A noise-robust post-processing pipeline for accelerated phase-cycled ²³Na Multi-Quantum Coherences MRI.

Area of Science:

• Medical Imaging
• Biophysics
• Neuroimaging

Background:

• Sodium (²³Na) Multi-Quantum Coherences (MQC) MRI is valuable for in vivo human brain imaging at 7 Tesla.
• Current MQC MRI techniques face challenges with low signal-to-noise ratio (SNR) and lengthy radiofrequency (RF) phase-cycling.
• Accurate separation of Single Quantum (SQ) and Triple Quantum (TQ) signals is essential for calculating the TQ/SQ ratio, a key diagnostic parameter.

Purpose of the Study:

• To develop an advanced post-processing pipeline for noise-robust, accelerated ²³Na MQC MRI of the in vivo human brain at 7 T.
• To enhance SNR and improve the separation of SQ and TQ signal components in ²³Na MQC MRI.
• To reduce acquisition time while maintaining high image quality.

Main Methods:

• Combined low-rank k-space denoising for SNR enhancement with Dynamic Mode Decomposition (DMD) for robust SQ and TQ signal separation.
• Validated the pipeline in silico, in vitro, and in vivo, comparing it against conventional denoising and Fourier Transform (FT) methods.
• Assessed pipeline robustness using ablation experiments simulating corrupted RF phase-cycling steps.

Main Results:

• The denoising algorithm doubled SNR compared to non-denoised images and improved SNR by up to 29% over Wavelet denoising.
• DMD effectively separated SQ and TQ signals, even with incomplete RF phase-cycling, achieving superior SSIM (0.89±0.024) and lower RMSE (0.055±0.008) compared to FT methods.
• Enabled high-quality 8x8x15mm³ in vivo ²³Na MQC MRI with acquisition time reduced from 48 to 10 minutes.

Conclusions:

• The proposed pipeline significantly enhances robustness in ²³Na MQC MRI through low-rank denoising and DMD-based signal separation.
• Achieved high-quality MR images for both SQ and TQ components, even under accelerated and incomplete RF phase-cycling conditions.
• This method offers a more efficient and reliable approach for advanced ²³Na MQC MRI of the human brain.