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Contrast materials

7. Contrast media

Author: Balázs Kovács

Semmelweis University Department of Radiology, Budapest


7.1. Aim of the chapter

In this chapter the main characteristics of the contrast media (or contrast agents) are introduced. It is important to be familiar with the types, classifications, indications, contraindications and side effects of contrast media.

I. What is a contrast medium?

Contrast medium is a substance in medical imaging that modifies the detected signal and enhances the contrast of structures within the body. Legally contrast medium is a pharmaceutical drug.

II. Why is a contrast agent required?

Radiocontrast - used to improve the visibility of internal bodily structures in an X-ray based imaging techniques – can increase or decrease the density of the given structure. Ultrasound contrast agents enhance the reflection of ultrasound waves by increasing the number of acoustic interfaces. MR contrast agents alter the relaxation times of tissues in which they are present. Using contrast agents the contrast of the medical image is increased which enhances the visibility and improves detection. Both static and dynamic images can be made during contrast examination. Static image (i.e. radiographic film) provides morphological information, while continuous monitoring during contrast administration provides additional functional information beside morphology (i.e. barium swallowing). In tomographic imaging (CT, MR) images are made at different phases after contrast administration, which provide information about contrast enhancement of a given organ or mass in function of time helping differential diagnosis.

7.2. Classification of contrast media

7.2.1.Radiocontrast Positive contrast media

A positive contrast agent means that it absorbs more x-ray than the surrounding tissue. Today’s contrast agents are mostly positive, and their further classification is based on their physical characteristics.

Iodine-based, water soluble contrast media
Nephrotrop, ionic contrast media
Ionic contrast media have higher osmolarity (above 1000 mOsm/kg), they are excreted by the kidney, and cause more side effects, therefore it is not in use intravenously anymore, however, it is used for enterography routinely. Due to its hyperosmolarity ionic contrast media exert laxative effect. (In adhesional ileus they also might have therapeutic effect.) Fractionated iodine-based, ionic contrast medium is given orally in certain abdominal CT studies to differentiate bowels from other structures, for example abscesses.

Nephrotrop, nonionic contrast media
Ionic contrast media are hypo- (290-800 mOsm/kg) or iso-osmolar (290 mOsm/kg) compared to blood. These types of contrast media are also excreted by the kidney. They are generally used for intravenous and intra-arterial administration because they are safer. They causing fewer side effects, their nephrotoxicity are less common, which are not related to their osmolarity, but the single molecules’ nephrotoxicity.

Hepatotrop contrast media
Intravenously administered contrast agents, which are excreted by the liver, therefore they are able enhance the biliary system. Not in use any more.

Iodinated, oily contrast media
Recently these oils are used for chemoembolization of certain tumors.

Non-iodinated contrast media
Most frequently used non-iodinated contrast agent is barium sulphate. It is can be used orally or rectally, as a suspension of fine particles in aqueous solution. Barium-sulphate is insoluble in water, and if it gets into the peritoneal or mediastinal cavity it can lead to barium peritonitis and mediastinitis respectively. In case of aspiration it can cause pneumonitis. For these reasons, using barium is contraindicated in suspected bowel perforation and in patients at risk for aspiration. Negative contrast media

Negative contrast agent means that it absorbs less x-ray than the surrounding tissue: air and carbon-dioxide are most frequently used. Air as a contrast agent is used mostly in double-contrast barium enema, in which procedure barium is administered rectally, followed by air injection.

7.2.2. MRI contrast agents

MRI contrast agents cause changes in local magnetic fields by inducing proton relaxation time shortening. According to their physical characteristics intravenous MRI contrast agents can be classified into the following two major groups. Paramagnetic contrast agents

Paramagnetic contrast agents shorten T1 relaxation time of the protons. Gadolinium, a rare-earth metal, is the most commonly used paramagnetic contrast agent. Gadolinium as a free ion is highly toxic, therefore stabile chelated gadolinium compounds, which is generally regarded as safe, are used in medical imaging. Apart from the gadolinium-containing contrast agents there are other, so-called organ- or tissue-specific contrast agents containing other metallic elements. Superparamagnetic contrast agents

Superparamagnetic contrast agents reduce the T2 relaxation time of the protons in absorbing tissues.
Similarly to abdominal CT, the enhancement of the gastrointestinal tract with oral contrast medium might be necessary during MR examination. A wide variety of oral contrast agents can be used for enhancement of the gastrointestinal tract, which includes manganese chelates, iron salts and natural products with high manganese concentration such as blueberry juice and green tea.

