Jodie A. Christner

Estimating Effective Dose for CT Using Dose–Length Product Compared With Using Organ Doses: Consequences of Adopting International Commission on Radiological Protection Publication 103 or Dual-Energy Scanning

OBJECTIVE The objective of our study was to compare dose-length product (DLP)-based estimates of effective dose with organ dose-based calculations using tissue-weighting factors from publication 103 of the International Commission on Radiological Protection (ICRP) or dual-energy CT protocols. MATERIALS AND METHODS Using scanner- and energy-dependent organ dose coefficients, we calculated effective doses for CT examinations of the head, chest, coronary arteries, liver, and abdomen and pelvis using routine clinical single- or dual-energy protocols and tissue-weighting factors published in 1991 in ICRP publication 60 and in 2007 in ICRP publication 103. Effective doses were also generated from the respective DLPs using published conversion coefficients that depend only on body region. For each examination type, the same volume CT dose index was used for single- and dual-energy scans. RESULTS Effective doses calculated for CT examinations using organ dose estimates and ICRP 103 tissue-weighting factors differed relative to ICRP 60 values by -39% (-0.5 mSv, head), 14% (1 mSv, chest), 36% (4 mSv, coronary artery), 4% (0.6 mSv, liver), and -7% (-1 mSv, abdomen and pelvis). DLP-based estimates of effective dose, which were derived using ICRP 60-based conversion coefficients, were less than organ dose-based estimates for ICRP 60 by 4% (head), 23% (chest), 37% (coronary artery), 12% (liver), and 19% (abdomen and pelvis) and for ICRP 103 by -34% (head), 37% (chest), 74% (coronary artery), 16% (liver), and 12% (abdomen and pelvis). All results were energy independent. CONCLUSION These differences in estimates of effective dose suggest the need to reassess DLP to E conversion coefficients when adopting ICRP 103, particularly for scans over the breast. For the evaluated scanner, DLP to E conversion coefficients were energy independent, but ICRP 60-based conversion coefficients underestimated effective dose relative to organ dose-based calculations.

Virtual monochromatic imaging in dual-source dual-energy CT: Radiation dose and image quality

PURPOSE To evaluate the image quality of virtual monochromatic images synthesized from dual-source dual-energy computed tomography (CT) in comparison with conventional polychromatic single-energy CT for the same radiation dose. METHODS In dual-energy CT, besides the material-specific information, one may also synthesize monochromatic images at different energies, which can be used for routine diagnosis similar to conventional polychromatic single-energy images. In this work, the authors assessed whether virtual monochromatic images generated from dual-source CT scanners had an image quality similar to that of polychromatic single-energy images for the same radiation dose. First, the authors provided a theoretical analysis of the optimal monochromatic energy for either the minimum noise level or the highest iodine contrast to noise ratio (CNR) for a given patient size and dose partitioning between the low- and high-energy scans. Second, the authors performed an experimental study on a dual-source CT scanner to evaluate the noise and iodine CNR in monochromatic images. A thoracic phantom with three sizes of attenuating rings was used to represent four adult sizes. For each phantom size, three dose partitionings between the low-energy (80 kV) and the high-energy (140 kV) scans were used in the dual-energy scan. Monochromatic images at eight energies (40 to 110 keV) were generated for each scan. Phantoms were also scanned at each of the four polychromatic single energy (80, 100, 120, and 140 kV) with the same radiation dose. RESULTS The optimal virtual monochromatic energy depends on several factors: phantom size, partitioning of the radiation dose between low- and high-energy scans, and the image quality metrics to be optimized. With the increase of phantom size, the optimal monochromatic energy increased. With the increased percentage of radiation dose on the low energy scan, the optimal monochromatic energy decreased. When maximizing the iodine CNR in monochromatic images, the optimal energy was lower than that when minimizing noise level. When the total radiation dose was equally distributed between low and high energy in dual-energy scans, for minimum noise, the optimal energies were 68, 71, 74, and 77 keV for small, medium, large, and extra-large (xlarge) phantoms, respectively; for maximum iodine CNR, the optimal energies were 66, 68, 70, 72 keV. With the optimal monochromatic energy, the noise level was similar to and the CNR was better than that in a single-energy scan at 120 kV for the same radiation dose. Compared to an 80 kV scan, however, the iodine CNR in monochromatic images was lower for the small, medium, and large phantoms. CONCLUSIONS In dual-source dual-energy CT, optimal virtual monochromatic energy depends on patient size, dose partitioning, and the image quality metric optimized. With the optimal monochromatic energy, the noise level was similar to and the iodine CNR was better than that in 120 kV images for the same radiation dose. Compared to single-energy 80 kV images, the iodine CNR in virtual monochromatic images was lower for small to large phantom sizes.

