At one time, if doctors wanted to look at your brain they'd have had to cut open your skull. Fortunately, brain-imaging techniques are providing doctors and scientists with astounding insight into the brain without the need for scalpels and bone saws. Here are some of the ways doctors get into your head.
CAT scans are sophisticated X-ray images that can be assembled into stunning three-dimensional pictures of the brain. A CAT scan machine shoots X-rays through your head from many different directions. A computer analyzes multiple X-ray images, and deduces from them what a single section of brain looks like. After photographing many sections, the pictures be assembled into a model of the whole brain.
Although a CAT scan exposes patients to a significant amount of X-ray radiation, it is a widely available and important alternative to exploratory surgery.
The positrons in Positron Emission Tomography come from a radioactive material that a doctor injects into your blood stream or asks you to breathe in. For example, a doctor may have you inhale a radioactive form of oxygen. The radioactive material breaks down and emits antimatter particles called positrons. When a positron runs into an electron somewhere in your body, both the electron and the positron disappear and gamma rays are produced. A gamma ray detector determines where the gamma rays originated, which in turn indicates where the radioactive material was when it broke down. In the case of inhaled radioactive oxygen tests, a PET scan of the brain shows which regions are most active, because those regions use the most oxygen and should have the highest concentration of the radioactive oxygen that you breathed in.
Magnetic Resonance Imaging. Many of the atoms and molecules in your body have magnetic fields, much like the magnetic field of the earth or a simple bar magnet. Magnets line up in a strong magnetic field (that's why a compass needle points north). When you get an MRI of your brain, you stick your head inside a powerful magnet that lines up the atomic and molecular magnets in your brain.
Although the atoms and molecules tend to line up pointing in the direction of the magnetic field, they can sometimes absorb energy from a radio signal and flip over to point in the opposite direction of the field for a short time. They eventually flip back over and produce a small signal as they line up along the field again.
To produce an image of your brain, the doctor turns on a radio signal that can be tuned to make some of the atoms in your head flip over. But it doesn't help much to flip over all the atoms in your brain at the same time, so doctors use a set of magnets called the gradient magnets to make sure that the magnetic field and radio signal are just right for flipping atoms at only one place at a time.
Once the atoms at a certain point have been flipped, the radio signal shuts off, the atoms flip back over, and the resulting signal is detected with an array of antennas. The MRI machine produces a brain image by moving point-by-point through your brain, flipping atoms, and measuring the signal strength when the atoms flip back over. In effect, an MRI maps out the distribution of atoms in your brain. Because atoms in different types of tissue respond to the MRI signals slightly differently, the MRI ultimately produces an image of your brain tissue.
MRIs are good for evaluating soft tissue damage, such as the type that result from concussions, without exposing patients to X-rays or radioactive materials.
Angiography is another X-ray imaging technique, but instead of imaging tissue directly, the X-rays produce images of dyes injected into the blood stream. It is particularly useful for mapping blood vessels in the brain and organs, and for detecting circulatory problems that could lead to strokes or heart attacks.