Purpose: Imaging biomarkers with increased myelin specificity are needed to
better understand the complex progression of neurological disorders.
Inhomogeneous magnetization transfer (ihMT) imaging is an emergent technique
that has a high degree of specificity for myelin content but suffers from low
signal-to-noise ratio (SNR). This study used simulations to determine optimal
sequence parameters for ihMT imaging for use in high-resolution cortical
mapping. Methods: MT-weighted cortical image intensity and ihMT SNR were
simulated using modified Bloch equations for a range of sequence parameters.
The acquisition time was limited to 4.5 min/volume. A custom MT-weighted RAGE
sequence with center-out k-space encoding was used to enhance SNR at 3 Tesla.
Pulsed MT imaging was studied over a range of saturation parameters and the
impact of the turbo-factor on effective ihMT was investigated. 1 mm isotropic
ihMTsat maps were generated in 25 healthy adults using an optimized protocol.
Results: Greater SNR was observed for larger number of bursts consisting of 6-8
saturation pulses each, combined with a high readout turbo-factor. However,
that protocol suffered from a point spread function that was more than twice
the nominal resolution. For high-resolution cortical imaging, we selected a
protocol with a higher effective resolution at the cost of a lower SNR. We
present the first group-average ihMTsat whole-brain map at 1 mm isotropic
resolution. Conclusion: This study presents the impact of saturation and
excitation parameters on ihMTsat SNR and resolution. We demonstrate the
feasibility of high-resolution cortical myelin imaging using ihMTsat in less
than 20 minutes