Performance of Contrast Enhanced Magnetic Resonance Angiography 57 No relationship between aneurysm location and accuracy of detection was found. One observer reported 4 aneurysms at CEMRA -all very small- that were not confirmed at DSA. One of these aneurysms, which was reported by both observers, appeared to be an infundibular widening of the origin of the left posterior communicating artery (Figure 3). One other aneurysm unconfirmed by DSA was (temporarily) thrombosed at the time DSA was performed. Because the study protocol states that DSA is the standard of reference, considering this finding as a true aneurysm would be a violation of the study protocol. The other two aneurysms were seen only by Observer 1, located on the basilar tip (1.5x1.5mm) and right middle cerebral artery (2x2 mm) respectively. These false positive findings represent either artifacts or irregularities of the vessel wall. None of these four false positive aneurysms was registered by CTA. At CTA both observers scored one false positive aneurysm, both located at the PICA, but in two different patients. In one of these two cases there was a dysplastic vertebral artery, rather than a true aneurysm. This was not considered as the bleeding source, but was reported by Observer 2 as an aneurysm. The other false-positive PICA aneurysm reported at CTA was not seen on CEMRA or DSA. Figure 3 a. Lateral projection of a volume rendered reconstruction of the CEMRA scan of a patient presenting with an SAH. Both observers described a small left posterior communicating artery aneurysm. b. Lateral projection of the left internal carotid artery made by DSA of the same patient as in figure 3a. It shows a posterior communicating artery with an infundibular widening. In our study population 50.8% (33/65) of the aneurysms were smaller than 5 mm and 18.5% (12/65) were smaller than 3mm. The sensitivities for detection of these small aneurysms are given in Table 6. Table 6. Sensitivity of CTA and CEMRA for the detection of small aneurysms All Obs1 90.8 84.8 75 CTA Obs2 92.3 84.8 75 Obs1 96.9 96.9 91.7 CEMRA Obs2 93.8 90.9 83.3
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