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Development of Small Animal PET-insert Compatible RF Coils for Simultaneous ¹²⁹Xe MRI/¹⁵O₂-PET Brain Perfusion Measurements

Elise Noelle Woodward¹, Heeseung Lim^2,3, Ramanpreet Kaur Sembhi¹, Matthew S Fox^1,2, and Alexei Ouriadov^1,4
¹Physics and Astronomy, Western University, London, ON, Canada, ²Lawson Health Research Institute, London, ON, Canada, ³Siemens Healthcare Limited, London, ON, Canada, ⁴2Lawson Health Research Institute, London, ON, Canada

Synopsis

Keywords: RF Arrays & Systems, Hyperpolarized MR (Gas)

This study aims to present two birdcage radiofrequency (RF) coils for the use of simultaneous hyperpolarized Xenon-129 (¹²⁹Xe) MRI and ¹⁵O₂ PET imaging for brain perfusion measurements. Coils were designed for use with a 2.89T Siemens scanner, or 34.05MHz, one using a low pass filter design, and the other a high pass filter design.

Introduction

The use of laser-polarized Xenon-129¹ (¹²⁹Xe) as a novel contrast agent for magnetic resonance imaging (MRI) has been shown to be effective for functional² and structural imaging of the brain³ and other organs^4, ⁵. The original interest of one of the co-inventors was to use ¹²⁹Xe to better understand the brain, namely directly imaging the effect of anesthesia on brain function and cognition⁶. As a result of over two decades of research and development within the field, both brain^{2, 3} and lung imaging^{7, 8} have been made possible in animals and humans, reliable polarizers are now commercially available⁹. ¹²⁹Xe-based lung imaging for imaging disease progression is clinically accepted in the United Kingdom¹⁰, and FDA approval in the United States is expected this year¹¹. Our research program focuses on the development of advanced novel imaging techniques for assessing brains and consequently the development of hardware needed to conduct those studies. ¹²⁹Xe-based imaging could transform our methods of mapping grey and white matter^12-14, perfusion³, and targeted drug and cell tracking techniques¹⁵ by improving sensitivity over other MRI methods such as conventional proton MR imaging, and reaching beyond the resolution limitations of PET. Birdcage coils have been widely used in MR imaging as their RF homogeneity and SNR exceed that of common linear coils¹⁶. High-pass birdcage coils are constructed by adding two capacitors of equal value on either end of the equivalent circuit, these capacitors sit perpendicular to the main body of the coil. Low-pass birdcage coils have one capacitor in the middle of the rung, aligned parallel with the main body of the coil¹⁷. In this proof-of-concept study, we propose to build two same-size ¹²⁹Xe rat-head-sized RF coils for conducting simultaneous [¹⁵O] PET and hyperpolarized ¹²⁹Xe MRI brain perfusion imaging. Two main birdcage design RF coils (low- and high-pass) were built to determine the optimal coil design for the multi-modalities brain perfusion measurements inside a NuPET small animal PET insert (Cubresa Inc, Winnipeg Manitoba).

Methods

Both high-pass and low-pass coils were tuned to 34.05MHz at 2.89T inside the PET insert, with 8 rungs on each coil. The coil was built to 75 mm in length, with a diameter of 45mm. Coils were matched to approximately 50Ω. Rungs were constructed with copper tape, and each loop element was tuned with non-magnetic capacitors (Passive Plus, USA) and inductors (CoilCraft, USA). The dominant resonant frequency mode was chosen due to its low impedance¹⁷. Hyperpolarized ¹²⁹Xe gas was obtained from a turn-key, spin-exchange polarizer system (Polarean 9800 ¹²⁹Xe polarizer). The initial ¹²⁹Xe polarization was 15%. ¹²⁹Xe spectrum and images were acquired in a 3T PET/MRI (Siemens Biograph mMR, Siemens Healthineers, Erlangen, Germany) scanner using whole-body gradients. A Fast Gradient Recalled Echo sequence was utilized using the following parameters: Matrix Size=64x64; Slice thickness=250mm; TE/TR=2.04/20ms; BW=660Hz/pixel; Flip angle=11^o (a Constant Flip Angle approach was used); and FOV=100x100mm².

Results

The low-pass and high-pass RF coils are shown in Fig.1. Coils were measured using a vector network analyzer. The low-pass circuit has a sensitivity of -19.8 dB for channel 1 and -27.6 dB for channel 2 (Fig.2, right). The low-pass-coil is matched to 44.3Ω with a 2.3 Ω phase (Fig.3, right). Input power was calculated to be 7V. The high-pass-circuit had a sensitivity of -22.7 dB for channel 1 and -29.3 dB for channel 2 (Fig.2, left). The impedance of the coil was 49.4Ω with a 3.0Ω phase (Fig.3, left). Input power was calculated to be 5V. Fig.4 shows the ¹²⁹Xe spectra obtained for both coils and the 2D ¹²⁹Xe phantom image obtained with the high-pass RF coil.

Discussion and Conclusions

We were able to build two rat-head-sized ¹²⁹Xe RF coils fitting into NuPET small animal PET insert resonating at the ¹²⁹Xe resonance frequency at the 3T MRI system. Initial phantom scans indicated that both coils have demonstrated good sensitivity, and therefore they can be used in the simultaneous in-vivo [15O] PET and hyperpolarized ¹²⁹Xe MRI brain perfusion imaging measurement. Further in-vivo PET/MRI scans should help us to decide which circuit design provides better SNR on the ¹²⁹Xe side and causes minimal PET attenuation. While both coils provided clear ¹²⁹Xe images, the high-pass birdcage RF coil circuit design places capacitors at the end of the rungs, allowing for axial and radial symmetry and likely less potential for PET signal attenuation coming from the central part of the high-pass coil where the animal head is normally placed during the PET/MRI scan. On the other hand, low-pass coils are only radially symmetric, as the capacitors are at the middle of the rungs, and therefore the PET transparency of the central part of the coil is affected. High-pass coils set the capacitors and other main coil components outside of the FOV of the PET insert, and therefore are better suited for use in an MR/PET setting. This work will be expanded upon by imaging with and without the PET insert.

Acknowledgements

We acknowledge the support of the Natural Sciences and Engineering Research Council of Canada, R5942A04.

References

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Figures

Figure 1. A Low pass ¹²⁹Xe RF coil (left) and high path ¹²⁹Xe RF coil (right) inside the enclosure for the animal-sized PET-insert.

Figure 2. Tuning of the High Pass ¹²⁹Xe coil shown with channel 01 on the left (dB scale -5dB, -29.3dB amplitude) and Low Pass ¹²⁹Xe coil on the right (dB scale -10db, -27.0dB amplitude).

Figure 3. Smith Charts for the High pass coil (left) and Low pass coil (right). The high pass coil shows an inductance of 49.4Ω and the low pass coil was matched to 44.3 Ω.

Figure 4. ¹²⁹Xe MRI signal (spectra) obtained for the high-pass RF coil (top panel) and low-pass RF coil (middle-panel). The bottom panel shows the ¹²⁹Xe phantom (60ml plastic syringe) image obtained for the high-pass RF coil for the axial view. The FOV was 10x10cm² and the image matrix was 64x64.

Proc. Intl. Soc. Mag. Reson. Med. 31 (2023)

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DOI: https://doi.org/10.58530/2023/4400