Nuclear Magnetic Resonance Imaging Scanners can produce detailed images of the body from different views which allow clinicians to diagnose and treat diseases, and researchers to investigate specific structures and their relationship to various conditions.

An MRI Scanner generates a strong, stable and homogeneous magnetic field with a superconducting magnet. In conjunction with another gradient magnet(s) and Radio Frequency (RF) pulses of energy, a 2D or 3D point by point tissue map of the body is created. These points, cube-shaped voxels, can be as small as half a millimeter on each side, allowing for excellent detail. A functional MRI Scanner (fMRI) can even show blood flow in virtually any part of the body.

Image of an MRI Scanner.

How does this work? Our body, like anything else in nature, is made of atoms. Hydrogen atoms are most abundant and easiest to study because they have only one proton per nucleus. When hydrogen atoms of our body are placed in a magnetic field (the scanner), they line up in the direction of the magnetic field. Because their lineup can be in either direction, most of the nuclei cancel each other out; however, a sufficient number more will be in one of the two directions and are ready to give us the image we need.

At this point, the MR machine applies a RF pulse specific to hydrogen to an area of body that is to be studied (in our case, that would be the head). The pulse causes the hydrogen protons to absorb energy and then emit energy at a frequency that depends on the strength of the magnetic field. There are additional gradient magnets inside the main magnet that can alter the magnetic field locally and allow us to take images of “slices” in any direction of any particular area we want.

After the protons absorb energy, the RF pulse is turned off and the nuclei eventually release this energy to return to their initial state of equilibrium. The return of the nuclei to their equilibrium state takes place over some time and is governed by two physical processes:

This transmission of energy by the nuclei as they return to their initial state is what is observed as the MRI signal. The strength of the MRI signal depends primarily on three parameters:

The contrast between brain tissues is dependent upon how these 3 parameters differ between tissues. Different body tissue types, because of their different contents of excitable nuclei, generate signals of different intensity over a gray scale leading to an image more detailed than X-ray or CAT images. The gray scale below shows relative signal distribution typical for a Proton Density Image.

It is possible to manipulate the MR signal by changing the way in which the nuclei are initially subjected to electromagnetic energy. This manipulation can change the dependence of the observed signal on the three parameters: proton density, T₁ and T₂. Hence, one has a number of different MR imaging techniques ("weightings") to choose from, which accentuate some properties and not others.

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