Marsha L. Roskopf

Effective doses to patients undergoing thoracic computed tomography examinations

The purpose of this study was to investigate how x-ray technique factors and effective doses vary with patient size in chest CT examinations. Technique factors (kVp, mAs, section thickness, and number of sections) were recorded for 44 patients who underwent a routine chest CT examination. Patient weights were recorded together with dimensions and mean Hounsfield unit values obtained from representative axial CT images. The total mass of directly irradiated patient was modeled as a cylinder of water to permit the computation of the mean patient dose and total energy imparted for each chest CT examination. Computed values of energy imparted during the chest CT examination were converted into effective doses taking into account the patient weight. Patient weights ranged from 4.5 to 127 kg, and half the patients in this study were children under 18 years of age. All scans were performed at 120 kVp with a 1 s scan time. The selected tube current showed no correlation with patient weight (r2=0.06), indicating that chest CT examination protocols do not take into account for the size of the patient. Energy imparted increased with increasing patient weight, with values of energy imparted for 10 and 70 kg patients being 85 and 310 mJ, respectively. The effective dose showed an inverse correlation with increasing patient weight, however, with values of effective dose for 10 and 70 kg patients being 9.6 and 5.4 mSv, respectively. Current CT technique factors (kVp/mAs) used to perform chest CT examinations result in relatively high patient doses, which could be reduced by adjusting technique factors based on patient size.

Patient size and x‐ray technique factors in head computed tomography examinations. I. Radiation doses

We investigated how patient age, size and composition, together with the choice of x-ray technique factors, affect radiation doses in head computed tomography (CT) examinations. Head size dimensions, cross-sectional areas, and mean Hounsfield unit (HU) values were obtained from head CT images of 127 patients. For radiation dosimetry purposes patients were modeled as uniform cylinders of water. Dose computations were performed for 18 x 7 mm sections, scanned at a constant 340 mAs, for x-ray tube voltages ranging from 80 to 140 kV. Values of mean section dose, energy imparted, and effective dose were computed for patients ranging from the newborn to adults. There was a rapid growth of head size over the first two years, followed by a more modest increase of head size until the age of 18 or so. Newborns have a mean HU value of about 50 that monotonically increases with age over the first two decades of life. Average adult A-P and lateral dimensions were 186+/-8 mm and 147+/-8 mm, respectively, with an average HU value of 209+/-40. An infant head was found to be equivalent to a water cylinder with a radius of approximately 60 mm, whereas an adult head had an equivalent radius 50% greater. Adult males head dimensions are about 5% larger than for females, and their average x-ray attenuation is approximately 20 HU greater. For adult examinations performed at 120 kV, typical values were 32 mGy for the mean section dose, 105 mJ for the total energy imparted, and 0.64 mSv for the effective dose. Increasing the x-ray tube voltage from 80 to 140 kV increases patient doses by about a factor of 5. For the same technique factors, mean section doses in infants are 35% higher than in adults. Energy imparted for adults is 50% higher than for infants, but infant effective doses are four times higher than for adults. CT doses need to take into account patient age, head size, and composition as well as the selected x-ray technique factors.

Patient size and x-ray technique factors in head computed tomography examinations. II. Image quality

We investigated how patient head characteristics, as well as the choice of x-ray technique factors, affect lesion contrast and noise values in computed tomography (CT) images. Head sizes and mean Hounsfield unit (HU) values were obtained from head CT images for five classes of patients ranging from the newborn to adults. X-ray spectra with tube voltages ranging from 80 to 140 kV were used to compute the average photon energy, and energy fluence, transmitted through the heads of patients of varying size. Image contrast, and the corresponding contrast to noise ratios (CNRs), were determined for lesions of fat, muscle, and iodine relative to a uniform water background. Maintaining a constant image CNR for each lesion, the patient energy imparted was also computed to identify the x-ray tube voltage that minimized the radiation dose. For adults, increasing the tube voltage from 80 to 140 kV changed the iodine HU from 2.62 x 10(5) to 1.27 x 10(5), the fat HU from -138 to -108, and the muscle HU from 37.1 to 33.0. Increasing the x-ray tube voltage from 80 to 140 kV increased the percentage energy fluence transmission by up to a factor of 2. For a fixed x-ray tube voltage, the percentage transmitted energy fluence in adults was more than a factor of 4 lower than for newborns. For adults, increasing the x-ray tube voltage from 80 to 140 kV improved the CNR for muscle lesions by 130%, for fat lesions by a factor of 2, and for iodine lesions by 25%. As the size of the patient increased from newborn to adults, lesion CNR was reduced by about a factor of 2. The mAs value can be reduced by 80% when scanning newborns while maintaining the same lesion CNR as for adults. Maintaining the CNR of an iodine lesion at a constant level, use of 140 kV increases the energy imparted to an adult patient by nearly a factor of 3.5 in comparison to 80 kV. For fat and muscle lesions, raising the x-ray tube voltage from 80 to 140 kV at a constant CNR increased the patient dose by 37% and 7%, respectively. Our two key findings are that for head CT examinations performed at a constant CNR, the mAs can be substantially reduced when scanning infants, and that use of the lowest x-ray tube voltage will generally reduce patient doses.

