Automatic exposure control in pediatric and adult computed tomography examinations: can we estimate organ and effective dose from mean MAS reduction?

OBJECTIVES

: The purpose of this study was (i) to measure absorbed dose to radiosensitive organs and estimate the effective dose associated with fixed tube current and automatic exposure control (AEC)-activated standard computed tomography (CT) examinations, and (ii) to investigate the relationship between the average reduction of tube current achieved by AEC and the reduction in organ and effective dose.

MATERIALS AND METHODS

: Four physical anthropomorphic phantoms that represent the average individual as neonate, 1-year-old, 5-year-old, 10-year-old child, and the Rando phantom that simulates the average adult individual were employed. The phantoms were subjected to standard head and neck, thorax, and abdomen and pelvis scans using a 16-slice CT system. The scans were performed both with fixed tube current and with AEC. Dose measurements were performed for each scan using thermoluminescent dosimeters placed at internal locations in the phantoms and on the phantoms' surface. Dose measurements were performed for all radiosensitive organs according to the 2007 recommendations of the International Commission on Radiologic Protection. Effective dose was estimated on the basis of weighted sum of measured organ absorbed doses (EDMEAS). Percent reduction of organ absorbed dose and effective dose were compared with the mean percent reduction of the tube current.

RESULTS

: The percent organ dose reduction achieved when AEC was activated in standard head and neck CT scans ranged from 26.6% to 42% for neonate, 8.1% to 63.8% for 1-year-old, -2.9% to 22.5% for 5-year-old, -8.7% to 44.9% for 10-year-old, and 16.3% to 50.1% for an adult. The corresponding values for thorax scans were found to range from -26.1% to 9.9% for neonate, -2.5% to 37.7% for 1-year-old, -20.8% to 15.4% for 5-year-old, -61.9% to 9.3% for 10-year-old, and 5.6% to 42.2% for an adult, whereas the corresponding values for abdomen and pelvis scans were found to range from -12.1% to 29.1% for neonate, -4.9% to 26.6% for 1-year-old, -11.7% to 38.9% for 5-year-old, -62.4% to -17.3% for 10-year-old, and 31.0% to 56.8% for an adult. In neonate, the EDMEAS values ranged from 1.18 to 3.23 mSv for fixed tube current and 1.31 to 1.73 mSv for AEC scans. In 1-year-old phantom, the EDMEAS values ranged from 1.71 to 2.82 mSv for fixed tube current and 0.99 to 2.38 mSv for AEC scans. In 5-year-old phantom, the EDMEAS values ranged from 2.03 to 3.72 mSv for fixed tube current and 1.57 to 3.35 mSv for AEC scans. In 10-year-old phantom, the EDMEAS values ranged from 1.56 to 2.88 mSv for fixed tube current and 1.63 to 3.14 mSv for AEC scans. In adult phantom, the EDMEAS values ranged from 3.39 to 8.06 mSv for fixed tube current and 2.28 to 3.83 mSv for AEC scans. Mean mAs reduction is linearly related to the EDMEAS reduction (r = 0.807, P < 0.0001). The absolute percent difference between percent tube current and %EDMEAS reduction was in most cases higher than 15%.

CONCLUSIONS

: The reduction in the modulated tube current achieved by AEC should not be used to estimate the reduction in the absorbed dose to exposed radiosensitive organs. Moreover, the reduction in the modulated tube current may only be considered as a rough approximation of the corresponding effective dose reduction.