MINIMUM DETECTABLE DIFFERENCE OF CT ANGIOGRAPHY SCANS AT VARIOUS CARDIAC BEATS: EVALUATION VIA A CUSTOMIZED OBLIQUE V-SHAPED LINE GAUGE AND PMMA PHANTOM

The minimum detectable difference (MDD) at various beats/min (BPM) of CT angiography (CTA) was evaluated using an oblique V-shape line gauge and poly methyl methacrylate (PMMA) phantom in this study. The customized phantom with the size of 30 x 30 x 30 cm(3) was made from a 1 cm-thick PMMA. The reciprocating mechanism in the phantom was run by a step motor with an eccentric gear connected to a crank rod to provide a stable harmonic motion, simulating the cardiac beats. The MDD has a unique feature in defining the quality characteristic of CT-scanned images and provides more information than simple line pair/cm in the previous studies. The derived MDD was quantified according to various BPM, and the CTA factor combination was preset following either the conventional recommendation or the optimal one. In doing so, the performance was substantiated by the Taguchi-based signal-to-noise ratio and integrated by another index, namely, figure of merit (FOM). The MDD and corresponding eta (dB) changed from 1.70 +/- 0.01mm to 2.66 +/- 0.01mm and from 16.7dB to 14.2dB, respectively, for conventional settings; while those obtained for the optimal preset changed from 1.71 +/- 0.03mm to 2.58 +/- 0.02mm and from 12.2 dB to 16.4 dB, respectively of CTA at 0-90 BPM. The integrated FOM values for conventional or optimal cases were 1240 and 1337, respectively. The MDD proved to be a useful technique in justifying the CTA-scanned images. For compliance with previous studies, MDD results can be converted to the line pair/cm results, but it is more informative than the quantized number of line pairs.