MR Angiography at 3T versus Digital Subtraction Angiography in the Follow-up of Intracranial Aneurysms Treated with Detachable Coils

Patients

Between November 2003 and July 2004, 20 consecutive patients (nine men, 11 women; age range, 18–74 years; mean age, 49 years) with 21 aneurysms (20 ruptured, one unruptured) underwent DSA and MR imaging on the same day, at a mean follow-up period of 6 months (range, 4–14 months) after endovascular treatment with detachable coils (Guglielmi detachable coils; Boston Scientific, Freemont, CA). The local ethics committee approved the study, and written informed consent was obtained from all patients. The locations of the aneurysms were as follows: anterior communicating artery (n=6), posterior communicating artery (n=5), middle cerebral artery (n=2), internal carotid artery (n=1), basilar tip (n=3), posterior inferior cerebellar artery (n=2), superior cerebellar artery (n=1), and anterior inferior cerebellar artery (n=1). The size of the aneurysms was 5 mm or less in six cases, 6–10 mm in 11, and more than 10 mm in four.

MR Imaging Techniques

All MR examinations were performed with a 3T system (Intera R10; Philips Medical Systems, Best, the Netherlands) by using the sensitivity encoding (SENSE) phased-array head coil (MRI Devices; Gainesville, FL). All patients underwent the same MR imaging protocol that included axial T2-weighted fast spin-echo, nonenhanced and enhanced axial T1-weighted spin-echo, and MOTSA 3D TOF MRA sequences. Imaging parameters for the T1-weighted spin-echo sequence were 570/12 (TR/TE), 256 × 256 matrix (reconstructed to 512 × 512), 180-mm FOV, 90% rectangular FOV, 3-mm thick sections with a 0.3-mm gap. Parameters for the T2-weighted fast spin-echo sequence were 3394/80, 400 × 400 matrix (reconstructed to 512 × 512), 230-mm FOV, 70% rectangular FOV, 5-mm thick sections with a 0.5-mm gap. The volume of the MOTSA 3D TOF MRA was localized on a sagittal 2D phase-contrast scout image. A presaturation band was applied above the imaging volume to saturate incoming venous blood. For the MOTSA 3D TOF MRA sequence, the parameters were as follows: 3D fast field echo T1-weighted sequence, 21/4, flip angle 20°, 512 × 512 matrix (reconstructed to 1024 × 1024), 200-mm FOV, 85% rectangular FOV, 1.0-mm-thick sections interpolated to 0.5 mm, 160 sections acquired in eight chunks. The measured voxel size of the MOTSA 3D TOF MRA image was 0.39 × 0.61 × 1 mm, and the reconstructed voxel size was 0.2 × 0.2 × 0.5 mm. Imaging time of the high-spatial-resolution MOTSA 3D TOF sequence was reduced by parallel imaging. For parallel imaging, we used SENSE, a technique that uses multiple coil elements to encode spatial information in addition to traditional gradient encoding. Less gradient encodings are required, resulting in shorter imaging times (27). By using the SENSE head coil with a SENSE reduction factor of 1.5, we could limit the acquisition time to 7 minutes 14 seconds. Tilted optimized nonsaturating excitation was used in this protocol to optimize excitation profiles (3).

After the intravenous administration of 0.2 mL of gadopentetate dimeglumine (Magnevist; Schering AG, Berlin, Germany) per kilogram of bodyweight, the MOTSA 3D TOF MRA and axial T1-weighted spin-echo sequences were repeated. The T1- and T2-weighted images were obtained to evaluate the degree to which coils produce artifacts at 3T. Also, the T1-weighted spin-echo images were obtained to detect T1 shortening due to thrombus that may occasionally be interpreted as residual flow within the aneurysm on the MRA image. Total examination time for MR imaging was 45 minutes.

