An open 8‐channel parallel transmission coil for static and dynamic 7T MRI of the knee and ankle joints at multiple postures
The development of RF coil arrays at UHF with parallel transmission (pTx) capability provides a promising approach to balance the three competing factors of inhomogeneous excitation profiles, local SAR restrictions and limited peak RF power (typically 8 or 16 kW at the RF power amplifier level) 7. Specifically, pTx techniques 8 require independent control of the simultaneous transmission channels to tailor the RF waveforms and achieve a targeted excitation efficiently. B1 shimming 10 optimizes the magnitudes and phases of the otherwise identical RF waveform from all channels and provides solutions to the aforementioned issues 7. When combined with these techniques, dedicated multichannel array coils with independent parallel channels offer an attractive solution for maximizing the performance of UHF MRI in humans.
Dedicated UHF RF coils for MRI of the musculoskeletal system have the capacity for superior signal‐to‐noise ratio, enabling higher spatial resolution to facilitate improved assessment of small structures with oblique orientation 13. In addition to providing conventional static images with excellent contrast between muscles, menisci, ligaments, and tendons 15, RF coils with innovative design offer avenues for dynamic (kinematic) imaging to investigate joints positioned in multiple postures and/or during active movement. Dynamic musculoskeletal imaging has been performed at lower fields 17, providing kinematic information not intrinsically available in static scans 18. Specifically, it allows examination of in vivo kinematics and interactions between tissues to assess aspects such as impingement, mal‐tracking, or deformation of structures within and around complex joints such as the knee and ankle.
At present, commercial RF coils for musculoskeletal imaging at 7T are mostly limited to the knee and typically without pTx capability. Existing examples include the 1‐Tx/28‐Rx knee coil from Quality Electrodynamics (QED; Mayfield Village, Ohio, USA) 19, the transceive quadrature knee coils from In‐Vivo Corporation (Gainesville, Florida, USA) 6 and Nova Medical (Wilmington, Massachusetts, USA) 22 and the transceive quadrature 23Na knee coil from Rapid MR International (Columbus, Ohio, USA) 24. Several important custom knee pTx coil developments have been implemented to improve transmission homogeneity, including a two‐channel transmit coil in combination with the QED coil for reception at 7T 19 and a four‐channel proton birdcage sodium transceiver array for multinuclei imaging 25. A multipurpose 16‐channel pTx RF array has been developed by attaching microstrip transmission line elements to a malleable canvas, capable of conformable imaging of the knee (with eight active channels) 26. However, these custom 7T knee pTx coils cannot readily accommodate flexed joint postures for either static or dynamic imaging. To assist kinematic imaging, a “stretchable” receive array at 3T has been presented 27. However, this technique may not be suitable for transceive array applications at 7T due to higher resistive loss and variations of coil loading/coupling when stretched. A custom eight‐channel pTx coil 28 has been developed for ankle imaging at 7T with an absence of dedicated commercial coils for this anatomical region.