CT of the head by use of reduced current and kilovoltage: relationship between image quality and dose reduction.

BACKGROUND AND PURPOSE CT is a frequent examination that is performed using ionizing radiation. We sought to assess image-quality changes on CT scans of the head when the radiation dose is reduced by changing tube current and kilovoltage. METHODS A formalin-fixed cadaver was examined in conventional and helical mode by use of two CT-scanners. Surface dose was measured with standard scanning parameters, and after reduction of tube current and kilovoltage. Five experienced examiners independently evaluated subjective image quality. RESULTS In the conventional mode, the highest surface dose was 83.2 mGy (scanner 1: helical mode, 55.6 mGy), and 66.0 mGy (scanner 2: helical mode, 55.9 mGy). By changing kVp and mAs, a dose reduction of up to 75% (scanner 1), and 60% (scanner 2) was achieved. No observable differences in image quality between scans obtained with doses from 100% to 60% of standard settings were noted. Ten of 20 images obtained with the highest dose and 13 of 20 images obtained with lowest dose (19-29.4 mGy) were reliably identified by subjective quality assessment. Scans produced with a surface dose of less than 30 mGy were judged uninterpretable. CONCLUSION Standard parameters used in cranial CT are oriented toward best image quality. A dose reduction up to 40% may be possible without loss of diagnostic image quality.