Computed tomography (CT) scans are a common method used to take pictures of the brain. While the images are not as high-resolution as an MRI scan, CT scans are faster and less expensive options that are especially good at detecting major problems like blood or fractures inside the skull.
To understand how a CT scan works, it's important to look back a bit in history. Originally, the only way to take a picture of what was inside someone's head was by using an X-ray. X-rays are beams of radiation that are absorbed to different extents by different types of tissues. For example, air hardly absorbs any x-rays, whereas bone absorbs a great deal. By putting a film opposite the source of the x-ray, we can get a sense of the number of X-rays that have penetrated the object (in our case, a head), and use that information to infer something about the nature of the tissue being investigated.
For example, because X-rays don't pass through dense bone, very few X-rays will hit the film if bone is between the X-ray source and the film. In this case, the film will remain white in the shape of the skull.
How a CT Scan Works
Computerized tomography was developed from X-ray technology, and many of the principles are the same. In CT, rather than just taking one shot of the patient, the X-ray beam is rotated around the head at different levels. The X-ray information is compiled by a computer to create a series of images that look as if the brain had been sliced somewhat like a loaf of bread. The slices start at the top of the brain and work down towards the base of the skull, depicting structures such as soft tissue, liquid, bone and air.
Like a traditional X-ray, dense structures appear lighter in color on a CT scan, and are referred to as hyperdensities. Darker areas, in contrast, are called hypodensities. For example, bone appears bright white on a CT scan, and cerebrospinal fluid appears dark. The brain appears in shades of grey.
How Abnormalities Appear on a CT Scan
A CT scan can detect several different problems in the skull.
- Hemorrhage CT scans are especially useful at detecting blood where it doesn't belong. Fresh intracranial hemorrhage coagulates almost immediately, becoming dense and therefore glowing brightly on CT scans. Eventually, the clot is broken down by the body, becoming the same density as the brain after about one week, and then appearing dark after two to three weeks.
- Ischemic Stroke Unlike hemorrhage, ischemic strokes are not usually immediately detectable on a CT scan. After about three hours, subtle signs can be appreciated by skilled readers of CT scans, and after 6 to 12 hours, a more obvious hypodensity becomes apparent in the area of the stroke. This density will become even darker with time as brain tissue is resorbed and replaced by cerebrospinal fluid.
- Tumors Tumors have differing appearances on a CT scan depending on the type of tumor and how advanced the cancer has become. Some tumors have calcification that glows brightly, and others form hypodense, fluid-filled cysts. Intravenous contrast dye can be useful in identifying tumors on a CT scan.
- Abscesses An abscess is an infection that the immune system has encapsulated as a way of sealing it off from the rest of the body. Abscesses usually appear spherical, and with contrast the rim of the sphere may appear to glow.
- Mass Effect When pressure builds behind part of the brain, it can move and compress important structures, distorting the brain's normal anatomy. On a CT scan, this mass effect can be seen as an asymmetry of normal structures like ventricles or sulci.
More Neurological Applications of CT Scans
CT scans can be combined with different techniques in order to better investigate specific parts of the nervous system.
For example, in order to get a better picture of the blood vessels in the brain, a CT angiogram can be done. In this study, contrast is injected into the arteries in order to highlight vessels of the brain. This is useful at detecting aneurysms and other vascular malformations.
A CT myelogram can be used to investigate the cerebrospinal fluid space in the spine. To do this, iodinated contrast dye is injected into the space by lumbar puncture. This can be useful in looking for nerve root or spinal cord compression.
CT perfusion studies again involve injecting contrast into the arteries, but this time the contrast is followed in real time as it travels through brain tissue. This is a technique sometimes used to investigate blood vessel function prior to endovascular treatment of acute stroke.
Properly performed, CT scans can be invaluable in the investigation of neurological diseases, especially in emergency settings.
Blumenfeld H, Neuroanatomy through Clinical Cases. Sunderland: Sinauer Associates Publishers 2002.
Robert I. Grossman and David M. Yousem. Neuroradiology: The Requisites 2nd ed. St. Louis, MO: Mosby; 2003.