Use Of Tube Current Modulation Research Articles

TU‐H‐207A‐07: Comparison of Performance of Tube‐Current Modulation of Three Commercial MDCT Scanners

Purpose:To compare performance of tube‐current modulation (TCM) of three commercial MDCT scanners (TOSHIBA Aquilion One, SIEMENS SOMATOM Definition AS+, PHILIPS Brilliance iCT) from tube current measurement, image noise measurement, and Monte Carlo (MC) simulation based on time‐resolved air kerma measurement.Methods:CT image acquisitions were performed with and without TCM by means of TCM phantoms with length of 40 cm which consists of three oval phantoms (10×20 cm, 14×28 cm, 18×36 cm) and one cylindrical phantom with diameter of 10 cm. Tube currents for each slice were obtained from the DICOM header information. The image noise was measured as standard deviation of CT numbers in ROI at each CT slice. TCM was tested for two oval phantoms. A time‐resolving ionization chamber was positioned at 0 degree and 270 degree and the time‐resolved air kerma was measured with and without TCM. Three MDCT scanners were modeled using GMctdospp (IMPS, Germany) based on the EGSnrc user code. MC‐calculated dose curves per angle were compared to those measured. MC calculations for TCM were set to maximal tube‐current reduction factors (TCM limit) from 0.3 to 0.8.Results:The tube current changed depending on a phantom size. However, excessive tube current was observed at the boundary of phantoms. This is because the tube current changed gradually in anticipation of thickness change. The variation of image noise was improved by the use of TCM. MC‐calculated dose curves without TCM were in good agreement with the measurement. The variation of dose curves with TCM increased with decreasing TCM limit. The MC‐calculated curve with TCM limit of 0.3 agreed with the measurement in all scanners.Conclusion:TCM is useful to improve the image quality and reduce radiation dose. However, the excessive tube current was observed due to limitation of tube current control.

Volume of interest CBCT and tube current modulation for image guidance using dynamic kV collimation.

The focus of this work is the development of a novel blade collimation system enabling volume of interest (VOI) CBCT with tube current modulation using the kV image guidance source on a linear accelerator. Advantages of the system are assessed, particularly with regard to reduction and localization of dose and improvement of image quality. A four blade dynamic kV collimator was developed to track a VOI during a CBCT acquisition. The current prototype is capable of tracking an arbitrary volume defined by the treatment planner for subsequent CBCT guidance. During gantry rotation, the collimator tracks the VOI with adjustment of position and dimension. CBCT image quality was investigated as a function of collimator dimension, while maintaining the same dose to the VOI, for a 22.2 cm diameter cylindrical water phantom with a 9 mm diameter bone insert centered on isocenter. Dose distributions were modeled using a dynamic BEAMnrc library and DOSXYZnrc. The resulting VOI dose distributions were compared to full-field CBCT distributions to quantify dose reduction and localization to the target volume. A novel method of optimizing x-ray tube current during CBCT acquisition was developed and assessed with regard to contrast-to-noise ratio (CNR) and imaging dose. Measurements show that the VOI CBCT method using the dynamic blade system yields an increase in contrast-to-noise ratio by a factor of approximately 2.2. Depending upon the anatomical site, dose was reduced to 15%-80% of the full-field CBCT value along the central axis plane and down to less than 1% out of plane. The use of tube current modulation allowed for specification of a desired SNR within projection data. For approximately the same dose to the VOI, CNR was further increased by a factor of 1.2 for modulated VOI CBCT, giving a combined improvement of 2.6 compared to full-field CBCT. The present dynamic blade system provides significant improvements in CNR for the same imaging dose and localization of imaging dose to a predefined volume of interest. The approach is compatible with tube current modulation, allowing optimization of the imaging protocol.

Characteristic image quality of a third generation dual-source MDCT scanner: Noise, resolution, and detectability.

