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Sixteen–Detector Row CT of Abdomen and Pelvis: Study for Optimization of Z-Axis Modulation Technique Performed in 153 Patients¹

¹ From the Department of Radiology, Division of Abdominal Imaging and Intervention, 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.); GE Yokogawa Medical Systems, Tokyo, Japan (T.H.); and GE Medical Systems, Waukesha, Wis (T.L.T.). From the 2003 RSNA scientific assembly. Received September 17, 2003; revision requested December 2; revision received January 6, 2004; accepted February 2. Address correspondence to M.K.K. (e-mail: mannudeep_k_kalra@yahoo.com).

PURPOSE: To retrospectively determine the optimal noise indexes required to obtain diagnostically acceptable computed tomographic (CT) images of the abdomen and pelvis with z-axis modulation.

MATERIALS AND METHODS: Ninety-five patients underwent 16-section multi–detector row CT of the abdomen and pelvis with z-axis modulation at noise indexes of 10.5, 11.0, 11.5, and 12.0 HU with 10–380 mA. Subsequently, 58 patients were scanned at noise indexes of 12.5 and 15.0 HU with 75–380 mA. The weights of all subjects were recorded, and transverse and anteroposterior diameters were measured. The CT images were evaluated for abnormalities and graded for image quality in terms of noise and diagnostic acceptability by using a five-point scale. Objective noise in the liver parenchyma was measured, and the tube current was recorded at each section in all 153 patients. Statistical analyses were performed to determine the appropriate noise index and to assess the effect of patient weight and abdominal diameters on image noise and diagnostic acceptability at different noise indexes. Tube current–time products (in milliampere seconds) at various noise indexes were compared with those at CT previously performed without z-axis modulation.

RESULTS: No significant difference in subjective image noise or diagnostic acceptability was found at noise indexes of 10.5–15.0 HU (P = .14), and objective noise was significantly inferior only at a noise index of 15.0 HU (P = .009). Compared with CT scanning at a 10.5-HU noise index, CT scanning at 12.5- and 15.0-HU noise indexes yielded, respectively, 10.0% and 41.3% reductions in radiation exposure. Patient weight and abdominal diameters affected subjective image quality.

CONCLUSION: Use of a 15.0-HU noise index at 75–380 mA results in acceptable subjective image noise and diagnostic acceptability but significantly greater objective image noise at routine abdominal-pelvic CT. For greater image quality demands, a noise index of 12.5 HU results in acceptable image quality and a 19.6% reduction in radiation exposure.

© RSNA, 2004

Index terms: Abdomen, CT, 70.12111 • Computed tomography (CT), image quality • Computed tomography (CT), radiation exposure • Pelvis, CT, 813.12111

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