CT (computed tomography), sometimes called CAT scan, uses special x-ray equipment to obtain image data from different angles around the body, and then uses computer processing of the information to show a cross-section of body tissues and organs.
CT imaging is particularly useful because it can show several types of tissue-lung, bone, soft tissue and blood vessels-with great clarity. Using specialized equipment and expertise to create and interpret CT scans of the body, radiologists can more easily diagnose problems such as cancers, cardiovascular disease, infectious disease, trauma and musculoskeletal disorders.
Because it provides detailed, cross-sectional views of all types of tissue, CT is one of the best tools for studying the chest and abdomen. It is often the preferred method for diagnosing many different cancers, including lung, liver and pancreatic cancer, since the image allows a physician to confirm the presence of a tumour and measure its size, precise location, and the extent of the tumour’s involvement with other nearby tissue (staging the tumour). CT examinations are often used to plan and properly administer radiation treatments for tumours, to guide biopsies and other minimally invasive procedures, and to plan surgery and determine surgical resectability.
CT can clearly show even very small bones, as well as surrounding tissues such as muscle and blood vessels. This makes it invaluable in diagnosing and treating spinal problems and injuries to the hands, feet and other skeletal structures. CT images can also be used to measure bone mineral density for the detection of osteoporosis. In cases of trauma, CT can quickly identify injuries to the liver, spleen, kidneys, or other internal organs. CT can also play a significant role in the detection, diagnosis and treatment of vascular diseases that can lead to stroke, internal bleeding, kidney failure, or even death.
You should wear comfortable, loose-fitting clothing for your CT exam. Metal objects can affect the image, so avoid clothing with zippers and snaps. You may also be asked to remove hairpins, jewellery, eyeglasses, hearing aids and any removable dental work, depending on the part of the body that is being scanned. You may be asked not to eat or drink anything for one or more hours before the exam. Women should always inform their doctor or x-ray technologist if there is any possibility that they are pregnant.
The CT scanner is a circular, ring-like machine with a large hole in the centre, something like a doughnut. The patient lies still on a table that can move up or down, and slide into and out from the center of the ring. Within the machine, an x-ray tube on a rotating gantry moves around the patient’s body to produce the images, making clicking and whirring noises as the arm moves. Though the technologist will be able to see and speak to you, you will be alone in the room during the exam.
In many ways, CT scanning works very much like other x-ray examinations. Very small, controlled amounts of x-ray radiation are passed through the body, and different tissues absorb radiation at different rates. With plain x-rays, when special film is exposed to the absorbed x-rays, an image of the inside of the body is captured. With CT, the film is replaced by an array of detectors, which measure the x-ray profile.
Inside the CT scanner is a rotating gantry that has an x-ray tube mounted on one side and an arc-shaped detector mounted on the opposite side. An x-ray beam is emitted in a fan shape as the rotating frame spins the x-ray tube and detector around the patient. Each time the x-ray tube and detector make a 360 degree rotation and the x-ray passes through the patient’s body, the image of a thin section is acquired. During each rotation, the detector records about 1,000 images (profiles) of the expanded x-ray beam. Each profile is then reconstructed by a dedicated computer into a two-dimensional image of the section that was scanned. Multiple computers are typically used to control the entire CT system.
You might think of it as looking into a loaf of bread by cutting the bread into thin slices. When the image slices are reassembled by computer, the result is a very detailed, multidimensional view of the body’s interior.
A relatively new technique, spiral (helical) CT has improved the accuracy of CT for many diseases. A new vascular imaging technique-spiral CT angiography-is noninvasive and less expensive than conventional angiography, and allows doctors to see blood vessels without the need for more invasive procedures.
The term “spiral CT” comes from the shape of the path taken by the x-ray beam during scanning. The examination table advances at a constant rate through the scanner gantry while the x-ray tube rotates continuously around the patient, tracing a spiral path through the patient. This spiral path gathers continuous data with no gaps between images.
With spiral-or helical-CT, refinements in detector technology support faster, higher-quality image acquisition with less radiation exposure. The current spiral CT scans are called multidetector CT and are most commonly four- or 16-slice systems. CT scanners with 32, 40 and 64 detectors now becoming available. These instruments provide either faster scanning or higher resolution images. Using 16-slice scanner systems the radiologist can acquire 32 image slices per second. A spiral scan can usually be obtained during a single breath hold (less than 30 seconds). Such speed is beneficial in all patients, but especially in populations in which the length of scanning was often problematic, such as elderly, paediatric or critically-ill patients. The multidetector CT also allows applications like CT angiography to be more successful.
With conventional CT, small lesions may frequently go undetected when a patient breathes differently on consecutive scans, as a lesion may be missed by unequal spacing between scans. The speed of spiral scanning and a single breath hold increase the rate of lesion detection.
The technologist begins by positioning the patient on the CT table. The patient’s body may be supported by pillows to help hold it still and in the proper position during the scan. As the study proceeds, the table will move slowly into the CT scanner “doughnut.” Depending on the area of the body being examined, the increments of movement may be so small that they are almost undetectable, or large enough that the patient feels the sensation of motion.
A CT examination often requires the use of different contrast materials to enhance the visibility of certain tissues or blood vessels. The contrast material may be injected through an IV directly into the blood stream, swallowed or administered by enema, depending on the type of examination. Before administering the contrast material, the radiologist or technologist will ask whether the patient has any allergies, especially to medications or iodine, and whether the patient has a history of diabetes, asthma, a heart condition, kidney problems, or previous reaction to intravenous x-ray contrast. These conditions may indicate a higher risk of reaction to the contrast material or potential problems eliminating the material from the patient’s system after the exam.
A CT examination usually takes five to 15 minutes. When the exam is over, the patient may be asked to wait until the images are examined to determine if more images are needed.
CT scanning causes no pain, and with spiral CT, the need to lie still for any length of time is reduced. For different parts of the body, the patient preparation will be different. You may be asked to swallow either water or a positive contrast material, a liquid that allows the radiologist to better see the stomach, small bowel and colon. Some patients find the taste of the contrast material mildly unpleasant, but most can easily tolerate it. Your exam may require the administration of the material by enema if the colon is the focus of the study. You will experience a sense of abdominal fullness and may feel an increasing need to expel the liquid. Be patient; the mild discomfort will not last long.
Commonly, a contrast material is injected into a vein to better define the blood vessels and kidneys, and to accentuate the appearance between normal and abnormal tissue in organs like the liver and spleen. Some people report feeling a flush of heat and sometimes a metallic taste in the back of the mouth. These sensations usually disappear within a minute or two. Some people experience a mild itching sensation. If it persists or is accompanied by hives (small bumps on the skin), the itch can be treated easily with medication. In very rare cases, a patient may become short of breath or experience swelling in the throat or other parts of the body. These can be indications of a more serious reaction to the contrast material that should be treated promptly, so tell the technologist immediately if you experience these symptoms. Fortunately, with the safety of the newest contrast materials, these adverse effects are very rare.
You will be alone in the room during the scan; however, the technologist can see, hear and speak with you at all times. When scanning children, a parent may be allowed in the room with the patient to alleviate fear, but will be required to wear a lead apron to prevent radiation exposure.