Sunday, 16 November 2014

Magnetic resonance imaging—how does it work?

Magnetic resonance imaging—how does it work?

Alberta Children's Hospital 3T MRI @UofCr4kids and a 13C phantom

The two terms in the name, magnetic and resonance, are there because MRI involves magnetism and atoms spinning at a particular frequency or resonance. There is no ionizing radiation. Imaging methods that include x-ray, CT, PET, and SPECT all use radiation of some type—either seeing how much passes through the body (x-ray and CT) or adding a radioactive tracer and then monitoring where it is in the body (SPECT, PET). The fact that magnetism is the key component of MRI makes this one of the safest imaging methods for repeated scanning.

As an aside, this method started out in the chemistry labs. It is still there and is called NMR—nuclear magnetic resonance. When commercial medical systems came onto the market in the 80’s, the vendor and buyers didn’t want the patients to hear the word nuclear—so it was changed to MRI. The word nuclear was there because the thing that is spinning, that MRI detects, is the nucleus of the atom. The name also differentiated the method from electron paramagnetic resonance, a method that detects a spinning electron.

So here is how it works. Every nucleus spins. They spin at a frequency that is proportional to the local magnetic field. The larger the field, the faster they spin. Many nuclei have a magnetic moment, they act like tiny magnets. I like to think of the idea that if you spin a bar magnet under a piece of wire you can induce a current in the wire. Now you take some wire, bend it around the body and add capacitors to make it an FM antenna (sensitive in the MHz frequency range), and listen. If there are magnets spinning at the frequency of the coil, a current will be induced in the coil which can be digitized. That is where the signal comes from in MRI. It is all based on detecting those nuclei that spin at the resonant frequency of the coil.

We use protons for most imaging because the concentration of water in the body is high—about 80% of your weight.

But listening is not enough. Since all the little proton magnets are spinning randomly, there is no net signal. Imagine millions of spinning bar magnets, all in different orientations and positions. The FM antenna would not get an induced current. You need to get the nuclei to spin together.

So, enter the big magnet. Most of the structure that you see in MRI, the tube, is a large magnet encased in layers to keep it cold. The wire is superconducting, and so must be kept at liquid helium temperatures. Helium boils off and is expensive, so it is surrounded by a vacuum, then liquid nitrogen and another vacuum. When you run a current through the superconducting wire, the system becomes charged or magnetized. Since it is superconducting, once it is at the desired magnetic field, no more current goes in. The MRI’s are always on and are not taking any additional current to keep them at that magnetic field. Because they are always magnetic, the main danger in an MRI is walking between the MRI and a large metal object—since that metal object will be strongly attracted and you don’t want to be caught in the middle.

When you are in the MRI, your nuclei are spinning faster since the magnetic field is stronger. The frequency of a standard 1.5T  (T=tesla, a unit of magnetic strength) MRI is about 64 MHz on your FM dial. Since it is a commonly used frequency, the MRI rooms are encased in a shield to keep out radiofrequency noise. As the nuclei are little magnets, they also align along the main magnetic field. They start to ‘line up’ and spin together. There is no signal in the coil though as the spin is in the wrong orientation to pick up signal.

When you go into the MRI, the technologists positions an MR coil around the region they want to scan—this is the FM antenna. A small current is pulsed through the coil. This creates a local field for a short time and tips the spins of the nuclei. Now you have many spins lined up, spinning together, in an orientation that makes them sensitive to the MR coil. A signal is induced in the wire. Current is detected. Thus, no ionizing radiation—just the natural magnetism within your body is detected.

The creation of an image requires that the machine knows where in space the signal came from. Recall that the nuclei spin at a specific frequency for a specific magnetic field. If you put a gradient of magnetic field across the MRI, you can detect all the different proton frequencies and put them in the correct position in space, just by knowing their frequency. This magnetic field gradient is created by room temperature electromagnets that line the bore of the big MRI. As the strength and orientation of the gradient changes, there is tension caused between the electromagnet structure and the structure of the main MRI system. Try holding two bar magnets aligned the same way. They resist. When the gradients change orientation they cause tension in the hardware. This is what causes the banging you hear. The larger the gradient strength or the faster the change in orientation, the louder the banging. Manufacturers are working to make them quieter.

I hope that helped.

There is a ton of information on how MRI works on the net. If you like videos, the late Paul Callaghan made some great teaching videos on Youtube. Paul, you are missed.

If you want to go technical, I like the “Basics of MRI” by Joe Hornak

If you want to be amused at a low quality first pass at video-teaching… here was a stab of mine.

So, don’t worry about the noise in the MRI. Just relax and know you are safe. Many MR scientists volunteer for controls in MRI studies to get some much needed rest.

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