be performed with a water-containing cylinder whose cross-sectional mass is equal to that of the subject. This method has the potential to substantially improve evaluations of patient dose from clinical CT scans, compared to CTDIvol, size-specific dose estimate (SSDE), or the dose evaluated for a TCM scan with a constant tube current equal to the average tube current of the TCM scan.

Validation of a Monte Carlo model used for simulating tube current modulation in computed tomography over a wide range of phantom conditions/challenges.

Monte Carlo (MC) simulation methods have been widely used in patient dosimetry in computed tomography (CT), including estimating patient organ doses. However, most simulation methods have undergone a limited set of validations, often using homogeneous phantoms with simple geometries. As clinical scanning has become more complex and the use of tube current modulation (TCM) has become pervasive in the clinic, MC simulations should include these techniques in their methodologies and therefore should also be validated using a variety of phantoms with different shapes and material compositions to result in a variety of differently modulated tube current profiles. The purpose of this work is to perform the measurements and simulations to validate a Monte Carlo model under a variety of test conditions where fixed tube current (FTC) and TCM were used. A previously developed MC model for estimating dose from CT scans that models TCM, built using the platform of mcnpx, was used for CT dose quantification. In order to validate the suitability of this model to accurately simulate patient dose from FTC and TCM CT scan, measurements and simulations were compared over a wide range of conditions. Phantoms used for testing range from simple geometries with homogeneous composition (16 and 32 cm computed tomography dose index phantoms) to more complex phantoms including a rectangular homogeneous water equivalent phantom, an elliptical shaped phantom with three sections (where each section was a homogeneous, but different material), and a heterogeneous, complex geometry anthropomorphic phantom. Each phantom requires varying levels of x-, y- and z-modulation. Each phantom was scanned on a multidetector row CT (Sensation 64) scanner under the conditions of both FTC and TCM. Dose measurements were made at various surface and depth positions within each phantom. Simulations using each phantom were performed for FTC, detailed x-y-z TCM, and z-axis-only TCM to obtain dose estimates. This allowed direct comparisons between measured and simulated dose values under each condition of phantom, location, and scan to be made. For FTC scans, the percent root mean square (RMS) difference between measurements and simulations was within 5% across all phantoms. For TCM scans, the percent RMS of the difference between measured and simulated values when using detailed TCM and z-axis-only TCM simulations was 4.5% and 13.2%, respectively. For the anthropomorphic phantom, the difference between TCM measurements and detailed TCM and z-axis-only TCM simulations was 1.2% and 8.9%, respectively. For FTC measurements and simulations, the percent RMS of the difference was 5.0%. This work demonstrated that the Monte Carlo model developed provided good agreement between measured and simulated values under both simple and complex geometries including an anthropomorphic phantom. This work also showed the increased dose differences for z-axis-only TCM simulations, where considerable modulation in the x-y plane was present due to the shape of the rectangular water phantom. Results from this investigation highlight details that need to be included in Monte Carlo simulations of TCM CT scans in order to yield accurate, clinically viable assessments of patient dosimetry.

The impact on CT dose of the variability in tube current modulation technology: a theoretical investigation

Body CT scans are routinely performed using tube-current-modulation (TCM) technology. There is notable variability across CT manufacturers in terms of how TCM technology is implemented. Some manufacturers aim to provide uniform image noise across body regions and patient sizes, whereas others aim to provide lower noise for smaller patients. The purpose of this study was to conduct a theoretical investigation to understand how manufacturer-dependent TCM scheme affects organ dose, and to develop a generic approach for assessing organ dose across TCM schemes. The adult reference female extended cardiac-torso (XCAT) phantom was used for this study. A ray-tracing method was developed to calculate the attenuation of the phantom for a given projection angle based on phantom anatomy, CT system geometry, x-ray energy spectrum, and bowtie filter filtration. The tube current (mA) for a given projection angle was then calculated as a log-linear function of the attenuation along that projection. The slope of this function, termed modulation control strength, α, was varied from 0 to 1 to emulate the variability in TCM technology. Using a validated Monte Carlo program, organ dose was simulated for five α values (α = 0, 0.25, 0.5, 0.75, and 1) in the absence and presence of a realistic system mA limit. Organ dose was further normalized by volume-weighted CT dose index (CTDIvol) to obtain conversion factors (h factors) that are relatively independent of system specifics and scan parameters. For both chest and abdomen-pelvis scans and for 24 radiosensitive organs, organ dose conversion factors varied with α, following second-order polynomial equations. This result suggested the need for α-specific organ dose conversion factors (i.e., conversion factors specific to the modulation scheme used). On the other hand, across the full range of α values, organ dose in a TCM scan could be derived from the conversion factors established for a fixed-mA scan (hFIXED). This was possible by multiplying hFIXED by a revised definition of CTDIvol that accounts for two factors: (a) the tube currents at the location of an organ and (b) the variation in organ volume along the longitudinal direction. This α-generic approach represents an approximation. The error associated with this approximation was evaluated using the α-specific organ dose (i.e., the organ dose obtained by using α-specific mA profiles as inputs into the Monte Carlo simulation) as the reference standard. When the mA profiles were constrained by a realistic system limit, this α-generic approach had errors of less than ~20% for the full range of α values. This was the case for 24 radiosensitive organs in both chest and abdomen-pelvis CT scans with the exception of thyroid in the chest scan and bladder in the abdomen-pelvis scan. For these two organs, the errors were less than ~40%. The results of this theoretical study suggested that knowing the mA modulation profile and the fixed-mA conversion factors, organ dose may be estimated for a TCM scan independent of the specific modulation scheme applied.

