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McGill Reporter
September 7, 2006 - Volume 39 Number 02
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The hows and whys of MRIs

An MRI machine is a very powerful magnet, and there's iron in our bodies. Why is it that this iron does not get attracted to the magnet?

Elie Tordjman,
Information Systems Resources

Generally, MRI scanners aren't calibrated to register the iron's presence. The magnets inside the scanner serve to produce the magnetic field (100,000 times stronger than the Earth's), which is created inside an MRI scanner by electrical current passing through a superconducting coil.

Iron, cobalt and nickel are ferromagnetic materials, which means they have a large and positive susceptibility to an external magnetic field. In healthy 75 kg human there are 3g to 4g of iron found in proteins and chemicals, such as hemoglobin. Due to the structure of these molecules, the iron is not ferromagnetic. Most of the iron-containing molecules in the human body are in paramagnetic and diamagnetic states, and therefore the interaction exists but is very weak. In other words, there is not enough iron present in any single concentration anywhere in our bodies to produce the reaction we normally associate with iron and magnets.

The chemical of most interest for MRI studies is proton, which is in hydrogen. The protons emit a certain frequency under the magnetic field, which the MRIs are calibrated to pick up.

In the MRI bore, the magnetic field imposes a steady state condition in which the protons align parallel to the magnetic field of the scanner. When a MRI pulse disrupts this steady state condition, the transiently non-aligned protons reassume the steady state imposed by the magnetic field. Relaxation times are necessary for the protons to return back to the steady state.

MRI images are spatial maps of the relaxation times that protons undergo during an MRI evaluation. Thus, MRIs reveal anatomy, functional states and chemistry in the human body's soft tissues.

Pedro Rosa-Neto,
McConnell Brain Imaging Centre, Montreal Neurological Institute

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