The CT scan is a medical imaging procedure that uses x-rays and digital computer technology to create detailed two- or three-dimensional images of the body. Unlike other forms of medical imaging, the CT scan can make an image of every type of body structure at once, including bone, blood vessels and the soft tissues.
The CT scanning equipment consists of a large gantry with a circular hole (like a large doughnut or Polo mint). Inside the gantry is a rotating ring that carries the x-ray source and electronic x-ray detectors. An attached table slides the lying down person through the hole.
Multiple x-ray projections are taken in thin cross-sections along the person's body (like a loaf of sliced bread). The detectors collect the x-ray information from each cross-section and send them to a special computer that combines them into an image.
The CT scan may also be referred to by its older name of computed axial tomography or 'CAT' scan.
When are CT scans used?
Some of the common uses of a CT scan include:
Assessment of a body part's structure or shape
Acute trauma - especially to the head and torso and assessment of fractures or dislocations
Oncology - diagnosis, staging and follow up to assess response to treatments.
Diagnosis of vascular disease
Acute neurological syndromes - especially in the management of stroke
An aid to planning some surgery
For certain interventional procedures (going inside the body) such as biopsy or needle aspiration
Some CT examinations involving the use of contrast medium may require a small amount of special preparation. The radiology department will give you instructions on how to prepare for the test and what to expect. The contrast medium may be taken orally or by intravenous injection (or both).
CT scan procedure
Generally, as part of the procedure:
You may be given a drink to consume before the scan. This drink is an oral contrast agent that helps to enhance pictures of the stomach, small intestines and bowel.
You will be asked to undress and wear a hospital gown. You must remove all metal objects including jewellery.
You lie down on the scanner table. The radiographer may use straps and foam pillows to position your body and help keep you still.
You may be given an injection of an iodinated contrast medium (a type of dye) to help produce better images. This substance may cause a strange warm feeling that lasts for a few seconds, a funny metallic taste in the mouth or the sensation that you have 'wet' yourself.
The table slides into the circular hole in the machine.
Depending on the body part and condition being investigated, a number of images may be taken as the table moves in and out of the circular hole.
The x-ray tube moves in a circle inside the gantry around you as it takes the x-ray images. Each rotation of the gantry takes less than a second and there may be a number of rotations depending on the examination.
Lie very still. This is very important, as movement will blur the images. You may be asked to hold your breath for a few seconds while the scanner takes the images.
The equipment makes clicks and buzzes while taking the images. Don't be alarmed - this is normal.
Depending on the type of medical investigation, the CT scan may take anywhere from a few minutes to half an hour.
Immediately after the CT scan you may be asked to wait while the radiographer reviews the images. In some cases, more images need to be taken. Once the radiographer has enough good images, you can dress and leave. The radiologist assesses the scans and the results are usually sent to your referring doctor.
Complications of a CT scan
The CT scan is a relatively safe procedure. On rare occasions, some people who have an injection of iodinated contrast material may have an allergic reaction to this substance. CT does have a relatively high radiation dose, same as you would receive from the general environment in 0.5-4.5 years dependant on what you have scanned. Despite the dose the benefits of having a CT scan generally outweigh any potential risks.