Variable density magnetization transfer (vdMT) imaging for 7T MR imaging

Abstract

As the use of ultra-high field (UHF; ≥7T) magnetic resonance (MR) imaging expands, there is an increasing need to establish high-resolution MR imaging protocols for patients with neurological disease. Magnetization transfer (MT) imaging has been used to provide information about changes in the magnitude of the restricted protons that are caused by tissue damages. Several studies have found that MTR has a good sensitivity to measure changes in myelin concentration within the brain. Because of the much higher specific absorption rate (SAR) of tissue and longer acquisition time required for UHF, however, in-vivo studies using conventional pulsed MT sequences at UHF have not been well utilized. In this study, we introduce a new MT data acquisition approach using a 7T MR system, variable density magnetization transfer (vdMT) imaging, which can be reasonably included in a routine patient scan protocol with a much shorter scan time and reduced discomfort to the patient. To reduce SAR and scan time while maintaining at least similar MTR image quality to that obtained with the conventional method, a higher density of MT RF pulses was applied in the center of k-space, and sparsely applied MT RF pulses were used in the outer part of k-space. The fraction of k-space receiving 100% MT RF density and TR were optimized based on in-vivo ROI analysis, and results were confirmed with high-resolution MTR map using a vdMT approach from healthy controls and patients with multiple sclerosis (MS). The experimental results confirmed that vdMT imaging can cover a whole brain volume in an acceptable scan time for routine patient scans while providing MTR image quality at least similar to that obtained with conventional MT imaging (correlation coefficient=0.95 in an agar-gel phantom [MT offset frequency=1kH], 0.90 in a postmortem MS brain, and 0.85 in the 4 healthy volunteers). MS lesions were associated with signal reductions in the postmortem MS brains and in the patients with MS. In this study, we have described a new approach for acquiring high-resolution MTR map of the whole brain volume using 7T MR imaging. This vdMT method provides similar image quality to that obtained with the conventional method, and shortens the scan time by reducing SAR. These results suggest that vdMT approach is a method that could be used for UHF scans of patients with neurological disease.

Keywords: 7T; MT; MTR; Magnetization transfer imaging; Magnetization transfer ratio; UHF; Ultra-high field; Variable density MT.