Magnetic resonance imaging (MRI) uses extremely strong magnets

MRI scanners in hospitals and radiology practices require powerful magnets, with field strengths of up to 3 Tesla. This is about 100,000 times stronger than the Earth's magnetic field! The MPI for Biological Cybernetics also has two MRI scanners with 9.4 and even 14.1 Tesla. Generating such strong magnetic fields requires a lot of effort, which is why MRI machines are usually large, heavy, and expensive. Why is this necessary? Let's take a look at how magnetic resonance imaging works.

MRI uses the magnetic properties of atomic nuclei, particularly of hydrogen. There is one proton in a hydrogen atom, and it aligns itself with the strong magnetic field of the MRI scanner – like a compass needle pointing north. In an MRI scanner, high-frequency electromagnetic pulses knock the proton briefly out of alignment. Like a spinning top, it then tumbles around its axis while gradually realigning itself with the scanner's magnetic field.
As this happens, the protons emit magnetic signals. When they finally point in the original direction again, the scanner detects a strong signal in that direction. However, depending on the tissue, the protons may realign more or less quickly. By comparing the times, it is possible, for example, to distinguish between grey and white matter in the brain.

The stronger the magnet, the better the image

But there is also another trick: Immediately after being knocked out of alignment, the “proton spinning tops” all tumble in unison. But over time, some become faster, others slower, partly because they influence each other. As a result, the signals partially cancel each other out; the signal transverse to the magnetic field becomes weaker and weaker. This happens very quickly in bone tissue, but takes longer in water or blood.
A crucial point for both methods: The stronger the magnetic field of the MRI scanner, the stronger the signal emitted by the proton nuclei. Doubling the magnetic field strength results in a signal four times as strong, and tripling it produces a signal nine times as strong. This is why high-field MRI scanners, such as those at the MPI for Biological Cybernetics, produce particularly precise images.
Researchers in the High Field Magnetic Resonance department, led by Klaus Scheffler, are working to make MRI faster and more precise. One mission, for example, is to make the strong magnetic field even more uniform. They are also developing highly specialized transmitters and receivers for the magnetic signals. Another challenge is how to deal with image errors caused, for example, by the movement of the patient. The researchers use artificial intelligence, among other methods, to address these problems.

Cheaper, transportable alternatives on the horizon

By the way: The MPI for Biological Cybernetics is also developing low-field MRI techniques. This method uses much weaker magnets, comparable to the Earth's magnetic field. This is made possible by so-called hyperpolarization: Normally, about half of the atomic nuclei point toward the north pole and the other half toward the south pole of the magnetic field. In a hyperpolarized sample, however, the majority of the nuclei point toward the same pole. This makes the measurable MRI signal much stronger.
Without strong magnets, MRI scanners can be much cheaper and easier to transport. They could one day be used in mobile applications such as ambulances. Low-field MRI scanners also have the potential to become a low-cost diagnostic method.