Improving PCASL at ultra-high field using a VERSE-guided parallel transmission strategy

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Neuroimaging
• Biophysics

Background:

• Pseudo-continuous arterial spin labeling (PCASL) is a non-invasive technique to measure brain perfusion.
• High field MRI (7T) offers potential for improved signal-to-noise ratio but faces challenges in radiofrequency (RF) homogeneity and specific absorption rate (SAR).
• Parallel transmission (pTx) technology enables spatial RF field control to improve homogeneity.

Purpose of the Study:

• To enhance the labeling efficiency of PCASL at 7T.
• To optimize RF shimming and pulse design using pTx for improved labeling homogeneity.
• To compare the performance of optimized 7T PCASL with standard 3T PCASL.

Main Methods:

• Utilized an 8-channel transmit 7T MRI scanner with pTx capabilities.
• Acquired time-of-flight angiography and B0 maps for precise labeling pulse design.
• Implemented complex RF shimming with Variable-Rate Selective Excitation (VERSE) pulses for labeling.
• Acquired whole-brain perfusion-weighted images and compared results with a 3T scanner.

Main Results:

• VERSE with RF shimming improved flip-angle homogeneity by 90% across regions of interest compared to standard circularly polarized (CP) mode.
• Temporal signal-to-noise ratio (SNR) was enhanced by 375% with VERSE and RF shimming versus CP mode.
• The optimized 7T sequence did not outperform a matched 3T sequence in terms of SNR.

Conclusions:

• Demonstrated improved PCASL tagging at 7T using VERSE with RF shimming under conservative SAR limits.
• Further optimization, potentially with less restrictive SAR limits, may be needed to fully leverage 7T advantages over 3T.
• This technique shows promise for advanced neuroimaging at ultra-high fields.