How Does An MRI Machine Work?

You would think the one thing everyone would know about MRI machines is that they involve very powerful magnets. It is, after all, right there in the name – Magnetic Resonance Imaging. However, it seems a remarkable number of people either don’t know this, or forget, given the disasters that get reported when someone does something as mindbogglingly stupid as taking a loaded gun into an MRI or wearing a heavy chain next to one.

Most of us, however, are aware there are very powerful magnetic fields involved. It’s just not obvious how this creates images of internal organs, however, let alone such detailed ones.

Mapping the spin

MRIs take advantage of the fact that protons have a property we call spin. You can argue over whether the name is appropriate, but quantum spin gives protons a small magnetic moment, which you can think of as like a tiny bar magnet. In the presence of a powerful external magnetic field the protons’ magnetic moment become aligned with the field. That’s harmless to us, as it doesn’t cause the protons to move around the body, their magnetic moments just follow the direction of the field.

Intense radio waves can change the direction of the protons’ alignment to make some either directed against the field, or at right angles to it. This give the protons potential energy, just as raising a weight in a gravitational field does.

When the radio waves are turned off, the protons rotate back to align with the field. In the process, they release the potential energy as tiny amounts of electromagnetic energy in the form of radio waves of their own.

If all protons were equally aligned by the radio waves, and realigned together, this would not be helpful. However, protons that are on their own (i.e. hydrogen atoms) behave differently under these conditions from those that are bound in a nucleus with neutrons and other protons. The energy release can be detected, and hydrogen atoms produce a distinctive signal that can be differentiated from protons in any other atoms, including by the speed with which it happens. Consequently, an MRI scan can map hydrogen concentrations in the body, which allows us to identify the presence of water and fat.

The distribution of hydrogen varies among organs and between healthy tissue and tumors. Consequently, MRIs reveal the presence of abnormalities that can signal disease.

Contrast agents such as gadolinium can bring features into greater clarity because their magnetic properties make spins align more quickly; although since these need to be administered intravenously, they’re only used where necessary.

Are MRIs safe?

Powerful magnetic fields and radio waves intense enough to change the spins of our atomic nuclei sound alarming. However, MRIs are much safer than X-rays, which can be used in CT scans to map many of the same features. X-rays’ ionizing radiation can damage DNA or make cells malfunction and so cause cancer if applied too frequently; if there’s a problem with excessive MRI use it’s more likely to be cost than danger.

There are two exceptions to this, however. One is when metal, specifically anything ferromagnetic gets into the magnetic field. The stories of guns and chains mentioned above are rare, but older pacemakers or certain implants can be catastrophic in MRIs.

MRIs can also be unsafe for people with claustrophobia, often exacerbated by the loud noises produced by the coils that create the magnetic field, but some machines are designed to address that (see below).

The helium problem

The powerful magnetic fields required by MRI machines naturally require extremely strong magnets. Although permanent magnets can in theory make such fields, it is much easier and cheaper to use superconductors. Although “high temperature” (still extremely cold by the standards of most of us) superconducting materials have been made, the most robust superconductivity requires cooling to temperatures below 9 K (-443°F), which is only really practical using liquid helium.

Although the Solar System formed out of a cloud where helium was the second most common element, the combination of lightness and non-reactivity with other elements means all primordial helium has long since escaped. The helium we have today is the product of nuclear decay releasing alpha radiation, which captures electrons to become helium atoms. These are sometimes trapped underground, often in the same cracks in the Earth that hold methane.

There’s enough easily available helium on Earth to run all the MRI machines we are likely to want for thousands of years, if that was the only thing we used it for. However, helium has other uses, of which the most familiar is party balloons, although some other scientific equipment also relies on helium-cooled superconductors.

There’s considerable concern that if we don’t ration helium, or price it properly, we will run out. If you see a MRI operator or neuroimager looking very glum at an event where escaping helium balloons are supposed to lift people’s spirits it might be because they’re worried about the effects on sealife when the balloons come down. Alternatively, they may also be thinking about an MRI-less future because we squandered our helium resource.

Open MRI

Some MRI machines do work without superconductors, however, relying on permanent magnets instead. That’s not yet because of a helium shortage. Instead, it is because ordinary MRI machines tend to trigger people with claustrophobia, particularly if they’re also sensitive to loud noise.

Using permanent magnets leads to less powerful magnetic fields, and lower resolution. However, it also provides an opportunity to make less enclosed MRI machines that can make patients more comfortable. It also avoids the need for helium.

All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.