Physics and medicine have a close but changing relationship. Many discoveries in physics are regarded as major breakthroughs precisely when they bring medical benefits. Stethoscopes use membranes to amplify acoustic waves, thermometers utilize the thermal expansion of mercury to measure temperature, and sonography uses ultrasound to take a first look at the unborn baby. However, some discoveries in physics turn out to be a double-edged sword. X-rays, for example, allow us to look inside the body to recognize and correctly treat bone fractures. There is only one small disadvantage: X-rays destroy our bodies and can kill us.
X-rays penetrate our bodies and are attenuated to different degrees by different tissues. Bones do not let the radiation through as well as muscles and therefore cast shadows on the X-ray image. Unlike visible light, X-rays can penetrate so deeply into our bodies because they are very high in energy. On their way, however, they make a furrow of destruction through our body and, in the worst case, cause cancer.
Fortunately, however, it is part and parcel of science to constantly find (or cause) new problems that we can then solve again. Almost 70 years after the development of X-ray machines in 1896, magnetic resonance imaging was invented — also known by its nickname MRI. It does not use harmful radiation and only requires magnetic fields and radio waves for imaging: the more harmless and entertaining sister of X-rays.
The physical principle behind MRI is called nuclear magnetic resonance, or NMR for short. Before I explain in more detail what nuclear spin is and what resonances play a role, here is the basic idea behind MRI: X-rays cut their way through the body with a machete and as soon as the blade meets resistance you know that a tree (or bone) was in the way. MRI, on the other hand, shouts into the forest and listens carefully to see if an echo comes back.
Dancing particles
To understand nuclear magnetic resonance, we need to take a brief excursion into nuclear physics. Atoms consist of an electron shell and a nucleus. Atomic physics is concerned with the properties of the electron shell and assumes the nucleus to be a single particle. Nuclear physics, on the other hand, is interested in the nucleus of the atom. In the simplest case, the atomic nucleus consists of just a single proton, but in most cases it is made up of several protons and neutrons. This means that the atomic nucleus is always positively charged.
All these fundamental particles — electrons, neutrons, protons and many more — have a fascinating property: they whirl around their own axis like tiny little ballerinas. This property is called spin. Figuratively speaking, it indicates how much momentum a particle has as it spins around its own axis. For the sake of completeness, I would like to point out that this is only a comparison: spin is a quantum mechanical quantity and is more complex than a simple rotation. However, for most cases, and also for NMR, the image of pirouetting particles is sufficient. In physics, the “momentum” of a rotation is called angular momentum. Everything that spins has angular momentum: a ballerina or figure skater pirouetting; the earth spinning on its axis; or us spinning on office chairs.
An atomic nucleus usually consists of several protons and neutrons, all of which have a spin. The nuclear spin, i.e. the sum of the spins of all particles in the nucleus, in combination with its overall positive charge leads to the effect that atomic nuclei are magnetic. It has long been known in classical physics that moving charges generate a magnetic field. For example, a magnetic field is always created around a power cable through which electrons are travelling. The same applies to pirouetting atomic nuclei.
Remember: Atomic nuclei have a positive charge and a spin – and therefore behave like rotating bar magnets.
Precise precession
We turn a little further. For NMR spectroscopy, a magnetic field is applied around the atomic nuclei. In this field, the spins of the atomic nuclei behave exactly like spinning tops. So let’s take a closer look at the spinning top.
An ordinary toy spinning top is often made of wood, is symmetrical and tapers towards the bottom. If you balance it on its tip and let go, it simply tips over. However, if you turn it vigorously beforehand, it will defy gravity and remain upright. It owes this solely to its rotation, or more precisely, its angular momentum. Gradually, however, the spinning top will tilt to one side and gravity should simply pull it to the ground. But instead of tipping over, the spinning top turns sideways and continues to rotate. We call this movement precession. Only a long time later will the spinning top tip over due to friction losses (at least if you are not in a dream).
