How MRI Works

“I’m picking up good vibrations” If you can remember when that Beach Boys song came out you are of the same vintage as me! In the thirty odd years since then we have been offered more advances in technology than at the height of the Industrial Revolution.

Many of those advances have been in the field of medicine, and one of the most notable of those has been the NMR scanner (also called an MRI, Magnetic Resonance Imaging), for which one of the developers won a joint Nobel Prize. These very expensive, very large machines give us incredibly detailed images of the interior of the human body that make Xrays look the pale shadows that they actually are.

NMR stands for “Nuclear Magnetic Resonance”, let’s take these in order.

Nuclear: our bodies are made from thousands of different substances, proteins, amino acids, lipids etc. etc. Each of those is made up of collections of atoms and each atom has a nucleus. Each nucleus is made up of two other components, protons and neutrons (except for the very important hydrogen, which is part of the water atom and only has a single proton). If we could “tweak” those protons, twist them out of place and then let go, they would then spin back to their original position. Certain of the substances in the body contain protons capable of doing this.

Magnetic: these protons are a little like compass needles, put them in a powerful enough magnetic field and they will turn to line up with the magnets. If you have seen an NMR machine it is a very large drum, and some of that bulk is insulation to protect the incredibly powerful magnet, cooled by liquid gas, that is used for this purpose.

Resonance: here we find those “good vibrations”. If you “twang” a ruler that is hanging out over edge of a table it vibrates at its “resonant” frequency. Chose a thicker ruler and that note goes deeper, a thinner ruler and it goes higher. Each of those substances in the body that have protons capable of being twisted also has a unique resonant frequency. Subject the substance to that frequency and the protons all line up, go it and they all go back to where they were. So, play a series of “notes” to the body and, in turn, different substances will “twang” at different points on the scale. Though it has to be said that this “note” is more akin to that picked up by a radio than the human ear!

In this case the “ears” are radio receivers that can detect the direction the signal comes from. One checks the “up-down” direction, the other “left-right”. “End-to-end” is achieved by moving the body through the scanner a little bit at a time, scanning in a series of “slices”.

That was actually the fairly easy bit. We need to be able to see these substances to gather the information about the illness we are checking for, we need to be able to turn all those billions of twanging protons into a picture. What else could we use but the inevitable computer. The signals are recorded as a location by three numbers, how far up-down, left-right and end-to-end they are from a fixed point. The computer then converts these numbers into a form that can be displayed on a screen, or printed onto paper, as a series of slices. These sliced images can actually be used to make a solid, three dimensional, plastic model in an automatic process ideal for making substitute bone parts for shattered faces etc. Simple isn’t it? NO, this job requires computers and software that make the PC and word processor that I am typing this into seem rather simple in comparison. The scanner can also be set up to detect the presence and distribution (or absence) of specific chemicals that may aid diagnosis and the computer can calculate the size and volume of organs or tumors.

Many people, many years, many technologies and a good dose of inspiration and determination have gone together to produce a safe device for exploring the innermost details of the body, and it will inevitable do that with better results with each new model that comes out. The NMR or MRI scanner, in its various forms, is probably the most important diagnostic tool in medicine at the moment.

We can’t all work on technology that has such a large effect on our welfare, but, at any level the excitement and sense of achievement felt in solving a technical problem, making a discovery or inventing a new device is still a big thrill. Many inventors are still big kids playing, and the behaviour in the lab I work in demonstrates this graphically at times! But, that sense of play, of excitement, of wonder is a very important talent in looking a little beyond the boundaries of today.