Colin Anderson-Evans

Radiation Dose Savings for Adult Pulmonary Embolus 64-MDCT Using Bismuth Breast Shields, Lower Peak Kilovoltage, and Automatic Tube Current Modulation

OBJECTIVE The purpose of this study was to assess whether radiation dose savings using a lower peak kilovoltage (kVp) setting, bismuth breast shields, and automatic tube current modulation could be achieved while preserving the image quality of MDCT scans obtained to assess for pulmonary embolus (PE). MATERIALS AND METHODS CT angiography (CTA) examinations were performed to assess for the presence or absence of pulmonary artery emboli using a 64-MDCT scanner with automatic tube current modulation (noise level=10 HU), two kVp settings (120 and 140 kVp), and bismuth breast shields. Absorbed organ doses were measured using anthropomorphic phantoms and metal oxide semiconductor field effect transistor (MOSFET) detectors. Image quality was assessed quantitatively as well as qualitatively in various anatomic sites of the thorax. RESULTS Using a lower kVp (120 vs 140 kVp) and automatic tube current modulation resulted in a dose savings of 27% to the breast and 47% to the lungs. The use of a lower kVp (120 kVp), automatic tube current modulation, and bismuth shields placed directly on the anterior chest wall reduced absorbed breast and lung doses by 55% and 45%, respectively. Qualitative assessment of the images showed no change in image quality of the lungs and mediastinum when using a lower kVp, bismuth shields, or both. CONCLUSION The use of bismuth breast shields together with a lower kVp and automatic tube current modulation will reduce the absorbed radiation dose to the breast and lungs without degradation of image quality to the organs of the thorax for CTA detection of PE.

Radiation Dose for Body CT Protocols: Variability of Scanners at One Institution

OBJECTIVE The objective of our study was to determine, using an anthropomorphic phantom, whether patients are subject to variable radiation doses based on scanner assignment for common body CT studies. MATERIALS AND METHODS Twenty metal oxide semiconductor field effect transistor dosimeters were placed in a medium-sized anthropomorphic phantom of a man. Pulmonary embolism and chest, abdomen, and pelvis protocols were used to scan the phantom three times with GE Healthcare scanners in four configurations and one 64-MDCT Siemens Healthcare scanner. Organ doses were averaged, and effective doses were calculated with weighting factors. RESULTS The mean effective doses for the pulmonary embolism protocol ranged from 9.9 to 18.5 mSv and for the chest, abdomen, and pelvis protocol from 6.7 to 18.5 mSv. For the pulmonary embolism protocol, the mean effective dose from the Siemens Healthcare 64-MDCT scanner was significantly lower than that from the 16- and 64-MDCT GE Healthcare scanners (p < 0.001). The mean effective dose from the GE 4-MDCT scanner was significantly lower than that for the GE 16-MDCT scanner (p < 0.001) but not the GE 64-MDCT scanner (p = 0.02). For the chest, abdomen, and pelvis protocol, all mean effective doses from the GE scanners were significantly different from one another (p < 0.001), the lowest mean effective dose being found with use of a single-detector CT scanner and the highest with a 4-MDCT scanner. For the chest, abdomen, and pelvis protocols, the difference between the mean effective doses from the GE Healthcare and Siemens Healthcare 64-MDCT scanners was not statistically significant (p = 0.89). CONCLUSION According to phantom data, patients are subject to different radiation exposures for similar body CT protocols depending on scanner assignment. In general, doses are lowest with use of 64-MDCT scanners.

