During many computed tomography examinations, patients may be asked to take a special contrast agent (orally, rectally or via injection). Intravenous, oral and rectal CT contrast are pharmaceutical agents (liquids) and are sometimes referred to as "dye". CT contrast is used to make specific organs, blood vessels and/or tissue types "stand out" with more image contrast to better show the presence of disease or injury. Thus CT contrast highlights specific areas of the resultant CT image or "dyes" it.
Note: It is important that patients consult the imaging location performing their CT exam for specific instructions to follow when contrast will be used. The information contained herein is only a general guideline.
There are four types of contrast agent used in CT:
- The type that is given via intravenous (through a vein) injection
- The type that is given orally
- The type that is given rectally
- A much less common type of contrast used in CT is inhaled as a gas and used for special lung and brain imaging. This technique (called Xenon CT) is only available at a small number of locations throughout the world and is only performed for rare cases.
CT Contrast Given Via Intravenous Injection
Intravenous contrast is used in CT to help highlight blood vessels and to enhance the tissue structure of various organs such as the brain, spine, liver and kidneys. "Intravenous" means that the contrast is injected into a vein using a small needle. Some imaging exams of the abdomen and gastrointestinal system use both the intravenous iodine and orally administered barium contrast for maximum sensitivity.
The intravenous CT contrast is clear like water and has a similar consistency. It is typically packaged in glass bottle or vial. A sterile syringe is used to draw it from the bottle or a power injector is used to administer the contrast. Typically between 75 cc to 150 cc (about 2.5 oz. to 5 oz) of contrast is used depending upon the patient's age, weight, area being imaged and cardiovascular health.
How does Intravenous CT Contrast Work?
A small needle is first placed into a vein in the hand or arm by the radiologist, technologist or a nurse and held in place with tape or a strap. Once the needle is in place, the vein is flushed with saline solution. Typically the contrast is loaded into a power-assisted injector, which injects the CT contrast using tubing through the needle into the body during a specific period in the CT exam. The injection is fully under the control of the technologist or radiologist. The injector is either mounted on a small trolley or hung from a ceiling mounted suspension next to the CT scanner. The contrast may also be hand injected using a large syringe connected to the needle via tubing.
Once the iodine contrast has been injected into the blood stream, it circulates through the heart and passes into the arteries, through the body's capillaries and then into the veins and back to the heart. As CT images are being acquired, the CT's x-ray beam is attenuated (weakened) as they pass through the blood vessels and organs flush with the contrast. This causes the blood vessels and organs filled with the contrast to "enhance" and show up as white areas on the x-ray or CT images. The kidneys and liver eliminate the contrast from the blood.
What Preparation is Needed Before Receiving Intravenous Contrast?
Sometimes it is necessary to not drink anything for an hour to several hours before the exam. The preparation time varies depending on the actual exam as well as the imaging center's requirements. Always ask the staff where the exam is scheduled for exact guidelines.
Is Intravenous CT Contrast Safe?
Typically, a patient will be asked to sign an "informed consent form" prior to having an CT exam which uses iodine contrast. This form will outline the potential side effects of the iodine. Overall, iodine is safe and has been used for many years and in millions of x-ray, CT and angiogram studies without serious side effects. Iodine contrast increases the sensitivity of the CT study. Thus the benefits of using iodine contrast typically outweighs the risks.
Patients should inform the radiologist or technologist if they have a history of allergies (especially to medications, previous iodine injections, or shellfish), diabetes, asthma, a heart condition, kidney problems, or thyroid conditions. These conditions may indicate a higher risk of iodine reactions or problems with eliminating the iodine after the exam.
The most common side effect of iodine includes a warm or hot "flushed" sensation during the actual injection of the iodine and a "metallic" taste in the mouth, which usually lasts less than a minute or so. This can vary depending on the type of iodine used, the rate at which it is administered, and individual patient sensitivity. There is no treatment necessary for this sensation
Another mild reaction that can take place following the administration of iodine is itching over various parts of the body with hives (bumps on the skin). This reaction can last from several minutes to several hours after the injection. This type of reaction is usually treated with medication administered by the radiologist, nurse, technologist or other physician.
More serious reactions, although much less likely, may include breathing difficulty, swelling of the throat, or swelling of other parts of the body. These reactions can be more serious if not treated immediately.
With newer types of "non-ionic" contrast (non-ionic means that the iodine has a different chemical structure than normal iodine contrast), the risk of an allergic reaction can be even less. Patients should discuss all of their questions with the imaging staff when they arrive and make sure they read and understand the "informed consent" form before having the exam.
In some cases, a CT can still provide valuable information without the administration of a contrast agent, and the physician may decide this is the best course for the patient at risk of reaction to contrast.
Oral CT Contrast
Note: It is important that patients consult the imaging location performing their CT exam for specific instructions to follow when contrast will be used. The information contained herein is only a general guideline.
