A CT (Computed Tomography or CAT) scan is a special type of X-ray used to create two or three dimensional images similar to an MRI scan.
During a normal X-ray procedure, an X-ray machine sends out a single beam of X-rays through your body in the area being examined. In a CT scan, the scanner machine sends several beams through your body from many different angles. A specialized computer then collates the results from the different X-ray beams to create a cross-section image through your body.
CT Scanner (courtesy Siemens UK)
Beams that have passed through less dense tissue, such as the lungs, will be stronger, whereas beams that have passed through denser tissue such as bone, will be weaker. This allows the Consultant to get a clearer idea of the structure of the tissues in the area which has been scanned.
A common type of CT scan is a scan of the sinuses. This will show any variations in bony structure of the sinuses that may cause problems with drainage. If your Consultant decides to operate on your sinuses, he will follow the CT scan closely to check he is treating the exact area of your sinus that is causing the problem.