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What Is A CT Scan?
Computed tomography (CT), also known as Computed Axial Tomography (CAT), is a painless, sophisticated x-ray procedure. Multiple images are taken during a CT or CAT scan, and a computer compiles them into complete, cross-sectional pictures (“slices”) of soft tissue, bone, and blood vessels.
A CT scan obtains images of parts of the body that cannot be seen on a standard x-ray. Therefore, these scans often result in earlier diagnosis and more successful treatment of many diseases.
A CT scan is considered to be a safe examination. While CT imaging does involve x-rays, the diagnostic benefits generally outweigh the risks of x-ray (radiation) exposure.
In some CT scans, contrast agents or sedatives may be used. A
CT scanning was developed during the mid-1970s. The original systems were dedicated to head imaging and were very slow-it took hours to acquire the images for each individual slice. The newest scanners collect as many as four slices of data in less than 350 microseconds.
This great improvement in the speed of CT scanning has been accompanied by increased patient comfort and higher resolution images. And, as scan times have become faster, the time of x-ray exposure has decreased, providing better image quality at lower x-ray doses.
How Does A CT Scan Work?
CT uses a computer and a rotating x-ray device to create detailed, cross-sectional images, or slices, of organs and body parts.
A CT machine resembles a large, square doughnut. A flat “patient couch” is situated in the circular opening, which is about 24 to 28 inches in diameter. The patient lies on the couch, which can be moved up, down, forward, and backward to position the patient for imaging.
The CT scanner itself is a circular, rotating frame with an x-ray tube mounted on one side and a banana-shaped detector mounted on the other. A fan-shaped beam of x-rays is created as the rotating frame spins the x-ray tube and detector around the patient. For each complete rotation, one cross-sectional slice of the body is acquired.
As the scanner rotates, the detector takes numerous snapshots called “profiles.” Typically, about 1,000 profiles are taken in one rotation. Each profile is analyzed by computer, and the full set of profiles from each rotation is compiled into to form the slice-a two-dimensional image.
When Is A CT Scan Needed?
CT scanning has the unique ability to image a combination of soft tissue, bone, and blood vessels. Among all available imaging techniques, it is one of the best tools for studying the lungs and abdomen. It is also invaluable in cancer diagnosis, and is the preferred method for diagnosing lung, liver, and pancreatic cancer.
Other applications include:
- Diagnosis and treatment evaluation for heart disease
- Diagnosis of acute stroke
- Diagnosis and treatment evaluation for vascular diseases
- Measurement of bone mineral density (to detect the bone disease
- Diagnosis and assessment of traumatic injuries
CT is also used to diagnose problems of the sinuses and inner ear because it can generate high-resolution images of the soft tissues and fine bones of these structures.
CT provides detailed information for nearly every part of the body, including:
- The brain and its vessels, eyes, inner ear, and sinuses
- The chest, heart, aorta, lungs
- The neck, shoulders, and cervical spine
- The pelvis and hips, male and female reproductive systems, bladder and gastrointestinal tract
- The skeletal system
Advances In CT
Original CT scanners (1974 to 1987) would spin 360° in one direction and make an image (or slice), then spin 360° in the other direction to make a second slice. Between each slice, the machine would stop completely and reverse directions while the patient table was moved forward by an increment equal to the thickness of a slice.
In the mid-1980s, an innovation called the “power slip ring” allowed scanners to rotate continuously. This development led to a new type of CT called “spiral” or “helical” scanning.
Spiral And Helical CT
Spiral CT scanners image entire anatomic regions (like the abdomen or lungs) in 20 to 30 seconds. The scanner rotates continuously as the patient couch glides. And at that speed, most patients can hold their breath for the entire imaging session. That eliminates the possibility that image quality will suffer due to the motion associated with breathing.
The continuous nature of the images-there are no gaps between slices obtained through spiral scanning-means that the data can be reconstructed to provide three-dimensional images, displaying the entire volume of organs and vessels. This increases the likelihood that very small lesions will be detected.
Spiral CT has become the primary imaging technique for the chest, lungs, abdomen, and bones because of its ability to combine fast data acquisition and high resolution.
