The median effective dose from cardiac CT with standard filtered back-projection was comparable with the effective dose from invasive coronary angiography, even with application of the most contemporary ICRP tissue weightings and use of cardiac specific volumes. Cardiac CT scanning incorporating iterative reconstruction resulted in a significant reduction in the effective dose.

Objective: To assess submillimetre coronary computed tomographic angiography (CTA) in comparison with invasive quantitative coronary angiography as the gold standard and to examine the effect of significant coronary artery calcification (CAC), which is known to impede lumen visualisation, on the accuracy of the examination. Methods: After invasive coronary angiography, 58 patients underwent coronary imaging with a GE Lightspeed 16 computed tomography (CT) system. CAC was quantified after an ECG triggered acquisition with a low tube current. Coronary CTA was performed with retrospective ECG gating and a 16 6 0.63 mm collimation and was reconstructed with an effective 65-250 ms temporal resolution. All 13 major coronary artery segments were evaluated for the presence of > 50% stenosis, and compared with the gold standard. Results: One patient moved and could not be evaluated. All segments (except occluded segments) were evaluated for 57 patients. Overall the accuracy of coronary CTA for detection of > 50% stenosis was: sensitivity 83%, specificity 97%, positive predictive value 80%, and negative predictive value 97%. The number of diseased coronary arteries was correctly diagnosed in 34 of 38 (89%) patients overall. Altogether 21 of 57 (37%) patients had a CAC score > 400, which was predefined as representing significant CAC. Excluding these patients from the analysis improved the accuracy of coronary CTA to a sensitivity of 89%, specificity 98%, positive predictive value 79%, and negative predictive value 99%. Conclusions: Non-invasive coronary angiography with submillimetre CT is reliable and accurate. It appears that a subgroup of patients may be selected based on CAC score in whom the investigation has even higher accuracy. Coronary CTA has reached the stage where it should be considered for a clinical role. Further research is required to define this role.T he emergence of ECG gated multislice computed tomography (CT) has stimulated great interest among cardiologists. This is primarily because this new technology may provide a clinically useful method for performing non-invasive coronary angiography. Four slice CT has been extensively evaluated in this context and is lacking in both reliability of image quality and accuracy of results. [1][2][3][4][5] However, these studies have identified certain patient related factors that appear to affect the provision of a clear image of the contrast enhanced coronary artery lumen. Specifically, high heart rates induce motion artefact and excessive coronary artery calcification (CAC) impedes accurate lumen visualisation and may produce blooming artefacts. 6 Recently four slice CT has been superseded by 16 slice CT. For cardiac applications this technology provides not only a submillimetre collimation but also improved temporal resolution. Preliminary studies have been performed with this type of technology.7-9 Although the results of these studies have been favourably received, improvements are still needed before clinical use. 10

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