Since our topic this week will be related to the use of Magnetic Resonance Imaging, or MRI, I thought I would write up a few words about how an MRI works. However, as I was googling around the internet, I came upon a pretty good write up on just that. I’ll give you the link further down the page since I don’t want to reinvent the wheel, but I just wanted to discuss one thing first. While reading some of these MRI articles, I realized that for most of them a knowledge of the physics of angular momentum was assumed. Angular momentum is an interesting phenomenon, it is one that keeps the earth spinning so we have day and night, keeps us stable on a bicycle, and is a key component of how an MRI works. So lets talk a bit about angular momentum.

Have you ever played with a top — you know the toy you spin that balances itself until it slows down too much? Or maybe, you’ve gone to the science museum and picked up a high tech top, a.k.a. a gyroscope.

There are two main concepts we talk about when describing the motion of a top, there is the spin, that is, motion around itself; and the precession, that is, the motion about an imaginary vertical line drawn from our table toward the ceiling. (If you think about the Earth, the axis of rotation is more or less a line drawn from the North Pole to the South Pole.) If you spin a top perfectly on a perfectly flat table it will just spin happily with no visible wobble (i.e. no precession) — in this case the axis of the spinning is exactly vertical and thus aligned with the axis of the precession. If we mounted a laser inside the the center of the top so that it is aligned with the spin axis and shoots a beam up towards our ceiling, then as the top spins steady on the table the laser with point at one spot on our ceiling and not move. In fact, if you ever-so-slightly incline the table, the top would move down the table and the laser would move along our ceiling in a straight line following the motion of the top across the table.

But if you lightly tap your top without disturbing its spinning, then its motion takes on another dimension — the spin axis will start to precess around that imaginary vertical line (or what is usually called the axis of precession). That is, it’s spin axis will rotate around the axis of the precession so that our laser would continually trace a large circle on the ceiling.

As an aside, have you ever played with a gyroscope? A good toy top will spin for a few seconds or maybe up to a minute, but even a cheap five-dollar gyroscope from your local science museum can spin and maintain it’s balance for long periods of time. The reason for this is due to their shape. The shape of a gyroscope is designed to max out the amount of angular momentum that it can store. As you have probably noticed already, I am a fan of Wikipedia, but the article for angular momentum goes a bit further than I want to discuss now. If you are interested check it out.

Ok, so what does all this have to do with an MRI? Well, I just wanted to get your mind thinking a bit about angular momentum because the MRI uses this concept, not with tops or gyroscopes or the Earth, but with individual atoms. The MRI manipulates the precession of the atoms in our body in a specific manner such that it can detect changes in the atom’s angular momentum! So without further ado, check out this article. The article is rather thorough, so if you are just interested getting down to the brass tacks, Section 7 How it works: Atoms and Section 8 MRI Machine provide all the essentials.