Synopsis

Keywords: Non-Array RF Coils, Antennas & Waveguides, RF Arrays & Systems

Current state-of-the-art RF coils for X-nuclear MRS/MRI studies typically use two separate sets of RF coils operating at the X-nuclear and proton frequencies, respectively. Here, we introduce a new coil concept whereby large-size loop coil with split capacitors which can be simultaneously tuned and matched to ¹⁷O and ¹H Larmor frequencies for 7T human imaging application. Importantly, this novel coil exhibits excellent performance in proton and X-nuclear imaging, therefore, it provides a simple RF coil solution, particularly for ultrahigh field (UHF) multinuclear brain MR imaging applications.

Introduction:

In vivo X-nuclear MRSI at ultrahigh field (UHF) offers unique capabilities for studying brain energy metabolism in normal and disease states ¹. However, it requires the coil operating at the X-nuclear frequency and ¹H frequency (for B₀ shimming and anatomical imaging) ². Current state-of-the-art X-nuclear/¹H coils typically use two separate sets of coils for X-nuclear and ¹H operations ^3-5, which may degrade ¹H and/or X-nuclear coil performance due to cross coil interactions. Here we combine X-nuclear and ¹H operations into a single coil and introduce the DODO (DOuble tuned and DOuble matched large size loop) coil that can be tuned and matched to ¹⁷O and ¹H frequencies at the same time, showing excellent imaging performance at both the operation frequencies.

Methods:

Coil schematics and bench measurements:
The 15cm diameter single-loop ¹⁷O-¹H dual-frequency DODO coil shown in Fig. 1A can be independently tuned/matched at the same time to 7T ¹⁷O (40.3MHz) and ¹H (297MHz) Larmor frequencies with a huge frequency difference of >250 MHz. The DODO coil circuit is shown in Fig. 1C. We also made a quadrature two-loop DODO array coil using two identical DODO coils (Fig. 1B).
To evaluate the DODO coil performance, we built another 15cm diameter single-loop ¹⁷O-¹H dual-frequency coil as control (Fig. 1D) based on the similar coil design ^6,7 without the split capacitors, which can be tuned and matched to 40.3MHz or 297MHz, respectively.
Phantom study at 7T:
We acquired 7T ¹⁷O FSW-weighted CSIs ⁸ (1ms square pulse, 9×9×7 matrix size, 200ms TR, 12×12×12cm³ FOV and 30k spectral bandwidth) from a head-sized NaCl solution phantom using the DODO and control coils as transceiver. The CSI data acquired with varied RF pulse voltages were fitted with a “sine” function to generate two types of RF magnetic field (B₁) maps: transmission (|B₁⁺|) and reception (|B₁^-|) field ⁹. The SNRs are calculated from fitted ¹⁷O water peak signals divided by the CSI baseline noise level. Finally, we collected ¹H localizers with the head-shape phantom(ε_r= 45 at 298 MHz) for ¹H performance evaluation.
Electromagnetic (EM) simulations of DODO coil:
We simulated the single-loop DODO coil loaded with “Duke” human model using the CST Studio 2019 with Hexahedral Time Domain solver to generate the H-field spatial distributions and the coil surface current density at 40.3MHz and 297 MHz driven by 1 volt forward RF pulse voltage.

Results:

The single-loop DODO coil was tuned and matched to 40.3 MHz and 297 MHz simultaneously with S₁₁< - 20 dB (Figs. 2A&2B). The two overlapping-DODOs (Fig. 1B) are decoupled at 40.3MHz with decoupling efficiency better than -20dB (Fig. 2D). At this condition, moderate decoupling around -10dB (Fig. 2C) was also achieved at 297MHz.
The ¹H MRI and ¹⁷O B₁ maps of the control and single-loop DODO coils are shown in Fig. 3. The DODO coil provides higher quality ¹H MRI (Fig. 3B vs. Fig. 3A), and superior ¹⁷O CSI performance as shown by the |B₁^-| and SNR maps (Fig. 3D vs. Fig. 3C). Furthermore, quadrature-driven DODO array coil showed further improvement in both ¹H MRI (Fig. 4A) and ¹⁷O CSI B₁ strength (Fig. 4B) compared to the single-loop DODO coil (Fig. 3) at 7T. Figure 4C shows the SNR comparison results among the three coils. The single-loop DODO coil has two times higher SNR compared to control coil, and the quadrature-driven DODO array coil has ~1.8 times higher SNR gain compared to single-loop DODO coil.
The EM simulations show highly efficient H-field (Fig. 5B) and good coverage in the “Duke” (model) brain for the single-loop DODO coil at both ¹H and ¹⁷O operating frequencies. The real part of the coil surface current density is shown in Fig. 5C. The surface current at 297MHz is relatively uniform, with majority of the currents reside on the lower part of the large loop away from the feeds. Lastly, at 40.3 MHz, the surface currents are uniformly distributed on the large coil loop.

Discussion:

We presented a novel concept of the dual-frequency DODO coil design that demonstrated good performance on both 7T ­¹H and ¹⁷O imaging using a large single-loop coil. Additionally, we have shown that it is possible to construct array coils using multiple overlapping DODO coils, for instance, the quadrature DODO array coil as demonstrated herein. Adding split capacitors (60-80 pF) on the large coil loop makes the coil surface currents at the ­¹H and ¹⁷O frequencies uniform on the large resonating loop, which is different from the previous coil design in which most of the ¹H surface current resides on the matching circuit near the feed ⁶, therefore, the ¹H B₁ strength in the imaging object is significantly attenuated.

Conclusion:

Through 7T phantom imaging measurements and EM simulations, we demonstrate a novel and simple dual-frequency DODO coil design with a large loop size that can be simultaneously tuned and matched to the 7T ¹H and ¹⁷O frequencies, exhibiting excellent performance at both frequencies. This advanced RF coil should be highly valuable for performing X-nuclear MRSI in the human brain at UHF.

Acknowledgements

This work was supported in part by NIH grants of U01 EB026978, R01 CA240953 and P41 EB027061.

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