G. Shajan

Purpose: A multinuclei imaging setup with the capability to acquire both sodium (²³Na) and proton (¹H) signals at 9.4 Tesla is presented. The main objective was to optimize coil performance at the ²³Na frequency while still having the ability to acquire satisfactory ¹H images.Methods: The setup consisted of a combination of three radio frequency (RF) coils arranged in three layers: the innermost layer was a 27‐channel ²³Na receive helmet which was surrounded by a four‐channel ²³Na transceiver array. The outer layer consisted of a four‐channel ¹H dipole array for B₀ shimming and anatomical localization. Transmit and receive performance of the ²³Na arrays was compared to a single‐tuned ²³Na birdcage resonator.Results: While the transmit efficiency of the ²³Na transceiver array was comparable to the birdcage, the ²³Na receive array provided substantial signal‐to‐noise ratio (SNR) gain near the surface and comparable SNR in the center. The utility of this customized setup was demonstrated by ²³Na images of excellent quality.Conclusion: High SNR, efficient transmit excitation and B₀ shimming capability can be achieved for ²³Na MRI at 9.4T using novel coil combination. This RF configuration is easily adaptable to other multinuclei applications at ultra high field (≥ 7T). Magn Reson Med 75:906–916, 2016.

Purpose: A multinuclei imaging setup with the capability to acquire both sodium (²³Na) and proton (¹H) signals at 9.4 Tesla is presented. The main objective was to optimize coil performance at the ²³Na frequency while still having the ability to acquire satisfactory ¹H images.Methods: The setup consisted of a combination of three radio frequency (RF) coils arranged in three layers: the innermost layer was a 27‐channel ²³Na receive helmet which was surrounded by a four‐channel ²³Na transceiver array. The outer layer consisted of a four‐channel ¹H dipole array for B₀ shimming and anatomical localization. Transmit and receive performance of the ²³Na arrays was compared to a single‐tuned ²³Na birdcage resonator.Results: While the transmit efficiency of the ²³Na transceiver array was comparable to the birdcage, the ²³Na receive array provided substantial signal‐to‐noise ratio (SNR) gain near the surface and comparable SNR in the center. The utility of this customized setup was demonstrated by ²³Na images of excellent quality.Conclusion: High SNR, efficient transmit excitation and B₀ shimming capability can be achieved for ²³Na MRI at 9.4T using novel coil combination. This RF configuration is easily adaptable to other multinuclei applications at ultra high field (≥ 7T). Magn Reson Med 75:906–916, 2016.

Purpose: Arranging transmit array elements in multiple rows provides an additional degree of freedom to correct B₁⁺ field inhomogeneities and to achieve whole‐brain excitation at ultrahigh field strengths. Receive arrays shaped to the contours of the anatomy increase the signal‐to‐noise ratio of the image. In this work, the advantages offered by the transmit and receive array techniques are combined for human brain imaging at 9.4 T.Methods: A 16‐element dual‐row transmit array and a 31‐element receive array were developed. Based on an accurate numerical model of the transmit array, the deposited power was calculated for different head sizes and positions. The influence of the receive array on the transmit field was characterized. Parallel imaging performance and signal‐to‐noise ratio of the receive array were evaluated.Results: On average, a two fold increase in signal‐to‐noise ratio was observed in the whole‐brain volume when compared with a 16‐channel elliptic microstrip transceiver array. The benefits of combining the two arrays, B₁⁺ shimming in three directions and high receive sensitivity, are demonstrated with high‐resolution in vivo images.Conclusion: The dual‐row transmit array provides whole‐brain coverage at 9.4 T, which, in combination with the helmet‐shaped receive array, is a valuable radio frequency configuration for ultra‐high field magnetic resonance imaging of the human brain. Magn Reson Med 71:870–879, 2014.

For human brain imaging at ultrahigh fields, the traveling wave concept can provide a more uniform B₁⁺ field over a larger field of view with improved patient comfort compared to conventional volume coils. It suffers, however, from limited transmit efficiency and receive sensitivity and is not readily applicable in systems where the radiofrequency shield is too narrow to allow for unattenuated wave propagation. Here, the near field of a capacitively adjustable patch antenna for excitation is combined with a receive‐only array at 9.4 T. The antenna is designed in compact size and placed in close proximity to the subject to improve the transmit efficiency in narrow bores. Experimental and numerical comparisons to conventional microstrip arrays reveal improved B₁⁺ homogeneity and longitudinal coverage, but at the cost of elevated local specific absorption rate. High‐resolution functional and anatomical images demonstrate the use of this setup for in vivo human brain imaging at 9.4 T. Magn Reson Med, 2013.

For MRI at 16.4T, with a proton Larmor frequency of 698 MHz, one of the principal RF engineering challenges is to generate a spatially homogeneous transmit field over a larger volume of interest for spin excitation. Constructing volume coils large enough to house a receive array along with the subject and to maintain the quadrature symmetry for different loading conditions is difficult at this frequency. This calls for new approaches to RF coil design for ultra‐high field MR systems. A remotely placed capacitively tunable patch antenna, which can easily be adjusted to different loading conditions, was used to generate a relatively homogeneous excitation field covering a large imaging volume with a transversal profile similar to that of a birdcage coil. Since it was placed in front of the animal, this created valuable free space in the narrow magnet bore around the subject for additional hardware. To enhance the reception sensitivity, the patch antenna was combined with an actively detunable 3‐channel receive coil array. In addition to increased SNR compared to a quadrature transceive surface coil, we were able to get high quality gradient echo and spin‐echo images covering the whole rat brain. Copyright © 2012 John Wiley & Sons, Ltd.

At the field strength of 9.4 T, the highest field currently available for human MRI, the wavelength of the MR signals is significantly shorter than the size of the examined structures. Even more than at 7 T, constructive and destructive interferences cause strong inhomogeneities of the B₁ field produced by a volume coil, causing shading over large parts of the image. Specialized radio frequency hardware and B₁ management methods are required to obtain high‐quality images that take full advantage of the high field strength. Here, the design and characteristics of a radio frequency front‐end especially developed for proton imaging at 9.4 T are presented. In addition to a 16‐channel transceiver array coil, capable of volume transmit mode and independent signal reception, it consists of custom built low noise preamplifiers and TR switches. Destructive interference patterns were eliminated, in virtually the entire brain, using a simple in situ radio frequency phase shimming technique. After mapping the B profile of each transmit channel, a numerical algorithm was used to calculate the appropriate transmit phase offsets needed to obtain a homogeneous excitation field over a user defined region. Between two and three phase settings are necessary to obtain homogeneous images over the entire brain. Magn Reson Med, 2011. &