Computed Axial Tomography
(CAT Scan, CT Scan)

he CAT scan (also called CT scan) is well-known by name, but do you really know what it is and understand how it works? A CT scan is usually one of the first tests done in a stroke evaluation, particularly during an acute stroke in the emergency room. This test can show areas of abnormalities in the brain, and can help to determine if these areas are caused by insufficient blood flow (ischemic stroke), a ruptured blood vessel (hemorrhage), or a different kind of a problem.  CT scans can be obtained on any part of the body, but the information here applies only to CT scans of the head.

What is a CT scan?
A CT scan uses x-rays to take pictures of your skull and brain.  The patient lies in a tunnel-like machine.   The inside of the machine rotates and takes x-rays of the head from different angles, which are later used by computers to make an image of a “slice” (or cross-section) of the brain.

Why do doctors use CT scans?
CT scans use computers and rotating x-ray machines to create images of slices, or cross-sections, of the brain.  Unlike other techniques, CT scans (and MRI scans) can show the inside of the head, including soft tissue, bones, brains and blood vessels.  CT scans can often show the size and locations of brain abnormalities caused by tumors, blood vessel defects, blood clots, and other problems.   CT scans are a primary method of determining whether a stroke is ischemic or hemorrhagic (see the “About Stroke” section).

Does a CT scan always diagnose a stroke?
No.  Even if you are having a stroke, it might not be seen on CT scan for several reasons.   In many cases, the involved area of the brain does not appear abnormal for the first several hours after the onset of stroke.  Also, the stroke region may be too small to be seen on CT scan, or it may be in a part of the brain (brainstem or cerebellum) which the CT scan does not image well.   Depending on the results of the CT scan, your doctor may wish to obtain additional testing, including an MRI scan.   MRI can be more accurate for stroke and other conditions, but it takes longer and is often not available under emergency conditions.  

What happens during a CT scan?
You will remove any metallic objects which could diminish the quality of the images (this includes jewelry, glasses, dentures, and hairclips).  You may also be asked remove your clothing and put on a patient gown.  A technologist will help you to lie face up on the  scanner table, with your head toward the "donut hole" of the CT scanner.  The technologist will position you on the table, and a device to hold your head in place may be used.  Then he or she leaves the exam room and goes to the control room, where you can still communicate by intercom.

An intravenous dye (contrast dye) may be given, through injection. This can help to highlight any areas of abnormality in the scan. 

While CT images are being taken, it is important to lie still on the table, which will be moving very slowly in order to image the brain.  It is normal for the CT scanner to make a whirring noise during the exam, so you should not be alarmed.  The table will be moving a few millimeters at a time in order to obtain images of small slices of the brain, until the exam is finished.  The procedure usually takes between 20 minutes and an hour.

What are the risks of CT scans?
The test is painless and there are few side effects.   The CAT scan uses very little x-ray radiation. If you receive contrast dye, there is a chance of an allergic reaction.   This reaction can be serious, and may require treatment with appropriate medication.  If you have allergies to any foods or medicines, particularly seafood or iodine, it is important to inform the technologist before the procedure.   You should also tell the technologist if you could be pregnant.

How does a CT scan work?
CT is based on the same principles as regular x-ray.  The x-rays are absorbed differently by the different parts of the body.  Bone absorbs the most x-rays, so the skull appears white on the image.  Water (in the cerebral ventricles, fluid-filled cavities in the middle of the brain) absorbs little, and appears black.   The brain has intermediate density and appears grey.  Most ischemic strokes are less dense (darker) than normal brain, whereas blood in hemorrhage is denser and looks white on CT.

In brain CT imaging, a fan beam of x-rays is sent out through the skull, and a device on the other side of the scanner picks up the different strengths of the x-rays.  After the x-ray tube and detector have made one 360° rotation, the image of one cross-section (a few millimeters in width) has been taken.  During this rotation, hundreds of snapshots are taken, which are later used by a computer to make the final image.

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