Computerized tomography, more commonly known as a CT or CAT scan, is a diagnostic medical test that produces multiple images or pictures of the inside of the body (similar to traditional x-rays).
Unlike x-rays, the cross-sectional images generated during a CT scan can be reformatted in multiple positions and directions, and can even generate three-dimensional images. These images can be viewed on a computer monitor, printed on film, or transferred to a CD or DVD.
CT images of internal organs, bones, soft tissue, and blood vessels typically provide greater detail than traditional x-rays, particularly of soft tissues and blood vessels.
A CT scan of the face produces images that also show a patient’s paranasal sinus cavities. The paranasal sinuses are hollow, air-filled spaces located within the bones of the face and surround the nasal cavity, the system of air channels connecting the nose with the back of the throat. There are four pairs of sinuses, each connected to the nasal cavity by small openings.
What are some common uses of CT scans?
A CT scan of the sinuses primarily is used to:
- Detect the presence of inflammatory diseases.
- Aid in the planning for surgery by defining anatomy or giving further information about tumors of the nasal cavity and sinuses.
- Evaluate sinuses that are filled with fluid or thickened sinus membranes.
- Help diagnose sinusitis.
How should I prepare for a CT scan?
You should wear comfortable, loose-fitting clothing to your appointment. You may be given a gown to wear during the procedure.
Metal objects, including jewelry, eyeglasses, dentures, and hairpins, may affect the CT images and should be left at home or removed prior to your scan. You may also be asked to take out hearing aids and removable dental work. Women will be asked to remove bras containing metal underwire. You may be asked to remove piercings, if possible.
You will be asked not to eat or drink anything for a few hours beforehand, as contrast material (dye) will be used in your CT scan. You should inform your physician of all medications you are taking and if you have any allergies. If you have a known allergy to contrast material, your doctor may prescribe medications (usually a steroid) to reduce the risk of an allergic reaction. These medications generally need to be taken 12 hours prior to the administration of the contrast material. To avoid unnecessary delays, contact your doctor before the exact time of your scan.
Also inform your doctor of any recent illnesses or other medical conditions and if you have a history of heart disease, asthma, diabetes, kidney disease, or thyroid problems. Any of these conditions may increase the risk of an unusual, adverse side effect.
Women should always inform their physician and the CT technologist if there is any possibility that they may be pregnant.
What does the equipment look like?
The CT scanner is typically a large, box-like machine with a hole, or short tunnel, in the center. You will lie on a narrow examination table that slides into and out of this tunnel. The x-ray tube and electronic x-ray detectors are located opposite each other in a ring, called a gantry, that rotates around you. The computer workstation that processes the imaging information is located in a separate control room where the technologist operates the scanner and monitors your examination. He or she remains in direct visual contact with you and usually has the ability to hear you and talk to you with the use of a speaker and microphone.
How does the procedure work?
In many ways, CT scanning works very much like other x-ray examinations. Different body parts absorb the x-rays in varying degrees. It is this crucial difference in absorption that allows the body parts to be distinguished from one another on an x-ray film or CT electronic image.
In a conventional x-ray, a small amount of radiation is aimed at and passes through the part of the body being examined, recording an image on a special electronic image recording plate. Bones appear white on the x-ray; soft tissue, such as organs like the heart or liver, shows up in shades of gray; air appears black.
With CT scanning, numerous x-ray beams and a set of electronic x-ray detectors rotate around you, measuring the amount of radiation being absorbed throughout your body. Sometimes, the examination table will move during the scan so that the x-ray beam follows a spiral path. A special computer program processes this large volume of data to create two-dimensional cross-sectional images of your body, which are then displayed on a monitor. CT imaging is sometimes compared to looking into a loaf of bread by cutting the loaf into thin slices. When the image slices are reassembled by computer software, the result is a very detailed multi-dimensional view of the body’s interior.
Refinements in detector technology allow nearly all CT scanners to obtain multiple slices in a single rotation. These scanners, called multi-slice CT or multi-detector CT, allow thinner slices to be obtained in a shorter period of time, resulting in more detail and additional view capabilities.
Modern CT scanners are so fast that they can scan through large sections of the body in just a few seconds, and are even faster in small children. Such speed is beneficial for all patients but especially children, the elderly, and the critically ill, all of whom may have difficulty remaining still, even for the brief time necessary to obtain images.
