Introduction and historical background 15 patient without pausing for each section.127-129 The first results of studies assessing the possibility of diagnosing intracranial aneurysms with CTA were reported in the early 1990s.130-134 Since then, many articles have described the performance of CTA in detecting intracranial aneurysms, and the first meta-analysis was published in 2000 by White and Wardlaw.135 Studies that evaluated non- invasive angiographic techniques against DSA as the standard of reference were included in this meta- analysis and a total of sixteen studies with CTA as the non-invasive diagnostic modality comprising a total of 677 patients were analyzed. The overall sensitivity of CTA for detecting aneurysms was found to be 92% (95% Confidence Interval (CI): 89-95%), but this sensitivity decreased sharply for smaller aneurysms: sensitivity for aneurysms larger than 3mm was 96% (95% CI: 94-98%), and only 61% (95% Cl: 51-70%) for aneurysms smaller than 3mm. It should be noted that not all sixteen studies employed spiral technique (in 2 of 16 studies sequential CT scans were used). The meta-analysis of White and Wardlaw135 was updated in 2002136 with special focus on the effect of aneurysm size on diagnostic performance. Nine studies with a total of 619 individual cases were included. The sensitivity of CTA found in this meta-analysis ranged from 53% (95% CI 44-62%) for aneurysms 2mm or less in size to 95% (95% CI 92-97%) for aneurysms larger than 7mm. Another meta-analysis was published at about the same time137 including some of the same studies used by White and Wardlaw. Twenty-one studies met the inclusion criteria for this meta-analysis and 1251 patients were evaluated, resulting in a mean sensitivity of 93.3% and specificity of 87.8%. When the studies were weighted for the number of patients in each study, the sensitivity decreased slightly, to 92.7% and the specificity to 77.2%. The authors concluded that DSA should remain the standard method for detecting intracranial aneurysms, but that CTA could be used as the first diagnostic modality followed by DSA in cases where an aneurysm was not identified or clearly delineated. In the meantime, spiral or helical computed tomographic technology kept improving with faster gantry rotation, more powerful X-ray tubes and improved interpolation algorithms. However, the greatest advance came with the introduction of multi-detector-row computed tomography scanners which provided similar scan quality at speeds 3-6 times greater than single-detector-row scanners. This improvement had a profound impact on the performance of CT angiography, resulting in greater anatomic coverage, lower iodinated contrast doses, and images with higher spatial resolution138-140. Needless to say these multi-slice or multi-detector CT (MDCT) scanners were a welcome new tool, and they were immediately implemented for intracranial aneurysm imaging.141-147 Another great improvement in CT scanning was the introduction of subtraction techniques.148-155 Aneurysms near the skull base are sometimes difficult to detect because the contrast-filled vessels and aneurysms have approximately the same attenuation coefficient (Hounsfield Units) as bone; effective subtraction of bony structures solves this problem. Recent meta-analyses including studies using multi-detector-row CT scanners report significantly better diagnostic accuracies.156,157 Westerlaan et al. included 50 studies with 4,097 patients in their meta- analysis,157 and found a pooled sensitivity of 98% (95% CI: 97 – 99%) and a pooled specificity of 100% (95% CI: 97 – 100%). The authors stated that multi-detector-row scanners were used in all included studies, 34 of which using 4-row scanners. However, closer inspection reveals that at least 20 of the aforementioned 34 studies appear to have been performed using single-detector-row scanners.158-177 In five studies distal pericalossal and posterior inferior cerebellar aneurysms were missed because they
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