How Effective Is Effective Dose as a Predictor of Radiation Risk

OBJECTIVE This article discusses the relatively recent adoption of effective dose in medicine that allows comparison between different imaging techniques, and describes the principles, pitfalls, and potential value of effective dose. The medical community must use this information wisely, realizing that effective dose represents a generic estimate of risk from a given procedure for a generic model of the human body. CONCLUSION Effective dose is not the risk for any one individual. Due to the inherent uncertainties and oversimplifications involved, effective dose should not be used for epidemiologic studies or for estimating population risks.

Size-specific Dose Estimates for Adult Patients at CT of the Torso

PURPOSE To determine relationships among patient size, scanner radiation output, and size-specific dose estimates (SSDEs) for adults who underwent computed tomography (CT) of the torso. MATERIALS AND METHODS Informed consent was waived for this institutional review board-approved study of existing data from 545 adult patients (322 men, 223 women) who underwent clinically indicated CT of the torso between April 1, 2007, and May 13, 2007. Automatic exposure control was used to adjust scanner output for each patient according to the measured CT attenuation. The volume CT dose index (CTDI(vol)) was used with measurements of patient size (anterioposterior plus lateral dimensions) and the conversion factors from the American Association of Physicists in Medicine Report 204 to determine SSDE. Linear regression models were used to assess the dependence of CTDI(vol) and SSDE on patient size. RESULTS Patient sizes ranged from 42 to 84 cm. In this range,CTDI(vol) was significantly correlated with size (slope = 0.34 mGy/cm; 95% confidence interval [CI]: 0.31, 0.37 mGy/cm; R(2) = 0.48; P < .001), but SSDE was independent of size (slope = 0.02 mGy/cm; 95% CI: -0.02, 0.07 mGy/cm; R(2) = 0.003; P = .3). These R(2) values indicated that patient size explained 48% of the observed variability in CTDI(vol) but less than 1% of the observed variability in SSDE. The regression of CTDI(vol) versus patient size demonstrated that, in the 42-84-cm range, CTDI(vol) varied from 12 to 26 mGy. However, use of the evaluated automatic exposure control system to adjust scanner output for patient size resulted in SSDE values that were independent of size. CONCLUSION For the evaluated automatic exposure control system,CTDI(vol) (scanner output) increased linearly with patient size; however, patient dose (as indicated by SSDE) was independent of size.

Dose Reduction in Helical CT: Dynamically Adjustable z-Axis X-Ray Beam Collimation

OBJECTIVE The purpose of this study was to measure the dose reduction achieved with dynamically adjustable z-axis collimation. MATERIALS AND METHODS A commercial CT system was used to acquire CT scans with and without dynamic z-axis collimation. Dose reduction was measured as a function of pitch, scan length, and position for total incident radiation in air at isocenter, accumulated dose to the center of the scan volume, and accumulated dose to a point at varying distances from a scan volume of fixed length. Image noise was measured at the beginning and center of the scan. RESULTS The reduction in total incident radiation in air at isocenter varied between 27% and 3% (pitch, 0.5) and 46% and 8% (pitch, 1.5) for scan lengths of 20 and 500 mm, respectively. Reductions in accumulated dose to the center of the scan were 15% and 29% for pitches of 0.5 and 1.5 for 20-mm scans. For scan lengths greater than 300 mm, dose savings were less than 3% for all pitches. Dose reductions 80 mm or farther from a 100-mm scan range were 15% and 40% for pitches of 0.5 and 1.5. With dynamic z-axis collimation, noise at the extremes of a helical scan was unchanged relative to noise at the center. Estimated reductions in effective dose were 16% (0.4 mSv) for the head, 10% (0.8 and 1.4 mSv) for the chest and liver, 6% (0.8 mSv) for the abdomen and pelvis, and 4% (0.4 mSv) and 55% (1.0 mSv) for coronary CT angiography at pitches of 0.2 and 3.4. CONCLUSION Use of dynamic z-axis collimation reduces dose in helical CT by minimizing overscanning. Percentage dose reductions are larger for shorter scan lengths and greater pitch values.

