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Does a Combination of Dose Modulation with Fast Gantry Rotation Time Limit CT Image Quality?

¹ All authors: Department of Diagnostic Radiology, Yale University School of Medicine, 333 Cedar St., PO Box 208042, New Haven, CT 06520-8042.

OBJECTIVE. The objective of our study was to determine the degree to which CT tube current saturates (tube current reaching its maximal capacity) if dose modulation and fast gantry rotation speeds are used when imaging the abdomen and pelvis and to determine whether saturated tube current affects image quality.

MATERIALS AND METHODS. We evaluated the CT scans of patients who underwent imaging of the abdomen and pelvis using dose modulation and the fastest gantry rotation time available on two different CT scanners. Ninety-five patients were scanned with a 64-MDCT scanner (noise index, 11; tube rotation speed, 0.5 second) with a maximal x-ray tube capacity of 695 mA. Ninety-four patients were scanned with a 16-MDCT scanner (noise index, 11.6; tube rotation speed, 0.6 second), which has a maximal x-ray tube capacity of 440 mA. The total number of images per examination, total number of images obtained at saturated tube current, image noise (SD of fluid attenuation), and patient width were recorded. A qualitative evaluation of image quality, with images obtained below and at the maximal tube current grouped separately, was performed by two independent radiologists who were not blinded to the type of scanner used using a scale of from 1 (best) to 4 (worst). Statistical analyses included the Kruskal-Wallis one-way analysis of ranks test for nonparametric ordinal data, the unpaired two-tailed Student's t test, and the chi-square test.

RESULTS. For images obtained with the stronger x-ray tube (maximum tube current = 695 mA), the average number of axial images per examination was 87.6. In 34 of 95 (36%) patients, at least one image was acquired with the tube current saturated. The average image noise was 12.4 H. Subjective evaluation yielded an average image quality score of 1.2 for images below saturated tube current and 1.2 for images at saturated tube current. For images obtained with the weaker x-ray tube (maximum tube current = 440 mA), the average number of axial images per examination was 88.9. In 84 of 94 (89%) patients, at least one image was acquired with the tube current saturated. The average image noise was 16.8 H. Qualitative evaluation showed average image quality scores of 1.3 and 1.8 for images below and at the saturated tube current, respectively. The percentage of images acquired at the saturated tube current was significantly greater for the weaker x-ray tube than the stronger x-ray tube (p < 0.0001), and qualitative analysis of images obtained at saturated tube current showed significantly decreased quality for the weaker x-ray tube when compared with images obtained with nonsaturated current (p = 0.001).

CONCLUSION. On the MDCT scanners investigated, when dose modulation is combined with fast tube rotation times, tube current saturation occurs with weaker x-ray tubes resulting in deterioration of image quality.

Keywords: abdominal imaging • CT technique • dose modulation • gantry rotation time • MDCT • tube current

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