The purpose of this work was to assess the inherent image quality characteristics of a new multidetector computed tomography system in terms of noise, resolution, and detectability index as a function of image acquisition and reconstruction for a range of clinically relevant settings. A multisized image quality phantom (37, 30, 23, 18.5, and 12 cm physical diameter) was imaged on a SOMATOM Force scanner (Siemens Medical Solutions) under variable dose, kVp, and tube current modulation settings. Images were reconstructed with filtered back projection (FBP) and with advanced modeled iterative reconstruction (ADMIRE) with iterative strengths of 3, 4, and 5. Image quality was assessed in terms of the noise power spectrum (NPS), task transfer function (TTF), and detectability index for a range of detection tasks (contrasts of approximately 45, 90, 300, -900, and 1000 HU, and 2-20 mm diameter) based on a non-prewhitening matched filter model observer with eye filter. Image noise magnitude decreased with decreasing phantom size, increasing dose, and increasing ADMIRE strength, offering up to 64% noise reduction relative to FBP. Noise texture in terms of the NPS was similar between FBP and ADMIRE (<5% shift in peak frequency). The resolution, based on the TTF, improved with increased ADMIRE strength by an average of 15% in the TTF 50% frequency for ADMIRE-5. The detectability index increased with increasing dose and ADMIRE strength by an average of 55%, 90%, and 163% for ADMIRE 3, 4, and 5, respectively. Assessing the impact of mA modulation for a fixed average dose over the length of the phantom, detectability was up to 49% lower in smaller phantom sections and up to 26% higher in larger phantom sections for the modulated scan compared to a fixed tube current scan. Overall, the detectability exhibited less variability with phantom size for modulated scans compared to fixed tube current scans. Image quality increased with increasing dose and decreasing phantom size. The CT system exhibited nonlinear noise and resolution properties, especially at very low-doses, large phantom sizes, and for low-contrast objects. Objective image quality metrics generally increased with increasing dose and ADMIRE strength, and with decreasing phantom size. The ADMIRE algorithm could offer comparable image quality at reduced doses or improved image quality at the same dose. The use of tube current modulation resulted in more consistent image quality with changing phantom size.

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 profound effects of patient arm positioning on organ doses from CT procedures calculated using Monte Carlo simulations and deformable phantoms.

The purpose of this study was to evaluate the organ dose differences caused by the arms-raised and arms-lowered postures for multidetector computed tomography procedures. Organ doses were calculated using computational phantoms and Monte Carlo simulations. The arm position in two previously developed adult male and female human phantoms was adjusted to represent 'raised' and 'lowered' postures using advanced BREP-based mesh surface geometries. Organ doses from routine computed tomography (CT) scan protocols, including the chest, abdomen-pelvis, and chest-abdomen-pelvis scans, were simulated at various tube voltages and reported in the unit of mGy per 100 mAs. The CT scanner model was based on previously tested work. The differences in organ dose per unit tube current between raised and lowered arm postures were studied. Furthermore, the differences due to the tube current modulation (TCM) for these two different postures and their impact on organ doses were also investigated. For a given scan parameter, a patient having lowered arms received smaller doses to organs located within the chest, abdomen or pelvis when compared with the patient having raised arms. As expected, this is caused by the attenuation of the primary X rays by the arms. However, the skin doses and bone surface doses in the patient having lowered arms were found to be 3.97-32.12% larger than those in a patient having raised arms due to the fact that more skin and spongiosa were covered in the scan range when the arms are lowered. This study also found that dose differences become smaller with the increase in tube voltage for most of organs or tissues except the skin. For example, the liver dose differences decreased from -15.01 to -11.33% whereas the skin dose differences increased from 21.53 to 25.24% with tube voltage increased from 80 to 140 kVp. With TCM applied, the organ doses of all the listed organs in patient having lowered arms are larger due to the additional tube current necessary to overcome the presence of the arms while maintaining sufficient image quality Arm position affects the dose to internal organs from CT scans by as much as 25.3%. The presence of arms in the scan range results in a dose increase for the skin and bone surface, but a dose decrease for organs located in the torso. Considering the use of TCM, which is common in many clinics, the patient having lowered arms may receive 50% higher radiation dose to most of the organs because of the increased tube current. The use of higher tube voltage might narrow such dose differences between patients of these two postures due to the greater penetration of higher-energy X rays. Therefore, when calculating or reporting patient doses from CT scans, it is prudent to select an appropriate phantom that accurately represents the patient posture.

