Strategies for Artifact Correction and Motion Monitoring in MRI Through Innovations in Radiofrequency Coil Design
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In the reconstruction of magnetic resonance (MR) images, two important assumptions that are made are that the main magnetic field B_0 is homogeneous and that there is no bulk movement of the subject. This work proposes various strategies using innovations in radio-frequency (RF) coil design to address the problems that arise when these assumptions are broken.
Typically, B_0 inhomogeneities are caused by susceptibility differences at air/tissue interfaces and result in image artifacts such as geometric distortions and signal loss. B_0 inhomogeneities can be corrected through a process called shimming, which generates magnetic field patterns that have a similar spatial distribution but are opposite in polarity. 2\nd order spherical harmonic shimming is used on most clinical scanners, however, it is unable to correct highly localized \B_0 inhomogeneities found in the inferior frontal and temporal brain regions. By integrating a direct current (DC) path onto an RF surface coil, thereby allowing both DC current and an RF current at the Larmor frequency to flow on the same coil, localized B_0 shimming and MR imaging can be performed with the same coil array. This technology, referred to as an integrated parallel reception, excitation, and shimming (iPRES) coil array, was previously used to correct for distortions in spin-echo echo-planar imaging (EPI) and is further developed here to also recover signal loss in gradient-echo EPI, which is used for blood-oxygenation level-dependent (BOLD) functional MRI (fMRI). This was done through modification of the cost function used in the shim optimization, which typically uses a single term representing the B_0 inhomogeneity, 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. 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 spherical harmonic shimming with either cost function. In breath-holding BOLD fMRI experiments, the ∆B_0 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.
In addition to the integration of DC currents, currents at a Wi-Fi frequency can be integrated onto RF coils as well to perform simultaneous MR imaging and wireless communication. This technology, called an integrated RF/wireless (iRFW) coil, has previously been used for wireless respiratory monitoring with a respiratory belt or to wirelessly control shim currents, and is further developed here to wirelessly transmit ultrasound data acquired with an organ-configuration motion (OCM) sensor. OCM sensors are small, ultrasound based sensors that attach to the skin, move with the subject, and provide information about internal physiological motion. They can be used to create synthesized MR images through machine learning techniques and to monitor patient motion, and ultimately can be used to improve various treatments and therapies. However, they currently require electronics that must remain outside of the scanner as well as various wired connections to those electronics, which limits their portability. By making the OCM sensors wireless and their associated electronics MR-compatible, setup time is decreased and the OCM sensor can accompany the patient throughout the hospital while monitoring motion. This was done by developing MR-compatible, battery-powered electronics to trigger the ultrasound sensor, digitize the received ultrasound signal, modulate it to a Wi-Fi frequency, and wirelessly transmit it via the iRFW coil to a nearby access point (AP). Phantom experiments were performed to ensure that 1) the MR data quality was unaffected with and without wirelessly transmitting ultrasound data and that 2) the ultrasound data was unaffected with and without acquiring MR images. In vivo experiments were performed to demonstrate the portability of the wireless ultrasound device and its ability to monitor motion.
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