Subject-Loaded Helical-Antenna Radio-Frequency Coil for MRI
At A Glance
The main area of engineering research in advancing next-generation MRI scanners at high (B0 = 3 T) and ultra-high (B0 > 3 T) polarizing fields is in improving RF coils and RF magnetic fields (B1) in an MRI bore loaded with a human body or a phantom. This invention features a new RF coil, the subject-loaded multi-channel helical-antenna RF volume coil, which, when compared to the existing RF coil concepts and designs, provides:
- improved uniformity of the B1 field,
- better circular polarization of the field
- increased field of view in the axial direction
- use in parallel transmission to enable all multi-transmit channel RF technology.
Investigators at Colorado State University have developed a novel method and apparatus for excitation of radio-frequency (RF) magnetic fields in high-field (HF) and ultra-high-field (UHF) magnetic resonance imaging (MRI) systems (with main polarizing static magnetic field B0 ≥ 3 T), so for state-of-the-art clinical scanners and near-future, next-generation clinical systems, using a novel category of multi-channel RF coil structures with volume coverage, based on a subject-loaded multifilar helical-antenna RF volume coil. The novel helical antenna RF coil provides RF magnetic field B1+ with high field-uniformity, excellent circular polarization (CP), and large field of view (FOV). The new coil is aimed for replacement of traditional near-field body RF coils, such as a birdcage coil in state-of-the-art clinical HF scanners at 3 T, and provides at the same time better solution at UHF fields (B0 > 3 T) than state-of-the-art traveling-wave antennas. It has clear advantages over patch antennas implemented in B0 ≥ 7 T MRI systems, as well as over existing traveling-wave exciters and coils, and is simultaneously a solution for a body RF coil at HF fields. The novel method for RF excitation in MRI systems is universal and not limited to any particular field strength and any particular frequency. The invention embodies applications of helical antennas as RF coils to MRI systems at 3 T, 7 T, 10.5 T, and any other B0 field strength. It thus includes the entire spectrum of available and future scanners 3-T and up. Helical antennas can also be used as head coils, and coils for scanning arms, hands, wrists, legs, knees, ankles, etc. Potential applications include research, pre-clinical, and clinical MRI systems.
Single-channel – or monofilar – (uniformly-wound) helical antenna (in free space) is the simplest of all helical antenna designs, summarized as a metallic wire antenna wound periodically with many wire turns and a certain pitch angle about an imaginary (or dielectric) cylinder. The antenna is fed at one wire end against a circular back-plate, acting as a ground plane (hollow plate is also possible). Taking advantage of the multi-channel RF technology, a more advanced RF coil design approach is proposed, based on either a quadrifilar (4-channel) or an octafilar (8-channel) helical antenna, where four (or eight) helices are wound coaxially and fed in time-phase quadrature, i.e., with proper phase increments with respect to each other against the common back plate. The goal of this multi-channel approach is to enable multi-channel RF methods (e.g. B1 shimming) to further mitigate B1+ field heterogeneity. This new coil exploits both near-field and far-field regimes, and allows for uniquely combining traveling-wave behavior through the overall coil wire structure while preserving near-field RF interaction between the inner side of the conducting elements and the imaged tissues. In other words, this design benefits from the congruence of two regimes: a far-field regime that concerns the current path over the wires of the coil and a near-field regime that is involved in local interactions between the sample and the coil wires. Furthermore, it has a potential for being used as a special purpose coil for parallel transmission. Multiple channels are utilized (4 and 8 in our prototype configurations) to enable all multi-transmit channel RF technology. This is expected to further expand the capability to mitigate B1+ heterogeneity.
The phantom data obtained at 7 T showed excellent consistency between numerical simulations and experimental results with 4- and 8-channel helix coils. The examples have shown, through simulations and measurements, that designed helical-antenna exciters, both long and short, provide good circular polarization, B1+ field strength and uniformity, flip angle, and FOV, as well as a diverse interleaved field/phase pattern due to the four or eight helices (four or eight channels), and thus capability for parallel imaging. The power efficiency is good and the amount of power delivered to the imaged phantom is sufficiently high for all experiments and MRI processing. Data are obtained for absolute B1 maps, g-factor, and GRAPPA X3 acceleration. Experimental and simulation results at 10.5 T have demonstrated the scalability and versatility of the coil design. In addition, the simulation results for the quadrifilar helical-antenna RF body coils at 3 T show that such coils of different lengths can, for instance, easily provide an excellent CP and highly uniform B1+-field within the MRI maximum FOV length of 50 cm, and even 100 cm. The novel 3-T MRI RF helix coil yields a remarkable improvement in the field uniformity in the longitudinal direction, for various phantoms, with comparable efficiency and SAR levels.
- More homogeneous field distribution
- Better signal-to-noise ratio
- Larger field of view
- Excellent coupling of the field/wave with the phantom
- Strong field penetration
- Appropriate Specific Absorption Rate (much below the acceptable and allowable levels) at every point
- Increased sensitivity and functionality, overall
- Not limited to any particular frequency or field strength
- Applications in research, pre-clinical, and clinical MRI systems
- Helical antennas can also be used as head coils, and coils for scanning arms, hands, wrists, legs, knees, ankles, etc.
Last updated: April 2020