Chapter 113. Scanning the Brain

1. The human brain is hidden within the skull, but its shape and activity can be observed and measured with the modern tools of neuroscience. Bombarding the brain with x-rays and magnetic pulses can reveal the structure of each brain region and can detect abnormalities, including tumors and damage from strokes. The faint electrical and magnetic signals generated by individual neurons, as well as changes in blood flow to brain areas, can be captured and analyzed to paint a picture of the moment-by-moment pattern of brain activity.

2. The oldest of these tools is the electroencephalogram (EEG)—an amplified recording of the electrical signals generated by the synchronized activity of thousands of neurons in each region of the brain. These “brain waves” are measured by electrodes placed on the scalp. EEG is used extensively in research on sleep, as it can track the changes in overall brain activity as the sleeper moves through the stages of sleep. EEG can also identify the location of seizures in a patient with epilepsy.

3. Next came a type of brain scan called CT (computed tomography). This is a series of x-ray photographs taken from different angles in the same plane and combined by computer to create a single image that shows a “slice” through the brain. CT scans provide a crude view of the structure of the brain, and are used mainly to identify abnormalities in brain tissue caused by cancer, stroke, or fluid-filled cysts.

4. The brain requires glucose as fuel. Researchers developed a way of injecting a radioactive form of glucose into the bloodstream, and then using PET (positron emission tomography) to detect where that glucose flows while the brain performs a cognitive task. A PET scan provides a visual display of brain activity that shows areas of high glucose use in yellow and red, while areas of low glucose use are shown in green and blue. These PET scan images demonstrate that “looking” activates the visual areas at the rear of the brain, while “listening” activates the auditory areas on the sides of the brain.

5. MRI (magnetic resonance imaging) provides the clearest, most detailed view of the individual structures within the brain. This technique uses radio waves to disrupt magnetically aligned atoms in the brain. As the atoms snap back into position, the scanner detects the movement and produces a computer-generated image at such high resolution that it looks like a textbook illustration. On this MRI image, can you recognize and name some of the important structures within the brain?

6. A single MRI scan shows the structure of the brain, but also can detect the blood concentration of oxygen, another type of fuel that neurons require. By performing multiple MRI scans in quick succession, fMRI (functional magnetic resonance imaging) tracks changes in blood flow during different cognitive tasks. This scan shows that, when a person is engaged in a conversation, a high level of oxygen flows to the language areas of the brain.

7. The newest method of brain scanning is called MEG (magnetoencephalography). It detects the activity of clusters of neurons in specific regions of the brain. Rather than measuring the neurons’ electrical signals directly, MEG uses multiple detectors around the head to measure the magnetic fields generated by that electrical activity. Like EEG, MEG can track moment-by-moment changes in brain activity, but does a better job than EEG in identifying the exact location of the activity.