Introduction

In MRI, data transfer between the scanner subsystems (e.g., RF coils and shim coils) or peripheral systems (e.g., respiratory and cardiac monitoring) and the computers outside the scanner room currently requires a network of carefully routed wired connections, which take up space within the scanner and require costly RF filters, baluns, and cable traps to preserve data integrity and MR image quality. To reduce the number of wired connections, a novel integrated RF/wireless coil design has recently been developed, in which RF currents at the Larmor frequency (e.g., 127 MHz for 3T scanners) and in a wireless communication band (e.g., 2.4 GHz for WiFi) can flow on the same coil to enable simultaneous MR image acquisition and wireless data transfer for different applications, respectively, without requiring any scanner modifications or additional antenna systems within the scanner bore^1,2.

Previously, this coil design was combined with the integrated parallel reception, excitation, and shimming (iPRES) coil design³, in which a DC current can also flow on the same coil to enable wireless localized B₀ shimming^1,4. In this iPRES-W coil design, the coil wirelessly receives commands from outside the scanner room to control an MR-compatible power supply in the scanner bore, which delivers the DC currents for shimming. In this work, the integrated RF/wireless design is implemented onto a 48-channel head coil array to enable two independent wireless data streams for two separate applications, specifically, wireless 1) control of the DC currents used for B₀ shimming and 2) respiratory tracking with a respiratory belt.

Methods

First, the 16 anterior coil elements of the 48-channel head coil array were modified into iPRES coil elements by adding: inductors to bypass the capacitors; an MR-compatible battery pack to provide the DC currents for B₀ shimming (range: ±2 A, resolution: 4 mA); and inductive chokes to provide RF-isolation between the coil elements and the battery pack to maintain a high SNR³. Next, two of the iPRES coil elements were modified into iPRES-W coil elements by adding 127-MHz and 2.4-GHz band-stop filters between each of these coil elements and the WiFi micro-controller or the preamplifier, respectively¹, to enable two wireless single-input single-output data streams (Fig. 1a, orange). The two iPRES-W coil elements were chosen to be spatially orthogonal to provide a high RF-isolation (S₂₁ ~ -35 dB) between them, which reduces coupling and increases the amount of power radiated from each coil element in the WiFi frequency band (Fig. 1b).

One of the iPRES-W coil elements (Fig. 1a, coil 1) was used to wirelessly control the DC currents for B₀ shimming. In this work, 8 of the iPRES coil elements were connected to the battery pack and used for shimming (Fig. 1a, blue). The optimal DC currents to shim a slice were determined by minimizing the root-mean-square error (RMSE) between a combination of basis B₀ maps acquired on a phantom³ with 1 A separately applied in each iPRES coil element and a B₀ map acquired in the brain of a healthy volunteer. The commands to control the DC currents were then wirelessly transmitted from outside the scanner room to the WiFi micro-controller of the iPRES-W coil element via an access point (AP) placed on the scanner room wall. B₀ maps and spin-echo EPI images (2 x 2 mm resolution) were acquired on a 3T scanner with and without DC currents to assess the B₀ shimming performance.

The other iPRES-W coil element (Fig. 1a, coil 13) was used to perform real-time wireless respiratory tracking during MR image acquisition and shimming. The signal from a force-sensitive respiratory belt placed around the subject’s abdomen (Fig. 1a, magenta) was wirelessly transmitted from the iPRES-W coil element to the AP, then recorded on a computer outside the scanner room.

Results

The iPRES-W coil element modifications did not significantly impact the SNR with or without wireless transmission of the respiratory data (Fig. 2). Wireless shimming significantly reduced the B₀ field inhomogeneity (-41%) and the geometric distortions in the spin-echo EPI images (Fig. 3, green arrows). The B₀ shimming performance can be further improved by implementing iPRES into all coil elements. Additionally, the wirelessly transmitted respiratory signal was similar with or without simultaneous MR image acquisition and wireless localized B₀ shimming (Fig. 4).

Discussion and Conclusion

These results demonstrate that the dual-stream iPRES-W head coil array can perform simultaneous image acquisition and wireless data transfer for multiple applications, specifically, for wireless localized B₀ shimming and respiratory tracking. In future applications, the respiratory signal can be used to perform dynamic shimming with the iPRES coil elements and correct for respiration-induced B₀ variations, which has previously been done using wired rather than wireless connections⁵. In addition, the integrated RF/wireless coil design can be used for wireless data transfer in other applications, such as cardiac monitoring or field monitoring with NMR probes.