Basic concept of CT
CT scan is a diagnostic imaging procedure that combines the use of X–rays with computer technology to produce cross–sectional images of the body. The images produced by this scan are more detailed than those of an ordinary X–ray. A thin cross section of the body, i.e. a tomography is examined from multiple angles with a thin beam. After the beam passes through the body, the transmitted radiation is picked up by a series of detectors, fed into a computer for analysis by a mathematical algorithm and reconstructed as a tomographic image.As a result, the entire body can be visualized in a series of cross–sectional images. CT scanning produces clear, detailed images of all structures (bone muscle, fat, and organs) in the part of the body being examined.

Indications

• Tumors of various parts of the body.
• Aneurysms.
• Bleeding or injuries.
• After an accident to check for damage to internal body structures.

Preparation for CT

• During a CT scan, the patient lies still on a table.
• He or she removes all jewelry and metal items.
• The table slides into the CT unit, which is like a small tunnel.
• It is important not to move during the scan, although you may breathe normally.
• The scanner rotates around the table and may make clacking noises.
• Communication is maintained throughout the procedure.
• If a person becomes panicky the procedure may be stopped at any time. Some people have a fear of enclosed spaces.
• The doctor may give a mild sedative to restless or anxious people.
• Prior fasting for 3–4 hours is essential in all scans where IV contrast is to be administered.
• All scans related to abdomen and pelvis need opacification of the bowel (large and small) 20–30 minutes before commencing the study. This is done by asking the patient to drink 6–7 glasses of a flavored drink which contains a contrast material.
• Other scans not requiring any IV/oral contrast do not require any specific preparation.

Procedure
Common Indications for CT Scans
During a CT scan, the patient lies still on a table. He or she removes all jewelry and metal item. The table slides into the CT unit, which is like a small tunnel. It is important not to move during the scan, although you can breathe normally. The scanner rotates around the table, and may make clacking noises. Communication is maintained throughout the procedure. If a person becomes panicky, the procedure may be stopped at any time. Some people have a fear of enclosed spaces. The doctor may give a mild sedative to restless or anxious people. In some cases, an intravenous injection of a dye containing iodine is given before the scan. The dye helps to make blood vessels, organs, or abnormalities (such as tumors) show up better on the scan. With an abdominal scan, you may be asked to drink a barium solution several hours before the procedure. This solution lines the bowel and makes it more visible on the scan. You will be asked to lie on a table that slides into a large, circular opening in the scanning machine. As the table moves into the opening, a succession of X–ray beams are passed through the body from many different directions. Detectors in the scanner receive these beams and send signals to a computer. The computer processes these signals and produces a cross–sectional image on a viewing monitor.

Side Effects of CT Scan
Scanning typically lasts from 30 to 90 minutes. It is painless. The person may receive an injection of dye to create contrast. If so, the person is instructed to fast for 4 hours before the test. Some people are sensitive to the contrast dye. The dye may cause warm feelings, flushed face, temporary headache, salty taste, or nausea and vomiting.


Parts of the body that can be examined by CT

• Head and brain.
• Paranasal sinuses/orbits.
• Temporal bones (Ear structure).
• Neck.
• Abdomen/pelvis.
• All joints.
• Extremities.

Common Indications for CT Scans

• Brain.
• Spine.
• Head (excluding brain) and Neck.
• Musculoskeletal System.
• Interventional CT Guided Procedures.

Side effects of CT Scan and X–ray exposure
Some patients may also experience side effects as a result of contrast media reactions. Even though a thin pencil beam X–ray is being used, other parts of the body also receive some scatter radiation during the examination.
As far as possible, pregnant women should not be scanned, especially in the first trimester, unless absolutely indicated. In that situation adequate radiation protection measures should be adopted. After a CT scan, one should watch for signs of delayed reaction to the contrast dye. These include itching, rash, sweating, or difficulty breathing. The person should drink plenty of fluids to rid the body of the dye. CT scanning of the abdomen should not be performed during pregnancy.

Results
This scan shows a darkened area in the brain, which indicates that the blood supply to that area has been blocked. This is confirms the doctor’s initial diagnosis of stroke.

CT scan of brain showing the darkened area in the brain (arrows).