Radiation Dose Estimations to the Thorax Using Organ-Based Dose Modulation

OBJECTIVE The purpose of this study was to assess the radiation dose distribution and image quality for organ-based dose modulation during adult thoracic MDCT. MATERIALS AND METHODS Organ doses were measured using an anthropomorphic adult female phantom containing 30 metal oxide semiconductor field-effect transistor detectors on a dual-source MDCT scanner with two protocols: standard tube current modulation thoracic CT and organ-based dose modulation using a 120° radial arc. Radiochromic film measured the relative axial dose. Noise was measured to evaluate image quality. Breast tissue location across the anterior aspect of the thorax was retrospectively assessed in 100 consecutive thoracic MDCT examinations. RESULTS There was a 17-47% decrease (p = < 0.05) in anterior thoracic organ dose and a maximum 52% increase (p = < 0.05) in posterior thoracic organ dose using organ-based dose modulation compared with tube current modulation. Effective dose (SD) for tube current modulation and organ-based dose modulation were 5.25 ± 0.36 mSv and 4.42 ± 0.30 mSv, respectively. Radiochromic film analysis showed a 30% relative midline anterior-posterior gradient. There was no statistically significant difference in image noise. Adult female breast tissue was located within an average anterior angle of 155° (123-187°). CONCLUSION Organ-based dose modulation CT using an anterior 120° arc can reduce the organ dose in the anterior aspect of the thorax with a compensatory organ dose increase posteriorly without impairment of image quality. Laterally located breast tissue will have higher organ doses than medially located breast tissue when using organ-based dose modulation. The benefit of this dose reduction must be clinically determined on the basis of the relationship of the irradiated organs to the location of the prescribed radial arc used in organ-based dose modulation.

Radiation Dose Estimation for Prospective and Retrospective ECG-Gated Cardiac CT Angiography in Infants and Small Children Using a 320-MDCT Volume Scanner

OBJECTIVE The purpose of this study is to determine patient dose estimates for clinical pediatric cardiac-gated CT angiography (CTA) protocols on a 320-MDCT volume scanner. MATERIALS AND METHODS Organ doses were measured using 20 metal oxide semiconductor field effect transistor (MOSFET) dosimeters. Radiation dose was estimated for volumetrically acquired clinical pediatric prospectively and retrospectively ECG-gated cardiac CTA protocols in 5-year-old and 1-year-old anthropomorphic phantoms on a 320-MDCT scanner. Simulated heart rates of 60 beats/min (5-year-old phantom) and 120 beats/min (1- and 5-year-old phantoms) were used. Effective doses (EDs) were calculated using average measured organ doses and International Commission on Radiological Protection 103 tissue-weighting factors. Dose-length product (DLP) was recorded for each examination and was used to develop dose conversion factors for pediatric cardiac examinations acquired with volume scan mode. DLP was also used to estimate ED according to recently published dose conversion factors for pediatric helical chest examinations. Repeated measures and paired Student t test analyses were performed. RESULTS For the 5-year-old phantom, at 60 beats/min, EDs ranged from 1.2 mSv for a prospectively gated examination to 4.5 mSv for a retrospectively gated examination. For the 5-year-old phantom, at 120 beats/min, EDs ranged from 3.0 mSv for a prospectively gated examination to 4.9 mSv for a retrospectively gated examination. For the 1-year-old phantom, at 120 beats/min, EDs ranged from 2.7 mSv for a prospectively gated examination to 4.5 mSv for a retrospectively gated examination. CONCLUSION EDs for 320-MDCT volumetrically acquired ECG-gated pediatric cardiac CTA are lower than those published for conventional 16- and 64-MDCT scanners.

Organ-specific radiation dose rates and effective dose rates during percutaneous nephrolithotomy.

BACKGROUND AND PURPOSE Radiation exposure during medical procedures continues to be an increasing concern for physicians and patients. We determined organ-specific dose rates and calculated effective dose rates during right and left percutaneous nephrolithotomy (PCNL) using a validated phantom model. MATERIALS AND METHODS A validated anthropomorphic adult male phantom was placed prone on an operating room table. Metal oxide semiconductor field effect transistor dosimeters were placed at 20 organ locations in the model and were used to measure the organ dosages. A portable C-arm was used to provide continuous fluoroscopy for three 10 minute runs each to simulate a left and right PCNL. Organ dose rate (mGy/s) was determined by dividing organ dose by fluoroscopy time. The organ dose rates were multiplied by their tissue weighting factor and summed to determine effective dose rate (EDR) (mSv/s). Two-dimensional radiation distribution in the abdomen during a left-sided PCNL was visually determined using radiochromic film. RESULTS The EDR for a left PCNL was 0.021 mSv/s ± 0.0008. The EDR for a right PCNL was 0.014 mSv/s ± 0.0004. The skin entrance was exposed to the greatest amount of radiation during left and right PCNL, 0.24 mGy/s and 0.26 mGy/s, respectively. Radiochromic film demonstrates visually the nonuniform dose distribution as the x-ray beam enters through the skin from the radiation source. CONCLUSIONS The effective dose rate is higher for a left-sided PCNL compared with a right-sided PCNL. The distribution of radiation exposure during PCNL is not uniform. Further studies are needed to determine the long-term implications of these radiation doses during percutaneous stone removal.

