TY - JOUR
T1 - Mapping magnetization transfer saturation (MTsat) in human brain at 7T
T2 - Protocol optimization under specific absorption rate constraints
AU - Olsson, Hampus
AU - Andersen, Mads
AU - Wirestam, Ronnie
AU - Helms, Gunther
PY - 2021/6/30
Y1 - 2021/6/30
N2 - Purpose: To optimize a whole-brain magnetization transfer saturation (MTsat) protocolat 7T, focusing on maximizing obtainable MTsat under the constraints of specificabsorption rate (SAR) and transmit field inhomogeneity, while avoiding bias and keeping scan time short.Theory and Methods: MTsat 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 MTsat 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 MTsat, 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 MTsat 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.
AB - Purpose: To optimize a whole-brain magnetization transfer saturation (MTsat) protocolat 7T, focusing on maximizing obtainable MTsat under the constraints of specificabsorption rate (SAR) and transmit field inhomogeneity, while avoiding bias and keeping scan time short.Theory and Methods: MTsat 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 MTsat 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 MTsat, 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 MTsat 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.
U2 - 10.1002/mrm.28899
DO - 10.1002/mrm.28899
M3 - Article
C2 - 34196043
SN - 1522-2594
JO - Magnetic Resonance in Medicine
JF - Magnetic Resonance in Medicine
ER -