Practical Strategies to Reduce Pediatric CT Radiation Dose

Estimates of individual patient risk, and epidemiologic studies assessing potential late effects, must use patient size–specific dose estimates—they cannot use only scanner output (volume CT dose index or dose-length product).

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For the optimisation of radiation dose in computed tomography (CT), dose reduction is attempted while preserving diagnostic performance. To reduce the overall radiation dose related to CT, the dose from scout radiography may be reduced by decreasing the tube voltage and tube current. We evaluated the effects of scout imaging conditions on CT radiation dose. An anthropomorphic chest phantom was imaged on two CT scanners (scanners A and B) from different vendors, manipulating the scout imaging conditions in terms of imaging direction, tube voltage and tube current, and assessed the tube current modulation in subsequent CT and pixel values in scout radiographs. The direction of the scout radiography influenced the shape of the tube current modulation curve and total radiation dose in subsequent CT. When compared with the use of the lateral projection, use of the posteroanterior or anteroposterior projection alone increased the radiation dose substantially on scanner A, but did not change, or mildly decreased, the dose on scanner B. When imaged using the lateral scout on scanner A, reduction in tube voltage and tube current for scout radiography decreased the CT dose in the cranial part of the scan range and, to a lesser degree, the total radiation dose. On the low-voltage, low-current lateral radiograph, the image contrast was impaired and pixel values were underestimated around the lung apex. Without the use of the lateral radiograph, neither the tube voltage nor tube current for scout radiography influenced the CT dose. On scanner B, reduced tube voltage for scout radiography increased the CT dose. In conclusion, reduced tube voltage and tube current may affect scout radiographs, resulting in alteration of the tube current modulation pattern and total radiation dose in subsequent CT. These effects vary depending on the CT scanners and scout direction.

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Children are at greater risk of radiation exposure than adults because the rapidly dividing cells of children tend to be more radiosensitive and they have a longer expected life time in which to develop potential radiation injury. Some studies have surveyed computed tomography (CT) radiation doses and several studies have established diagnostic reference levels according to patient age or body size; however, no survey of CT radiation doses with a large number of patients has yet been carried out in South Korea.The aim of the present study was to investigate the radiation dose in pediatric CT examinations performed throughout South Korea.From 512 CT (222 brain CT, 105 chest CT, and 185 abdominopelvic CT) scans that were referred to our tertiary hospital, a dose report sheet was available for retrospective analysis of CT scan protocols and dose, including the volumetric CT dose index (CTDIvol), dose-length product (DLP), effective dose, and size-specific dose estimates (SSDE).At 55.2%, multiphase CT was the most frequently performed protocol for abdominopelvic CT. Tube current modulation was applied most often in abdominopelvic CT and chest CT, accounting for 70.1% and 62.7%, respectively. Regarding the CT dose, the interquartile ranges of the CTDIvol were 11.1 to 22.5 (newborns), 16.6 to 39.1 (≤1 year), 14.6 to 41.7 (2–5 years), 23.5 to 44.1 (6–10 years), and 31.4 to 55.3 (≤15 years) for brain CT; 1.3 to 5.7 (≤1 year), 3.9 to 6.8 (2–5 years), 3.9 to 9.3 (6–10 years), and 7.7 to 13.8 (≤15 years) for chest CT; and 4.0 to 7.5 (≤1 year), 4.2 to 8.9 (2–5 years), 5.7 to 12.4 (6–10 years), and 7.6 to 16.6 (≤15 years) for abdominopelvic CT. The SSDE and CTDIvol were well correlated for patients <5 years old, whereas the CTDIvol was lower in patients ≥6 years old.Our study describes the various parameters and dosimetry metrics of pediatric CT in South Korea. The CTDIvol, DLP, and effective dose were generally lower than in German and UK surveys, except in certain age groups.

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To evaluate effects of varying tube current and voltage on radiation dose, image noise, and image contrast with different phantom sizes and shapes. Four round lucite phantoms with 8-32-cm diameters were scanned with multi-detector row computed tomography (CT) and 80-120 kVp. Radiation dose was based on CT dose index, image noise, and iodine contrast and measured with constant and variable tube currents that were age appropriate for each tube voltage. Radiation dose and image noise and contrast were compared in round and oval 24-cm phantoms. For various combinations of technical factors and phantom sizes and shapes, percentage differences were calculated for radiation dose and image noise and contrast. Associations between tube voltage and radiation dose, image noise, and image contrast in round and oval phantoms were determined by fitting second-degree polynomials to data. Differences in radiation dose and image noise and contrast, which were attributable to differences in tube voltage, were tested with paired t tests. With 165-mAs tube current, radiation doses with 140- and 80-kVp tube voltages were 103% ([41.9 mGy - 20.6 mGy]/20.6 mGy) and 58% ([10.2 mGy - 4.2 mGy]/10.1 mGy) higher in the 8-cm phantom than in the 32-cm phantom. When tube current was adapted for phantom size, radiation dose at 80 kVp in the 8-cm phantom was reduced by 82% ([10.1 mGy - 1.8 mGy]/10.1 mGy). In the 8-cm phantom, tube voltage was decreased from 120 to 80 kVp and tube current remained at 165 mAs, resulting in a 68% noise increase ([3.1 HU - 1.8 HU]/1.8 HU). With variable tube current, 80-kVp tube voltage in the 8-cm phantom led to a 138% noise increase ([7.3 HU - 3.1 HU]/3.1 HU). With reduced tube voltage, image contrast increased. In the 8-cm phantom, with a constant 165-mAs tube current and a decrease in tube voltage from 120 to 80 kVp, there was a 35% ([333 HU - 217 HU]/333 HU) increase in contrast. No difference was noted in radiation dose or noise between round and oval phantoms (P = .604 and P = .06, respectively), but a small statistically significant difference (1%) in contrast attenuation was demonstrated (P = .025). Reduced tube voltage for pediatric contrast material-enhanced CT reduces radiation dose and maintains image contrast. Image noise increases, but the effect is minimal in smaller phantoms. An additional reduction in tube current further reduces radiation dose.

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