Radio Frequency Coil Design for Ultra High Field Magnetic Resonance Imaging

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Multichannel radio frequency (RF) coil arrays, composed of multiple resonant elements, are a critical component in magnetic resonance imaging (MRI) acquisition. Parallel transmit (pTx) arrays provide individual sensitivity profiles that when used in concert with optimized gradient and RF waveforms can accelerate the traversal of excitation kspace. This principle can be used to accelerate multidimensional selective excitation, perform B1+ or RF shimming to overcome inhomogeneity at ultrahigh field (UHF), or reduce specific absorption rate (SAR). Similarly, parallel receive (pRx) arrays exploit the locally high signal-to-noise ratio (SNR) of surface coils to the MRI signal (B1-) and extend it across a full field of view while simultaneously performing spatial encoding, utilized in accelerated imaging.    The electromagnetic fields responsible for exciting spins during RF transmission, as well as receiving signal from the transverse magnetization post-excitation, transition from purely reactive nearfield interaction towards a mixture of both near and far fields as the main magnetic field strength increases. Due to this, electric dipole antennas are finding increasing utility at UHF when compared to more conventional RF loop elements and combinations of loop and dipole array elements into a single RF coil assembly are expected to show performance gains in SAR efficiency per unit B1+ and SNR.    The evolution towards using electric dipole elements and/or combining dissimilar elements present a technical challenge. The radiation patterns produced from the electric dipole, in conjunction with its geometry, are limited by the applicability of conventional decoupling methods. This is best demonstrated in densely populated dipole receive arrays where unmitigated coupling and enhanced noise correlations degrade measured SNR, regardless of an increased sensitivity to the sample. We have developed a generalized decoupling approach that allows complex RF arrays of mixed element types and variable coupling coefficients to be efficiently decoupled with a ladder filter approach. Using this approach, we will simulate, design and build multichannel transmit  arrays that combine loops and dipoles in a manner that captures as much of the transmit efficiency as possible for human head imaging. Once the hardware is optimized to generate the best B1+ homogeneity via RF shimming, RF pulses will be used to remove the residual inhomogeneity. Real-time pulse optimization has been a challenge, so compromised approaches are currently used. Using B1+ and static magnetic field maps from subjects and from electromagnetic simulations, we will use machine learning approaches to help design the optimal solutions in real-time that take into account RF homogeneity with the added constraints of SAR.    In combination, these approaches will surmount the remaining barriers to the adoption of multichannel pTx for 7 T scanners in clinical and basic neuroscience settings.