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Radiation from "Extra" Images Acquired with Abdominal and/or Pelvic CT: Effect of Automatic Tube Current Modulation¹

¹ From the Division of Abdominal Imaging and Intervention, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, White 270-E, 55 Fruit St, Boston, MA 02114 (M.K.K., M.M.M., R.S.K., E.F.H., S.S.); and GE Medical Systems, Waukesha, Wis (T.L.T.). Received July 25, 2003; revision requested October 2; final revision received December 18; accepted January 29, 2004. Address correspondence to M.K.K. (e-mail: mannudeep_k_kalra@yahoo.com).

PURPOSE: 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.

MATERIALS AND METHODS: 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.

RESULTS: 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).

CONCLUSION: 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.

© RSNA, 2004

Index terms: Abdomen, CT, 78.1211 • Computed tomography (CT), radiation exposure • Computed tomography (CT), technology • Computed tomography (CT), thin-section, 78.12118, 88.12118 • Pelvis, CT, 88.1211 • Special Reports

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