“Multi-Slice” Spiral CT Scanners
The newest “multi-slice” spiral CT scanners can acquire up to four slices in a single rotation and collect as much as eight times more data than previous state-of-the-art spiral CT scanners. This new technology will provide for more non-invasive imaging of a wider range of conditions in less time and with greater patient comfort.
“Virtual Reality” Imaging And Advanced 3D CT
New computer software and advanced computer systems combine with spiral CT to produce three-dimensional images that enable a growing number of non-invasive “virtual
Endoscopy involves the use of an endoscope–a tiny camera at the end of a thin tube–to visualize the inside of certain organs, such as the colon. Virtual endoscopy performed with CT allows visualization of these same organ interiors without using an invasive endoscope.
Some virtual endoscopy procedures, like the placing of a stent inside a major blood vessel, were not possible with conventional endoscopy. Endoscopes could not be used to visualize the inside of blood vessels, but spiral CT can.
Multi-slice CT, combined with 3D reconstruction, is the newest technology for the management of heart disease and stroke.
How Do I Prepare For A Conventional CT Scan?
In general, no special preparation is required for a CT scan. Comfortable, loose clothing should be worn, although in some cases a patient will be asked to change into a hospital gown for the examination. It is also important to remove any metal prior to the exam: jewelry, dentures, eyeglasses, belt buckles, and metal zippers and buttons can interfere with the images.
A patient may be asked to limit eating and drinking to clear liquids, such as water, black coffee and tea, and broth for several hours prior to the appointment for a CT scan. This request will be dependent upon the facility and the type of examination.
For many CT examinations, a
When Is A Contrast Agent Required?
CT contrast agents, sometimes referred to as “dyes,” are used to highlight specific areas so that the organs, blood vessels, or tissues are more visible. By increasing the visibility of all surfaces of the organ or tissue being studied, they can help the radiologist determine the presence and extent of disease or injury.
Contrast agents are available in several different forms, but in general a CT contrast agent is a pharmaceutical substance. Some of the more common contrast agents used are:
- Barium sulfate
Contrast agents for CT examinations are administered in four different ways:
- Intravenous injection
- Oral administration
- Rectal administration
- Inhalation-this is a relatively uncommon procedure in which xenon gas is inhaled for a highly specialized form of lung or brain imaging. The technique, xenon CT, is only available at a small number of locations worldwide and is used only for rare cases.
Intravenous contrast is used to highlight blood vessels and to enhance the structure of organs like the brain, spine, liver, and kidney. The contrast agent (usually an iodine compound) is clear, with a water-like consistency. Typically the contrast is contained in a special injector, which injects the contrast through a small needle taped in place (usually on the back of the hand) during a specific period in the CT exam.
Once the contrast is injected into the bloodstream, it circulates throughout the body. The CT’s x-ray beam is weakened as it passes through the blood vessels and organs that have “taken up” the contrast. These structures are enhanced by this process and show up as white areas on the CT images. When the test is finished, the kidneys and liver quickly eliminate the contrast from the body.
Need To Know:
Is Iodine a Safe Contrast Agent?
Iodine is considered to be a safe contrast agent. It has been used for many years without serious side effects. Because iodine contrast increases the visibility of target tissues on the images, the benefits are considered to outweigh the risks.
The most common side effect of iodine is a warm or “flushed” sensation during the actual injection of the iodine, followed sometimes by a metallic taste in the mouth that usually lasts for less than a minute. No treatment is necessary for this sensation, if experienced.
Another mild reaction is itching over various parts of the body. This reaction lasts from several minutes to a few hours after the injection. When this reaction occurs, medication is usually administered to counteract the itching.
More serious allergic reactions, while uncommon, include difficulty breathing and swelling of the throat or other parts of the body. These reactions, if experienced, are treated immediately.
Newer forms of contrast help to reduce the risk of an allergic reaction. If you have had an allergic reaction to iodine or a contrast agent in the past, the physician may recommend one of these newer agents.
