Quantitative Magnetization Transfer Imaging of Human Cervical Spinal Cord at 3T

Introduction: The goal of this study was to determine the feasibility of performing quantitative magnetization transfer (qMT) at high resolution in the spinal cord on clinical 3T systems. While magnetization transfer (MT) imaging has been used to assess brain tissue microstructure, similar studies in the human spinal cord have been limited. This can largely be attributed to the difficulties associated with imaging the spinal cord, which include: i) high resolution demands (cord diameter ≈ 1.5 cm), ii) motion (CSF pulsation and respiration), and iii) susceptibility gradients (bone/tissue interfaces). Despite these difficulties, MT has been characterized in the spinal cord via the magnetization transfer ratio normalized to the CSF (MTCSF), a measure that has been shown to be sensitive to myelination changes in patients with adrenomyeloneuropathy (AMN) [1,2]. Unfortunately, MTCSF is also sensitive to changes in tissue relaxation times and flow effects in CSF, making it difficult to discriminate between myelin and inflammation effects. To alleviate this sensitivity, quantitative MT (qMT) approaches have been proposed. Previous work at 1.5T [3] has indicated that the ratio of the macromolecular to free water pool sizes, or pool size ratio (PSR), may be sensitive to myelination changes in the spinal cord. Given the resolution demands of imaging the spinal cord, qMT studies of the spinal cord would presumably benefit from the increased SNR at 3T; however, such studies are hampered by the fact that SAR scales according the field strength squared. To address these issues, we have developed a protocol for high resolution qMT imaging of the cervical spinal cord at 3T and here we report data acquired in healthy subjects.