A computed tomography (CT) scan is a special type of X-ray that can produce detailed pictures of structures inside the body. A CT scan is also called a computerized axial tomography (CAT) scan.
A CT scanner directs a series of X-rays through the head that are analyzed by a computer to produce a detailed picture of a "slice" of the area being studied. Each X-ray pulse lasts only a fraction of a second, and it takes only a few seconds for the machine to record each slice. A CT scan produces pictures of the brain that are clearer than regular X-rays. Information from a CT scan can be saved and stored on a computer for further study. Photographs of selected views can also be made.
Dense tissues, such as bones, appear white in the pictures produced by a CT scan. Less dense tissues, such as brain tissue or muscles, appear in shades of gray. Fluid-filled spaces (such as the openings in the brain, or ventricles) appear dark gray. Air-filled spaces (such as the facial sinus cavities) appear black.
For a CT scan of the head, the head is positioned inside a cylinder that is part of the CT scanner. The entire scanner can tilt, and the X-ray scanning cylinder within it can rotate to obtain the views needed. For a head scan, 10 to 30 slices are usually taken. The results are highly detailed images of the head, including the brain and bones of the skull.
A CT scan of the head can provide limited information about the eyes (orbits), facial bones, and the air-filled cavities (sinuses) within the bones around the nose. If these areas are of concern, additional CT scanning of the face (including 30 to 60 more slices) is necessary.
Sometimes a dye that contains iodine (contrast material) is injected into the blood during a CT scan of the head. The dye makes blood vessels and certain structures inside the head more visible on the CT scan images.
Why It Is Done
A CT scan of the head is done to:
- Evaluate certain symptoms (such as confusion, paralysis, numbness, vision problems, vertigo, or headaches) that might indicate a brain injury, a brain tumor, a ruptured aneurysm, or bleeding within the skull.
- Determine the extent of damage caused by a stroke and to help determine the effectiveness of treatment for a stroke.
- Help determine the cause of a loss of consciousness or a decreasing level of consciousness.
- Detect blood clots, tumors, bleeding, infection, or increased pressure in the skull or brain.
- Evaluate the effectiveness of treatment for a brain tumor.
CT scans of the eyes, facial area, and sinuses may be done to:
- Investigate problems of the eyes and the optic nerve. The test may detect fractures of the bones around the eyes or foreign objects in the eye.
- Investigate problems of the middle ear bones and the auditory nerve.
- Evaluate problems or diseases of the air-filled cavities (sinuses) within the bones around the nose.
- Evaluate problems with the bones and joints of the jaw, face, and skull (such as temporomandibular disorder or Paget's disease).