Patient size and x-ray transmission in body CT.

Abstract— Physical characteristics were obtained for 196 patients undergoing chest and abdomen computed tomography (CT) examinations. Computed tomography sections for these patients having no evident pathology were analyzed to determine patient dimensions (AP and lateral), together with the average attenuation coefficient. Patient weights ranged from approximately 3 kg to about 120 kg. For chest CT, the mean Hounsfield unit (HU) fell from about −120 HU for newborns to about −300 HU for adults. For abdominal CT, the mean HU for children and normal-sized adults was about 20 HU, but decreased to below −50 HU for adults weighing more than 100 kg. The effective photon energy and percent energy fluence transmitted through a given patient size and composition was calculated for representative x-ray spectra at 80, 100, 120, and 140 kV tube potentials. A 70-kg adult scanned at 120 kVp transmits 2.6% of the energy fluence for chest and 0.7% for abdomen CT examinations. Reducing the patient size to 10 kg increases transmission by an order of magnitude. For 70 kg patients, effective energies in body CT range from ∼50 keV at 80 kVp to ∼67 keV at 140 kVp; increasing patient size from 10 to 120 kg resulted in an increase in effective photon energy of ∼4 keV. The x-ray transmission data and effective photon energy data can be used to determine CT image noise and image contrast, respectively, and information on patient size and composition can be used to determine patient doses.

In-patient to isocenter KERMA ratios in CT.

PURPOSE To estimate in-patient KERMA for specific organs in computed tomography (CT) scanning using ratios to isocenter free-in-air KERMA obtained using a Rando phantom. METHOD A CT scan of an anthropomorphic phantom results in an air KERMA K at a selected phantom location and air kerma K(CT) at the CT scanner isocenter when the scan is repeated in the absence of the phantom. The authors define the KERMA ratio (R(K)) as K∕ K(CT), which were experimentally determined in a Male Rando Phantom using lithium fluoride chips (TLD-100). R(K) values were obtained for a total of 400 individual point locations, as well as for 25 individual organs of interest in CT dosimetry. CT examinations of Rando were performed on a GE LightSpeed Ultra scanner operated at 80 kV, 120 kV, and 140 kV, as well as a Siemens Sensation 16 operated at 120 kV. RESULTS At 120 kV, median R(K) values for the GE and Siemens scanners were 0.60 and 0.64, respectively. The 10th percentile R(K) values ranged from 0.34 at 80 kV to 0.54 at 140 kV, and the 90th percentile R(K) values ranged from 0.64 at 80 kV to 0.78 at 140 kV. The average R(K) for the 25 Rando organs at 120 kV was 0.61 ± 0.08. Average R(K) values in the head, chest, and abdomen showed little variation. Relative to R(K) values in the head, chest, and abdomen obtained at 120 kV, R(K) values were about 12% lower in the pelvis and about 58% higher in the cervical spine region. Average R(K) values were about 6% higher on the Siemens Sensation 16 scanner than the GE LightSpeed Ultra. Reducing the x-ray tube voltage from 120 kV to 80 kV resulted in an average reduction in R(K) value of 34%, whereas increasing the x-ray tube voltage to 140 kV increased the average R(K) value by 9%. CONCLUSIONS In-patient to isocenter relative KERMA values in Rando phantom can be used to estimate organ doses in similar sized adults undergoing CT examinations from easily measured air KERMA values at the isocenter (free in air). Conversion from in-patient air KERMA values to tissue dose would require the use of energy-appropriate conversion factors.

