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Radiation Dose Reduction without Degradation of Low-Contrast Detectability at Abdominal Multisection CT with a Low–Tube Voltage Technique: Phantom Study¹

¹ From the Department of Radiological Sciences, School of Health Sciences (Y.F., M.S.), and Department of Diagnostic Radiology, Graduate School of Medical Sciences (K.A., Y.N., S.S., S.M., Y.Y.), Kumamoto University, 4.24.1 Kuhonji, Kumamoto 862-0976, Japan; Department of Radiology, Kumamoto University Hospital, Kumamoto, Japan (K.K., N.N.); and Philips Medical Systems, Minato-ku, Tokyo, Japan (N.S.). Received September 23, 2004; revision requested November 30; revision received January 23, 2005; accepted February 23. Address correspondence to Y.F.

View larger version (149K): [in a new window]   Figure 1. Phantom (small arrow) and body annulus (large arrow). The phantom contained several cylindrical low-contrast objects of different sizes arranged in a circle; the background was of uniform density. The annulus, which consisted of acrylic, was set around the phantom to mimic attenuation during body imaging. The diameter and length along the z-axis of the acrylic annulus were 32.0 cm and 10.0 cm, respectively.

View larger version (158K): [in a new window]   Figure 2. Image of the phantom and body annulus. The extracted image includes a low-contrast object and a rectangular area of 10 x 30 mm. We used 9.0- and 15.0-mm-diameter low-contrast objects with a contrast difference of 1.0% to evaluate low-contrast detectability.

View larger version (28K): [in a new window]   Figure 3. Graph illustrates the relationship between the CNR and the radiation dose obtained in the center of the phantom at both 90 and 120 kV. The Pearson correlation coefficient (r) and the corresponding P values were r = 0.99 and P < .001 at 90 kV and r = 0.94 and P < .001 at 120 kV. At identical radiation doses, the CNR at 90 kV was 1.29 times higher than the CNR at 120 kV. At identical CNR, the radiation dose at 90 kV was 0.71 times higher than that at 120 kV.

View larger version (51K): [in a new window]   Figure 4. Graph shows averaged receiver operating characteristic curves of five radiologists for low-contrast detectability. Data are from readings with 120 kV and 300 mAs and 90 kV and 250–560 mAs. At 90 kV, the mean Az value—0.744 at 250 mAs, 0.804 at 300 mAs, 0.837 at 350 mAs, and 0.865 at 400 mAs—was significantly lower than it was at 120 kV and 300 mAs. The difference between the mean Az value at 120 kV and 300 mAs and the values obtained at 90 kV at any of the other tube current–time product settings investigated was not statistically significant.