Not everyone has a magnetic personality, but we all do have magnetic bodies. Spinning hydrogen nuclei in water behave like tiny magnets, and since the human body is mostly water, it’s just chock-full of magnetism. That’s been a boon for radiologists, who take advantage of the body’s magnetic fields in the high-tech diagnostic technique known as magnetic resonance imaging.
During an MRI exam a patient is placed inside a narrow, tube- shaped magnet. The external magnetic field causes the magnetic axes of those spinning hydrogen nuclei to line up in parallel. When a technician transmits a radio signal into the tube, however, the axes tip over. When the signal stops, the nuclei relax back to their original alignment, and in doing so they emit their own signal. Magnetic coils in the scanner read these signals and translate them into images of the body’s internal anatomy. Structures with a lot of hydrogen, such as fat and water-filled cysts, show up as bright areas. Dark areas are those with little hydrogen, such as bone and some cancerous tumors.
MRI scans are invaluable in diagnosing diseases and injuries, but there have always been some limitations on how much they can see. Since the technology relies on the magnetic fields of hydrogen nuclei, it’s trouble for the scanner when those fields are distorted anywhere. The culprit is usually oxygen molecules, which interfere with the magnetic field produced by the scanner, weakening the signal the hydrogen nuclei emit as their axes realign. The more oxygen molecules there are, the greater the signal loss.
The most troublesome spot is directly in front of the brain-- accurate readings of the area are hampered by pockets of air in the mouth, throat, ears, nose, and sinus cavities. In fact, with some MRI techniques the signal loss there is so great that the scan ends up with a big dark spot just above the nasal cavity. That means you have a black hole for the information from that part of the brain, says Michael Smith, director of Penn State’s MRI research lab.
There are other brain-mapping techniques, such as PET and CT scans, that can see the part of the brain obscured by the black spot. But MRI is cheaper and safer and produces sharper images than these other techniques. We would like to be able to use MRI to see what’s going on in that part of the brain, says Smith. We want to see what’s in the black hole.
Smith and his collaborators, graduate student Chris Collins and physicist Shizhe Li, are well on their way to seeing into that black hole. Their new computer models detail the magnetic distortions down to a hundredth of a millimeter and will enable radiologists to compensate for them when reading an MRI scan.
Because the magnetic field in the head varies so much over short distances, Smith’s models need to be extremely detailed. He got a head start from Viewpoint, a company that produces special effects for movies. Viewpoint had already developed detailed models of the body, which it modified to meet Smith’s medical needs.
Smith subdivided Viewpoint’s model of the head into 80,000 regions shaped as tetrahedrons, each one of which, on average, is only a few millimeters across. Smith’s team then calculated the magnetic field within each tiny tetrahedron, based on the abundance of water and oxygen molecules in that area. From these calculations they built their finely detailed, point-by-point model of the head’s magnetic field.
With these models Smith can determine any distortion of the magnetic field caused by oxygen molecules. He has already found a way to filter out the black spot in front of the brain after the scan is complete, but the method reduces the quality of the image. The next step will be to use the model to compensate for the loss of signal as an MRI exam takes place.
With the ability to compensate for distortion, radiologists will finally be able to take full advantage of new high-speed imaging techniques, which are even more sensitive to distortions in magnetic fields. The world is going toward faster imaging, notes Smith. We’d like to cut down the time of the MRI exam to reduce health care costs. Ultimately, the name of the game is, if we can’t do it quickly, we can’t perform that kind of medicine anymore.
I don’t think people were expecting that there would be a solution for maybe ten years, Smith adds. This, combined with other advances in imaging, will take MRI to the next level.