Standardization and Optimization of CT Protocols to Achieve Low Dose

The increase in radiation exposure due to CT scans has been of growing concern in recent years. CT scanners differ in their capabilities, and various indications require unique protocols, but there remains room for standardization and optimization. In this paper, the authors summarize approaches to reduce dose, as discussed in lectures constituting the first session of the 2013 UCSF Virtual Symposium on Radiation Safety and Computed Tomography. The experience of scanning at low dose in different body regions, for both diagnostic and interventional CT procedures, is addressed. An essential primary step is justifying the medical need for each scan. General guiding principles for reducing dose include tailoring a scan to a patient, minimizing scan length, use of tube current modulation and minimizing tube current, minimizing tube potential, iterative reconstruction, and periodic review of CT studies. Organized efforts for standardization have been spearheaded by professional societies such as the American Association of Physicists in Medicine. Finally, all team members should demonstrate an awareness of the importance of minimizing dose.

Radiation dose reduction in thoracic and abdomen-pelvic CT using tube current modulation: a phantom study.

This phantom study was designed to compare the radiation dose in thoracic and abdomen–pelvic CT scans with and without use of tube current modulation (TCM). Effective dose (ED) and size‐specific dose estimation (SSDE) were calculated with the absorbed doses measured at selective radiosensitive organs using a thermoluminescence dosimeter‐100 (TLD‐100). When compared to protocols without TCM, the ED and SSDE were reduced significantly with use of TCM for both the thoracic and abdomen–pelvic CT. With use of TCM, the ED was 6.50±0.29 mSv for thoracic and 6.01±0.20 mSv for the abdomen–pelvic CT protocols. However without use of TCM, the ED was 20.07±0.24 mSv and 17.30±0.41 mSv for the thoracic and abdomen–pelvic CT protocols, respectively. The corresponding SSDE was 10.18±0.48 mGy and 11.96±0.27 mGy for the thoracic and abdomen–pelvic CT protocols with TCM, and 31.56±0.43 mGy and 33.23±0.05 mGy for thoracic and abdomen–pelvic CT protocols without TCM, respectively. The highest absorbed dose was measured at the breast with 8.58±0.12 mGy in the TCM protocols and 51.52±14.72 mGy in the protocols without TCM during thoracic CT. In the abdomen–pelvic CT, the absorbed dose was highest at the skin with 9.30±1.28 mGy and 29.99±2.23 mGy in protocols with and without use of TCM, respectively. In conclusion, the TCM technique results in significant dose reduction; thus it is to be highly recommended in routine thoracic and abdomen–pelvic CT.PACS numbers: 87.57.Q‐, 87.57.qp, 87.53.Bn

TH-E-217BCD-04: MA Modulation and Iterative Reconstruction: Evaluation Using a New CT Phantom

