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What is a Contrast Scan?

By Brenda Scott
Updated May 17, 2024
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Before the development of computerized imaging technology, doctors often needed to perform exploratory surgery in order to identify abnormalities in organs and soft tissues. Now medical professionals have a wide range of imagining equipment to aid in the diagnosis and treatment of disease. A contrast scan is an imaging test which uses a contrast material, or dye, to highlight a particular area of the body. These tests allow doctors to identify some tumors and other problems which would not be visible by any other medium.

One of the most popular imaging machines is the computerized axial tomography scan, knows as a CT or CAT scan. This machine can examine almost every part of the body, with the exception of breasts, the soft tissue around joints and obstetrics. A CT uses computers and ionizing radiation to create images of internal organs, blood vessels and three-dimensional images of bones.

While not every CT test is a contrast scan, many do require the use of a contrast agent. Iodine is dye most commonly used with CT or x-ray. It absorbs the x-ray photons so they are not picked up by the x-ray. This creates a highlighted affect, making the organ, blood vessel or tissue area stand out.

A contrast scan can also been performed with magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) equipment. These machines use magnets and radio waves to create images of internal tissues or blood vessels. Depending upon the test, an MRI exam may or may not use a contrast agent. MRA tests always use a contrast agent.

An MRI and MRA contrast scan generally uses the element gadolinium, which alters the magnet properties of the tissues. This makes the tissues appear white, differentiating them from the surrounding matter. While there are fewer reactions to gadolinium than to the iodine, it can cause a life-threatening disease in patients with kidney dysfunctions.

The dye used in a contrast scan can be administered orally, intravenously (IV), rectally, or in rare instances, through inhalation. Some tests involve a combination of these methods. The method of administration depends upon the test being performed. An IV contrast scans is used to highlight blood vessels or the tissue structure of organs such as the brain, heart, spine, liver and kidneys. The most popular IV contrast agent is iodine, which passes through the body quickly.

Orally administered dye is usually used with a contrast scan examining the abdomen and pelvis. The most common oral contrast agent is barium sulfate, which weakens the x-ray beam, highlighting the area containing the agent. Rectally administered dyes also contain barium. These tests generally exam the large intestine, bladder, uterus or other organs in the lower region of the abdomen. Inhalation of a gas contrast agent is very rare, and only used for certain brain and lung exams.

Side affects of barium can include constipation and a metallic taste. Iodine agents, which are administered by IV, can cause a wider range of reactions. The most common of these are a flushed feeling, redness, nausea or dizziness. More serious affects include shortened breath, blood clots and allergic reactions. Patients who have allergies, asthma, heart conditions, diabetes, and kidney or thyroid conditions are at greater risk.

A contrast scan is an extremely valuable tool for diagnosing diseases, and assessing internal damage and other abnormalities. While there is a possibility of reactions to the contrast agent, these are rare, and undergoing the test is generally preferable to leaving a disease or condition untreated. When dealing with a patient who has had reactions in the past, or is considered higher risk, the technician usually administers an antihistamine as a preventative measure.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
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