Specific absorption rate intersubject variability in 7T parallel transmit MRI of the head

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Biomedical Engineering
• Computational Electromagnetics

Background:

• High-field MRI (7T) requires precise radiofrequency (RF) shimming for optimal image quality.
• Patient-specific specific absorption rate (SAR) monitoring is crucial for safety during RF shimming.
• Numerical simulations are effective for SAR assessment but often rely on patient-specific dielectric models.

Purpose of the Study:

• To investigate the feasibility of using generic head models for SAR estimation in 7T MRI.
• To determine if a safety factor can account for population variations in SAR behavior.
• To evaluate the effectiveness of generic models for SAR-constrained RF shimming.

Main Methods:

• Simulated RF fields for an eight-channel transmit array using six detailed head models at 7T.
• Analyzed generic SAR behavior by comparing eigenvalues/eigenvectors of local Q-matrices.
• Performed numerical RF shimming with and without SAR constraints, evaluating SAR on generic models.

Main Results:

• Largest eigenvalue distribution showed comparable SAR behavior across different generic models.
• RF shimming without constraints improved |B +1| homogeneity but significantly increased SAR.
• SAR estimation using a generic model with a safety factor of 1.4 proved effective for shimming constraints.

Conclusions:

• Generic head models are a practical alternative to patient-specific models for SAR assessment in 7T MRI.
• Effective |B +1| shimming with SAR constraints is achievable using generic models.
• A safety factor of 1.4 adequately addresses population variability in SAR for 7T head imaging.