Oxford boosts its MRI

The University of Oxford recently received a 7 Tesla magnetic resonance imaging (MRI) system; one of only two currently operating at this field strength in the United Kingdom. The scanner is primarily going to be used for neuroscience research. Part of the reason why these systems are so rare is that the technology doesn’t come cheap. The system was part of a £8.2 million project funded by the University of Oxford, the Medical Research Council, and the Wolfson Foundation. Researchers are eager to start new imaging projects with the system, and hope the state-of-the-art scanner will keep Oxford at the forefront of brain imaging in the UK.

To understand why this scanner is an exciting advancement for neuroimaging in Oxford, it is important to understand a bit more about what the MRI scanner does, and what 7 Tesla (T) actually means in terms of imaging.

Many people will be familiar with what MRI is, through personal experience or from the often-embellished world of TV shows such as House. Briefly, it is an imaging technique to visualize internal body structures. MRI can differentiate between soft tissues, so it is often used to image the brain, muscles, and heart. It also has high sensitivity and is non-invasive, which has lead to its position as the most used imaging method in neuroscience.

MRI uses a static magnetic field in combination with radio waves. Firstly, a large magnet aligns all the protons found in water molecules within the body. Radio frequency transmitters are then used to create localized electromagnetic fields, which push some of these protons out of alignment. When the radio transmitters are turned off the displaced protons flip back into alignment, generating a radio signal as they do so. It is this tiny signal that is picked up, and after thousands of repeats forms the detailed image.

Each scanner is defined by the strength of its large magnet. This is because the magnet is the limiting factor on resolution; the stronger the magnet is, the more clarity the image has. Oxford’s new 7T magnet is really strong. To put it in perspective it is about 5 times stronger than the giant magnets in junkyards that can pick up cars and the magnetic field of the earth itself comes in at just 0.00005T.

The majority of scanners used for clinical applications range from 1.5-3T, but scanners used for human research purposes can reach up to 11.7T. The scanners at Oxford are used just for research, and this new 7T scanner is an addition to an existing 3T one. The smaller 3T machine used to be at the cutting edge of research technologies, but as always times have changed and 3T scanners are now common clinical tools. 7T scanners are the new frontier.

Moving up to 7T from 3T has a few key benefits.  As the field strength increases, so does the raw signal of the scans, and the signal-to-noise ratio (SNR) is boosted. This increase is great for images of brain structure because it allows higher spatial resolution and much finer detail. The improvement may allow researchers to locate lesions that were previously too fine to see, as well as signs of neurodegenerative disorders. Functional MRI (fMRI) – the imaging of brain activity as measured by blood flow – may also be improved at higher field strengths. Scanning times can be reduced for certain types of MRI scans at 7T, which is helpful as patients are required to lie still in the scanner.

However, there are challenges to work around when scanning at 7T. Much of the neuroimaging research in recent years has been carried out on 3T scanners and so protocols and equipment in facilities have been developed for the lower field strength. Everything has been optimized for 3T scanners; time, money and effort must now go into adapting research facilities for the new 7T machines. A further complication particularly effects fMRI; the increased sensitivity means better detection of blood flow in the part of the body you are interested in, but it also means that physiological noise is picked up from all sorts of other bodily functions like fluctuations in brain metabolism and cardiac and respiratory variations. Finally, inhomogeneities in both the large magnet field and the radio field become more pronounced in higher fields leading to a greater degree of artefact and distortion in the brain images. Whilst these technical challenges can be tough to resolve, various hardware and software approaches are being developed to deal with the difficulties.

7T is not expected to replace 3T scanners for clinical use and diagnosis any time soon. Rather, it is an additional research tool for specific types of scans that can supplement the existing methods. As engineers race to keep up with the challenges presented by using these new machines we can be excited about the technological advances in their own right, the potential to gain a better understanding of how the brain works and how diseases may affect it, and that Oxford is leading the way.