Radiation from "extra" images acquired with abdominal and/or pelvic CT: effect of automatic tube current modulation

To retrospectively determine the number and usefulness of images acquired beyond the intended anatomic area of interest with abdominal and/or pelvic computed tomography (CT) and to assess the effect of automatic tube current modulation (ATCM) on associated radiation.

Superior and inferior levels at routine abdominal and/or pelvic CT were defined as the dome of the diaphragm and the inferior margin of the pubic symphysis, respectively. Records of 106 consecutive examinations (male-to-female ratio, 45:61; age range, 21-86 years) performed from June 1 to June 30, 2003, were reviewed to determine the number of "extra" images. Sixty-two abdominal and/or pelvic CT examinations performed concurrently with chest or thigh CT or for trauma were not included in the 106. Abdominal and/or pelvic CT was performed with either ATCM (n = 44) or manual selection of tube current (n = 62). CT parameters recorded for each extra image included tube current, peak kilovoltage, and gantry rotation time. Mean and median tube current-time products were calculated for extra images. Extra images were analyzed for pathologic findings. Statistical analysis was performed with the Student t test.

Extra images were acquired above the dome of the diaphragm in 103 (97%) of 106 examinations and below the pubic symphysis in 100 (94%) of 106. A total of 1,280 extra images were acquired in 106 examinations (mean, 12 images per examination). Nineteen additional findings were observed on extra images. With ATCM, mean tube current-time product was 74.5 and 120.6 mAs for extra images acquired above the diaphragm and below the pubic symphysis, respectively; with manual selection, mean tube current-time products were 167.5 and 168.3 mAs (P <.05).

Most extra images acquired at abdominal and/or pelvic CT contributed no additional information. With ATCM, the radiation dose was reduced by a mean of 56% (median, 72%) for extra images above the diaphragm and 29% (median, 36%) for images below the pubic symphysis, compared with dose levels with manual selection.