2.3. In vivo MRI

All MRI data were acquired on a Varian DirectDrive™ horizontal 9.4 T scanner using a saddle-shaped transmit-receive surface coil positioned around the cervical spine region. The image field of view was centered at the level where the lesion was targeted. The MRI acquisition protocol was similar to that employed in our previous study (Wang et al., 2016) and details are provided in the supporting information. After acquiring high resolution structural images with magnetization transfer contrast (MTC) to guide the placements of image slices, qMT data were obtained from a coronal slice where the dorsal columns and dorsal horns reside (Fig. 1A–C) using a 2D MT-weighted spoiled gradient echo sequence (TR 24 ms, flip angle =7°, resolution = ~0.313 × 0.313 × 1 mm³). We obtained qMT data using 12 different RF offsets (Fig. 1E) between 1 and 100 kHz and two saturation powers (θ_sat = 220° and 820°, pulse width = 12 ms).

Quantitative MT imaging of cervical spinal cord with a unilateral dorsal column lesion. (A-C) Sagittal, axial and coronal images with MT contrast. The arrows indicate the lesion site. White dashed lines indicate the location of coronal imaging plane. DPC (dorsal pathway), LPC (lateral pathway), VPC (ventral pathway) and GMC (gray matter) are on the non-lesion control side; DPL, LPL, VPL and GML indicate respective regions on the lesion side. (D) Schematic illustration of the reconstructed lesion of dorsal column on the right side (black patch). (E) MT in vivo data showing the normalized intensity extracted from normal spinal cord GM at different RF offsets and two flip angles of θ_sat 820° (black circles) and 220° (red squares).