Imaging Techniques

Structural Techniques

Computed tomography (CT), also called computed axial tomography (CAT), is an imaging procedure that emits X-rays from different angles to specific areas of the body (such as the brain) to reveal structure. A series of two-dimensional images, or “slices,” are combined by a computer to form a three-dimensional cross-sectional view of the brain. MRI can also be used to obtain a structural image. Instead of emitting X-rays, MRI uses magnetic fields and radio waves to image the nuclei of atoms inside the body. MRI yields more detailed images than are possible with X-rays.

Activation Techniques: Electrophysiological Methods

Electroencephalography (EEG) is used to measure the electrical activity that occurs when neurons in the brain communicate. The electrical activity is detected by electrodes that are placed on the [Page 582]scalp. The recordings made by the electrodes are commonly referred to as brain waves because of their cyclic, rhythmic nature. Brain waves have traditionally been divided into bands according to their frequency (measured in number of oscillations per second—hertz [Hz]). Gamma waves (~ 30 to 100 Hz) are present when one is awake and engaged in mental activity. They are implicated in conscious perception and memory processes and may reflect communication between different brain areas. Beta waves (~ 12 to 30 Hz) are present when one is consciously alert, particularly during active thought and concentration. Alpha waves (~ 8 to 12 Hz) are present when one is in a mentally and physically relaxed state. Theta waves (~ 4 to 8 Hz) are present in a state of near-sleep. Delta waves (~ 0.1 to 4 Hz) are present during unconscious states like sleeping.

Magnetoencepholography (MEG) measures the magnetic fields created by the electrical activity of neurons and is thus closely related to EEG. The magnetic fields are very small and can only be detected by sensitive magnetometers called superconducting quantum interference devices (SQUIDS). EEG and MEG are similar in that they both record signals that arise (directly or indirectly) from the electrical activity of neurons. Each technique also has unique strengths and weaknesses. The magnetic fields measured by MEG decay more quickly over distance than electric fields. As a result, neural activity that is deeper in the brain can be difficult to detect with MEG. However, electric fields become very blurred and distorted when passing through the skull and scalp, whereas magnetic fields pass through unaffected; thus MEG can locate the neural activity source more precisely than EEG.

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