Purpose: The routine clinical CT use of tube current modulation (TCM) and iterative reconstruction algorithms, which can be used to reduce radiation dose, necessitates evaluating their effects on image quality. This study's purpose was to use a new phantom (Mercury Phantom) to evaluate the quality of images acquired with TCM and iterative reconstruction algorithms as implemented by two different CT manufacturers. Methods: The phantom consists of four cylindrical and three tapered regions. The phantom was designed to measure noise-power spectrum (NPS) and modulation transfer function (MTF) across a range of diameters'” 16, 23, 30, and 37 cm'”and at multiple, clinically relevant insert materials from air to iodinated tissue. Images were acquired on a Siemens Flash and GE 750HD using a range of TCM and proprietary reconstruction algorithms: ASIR, IRIS, and SAFIRE. Custom software measured the non-stationarity of NPS as a function of reconstruction and TCM. Contrast and MTF were measured for each insert for each reconstruction algorithm. Results: Measured HU of different materials varied minimally as a function of reconstruction algorithm and dose reduction technique. However, as a function of phantom size from the 16-cm to 37-cm diameter, the [air,Polystyrene,Acrylic,Teflon] HU of 120-kVp images changed by approximately [50,−20,−8,−130] for the 750HD and [35,0,6,−75] for the Flash. MTFs of the linear reconstruction algorithms were independent of size, TCM, and contrast, but for iterative algorithms, the MTFs depended on the object contrast and improved by as much as 25%. Peak NPS decreased from 37-cm to 16-cm (∼86%), decreased for iterative reconstruction relative to linear algorithms (∼21%), and decreased radially from the phantom's center (∼25%). mA modulation performance differed considerably between the two systems. Conclusion: The Mercury Phantom can be used to assess trade-offs in CT image quality and dose reduction techniques for a range of sizes incorporating TCM and iterative reconstruction. Joshua M. Wilson Funded, in part, by Research Agreement with GE Healthcare. Ehsan Samei Research grant: NIH R01 EB001838; Research grant: General Electric; Research grant: Siemens Medical; Research grant: Carestream Health; Share-holder: Zumatek Inc; Advisory Board: IBA

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.

The effect of heart rhythm on patient radiation dose with dual-source cardiac computed tomography

To lower the radiation exposure associated with cardiac CT, it is essential to identify all factors that influence radiation dose. We explored the effect of heart rhythm during scan acquisition on radiation dose with a 64-slice dual-source cardiac CT. Patient and scan data were collected prospectively in 302 consecutive patients referred for a clinical dual-source cardiac CT. Electrocardiograms recorded during acquisition were interpreted by a cardiologist and categorized as (1) normal sinus rhythm (NSR), (2) premature atrial contraction (PAC) or premature ventricular contraction (PVC), or (3) atrial fibrillation or flutter. Of the 302 patients, 227 (75.2%) were in NSR and had no ectopy, 55 (18.2%) had PAC/PVC, and 20 (6.6%) had atrial fibrillation or flutter during the scan. Patients with irregular rhythm (PAC/PVC and atrial fibrillation or flutter) were older than patients with regular rhythm (61.0 vs 54.8years; P = 0.006). Patients with NSR had the lowest estimated radiation dose, followed by PAC/PVC and atrial fibrillation/flutter (9.4, 14.5, 20.9 mSv; P < 0.001). The difference remained significant after adjustments for differences in examination type, tube current and voltage, scan length, pitch, and use of tube current modulation (9.8, 14.1, 17.9 mSv; P < 0.001). No significant association was observed between heart rhythm and subjective image quality although scans with regular rhythm and no ectopy had higher signal-to-noise and contrast-to-noise ratios (P < 0.01). Compared to patients with NSR, patients with atrial fibrillation/flutter had the highest radiation exposure, followed by those with PAC/PVC. Even after adjustment for factors associated with radiation exposure, a significant difference in radiation dose persisted. These findings can be used to identify patients who are more likely to receive higher radiation dose when undergoing cardiac CT and to develop future more-efficient scanner algorithms for use in patients with arrhythmias.

Direct Quantification of Breast Dose During Coronary CT Angiography and Evaluation of Dose Reduction Strategies