Reconstruction filters and contrast detail curves in CT

In this study, we investigated the effect of CT reconstruction filters in abdominal CT images of a male anthropomorphic phantom. A GE Light Speed CT 4-slice scanner was used to scan the abdomen of an adult Rando phantom. Cross sectional images of the phantom were reconstructed using four reconstruction filters: (1) soft tissue with the lowest noise; (2) detail (relative noise 1.7); (3) bone (relative noise 4.5); and (4) edge (relative noise 7.7). A two Alternate Forced Choice (AFC) experimental paradigm was used to estimate the intensity needed to achieve 92% correct (i.e., I92%). Four observers measured detection performance for five lesions with size ranging from 2.5 to 12.5 mm for each of these four reconstruction filters. Contrast detail curves obtained in images of an anthropomorphic phantom were not straight lines, but best fitted to a second order polynomial. Results from four readers show similar trends with modest inter-observer differences with the measured coefficient of variation of the absolute performance levels of ~22%. All reconstruction filters had similar shaped contrast detail curves except for smallest details where the frequency response of filters differed most significantly. Increasing the noise level always reduced detection performance, and a doubling of image noise resulted in an average drop in detection performance of ~20%. The key findings of this study are that (a) the Rose model can provide reasonable predictions as to how changes in lesion size affect observer detection; (b) the shape of CT contrast detail curves is affected only very slightly with reconstruction filter; (c) changes in reconstruction filter noise can predict qualitative changes in observer detection performance, but are poor direct predictors of the quantitative changes of imaging performance.

Inter-reader variability in alternate forced choice studies

In this study, we investigated differences in detection performance for twelve observers who each generated a CT contrast detail curve. An anthropomorphic newborn phantoms abdomen was imaged using a GE Light Speed CT scanner (4-slice). Alternate Forced Choice (AFC) experiments were performed with lesions sizes ranging from 2.5 to 12.5 mm to determine the intensity needed to achieve 92% correct (I92%). Following training, twelve readers consisting of (2 technologists, 4 college students, 4 medical students, and 2 radiology residents) generated a single contrast detail curve. Eight readers produced approximately linear contrast detail curves while the remaining four readers required a second order polynomial fit because of reduced performance when detecting the largest (i.e., 12.5 mm) lesion. For the three smallest lesions, the coefficient of variation between the twelve readers was ~12%, which increases with increasing lesion size to ~23% for 12.5 mm lesion size. The ratio of the maximum I92% to minimum I92% values was ~1.6 for the smallest lesions, which increased to a factor of ~2.1 for the 12.5 mm lesion. Our results show that minimizing inter-reader variability in our AFC experiments could be achieved by eliminating the largest lesion that cause detection problems in one third of observers. The combined experimental data showed that the slope of the contrast detail curve was -0.42, lower than the value of -1.0 predicted by the Rose model, suggesting that the noise texture in CT associated with both quantum mottle and anatomic structure is an important factor affecting detection of these lesions.

How do kV and mAs affect CT lesion detection performance

The purpose of this study was to investigate how output (mAs) and x-ray tube voltage (kV) affect lesion detection in CT imaging. An adult Rando phantom was scanned on a GE LightSpeed CT scanner at x-ray tube voltages from 80 to 140 kV, and outputs from 90 to 360 mAs. Axial images of the abdomen were reconstructed and viewed on a high quality monitor at a soft tissue display setting. We measured detection of 2.5 to 12.5 mm sized lesions using a 2 Alternate Forced Choice (2-AFC) experimental paradigm that determined lesion contrast (I) corresponding to a 92% accuracy (I92%) of lesion detection. Plots of log(I92%) versus log(lesion size) were all approximately linear. The slope of the contrast detail curve was ~ -1.0 at 90 mAs, close to the value predicted by the Rose model, but monotonically decreased with increasing mAs to a value of ~ -0.7 at 360 mAs. Increasing the x-ray tube output by a factor of four improved lesion detection by a factor of 1.9 for the smallest lesion (2.5 mm), close to the value predicted by the Rose model, but only by a factor of 1.2 for largest lesion (12.5 mm). Increasing the kV monotonically decreased the contrast detail slopes from -1.02 at 80 kV to -0.71 at 140 kV. Increasing the x-ray tube voltage from 80 to 140 kV improved lesion detection by a factor of 2.8 for the smallest lesion (2.5 mm), but only by a factor of 1.7 for largest lesion (12.5 mm). We conclude that: (i) quantum mottle is an important factor for low contrast lesion detection in images of anthropomorphic phantoms; (ii) x-ray tube voltage has a much greater influence on lesion detection performance than x-ray tube output; (iii) the Rose model only predicts CT lesion detection performance at low x-ray tube outputs (90 mAs) and for small lesions (2.5 mm).

