Technical examinations for improvement of the detectable limit for abdominal lesions. Low-contrast resolution and imaging dose

Since to know the low-contrast resolution of an apparatus is essential for improving the detectable limit for abdominal lesions in CT imaging, the objective detectable limit values were defined from the quality evaluation of the apparatus and from clinical purposes, to obtain standard values for optimal imaging dose determination. The apparatus used was GE Hispeed Advantage/RP. Absorbed dose was measured with Radcal Dosimeter 9015 and the phantom for evaluation of high speed CT quality (Kyoto Kagaku Co.); low contrast resolution and image noise, with water phantom, the first and second recommendation phantoms, Catphan 966 (CTP263) phantom (The Phantom Lab.) and the Kyoto phantom above; and slice thickness, with 0.5 mm Pb sphere phantom and JIS phantom. Followings were examined: the resolutions with absorbed doses and sizes of imaging object, the resolution for body axis with slice thickness, the objective detectable limit values and the standard values. The resolution was defined to be 8 mmφ and 0.5% objective value; the image noise, <1.0%; absorbed dose, <10 mGy; and slice thickness, <10 mm. For helical scanning with tube voltage 120 kV and imaging dose 250 mAs, 10, 7 and 10 mm/r values were obtained for simple, arterial and equilibrium phases, respectively, with 7 mm slice thickness and 7 mm reconstruction interval as for author's apparatus. (K.H.)