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Parallel imaging is one of the key MRI technologies that allow reduction of image acquisition time. However, the parallel imaging reconstruction commonly leads to a signal‐to‐noise ratio (SNR) drop evaluated using a so‐called geometrical factor (g‐factor). The g‐factor is minimized by increasing the number of array elements and their spatial diversity. At the same time, increasing the element count requires a decrease in their size. This may lead to insufficient coil loading, an increase in the relative noise contribution from the RF coil itself, and hence SNR reduction. Previously, instead of increasing the channel number, we introduced the concept of electronically switchable time‐varying sensitivities, which was shown to improve parallel imaging performance. In this approach, each reconfigurable receive element supports two spatially distinct sensitivity profiles. In this work, we developed and evaluated a novel eight‐element human head receive‐only reconfigurable coaxial dipole array for human head imaging at 9.4 T. In contrast to the previously reported reconfigurable dipole array, the new design does not include direct current (DC) control wires connected directly to the dipoles. The coaxial cable itself is used to deliver DC voltage to the PIN diodes located at the ends of the antennas. Thus, the novel reconfigurable coaxial dipole design opens a way to scale the dynamic parallel imaging up to a realistic number of channels, that is, 32 and above. The novel array was optimized and tested experimentally, including in vivo studies. It was found that dynamic sensitivity switching provided an 8% lower mean and 33% lower maximum g‐factor (for Ry × Rz = 2 × 2 acceleration) compared with conventional static sensitivities. [ABSTRACT FROM AUTHOR]

Homogeneity and longitudinal coverage of transmit (Tx) human head RF coils at ultrahigh field (UHF, ≥7 T) can be improved by 3D RF shimming, which requires using multi‐row Tx arrays. Examples of 3D RF shimming using double‐row UHF loop transceiver (TxRx) and Tx arrays have been described previously. Dipole antennas provide unique simplicity and robustness while offering comparable Tx efficiency and signal‐to‐noise ratio to conventional loop designs. Single‐row Tx and TxRx human head UHF dipole arrays have been previously described by multiple groups. Recently, we developed a novel type of dipole antenna, a folded‐end dipole, and presented single‐row eight‐element array prototypes for human head imaging at 7 and 9.4 T. These studies have shown that the novel antenna design can improve the longitudinal coverage and minimize peak local specific absorption rate (SAR) as compared with common unfolded dipoles. In this work, we developed, constructed, and evaluated a 16‐element double‐row TxRx folded‐end dipole array for human head imaging at 9.4 T. To minimize cross‐talk between neighboring dipoles located in different rows, we used transformer decoupling, which decreased coupling to a level below −20 dB. The developed array design was demonstrated to be capable of 3D static RF shimming and can be potentially used for dynamic shimming using parallel transmission. For optimal phase shifts between the rows, the array provides 11% higher SAR efficiency and 18% higher homogeneity than a folded‐end dipole single‐row array of the same length. The design also offers a substantially simpler and more robust alternative to the common double‐row loop array with about 10% higher SAR efficiency and better longitudinal coverage. [ABSTRACT FROM AUTHOR]

Purpose: Development of a novel quadrature inductively driven transceive wireless coil for breast MRI at 1.5 T. Methods: A quadrature wireless coil (HHMM‐coil) design has been developed as a combination of two linearly polarized coils: a pair of 'metasolenoid' coils (MM‐coil) and a pair of Helmholtz‐type coils (HH‐coil). The MM‐coil consisted of an array of split‐loop resonators. The HH‐coil design included two electrically connected flat spirals. All the wireless coils were coupled to a whole‐body birdcage coil. The HHMM‐coil was studied and compared to the linear coils in terms of transmit and SAR efficiencies via numerical simulations. A prototype of HHMM‐coil was built and tested on a 1.5 T scanner in a phantom and healthy volunteer. We also proposed an extended design of the HHMM‐coil and compared its performance to a dedicated breast array. Results: Numerical simulations of the HHMM‐coil with a female voxel model have shown more than a 2.5‐fold increase in transmit efficiency and a 1.7‐fold enhancement of SAR efficiency compared to the linearly polarized coils. Phantom and in vivo imaging showed good agreement with the numerical simulations. Moreover, the HHMM‐coil provided good image quality, visualizing all areas of interest similar to a multichannel breast array with a 32% reduction in signal‐to‐noise ratio. Conclusion: The proposed quadrature HHMM‐coil allows the B1+$$ {\mathrm{B}}_1^{+} $$‐field to be significantly better focused in the region‐of‐interest compared to the linearly polarized coils. Thus, the HHMM‐coil provides high‐quality breast imaging on a 1.5 T scanner using a whole‐body birdcage coil for transmit and receive. [ABSTRACT FROM AUTHOR]

MRI at ultra‐high field (UHF, ≥7 T) provides a natural strategy for improving the quality of X‐nucleus magnetic resonance spectroscopy and imaging due to the intrinsic benefit of increased signal‐to‐noise ratio. Considering that RF coils require both local transmission and reception at UHF, the designs of double‐tuned coils, which often consist of several layers of transmit and receive resonant elements, become quite complex. A few years ago, a new type of RF coil, ie a dipole antenna, was developed and used for human body and head imaging at UHF. Due to the mechanical and electrical simplicity of dipole antennas, combining an X‐nucleus surface loop array with 1H dipoles can substantially simplify the design of a double‐tuned UHF human head array coil. Recently, we developed a novel bent folded‐end dipole transceiver array for human head imaging at 9.4 T. The new eight‐element dipole array demonstrated full brain coverage, and transmit efficiency comparable to that of the substantially more complex 16‐element surface loop array. In this work, we developed, constructed and evaluated a double‐tuned 13C/1H human head 9.4 T array consisting of eight 13C transceiver surface loops and eight 1H transceiver bent folded‐end dipole antennas all placed in a single layer. We showed that interaction between loops and dipoles can be minimized by placing four 1H traps into each 13C loop. The presented double‐tuned RF array coil substantially simplifies the design as compared with the common double‐tuned surface loop arrays. At the same time, the coil demonstrated an improved 1H longitudinal coverage and good transmit efficiency. [ABSTRACT FROM AUTHOR]