Multiphoton parallel transmission (MP-pTx): Pulse design methods and numerical validation.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Engineering
• Biomedical Engineering

Background:

• High-field MRI (≥7 T) faces challenges with flip angle inhomogeneities.
• Conventional parallel transmission (pTx) methods can be complex and costly.
• Existing techniques struggle to achieve uniform RF excitation at high fields.

Purpose of the Study:

• To introduce and evaluate Multiphoton Parallel Transmission (MP-pTx) for mitigating flip angle inhomogeneities in high-field MRI.
• To demonstrate MP-pTx's potential as a cost-effective alternative to conventional pTx.
• To simplify Specific Absorption Rate (SAR) analysis in high-field MRI.

Main Methods:

• MP-pTx utilizes a single birdcage coil combined with low-frequency (kHz) irradiation from shim or gradient arrays.
• Simulations at 7 T assessed MP-pTx's ability to create uniform, nonselective brain excitations.
• Optimization of RF, shim, and gradient waveforms was performed using genetic algorithms and sequential quadratic programming.

Main Results:

• MP-pTx reduced transverse magnetization error from 29% to 6.6% using a 32-channel shim array.
• This represents a nearly 40% improvement over conventional birdcage coil excitation with optimized kT-points.
• The method achieved this with comparable pulse power and current constraints (<50 Amp-turns).

Conclusions:

• MP-pTx offers an alternative to conventional pTx by using lower-cost shim channels.
• The method achieves flip angle uniformity comparable to 7 T pTx and superior to 3 T CP-mode.
• MP-pTx retains the straightforward SAR characteristics of conventional birdcage coils due to negligible SAR from low-frequency fields.