High field magnets are superconducting alloys that must be kept under liquid helium to remain below their critical temperature. If their temperature rises above the critical temperature, they become Ohmic resistors and can melt -- or at least boil off all their cryogenic helium and nitrogen rapidly enough to be dangerous (so called quenching).

Low field magnets are usually just solenoid coils. There are a number of low-field magnetic resonance devices, including benchtop relaxometers, which use either permanent magnets or electromagnets. This one just uses an electromagnet, I believe, and it looks like it's in a Helmholtz configuration. This is a smaller version of a design used primarily in veterinary contexts, e.g. so you can walk an upright horse into it.

(Edit: Someone below says it's permanent magnets here. Can probably look up their patent or FDA application to be sure.)

I think fixed machines are like 1.4 - 3 Tesla field strength. The portable one is like .05 Tesla... So maybe it doesn't even use superconducting magnets.

But you can get high temperature REBCO tape now and get super conductors that can be cooled with liquid nitrogen.

MRI machines go up to 7T now.

Wow, makes me wonder about adverse effects. Could you magnetize the hemoglobin in your blood?

.05 Tesla! Seems like there would be poor contrast and SNR. I need images.

I understand that there are now graphene based magnetic field sensors which offer orders of magnitude improvements in sensitivity, from the likes of Paragraf. I'd be surprised if they had already made it into product but low intensity MRI is certainly an application touted.

MRI/NMR doesn’t sense magnetic fields, so I don’t think that kind of sensor would help. You have to put your sample in a strong magnetic field for this method to work, but after that it’s just a matter of emitting some RF pulses and receiving some RF signals.

I thought they had to turn the magnet on and off. This would first align all the magnetic moments of the specific atoms and then as they randomize when the field is off, they interfere with a superimposed radio field creating peaks and troughs. Tomographic calculations are then applied to the collected radio signals to give an image. How can a permanent magnet be used?

There are a lot of NMR imaging techniques, and I'm not an expert, but my understanding (based on discussion with experts) is that the main magnetic field is static during all MRI scans. The gradient fields from electromagnets change, though, to alter the Larmor frequency across the thing being imaged.

The field is always on. Safety first. :)

If I understand it correctly, the machine uses permanent magnets rather than a giant solenoid.

The magnetic field is way less strong (0.064 Tesla compared to at least 1 in most machines), but the machine doesn't have to generate heat while producing a magnetic field

An MRI magnet doesn’t produce heat while producing a magnetic field. That’s not why they are cooled. They are superconductors, the resistance is 0.

> They are superconductors, the resistance is 0.

So long as they stay under their critical temperature. If that changes… well, a lot of liquid helium is about to stop being liquid.

And that's one of the reasons why a permanent-magnet MRI is so interesting -- it doesn't depend on a constant supply of power (for the helium cooler) and gaseous helium (to replace lost gas), so the system is portable, and maintenance costs are likely to be much lower.

I have some experience with NMR from a chemistry perspective. The magnetic field is generated through a super conductive coil. Cool it down with liquid nitrogen, generate the current and you have a superconducting magnetic field as long as you keep it cold.

If it warms up, it’s no longer superconducting and all that current starts generating heat which boils the liquid nitrogen off, which can be quite spectacular. Plenty of videos on YouTube.

So yes, turning the magnet on and off is not trivial at all, but as long as you keep it cold, you’re good.

The ones in hospitals normally use liquid helium, not liquid nitrogen, which accounts for the quench being so spectacular and expensive. The nitrogen just keeps the helium from boiling off too fast.

There is a big red button to quench the magnet in emergencies, but it can cost a lot of money to get the MRI going again.

Money and time. I recall one clinical machine that was still not operational I think something like 15 months after unintentional quench. That’s expensive real estate to be sitting idle.

15 months is exceptionally long. With a decent support contract, it can all be sorted out in a few weeks.

In theory it can, but depends a bit of damage and the state of your support . I suspect that one was due to arguing about the latter than the former.

There are superconductors that can use cheap liquid nitrogen now.