Effect of RF Pulse Sequence on Temperature Elevation for a Given Time-Average SAR.

Area of Science:

• Biomedical Engineering
• Medical Imaging Physics

Background:

• Accurate temperature rise prediction during MRI is crucial for patient safety.
• Current methods often average Specific Absorption Rate (SAR) over the repetition time (TR).

Purpose of the Study:

• To investigate the validity of using time-averaged SAR versus pulse-specific heating for MRI temperature calculations.
• To assess the impact of different pulse sequences on temperature increase during MRI scans.

Main Methods:

• Numerical calculations of SAR and temperature increase for a human head model in a volume coil at 64 MHz and 300 MHz.
• Analysis of three distinct pulse sequences with identical TR and time-average whole-head SAR.
• Semi-analytical calculations for a gradient-echo sequence with high SAR delivered in a short pulse.

Main Results:

• Temperature differences resulting from explicit RF pulse heating were found to be small and transient.
• Calculations across different frequencies and pulse sequences showed minimal variation in temperature profiles.
• The temporal effect of pulse sequence on temperature rise was negligible for practical safety assessments.

Conclusions:

• Averaging SAR over the repetition time (TR) is an adequate assumption for calculating MRI-induced temperature increases.
• Explicitly modeling the transient heating during RF pulses is generally unnecessary for safety evaluations.
• The standard practice of using time-average SAR provides sufficient accuracy for thermal safety assessments in MRI.