Variable power combiner for RF mode shimming in 7-T MR imaging.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• RF Engineering
• Coil Array Design

Background:

• RF mode shimming in MRI enables spatial mode superposition for improved imaging.
• Current mode-shimming techniques can lead to significant underutilization of available RF power due to varying mode power requirements.

Purpose of the Study:

• To propose and evaluate a variable power combiner (VPC) for enhancing RF power utilization in MRI systems employing RF mode shimming.
• To compare the performance of the VPC configuration against the standard transmit chain configuration.

Main Methods:

• Implementation of a variable power combiner (VPC) as an additional matrix network in the MRI transmit system.
• The VPC controls phase and amplitude of amplifier input signals to achieve desired power distribution to the coil array's mode ports.
• Comparison of VPC configuration with standard transmit chain using an 8-channel transmit system with 16 coils.

Main Results:

• Significant improvements in power utilization were observed: 17% to 60% for an elliptical phantom and 14% to 55% for an abdominal region of interest.
• A 1 dB increase in power utilization was noted for a region of interest in the upper leg.
• The VPC demonstrated substantial power utilization gains when shimming solutions required energizing only a few modes.

Conclusions:

• The proposed variable power combiner effectively enhances RF power utilization in MRI mode shimming.
• The VPC is most beneficial when shimming solutions necessitate energizing a limited number of modes.
• The technique may lead to reduced power utilization if many modes require high power levels.