Mapping magnetization transfer saturation (MT_sat ) in human brain at 7T: Protocol optimization under specific...

Area of Science:

• Magnetic Resonance Imaging
• Biophysics

Background:

• Magnetization Transfer Saturation (MT_sat) is a semi-quantitative MRI metric.
• Optimizing MT_sat at 7 Tesla (7T) requires balancing SAR and B1 field inhomogeneity.
• Existing protocols need refinement for efficient whole-brain coverage.

Purpose of the Study:

• To optimize a whole-brain Magnetization Transfer Saturation (MT_sat) protocol at 7T.
• Maximize obtainable MT_sat within SAR and B1 field constraints.
• Minimize scan time and avoid image bias.

Main Methods:

• Utilized a 7T dual flip angle protocol.
• Focused on optimizing MT pulse parameters, readout flip angle, TR, and offset frequency (Δ).
• Implemented separate flip angle mapping for B1 field correction.

Main Results:

• Achieved 100% SAR with a 180° MT pulse (4 ms duration) and TR of 26.5 ms.
• Negative offset frequency (Δ = -2.0 kHz) yielded 45% higher MT_sat in white matter compared to positive Δ.
• A 4° readout flip angle minimized bias and maintained good SNR; scan time was 4:58 min.

Conclusions:

• MT_sat at 7T is limited but can be improved by exploiting macromolecular lineshape asymmetry via Δ sign.
• Compact MT pulses and short TR/readout flip angles reduce scan time.
• Separate flip angle mapping is essential for accurate B1 field correction.