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Objective: The minimum detectable difference (MDD) of computed tomography (CT) scanned images was quantified and optimized according to an indigenous hepatic phantom, line group gauge and Taguchi ${\text{L}}_{18}$ optimization analysis in this work. Methods: Optimal combinations of CT scan factors in every group with the level organization were judged using the Taguchi analysis, in which every factor was organized into only 18 groups, creating evaluated outcomes with the same confidence as if every factor was analyzed independently. The five practical factors of the CT scan were (1) kVp, (2) mAs, (3) pitch increment, (4) field of view (FOV) and (5) rotation time for one loop of CT scan. Insofar as each factor had two or three levels, the total number of 162 (i.e., $2\times 3\times 3\times 3\times 3$) combinations was considered. Results: The optimal setting was 120kVp, 300mAs, 0.641 pitch, 320mm FOV and 1.0s of rotation time of CT scan. The minimal MDD was 2.65mm under 0.39mm of the slit depth from the revised Student’s $t$-test with a 95% confidence level. In contrast, the MDD of conventional and the best one (no. 7) among all original 18 groups were 3.27mm and 2.93mm for 0.43mm and 0.41mm slit depths, respectively. Conclusion: The Taguchi analysis was found very lucrative for the design of imaging analysis in practical diagnosis. The indigenous line group gauge and hepatic phantom also proved to be suitable in simulating the human body in real hepatic carcinoma examination.

The CT scan protocol optimization for peripheral arterial occlusive disease (PAOD) syndrome was performed by organizing seven CT factors [kVp, mAs, pitch, field of view (FOV) (mm), time of rotation (s), slice thickness (mm), and matrix size] into Taguchi unique $L_{18}$ orthogonal array. The minimum detectable difference (MDD) in the optimizing process was quantified by adopting a customized line group gauge. Besides, three qualified experts in radiology examined by the double-blind criterion the gauge scanned images and ranked them, yielding the optimal setting of CT scan protocols. The latter setting for PAOD included the kVp of 100, mAs of 240, pitch of 0.513, FOV of 320mm, rotation time of 0.75s, slice thickness of 4.0, and matrix size of $768\times 768$. The ANOVA and revised Student’s $t$-test verified the smallest MDD as 1.43mm at a 0.45-mm gauge depth. The ranking process, which makes it possible to magnify and emphasize the imaging correlation among groups, was found to be preferable to grading in the optimization process. The comparative analysis of various MDDs obtained from different medical facilities and literary sources was performed, which revealed that the cardiac X-ray provided the finest spatial resolution according to the quantified MDD. Meanwhile, the CT scan protocol for PAOD adopted in this study had finer MDD than that for the abdomen due to comparatively low kVp or/and mAs.