• CT scanning adds X-ray images with a computer’s aid to generate cross-sectional views of a patient’s anatomy.
• CT scanning can identify normal and abnormal structures and be used to guide procedures.
• CT scanning is painless and often performed in the outpatient setting.
• Iodine-containing contrast material is sometimes used in CT scanning. Patients with a history of allergy to iodine or contrast materials should notify their physicians and radiology staff.
Computerized (or computed) tomography, and often formerly referred to as computerized axial tomography (CAT) scan, is an X-ray procedure that combines many X-ray images with the aid of a computer to generate cross-sectional views and, if needed, three-dimensional images of the internal organs and structures of the body. Computerized tomography is more commonly known by its abbreviated names, CT scan or CAT scan. A CT scan is used to define normal and abnormal structures in the body and assist in procedures by accurately guiding instruments or treatments.
A large donut-shaped X-ray machine or scanner takes X-ray images at many different angles around the body. A computer processes these images to produce cross-sectional pictures of the body. In each of these pictures, the body is seen as an X-ray “slice” of the body recorded on a film. This recorded image is called a tomogram. “Computerized axial tomography” refers to the recorded tomogram “sections” at different body levels.
Imagine the body as a loaf of bread, and you are looking at one end of the loaf. As you remove each slice of bread, you can see the entire surface of that slice from the crust to the center. The body is similarly seen on CT scan slices from the skin to the body’s central part being examined. When these levels are further “added” together, a three-dimensional picture of an organ or abnormal body structure can be obtained.
In general, both CT and MRI scans are relatively safe. However, there can be problems. MRI scans should not be done on patients with aneurysm clips (clips of the vessels within the brain) unless these clips are known to be MRI safe, as these clips can be pulled off, and the patient could die from bleeding into the brain.
Another problem with the MRI is some cardiac pacemakers or defibrillators’ presence because the magnets can cause malfunctions in these battery-operated devices. Any metal devices that can interact with a magnetic field, for example, the presence of metal shavings in an organ, the eye, or extremity, may be pulled out by the magnetic field. Moreover, other canisters that are metal (like some oxygen tanks) need to be kept away from MRI machines because they can be attracted to the magnet and injure or kill the patient.
CT scans do not have these problems; however, they expose the patient to radiation, though it’s a relatively low dose. Certain types of CT scans may not be appropriate during pregnancy.
CT scans are performed to analyze the internal structures of various parts of the body. This includes the head, where traumatic injuries (such as blood clots or skull fractures), tumors, and infections can be identified. In the spine, the vertebrae’s bony structure can be accurately defined, as can the anatomy of the intervertebral discs and spinal cord. CT scan methods can measure the density of the bone in evaluating osteoporosis accurately.
Occasionally, contrast material (an X-ray dye) is placed into the spinal fluid to enhance further the scan and the various structural relationships of the spine, the spinal cord, and its nerves. Contrast material is also often administered intravenously or through other routes before obtaining a CT scan (see below). CT scans are also used in the chest to identify tumors, cysts, or infections that may be suspected on a chest X-ray. CT scans of the abdomen are extremely helpful in defining body organ anatomy, including visualizing the liver, gallbladder, pancreas, spleen, aorta, kidneys, uterus, and ovaries. CT scans in this area are used to verify the presence or absence of tumors, infection, abnormal anatomy, or body changes caused by trauma.
The technique is painless and can provide extremely accurate images of body structures and guide the radiologist in performing specific procedures, such as biopsies of suspected cancers, removing internal body fluids for various tests, and draining abscesses deep in the body. Many of these procedures are minimally invasive and have markedly decreased the need to perform surgery to accomplish the same goal.
A CT scan is a very low-risk procedure. The most common problem is an adverse reaction to intravenous contrast material. Intravenous contrast is usually an iodine-based liquid given in the vein, which makes many organs and structures, such as the kidneys and blood vessels, much more visible on the CT scan. There may be resulting itching, a rash, hives, or a feeling of warmth throughout the body. These are usually self-limiting reactions that go away rather quickly. If needed, antihistamines can be given to help relieve the symptoms. A more serious allergic reaction to intravenous contrast is called an anaphylactic reaction. When this occurs, the patient may experience severe hives and or extreme difficulty in breathing. This reaction is quite rare but is potentially life-threatening if not treated. Medications, which may include corticosteroids, antihistamines, and epinephrine, can reverse this adverse reaction.
Toxicity to the kidneys, which can result in kidney failure, is an infrequent complication of the intravenous contrast material used in CT scans. People with diabetes, dehydrated people, or patients who already have impaired kidney function are most prone to this reaction. Newer intravenous contrast agents have been developed, such as Isovue, which have nearly eliminated this complication.
The amount of radiation a person receives during a CT scan is minimal. In men and nonpregnant women, it has not been shown to produce any adverse effects. If a woman is pregnant, there may be a potential risk to the fetus, especially in the first trimester of the pregnancy. If a woman is pregnant, she should inform her doctor of her condition and discuss other potential imaging methods, such as an ultrasound, which are not harmful to the fetus. However, radiation exposure during a CT scan may cause a minimal increase in a person’s lifetime risk of developing cancer. This concern is often viewed as more critical for children because the cancer risk per dose of radiation is higher for younger patients than adults, and younger patients have a longer life expectancy. Still, the dangers of radiation exposure must be weighed against the benefits of using CT scanning to diagnose or treat illness. CT scanners can be modified to deliver exposures that are more appropriate for pediatric patients. Most physicians suggest that all radiation exposure to patients should be kept to a minimum; those patients that “doctor shop” or repeatedly go to emergency departments for a “CT” put themselves at risk for radiation-caused problems.
In preparation for a CT scan, patients are often asked to avoid food, especially when contrast material is used. Contrast material may be injected intravenously or administered by mouth or by an enema to increase the distinction between various organs or areas of the body. Therefore, fluids and food may be restricted for several hours before the examination. Suppose the patient has a history of allergy to contrast material (such as iodine). In that case, the requesting physician and radiology staff should be notified. All metallic materials and specific clothing around the body are removed because they can interfere with the images’ clarity.
Patients are placed on a movable table. The table is slipped into the center of a large donut-shaped machine that takes the X-ray images around the body. The actual procedure can take from half an hour to an hour and a half. If the radiologist performs specific tests, biopsies, or interventions during CT scanning, additional time and monitoring may be required. During the CT scan procedure, the patient minimizes any body movement by remaining as still and quiet as possible. This significantly increases the clarity of the X-ray images. The CT scan technologist tells the patient when to breathe or hold his/her breath during scans of the chest and abdomen. If any problems are experienced during the CT scan, the technologist should be informed immediately. The technologist directly watches the patient through an observation window during the procedure. There is an intercom system in the room for added patient safety.
CT scans have vastly improved doctors’ ability to diagnose many diseases earlier in their course and with much less risk than previous methods. Further refinements in CT scan technology continue to evolve, which promise even better picture quality and patient safety. CT scans known as “spiral” or “helical” CT scans can provide more rapid and accurate visualization of internal organs. For example, many trauma centers use these scans to diagnose internal injuries after a severe body trauma rapidly. High-resolution CT scans (HRCT) are used to assess the lungs for inflammation and scarring accurately. CT angiography is a newer technique that allows noninvasive imaging of the coronary arteries. Note that some CT scanners may not be able to accommodate patients that weigh over 400 pounds.
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