Application of MOSFET Detectors for Dosimetry in Small Animal Radiography Using Short Exposure Times

Abstract Lin, M. D., Toncheva, G., Nguyen, G., Kim, S., Anderson-Evans, C., Johnson, G. A. and Yoshizumi, T. T. Application of MOSFET Detectors for Dosimetry in Small Animal Radiography Using Short Exposure Times. Radiat. Res. 170, 260– 263 (2008). Digital subtraction angiography (DSA) X-ray imaging for small animals can be used for functional phenotyping given its ability to capture rapid physiological changes at high spatial and temporal resolution. The higher temporal and spatial requirements for small-animal imaging drive the need for short, high-flux X-ray pulses. However, high doses of ionizing radiation can affect the physiology. The purpose of this study was to verify and apply metal oxide semiconductor field effect transistor (MOSFET) technology to dosimetry for small-animal diagnostic imaging. A tungsten anode X-ray source was used to expose a tissue-equivalent mouse phantom. Dose measurements were made on the phantom surface and interior. The MOSFETs were verified with thermoluminescence dosimeters (TLDs). Bland-Altman analysis showed that the MOSFET results agreed with the TLD results (bias, 0.0625). Using typical small animal DSA scan parameters, the dose ranged from 0.7 to 2.2 cGy. Application of the MOSFETs in the small animal environment provided two main benefits: (1) the availability of results in near real-time instead of the hours needed for TLD processes and (2) the ability to support multiple exposures with different X-ray techniques (various of kVp, mA and ms) using the same MOSFET. This MOSFET technology has proven to be a fast, reliable small animal dosimetry method for DSA imaging and is a good system for dose monitoring for serial and gene expression studies.

Kerma Area Product Method for Effective Dose Estimation During Lumbar Epidural Steroid Injection Procedures: Phantom Study

OBJECTIVE The purpose of this study was to derive from the kerma area product the dose conversion coefficient for estimating the effective dose for lumbar epidural steroid injection procedures. MATERIALS AND METHODS A mobile fluoroscopy system was used for fluoroscopic imaging guidance of lumbar epidural steroid injection procedures. For acquisition of organ dose measurements, 20 diagnostic metal oxide semiconductor field effect transistor detectors were placed at each organ in an anthropomorphic phantom of a man, and these detectors were attached to four mobile metal oxide semiconductor field effect transistor wireless bias supplies to obtain the organ dose readings. The kerma area product was recorded from the system console and independently validated with an ion chamber and therapeutic x-ray film. Fluoroscopy was performed on the phantom for 10 minutes for acquisition of the dose rate for each organ, and the average clinical procedure time was multiplied by each organ dose rate for acquisition of individual organ doses. The effective dose was computed by summing the product of each organ dose and the corresponding tissue weighting factor from International Commission on Radiologic Protection publication 60. RESULTS The effective dose was computed as 0.93 mSv for an average lumbar epidural steroid injection procedure (fluoroscopic time, 40.7 seconds). The corresponding kerma area product was 2.80 Gy.cm(2). The dose conversion coefficient was derived as 0.33 mSv/(Gy.cm(2)). CONCLUSION The effective dose for lumbar epidural steroid injection can be easily estimated by multiplying the derived dose conversion coefficient by the console-displayed kerma area product.

