Premium
Mapping magnetization transfer saturation (MT sat ) in human brain at 7T: Protocol optimization under specific absorption rate constraints
Author(s) -
Olsson Hampus,
Andersen Mads,
Wirestam Ronnie,
Helms Gunther
Publication year - 2021
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.28899
Subject(s) - magnetization transfer , specific absorption rate , saturation (graph theory) , nuclear magnetic resonance , absorption (acoustics) , protocol (science) , magnetization , transfer (computing) , materials science , computational physics , biological system , chemistry , computer science , physics , mathematics , magnetic resonance imaging , optics , biology , magnetic field , medicine , pathology , telecommunications , combinatorics , antenna (radio) , radiology , alternative medicine , quantum mechanics , parallel computing
Purpose To optimize a whole‐brain magnetization transfer saturation (MT sat ) protocol at 7T, focusing on maximizing obtainable MT sat under the constraints of specific absorption rate (SAR) and transmit field inhomogeneity, while avoiding bias and keeping scan time short. Theory and Methods MT sat is a semi‐quantitative metric, obtained by spoiled gradient‐echo MRI in the imaging steady‐state. Optimization was based on an established 7T dual flip angle protocol, and focused on MT pulse, readout flip angle, repetition time (TR), offset frequency (Δ), and correction of residual effects from transmit field inhomogeneities by separate flip angle mapping. Results A 100% SAR level was reached at a 180° MT pulse flip angle, using a compact sinc main lobe (4 ms duration) and minimum TR = 26.5 ms. The use of Δ = +2.0 kHz caused no discernible direct saturation, while Δ = −2.0 kHz resulted in 45% higher MT sat in white matter (WM) compared to Δ = +2.0 kHz. A 4° readout flip angle eliminated bias while yielding a good signal‐to‐noise ratio. Increased TR yielded only a little increase in MT sat , and TR = 26.5 ms (scan time 04:58 min) was thus selected. Post hoc transmit field correction clearly improved homogeneity, especially in WM. Conclusions The range of MT sat is limited at 7T, and this can partly be overcome by the exploitation of the asymmetry of the macromolecular lineshape through the sign of Δ. To reduce scan time, a compact MT pulse with a sufficiently narrow frequency response should be used. TR and readout flip angle should be kept short/small. Transmit field correction through separate flip angle mapping is required.