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

I did very basic GCSE physics, which I understood perfectly well, but I didn't do A Level.

I have now got a desire to understand how my MRI scanner works, and all the books I can find in the university library contain concepts I don't understand. Spin for instance, and quantum.

Are there any good physics A Level Textbooks I could read though.

The basics are not too difficult. If you need more depth of knowledge than this, it gets very inconvenient indeed.

The MRI produces a very powerful electromagnetic field. This causes the protons (hydrogen nuclei) in the bodies water atoms to become aligned. It's important to remember the term 'spin', in quantum terms such as these, does not refer to anything actually spinning; it is merely a way of distinguishing between sub-atomic particles - in a very simple sense, they are two marginally different energy levels named spin-up and spin-down. If you need to go into any more detail about spin, then that's when it gets seriously complicated.

Anyway, the MRI excites all (well, the vast majority) of the protons in the hydrogen nuclei of water to the same spin-up state. When the electromagnetic field is released, many protons will 'relax' back to their original spin-down energy level. And when sub-atomic particles move to a lower energy level, a photon of energy is emitted. These photons are detected and processed into the MRI image in much the same way as a digital photograph would be.

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Thanks - that's a useful summary.

That's the problem - I need more in depth knowledge, and the books with indepth knowlegdge require more physics background than I actually have. I think the key is to backfill the physics background.

So I need an A Level textbook with exercises! I read 'Quantum Physics cannot hurt you' by Marcus Chaucan, which was great, but it didn't include enough maths. I really need to work through problems a few times to get it!

Mmm, I posted earlier, where's it gone?

This was beyond A-level in my day. We touched on NMR (the same thing as MRI) in my A-level chemistry to the barest minimum point.
Physics had almost no quantum theory at all in it back then either. I think we touched on the concept of a wave function and that was about it.

A-levels haven't got any harder since (I did mine in 1989)

So you'll have to go beyond A-level for what you want.

I studied the maths of quantum mechanics in first-year physics in University, and it wasn't easy by any means. That was just the quantum mechanics of a single proton in an energy-well, the simplest combination. It only gets harder after that.

Spin is not separate from quantum theory by the way.

What Mu Beta said earlier is pretty good. Spin is, as he said, no really anything to do with what you might think of as spinning. There are mathematical properties associated with it that are handled using angular momentum equations though, hence the name - also it is related, spectroscopically to a similar property found earlier in electrons that was also called spin. (NMR and MRI measure transitions between nuclear spin quantum states).

What spin is in fact is a small magnetic moment associated with certain atoms nuclei.

So when you apply a *huge* magnetic field across a substance the nuclear spins can either align with the field or against it creating two energy levels that can be detected when you excite transitions between these two states by irradiating with radiofrequency electromagnetic radiation (radio waves).


I suspect much of that is gobbledegook as you will need to have a concept of spectroscopy to really understand it.

I would therefore seek out simple, 1st year undergrad level textbooks on spectroscopy and look for keywords such as Zeeman effect, Fine structure, hyperfine structure (this is the really important one), Atomic structure and then move on to NMR/MRI itself.
It might even be useful too look at UV-visible, infra-red and microwave spectroscopy as the concept is much the same, just with different energies involved and so different types of molecular/atomic transitions occurring.
(UV/vis looks at the excitation of electrons from one energy level to another, IR at vibrational transitions in molecules, microwave at rotational transitions)

'So when you apply a *huge* magnetic field across a substance the nuclear spins can either align with the field or against it creating two energy levels that can be detected when you excite transitions between these two states by irradiating with radiofrequency electromagnetic radiation (radio waves).'

Thanks Orcus, it's not gobblegook, I understand it, but I don't fundamentally *get* it. I'm not sure why that it. Perhaps the lack of knowledge fo spectroscopy.

I'm on the verge of having to venture out of the nice teaching hospital and into The Real World, of The Campus, which is an awkward drive away.

I don't really think you need to delve too deeply into quantum theory to understand NMR/MRI by the way.

It is enough I think to accept that nuclear spin (magnetic moment) exists in certain nuclei (the proton being the most important as far as MRI is concerned). And that this results in essentially two energy levels, the energy gap of which is equivalent to the radiofrequency region of the em spectrum

(E = hv --> the simplest equation in quantum theory is relevant here.
energy (E) = Planck's constant(h) x frequency (v) for one quantum of em radiation - energy is directly proportional to the frequency of light)


As to the workings of an MRI scanner then I believe one is looking at the equivalent of measuring 2D and 3D NMR spectra - this is an unbelievably difficult concept and I don't really follow much of it myself. You need first to understand the simple NMR theory - then you need to appreciate that modern NMR machines excite all frequencies within the range of the spectrum required in one go using square wave pulses. You then have to work in a rotating frame of reference and must be able to understand the concepts of signal relaxation in both the z-axis and the xy-plane and over time (bulk magnetisation is a mathrmatical tensor). Also needed is Fourier theory (NMR/MRI are Fourier transforms of the detected signals) and be able to understand the extremely cryptic pulse sequences NMR/MRI spectroscopists use.


In short, NMR/MRI is simple at the conceptual level but rapidly becomes extremely tricky. You may find this is a

I've made huge progress just reading a couple of review articles this morning. I would rather not get involved in this, but our phsycist is leaving the team and I want to be able to discuss the pros and cons of our novel MR technique.

I do wish I'd carried on physics further. It seems odd that when my teachers said 'you don't need physics to go to medical school' they meant 'You don't need physics to get into medical school'. It certainly would have been helpful later - it would have helped me get a grasp on scanning quicker.

Have a go at PET imaging first - that's much easier.