DSA Technique

DSA was performed after the MR examination on the same day with a single-plane angiographic unit (Integris Allura Neuro; Philips Medical Systems). Six to eight milliliters of nonionic contrast material (iodixanol, Visipaque 320 mgI/mL; Amersham Health, Cork, Ireland) was injected into the internal carotid or vertebral artery with a power injector at 4–6 mL/s. Three views were acquired in each patient, including the view that showed the aneurysm best at the time of embolization. Three-dimensional angiography was performed before endovascular treatment but was not performed for follow-up. Complications of angiography were recorded.

Image Analysis

Findings at follow-up DSA were classified by two neuroradiologists (C.B.L.M.M., M.S.) in consensus, in conjunction with the pretreatment DSA and the DSA performed during the coiling procedure. A remnant was defined as residual aneurysm filling (including a neck remnant, dog ear, or residual filling in the aneurysm sac) present on the DSA image immediately after coil placement. It was also called a remnant at the follow-up study if it did not increase in size. An aneurysm recurrence was defined as filling of the aneurysm at the follow-up study that was not present on the DSA image obtained immediately after coil placement or as an enlargement of a remnant.

Two observers (M.E.S., W.J.J.R.), blinded to follow-up DSA images, assessed MR images independently, together with the pretreatment DSA images and the DSA images obtained during the coiling procedure. Source images and multiple maximum intensity projections of MRA images were both used. MR images were evaluated for artifacts, presence and size of aneurysm remnants and recurrences, patency of parent and branch vessels, and added value of contrast material enhancement. The contrast-enhanced MRA images were scored separately from the nonenhanced images with an interval of 2 months. These were also evaluated for the presence of venous overlap. For MRA, sizes of aneurysm remnants or recurrences were directly measured on a workstation, and for DSA these sizes were estimated by comparison with the diameter of the internal carotid or basilar artery. Interobserver variability was assessed with κ statistics. After the blinded study, discrepancies were resolved by consensus. Finally, the consensus data of the MRA images were compared with DSA findings.

Contrast Material Enhancement

Contrast enhancement may reduce saturation effects on 3D TOF MRA and dynamic ultrafast MRA images (18, 20, 22, 23, 30–32), improving the visualization of giant aneurysms (32) and large remnants or recurrences of aneurysms treated with coils (18).

Contrast-enhanced 3D TOF MRA also benefits from imaging at 3T. T1 shortening in enhanced blood combined with T1 lengthening due to increased field strength in background tissues improves blood-to-background contrast (33). However, the use of intravenous contrast material had no additional value in the current study. Saturation reduction by the use of the MOTSA technique and the absence of large remnants or recurrences in our relatively small study of 21 aneurysms may explain the lack of added value of contrast material administration.

Coil-Related Artifacts

The platinum coil wires can produce susceptibility artifacts, although previous studies found a relative lack of susceptibility effects of coils in vitro and in vivo at 1.5 T (9, 10, 14). At higher field strength, increased susceptibility artifacts from paramagnetic substances are expected. In an in vitro study of MR compatibility of detachable coils at 3T, however, only minor susceptibility artifacts were found in the readout direction on gradient-echo images (11). Magnetic field mapping showed no induced field inhomogeneity (11). Also, no change in temperature was measured during movement into the imager bore or within the bore, and no evidence of deflection of the coil mass was found (24). At 3T, we found a high-signal-intensity rim artifact on T1-weighted spin-echo and T2-weighted fast spin-echo images around the coil mesh in most aneurysms, but on MRA images around only three aneurysms. This artifact did not interfere with interpretation of the occlusion status because the high-signal-intensity rim was not observed on the 3D TOF axial source images at the neck area.

Coil-induced signal intensity loss mimicking parent and branch vessel narrowing or occlusion has been described previously on 3D TOF MRA images (16–18). This signal intensity loss may prevent evaluation of the parent artery and aneurysm neck in up to 11% of cases on 1.5-T images (18). Although this artifact was observed in seven (33%) of 21 coil-treated aneurysms in our study, it was not so severe as to prevent evaluation of the aneurysm neck.