How MRI works

Magnetic resonance imaging, abbreviated to MRI, is nowadays very common in hospitals. Almost every hospital has at least one MRI scanner. Many people have had or will have a MRI scan in their life, and the images created with MRI show up everywhere more often. But have you ever wondered how such an image is created? This article will explain to you how magnetic resonance imaging works.

First of all do not confuse MRI with CT (computed tomography). The MRI and CT scanners look like each other, both scanners are impressively large machines that fill up the majority of the examination room, and both machines have a hole in the middle where the person to be imaged is placed. But there are some differences to the eye of a careful observer. CT scanners are flat; the hole is just several tens of centimeters thick, in contrast to the MRI scanners which have considerable depth; the hole is more of a short tunnel. Besides the differences in physical appearance, CT is based on imaging with x-rays and is mainly used for bone imaging, whereas MRI mainly images soft tissues.

Now that you have the right scanner in mind, let me introduce you to the concepts of magnetic resonance imaging. The first word we come across is magnetic. The main component of a MRI scanner is a very large and very strong magnet. Compared to the magnetic field of the earth, the magnetic field of the scanner is ten thousand times stronger, so if you walk with a compass in the vicinity of a MRI scanner, the needle will show you where the scanner is instead of where the north pole of the earth is. And if you are within a meter of the scanner, you will probably even feel the scanner’s magnet gently pulling at your compass needle. The magnet of the scanner has the shape of a tube or tunnel, this shape creates a magnetic field that is equally strong everywhere inside the tunnel, which is convenient when we want to use the magnetic field for imaging.

What happens when we place a person in this strong magnetic field? Our body consists mainly of water and water is build from oxygen and hydrogen atoms. As it happens, the nuclei of these hydrogen atoms behave like small compass needles. And just as compass needles point to the north pole, a majority of the hydrogen nuclei will point in the same direction along the tunnel. This sounds strange, but you won’t feel anything and it is not harmful.

With all hydrogen nuclei neatly aligned, they are ready for action; this action comes from our second word resonance. Imagine the hydrogen nuclei as people on a dance floor, when we turn on the music everybody will start dancing on the rhythm, in other words, the people start resonating to the music. Our hydrogen nuclei, however, like only one sort of music, an electro magnetic pulse, but they like it so much that after the pulse is turned off they will keep on dancing for a while. This is the moment at which we can listen to the music their ‘feet’ produce, in other words, measure the resonance signal.

Let us take a closer look at the music produced by our body. There are three factors determining the signal. The first is simple: the more hydrogen there is, the more signal. For the second and third factors, however, we have to return to the dance floor. When we ask the people to keep on dancing after the music is turned off, you will notice that people have a hard time sticking to the original beat. Slowly, everybody will adopt their own rhythm or stop dancing at all. The same happens with the hydrogen nuclei, the resonance signal will slowly fade out. When we ask the people on the dance floor to restrict their movements a little bit, they will probably not hold the rhythm as long as they would if they were moving freely. Also, when we allow the people to watch each other, they will be influenced by the other movements they see and it will be harder for them to keep the original rhythm. In case of the hydrogen nuclei, the tissue type determines how freely the hydrogen atoms can move and whether they can be distracted by other hydrogen nuclei. For example, in blood hydrogen can move freely and the signal will fade out slowly, whereas in fat hydrogen are closely bound and the signal will fade quickly.

The last question is how to do imaging with the resonance signal. It is easy to receive the signal produced by the hydrogen nuclei, but the difficult part is to differentiate from which location in the body the signal came from. To be able to image with magnetic resonance we have to encrypt the resonance signal with information about its origin. As I mentioned above – as far as resonance is concerned, hydrogen nuclei like only one sort of music. To be more specific, they like only one tone and the tone they like can be influenced by the strength of the magnetic field. By varying the magnetic field, we can encrypt the resonance signal with different tones. After reception of the signal we can do a frequency analysis to construct an image from the signal. Maybe you have never heard of a frequency analyzer but probably even your stereo set has one. It is the part where you can adjust for different tones (bass, mid, high). The more sophisticated your stereo is, the more categories of tones you can choose from. The MRI scanner has to be very sophisticated to be able to analyze the resonance signal for a detailed image.

Time for a summary, so far you know that we line up our hydrogen nuclei by the magnetic field and that we produce the signal by resonance from a radiofrequency pulse. You also learned that the signal depends on the amount of hydrogen present and the tissue type they are in. For imaging, small variations in the magnetic field encrypt the resonance signal with information about the origin and by frequency analysis the signal is unscrambled again to construct the final image. Magnetic Resonance Imaging is a beautiful but complex way of imaging the inside of the human body, I hope I have lifted a corner of the veil for you, enough to make you curious to see your own inner self.