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Parallel magnetic resonance imaging (MRI) techniques have been widely utilized to shorten scan times or improve spatial resolution (1, 2). Parallel imaging reduces the number of phase-encoding steps by a reduction (R) factor, which refers to the ratio of original phase-encoding steps to reduced phase-encoding steps. The R factor ranges from 1.5 to 4 in most commercially available applications (1, 2). Although applications with much higher R factors are possible, they are currently limited by poor signalto-noise ratio (SNR) and image reconstruction artifacts (1-3). The use of diffusion tensor tractography (DTT) of the cerebral white matter has become widespread in research into a broad spectrum of diseases (4-6). DTT is an imaging method based on diffusion weighted imaging data, which is usually obtained by single-shot echo-planar imaging (EPI). Single-shot EPI has several inherent drawbacks, including low SNR and vulnerability to susceptibility artifacts (1, 2). These drawbacks can be partially overcome by parallel imaging; the reduction in phase-encoding steps increases the SNR via a reduction in the echo time Diffusion Tensor Tractography of the Brainstem Pyramidal Tract: A Study on the Optimal Reduction Factor in Parallel Imaging 뇌간 추체로의 확산텐서 신경다발 영상: 병렬 영상에서의 적정 감소 인자에 대한 연구

Purpose: We aimed to evaluate the utility of two types of MR cisternography [fast spin echo sequence and steady-state coherent gradient echo (GRE) sequence] in addition to phase contrast-cine imaging (PC-cine), for assessing patency at the aqueduct and third ventriculostomy site. Materials and Methods: 43 patients (35 patients with suspected aqueductal stenosis and 8 patients with third ventriculostomy) were retrospectively analyzed. PCcine, 3 dimensional sagittal fast spin echo sequence [driven-equilibrium imaging (DRIVE) or volumetric isotrophic T2-weighted acquisition (T2 VISTA)] and steadystate coherent fast GRE sequence (balanced turbo field echo; bTFE) imaging were performed in all patients. The patency of the aqueduct or third ventriculostomy site was scored. Some pitfalls of each sequence were also analyzed in individual cases. Results: 93% of all cases showed consistent scores in PC-cine, DRIVE/T2 VISTA, and bTFE imaging. DRIVE/T2 VISTA imaging provided functional information of cerebrospinal fluid flow with flow-related artifacts, while bTFE imaging allowed direct visualization of the aqueduct or ventriculostomy site. However, evaluation of anatomical structures was difficult in three cases with strong flow-related artifacts on DRIVE/T2 VISTA and in 2 cases with susceptibility artifacts on bTFE. Conclusion: Both DRIVE/T2 VISTA and bTFE imaging have complementary roles in evaluating the patency of the aqueduct and 3rd ventriculostomy site. Index terms Aqueductal Stenosis Third Ventriculostomy Volumetric Isotrophic T2-Weighted Acquisition Driven-Equilibrium Imaging Balanced Turbo Field Echo

Purpose: We wanted to determine whether or not the “susceptibility asymmetry index” (SAI) of acute stroke on the T2*-weighted image is related with successful recanalization using multimodal intra-arterial thrombolysis (IAT). Materials and Methods: The 81 patients who underwent multimodal IAT for middle cerebral artery (MCA) territory acute stroke were included in this retrospective study. The multimodal IAT included intra-arterial urokinase infusion, clot disruption by a microwire, microcatheter and balloon manipulation, and balloon angioplasty and/or stenting for the flow-limiting stenosis. The diameter of the susceptibility vessel sign was measured on the T2*-weighted gradient echo imaging (GRE), and the diameter of the contralateral normal MCA at the corresponding level was measured on magnetic resonance angiography (MRA); the ratio between these two diameters was defined as the susceptibility asymmetry index. The relation between the TICI (Thrombolysis In Cerebral Infarction) score of 2-3 after multimodal IAT and the SAI was assessed. The receiver operating characteristic (ROC) curve analysis was performed on the SAI to predict a TICI score of 2-3 after multimodal IAT. Results: The mean SAI of 81 patients was 1.66 ± 0.66. Seventy nine percent of the patients had a TICI of 2-3 after multimodal IAT. According to the ROC curve analysis, an SAI less than 1.3 was optimal for predicting the presence of stenotic lesion after recanalization (area under the curve: 0.821, sensitivity: 88.2%, specificity: 69.8%, p=0.0001), and the SAI ≤1.61 (area under the curve: 0.652, sensitivity: 60.9%, specificity: 70.6%, p=0.0226) could predict a TICI score of 2-3. The TICI score of 2-3 after multimodal IAT was achieved in 88.6% of the cases with a SAI ≤ 1.61 and in 67.6% of the cases with a SAI >1.61 (p=0.028). Conclusion: The lower SAI on T2*-GRE could predict stenotic lesion and successful recanalization after performing IAT.