SU‐FF‐I‐16: Calibration of TLD Chips to Maximize Accuracy in Radiographic Phantom Dosimetry

Purpose: To develop an efficient annealing/readout protocol for TLDdosimetry that will maximize the accuracy and precision in radiographicdosimetry measurements. Method and Materials: 500 TLD chips were grouped in batches of 100 and subjected to varying annealing protocols and then irradiated to varying exposure levels. Three different annealing/readout protocols were tested. In protocol ♯1, the chips were annealed at 400 C for 1 hour followed by 2 hours at 100 C. The chips were exposed, and then allowed to rest for 24 hours before reading. In protocol ♯2, the anneal cycle was 400 C for 1 hour followed with a 30 minute cool‐down, followed by 20 hours at 80 C. Immediately after exposure, the TLDs were heated to 120 C for 10 minutes, then read. In protocol ♯3, the anneal cycle was 400 C for 1 hour followed by a 30 minute cool‐down, then 100 C for 2 hours. After exposure, the TLDs were pre‐heated and read as in protocol ♯2. Results: For protocol ♯1, the inter‐exposure uncertainty in the response (nC/mR) was approximately 4.2%. Using protocol ♯2, the intra‐batch uncertainty was reduced to 4.0%, and for protocol ♯3 the uncertainty was reduced to ∼3.7%. By using individual chip calibrations, the intra‐batch uncertainty for estimating consecutive exposures was reduced to 3.5%, 1.8%, and 1.7% for protocols ♯1, 2, and 3, respectively. By binning TLD signals over groups of 3 chips, the uncertainty in estimating exposures was reduced to ∼1.1% for protocol ♯3. Conclusion: With careful calibration and binning of results, an accuracy approaching 1% is readily obtained. The anneal/readout protocol that yielded the highest accuracy also required the least time for a complete cycle, with a batch of 100 chips being processed in <1 working day using a single furnace for annealing and a single‐chip reader.

A method for analyzing contrast-detail curves

The purpose of this study was to develop a concise way to summarize radiographic contrast detail curves. We obtained experimental data that measured lesion detection in CT images of a 5-year-old anthropomorphic phantom. Five lesion diameters (2.5 to 12.5 mm) were investigated, and contrast detail (CD) curves were generated at each of five tube current-exposure time product (mAs) values using twoalternative forced-choice (2-AFC) studies. A performance index for each CD curve was calculated as the area under the curve bounded by the maximum and minimum lesion sizes, with this value being normalized by the range of lesion sizes used. We denote this quantity, which is mathematically equal to the mean value of the CD curve, as the contrast-detail performance index (PCD). This quantity is inspired by the area under the curve (Az) that is used as a performance index in ROC studies, though there are important differences. PCD, like Az, allows for the reduction in the dimensionality of experimental results, simplifying interpretation of data while discarding details of the respective curve (CD or ROC). Unlike Az, PCD decreases with increasing performance, and the range of values is not fixed as for Az (i.e. 0 < Az < 1). PCD is proportional to the average SNR for the lesions used in the 2-AFC experiments, and allows relative performance comparisons as experimental parameters are changed. For the CT data analyzed, the PCD values were 0.196, 0.166, 0.146, 0.132, and 0.121 at mAs values of 30, 50, 70, 100, and 140, respectively. This corresponds to an increase in performance (i.e. decrease in required contrast) relative to the 30 mAs PCD value of 62%, 48%, 33%, and 18% for the 140, 100, 70, and 50 mAs data, respectively.