Hybrid 1H/19F Magnetic Resonance Imaging at 7T

Context:
Inflammation is involved in various human diseases including stroke, and improved ability to image inflammation will aid disease diagnosis, and prognosis in a multitude of disorders. A potential method for accurately measuring inflammation in humans is 19F MRI using fluorinated contrast agents in humans; since 19F is not endogenously present it can measure inflammation processes without any background interference.

19F MRI has not found applications in humans because of the lack of a clinically approved fluorinated contrast agent and the low signal to noise ratio (SNR) of 19F MRI. Our locally developed perfluorocarbon emulsion is currently undergoing clinical trials, and we propose to address the SNR challenge by developing a novel 19F/1H RF coil design and exploiting the increased SNR of 7T. Ultra high field MRI scanners increase the SNR and a state-of-the-art high-field 7T MRI scanner is being installed in Glasgow (operational early 2017). However, dedicated RF hardware solutions for 19F MRI of the human brain at 7T are currently not available.

Aims and objectives:
We propose to develop a hybrid 1H/19F MRI RF coil; this is a novel design that can improve the SNR and efficiency over the conventionally used dual-tuned approach. It consists of a unique eight-channel transceiver array for 1H imaging that is switchable to the 19F frequency in order to be used as the 19F transmit coil. A 24-channel receive array on a close fitting helmet is used to receive the 19F signal and maximize the SNR.
Because of dual-frequency operation, the subject does not need to be repositioned on to a different coil for image co-registration and B0 shimming. In addition to optimum SNR, this hardware solution will reduce the setup time and improve patient comfort.

Benefits and applications:
Our RF solution will address the current multi-nuclei imaging hardware limitation and will enable 1H/19F MRI of the human brain at 7T. The process of inflammation is involved, either directly or indirectly in various human diseases. The proposed solution will allow translation of 19F imaging into the clinic, while also providing a new tool for fundamental research into the mechanisms of inflammation and metabolism.

Non-1H MRI suffers from low signal to noise ratio (SNR) due to the lower concentration in the tissue and/or the lower gyromagnetic ratio of non-1H nuclei. Because of this, dedicated and optimized RF hardware solutions are often not available for non-proton MRI, but multi-nuclear MRI is becoming more attractive with the advent of ultra high field (UHF) scanners.

Pre-clinical studies have shown that 19F imaging using perfluorocarbons has potentially many applications such as non-invasive oximetry in tumours, or the monitoring and diagnosing inflammatory process. In addition, 19F MRI does not suffer from background interference because 19F is not endogenously present. However, 19F MRI has not found application in humans because of the lack of a clinically approved fluorinated contrast agent and the low SNR of 19F MRI methods. We propose to design and build a novel design for a hybrid 19F/1H RF coil to address the challenge of low SNR in 19F MRI. A perfluorocarbon emulsion developed in Glasgow is currently undergoing clinical trials and can potentially address the problem of the lack of a clinically approved 19F contrast agent.

We propose the development of a novel RF coil design that will allow hybrid 1H/19F MRI to take advantage of the increased SNR of our soon to be installed (early 2017) state-of-the art UHF scanner. Compared to conventional dual tuned design approach which compromises coil performance, we use a combination of a novel switchable coil for 1H/19F transmit and a 24-channel 19F receive array for increased SNR.

Furthermore, due to the dual-frequency operation, the subject does not need to be repositioned on a different coil for co-registration and B0 shimming; improving patient comfort, reducing scan time and improved image quality. The coil performance will be validated using phantom models and the complete RF hardware will be validated for human use as per the guidelines of the local ethics committee.

Pre-clinical research has shown that 19F MRI using perfluorocarbons has many applications such as non-invasive oximetry in tumours, monitoring and diagnosing inflammatory process. However, 19F MRI in humans is severely limited by the lack of a clinically approved fluorinated contrast agent and the low signal to noise ratio.

Ultra-high field MRI scanners are particularly beneficial for multi-nuclear applications because of the increase in signal to noise ratio (SNR) at high-field. This project is expected to create a powerful tool to enable 19F MRI in humans by combining the SNR advantage of the ultra-high field MRI scanner and a dedicated high performance 19F RF coil hardware.

In Glasgow we are in the advanced stages of planning clinical research in patients with acute stroke starting 2017, using the perflourocarbon emulsion containing perfluoro (t-butyl cyclohexane), which is MR visible. After demonstrating the RF coil performance in phantom studies, we can start research using 19F MR imaging in patients after appropriate ethical and regulatory approvals and be the first to assess post ischaemia/infarct inflammation in the brain using 19F MR imaging.

Given the increased interest in multi-nuclei imaging at high-field, we will be able to expand the hybrid 1H/19F MRI approach to other multi-nuclei imaging. Novel hybrid dual-frequency coil combinations are feasible for other nuclei such as 23Na and 31P using our proposed hardware architecture after appropriate design variations. Furthermore, this design concept is also transferrable to other anatomies of interest.