7T Transmit Four-Channel Receive Array for High-Resolution MRI of Trabecular Bone in the Distal Tibia

Introduction Osteoporosis is a disease involving loss of trabecular bone, and it has been shown that the quality of the trabecular network is an important determinant of a bone’s strength and resistance to fracture (1). Micro-MRI at intermediate field strengths (1.5 and 3 T) has been used to obtain three-dimensional images of trabeculae in bones of the appendicular skeleton such as in the distal radius and tibia (2). While such image data, combined with specialized algorithms for quantifying the trabecular network topology, have proven to be powerfully predictive of fracture risk and osteoporosis treatment efficacy in humans, the accuracy of these quantitative methods can be greatly benefited by increases in SNR. With the advent of new whole-body 7 T research scanners, we have therefore explored the potential of performing similar methods at 7 T. Methods Coil design: The coil was designed to be interfaced with a Siemens 7 T whole-body MRI scanner, operating at 297.2 MHz. Since there is no bodytransmit coil, a Helmholtz pair local transmit coil with a decoupled 4-element phased array receive coil was chosen based on success with an identical 3 T version of the 4-element array. Simulations were performed using custom-written software to estimate the B1 field maps of both transmit and receive coils. The Helmholtz pair was found to require a shield to reduce radiation losses and interactions with the bore of the magnet. The shield was further slotted to reduce possible effects of gradient-induced eddy currents. The final assembly was compact and provided intrinsic immobilization for the foot (Fig. 1).