Friday 20 October 2017

Abstract

Purpose: To assess whether image quality and adequacy for operative planning are maintained using low contrast dose and automated tube potential selection (ATPS) for CT aortography (CTA) on a dualsource, multi-detector CT scanner. Methods and Materials: 113 adult patients who underwent CT angiography of the thoraco-abdominal aorta with ATPS on our Siemens SOMATOM Force dual-source, 192-slice multi-detector CT scanner, between August 2015 and April 2016, were included in this retrospective study. Patients were divided into two groups. In 46 patients (Group 1), a contrast dose less than 50 ml of 350 mg iodine/ml contrast (or with equivalent iodine mass of 17.5 g) was used. The remaining 76 patients (Group 2) received a greater contrast dose. CT attenuation was measured at 8 regions of interest (ROIs) along the thoraco-abdominal aorta and common iliac arteries. Signal-to-noise (SNR) and contrast-to-noise (CNR) ratios were calculated for the purpose of quantitative assessment. Quality thresholds were defined as CT attenuation>250HU, SNR>5 and CNR>8. 1 ;2;3 Qualitative assessment was performed by a senior interventional radiologist and a senior vascular surgeon, who graded: definition of aortic outline, aortic enhancement homogeneity, subjective noise, presence of beam hardening artefact, level of renal arterial branch visualization and overall adequacy of images for operative planning. Results: In Group 1, contrast volumes used ranged from 20 to 50 ml (mean iodine mass: 14.5 g = 41 ml), while in Group 2, contrast volumes used ranged from 55 to 98 ml (mean iodine mass: 25 g = 71 ml) (p < 0.05). Mean selected tube potential was 89.1 kV in Group 1 and 90.9 kV in Group 2. Qualitative assessment between the groups was similar, with no significant difference in adequacy of image quality for operative planning (p = 0.24). Adequate overall quality was achieved in 44/46 patients in Group 1 and 61/67 patients in Group 2. Subjective noise, however, was increased in Group 1 (mean grade: 4.6/5 vs. 4.9/5; p = 0.01). CT attenuation, SNR and CNR were comparatively reduced in Group 1, mainly at proximal ROIs (ROI1-ROI4). The proportion of vessel segments not meeting the attenuation quality threshold of >250HU was significantly increased in Group 1 only at ROI1 (17.4% vs. 4.5%; p = 0.02). No difference in the proportion of vessel segments not meeting SNR or CNR quality thresholds was demonstrated at any ROI (p < 0.05). Conclusion: Contrast volume can be reduced to at least 41 ml to perform CTA when using ATPS whilst maintaining clinically satisfactory image quality, and levels of adequate SNR and CNR, compared to CTA using higher contrast dose.

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