Early First Trimester Fetal Dose Estimation Method in a Multivendor Study of 16- and 64-MDCT Scanners and Low-Dose Imaging Protocols

OBJECTIVE The purpose of this study was to corroborate the relation between the estimated absorbed fetal dose derived from directly measured uterine doses early in the first trimester and the volume CT dose index (CTDI(vol)) for 16- and 64-MDCT of the maternal chest, abdomen, and pelvis. MATERIALS AND METHODS Estimated absorbed fetal dose was measured with a metal oxide semiconductor field effect transistor (MOSFET) dosimeter placed in the expected uterine location in an anthropomorphic phantom of a woman and scanned with 16- and 64-MDCT units of one vendor and a 64-MDCT unit of another vendor. A trauma chest, abdomen, and pelvis protocol and an abdomen and pelvis protocol were used. Absorbed uterine dose was measured directly from the MOSFET detector. The CTDI(vol) for each protocol was recorded from the scanner console. Correlation between mean uterine dose and CTDI(vol) was tested with a goodness of fit model. RESULTS The absorbed uterine dose ranged from 9.25 to 37.7 mGy. Absorbed fetal dose in the early first trimester correlated with CTDI(vol) in a linear regression equation. For the 16-MDCT scanner, at 130 kVp, the fetal dose was 2.091 x CTDI(vol) - 9.489. For the 64-MDCT scanner from the same vendor, at 120 kVp, the fetal dose was 1.113 x CTDI(vol) + 1.773. For the 64-MDCT scanner from the other vendor, at 120 kVp, the fetal dose was 1.378 x CTDI(vol) - 1.014. The goodness of fit results (R(2)) for the equations were 0.97, 0.98, and 0.99. CONCLUSION Estimated absorbed fetal dose during the first trimester of pregnancy is linearly associated with CTDI(vol) regardless of beam energy, detector configuration, and scanner manufacturer.

Comparison of radiation dose estimates, image noise, and scan duration in pediatric body imaging for volumetric and helical modes on 320-detector CT and helical mode on 64-detector CT

BackgroundAdvanced multidetector CT systems facilitate volumetric image acquisition, which offers theoretic dose savings over helical acquisition with shorter scan times.ObjectiveCompare effective dose (ED), scan duration and image noise using 320- and 64-detector CT scanners in various acquisition modes for clinical chest, abdomen and pelvis protocols.Materials and methodsED and scan durations were determined for 64-detector helical, 160-detector helical and volume modes under chest, abdomen and pelvis protocols on 320-detector CT with adaptive collimation and 64-detector helical mode on 64-detector CT without adaptive collimation in a phantom representing a 5-year-old child. Noise was measured as standard deviation of Hounsfield units.ResultsCompared to 64-detector helical CT, all acquisition modes on 320-detector CT resulted in lower ED and scan durations. Dose savings were greater for chest (27–46%) than abdomen/pelvis (18–28%) and chest/abdomen/pelvis imaging (8–14%). Noise was similar across scanning modes, although some protocols on 320-detector CT produced slightly higher noise.ConclusionDose savings can be achieved for chest, abdomen/pelvis and chest/abdomen/pelvis examinations on 320-detector CT compared to helical acquisition on 64-detector CT, with shorter scan durations. Although noise differences between some modes reached statistical significance, this is of doubtful diagnostic significance and will be studied further in a clinical setting.

Dosimetric characterisation of bismuth shields in CT: measurements and Monte Carlo simulations

Although bismuth shields are frequently used in radiology to reduce radiation dose, its mechanism has not been fully investigated. Dosimetric characteristics of bismuth shields in computed tomography (CT) were assessed with ion chamber and Monte Carlo (MC) simulations. Primary attenuation and backscatter effects of paediatric (2-ply) and adult (4-ply) bismuth shields were measured. Simulated CT beams were used for ion chamber measurements. Radiation doses were measured free-in-air and in the tissue-equivalent slabs. MC simulations for the same settings were also performed. Mean dose reductions from primary attenuation were 23% (2-ply) and 40% (4-ply). The dose increase from backscatter was 2% for both shields. MC simulations for primary beam dose reduction were 20% (2-ply) and 38% (4-ply); the backscatter dose increase was around 6% for both shields. In summary, primary attenuation is the major factor that introduces the dose reduction in bismuth and the dose increase from backscatter is negligible.