In some cases, CT can still provide valuable diagnostic information without the administration of a contrast agent, so the physician may decide this is the best course of action.
Oral CT Contrast
Oral contrast is used to highlight gastrointestinal (GI) organs in the abdomen and pelvis. If oral contrast will be used during an examination, the patient will be asked to fast for several hours before administration.
Two types of oral contrast are used:
Barium sulfate, the most common oral contrast agent, resembles a milk shake in appearance and consistency. The compound, available in various flavors, is prepared by mixing with water.
Gastrografin is a yellowish, water-based drink mixed with iodine. It can have a bitter taste.
When oral contrast has been requested by the doctor, patients usually drink about 1,000 cc to 1,500 cc (the equivalent of three or four 12-ounce drinks).
After the contrast is swallowed, it travels to the stomach and gastrointestinal tract. Like intravenous iodine, barium and gastrografin weaken x-rays. On CT images, the organs that have “taken up” the contrast appear as highlighted white areas.
Need To Know:
Is Oral Contrast Safe?
In general, both barium and gastrografin contrast are safe and pass uneventfully through the gastrointestinal tract. Minor and temporary side effects, such as constipation, may occur.
Rectal CT Contrast
Rectal contrast is used when enhanced images of the large intestine and other lower GI organs are required. The same types of contrast used for oral contrast are used for rectal contrast, but in different concentrations.
Rectal CT contrast is usually administered by enema. When the contrast is administered, the patient may experience mild discomfort, coolness, and a sense of fullness. After the CT is complete, the contrast is drained and the patient may go to the bathroom.
The preparation for rectal contrast is similar to oral contrast, in that the patient should be fasting for several hours before the test. In addition, the patient will be required to use a Fleets Enema to cleanse the colon; it is usually used the night before the examination.
Need To Know:
Is Rectal Contrast Safe?
Rectal contrast is considered to be safe and passes through the gastrointestinal tract uneventfully. Minor and temporary side effects, such as constipation, can occur.
CT Scan: What Happens During The Procedure?
If an intravenous contrast agent will be used, the procedure will be explained and the patient will be asked to sign a consent form. The needle will then be placed and taped down.
The technologist settles the patient on the scanner’s “couch.” The technologist glides the couch into place within the opening of the gantry, using cross-hair positioning lights to put the “target” area (for example, the chest) in the path of the x-rays.
Some types of CT (involving the head, sinuses, throat, etc.) require that the head be immobilized during the procedure. In these cases, soft straps are used to extend the neck and hold the head in place.
After the patient is in position, the technologist usually leaves the CT room. The scanner is generally controlled by a computer in an adjacent room, which has a window facing the machine and patient. During this time, the technologist and patient can easily communicate through an intercom.
When images are being acquired, the patient is usually asked to hold his/her breath and remain motionless. Image acquisition typically lasts 20-30 seconds. When the scanner and patient couch move, the patient may hear whirring or clicking noises-this is normal. In addition, the scanner may tilt forward or back to capture images from the best angle during an examination of the head, sinus, inner ear, and spine.
It is very important to lie completely still while images are being taken. Any movement can reduce the clarity of the images, and the radiologist may then have difficulty interpreting them.
What Happens After The CT Scan?
When the scan is complete, the couch glides out of the scanner. If the patient changed into a hospital gown before the exam, he/she will be permitted to get dressed. However, the radiologist usually asks the patient to wait while the images are reviewed, in case more images are necessary.
After the CT images are reviewed, the patient is free to go. Generally, normal activities can be resumed at once. In some cases, when a contrast agent has been used, the doctor may provide specific after-care instructions.
Frequently Asked Questions: CT Scan
Here are some frequently asked questions related to CT scan.
Q: Does CT hurt?
A: CT imaging itself is painless. The patient is required to remain motionless during the examination (which is difficult for some people), but the actual scan causes no bodily sensation.
Q: How long will the CT take?
A: The length of a complete CT examination varies, depending on the type of CT required. Procedures usually take between 10 and 45 minutes. Some of the more complicated CT examinations take longer than 45 minutes.
