To enable clinical applications of quantitative magnetization transfer (qMT) imaging by developing a fast method to map one of its fundamental model parameters, the bound pool fraction (BPF), in the human brain.
Theory and Methods
The theory of steady‐state MT in the fast‐exchange approximation is used to provide measurements of BPF, and bound pool transverse relaxation time (). A sequence that allows sampling of the signal during steady‐state MT saturation is used to perform BPF mapping with a 10‐min‐long fully echo planar imaging‐based MRI protocol, including inversion recovery T1 mapping and B1 error mapping. The approach is applied in 6 healthy subjects and 1 multiple sclerosis patient, and validated against a single‐slice full qMT reference acquisition.
BPF measurements are in agreement with literature values using off‐resonance MT, with average BPF of 0.114(0.100‐0.128) in white matter and 0.068(0.054‐0.085) in gray matter. Median voxel‐wise percentage error compared with standard single slice qMT is 4.6%. Slope and intercept of linear regression between new and reference BPF are 0.83(0.81‐0.85) and 0.013(0.11‐0.16). Bland‐Altman plot mean bias is 0.005. In the multiple sclerosis case, the BPF is sensitive to pathological changes in lesions.
The method developed provides accurate BPF estimates and enables shorter scan time compared with currently available approaches, demonstrating the potential of bringing myelin sensitive measurement closer to the clinic.
Financed by the National Centre for Research and Development under grant No. SP/I/1/77065/10 by the strategic scientific research and experimental development program:
SYNAT - “Interdisciplinary System for Interactive Scientific and Scientific-Technical Information”.