Shajan Gunamony

Radio Frequency Coils for Ultra-High Field MRI

Volume resonators, highly successful at clinical field strength, cannot be used at ultra-high field (UHF) because of the inhomogeneous distribution of the RF field in tissue caused by the shorter wavelength. Hence, at UHF, transmit array coils are an essential tool to apply static or dynamic parallel transmission methods to mitigate the inhomogeneous spatial distribution of the RF field produced by the coil.

It is equally important to optimize the receive performance of the setup by using receive array coils designed to maximize the signal to noise ratio (SNR) and improved parallel imaging capability.

I am involved in the development of RF coils and RF hardware for the different MRI scanners in the institute. The main projects are:

• Design of a dual-row transmit array in combination with a receive array for optimum image SNR from the whole brain at 9.4 T.

• A ²³Na MRI setup designed to maximize the ²³Na SNR. The setup also has B₀ shimming capability.

• Shaped receive arrays for simultaneous electrophysiology and fMRI.

16-channel transmit and 31-channel receive array

Arranging transmit array elements in multiple rows provides an additional degree of freedom to correct B₁⁺ field in-homogeneities and to achieve whole-brain excitation at frequencies as high as 400 MHz. Receive arrays shaped to the contours of the human anatomy increase the signal-to-noise ratio (SNR) of the image. We developed a cutting-edge RF setup that exploits the benefits of UHF brain MRI: a dual-row transmit array with 16 mutually decoupled transmit elements was combined with a 31-channel tight-fitting receive array to achieve B₁⁺ field control and high-SNR parallel signal reception in the entire brain even at 9.4 T.

Multi-nuclei coils

UHF MR scanners constitute a significant benefit for non-proton imaging because the SNR has been found to scale at least linearly with the static magnetic field B₀. A multi-nuclei imaging setup with the capability to acquire both ²³Na and proton (¹H) signals at 9.4 T was developed. It consists of a combination of three RF coil arrays to cover the entire human brain. The main objective was to optimize coil performance at the ²³Na frequency while still having the ability to also acquire whole-brain ¹H images for B₀ shimming and anatomical localization.

The three layered coil design method originally proposed by our group for ²³Na MRI was adapted for phosphorus (³¹P) spectroscopy and ¹H imaging to perform ³¹P 3D chemical shift imaging (CSI) of the human brain at 9.4 T.

Shaped Arrays for non-human Primate Research

Simultaneous investigation of electrophysiology and fMRI in non-human primates presents several challenges on RF coil design. In addition to homogeneous excitation, the transmit coil structure should allow access for the electrodes from different orientations to allow recordings from different brain regions. Receive array designs aimed at maximizing the SNR of the fMRI experiment must be designed around head posts fixed on the animal head, leading to non-optimum coil orientations in the helmet. In cases with more than one head posts, the receive array structure must be splittable so that the two separable halves of the receive helmet can be fixed tightly on the monkey’s head. We developed an RF coil arrangement that optimizes the SNR and also allow access for recordings from different regions of the brain. The RF hardware development in combination with method development improved the signal sampling efficiency of fMRI by a considerable factor.