Q: Do all CT scans require the administration of a
A: Not all CT examinations require the use of a contrast agent. When a contrast agent is required, it is because the radiologist and referring physician determine that it is necessary for diagnosis. Contrast agents are considered to be safe and side effects are uncommon. The benefits associated with the improved imaging of particular organs generally outweigh the low risk of allergic reaction.
Q: Is it all right to have a CT during pregnancy?
A: Pregnant women should not have a CT, or any other x-ray examination, while in the first trimester (the first three-month period) of the pregnancy. Other exams, such as ultrasound, are available to help diagnose a medical condition in such cases.
Q: What is the difference between CT and MRI?
A: CT and MRI differ in two basic ways.CT uses x-rays to detect and record the radiation absorbed by different tissues, and sends the data to a computer to transform into images. MRI does not use x-rays. Instead, MRI employs a powerful magnetic field to monitor the nuclei of hydrogen atoms in water, the most abundant element in the body. When subjected to the magnetic field of an MRI, the hydrogen protons are knocked out of alignment and emit a radiofrequency signal that is detected by the MRI machine, which then processes the signals into images.CT scans usually show little differentiation in soft tissues, but highlight solid structures, like calcium deposits or kidney stones. MRI scans emphasize detailed tissue structures due to differences in water content.
Putting It All Together: CT Scan
Here is a summary of the important facts and information related to CT scan.
- Computed Tomography (CT), also known as Computed Axial Tomography (CAT), is a painless, sophisticated x-ray procedure that uses a computer to provide images of soft tissue, bone, and blood vessels.
- CT imaging is considered a safe examination. While CT does involve x-rays, the diagnostic benefits are usually considered to outweigh the risks of x-ray exposure or injections of contrast agents (or use of sedatives) during the procedure.
- CT combines a computer and a rotating x-ray device to create detailed cross-sectional images, or “slices” of organs and body parts.
- CT scanning has the unique ability to image a combination of soft tissue, bone, and blood vessels. Among all the other available imaging techniques, it is one of the best tools for studying the lungs and abdomen. It is also invaluable in cancer diagnosis, and is the preferred method for diagnosing lung, liver, and pancreatic cancer.
- The newest “multi-slice” spiral CT scanners can collect as much as eight times more data than previous state-of-the-art spiral CT scanners.
- In general, no special preparation is required for a CT scan. Comfortable, loose clothing should be worn, and any articles of clothing or jewelry that might degrade the images, such as earrings, glasses, dentures, belts, etc., should be removed.
- Many CT examinations require the oral or intravenous administration of a harmless
contrast agent, a liquid that enhances imaging of certain organs or blood vessels.
- Contrast agents for CT examinations are administered in three different ways:
- Intravenous injection
- Oral administration
- Rectal administration
- A relatively uncommon type of contrast that consists of a gas that is used for special lung and brain imaging. The technique is called Xenon CT and is only available at a small number of locations worldwide. It is used only for rare cases.
- It is very important to lie completely still during a CT examination while the scanner is taking images.
Glossary: CT Scan
Here are definitions of medical terms related to CT scan.
Contrast Agent: A substance used to “highlight” an organ or tissue during examination. Sometimes referred to as a “dye.” Common contrast agents are: iodine, barium, and gastrografin.
Endoscopy: Examination of a body cavity by means of an endoscope, a tube-like instrument with attached lenses and light source.
Magnetic Resonance Imaging (MRI ): A computerized picture made by magnetic fields that can show detailed, cross-sectional images of the inner body.
Osteoporosis: Loss of protein tissue from bone, causing it to become brittle and easily fractured.
Additional Sources Of Information: CT Scan
Here are some reliable sources that can provide more information on CT scan.
Radiology Information, presented by the Radiological Society of North America (RSNA) and American College of Radiology (ACR):
- CT Scanning of the Abdomen:http://www.radiologyinfo.org/content/ct-abdomen.htm
- Blood vessels:http://www.radiologyinfo.org/content/CT-angiography.htm
Imaginis: Computed Tomography Imaging http://www.imaginis.com/ct-scan/