Optimization of inhomogeneous magnetization transfer (ihMT) MRI contrast for preclinical studies using dipolar relaxation time (T1D) filtering

A pulsed inhomogeneous magnetization transfer (ihMT)-prepared fast imaging sequence was implemented at 11.75T for preclinical studies on mouse central nervous system. A strategy based on filtering the ihMT signal originating from short dipolar relaxation time (T-1D) components is proposed. It involves increasing the repetition time of consecutive radiofrequency (RF) pulses of the dual saturation and allows improved signal specificity for long T-1D myelinated structures. Furthermore, frequency offset, power and timing saturation parameters were adjusted to optimize the ihMT sensitivity. The optimization of the ihMT sensitivity, whilst preserving the strong specificity for the long T-1D component of myelinated tissues, allowed measurements of ihMT ratios on the order of 4-5% in white matter (WM), 2.5% in gray matter (GM) and 1-1.3% in muscle. This led to high relative ihMT contrasts between myelinated tissues and others (similar to 3-4 between WM and muscle, and >= 2 between GM and muscle). Conversely, higher ihMT ratios (similar to 6-7% in WM) could be obtained using minimal T-1D filtering achieved with short saturation pulse repetition time or cosine-modulated pulses for the dual-frequency saturation. This study represents a first stage in the process of validating ihMT as a myelin biomarker by providing optimized ihMT preclinical sequences, directly transposable and applicable to other preclinical magnetic fields and scanners. Finally, ihMT ratios measured in various central nervous system areas are provided for future reference.