Improved BOLD detection in the medial temporal region using parallel imaging and voxel volume reduction

Abstract

Using gradient-echo EPI, signal dropout due to macroscopic off resonance effects can prevent blood-oxygenation-level-dependent (BOLD) signal change detection. The anterior medial temporal lobe (MTL) is located near these susceptibility gradients and therefore shows considerable signal dropout with GE-EPI. Reducing the volume of the image voxel reduces susceptibility-related signal dropout. However, this is accompanied by a prohibitive reduction in signal-to-noise ratio (SNR). To compensate for SNR loss with smaller voxels, we used a multi-channel MRI receiver with an array of receive-only 16-element surface coils at 3 T. We demonstrate that the reduction of susceptibility artifacts, through use of high resolution images, coupled with the gains in image SNR from the array coil improves the temporal signal-to-noise ratio (TSNR) and enhances the contrast-to-noise ratio (CNR). Furthermore, a comparison of 2 mm with 4-mm-thick axial images both with the same in-plane resolution showed that thinner slices enhanced TSNR and CNR throughout the ventral-medial regions of the temporal lobes, with the greatest improvement in the most anterior regions of the MTL. Further improvements were seen when adjacent 2 mm slices were combined to match overall voxel volume. These results demonstrate that BOLD investigation of anterior MTL function can be enhanced by decreasing voxel size but only in combination with the SNR gained by using the 16-channel head coil system.