Just as a wooden spinning top precesses in the earth’s gravitational field, the atomic nucleus precesses as a magnetic spinning top in the magnetic field. And it does this at a very specific frequency, which depends on the strength of the external magnetic field and the type of atom. It is called the Larmor frequency and tells us a lot about the atomic nucleus and its environment.
Remember: In the magnetic field, the spin of the atomic nucleus precesses like a spinning top, i.e. it rotates around the magnetic field axis.
Once to the equator and back
While it is impossible to spin a wooden spinning top upside down on the ceiling, the nuclear spin can point upwards just as well as downwards. We therefore like to indicate the direction of the spin as an arrow pointing to any point on the surface of a sphere. And it is usually arbitrary too — if the atomic nuclei are free and untamed, their spin points in an arbitrary direction.
If the nucleus is located in an external magnetic field, there are two special cases. If the spin points to the north pole, i.e. it is parallel to the magnetic field, it is in its rest position. Its energy is minimal and it does not precess. In the other extreme case, the spin is at the equator of the sphere. Here the precession is strongest and the spin whirls in a circle. All other positions can be regarded as a mixture of these two extreme cases.
This difference between resting and excited spinning is the key to NMR spectroscopy. When the spin is at rest at the North Pole, we can turn it by giving it the necessary momentum. We know that the spin will spin at the Larmor frequency on the equator, so we have to shine electromagnetic rays at exactly this Larmor frequency onto the atomic nucleus, namely radio waves. If the frequency of the radio wave is equal to the Larmor frequency, we call it resonant. Just as a spinning top does not spin forever, nuclear spin does not spin forever either. After the nuclear spin has pirouetted for a while, it slowly returns to its resting position. However, as the energy that we have added to the atomic nucleus cannot simply disappear, it is released again in the form of radio waves.
Remember: In the rest position, the nuclear spin is aligned along the magnetic field. With the help of radio waves, it can be displaced and made to precess. The nuclear spin then returns to its resting position while emitting radio waves.
Marco Polo
How exactly can we use this excitation and de-excitation of the atomic nucleus for our purposes? How can we use it to look inside the body? The basic idea is: We irradiate the part of the body that we want to look at, for example the head, with radio waves and see whether the atomic nuclei send radio waves back. We shout “Marco” and wait to see if the nuclei shouts “Polo” back. But if all the nuclei in our head shout Polo in the same way, we don’t learn anything. It depends on which nuclei shout Polo, and how loudly.
It is important to note that the Larmor frequency of the atomic nucleus depends on the type of atom. Hydrogen rotates faster than oxygen, carbon rotates faster than oxygen. So only hydrogen will react to a radio wave with the Larmor frequency of hydrogen and nothing else. This is exactly what is done in MRI, as hydrogen is one of the most common atoms in the human body and reacts particularly strongly to magnetic fields. MRI images therefore do not show the tissue density like X-ray images, but the density of hydrogen nuclei.
The Larmor frequency also depends on the external magnetic field. This is the same for all atoms, as the magnetic field in the MRI tube surrounds the entire head. However, we remember that all atomic nuclei are small magnets themselves. So if a hydrogen atom sits next to an oxygen atom, they will feel each other’s magnetic field. The hydrogen nucleus therefore feels a stronger magnetic field when the oxygen atom is nearby than when it is alone. With its Polo-call, the atomic nucleus not only tells us which type it belongs to, but also who else is in the neighbourhood!
MRI not only allows us to look inside the body without harmful radiation, it also gives us much more information than a simple X-ray image. We can look into each other’s heads and hearts. We don’t have to make do with a simple, flat photo; MRI allows us to take three-dimensional images in real time. In the next article, I explain exactly how this works and why an MRI tube makes so much noise!
In this article, I explained MRI, which is one of the first-generation quantum technologies. Want to know what happens next? Then subscribe to my blog and never miss a new post!