Helmholtz-pair transmit coil with integrated receive array for high-resolution MRI of trabecular bone in the distal tibia at 7T

Abstract

A Helmholtz-pair local transmit RF coil with an integrated four-element receive array RF coil and foot immobilization platform was designed and constructed for imaging the distal tibia in a whole-body 7 T MRI scanner. Simulations and measurements of the B1 field distribution of the transmit coil are described, along with SAR considerations for operation at 7 T. Results of imaging the trabecular bone of three volunteers at 1.5 T, 3 T and 7 T are presented, using identical 1.5 T and 3 T versions of the 7 T four-element receive array. The spatially registered images reveal improved visibility for individual trabeculae and show average gains in SNR of 2.8x and 4.9x for imaging at 7 T compared to 3 T and 1.5 T, respectively. The results thus display an approximately linear dependence of SNR with field strength and enable the practical utility of 7 T scanners for micro-MRI of trabecular bone.

Internal Three-Dimensional Strains in Human Intervertebral Discs Under Axial Compression Quantified Noninvasively by Magnetic Resonance Imaging and Image Registration

“…1(a)) was constructed to interface with an Instron 8874 for load application. The loading frame incorporated locking bolts to maintain axial compression applied using the Instron, a sliding tank to maintain hydration and was designed to integrate with a two-piece RF coil (Helmholtz transmit, curved 4-channel receive array) [24] in a 7T whole-body MRI scanner (Magnetom, Siemens Medical Solutions) ( Fig. 1(b)).…”

Evaluation of intervertebral disc cartilaginous endplate structure using magnetic resonance imaging

“…For protocol optimization, the flip angle and TR that providing the best optimal NP-CEP contrast were selected using the analytical model simulation. All ex vivo imaging with specimens was done in a Siemens Magnetom 7T scanner (Siemens Medical Solutions, Erlangen, Germany) using a 4-channel ankle coil (Insight MRI) [35]. Due to the thinness of the endplate, a voxel size of 200 lm 3 was chosen.…”

A new RF transmit coil for foot and ankle imaging at 7T MRI

“…While this shows increase of power by a factor of 8.09 with the TTT design (in order to achieve the same average B 1 + ), the peak SAR (which would approach the FDA/IEC limits before the average SAR [40]) is 13.91 times lower, and therefore the SAR efficiency is substantially better with the TTT coil. While Wright et al [16] developed an innovative Helmholtz pair transmit-only with a 4-channel Rx-only array, no metric (e.g., B 1 + mean or peak, and/or SAR in the foot model simulations) that can be used to compare directly with this work was reported. Furthermore, the homogenous cylindrical phantoms (relative permittivity of ~78 and a conductivity of ~0.66 S/m) used in [16] are not comparable to the phantom utilized in this work (in the simulations and experiments) as it has a relative permittivity of 39 and a conductivity of 0.49 S/m, representing the average values of the lower leg of the Duke model.…”

“…While Wright et al [16] developed an innovative Helmholtz pair transmit-only with a 4-channel Rx-only array, no metric (e.g., B 1 + mean or peak, and/or SAR in the foot model simulations) that can be used to compare directly with this work was reported. Furthermore, the homogenous cylindrical phantoms (relative permittivity of ~78 and a conductivity of ~0.66 S/m) used in [16] are not comparable to the phantom utilized in this work (in the simulations and experiments) as it has a relative permittivity of 39 and a conductivity of 0.49 S/m, representing the average values of the lower leg of the Duke model.…”

“…To the best of our knowledge, there are few research groups that have developed RF coils specific for 7T imaging of the foot/ankle [16,17], as most of the studies use commercial knee coils [7,18–22] or head coils [23–26] for imaging these parts of the body. This is not optimal because the subject may be asked to stay in an uncomfortable position (e.g., foot in a high angle) and the coil may not be suitable and/or optimal for all foot sizes.…”