Magnetic resonance
imaging—how does it work?
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| 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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