Synopsis

We report details on the fabrication and performance of a transmit/receive ³He/¹H switched frequency 16 rung high-pass birdcage coil for neonatal imaging at 1.5 Tesla. The coil enables collection of ³He images of neonatal lungs inflated with hyperpolarized ³He gas and ultra-short echo time (UTE) ¹H images. Rotation of a shaft in the coil housing shifts the coil between ³He and ¹H operating modes. ³He and UTE ¹H images of explanted lungs inflated with hyperpolarized ³He gas collected using the coil demonstrate the excellent image quality achieved with the coil.

Introduction:

The production of hyperpolarized ³He and ¹²⁹Xe has opened exciting possibilities for imaging lung structure and function using MRI^1-4. Our institution has built a hyperpolarizer with the goal of using hyperpolarized ³He gas for MR imaging in neonatal lungs in a dedicated Neonatal Intensive Care Unit (NICU) 1.5T MR scanner developed at our institution^5-6. With this goal in mind, it is essential that the MRI coil used for imaging operate at both ³He and ¹H frequencies without a SNR penalty at either frequency and without the necessity to move the patient between ³He and ¹H scans.

Designs for transmit/receive (T/R) ³He/¹H dual tuned volume coils have been described in literature^7-9. However, in these designs, coil performance at one or both frequencies is reduced by addition of inductive and capacitive elements to improve isolation between the two circuits. We have implemented a novel switched frequency (SF) transmit/receive RF coil design for a 16 rung high-pass birdcage coil that can be used to collect high quality images at both ³He and ¹H frequencies (48.65 MHz and 63.86 MHz respectively at 1.5 Tesla). The SF coil described here is relatively simple with a common set of current carrying inductors used for both frequencies. The design does not include reactive elements between two tuned circuits. Advantages of this coil design include: 1) quadrature excitation and reception at both frequencies, 2) good SNR and B1 homogeneity at both frequencies, and 3) 5-10 second switching times between ³He and ¹H operation. This report provides details of the coil fabrication. Images of hyperpolarized ³He and ultra-short echo time (UTE) ¹H of ex-vivo explanted lungs demonstrate the excellent image quality obtained with the coil.

Materials and Methods:

The SF birdcage coil was constructed on a G10 (garolite) cylinder. The coil has an inner diameter of 7.0 in (178 mm) which will accommodate more than 90% of the neonates in our NICU. The coil rung length is 5.5 inches, which is adequate to cover the neonatal lung. A second G10 cylinder supports an integrated RF shield and serves as the outer shell of the coil.

Figure 1 shows the coil without its cover. A central component of the assembly is a white plastic carrier ring (red arrow) that holds 16 rectangular fiberglass strips (green arrow) with beryllium-copper contacts located at each end (blue arrows). As the carrier ring is rotated (black arrow), the contacts bridge gaps on the birdcage printed circuit board thereby adding additional capacitors in parallel with the birdcage coil’s main capacitors. This shifts the resonance frequency of the coil from ¹H down to ³He. Manually rotating a control shaft (yellow arrow) 180 degrees rotates the carrier ring inside the coil. As the shaft rotates, two Kevlar strings (purple arrows) attached to fiberglass strips pull the contact ring assembly, causing it to rotate. Clockwise rotation of the shaft adds the capacitance needed to operate the coil at the ³He frequency. Counter clockwise rotation removes the added capacitance and returns the coil back to ¹H frequency operation. After tuning the ¹H and ³He modes, the measured S parameter values were - ¹H mode: I -29dB S₁₁, Q -28dB S₁₁, -32dB S₂₁. ³He mode: I -20dB S₁₁, Q -14dB S₁₁, -27dB S_21.

Results and Outlook:

The coil’s ¹H SNR performance was assessed using a daily quality assurance (QA) phantom. A spin echo protocol was used to collect the image data. For SNR comparison, the same QA phantom was imaged with a 180 mm ID 16 rung low-pass birdcage product coil delivered with the scanner using the identical imaging protocol. Image results are shown in Figure 2A (¹H/³He coil) and 2B (product coil). The measured proton image SNR was 657.5 and 719.9 respectively for the two coils.

Ex-vivo imaging: Figure 3A shows ³He diffusion weighted images (DWIs) (one slice) of an explanted whole rabbit lung using nine b values incrementing in equal steps from 0 to 7.6 s/cm². Figure 3B shows five ¹H UTE images for the explanted whole rabbit lungs. Figure 4A shows ³He DWIs (one slice) of an explanted left lung of a 3 year old human using the same b values as used for the rabbit lung ³He images. Figure 3B shows five ¹H UTE images for the explanted left lung of a 3 year old human.

We have shown the ¹H/³He coil design described in this report provides excellent image quality and good SNR for both ³He and ¹H imaging.

Acknowledgements

References

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