CT scans show a slice, or cross-section, of the body. The image shows your organs and soft tissues more clearly than standard x-rays. Because the picture is made by a computer, it can be enlarged to make it easier to see and interpret.
Since the late 1970s, CT scans have been very useful in helping doctors find cancer. CT scans can show a tumor’s shape, size, and location, and even the blood vessels that feed the tumor – all without having to cut into the patient.
Doctors often use CT scans to help them guide a needle to remove a tissue sample. This is called a CT-guided biopsy. They can also be used to guide needles into tumors for some types of cancer treatments, such asradiofrequency ablation (using heat to destroy a tumor).
By comparing CT scans done over time, doctors can see how a tumor is responding to treatment or find out if the cancer is coming back after treatment.
How does it work?
CT scans use controlled amounts of x-rays – beams of high-energy radiation that are passed through the body – to make pictures. In a way, CT scans are like standard x-ray tests (see the “Radiographic studies” section). But an x-ray test uses a broad beam of radiation aimed from only one angle. A CT scan uses a pencil-thin beam to create a series of pictures taken from different angles. Each angle produces a slightly different view of the organs and soft tissues. The information from each angle is fed into a computer, which calculates how the images overlap. The computer then creates a black and white picture that shows a slice of a certain area of the body – much like looking at a single slice from a loaf of bread.
The picture can be made clearer by the use of special contrast materials which can be swallowed as a liquid, put into a vein, or put into the intestines through the rectum as an enema. Because body tissues absorb these materials differently, the CT image will show greater contrast between types of tissues. This allows things like tumors to be seen more clearly.
In recent years, spiral CT (also known as helical CT) has become the most common type of CT used. It’s a faster machine that uses less radiation than the original CT scanner.
By placing CT image slices on top of each other, the machine can create a 3-dimensional (3-D) scan, which provides even more information about certain cancers. The 3-D image can be rotated on a computer screen to look at different views.