High-resolution CT of the chest: radiation dose

The purpose of this study was to compare the skin radiation dose to the chest produced by high-resolution CT (HRCT) with the radiation dose produced by conventional CT. Previous studies have reported that radiation doses with HRCT are equal to or higher than those with conventional CT. These results, however, were based on the assumption that in HRCT, contiguous sections were scanned without the intersection gaps of 10 or 20 mm that are used clinically.

We used radiotherapy verification film to measure the skin radiation dose in 56 patients who had chest CT scans. Twenty-two had 1.5-mm collimation HRCT scans at 10-mm intervals, 15 had 1.5-mm collimation HRCT scans at 20-mm intervals, and 19 had 10-mm collimation conventional CT scans at 10-mm intervals. Scan parameters were identical in all cases: 120 kVp, 200 mA, 2 sec. Step wedges were used to generate calibration films with identical beam quality on the CT scanner, and exposure was measured with an ionization chamber. Calibration films and patients' radiotherapy verification films were digitized, and the radiation dose was calculated.

Mean skin radiation dose was 4.4 (standard error [SE], 0.2) mGy for 1.5-mm HRCT scans at 10-mm intervals, 2.1 (SE, 0.1) mGy at 20-mm intervals, and 36.3 (SE, 0.9) mGy for conventional 10-mm scans at 10-mm intervals.

HRCT scanning at 10- and 20-mm intervals produced 12% and 6%, respectively, of the radiation dose associated with conventional CT. This is considerably less radiation than suggested in earlier studies. Combining HRCT scans at 20-mm intervals with low-dose scan (20 mA, 2-sec scans) would result in an average skin dose comparable with the dose administered with chest radiography.