h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Entry: Magnetic Resonance Imaging (MRI) - A2091034
Author: infinite_shadow - U538839

Hi, I am an MRI physicist doing a postdoc at the Univeristy of Oxford. I thought some basic info on MRI's for anybody who is getting one or just interested might be useful in the guide.

Your entry is much shorter than I had expected when I read the title. That's good, though.

You mention 1.5 Tesla as a typical field strength. Could you relate this in terms of, say, refrigerator magnets?

I work at a medical centre where MRI is conducted, and as a computer tech, I see video displays that are whacked by the intense magnetism.

On a practical level, I've always wondered how much of this exposure the magnetic stripe on my credit card could endure.

Good information.

Hello infinite shadow.

Nice entry.

Just a couple of little niggles.

There is a grammatical problem with "part so the brain are being used", but I don't know whether singular or plural is best.

The last sentence gives MRI as improving our health, but surely its the improved diagnoses that MRI permits that lead to better treatments, not the MRI itself that improve health.

Finally, you do not mention that having an MRI scan can be (in my personal experience) much less pleasant than an X-ray. When I had my neck done, my head was literally wedged into a frame, and for the duration of the scan the 'music' was awful. The overall effect left a giddy feeling. I do not know how much of this you might want to mention.

Such a good entry that I look forward to a similarly succinct explanation of CT scans.

Well done.

One thing I'd like to see mentioned is the reason why Peter Mansfield and his mate was awarded the Nobel Prize for Medicine: the use of gradient fields to spatially encode the signal from the body, which allows imaging to take place anyhow.

Hi
thanks for the feedback, I have added to the entry accordingly. These days all monitors used near MRI's are lcd to avoid getting ruined, plus shielding is getting better around the rooms. As to the credit cards, they will certainly not survive a trip into an MRI, however I have seen entry swipe-cards live through a close encounter. This was a huge surprise to me, I guess they are designed to be tough and not to be rewritable like video tape is.
Thanks again

I think some commentary about the fact that MRI really is a magnet is appropriate. Metallic fillings in teeth, earrings, pocketwatches, and plates in one's head or hips are all of interest.

Do MRI machines have a radio emitter themselves, or do they just detect the protons of the water in the body? Are the O protons as regular, affectable and detectable as the H protons are? I always thought MRI worked by turning the magnet on and off (and also realigning it), and then sensing what the protons do in the way of emitting radio waves as they sproing back to normal.

I edit medical reports from an imaging center that also has MRI and I see T-1 and T-2 and nuclear contrast materials mentioned all the time. I don't really understand these things in their entirety, but they seem to be part of how the imaging works, so it's be interesting to learn just what the implications are.

Hello everyone.

Just read latest version, and echo comments of foolish mortal and Sea Change about the magnetic field. One does not go to the trouble and expense of superconducting coils unless the elctromagnet required is very strong indeed. Cheap, ordinary electromagnets can do such things as pick up cars in scrap yards, which many would think of as a very strong magnet. Is it a large field strength, a large area, or the two combined, that require such exotic coils to be used?

The MRI scanner is a comparatively new device, and few humans have had any contact with such high magnetic fields. When X-Ray was first developed, I fancy that it was regarded as safe, but now we know better. Should the statement regarding the safety of MRI be put in a slightly less definite way, or is the case cast-iron?

It remains a jolly good entry, despite my comments.

T1: spin-lattice relaxtion time - time taken for the bulk magnetisation of the excited nuclei to 'relax' back to the ground state
T2: spin-spin relaxtion time - time taken for the rotating bulk magnentisation of the nuclei to 'decohere'.

Imagine a load of gyroscopes set so that they are all precessing together in perfect harmony. After a while, they end up pointing all over the place (T2). Then, some of them run out of energy and end up pointing downwards (T1).

I did my 3rd year degree project on NMR

This is an excellent entry, and with a little work, it will be ready for the guide.

First, a question: if MRI tracks the Hyrdogen atoms in water, how can it monitor Oxygen usage in the brain? That involves O and Haemoglobin, doesn't it? Perhaps you could explain this in the entry in one or two sentences.

The paragraph starting "The magnetic field is created" needs to be rewritten slightly. You never actually state that the patient is placed inside the coil. You should first describe the coil, then say that the patient is put in the coil, talk about the claustrophobia, then mention the noise and its cause.

I suggest you split up the first sentence into two, and remove the word 'however':

"MRI is a form of medical imaging. Whereas X-rays look at hard tissue like bone, MRI actually images the water in your body."

I suggest you remove the word 'just' in the sentence "MRI just uses a very powerful ...".

Put in a comma after "even over time".

Change "radio frequency" to "radio-frequency".

Well done, good work!

You might want to add a sarcastic comment about it once being called NUCLEAR Magnetic Resonance Imaging, but they dumbed it down because the anti-scientific ignoramuses that make up the vast percentage of the British public would have got scared by that naughty word.

Is MRI a type of Computer Aided Tomography (CAT), or is that term reserved for systems which use X-rays?

Er, yeah, I suppose so. Tomography just implies taking a slice, and MRI uses computers.

I suppose it is technically a form of CT scanning, but in medicine currently (in the UK) CT is used to mean a scan involing X rays. That's made pretty clear in the entry.

I do wonder if the entry would be made a little more readable if there were a few sub headers?

I was actually avoiding the nuclear term for that very reason, start telling people that they are full of "nuclear particles" and *gasp* "chemicals", and they will try to blame someone for it and sue them.

It is on the whle much safer than most of the other diagnostic imaging tools, when used within the guide-lines, mobile phones are more likely to be a worry. As for large fields, you are right it is still early on in the science, but I have been working with them for years and find my collections of extra testicles quite a talking point at parties...

I have to admit my mother (who may fall into the catagory along with the rest of the public) is firmly convinced that microwaves may cause cancer. When my Dad brought her a microwave as a gift she refused to use it for ten years, and only then because the conventional oven had broken.

I was actually thinking of doing a second Advanced MRI entry for those who are more interested in the physics behind it, but I didn't want to put too much high-end science in this one so anybody could understand it. Sound like a good idea?

That sounds like a good idea to me, perhaps you could entitle it "The Physics of MRI".

I was also wondering if anyone would like to add a section on what it's actually like to have an MRI scan. Apparently it's not that nice?

I shall get started on that then.

I have about 3 scans a day (scientist and monkey all in one), and I have got very good at sleeping through them, mind you most people who are getting them clinically are usually not very happy or else they wouldn't be in the hospital in the first place.

Remember people who read this may be those who have to have a MRI that they may be very anxious about. I have meet some terminanally ill patients who have decided not to have MRIs because they find it such an unplesent experience.