The purpose of this study was to quantify the absorbed radiation dose received by the adult female breast during coronary CT angiography (CTA) and to evaluate the effectiveness of various dose reduction strategies. An adult female thoracic anthropomorphic phantom was scanned using eight different clinical coronary CTA protocols that varied in detector configuration (320 × 0.5 mm or 64 × 0.5 mm), x-ray tube activation (full R-R, 65% R-R, or 70-80% R-R), use of tube current modulation, and use of breast shields. Direct dosimetry measurements were performed using Gafchromic film to determine the absorbed breast dose. Retrospective helical data acquisition using a 64-detector array and a full cardiac cycle without dose modulation or breast shielding is associated with an average absorbed breast dose of 82.9 mGy. Optimization of coronary CTA technique using a 320-detector array and a 70-80% cardiac phase reduces the absorbed breast dose by 78.9% to 17.5 mGy, whereas breast shields used in isolation reduces breast dose by up to 46.8%. The implementation of clinically validated coronary CTA protocols using large-area detector acquisition and prospective ECG gating with limited x-ray tube activation results in substantial breast dose savings of up to 78.9% and should be used whenever possible in combination with bismuth breast shields to achieve further dose reduction.

100 kV versus 120 kV: effective reduction in radiation dose?

100 kV versus 120 kV: effective reduction in radiation dose?

Use of 100 kV versus 120 kV in cardiac dual source computed tomography: effect on radiation dose and image quality

To evaluate the effective radiation dose and image quality resulting from use of 100 vs. 120 kV among patients referred for cardiac dual source CT exam (DSCT). Prospective data was collected on 294 consecutive patients referred for DSCT. For each scan, a physician specializing in cardiac CT chose all parameters including tube current and voltage, axial versus helical acquisition, and use of tube current modulation. Lower tube voltage was selected for thinner patients or when lower radiation was desired for younger patients, particularly females. For each study, image quality (IQ) was rated on a subjective IQ score and contrast (CNR) and signal-to-noise (SNR) ratios were calculated. Tube voltage of 100 kV was used for 77 (26%) exams while 120 kV was used for 217 (74%) exams. Use of 100 kV was more common in thinner patients (weight 166 lbs vs. 199 lbs, P < .001). The effective radiation dose for the 100 and 120 kV scans was 8.5 and 15.4 mSv respectively. Among scans utilizing 100 and 120 kV, there was no difference in exam indication, use of beta blockers, heart rate, scan length and use of radiation saving techniques such as prospective ECG triggering and tube current modulation. The IQ score was significantly higher for 100 kV scans. While 100 kV scans were found to have higher image noise then those utilizing 120 kV, the contrast-to-noise and signal-to-noise were significantly higher (SNR: 9.4 vs. 8.3, P = .02; CNR: 6.9 vs. 6.0, P = .02). In selected non-obese patients, use of low kV results in a substantial reduction of radiation dose and may result in improved image quality. These results suggest that low kV should be used more frequently in non-obese patients.

Estimated cumulative radiation dose from PET/CT in children with malignancies: reply to Gelfand et al

Sir, We highly appreciate the comments of Dr. Michael Gelfand and his colleagues [1] regarding our article [2]. We agree that when appropriate administered activities of FDG and appropriate PET/CT protocols are used, the effective dose attributable to the addition of PET/CT to the previously required CT imaging can be much less than the 24.8 mSv stated in our study. In the discussion portion of our manuscript, we specifically pointed out that, while our original CT protocols were adjusted for patient size, we have instituted even more aggressive dose reduction protocols (as shown in Table 3 in our article [2]) that involve reducing kVp as well as the use of tube current modulation (a function that was not available on the original PET/CT scanner used in this study but is now available on the systems in our current clinical practice). We also described one key aspect of our practice, that the CT portion of the exam is used for diagnosis and not just for PET attenuation correction. This aspect itself has a significant impact on the radiation dose delivered to the patient. To obtain diagnostic-quality CT images requires more radiation dose than do images that are only used for attenuation correction. However, our radiologists believe that the information obtained is vital to patient management and so it is our practice to perform these scans at levels consistent with our diagnostic protocols. Further, it should be noted that our estimates of radiation dose were done in a very detailed manner and were not based solely on scanner-reported CTDI values, which represent dose to one of two standardized phantoms (either 16- or 32-cm phantom, depending on the manufacturer), and do not reflect actual patient dose. Instead, we used previously published Monte Carlo simulation results as well as published adjustments for patient size. We believe this provides a much more accurate estimate of actual radiation dose to individual pediatric patients than conventional estimates obtained using CTDIvol and Dose Length Product (DLP) values reported at the scanner. Finally, the point of our study was to review our existing institutional protocols, especially for pediatric patients with malignancies who undergo frequent and repeated PET/CT scanning and to look at not just the effects of an individual scan but at the cumulative impact on these patients, even for protocols that had been only somewhat tailored for children, and to evaluate their impact in terms of the radiation exposure. It should be remembered that many institutions do not have their protocols specifically adjusted for children and that many pediatric patients with malignancies are subjected to repeated studies to monitor disease status. Therefore, protocols should be designed with a balance of radiation exposure and benefit in mind, and both CT and PET/CT studies should be performed with specific pediatric guidelines, thus reducing the overall radiation dose to this patient population.

