Design and application of combined 8-channel transmit and 10-channel receive arrays and radiofrequency shimming for 7-t shoulder magnetic resonance imaging

Magnetic resonance imaging (MRI) of the shoulder has become the most valuable imaging modality for the evaluation of diseases involving the rotator cuff, articular cartilage, and labroligamentous structures. High signal-to-noise ratio (SNR) is important for the visualization of these structures, many of which are submillimeter in dimension and are only depicted with sufficient contrast on high-resolution images. Their location far from the body periphery and the appeal of applying parallel imaging to reduce scan time underline the need for high baseline SNR. Compared with shoulder MRI at 1.5 T, shoulder MRI at 3 T has been shown to provide greater spatial resolution and superior image quality, resulting in greater diagnostic confidence.1–3 Reports on 7-T musculoskeletal MRI of the knee, ankle, and wrist have demonstrated substantial SNR improvement in the cartilage, synovial fluid, and trabecular bone with superior resolution over 3 T.4–6 Magnetic resonance imaging of the shoulder at 7-T offers similar potential clinical advantages. Greater SNR at 7-T can be translated into higher spatial resolution, which could potentially improve depiction of the intricate shoulder anatomy and its pathology, which, at conventional field strength, is frequently manifested by subtle, difficult-to-visualize discontinuity or surface irregularity in these tissues (eg, linear hyperintense fluid signal within the labrum indicating a labral tear, or fibrillation of the cartilage surface indicating early cartilage degeneration). High-field imaging of the knee, ankle, and wrist has been successful in part because of convenient encircling coil structures afforded by their anatomic location and small size. For these tissues, reasonably homogeneous radiofrequency (RF) excitation (B1+) can be achieved using a standard azimuthally symmetric, fully surrounding transmit coil such as a birdcage or transverse electromagnetic coil. In contrast to these anatomic targets, high-field shoulder imaging poses unique RF-related challenges7,8 because of its asymmetric location in the body, which makes a fully surrounding transmit coil impractical. This restriction, along with the shoulder’s relatively large volume (compared with the RF wavelength in tissue) and associated high-field RF interference effects9,10 as well as a wide range of subject sizes, necessitates a custom transmit coil and an RF shim scheme to provide adequate B1+ efficiency and homogeneity. High-field shoulder imaging is further complicated by B1+ “twisting,”11–15 where conventional surface coils exhibit transmit and receive profiles that twist in opposite directions away from the geometric center of the element. B1+ twisting implies distinct B1+ patterns in the left and right shoulders, necessitating distinct RF shims. In this study, we describe the design of an 8-channel transmit array and corresponding RF shim methods for the left and right shoulders at 7-T. We further describe the design and implementation of a dedicated 10-channel receive array that functions inside the transmit array. Both transmit and receive arrays are mechanically flexible to minimize coil-tissue distance over a wide range of subjects. Further, the receive array is constructed on a close-fitting domed structure that conforms to the shoulder anatomy.