Chae et al. and Brinjikji et al. [8]). The meta-analysis of Brinjikji et al. (8) reported that incomplete occlusion rates for ruptured and unruptured aneurysms were similar (8). However, the ATENA study investigating a large series of unruptured intracranial aneurysm less than 15 mm, which was treated by the endovascular approach, showed a high incomplete occlusion rate (19%) and relatively low procedural rupture rate (3%), though there was no specification about the incomplete occlusion rates according to the size of aneurysms (7). Because our incompletely occluded aneurysms were unruptured, we might focus on the idea that aneurysm perforation during endovascular procedures should not occur and it might be the cause of not achieving a complete occlusion. Interventionist’s vigorous tries for complete occlusion may actually increase the chance of procedure-related rupture. Consequently, a high rate of incomplete occlusion in our series may be relevant with regards to the absence of procedural rupture. Of two thromboembolic complications, one (50%, 1 of 2) occurred in a patient with wide-neck aneurysm that was treated a remodeling technique using stent assistance. Balloon or stent assistance for the treatment of wide- necked and complex lesions has become a routine adjunct to endovascular treatment of intracranial aneurysms in some groups (10). However, interventionists should be reminded that additional endovascular manipulations during the insertion of a balloon or stent may increase procedure-related complications. The rate of aneurysm recurrence after coiling has been provided in various single and multicenter studies and has ranged from 5% to 33% (11-15). According to few reports investigating endovascular results in ruptured and unruptured very small aneurysms, the recurrence rate was 2-5% (5, 7, 8). In our series, none of the 27 aneurysms that were followed showed any recurrence. Nguyen et al. (16) reported that ruptured aneurysms and larger aneurysms were significantly associated with recanaliza- tion. Our experience and the previous few reports corrobo- rate this. Many paraclinoid ICA aneurysms were noted in our series, which were so-called side wall aneurysms, unlike other aneurysms of the bifurcation site. This type of aneurysms may be less influenced by hemodynamic forces including turbulent flow or direct jet pushing on the coiled mass, and as a result, may lead to the absence of recanal- ization. An initial incomplete occlusion or loose packing of the aneurysm was significantly associated with recanalization (16). For aneurysms that are not tightly packed, coiled masses are exposed to the ‘water-hammer effect,’ which is responsible for aneurysmal regrowth. Therefore, achieving 540 complete or near complete occlusion is the key to prevent- ing recurrent hemorrhage and regrowth of the aneurysm when coiling aneurysms. However, it is quite interesting that our five incompletely occluded aneurysms that were followed by MRA showed focal high signal intensities within the coiled aneurysmal sac, which represented arterial flow that disappeared on serial follow-up. The results suggest that even if occlusion is incomplete on immediate angiography after coil embolization, adequate occlusion may eventually be achieved. Though it is difficult to know what may progress to an adequate occlusion in these cases. We could suppose that the inflow zone can be more easily occluded in very small aneurysms because of the absolute small size of the inflow zone compared to larger aneurysms. However, our series is too small and has too short of a follow-up period; therefore, further studies with larger trials and longer follow-ups should be accumu- lated to ensure the course of incompletely occluded, unruptured, very small aneurysms. Our mean packing density was 34%. The incidence of recanalization was significantly lower in aneurysms with coil packing densities > 25% (12). The recurrence rate of endovascular treatment for very small aneurysms remains to be determined through additional studies. Very small aneurysms were common at our institution. They comprised 9% of all coiled aneurysms and consisted of up to 30% of surgically treated aneurysms. Moreover, incidental very small aneurysms are more frequently detected with the development of neuroimaging and the increase of availability. The results of the International Study of Unruptured Intracranial Aneurysms (ISUIA) suggested that small unruptured aneurysms have a benign natural course and a very low risk of bleeding when left untreated (17). There is no strict standard as to whether to treat or how to treat unruptured very small aneurysms in our institute. Each patient is evaluated in a joint meeting of neurosurgeons and interventional neuroradiologists. The decision to treat very small aneurysm is made on a case- by-case basis taking into consideration the following facts: 1) previous history of subarachnoid hemorrhage from another location, 2) presence of a family history of subarachnoid aneurysmal hemorrhage, 3) morphological factors of an aneurysm (daughter sac, irregular shape), and the potential modifications of the aneurysmal sac with time (increase in size, modification of the shape), and 4) patient preference. Once the decision to treat the aneurysm was made, endovascular treatment is preferred to surgery in the following cases: 1) a location with poor surgical accessi- bility, 2) clinical factors (age, medical condition), and 3) patient preference. From the view point of location only, it may be one of the most controversial issues (unruptured, Korean J Radiol 11(5), Sep/Oct 2010