Pediatric Movement Disorders - Diagnostic Methods

Structural Imaging

The three major forms of structural brain imaging are...

• Ultrasound
• Computed tomography (CT)
• Magnetic resonance imaging (MRI)

Ultrasound is mostly used in neonates, where the large fontanelle (i.e., "soft spot") in the head allows the ultrasound waves to penetrate the skull. Ultrasound may also be used to look at the spine in neonates. Ultrasound operates by using very high frequency sound waves that bounce off structures in the brain and are reflected back to a microphone.

Depending on how long it takes the sound to get back to the microphone, it is possible to determine the location of a given structure and then draw a picture of this structure. The advantage of ultrasound is that it may be done rapidly at the bedside, without the use of sedation. The disadvantage is that the quality of the images is not as good as CT or MRI and areas near the top of the brain are difficult to see.

Computed Tomography (CT) uses a computer to analyze the data from a series of X-ray images. It then reconstructs "slices" that show various levels of the brain. CT measures density and can show calcium (in bone) or fluid (in brain) with accuracy. CT can show changes in brain density that occur when there is excess fluid in one part of the brain (edema) or other damage. The advantage of CT is that it is relatively rapid (requiring usually less than 5 minutes) and inexpensive when compared to MRI. CT scanners are also widely available at modern hospitals. The disadvantages of CT are the child's exposure to X-rays and the image quality that is not as good as MRI. Also, it is difficult to visualize the bottom part of the brain (i.e., cerebellum and brainstem), as so much bone surrounds these structures.

Magnetic Resonance Imaging (MRI) measures the changes in the way hydrogen atoms spin when they are placed in a very strong magnetic field. A computer controls the magnetic field to look at different slices of the brain and uses a radio transmitter to generate pulses that probe the hydrogen atoms and then return the information to a radio receiver. The MRI scanner's magnet usually occupies most of a special room. Because this magnet is so strong, the room must be shielded and no iron-containing metal is allowed into the area. Any metal with iron is attracted right into the magnet. A single MRI scan of the brain can take an hour or longer. The scanner may be adjusted to measure many different properties of the hydrogen atoms in the brain. Usually there is more than one set of slices of the brain, each set giving a different type of picture.

The advantage of MRI is that it provides the best resolution and, to some degree, can determine chemical composition. MRI can also "see" anywhere throughout the brain and spinal cord. There are no X-rays and no known complications of the procedure. The disadvantage is that the scans take longer and the child must be very still. Since the machine itself is noisy, some children experience claustrophobia, as the inner tube of the magnet is small. This experience often necessitates the use of sedation or general anesthesia. MRI scanners may be used to measure more than the density of hydrogen atoms in the brain. They may also be used for magnetic resonance spectroscopy (MRS), which can determine levels of lactate, choline, acetyl-aspartate, and other chemicals in the brain. These findings are often helpful in diagnosing certain metabolic diseases. MRI can determine whether there is damage to some areas of the brain, including the basal ganglia. This is an evolving field of research. As the technique improves, it will be possible to measure an increasing number of different brain chemicals.