Dose Reduction to Anterior Surfaces With Organ-Based Tube-Current Modulation: Evaluation of Performance in a Phantom Study

OBJECTIVE The purpose of this study was to evaluate in phantoms the dose reduction to anterior surfaces and image quality with organ-based tube-current modulation in head and thoracic CT. MATERIALS AND METHODS Organ-based tube-current modulation is designed to reduce radiation dose to superficial radiosensitive organs, such as the lens of the eye, thyroid, and breast, by decreasing the tube current when the tube passes closest to these organs. Dose and image quality were evaluated in phantoms for clinical head and thorax examination protocols with and without organ-based tube-current modulation. Surface dose reduction as a function of position was measured using a 32-cm CT dose index (CTDI) phantom, an anthropomorphic adult phantom, and ion chambers. Surface dose reduction as a function of patient size was investigated using three semianthropomorphic phantoms with posteroanterior dimensions of 14, 25, and 31 cm. Image noise (the SD of CT numbers in regions of interest) was evaluated for the anthropomorphic and the semianthropomorphic phantoms. RESULTS For equivalent scanner output (volume CTDI), the dose to the midline of the anterior surface was reduced by 27-50%, depending on the anatomic region (head or thorax) and phantom size, and the dose to the posterior surface was correspondingly increased. Image noise was not significantly different between scans with and without organ-based tube-current modulation (p = 0.85). CONCLUSION Organ-based tube-current modulation can reduce the dose to the anterior surface of patients without increasing image noise by commensurately increasing the dose to the posterior surface. This technique can reduce the dose to anterior radiosensitive organs for head and thoracic CT scans.

Radiation dose reduction to the breast in thoracic CT: Comparison of bismuth shielding, organ‐based tube current modulation, and use of a globally decreased tube current

PURPOSE The purpose of this work was to evaluate dose performance and image quality in thoracic CT using three techniques to reduce dose to the breast: bismuth shielding, organ-based tube current modulation (TCM) and global tube current reduction. METHODS Semi-anthropomorphic thorax phantoms of four different sizes (15, 30, 35, and 40 cm lateral width) were used for dose measurement and image quality assessment. Four scans were performed on each phantom using 100 or 120 kV with a clinical CT scanner: (1) reference scan; (2) scan with bismuth breast shield of an appropriate thickness; (3) scan with organ-based TCM; and (4) scan with a global reduction in tube current chosen to match the dose reduction from bismuth shielding. Dose to the breast was measured with an ion chamber on the surface of the phantom. Image quality was evaluated by measuring the mean and standard deviation of CT numbers within the lung and heart regions. RESULTS Compared to the reference scan, dose to the breast region was decreased by about 21% for the 15-cm phantom with a pediatric (2-ply) shield and by about 37% for the 30, 35, and 40-cm phantoms with adult (4-ply) shields. Organ-based TCM decreased the dose by 12% for the 15-cm phantom, and 34-39% for the 30, 35, and 40-cm phantoms. Global lowering of the tube current reduced breast dose by 23% for the 15-cm phantom and 39% for the 30, 35, and 40-cm phantoms. In phantoms of all four sizes, image noise was increased in both the lung and heart regions with bismuth shielding. No significant increase in noise was observed with organ-based TCM. Decreasing tube current globally led to similar noise increases as bismuth shielding. Streak and beam hardening artifacts, and a resulting artifactual increase in CT numbers, were observed for scans with bismuth shields, but not for organ-based TCM or global tube current reduction. CONCLUSIONS Organ-based TCM produces dose reduction to the breast similar to that achieved with bismuth shielding for both pediatric and adult phantoms. However, organ-based TCM does not affect image noise or CT number accuracy, both of which are adversely affected by bismuth shielding. Alternatively, globally decreasing the tube current can produce the same dose reduction to the breast as bismuth shielding, with a similar noise increase, yet without the streak artifacts and CT number errors caused by the bismuth shields. Moreover, globally decreasing the tube current reduces the dose to all tissues scanned, not simply to the breast.