TH-C-201B-12: CT Dose Reduction with Tube Current Modulation: A Case Study of Fetal Doses to Pregnant Patients Using Monte Carlo Computational Phantoms

Purpose: To demonstrate CT fetal dose reduction with the use of tube current modulation (TCM) on pregnant patients using retrospective data. Method and Materials:Monte Carlo(MC) simulation techniques are used to model CT scanners the TCM schemes and the pregnant patients. Two MDCT scanners GE LightSpeed Pro 16 and GE LightSpeed™ 16 were modeled in MCNPX source code. The TCM data were selected from archived clinical records for pregnant patients who underwent CT procedures recently at Massachusetts General Hospital (MGH) in Boston. Three pregnant patient phantoms with different gestations were utilized. Results: According to the calculation results it is found that the fetal dose from TCM is always smaller than the fetal dose from non‐TCM. TCM schema can reduce fetal dose from 14% to 25%. Also fetal dose gets increased when fetus becomes larger. It is also found that results of ImPACT calculations are always larger than MC calculations performed in this research. The difference between the two sets of data can reach around 40%–50%. Conclusion: This work demonstrates that Monte Carlo method can be used to assess the organ doses and fetal doses of pregnant patients undergoing TCM CT examinations by modeling the CT scanner and TCM schema as well as patient phantoms. The comparison between TCM organ doses and non‐TCM organ doses indicates that the modulated current can effectively help reduce organ dose from CT scans from 14% to 25% according to the specific TCM schema.

Dose to Radiosensitive Organs During Routine Chest CT: Effects of Tube Current Modulation

The aims of this study were to estimate the dose to radiosensitive organs (glandular breast and lung) in patients of various sizes undergoing routine chest CT examinations with and without tube current modulation; to quantify the effect of tube current modulation on organ dose; and to investigate the relation between patient size and organ dose to breast and lung resulting from chest CT examinations. Thirty voxelized models generated from images of patients were extended to include lung contours and were used to represent a cohort of women of various sizes. Monte Carlo simulation-based virtual MDCT scanners had been used in a previous study to estimate breast dose from simulations of a fixed-tube-current and a tube current-modulated chest CT examinations of each patient model. In this study, lung doses were estimated for each simulated examination, and the percentage organ dose reduction attributed to tube current modulation was correlated with patient size for both glandular breast and lung tissues. The average radiation dose to lung tissue from a chest CT scan obtained with fixed tube current was 23 mGy. The use of tube current modulation reduced the lung dose an average of 16%. Reductions in organ dose (up to 56% for lung) due to tube current modulation were more substantial among smaller patients than larger. For some larger patients, use of tube current modulation for chest CT resulted in an increase in organ dose to the lung as high as 33%. For chest CT, lung dose and breast dose estimates had similar correlations with patient size. On average the two organs receive approximately the same dose effects from tube current modulation. The dose to radiosensitive organs during fixed-tube-current and tube current-modulated chest CT can be estimated on the basis of patient size. Organ dose generally decreases with the use of tube current-modulated acquisition, but patient size can directly affect the dose reduction achieved.