Oral contrast is often used to enhance CT images of the abdomen and pelvis. There are two different types of substances used for oral CT contrast. The first, barium sulfate, is the most common oral contrast agent used in CT. The second type of contrast agent is sometimes used as a substitute for barium and is called Gastrografin.
Barium contrast looks like and has a similar consistency as a milk shake. It is mixed with water and depending on the brand used, may have different flavors (for example, strawberry or lemon). Gastrografin contrast is a water-based drink mixed with iodine and has a tinted yellow color. When given orally, gastrografin may taste bitter.
Patients usually need to drink at least 1000 to 1500 cc (about three to four 12 oz. drinks) to sufficiently fill the stomach and intestines with oral contrast.
How does oral CT Contrast Work?
Barium and gastrografin are made up of substances which weaken (attenuate) x-rays. The oral contrast is swallowed and travels into the stomach and then into gastrointestinal tract. During the CT exam which follows, the CT x-ray beam is attenuated (weakened) as it passes through the organs containing the contrast, for example, the large intestine. The organs filled with the contrast are then "enhanced" and appear as highlighted white areas on the CT images.
What Preparation is Needed Before Taking Oral CT Contrast?
It is important to eliminate as much food as possible from the stomach and intestines in order to help the sensitivity of the CT exam using oral contrast. Food and food remains can mimic disease when the oral contrast is present. Thus a regimen of not eating and/or drinking for several hours before the CT exam is required. The preparation time varies depending on the actual exam as well as the imaging center's requirements. Patient preparation varies from center to center, depending on a number of issues. Some types of oral CT contrast are taken at home well before the CT examination.
Is Oral CT Contrast Safe?
In general, barium and gastrografin contrast are safe and pass through the gastrointestinal tract in the same fashion as food or drink. Minor side effects, such as constipation, may occur. Certain medical conditions such as a perforated ulcer may indicate the use of gastrografin instead or barium. The referring physicians and or radiologist will decide which type of contrast material is most appropriate based on a patient's specific condition. Some people have reported sensitivity to the flavoring substance used in oral barium contrast.
In some cases a CT can still provide valuable information without the administration of a contrast agent, and the physician may decide this is the best course for the patient at risk of reaction to contrast.
Rectal CT Contrast
Note: It is important that patients consult the imaging location performing their CT exam for specific instructions to follow when contrast will be used. The information contained herein is only a general guideline.
Rectal contrast is often used to enhance CT images of the large intestines and other organs in the pelvis. There two types of substances used for rectal CT contrast (barium and Gastrografin) are the same as the type used for oral CT contrast, but with different concentrations. The first, barium sulfate is the most common rectal contrast agent used in CT. The second type of contrast agent is sometimes used as a substitute for barium and is called Gastrografin. It is not uncommon to have an intravenous, rectal and/or oral contrast given for a CT exam which involves the pelvis.
Rectal CT contrast is usually given using an enema where a small plastic tip is inserted into the rectum while a patient lies on their side. This tip is connected to a bag filled with the barium or gastrografin contrast through a tube. After the tip is inserted, the patient lies flat and the bag is raised above head level to allow the contrast to fill the lower intestines. During this filling phase, the patient may feel mild discomfort, coolness, and generalized fullness. It is important to relax as much as possible during this phase until the CT scan is complete.
The rectal contrast helps to increase the sensitivity of the CT exam by outlining not only the large intestines (colon), but also the bladder, the uterus in female patients and other organs. After the exam is complete, the rectal contrast will be drained and the patient may go to the bathroom.
How does Rectal CT Contrast Work?
Barium and gastrografin are made up of substances which weaken (attenuate) x-rays. During the CT exam which follows, the CT scanners x-ray beam is attenuated (weakened) as it passes through the organs containing the contrast, for example, the large intestine. The organs filled with the contrast are then "enhanced" and appear as highlighted white areas on the CT images.
What Preparation is Needed Before Taking Rectal CT Contrast?
It is important to eliminate as much food remains as possible from the stomach, intestines and rectum in order to help the sensitivity of the CT exam using rectal contrast. Food and food remains can mimic disease when the oral contrast is present. Thus a regimen of not eating and/or drinking for several hours before the CT exam is required. In addition, for a pelvic CT exam where rectal contrast will be used, a Fleets Enema (available from most pharmacies) may be required to cleanse the colon the night before the exam. The preparation time varies depending on the actual exam as well as the imaging center's requirements. Patient preparation varies from center to center, depending on a number of issues. Always check with the imaging center where the actual exam will take place to confirm the actual preparation recommendations.
Is Rectal CT Contrast Safe?
In general, barium and gastrografin contrast are safe and pass through the gastrointestinal tract in the same fashion as food or drink. Minor side effects such as constipation may happen. Certain medical conditions such as a perforated ulcer, or certain colon conditions may indicate the use of gastrografin instead or barium. The referring physicians and or radiologist will decide which type of contrast material is most appropriate based on a patient's specific condition.
In some cases, a CT can still provide valuable information without the administration of a contrast agent, and the physician may decide this is the best course for the patient at risk of reaction to contrast.
Updated: June 11, 2008