Bismuth Shielding, Organ-based Tube Current Modulation, and Global Reduction of Tube Current for Dose Reduction to the Eye at Head CT

PURPOSE To compare the dose and image quality of three methods for reducing the radiation dose to the eye at head computed tomography (CT): bismuth shielding, organ-based tube current modulation (TCM), and global reduction of the tube current. MATERIALS AND METHODS An anthropomorphic head phantom was scanned under six conditions: (a) without any dose reduction techniques (reference scanning); (b) with one bismuth eye shield; (c) with organ-based TCM; (d) with reduced tube current to yield the same dose reduction as one bismuth shield; (e) with two layers of bismuth shields; and (f) with organ-based TCM and one bismuth shield. Dose to the eye, image noise, and CT numbers in the brain region were measured and compared. The effect of increasing distance between the bismuth shield and eye lens was also investigated. RESULTS Relative to the reference scan, the dose to the eye was reduced by 26.4% with one bismuth shield, 30.4% with organ-based TCM, and 30.2% with a global reduction in tube current. A combination of organ-based TCM with one bismuth shield reduced the dose by 47.0%. Image noise in the brain region was slightly increased for all dose reduction methods. CT numbers were increased whenever the bismuth shield was used. Increasing the distance between the bismuth shield and the eye lens helped reduce CT number errors, but the increase in noise remained. CONCLUSION Organ-based TCM provided superior image quality to that with bismuth shielding while similarly reducing dose to the eye. Simply reducing tube current globally by about 30% provides the same dose reduction to the eye as bismuth shielding; however, CT number accuracy is maintained and dose is reduced to all parts of the head.

Radiation Dose Levels for Interventional CT Procedures

OBJECTIVE The purpose of this study was to determine typical radiation dose levels to patients undergoing CT-guided interventional procedures. MATERIALS AND METHODS A total of 571 patients undergoing CT interventional procedures were included in this retrospective data analysis study. Enrolled patients underwent one of five procedures: cryoablation, aspiration, biopsy, drain, or injection. With each procedure, two scan modes were used, either intermittent (no table increment) or helical mode. Skin dose was estimated from the volumetric CT dose index (CTDI(vol)) and phantom measurements. Effective dose was calculated by multiplying dose-length product (DLP) and conversion factor (k factor) for helical mode, and using Monte Carlo organ dose coefficients for intermittent mode. RESULTS The mean (± SD) skin doses were 728 ± 382, 130 ± 104, 128 ± 81, 152 ± 105, and 195 ± 147 mGy, and the mean effective doses were 119.7 ± 50.3, 20.1 ± 11.0, 13.8 ± 9.2, 25.3 ± 15.4, and 9.1 ± 5.5 mSv for each of the five procedures, respectively. The maximum skin dose was 1.95 Gy. The mean effective dose across all procedure types was 24.1 mSv, with 2.3 mSv from intermittent scans and 21.8 mSv from helical scans. CONCLUSION Substantial dose differences were observed among the five procedures. The risk of deterministic effects appears to be very low, because the maximum observed skin dose did not exceed the threshold for transient skin erythema (2 Gy). The average risk of stochastic effects was comparable to that of 1-10 abdomen and pelvis CT examinations. Although the intermittent mode can contribute substantially to skin dose, it contributes minimally to the effective dose because of the much shorter scan range used.

Attenuation‐based estimation of patient size for the purpose of size specific dose estimation in CT. Part II. Implementation on abdomen and thorax phantoms using cross sectional CT images and scanned projection radiograph images

PURPOSE To estimate attenuation using cross sectional CT images and scanned projection radiograph (SPR) images in a series of thorax and abdomen phantoms. METHODS Attenuation was quantified in terms of a water cylinder with cross sectional area of A(w) from both the CT and SPR images of abdomen and thorax phantoms, where A(w) is the area of a water cylinder that would absorb the same dose as the specified phantom. SPR and axial CT images were acquired using a dual-source CT scanner operated at 120 kV in single-source mode. To use the SPR image for estimating A(w), the pixel values of a SPR image were calibrated to physical water attenuation using a series of water phantoms. A(w) and the corresponding diameter D(w) were calculated using the derived attenuation-based methods (from either CT or SPR image). A(w) was also calculated using only geometrical dimensions of the phantoms (anterior-posterior and lateral dimensions or cross sectional area). RESULTS For abdomen phantoms, the geometry-based and attenuation-based methods gave similar results for D(w). Using only geometric parameters, an overestimation of D(w) ranging from 4.3% to 21.5% was found for thorax phantoms. Results for D(w) using the CT image and SPR based methods agreed with each other within 4% on average in both thorax and abdomen phantoms. CONCLUSIONS Either the cross sectional CT or SPR images can be used to estimate patient attenuation in CT. Both are more accurate than use of only geometrical information for the task of quantifying patient attenuation. The SPR based method requires calibration of SPR pixel values to physical water attenuation and this calibration would be best performed by the scanner manufacturer.