IMPULSE: A scalable algorithm for design of minimum specific absorption rate parallel transmit RF pulses.

Area of Science:

• Medical Imaging
• Magnetic Resonance Imaging
• Radiofrequency Engineering

Background:

• Managing local specific absorption rate (SAR) in parallel transmission MRI is crucial for patient safety, requiring peak SAR to remain below regulatory limits.
• Patient safety is determined by cumulative, not instantaneous, SAR, necessitating joint optimization of RF pulses within a sequence.

Purpose of the Study:

• To introduce the Iterative Minimization Procedure with Uncompressed Local SAR Estimate (IMPULSE), an efficient algorithm for optimizing parallel transmit pulses.
• To enable joint optimization of RF pulses for all slices within a practical timeframe, handling uncompressed SAR matrices.

Main Methods:

• IMPULSE optimizes parallel transmit pulses for small-tip-angle slice selective excitation.
• It minimizes a single cost function including local SAR, global SAR, and per-channel power, averaged over a multislice scan, with constraints on excitation accuracy.
• Pulses were designed for an 8-channel 7T head coil and compared against generic optimization algorithms and VOPs.

Main Results:

• IMPULSE demonstrated lower SAR and reduced computation time compared to the VOP approach.
• Computation time was reduced by a factor of 5-6 compared to generic sequential quadratic programming algorithms.
• IMPULSE designed pulses for up to 120 slices jointly within 45 seconds, utilizing up to 6 million local SAR terms.

Conclusions:

• IMPULSE efficiently handles a larger number of SAR matrices and slices than conventional methods.
• It enables the use of uncompressed or partially compressed SAR matrices for multislice pulse design within practical time limits.
• This facilitates safer and more efficient parallel transmission MRI protocols.