Computed tomography (CT)-sometimes called CAT scan-uses special x-ray equipment to obtain image data from different angles around the body, 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 with great clarity, including organs such as the liver, spleen, pancreas and kidneys, particularly when intravenous contrast (x-ray dye) is utilized. Furthermore, calcified structures such as kidney or gallstones can be accurately be detected with CT, and abnormal fluid or enlarged lymph nodes in the abdomen or pelvis can be identified. Although the hollow organs such as the oesophagus, stomach and small and large bowel are not as optimally assessed, CT is often used to indirectly diagnose processes in these organs by detecting abnormalities in the adjacent soft tissues. Newer techniques such as CT Colonography (Virtual Colonoscopy) allows primary assessment of the distended colon to detect polyps, the precursor to colon cancer, and shows promise in screening for this disease.
Because it is a non-invasive procedure that provides detailed, cross-sectional views of all types of tissue, CT is becoming the preferred method for diagnosing many diseases of the bowel and colon, including diverticulitis and appendicitis, and for visualizing the liver, spleen, pancreas and kidneys. In cases of acute abdominal distress, CT can quickly identify the source of pain. Especially when pain is caused by infection and inflammation, the speed, ease and accuracy of a CT examination can reduce the risk of serious complications caused by a burst appendix or ruptured diverticulum and the subsequent spread of infection.
CT is often the preferred method for diagnosing many different cancers, including kidney and pancreatic cancer, since the image allows a physician to confirm the presence of a tumour and to measure its size, precise location, and the extent of the tumour’s involvement with other nearby tissue (staging the tumour). CT examinations of the lower GI tract can be used to plan and properly administer radiation treatments for tumours, and to guide biopsies and other minimally invasive procedures. CT also plays a pivotal role in abdominal trauma assessment, as it is very sensitive at picking up bleeding within and around the solid organs. CT can also play a significant role in the detection, diagnosis and treatment of vascular disorders such as abdominal aneurysms and narrowing or stenoses in blood vessels, through administration of intravenous contrast, and a special technique called CT Angiography.
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 be asked to remove hairpins, jewellery, eyeglasses, hearing aids and any removable dental work that could obscure the images. You also may be asked to refrain from eating or drinking anything for an hour or longer before the exam. Women should always inform their doctor or x-ray technologist if there is any possibility that they are pregnant. In certain patients receiving intravenous contrast, a blood test prior to the CT Examination may be necessary to ensure that kidney function is satisfactory.
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, while different tissues absorb the 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. During each full rotation, as the fan-shaped x-ray beam is emitted through the patient’s body, an image of a thin section is acquired. The detector records about 1,000 images-or profiles-of the expanded x-ray beam with each rotation. The profiles are then reconstructed by a dedicated computer into two-dimensional images of the sections that were 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.
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.