Thermal simulations in the human head for high field MRI using parallel transmission

Area of Science:

• Biomedical Engineering
• Medical Physics
• Radiology

Background:

• High-field magnetic resonance imaging (MRI) uses parallel transmission (PT) to improve image quality.
• Ensuring patient safety requires adherence to specific absorption rate (SAR) and temperature guidelines.
• The relationship between SAR and temperature in the human head during PT-MRI is not fully understood.

Purpose of the Study:

• To numerically investigate the compliance of specific absorption rate (SAR) versus temperature guidelines for the human head in high-field MRI procedures using parallel transmission.
• To assess the thermal safety of PT-MRI protocols in the human head.

Main Methods:

• Utilized a combination of finite element and finite-difference time-domain methods for temperature evolution calculations.
• Employed a detailed computational model of the human head comprising 20 anatomical structures.
• Simulated a large number of parallel transmission scenarios to evaluate radiofrequency field exposure.

Main Results:

• The recommended maximum local temperature of 39°C was not exceeded when the local 10-g average SAR threshold was met.
• Maximum temperatures approached guideline limits when the global SAR reached 3.2 W/kg.
• In both scenarios, the maximal temperature in the eye increased by more than 1°C.

Conclusions:

• Local 10-g SAR limits appear relevant for ensuring head temperature compliance during PT-MRI, but may allow significant temperature rises in the eye.
• Direct temperature monitoring may offer greater flexibility in PT-MRI pulse design compared to relying solely on SAR metrics.
• Further research into localized thermal effects in sensitive regions like the eye is warranted.