Local SAR in parallel transmission pulse design.

Area of Science:

• Biomedical Engineering
• Magnetic Resonance Imaging Physics
• Radiofrequency Engineering

Background:

• Managing radiofrequency power deposition, specifically local and global specific absorption rate (SAR), is critical for parallel transmission (pTx) systems in human subjects.
• The complex spatial distribution of local SAR in pTx arrays presents challenges not seen in single-channel systems, necessitating advanced pulse design.

Purpose of the Study:

• To propose and validate a novel pTx pulse design method that incorporates local SAR constraints within practical computation times.
• To develop an algorithm that bounds the achievable peak local SAR for a given excitation profile fidelity.

Main Methods:

• A compressed parameterization of local SAR distribution in numerical tissue models was utilized.
• The method integrated local SAR constraints into pTx pulse design suitable for in vivo magnetic resonance imaging scans.
• A 7 Tesla eight-channel transmit array and a numerical human head model were employed for performance evaluation.

Main Results:

• The proposed method achieved significant reductions in peak local 10 g SAR (14-66%) for slice-selective and 2D selective pTx excitations.
• Compared to designs constrained only by global SAR, the new method demonstrated superior local SAR management.
• A trade-off was observed, with an increase in global SAR (up to 34%) being acceptable when local SAR is the dominant constraint.

Conclusions:

• The developed pTx pulse design method effectively manages local SAR constraints in human subjects during MRI.
• This approach offers a practical solution for enhancing safety in pTx applications where local SAR limits are critical.
• The method provides a protocol-specific bound on peak local SAR, improving the predictability and safety of MRI procedures.