SU-D-103-06: Bismuth Shield Usage in MDCT Thoracic Scans: Organ Dose Vs. Image Quality

Purpose: To study Bismuth (Bi) shielding for the breast, organ dose and image quality were compared under the following conditions: (1) tube current modulation (TCM), (2) TCM with a Bi shield placed after topogram, and (3) manually reduced tube current (RTC) with no Bi. Methods: All measurements were performed with a 64‐slice scanner at 120 kVp. Organ dose was measured with MOSFETs using an adult male anthropomorphic phantom with supine breast attachments. The reference exposure and reduced exposure (with 4‐ply Bi shield) was measured with an ion chamber located at the level of the breast. The mA was reduced by normalizing the reference mA to the ratio of the reduced exposure to the reference exposure. Image quality was measured using a high contrast insert placed in the lung. Regions of Interest (ROIs) were drawn in thoracic organs to measure signal‐to‐noise ratio (SNR), percent contrast (%Contrast), noise, and HU values. Results: Organ doses (mGy) for the three scans (TCM, TCM with Bi, and RTC) were 11.1, 6.89, and 6.04 to the breast; 8.83, 8.01, and 7.62 to the lung; and 8.20, 7.36 and 8.40 to the heart, respectively. HU increase was greatest in the TCM with Bi scan for organs closest to the shield. The SNRs were 37.6, 34.1, and 43.3 and the %Contrast values were 349.2, 326.3, and 354.3 with TCM, TCM with Bi, and RTC, respectively. Conclusion: For thoracic CT, this RTC method provides a dose reduction to the breast similar to that of the TCM with Bi. Decrease in SNR and %Contrast in the TCM with Bi scan was expected due to decrease in photons reaching the detectors and beam hardening from the shield, respectively. Increased SNR in RTC scan was due to the increased mA compared to TCM scan at the level of measurement. U.S. NRC Health Physics Fellowship Grant No. NRC‐HQ‐12‐G‐38‐0022

TU‐G‐217BCD‐06: Reducing CT Dose during Routine Brain CT Using Attenuation Based Tube Current Modulation (TCM)

Purpose: Attenuation based Tube Current Modulation (TCM) is used routinely for chest, abdomen, and pelvis CT studies, but is often not used for routine head CT exams. The aim of this study was to determine whether using TCM for routine brain imaging has the potential to reduce CT dose (CTDI). Methods: A Toshiba Aquilion ONE CT was used to image a head phantom. Two acquisitions were performed, one fixed tube current scan with 165mAs and one TCM scan with the appropriate SUREExposure SD setting selected to achieve image noise (HU standard deviation) of brain tissue in the posterior fossa region equivalent to that of the fixed tube current scan. Assuming the mA of the fixed tube current scan is limited by the noise level required for the posterior fossa (since posterior fossa is the most challenging head region to image), it is reasonable to assume other regions of the head are receiving unnecessarily low noise levels and can accept further mA reduction; TCM should correct for this. Image noise was measured as average standard deviation of CT number for three ROIs in the posterior fossa and four ROIs in each of two axial locations representing large and small diameter regions. CT dose difference was calculated using scanner reported CTDIvol. Results: Compared to fixed tube current, the TCM scan reduced CT dose by 38%. Compared to fixed tube current, image noise from the TCM scan was equivalent in the posterior fossa region (as designed), nearly equivalent (3% increase) in the large diameter region of the head, and as expected, image noise increased in the smaller diameter brain (24% increase). Conclusions: Tube Current Modulation (TCM) has the potential to reduce radiation dose for routine head CT. Future studies will investigate clinical image quality between fixed tube current and TCM head CT scans. EA, DZ ‐ Employee Toshiba America Medical Systems, Inc.

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.

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.