Radiation Dose in a “Triple Rule-Out” Coronary CT Angiography Protocol of Emergency Department Patients Using 64-MDCT: The Impact of ECG-Based Tube Current Modulation on Age, Sex, and Body Mass Index

"Triple rule-out" coronary CT angiography (CTA) using 64-MDCT technology is a new approach for evaluating emergency department patients presenting with symptoms suggestive of acute coronary syndrome (ACS). Our objective was to evaluate the reduction in effective radiation dose through the use of tube current modulation in patients who underwent a triple rule-out coronary CTA evaluation and to document how effective radiation dose was impacted by patient age, sex, and body mass index (BMI). A retrospective analysis of triple rule-out coronary CTA examinations performed on a 64-MDCT scanner was ordered on a prospective cohort of 267 consecutive low- to moderate-risk emergency department patients with suspected ACS from a single university hospital between October 2006 and March 2008. Tube current modulation was generally used in patients with heart rates below 65 beats per minute during the second half of the study period as a way to reduce radiation exposure. We calculated effective radiation exposure using actual patient coronary CTA scanning parameters by age, sex, and BMI. Among the 172 patients evaluated without tube current modulation, effective dose averaged (+/- SD) 18.0 +/- 5.6 mSv (range, 9.9-31.3 mSv). Of the 95 patients who underwent CTA examination with tube current modulation, effective dose was significantly lower at 8.75 +/- 2.64 mSv (range, 5.4-16.6 mSv; p < 0.0001) and image quality was better (p < 0.0001) as compared with examinations without tube current modulation. There were no significant radiation differences by patient age, but tube current modulation decreased radiation exposure by at least half. Among the studies in which tube current modulation was not used, women received less radiation than men (17.0 vs 19.5 mSv, respectively; p < 0.001). For the studies with tube current modulation, there were no radiation differences by sex. Obese patients received significantly more radiation than overweight and normal-weight patients in the non-tube current modulation groups (20.9 mSv vs 15.0 and 14.9 mSv, respectively; p < 0.0001) and in the tube current modulation groups (10.3 mSv vs 7.6 and 7.1 mSv, p < 0.0001). The overall effective radiation dose for triple rule-out coronary CTA was reduced by more than 50% with ECG-based tube current modulation without loss of image quality. Tube current modulation should be used for triple rule-out coronary CTA examinations whenever possible.

Abstract 690: Variability and Predictors of Patient Radiation Dose with Dual Source Cardiac CT: Implications for Future Algorithms to Reduce Patient Radiation Dose

Intro : DSCT provides improved temporal resolution due to the simultaneous use of two x-ray sources &amp; detectors. Although use of two sources may increase radiation, the DSCT offers key mechanisms to reduce dose (i.e. pitch adaptation, tube current modulation (TCM) &amp; prospective triggering). Thus, our aim was to assess the patient radiation exposure associated with DSCT and identify variations based on the use of different scan related parameters. Methods : Prospective study of a single tertiary medical center where radiation and image quality related data was collected on 304 consecutive patients (pts) presenting for clinical CCT examination. Effective radiation dose was calculated by multiplying the dose-length product × (k=.017 mSv/mGy/cm). Image quality was rated on a subjective IQ score [1=poor to 4=excellent], as well as contrast (CNR) and signal-to-noise (SNR) ratios. Adjusted means of increased radiation dose were calculated based on linear regression models. Results: Among 304 consecutive studies (mean age 56.4, BMI 29.4 kg/m 2 , 37% Female), 60% were performed for coronary evaluation, 8% for CABG, 18% for pulmonary veins and 11% for aortic disease. The average radiation dose was 13.5±9.2mSv [range 0.5–55.5 mSv]. TABLE provides unadjusted and adjusted mean radiation dose for parameters which had a significant univariate association with radiation dose. Independent predictors of lower radiation included low kV, use of TCM, higher pitch, smaller scan length, and regular heart rhythm. Selected use of various TCM algorithms &amp; low KV resulted in no significant difference in IQ, CNR, or SNR. Conclusions : DSCT is associated with a wide range of patient radiation exposure. The variability in dose is due to both controllable parameters (i.e. use of TCM, low kV, scan length) as well as parameters that cannot be altered (i.e. irregular rhythm). These results suggest that individualizing scan protocols may result in lower radiation dose without compromising image quality. Table: Cardiac DSCT Parameters Affecting Patient Radiation Dose

