Coaxial waveguide for travelling wave MRI at ultrahigh fields

Area of Science:

• Medical Imaging
• Electrical Engineering
• Physics

Background:

• High magnetic fields in MRI scanners affect body coil performance due to waveguide effects.
• Undesirable strong radio frequency (RF) fields (B1+) can occur outside the target volume.
• Existing RF transmit systems may lead to inefficient power deposition and uneven B1+ fields.

Purpose of the Study:

• To propose and evaluate a novel RF transmit system for MRI that exploits the scanner bore's waveguide action.
• To introduce a coaxial waveguide section to improve RF field characteristics and safety.
• To compare the performance of the coaxial waveguide system with a travelling wave setup.

Main Methods:

• Implementation of a coaxial waveguide section between the RF antenna and the imaging region.
• Application of a novel RF matching principle based on transmission line impedance matching.
• Evaluation of RF transmission characteristics, including B1+ field homogeneity and power efficiency.
• Assessment of specific absorptive rate (SAR) deposition in body torso imaging.

Main Results:

• The coaxial waveguide system demonstrates superior RF transmission characteristics compared to a travelling wave setup.
• A novel RF matching principle enhances B1+ field homogeneity and power efficiency.
• Unwanted SAR deposition outside the target region is prevented, reducing peak SAR values by a factor of 5.
• A 3-fold increase in B1+ field gain in the prostate was achieved.

Conclusions:

• The proposed coaxial waveguide RF transmit system effectively utilizes the scanner bore's waveguide action for improved MRI.
• This system offers enhanced B1+ field homogeneity, power efficiency, and safety by reducing SAR deposition.
• The coaxial waveguide represents a significant advancement over previous travelling wave setups for high-field MRI applications.