SNR-Enhanced Whole-Brain Chemical Exchange Saturation Transfer Imaging by Optimized Variable Flip Angles.

Area of Science:

• Magnetic Resonance Imaging
• Biomedical Engineering
• Radiology

Background:

• Chemical Exchange Saturation Transfer (CEST) MRI offers valuable insights into tissue physiology.
• Whole-brain CEST imaging faces challenges with signal-to-noise ratio (SNR) and T2-decay-related blurring.
• Optimizing readout parameters is crucial for enhancing CEST sequence performance.

Purpose of the Study:

• To enhance whole-brain chemical exchange saturation transfer (CEST) imaging.
• To improve signal-to-noise ratio (SNR) and reduce blurring in SPACE-CEST sequences.
• To optimize refocusing flip angles for improved MRI quality.

Main Methods:

• Modeled SPACE-CEST sequence SNR as a function of flip angles.
• Incorporated a resolution penalty term to mitigate blurring.
• Optimized flip angles in Resolution-only and SNR+Resolution modes.
• Compared optimized angles to conventional methods regarding magnetization efficiency, storage, and SAR.
• Validated findings in phantoms, healthy volunteers, and brain tumor patients.

Main Results:

• Optimized flip angles in SNR+Resolution mode showed higher transverse magnetization usage efficiency and longitudinal storage.
• This mode achieved comparable signals to conventional methods but with significantly lower flip angles.
• Demonstrated robustness to B1 inhomogeneity and wide ranges of T1 and T2 values.
• Reported a 32.9% decrease in specific absorption rate (SAR).
• Achieved an average 14.2% SNR increase and 3.6% spatial resolution improvement in humans.

Conclusions:

• Optimizing refocusing flip angles in SPACE-CEST sequences increases transverse magnetization usage efficiency.
• This optimization leads to significant improvements in SNR and reductions in SAR.
• The proposed method enhances the diagnostic potential of whole-brain CEST MRI.