Introduction

Metabolic imaging is powerful in the detection of treatment effects or for early diagnoses of many diseases. However, metabolic imaging via ¹H MRSI is challenging primarily due to the dominant signals of water and lipids that can obscure the metabolite signals. When targeting other nuclei than ¹H, metabolic imaging can be simpler, and sufficient sensitivity can be obtained at high fields. Yet due to the requirement of differently tuned coils, metabolic scans are normally not applied to patient studies that rely on conventional MRI as well. This abstract presents the design and performance of a quintuple-tuned head setup for an MRI system focused on merging anatomic with metabolic MRI (META-scan). Our setup combines a broadband eight-channel RF transceiver for ¹H and ¹⁹F, as well as a triple-tuned local-receiver array for ³¹P, ²³Na and ¹³C. Bench-top or scan measurements for all 5 nuclei are presented to indicate the efficiency. For sodium, phosphorus and proton, in vivo results from a human brain are shown.

Methods

The META head coil (Figure 1a, 1d) contains an eight-channel dipole array (Figure 1b) as the RF transceiver for ¹H and ¹⁹F, a fifteen-channel surface-loop array (Figure 1c) as RF receiver for ³¹P, ²³Na and ¹³C, and a digital interface platform (Figure 1e) towards the scanner. The dipoles^[1] are tuned at 290MHz, which is between the Larmor frequencies of ¹H and ¹⁹F, taking advantage of the wide bandwidth of the dipole design. Each surface loop of the receiver array is dual-tuned for ³¹P and ²³Na, and the third frequency is realized by switching an extra capacitance with a PIN diode to decrease the lower resonance from 79MHz(²³Na) to 75MHz (¹³C). Each surface loop is directed to a low-loss diplexer to split signals of different frequencies. The split signals are fed to narrow-band preamplifiers and digital receivers, for ³¹P on one hand, ¹³C and ²³Na on the other hand. The coil was interfaced to a 7T whole body MR system (Philips Healthcare, Best, NL). The system was equipped with an 8-channel Tx/Rx setup for ¹H and ¹⁹F (8*2kW), a quadrature birdcage bore-coil^[3] tuned for 31P, connected to a 25kW amplifier, and a Helmholtz clamp for ²³Na excitation, connected to a 4 kW amplifier. In addition, the system was modified to accommodate multi-channel receiver array for all nuclei. B₁⁺ and SAR simulations were performed using DUKE^[4] (Sim4Life, Swiss) to assess safety and efficiency for ¹⁹F and ¹H. Bench-top measurement of Q-factors of the receiver-loops in loaded and unloaded conditions were obtained and was compared to single-tuned loops to ensure high efficiency. ³¹P channel was also verified with a phantom scan comparison to a quadrature ³¹P birdcage^[2]. An in vivo scan session was obtained with B₁⁺ phase shimming for ¹H, automated power adjustments, and B₀ shimming, followed by a 3D TFE sequence of 6.5 minutes. In addition, a 3D sodium scan was obtained in 10 minutes with a TE of 1.35ms and TR of 90ms. Finally a 3D ³¹P CSI FID was obtained in 10 minutes using a flip angle of 13deg and TR of 71.1ms.

Results

Proton and Fluorine: Figure 2 shows the simulation model, the simulated B₁⁺ field and the local SAR at both ¹⁹F and ¹H Larmor frequencies at 7T, with 1 Watt accepted power per channel, in quadrature mode. It indicates that, with the same amount of accepted power, the transmit fields for ¹H and ¹⁹F are highly similar. Figure 3 shows the TFE images. 18µT was achieved at the center of the ROI, with 922 Watt forward peak power per channel.
Phosphorus: The SNR in the center of META head coil is 11% higher compared to a highly efficient dual-tuned (³¹P and ¹H) head-birdcage^[2] under identical scan conditions (i.e. same flip angle, TR, and load). Figure 4c-d show the signal-level comparison. Figure 4a-b show a slice of the 3D CSI, indicating good coverage over the entire brain and high SNR.
Sodium: Figure 5a-b show ²³Na images of an in-vivo scan. Considering the very short T2* of ²³Na, the used echo time (1.35ms) is far from optimal.
Carbon: Bench measurements show that the receiving performance of the ¹³C elements is similar to that of ²³Na (at max 1dB loss): S₂₁ between two pick-up probes next to an unloaded surface-loop shows that the Q-factor of the unloaded surface-loop does not change when shifting from the resonance of ²³Na to that of ¹³C (Figure 5c).

Discussion and Conclusion

We have demonstrated a successful integration of an RF setup that could detect ²³Na, ¹³C, ³¹P, ¹⁹F and ¹H in a single scan session. Since tracers are generally required for ¹³C and ¹⁹F, we show natural-abundance 3D images of only ³¹P, ²³Na and ¹H, in a healthy human brain. Since the frequency of ¹³C is close to ²³Na and ¹⁹F close to ¹H, we do not expect the sensitivity to be different for these nuclei. Excluding the preparation time, the acquisition time for the ¹H, ³¹P and ²³Na scans can be less than 25 minutes in total, paving the way to incorporate metabolic MRI of all five nuclei in one scan session of acceptable scan time.