Rapid fat suppression for three-dimensional echo planar imaging with minimized specific absorption rate.

Area of Science:

• Magnetic Resonance Imaging
• Biophysics
• Medical Physics

Background:

• Functional MRI (fMRI) at ultrahigh fields (UHF) requires efficient water excitation.
• Conventional fat saturation methods can be power-intensive and sensitive to field imperfections.
• Optimizing radiofrequency (RF) power deposition is crucial for UHF MRI safety and efficiency.

Purpose of the Study:

• To evaluate a novel method using a single rectangular RF pulse (rect) for rapid water excitation.
• To assess its suitability as a replacement for conventional fat saturation in 3D EPI at 3 and 7 Tesla.
• To minimize RF power deposition while maintaining or improving image quality and speed.

Main Methods:

• Derivation of a pulse duration formula for lipid signal nulling, independent of the small-tip-angle approximation.
• Simulations and experimental validation at 3T and 7T.
• Comparison with conventional binomial-11 water excitation and fat saturation techniques.

Main Results:

• The single rect pulse method demonstrated superior selectivity and reduced sensitivity to shim imperfections compared to binomial-11 water excitation.
• Achieved a 25% measurement speedup and a significant reduction in specific absorption rate (SAR) from 44% to 1% at 7T.
• Reduced magnetization transfer effects, leading to up to 25% higher brain tissue signal-to-noise ratio.

Conclusions:

• The proposed single rect pulse method is highly suitable for whole-brain fMRI at ultrahigh fields.
• It maximizes sensitivity per unit time while minimizing RF power deposition.
• The method is easily implementable for other nonselective imaging techniques requiring efficient fat suppression.