Novel Radiofrequency Coils Facilitate MRI/MRS of Animal Model, Don't Require Implantation

Hyperpolarized ¹³C magnetic resonance spectroscopy (HP13C-MRS) is a promising technique for imaging enzyme-catalyzed metabolic transformations in vivo. However, obtaining good spatial resolution in small animals is still a challenge.

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In living mice, for example, most HP13C substrates or contrast agents have a T1 of less than 40 seconds. Therefore, unlike in conventional MRS, signal averaging over a long period isn't a viable strategy to improve the signal-to-noise (SNR) ratio and, therefore, the spatial resolution.

One solution under investigation for improving the SNR is to increase the radiofrequency coil sensitivity. Massachusetts General Hospital researchers have developed a series of inductively coupled transmit–receive coils that perform well for X-nucleus (non–proton nucleus) and proton MRI and MRS of several mammal species.

Atsushi M. Takahashi, PhD, assistant director of the Athinoula A. Martinos Center for Biomedical Imaging of Mass General, the Massachusetts Institute of Technology, and Harvard Medical School; Daniel P. Cahill, MD, PhD, a neurosurgeon at Mass General and neurosurgical oncologist in the Department of Neurosurgery; Yi-Fen Yen, PhD, director of the Hyperpolarized Imaging Lab at the Martinos Center; and colleagues describe the technology and the results of several experiments in the Journal of Magnetic Resonance Open.

Design Principle

Each coil set consists of a wireless resonant loop or a volume resonator tuned specifically to the nuclear resonance of interest and inductively coupled to a broadband pick-up loop connected to the scanner hardware. Implantation of the coil isn't necessary.

Conventional coil designs for X-nucleus MR require separate coils, each tuned to a specific resonance frequency. In contrast, the new coil can be used for proton MRI or any X-nucleus MR acquisition with only a minor and inexpensive adjustment.

Rodents

The simplest form of the coil was a resonant loop designed to be fit over a mouse head to achieve full coverage of the brain. The researchers tested the coil's performance for HP13C-MRS of pyruvate metabolism in a normal mouse and a glioblastoma mouse model at 4.7T. Because of the coil's close proximity to the brain, SNR exceeding 70:1 was obtained with good spatial resolution (1.53 × 1.53 mm) and good sensitivity.

For MR acquisitions of other X-nuclei, only new resonant loops needed to be made and the same pick-up coil was applied for all. For example, a similar inductively coupled loop was made in a few minutes and achieved high performance for ³¹P MRS of energy metabolism in rats.

Marmoset Monkeys

For imaging marmosets, the researchers built a pair of saddle coils, embedding the pick-up coil in the bottom half of the saddle loop. The split design made it simpler to place the animal in the ear bars and face mask, and high-resolution ¹H proton MRI was successful at 9.4T and 3T.

Other Advantages of the Technology

There are many constraints in the setup for animal imaging, such as a nose cone and tubing to deliver anesthetics, suction to remove excessive anesthetics, vital-sign sensors and ear bars. This coil design accommodates head restraint, vital-sign monitoring and anesthesia delivery yet simplifies the setup, improving workflow.

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