Introduction

High-density 7T receive arrays will improve the cortical signal-to-noise-ratio (SNR) without compromising the central SNR. In addition, significant gains in parallel imaging performance can be achieved by using high-density receive arrays^1-3. The NextGen 7T scanner⁴ is specifically built for high-resolution brain imaging. It is equipped with 128-receive channels, 16-transmit channels and a head gradient insert (“Impulse”) with Gmax 200 mT/m and slew rate 900 T/m/s.

RF coils for scanners with head gradient inserts are constrained by the limited space available on the patient table. For improved patient comfort and ease of positioning the patient, a split-top receive array with sliding mechanism is desirable. This work presents the design of a compact 8-channel transmit and a split-top 64-channel receive array which operates within the space constraints of the head gradient insert.

Methods

Transmit array: The transmit array consists of an 8-channel loop array constructed on a fiberglass tube of 290 mm inner diameter and extended 20 cm along the z-direction. The outer tube, at 30 mm from the coil, consisted of a slotted double layered RF shield. Each loop consisted of 13 fixed and one variable capacitor. All adjacent elements were overlapped to minimize coupling. The coupling between the second neighboring elements was reduced by counter-wound inductors⁵. Coil tune, match and decoupling were adjusted while the coil was loaded with a head and shoulder phantom⁶. A picture of the constructed transmit array is shown in figure 1.

Sliding mechanism: The transmit array slides on two long fiberglass rods positioned between the inner and outer tubes and connected between the two end plates of the coil structure (Figure 2a). The sliding rods are surrounded by another tube between the two ends of the transmit array and this prevents any fluids from entering inside the array electronics. The posterior half of the receive array is fixed on to the coil structure (Figure 2b). The anterior half slides on two other rods as shown in figure 2c.

Receive array: The receive array consists of 64 rectangular loops arranged in columns. This allowed to create an overlap between the anterior and posterior half of the receive array and decouple the adjacent elements between each half. There are 40 elements on the posterior half (fig. 3) and 24 elements on the anterior half. All adjacent elements were geometrically overlapped. Each loop consisted of one variable and three fixed capacitors (ATC 100B series) and connected to a low impedance preamplifier (Wantcom Inc, MN, USA). A shielded cable trap is connected between the coil input and the preamplifier. The equivalent circuit of the receive array element is as per Ref.6. The internal dimensions of the receive helmet is 178mm along the L/R direction and 215mm along the A/P direction.

Results

The s-parameter matrix of the transmit array is shown in figure 4. All channels were matched to better than -25dB. The average adjacent element decoupling was -15.35dB and the average next neighbor decoupling was -24.54dB. Figure 5 shows the B₁⁺ map of the transmit array in transceiver mode acquired by driving the coil in the circularly polarized (CP) phase configuration. For this measurement, a custom built 1x8 splitter with CP phase offsets was used to drive the coil and an external TR switch module was used to receive the signal. A reference voltage of 135v was required to achieve 90° flip angle in the center of the head and shoulder phantom. This included the losses introduced by the TR switch and power splitter.
The decoupling between the adjacent elements of the receive array was -12dB on average. Active detuning was better than -30dB and the average preamplifier decoupling is -20dB. Imaging results of the combined 8-channel transmit 64-channel receive is expected soon.

Conclusion

A novel 8-channel transmit array in combination with a 64-channel receive array has been developed to operate within the space constraints of the Impulse head gradient insert. The implemented sliding mechanism offers improved patient comfort. The transmit performance of the coil is evaluated using S-parameter measurements and B₁⁺ measurements in the scanner. Future work will include imaging performance evaluation of the 64-channel array with the industry standard 32-channel head coil and custom-built 96-channel receive array⁷.

Acknowledgements

No acknowledgement found.