Performance of Contrast Enhanced Magnetic Resonance Angiography 71 Table 3. Results of two experienced neurointerventionists on the feasibility of endovascular treatment of intracerebral aneurysms on the basis of the DSA’s. Coiling feasible obs 2 no yes Coiling feasible no 16 3 19 Obs 1 yes 16 30 46 Total 32 33 65 Size measurement is very consistent and robust for both modalities with an Intraclass Correlation Coefficient between two observers of 0.91 and 0.93 for CTA and CEMRA respectively. Both CTA and CEMRA appear to overestimate the size of the aneurysms by nearly 20% compared to DSA. One of the possible explanations for this discrepancy regarding MRA could be that this technique also generates a high signal in intraluminal thrombus, whereas DSA only displays the contrast-filled vascular lumen.30 Another reason for this discrepancy can be that with CTA and MRA the window/ level settings can be adjusted by the observer. These levels are often set in such a fashion that small aneurysms will be detected as well, thereby probably including more voxels with partial volume in the detected aneurysms. Size itself is not an important parameter for the assessment of feasibility of endovascular treatment, but this finding indicates a limitation in the accuracy of the non-invasive techniques. It could be of importance for very small aneurysms, because we consider aneurysms of 2mm or smaller as not suitable for endovascular treatment. In these very small aneurysms oversizing is not a significant factor however, because it is always proportional to the size of the aneurysm (ɴ in the equation DSA = ɲ + ɴ x (CTA or MRA)), and not an absolute overestimation in size (ɲ). For selecting the right size of the coils for a coiling procedure measurement of the diameter of the aneurysm is necessary. This is normally done on the DSA images made at the beginning of the coiling procedure. Although aneurysm size,31 size relative to vessel diameter32 or growth of the aneurysm33 might be predictors of the risk of rupture in as yet unruptured aneurysms, this was not the focus of this study. These aspects will play a role in screening for aneurysms and in follow-up of treated aneurysms. In vitro studies have shown that volumetric measurements with CTA and MRA are not very accurate.18,34 Conclusion There is no significant difference in the performance of CTA and CEMRA in guiding the decision whether or not to treat an intracranial aneurysm by endovascular coiling. This is an important function of non- invasive imaging in patients presenting with a SAH shown to harbor an intracranial aneurysm. If this performance could be improved, the need for additional diagnostic DSA studies would decrease. CTA and CEMRA both overestimate the size of intracranial aneurysms.
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