Evidence is mixed but there is significant evidence supporting CTA to screen for intracerebal aneurysm. CTA is performed with multi-slice helical CT scanners, capable of imaging blood vessels using intravenous contrast media. Images are taken during the arterial phase of blood flow, and then processed by a computer to produce a three-dimensional (3D) image of the arterial tree. Imaging can be carried out immediately after an unenhanced CT scan. A 2003 meta-analysis examined 21 studies (n=1251) comparing CTA with Digital Substraction Angiography (DSA) for the diagnosis of ICAs. Sensitivity and specificity, both weighted and unweighted for sample size, were calculated. Unweighted results revealed a sensitivity of 93.3% and a specificity of 87.8% for CTA, whereas weighted results were 92.7% and 77.2%, respectively, using DSA as the reference standard. Subsequent studies produced similar results. Wermer et al. constructed a decision model to examine the use of CTA to screen for new ICAs in patients who had already suffered one SAH. The strategies of "screening" and "no screening" were compared, taking into account risks of ICA recurrence, complications of CTA, and retreatment, as reported in the literature. The findings were not supportive of screening, leading the authors to conclude that at the present time, screening for new aneurysms after SAH cannot be recommended. They noted that screening may prevent new episodes of SAH but with too high a cost in terms of complications from preventive treatment The American College of Radiology (ACR) Appropriateness Criteria for cerebrovascular disease state, "to date, individuals with a history of aneurysm or SAH in a first-degree relative have been considered candidates for screening. Nevertheless, significant gaps in knowledge of the natural history (and thus risk of rupture) of intracranial aneurysms remain. Hence, while screening with MRA or CTA may be appropriate in patients with a positive family history, its impact on patient outcome is questionable." (ACR, 2006) Wermer MJ, Buskens E, van der Schaaf IC, et al. Yield of screening for new aneurysms after treatment for subarachnoid hemorrhage. Neurology. 2004;62(3):369-375. White PM, Wardlaw JM, Easton V. Can non-invasive imaging accurately depict intracranial aneurysms? A systematic review. Radiology. 2000;217(2):361-70. White PM, Wardlaw JM. Unruptured intracranial aneurysms. J Neuroradiol. 2003;30(5):336-50.