MRA artefacts with Nexus coils

Abstract

Sir, We read with interest the article ‘MR angiographic evaluation is limited in intracranial aneurysms embolized with Nexus coils’ by Kang HS et al. [1]. We are pleased to see that the authors have been able to substantiate our own observation of MR susceptibility artefacts associated with all Dendron, MTI and ev3 coils up to the Nexus generation. We presented a study of magnetic resonance angiography (MRA) artefacts following endovascular coiling at the World Federation of Interventional and Therapeutic Neuroradiology meeting in Venice in 2005 [2] and at the British Society of Neuroradiologists meeting in Edinburgh in 2005 showing that artefact associated with platinum:iridium coils (including Nexus) from ev3 was substantially greater than that seen with platinum:tungsten alloy coils at 1.5T and 3.0T. Our in vitro experiments at 1.5T and 3.0T with ev3 single coils (with and without the nitinol core wire) and coiled aneurysm models further agree with the authors that the susceptibility artefact is caused by the iridium component. We consider that all coiled aneurysms should have a digital subtraction angiogram (DSA) as well as an MRA for comparison at some stage, especially in centres where new devices are being assessed. As the authors correctly point out, even coils usually associated with the lowest susceptibility artefact (Micrus in our experience) can sometimes produce significant artefact. In centres that perform routine DSA follow-up after coiling, the MRA artefact is not a significant issue. Our standard imaging protocol following endovascular coiling includes a digital subtraction angiogram in addition to an MRA at 6 months post coiling. If there is good correlation between the two studies and no significant artefact, which is uncommon with Nexus coils, further follow-up is performed with MRA alone. Otherwise DSA is continued. We believe that Nexus coils offer many advantages including greater thrombogenicity, bioactivity, compaction resistance, softness and coil shape. They continue to be the most widely used coils in our department with 172 aneurysms in 162 patients treated purely with Nexus coils. We do not favour mixing coils from different manufacturers. Following our identification of the increased magnetic susceptibilty artefact associated with platinum:iridium coils, ev3 has produced a new coil series, Axium coils which use platinum:tungsten alloy. Our initial experience with these coils (submitted for publication) suggests that they produce no more artefact than other coil types. Axium coils are currently only available as bare coils, with further filament types due for launch in the near future. Until the filamentcoated Axium coils become available, we continue to use Nexus coils.