Application of an integrated radio-frequency/shim coil technology for signal recovery in fMRI.

PURPOSE: Gradient-echo echo-planar imaging (EPI), which is typically used for blood oxygenation level-dependent (BOLD) functional MRI (fMRI), suffers from distortions and signal loss caused by localized B0 inhomogeneities. Such artifacts cannot be effectively corrected for with the low-order spherical harmonic (SH) shim coils available on most scanners. The integrated parallel reception, excitation, and shimming (iPRES) coil technology allows radiofrequency (RF) and direct currents to flow on each coil element, enabling imaging and localized B0 shimming with one coil array. iPRES was previously used to correct for distortions in spin-echo EPI and is further developed here to also recover signal loss in gradient-echo EPI. METHODS: The cost function in the shim optimization, which typically uses a single term representing the B0 inhomogeneity, was modified to include a second term representing the signal loss, with an adjustable weight to optimize the trade-off between distortion correction and signal recovery. Simulations and experiments were performed to investigate the shimming performance. RESULTS: Slice-optimized shimming with iPRES and the proposed cost function substantially reduced the signal loss in the inferior frontal and temporal brain regions compared to shimming with iPRES and the original cost function or 2nd -order SH shimming with either cost function. In breath-holding fMRI experiments, the ΔB0 and signal loss root-mean-square errors decreased by -34.3% and -56.2%, whereas the EPI signal intensity and number of activated voxels increased by 60.3% and 174.0% in the inferior frontal brain region. CONCLUSION: iPRES can recover signal loss in gradient-echo EPI, which is expected to improve BOLD fMRI studies in brain regions suffering from signal loss.

Numerical simulations of an integrated radio-frequency/wireless coil design for simultaneous acquisition and wireless transfer of magnetic resonance imaging data.

Objective.A novel magnetic resonance imaging (MRI) radio-frequency (RF) coil design, termed an integrated RF/wireless (iRFW) coil design, can simultaneously perform MRI signal reception and far-field wireless data transfer with the same coil conductors between the coil in the scanner bore and an access point (AP) on the scanner room wall. The objective of this work is to optimize the design inside the scanner bore to provide a link budget between the coil and the AP for the wireless transmission of MRI data.Approach.Electromagnetic simulations were performed at the Larmor frequency of a 3T scanner and in a WiFi wireless communication band to optimize the radius and position of an iRFW coil located near the head of a human model inside the scanner bore, which were validated by performing both imaging and wireless experiments.Main Results.The simulated iRFW coil with a 40 mm radius positioned near the model forehead provided: a signal-to-noise ratio (SNR) comparable to that of a traditional RF coil with the same radius and position, a power absorbed by the human model within regulatory limits, and a gain pattern in the scanner bore resulting in a link budget of 51.1 dB between the coil and an AP located behind the scanner 3 m from the isocenter, which would be sufficient to wirelessly transfer MRI data acquired with a 16-channel coil array. The SNR, gain pattern, and link budget for initial simulations were validated by experimental measurements in an MRI scanner and anechoic chamber to provide confidence in this methodology. These results show that the iRFW coil design must be optimized within the scanner bore for the wireless transfer of MRI data.Significance.The MRI RF coil array coaxial cable assembly connected to the scanner increases patient setup time, can present a serious burn risk to patients and is an obstacle to the development of the next generation of lightweight, flexible or wearable coil arrays that provide an improved coil sensitivity for imaging. Significantly, the RF coaxial cables and corresponding receive chain electronics can be removed from within the scanner by integrating the iRFW coil design into an array for the wireless transmission of MRI data outside of the bore.