How CT Scans Work

A CT (computed tomography) scan or "cat scan" as they used to be called, is one of several diagnostic procedures that helps give doctors a better idea of what is going on inside a patient's body. Unlike X-rays, which only take one image of the body, CT scans take a series of images. Each image is a cross section of a portion of the body. When all the cross sections are put together with the help of the computer, they provide a much more in-depth look.

The advantage to having a CT scan done rather than an X-ray, is that a CT can "see" more features that would not be visible on a standard X-ray. For example, organs and tissues show up more readily on a CT scan. Additionally, a CT scan exposes the patient to less radiation than an X-ray, thereby making it a safer procedure. This is especially important since many cystic fibrosis patients will undergo dozens of X-rays in the course of a lifetime of treatment.

But how does it work?

The CT scanner device looks like a large doughnut that can rotate and tilt. X-ray beams on one side of the center opening are aimed directly across to a detector on the opposite side. The X-ray tube is positioned 180 degrees from the detector, similar to how a pitcher and catcher are positioned in baseball. The beams, like the baseball, are thrown from the x-ray tube (pitcher) to the detector (catcher.) This all happens at a very rapid pace. Moreover, the x-ray tube and detector maintain their relative positions to one another, all while continually rotating around the circle of the scanner.

The patient lies on a narrow table that is slowly moved through the center of the scanner. For the most part, the procedure is not altogether uncomfortable or confining. In some cases, such as scans of the sinuses or brain, the patient's head is stabilized or strapped down. This prevents the patient from moving while the scanner generates a 360 degree cross section.

The CT scan works in the following way: As the patient table slides toward the center of the ring, the scanner begins rotating. This is how it takes pictures of the body from every angle. After each series of pictures, the table moves a fraction of an inch-which is the equivalent to the depth of a slice of the total image-- as the scanner takes another series of pictures. Within minutes, the entire procedure is over, and the technicians have acquired an accurate cross-section image of a portion of the patient's body.

When are CT scans performed?

Computer tomography is a very powerful tool that can be used to assess nearly every portion of the body. CT scans are used when X-rays alone cannot provide an image with the level of detail required for the physician to make a diagnosis. For cystic fibrosis patients, CT scans are most often done to assess the need for sinus surgery. A doctor may also request a CT scan to look for abnormalities on the pancreas, gall bladder, or liver.

Although the CT scanner provides a very high resolution image for the doctor and radiology specialists to see, it cannot see everything. The tissues and sinews of the body may look more or less similar to one another, especially when viewed as a black and white image. In order to more readily identify an area of particular concern, a scan with contrast make be done.

For example, if a doctor suspects that a vein may contain a blood clot, a contrasting agent would be injected into the patient's vein. The contrast is a certain type of chemical dye that appears very dark on the scan. By identifying where on the scan the contrast appears or doesn't appear, enables the doctor to pinpoint a problem area.