Arrays of radiofrequency coils are widely used in magnetic resonance imaging to achieve high signal-to-noise ratios and flexible volume coverage, to accelerate scans using parallel reception, and to mitigate field non-uniformity using parallel transmission. However, conventional coil arrays require complex decoupling technologies to reduce electromagnetic coupling between coil elements, which would otherwise amplify noise and limit transmitted power. Here we report a novel self-decoupled RF coil design with a simple structure that requires only an intentional redistribution of electrical impedances around the length of the coil loop. We show that self-decoupled coils achieve high inter-coil isolation between adjacent and non-adjacent elements of loop arrays and mixed arrays of loops and dipoles. Self-decoupled coils are also robust to coil separation, making them attractive for size-adjustable and flexible coil arrays.
Purpose To design array-compressed parallel transmit radiofrequency (RF) pulses and compare them to pulses designed with existing transmit array compression strategies. Theory and Methods Array-compressed parallel RF pulse design is proposed as the joint optimization of a matrix of complex-valued compression weights that relate a full-channel physical array to a reduced-channel virtual array, along with a set of RF pulses for the virtual array. In this way, the physics of the RF pulse application determine the coil combination weights. Array-compressed pulse design algorithms are described for four parallel transmit applications: accelerated 2D spiral excitation, multislice RF shimming, small-tip-angle kT-points excitation, and slice-selective spokes refocusing. Array-compressed designs are compared in simulations and an experiment to pulses designed using four existing array compression strategies. Results In all cases, array-compressed pulses achieved the lowest root-mean-square excitation error among the array compression approaches. Low errors were generally achieved without increasing root-mean-square RF amplitudes or maximum local 10-gram specific absorption rate. Leave-one-out multisubject shimming simulations demonstrated that array-compressed RF shimming can identify useful fixed coil combination weights that perform well across a population. Conclusion Array-compressed pulse design jointly identifies the transmit coil array compression weights and RF pulses that perform best for a specific parallel excitation application.
In designing large-sized volume type phased array coils for human head imaging at ultrahigh fields, e.g., 7T, minimizing electromagnetic coupling among array elements is technically challenging. A new decoupling method based on induced current compensation or elimination (ICE) for a microstrip line planar array has recently been proposed. In this study, an eight-channel transmit/receive volume array with ICE-decoupled loop elements was built and investigated to demonstrate its feasibility and robustness for human head imaging at 7T. Isolation between adjacent loop elements was better than - 25 dB with a human head load. The worst-case of the isolation between all of the elements was about - 17.5 dB. All of the MRI experiments were performed on a 7T whole-body human MR scanner. Images of the phantom and human head were acquired and g-factor maps were measured and calculated to evaluate the performance of the coil array. Compared with the conventional capacitively decoupled array, the ICE-decoupled array demonstrated improved parallel imaging ability and had a higher SNR. The experimental results indicate that the transceiver array design with ICE decoupling technique might be a promising solution to designing high performance transmit/receive coil arrays for human head imaging at ultrahigh fields.
Radio-frequency coil arrays using dipole antenna technique have been recently applied for ultrahigh field magnetic resonance (MR) imaging to obtain the better signal–noise-ratio (SNR) gain at the deep area of human tissues. However, the unique structure of dipole antennas makes it challenging to achieve sufficient electromagnetic decoupling among the dipole antenna elements. Currently, there is no decoupling methods proposed for dipole antenna arrays in MR imaging. The recently developed magnetic wall (MW) or induced current elimination decoupling technique has demonstrated its feasibility and robustness in designing microstrip transmission line arrays, L/C loop arrays and monopole arrays. In this study, we aim to investigate the possibility and performance of MW decoupling technique in dipole arrays for MR imaging at the ultrahigh field of 7T. To achieve this goal, a two-channel MW decoupled dipole array was designed, constructed and analyzed experimentally through bench test and MR imaging. Electromagnetic isolation between the two dipole elements was improved from about −3.6 dB (without any decoupling treatments) to −16.5 dB by using the MW decoupling method. MR images acquired from a water phantom using the MW decoupled dipole array and the geometry factor maps were measured, calculated and compared with those acquired using the dipole array without decoupling treatments. The MW decoupled dipole array demonstrated well-defined image profiles from each element and had better geometry factor over the array without decoupling treatments. The experimental results indicate that the MW decoupling technique might be a promising solution to reducing the electromagnetic coupling of dipole arrays in ultrahigh field MRI, consequently improving their performance in SNR and parallel imaging.
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