Reduction of Radiation Dose Estimates in Cardiac 64-Slice CT Angiography in Patients After Coronary Artery Bypass Graft Surgery

The objectives of this prospective investigation in patients after bypass graft surgery were (1) to estimate radiation dose for routine bypass graft computed tomography (CT) angiography, (2) to study the impact of anatomically adapted and ECG-controlled tube current modulation on radiation dose estimates, and (3) effects on qualitative and quantitative image quality parameters. Radiation dose was estimated for 194 consecutive patients undergoing 64-slice CT angiography (Somatom Sensation 64 Cardiac, Siemens Medical Solutions). The impact of anatomically adapted tube current modulation was studied in 2 consecutive patients groups. Furthermore, the impact of ECG-controlled tube current modulation, applied as indicated, was evaluated in both groups. Mean radiation dose estimate for a 64-slice CT bypass graft study was 18.9 +/- 6.0 mSv (CTDIvol 42.3 +/- 12.9 mGy). The application of anatomically adapted tube current modulation had no effect on dose parameters (CTDIvol 43.3 +/- 13.2 vs. 40.1 +/- 12.1 mGy for those with versus those without anatomically adapted tube current modulation, P = 0.1). Additional implementation of ECG-controlled tube current modulation resulted in reduced dose parameters (CTDIvol of 32.9 +/- 2.6 vs. 58.9 +/- 3.9 mGy and radiation dose estimates: 14.7 +/- 1.9 mSv vs. 26.5 +/- 2.1 mSv for those with versus those without ECG pulsing, both P < 0.01). There was no deterioration in image quality with use of tube current modulation algorithms. The radiation burden associated with 64-slice bypass graft CT angiographies is substantial. Anatomically adapted tube current modulation does not reduce radiation dose parameters, whereas ECG-controlled tube current modulation was associated with a 45% reduction in dose estimates. Application of both tube current modulation algorithms did not result in reduced image quality.

Radiation Dose Reduction Strategy for CT Protocols: Successful Implementation in Neuroradiology Section

To retrospectively quantify the effect of systematic use of tube current modulation for neuroradiology computed tomographic (CT) protocols on patient dose and image quality. This HIPAA-compliant study had institutional review board approval, with waiver of informed consent. The authors evaluated the effect of dose modulation on four types of neuroradiologic CT studies: brain CT performed without contrast material (unenhanced CT) in adult patients, unenhanced brain CT in pediatric patients, adult cervical spine CT, and adult cervical and intracranial CT angiography. For each type of CT study, three series of 100 consecutive studies were reviewed: 100 studies performed without dose modulation, 100 studies performed with z-axis dose modulation, and 100 studies performed with x-y-z-axis dose modulation. For each examination, the weighted volume CT dose index (CTDI(vol)) and dose-length product (DLP) were recorded and noise was measured. Each study was also reviewed for image quality. Continuous variables (CTDI(vol), DLP, noise) were compared by using t tests, and categorical variables (image quality) were compared by using Wilcoxon rank-sum tests. For unenhanced CT of adult brains, the CTDI(vol) and DLP, respectively, were reduced by 60.9% and 60.3%, respectively, by using z-axis dose modulation and by 50.4% and 22.4% by using x-y-z-axis dose modulation. Significant dose reductions (P < .001) were also observed for pediatric unenhanced brain CT, cervical spine CT, and adult cervical and intracranial CT angiography performed with each dose modulation technique. Image quality and noise were unaffected by the use of either dose modulation technique (P > .05). Use of dose-modulation techniques for neuroradiology CT examinations affords significant dose reduction while image quality is maintained.