学术报告
题目：Magnetic resonance microscopy in non-human primates
报告人：Gang Chen
Instructor, Department of Radiology and Radiological Sciences
Vanderbilt University Institute of Imaging Science, U.S.A.
时间：2012年6月1日（周五），16:00-17:00 PM
地点：生命科学学院610会议室
Abstract:
Non-human primates have served as an important model for understanding functional brain organization in humans. The ability to conduct high resolution functional and anatomical magnetic resonance imaging (MRI) studies in monkeys promises to bridge the gap between human imaging studies and a large body of anatomical, electrophysiological, and functional optical imaging studies in non-human primates.
We used a high magnetic field 4.7T vertical scanner to perform MRI in awake macaque monkeys. High-field scanners provide higher blood oxygen level dependent signal (BOLD) and a greater signal-to-noise ratio (SNR). With a dedicated vertical primate scanner, monkeys may perform better for long scans with a more natural upright position. As magnetic field strength increases, there is degradation of signal due to motion and physiological noise as well as spatial distortions and loss of signal strength related to susceptibility artifact. We found that improvements in behavioral training were essential for obtaining greater data stability. The average translational movement decreased from over 500 µm to less than 80 µm in well-trained monkeys, a displacement close to that observed in anesthetized monkeys scanned in a 7 T horizontal scanner. Our results demonstrate that by using a combination of magnetic resonance microscopy and new MR contrasts, cortical layers can be identified in V1 from single 30-min runs at voxel size of 62.5 × 62.5 × 1000 μm3 (4 nl). Based on the phase and magnitude components of the T2*-weighted image, we were able to differentiate not only five cortical layers (I, II/III, IV, V, and VI) but also three sub-layers within layer IV (IVa, IVb, and IVc). To minimize susceptibility artifacts in functional MRI, the other major source of distortion at high fields, we used segmented, instead of single shot, gradient-echo (GE) echo-planar imaging (EPI) sequences. Comparison of images from functional runs using segments with these using single-shot EPI sequence revealed a roughly two-fold improvement in functional SNR and 50% decrease in distortion. With significant improvements by extensive behavioral training of monkeys and by the application of multi-shot sequences, we found the percent change of GE-BOLD signal is the highest at a depth corresponding to layer IV after excluding the influence from pial veins with an optical imaging based pial vein pattern analysis with a 2-cm surface coil which suggests that the intrinsic spatial resolution of GE-BOLD fMRI signal is high enough to reveal sub-millimeter functional structures in the awake monkey. Our findings lay the groundwork for future revealing of sub-millimeter functional domains and pathological change in awake subjects with magnetic resonance microscopy.
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