For children, the CT scanner technique is adjusted to their size and the area of interest to reduce the radiation dose.
For some CT exams, a contrast material (dye) is used to enhance visibility in the area of the body being studied.
What will I experience during and after the procedure?
CT exams are generally painless, fast, and easy. With multi-detector CT, the amount of time that the patient needs to lie still is reduced.
Although the scanning itself is painless, there may be some discomfort from having to remain still for several minutes. If you have a hard time staying still, are claustrophobic, or have chronic pain, you may find a CT exam to be stressful. The technologist or nurse (under the direction of a physician) may offer you some medication to help you tolerate the CT scanning procedure.
If an intravenous contrast material (dye) is used, you will feel a pin prick when the needle is inserted into your vein. You will likely have a warm, flushed sensation during the injection of the contrast materials and a metallic taste in your mouth that lasts for–at most–a minute or two. You may experience a sensation like you have to urinate; however, this is actually a contrast effect and subsides quickly.
When you enter the CT scanner room, special light lines may be seen projected onto your body, and are used to ensure that you are properly positioned. With modern CT scanners, you will hear only slight buzzing, clicking, and whirring sounds as the CT scanner’s internal parts (not usually visible to you) revolve around you during the imaging process.
You will be alone in the exam room during the CT scan, unless there are special circumstances. For example, with pediatric patients, a parent may be allowed in the room but will be required to wear a lead apron to minimize radiation exposure. However, the technologist will always be able to see, hear, and speak with you through a built-in intercom system.
After a CT exam, the intravenous line used to inject the contrast material will be removed by the technologist, and the tiny hole made by the needle will be covered with a small dressing. You can then return to your normal activities.
- A CT scan is one of the safest means of studying the head.
- CT is the most reliable imaging technique for determining if the sinuses are obstructed. It is the best imaging procedure for sinusitis.
- CT of the sinuses is now widely available and is performed in a relatively short time, especially when compared to magnetic resonance imaging (MRI).
- CT scanning is painless, noninvasive, and accurate.
- A major advantage of CT is its ability to image bone, soft tissue, and blood vessels all at the same time.
- Unlike conventional x-rays, CT scanning provides very detailed images of many types of tissue as well as the lungs, bones, and blood vessels.
- CT examinations are fast and simple; in emergency cases, they can reveal internal injuries and bleeding quickly enough to help save lives.
- CT has been shown to be a cost-effective imaging tool for a wide range of clinical problems.
- CT is less sensitive to patient movement than an MRI.
- CT can be performed if you have an implanted medical device of any kind, unlike an MRI.
- CT imaging provides real-time imaging, making it a good tool for guiding minimally invasive procedures such as needle biopsies and needle aspirations of many areas of the body, particularly the lungs, abdomen, pelvis, and bones.
- A diagnosis determined by CT scanning may eliminate the need for exploratory surgery and surgical biopsy.
- No radiation remains in a patient’s body after a CT examination.
- X-rays used in CT scans should have no immediate side effects.
- There is always a slight chance of cancer from excessive exposure to radiation. However, the benefit of an accurate diagnosis far outweighs the risk.
- The effective radiation dose for this procedure varies.
- Women should always inform their physician and x-ray or CT technologist if there is any possibility that they are pregnant.
- CT scanning is, in general, not recommended for pregnant women unless medically necessary because of potential risk to the baby in the womb.
- Manufacturers of intravenous contrast indicate mothers should not breastfeed their babies for 24-48 hours after contrast medium is given. However, both the American College of Radiology (ACR) and the European Society of Urogenital Radiology note that the available data suggest that it is safe to continue breastfeeding after receiving intravenous contrast. For further information please consult the ACR Manual on Contrast Media and its references.
- The risk of serious allergic reaction to contrast materials that contain iodine is extremely rare, and radiology departments are well-equipped to deal with them.
- Because children are more sensitive to radiation, they should have a CT exam only if it is essential for making a diagnosis and should not have repeated CT exams unless absolutely necessary. CT scans in children should always be done with a low-dose technique.
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CAT Scan (CT) – Sinuses – RadiologyInfo.org, http://www.radiologyinfo.org/en/info.cfm?pg=sinusct