26 Chapter 1 3T422,430,435,441 even though increased coil artefacts at 3T are reported.442 Other investigators did not find any difference between 1.5T and 3T.420,443 Anzalone et al found better results with TOF-MRA at 3T than at 1.5T, but both were still inferior to CEMRA at 1.5T.430 We performed a study evaluating the accuracy of CEMRA at 1.5T in the follow-up of coiled aneurysms. This report is described in Chapter 6: Follow-up of coiled intracranial aneurysms with Contrast Enhanced MRA with DSA as standard of reference. Long term follow-up The optimal strategy for monitoring the stability of ruptured aneurysms following coil embolization in terms of duration and time intervals is still unclear.444,445 As mentioned above, the annual rebleed rate of ruptured aneurysms after the first month is low, between 0.1 and 0.3%,323,390,391 and incomplete occlusion is a strong predictor of late rehemorrhage.392,393 Recanalization occurs in approximately 20% of coiled aneurysms and half of these aneurysms are retreated.445-449 A strategy in which patients with incomplete occlusion at six months are selected for retreatment or longer follow-up leads to a low incidence of late adverse events.394 The question is how long and in which frequency surveillance should continue after an aneurysm has been adequately occluded at six months. In order to answer this question, two factors need to be assessed: the risk of reopening of adequately occluded aneurysms at six months, and the risk of rebleed - from the treated aneurysm or from another aneurysm - in patients that have undergone adequate coiling at six months. In a cohort of 283 patients with aneurysms that were adequately coiled at six months, Schaafsma et al found three SAH’s occurring in the following eight years, which results in a cumulative incidence of recurrent SAH of 0.4%. This was lower than, but did not differ significantly from, the incidence they found in a cohort of 776 clipped aneurysms: 17 SAH’s recurred in eight years leading to a cumulative incidence of recurrent SAH of 2.6%.450 In the coiled group the hemorrhage probably occurred from rupture of the treated aneurysm. In the clipped group 13 of 17 hemorrhages were probably from an associated aneurysm in a different location. The incidence of recurrent SAH found was higher than that found by the CARAT investigators.391 The annual re-rupture rate of coiled aneurysms after more than one year in the CARAT study was 0.11%, but they did not focus on aneurysms that were adequately occluded at first follow- up.391 Other studies found slightly higher rates of SAH after coiling, but again they did not focus on adequately occluded aneurysms at early follow-up, and the timing of follow-up varied widely.323,390,451,452 In a study by Sprengers et al, including a small group of 104 patients with 111 aneurysms that were adequately occluded at six months, the risk of aneurysm recanalization was found to be approximately 3.6% in the first six years.453 This low incidence of late recurrences is in concordance with a previous observational study by Sluzewski et al.449 Other studies found more first time recurrences at longer follow-up periods,446-448 however, these studies used a wide variety of follow-up intervals and often it was not clear when the recurrence had actually taken place. A large multicenter study (LOTUS study) was set up in order to further investigate this risk of late recanalization: 400 patients with 439 aneurysms that were adequately coiled at six months were imaged
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