Rat brain MRI at 16.4T using a capacitively tunable patch antenna in combination with a receive array.

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.