Low contrast- and low radiation dose protocol for cardiac CT of thin adults at 256-row CT: usefulness of low tube voltage scans and the hybrid iterative reconstruction algorithm

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To evaluate the effect on image quality of a low contrast and radiation dose protocol for cardiac computed tomography (CT) using a low tube voltage, the hybrid-iterative reconstruction algorithm, and a 256-row CT scanner. Before clinical study, we performed phantom experiments to evaluate the hybrid iterative reconstruction technique. We randomly assigned 68 patients undergoing cardiac CT to one of two protocols; 33 were scanned with our conventional 120 kVp protocol, the contrast material (370 mgI/kg body weight) was delivered over 15 s. The other 35 patients underwent scanning at a tube voltage of 80 kVp; the contrast dose, reduced by 50 % (185 mgI/kg), was delivered at the same fractional dose (24.7 mgI/kg/s). The 80 kVp images were post-processed with the 60 % hybrid-iterative reconstruction technique. We evaluated the effective dose (ED), image noise, mean attenuation, and contrast-to-noise ratio (CNR) of each protocol. The hybrid-iterative reconstruction technique offers almost same spatial resolution and noise-power-spectrum curve as compared with filtered back projection reconstruction. There were no decrease in spatial resolution and no shift of spatial frequency in noise power spectrum. The average ED was 74 % lower with the 80- than the 120 kVp protocol (1.4 vs 5.4 mSv). Dunnett’s test showed that there were no significant differences in the image noise, mean attenuation, and CNR between hybrid-iterative-reconstructed 80 kVp scans and 120 kVp scans (28.6 ± 6.5 vs 25.3 ± 4.5, p = 0.18; 475.0 HU ± 87.0 vs 445.3 HU ± 67.7, p = 0.20; 17.1 HU ± 3.5 vs 17.8 HU ± 3.1, p = 0.53). The low kVp scan and hybrid-iterative reconstruction algorithm can dramatically decrease the radiation dose and contrast dose with adequate image quality at cardiac CT of thin adults using a 256-row CT scanner.