MRI Theory Basics 1

This page presents basic MRI theory using animations that cannot be displayed on our current learning management system pages.

MRI Theory is not scary!

Don’t be frightened! This site will lead you gently through the basics of magnetic resonance imaging physics, with hardly any maths at all.

The essential controls of an MRI scanner are this simple:

Making most MRI images really amounts to nothing more than adjusting these slider controls and pressing the red and blue buttons in a carefully defined sequence.

Cool, so any halfway competent DJ who isn’t completely off his/her face could be an MRI radiographer? Well, not quite. Maybe an alien DJ. It all has to be done with millisecond timing and inhuman precision. So the MRI radiographer tells a computer how to do it.

This basic MRI theory course explains what these controls do to the patient’s body and how this generates the signals we use to make an MRI image.

There are a couple of things you do need to keep in mind to understand MRI theory:

1.  Time

One of the potentially confusing things about MRI relates to the fact that we have to think about changes over time – how fast things happen, and when different things happen in relation to each other. Unlike X-ray and visible light imaging MRI is relatively slow and to make it work we have to manipulate the imaging system with an imaging sequence.

A complicated spaghetti diagram like this:

is just a way of describing the sequence of slider adjustments and button presses on the control panel above. It’s actually a lot easier to understand than a music score.

2.  Individual things, and populations of things
In MRI theory we sometimes need to think about the behavior of single nuclei, and sometimes we need to think about very large populations of nuclei. We need to understand how they behave individually AND as a large population.

Dismembering Nuclear Magnetic Resonance

Let’s start by breaking down the term ‘Nuclear Magnetic Resonance’.

1. NUCLEAR

The signals we measure in MRI relate to the behavior of atomic nuclei.  We don’t break nuclei up (nuclear fission) or stick them together (nuclear fusion) – those changes require and release huge amounts of energy.  In fact the electromagnetic energy we use in MRI is very weak – so weak that it has no effect on chemistry or physiology. The photons we use in MRI are much weaker than the ones we use to read a book (and much much weaker than the potentially harmful ones used in X-ray imaging).

2. MAGNETIC

Some atomic nuclei are magnetic – especially the nucleus of hydrogen.  That is very convenient for clinical MRI because the human body contains lots of water (two hydrogen nuclei per water molecule) and, in most of us, lots of fat (many  hydrogen nuclei per fat molecule).

3.  RESONANCE

The magnetic properties of atomic nuclei are very subtle. To be able to ‘see’ them, and make an image based on differences in their behavior in different parts of the body, we have to disturb their normal equilibrium. We do this by exciting (annoying?) them with radio waves pitched at exactly the right frequency — the frequency that causes resonance and the creation of measurable magnetisation.

 Swing resonance occurs if you apply pushes at just the right rate – at the resonance frequency of the swing.

The hydrogen nucleus

To understand the basics of MRI we only need to look at the hydrogen nucleus. Nearly all MRI scans are based on hydrogen.

There are three ‘versions’ of the hydrogen nucleus: 1H (hydrogen), 2H (deuterium), and 3H (tritium).  1H is by far the most common and that is the only one we need to consider.

The 1H nucleus comprises a single proton, which we can think of as a blob that has mass and a positive electric charge. This blob has two very important quantum mechanical properties that give rise to the phenomenon of nuclear magnetic resonance:

1. Angular momentum (Spin).

A serious physicist would be very grumpy about describing the hydrogen nucleus as a spinning ball, but this idea works quite nicely for basic MRI. We CAN understand a lot of magnetic resonance if we think of the nucleus as being like a ball (or wheel or top) spinning on an axle — it has angular momentum.

Unlike a real ball, the spin of a proton never ‘slows down’ or disappears, and it makes no sense to ask “how fast is the proton spinning?” Spin just is.  If this sounds silly, you can blame the quantum mechanics people for giving that particle property a misleading name.

Spin is such a fundamental nuclear property that we often refer to hydrogen nuclei simply as ‘spins’. Another common name is ‘protons’, a name that happens to be correct in the case of 1H nuclei.

2. Magnetism

If we DO think of the proton as a positively charged spinning blob, then the positive charge moving in a circle will act like an electric current flowing in a circle. This will give rise to a magnetic field oriented along the axis of rotation. We call this tiny magnetic field the Nuclear Magnetic Dipole, and typically use an arrow or vector to represent its strength and orientation. The nuclear magnetic dipole really does exist, but it is not fully explained by the ‘spinning charge’ idea.

But watch out, this little magnet does NOT behave just like a familiar bar magnet or compass needle.

 The ‘spinning charge’ model of the 1H nucleus