Science in plain english

How MRI (Magnetic Resonance Imaging) works


MRI_magnet
An MRI Magnet

The interior of the human body is largely made up of bones (the skull, tibia, fibula) and soft tissue (muscles, tendons, ligaments) most of which are invisible to the naked eye. Prior to the invention of MRI, the x-ray was the gold standard for getting a detailed view of the body’s innards without cutting it open. Even though the x-ray worked brilliantly for decades, it had a few drawbacks. First, excessive exposure to x-rays leads to deadly radiation poisoning. As a matter of fact, radiation poisoning actually claimed the life of the esteemed inventor of the x-ray (Marie Curie, 1867 – 1934). Second, it only allowed you to visualize things that were dense enough to block or significantly attenuate the x-rays. This meant that in visualizing the human body, the x-ray method was largely limited to the skeletal system. X-rays were perfect for diagnosing broken bones but were found wanting when it came to diagnosing torn muscles or severely sprained ligaments. Further, the encapsulating presence of the skull made it virtually impossible to visualize a living brain… a subject of intense interest till this very day. As is often the case in history, a medical necessity in the form of the need to visualize soft tissue and the brain itself served as the mother of an amazing invention: the magnetic resonance image.

MRI is one of the most astounding and influential technological tools in modern medicine. It is a cleverly designed system that takes advantage of the natural magnetic moment of a hydrogen proton to get a fairly detailed inside view of the human body. Unlike an x-ray, it can produce a complete image of part of the the human body’s interior… bone and soft tissue alike. When I first learned about MRI, a few questions came to my mind after it had recovered from being blown by the immediately preceding sentences… What sort of sorcery is this? How in the world can a giant magnet somehow produce such a detailed image of the body’s interior? In response to the first question, it isn’t sorcery at all… just the amazing wonder of science. In response to the second question, let me explain.

MRI lends itself well to imaging body parts because of the abundance of hydrogen present in the human body. It is common for one to think of oxygen as the predominant element associated with the human body because we all need to breathe it to stay alive. In actual fact though, it is hydrogen that is most plentifully present in the human body. Sounds weird… but think about it for a second. The human body is made up of ~70% water. The chemical formula of water is H2O which means 2 molecules of hydrogen bound to a single molecule of oxygen. In addition, hydrogen is also highly present in fat which is another major constituent of the body. The hydrogen atom also has another property that makes it suitable for magnetic resonance imaging… it has an odd number of protons at its center – 1. Any atom endowed with an odd number of protons will spin around its axis (just like the earth does) making it susceptible to the effects of a potent external magnetic field in a predictable way. A crucial property for MRI functionality as we will now see.

Applying a powerful external magnetic field leads to uniform alignment of hydrogen protons in the human body
Applying a powerful external magnetic field leads to uniform alignment of hydrogen protons in the human body

The first step in the generation of a high resolution image of the body’s interior is to artificially align a lion share of the hydrogen protons in the human body in the same direction. You can think of the hydrogen proton as a mini planet earth with a north and south pole, spinning around on its axis. In their natural state, these hydrogen protons are randomly aligned in a multitude of directions. With the application of a potent external magnetic field however, these protons start to precess and align themselves in the same general direction of the external magnetic field. You can liken this in your mind’s eye to the way the magnetic field of the earth pulls the needle of a compass. The additive effect of this alignment yields a net magnetization vector which is generally in the same direction as the external magnetic field. We will call this external magnetic field Bo. Pretty cool to think that from now on, whenever you see a giant circular magnet in any hospital you’ll be able to confidently tell people that its purpose is to get all the hydrogen protons in the subject’s body aligned in the same direction.

The next step is to exploit the artificially induced uniform alignment of the hydrogen protons to generate an image of the human body. Accomplishing this feat involves the intermittent introduction of a radio frequency pulse at a right angle (90 degrees) to the external magnetic field. The injection of this radio frequency pulse momentarily knocks the hydrogen protons out of alignment with Bo and into a high energy state which is at a right angle to the external magnetic field. The hydrogen proton does not “like” this high energy state and its natural tendency is to want to return to its original alignment with the external magnetic field. In a weird way, you can liken it to how we humans have a hard time changing deeply ingrained habits… always seeking to return to the natural urge to keep doing the same thing. When the radio frequency pulse is switched off, the protons relax to their original state which is unidirectionally aligned with the external magnetic field. Upon this relaxation the hydrogen protons release energy of a very distinct signature. These released packets of energy are picked up by receivers and elaborately reconstructed into images using the Fourier Transform (Jean-Baptiste Joseph Fourier 1768 – 1830).

MRI of a human knee
MRI of a human knee

Finely detailed reconstruction of representative images of the human body’s interior is made possible by the interesting fact that hydrogen protons in different parts of the body release different energy signatures. For instance, the hydrogen protons of the white matter in the brain have an energy signature that is readily distinguishable from that of the gray matter in the brain. This also holds true for the protons in the kidney, liver, heart etc. You can think of it as each component of the body having its own unique energetic fingerprint. This fundamental difference in energy signatures explains why we can readily reconstruct images of the human body with impressive attention to detail. Tangentially, it also is freaky to note that humans cannot feel a change in the polar alignment of all the protons in their body when under the effects of an MRI magnet. Further, no ill long term effects have been recorded in individuals who have been through an MRI scan. I for one can personally attest to not experiencing any ill after effects from my own MRI exam circa 2012.

Without Wax
Oyolu B.C. Ph.D

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