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What is contrast in a CT scan and what is it used for?

The images obtained in a computerized tomography (CT) Scan are generally black and white and the different structures have a very varied tonality within a large gray scale. This sometimes makes it necessary to use contrasts.

Contrasts are substances that are introduced into the body by different routes to distinguish some structure that we could not otherwise visualize because it has the same shade on the gray scale as another adjoining structure.

They are also used to study the vascularization of different organs or to assess the arteries and veins of any part of the body.

What precautions should be taken before administering the contrast?

For the use of any contrast, it is essential that the patient signs a consent.

The administered dose is calculated according to the weight and age of the patient and according to the concentration and composition of the contrast medium and the study that we are going to carry out.

The usual dose is 1 cc/kg of weight. In general, there are two large groups of contrasts, positive and negative:

  • The first are those that opaque the structure to be studied in a whiter tone (hyperdense) than the rest of the adjacent structures since they are opaque to the passage of X-rays. As the most used positive contrasts we have barium sulfate and salts of iodine and can be administered orally, rectally, intravenously, or intra-arterially. Other routes used are intravaginal, intraarticular or through catheters.

  • Negatives are those that cause said structure or cavity to stain a darker gray or black color than its surroundings (it becomes hypodense). In CT, the most used negative contrast is water and sometimes also air, and they are usually administered orally or rectally.

In the most common studies, oral (barium sulfate) and intravenous (iodinated solutions) contrast agents are used. The oral contrast is drank by the patient in the waiting room, little by little, for about 45 minutes to ensure that the entire intestine becomes opacified.

The intravenous contrast is usually introduced by puncturing a vein in the flexure of the elbow, when entering the scanner room.


Side effects that contrast agents can have

Certain complications may arise when administering contrasts (especially intravenous ones) that must be taken into account:

Extravasation of the contrast at the puncture point: generally due to rupture of the peripheral vein, which causes pain, swelling and inflammation in that place.

Mild allergic reaction: generally, to iodinated contrast agents. The patient reports nausea, sweating and the appearance of skin redness and hives, accompanied or not by itching.

Severe allergic reaction: generally, to iodinated contrast agents. The patient reports swelling in the throat and may be accompanied by hypotension, seizures, respiratory and/or cardiac arrest.

In any of these cases, the nurse will notify the radiologist, who will prescribe the necessary medication. If the patient has suffered an allergic reaction in any study, whether mild or severe, the nurse must report it whenever any imaging test that requires the use of contrasts is to be performed.

Today, allergic reactions are rare since contrasts have been greatly improved over time and we also have effective treatments for them, so we should not be afraid if it is necessary to administer a contrast to carry out a radiological study.

If you have any questions about CT contrast and how it will effect you please contact us, we're here to help with all your questions.

Article written by:

Laura Lopez Gonzalez- Doctor in medicine. Licensed Specialist in Radiodiagnosis at the University Assistance Complex of León



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