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Tissue‐mimicking reference phantoms are indispensable for the development and optimization of magnetic resonance (MR) measurement sequences. Phantoms have greatest utility when they mimic the MR signals arising from tissue physiology; however, many of the properties underlying these signals, including tissue relaxation characteristics, can vary as a function of magnetic field strength. There has been renewed interest in magnetic resonance imaging (MRI) at field strengths less than 1 T, and phantoms developed for higher field strengths may not be physiologically relevant at these lower fields. This work focuses on developing materials with specific relaxation properties for lower magnetic field strengths. Specifically, we developed recipes that can be used to create synthetic samples for target nuclear magnetic resonance relaxation values for fields between 0.0065 and 0.55 T. T1$$ {T}_1 $$ and T2$$ {T}_2 $$ mixing models for agarose‐based gels doped with a paramagnetic salt (one of CuSO4, GdCl3, MnCl2, or NiCl2) were created using relaxation measurements of synthetic gel samples at 0.0065, 0.064, and 0.55 T. Measurements were evaluated for variability with respect to measurement repeatability and changing synthesis protocol or laboratory temperature. The mixing models were used to identify formulations of agarose and salt composition to approximately mimic the relaxation times of five neurological tissues (blood, cerebrospinal fluid, fat, gray matter, and white matter) at 0.0065, 0.0475, 0.05, 0.064, and 0.55 T. These mimic sample formulations were measured at each field strength. Of these samples, the GdCl3 and NiCl2 measurements were closest to the target tissue relaxation times. The GdCl3 or NiCl2 mixing model recipes are recommended for creating target relaxation samples below 0.55 T. This work can help development of MRI methods and applications for low‐field systems and applications. [ABSTRACT FROM AUTHOR]

Purpose: To test an integrated "AC/DC" array approach at 7T, where B0 inhomogeneity poses an obstacle for functional imaging, diffusion‐weighted MRI, MR spectroscopy, and other applications. Methods: A close‐fitting 7T 31‐channel (31‐ch) brain array was constructed and tested using combined Rx and ΔB0 shim channels driven by a set of rapidly switchable current amplifiers. The coil was compared to a shape‐matched 31‐ch reference receive‐only array for RF safety, signal‐to‐noise ratio (SNR), and inter‐element noise correlation. We characterize the coil array's ability to provide global and dynamic (slice‐optimized) shimming using ΔB0 field maps and echo planar imaging (EPI) acquisitions. Results: The SNR and average noise correlation were similar to the 31‐ch reference array. Global and slice‐optimized shimming provide 11% and 40% improvements respectively compared to baseline second‐order spherical harmonic shimming. Birdcage transmit coil efficiency was similar for the reference and AC/DC array setups. Conclusion: Adding ΔB0 shim capability to a 31‐ch 7T receive array can significantly boost 7T brain B0 homogeneity without sacrificing the array's rdiofrequency performance, potentially improving ultra‐high field neuroimaging applications that are vulnerable to off‐resonance effects. [ABSTRACT FROM AUTHOR]

Purpose: Point‐of‐care MRI requires operation outside of Faraday shielded rooms normally used to block image‐degrading electromagnetic interference (EMI). To address this, we introduce the EDITER method (External Dynamic InTerference Estimation and Removal), an external sensor‐based method to retrospectively remove image artifacts from time‐varying external interference sources. Theory and Methods: The method acquires data from multiple EMI detectors (tuned receive coils as well as untuned electrodes placed on the body) simultaneously with the primary MR coil during and between image data acquisition. We calculate impulse response functions dynamically that map the data from the detectors to the time varying artifacts then remove the transformed detected EMI from the MR data. Performance of the EDITER algorithm was assessed in phantom and in vivo imaging experiments in an 80 mT portable brain MRI in a controlled EMI environment and with an open 47.5 mT MRI scanner in an uncontrolled EMI setting. Results: In the controlled setting, the effectiveness of the EDITER technique was demonstrated for specific types of introduced EMI sources with up to a 97% reduction of structured EMI and up to 76% reduction of broadband EMI in phantom experiments. In the uncontrolled EMI experiments, we demonstrate EMI reductions of up to 99% using an electrode and pick‐up coil in vivo. We demonstrate up to a nine‐fold improvement in image SNR with the method. Conclusion: The EDITER technique is a flexible and robust method to improve image quality in portable MRI systems with minimal passive shielding and could reduce the reliance of MRI on shielded rooms and allow for truly portable MRI. [ABSTRACT FROM AUTHOR]

Purpose: To test an integrated "AC/DC" array approach at 7T, where B 0 inhomogeneity poses an obstacle for functional imaging, diffusion-weighted MRI, MR spectroscopy, and other applications., Methods: A close-fitting 7T 31-channel (31-ch) brain array was constructed and tested using combined Rx and ΔB 0 shim channels driven by a set of rapidly switchable current amplifiers. The coil was compared to a shape-matched 31-ch reference receive-only array for RF safety, signal-to-noise ratio (SNR), and inter-element noise correlation. We characterize the coil array's ability to provide global and dynamic (slice-optimized) shimming using ΔB 0 field maps and echo planar imaging (EPI) acquisitions., Results: The SNR and average noise correlation were similar to the 31-ch reference array. Global and slice-optimized shimming provide 11% and 40% improvements respectively compared to baseline second-order spherical harmonic shimming. Birdcage transmit coil efficiency was similar for the reference and AC/DC array setups., Conclusion: Adding ΔB 0 shim capability to a 31-ch 7T receive array can significantly boost 7T brain B 0 homogeneity without sacrificing the array's rdiofrequency performance, potentially improving ultra-high field neuroimaging applications that are vulnerable to off-resonance effects., (© 2021 International Society for Magnetic Resonance in Medicine.)