A second-order and slice-specific linear shimming technique to improve spinal cord fMRI

Area of Science:

• Neuroimaging
• Biomedical Engineering
• Physics in Medicine

Background:

• Functional Magnetic Resonance Imaging (fMRI) of the human cervical spinal cord is challenged by signal loss and distortions.
• Improving B0 homogeneity and optimizing shim settings are crucial for high-quality spinal cord fMRI.

Purpose of the Study:

• To develop and evaluate a novel second-order and slice-specific linear shimming technique.
• To mitigate signal loss and distortions in human cervical spinal cord fMRI.
• To increase temporal signal-to-noise ratio (tSNR) for improved functional signal detection.

Main Methods:

• Scans performed on a 3T MRI scanner using a head, neck, and spine coil.
• Second-order shims (SOS) optimized using field maps and manually defined regions of interest.
• Slice-specific linear shims (x, y, z) optimized to maximize spinal cord signal, using spectral-spatial excitation pulses.
• Technique validated by comparing tSNR with standard linear shimming in 8 healthy volunteers and fMRI in 14 participants.

Main Results:

• Standard linear shimming resulted in severe signal loss below the C5 vertebral level.
• The developed SOS and slice-specific shimming technique significantly reduced signal loss, particularly at C6 and C7 levels.
• Significantly higher tSNR was observed across all vertebral levels (C3-C7) with the new technique compared to standard shimming.

Conclusions:

• A comprehensive shimming approach combining spectral-spatial pulses with second-order and slice-specific linear optimization effectively reduces signal loss.
• This technique enhances tSNR in the cervical spine (C3-C7).
• The improved image quality facilitates more reliable recording of functional signals from the human spinal cord.