7.2.3. Ultrasonic contrast agents

A contrast agent can be applied also during ultrasound examinations, although their usage has not been worldwide spread. Commercially available contrast media are gas-filled microbubbles that are administered intravenously to the systemic circulation. Microbubbles with their high degree of echogenicity increase the number of acoustic interfaces in the examined region leading to ultrasound wave reflection. Thus, tissues with high contrast accumulation become whiter, so-called hyper echoic on B-mode images.

7.3. Side effects of contrast agents and treatment of adverse reactions

Legally contrast media are registered as pharmaceutical drugs, so thus they might cause side effects and adverse reactions. Their clinical use is regulated strictly. Not only radiologists, but also the referring physicians must be familiar with contraindications of the contrast agents.

7.3.1. Oral contrast media

Oral contrast media are classified by their water solubility:

Insoluble: barium sulphate
Barium contrast administration is contraindicated in suspected bowel perforation and in patients at risk for aspiration.

Soluble: iodinated contrast media
Due to their hyperosmolarity they exert laxative effect, and have to be used with caution in dehydration and in patients with electrolyte imbalance.

7.3.2. Intravenous contrast agents

One of the adverse effects is extravasation at the intravenous injection site. Small amount of extravasation causes mild local pain, however, larger amount might lead to necrosis or compartment syndrome, both of which urges surgery. Extravasation is avoidable by careful manual injection; however, modern injectors are stopped automatically by sudden pressure elevation.

After injection early and delayed adverse effects might develop. Their severity range is from a mild inconvenience, such as itching to life-threatening emergency, such as laryngeal edema and hypovolemic shock. Delayed adverse reactions include contrast-induced renal impairment and thyreotoxic crisis.

All contrast materials might cause allergic reactions (e.g. itching, urticaria, diarrhea, flush, skin redness, nausea, vomiting and anaphylaxis). It is important to report which contrast material evoked the adverse effect. If a patient is proved to be sensitive to a given contrast agent, other type of contrast agent can be administered with precaution.

Modern contrast agents are excreted by the kidney, which put extra load on the kidneys. Thus, in non-urgent cases renal function has to be checked prior to intravenous contrast administration (serum creatinine, GFR). In impaired renal function (30<GFR<60) the use of iso- or hypoosmolar contrast is mandatory. In patients with GFR under 30 it is recommended to avoid intravenous iodinated contrast administration, or to refer the patient to other medical imaging modality.

Before intravenous contrast administration metformin and certain antibiotics which are excreted by the kidney should be stopped temporarily. In patients with normal renal function metformin should be stopped the day of the procedure, and you should not restart the medication until at least two days after the procedure. In patients with impaired renal function withholding administration of metformin for two days before and after administration of an intravenous contrast agent is generally recommended. Patients should be kept well hydrated before receiving the intravenous contrast agent.

In untreated hyperthyreosis, the administration of iodinated contrast material is contraindicated, since it might lead to thyreotoxicosis 3-5 days after the study. Treated hyperthyreosis does not contraindicate these studies.

In pregnancy contrast administration is generally not recommended. However, in emergency situations contrast administration might become truly necessary. In these cases, all neonates exposed to iodinated contrast in utero should have their thyroid function tested in the first week of life due to the theoretical risk of contrast-induced hypothyroidism. MR contrast agents

Gadolinium MRI contrast agents have proved safer than the iodinated contrast agents. Anaphylactic reactions are rare, but might occur similarly to X-ray radiography and computed tomography. There is a risk of a rare but serious illness called nephrogenic systemic fibrosis that has been linked to the use of gadolinium-containing MRI contrast agents. Although its pathophysiological background is unknown, renal impairment carries a risk of its development. For this reason contrast MR is contraindicated in patients with GFR under 30 and in children under the age of one year. New researches showed that small amount of gadolinium accumulates in the basal ganglia regardless of proper kidney and hepatobiliary function, its clinical significance is yet to be explained however it is important to carefully follow the protocols.

Take-home points

  • Legally contrast medium is a pharmaceutical drug; therefore it has contraindications and might have side effects.
  • Barium sulphate, an insoluble oral radiocontrast, is contraindicated in suspected bowel perforation and in patients at risk for aspiration.
  • Kidney function has to be checked prior to intravenous contrast administration. In hyperthyroidism the thyroid function has to be checked before a iodinated contrast study.
  • It has been recommended that all drugs which are excreted by the kidney be stopped temporarily before intravascular administration of contrast media.


    • When is a contrast agent required?
    • How can the contrast media be classified?
    • What are the contraindications for oral contrast agents?
    • What are the contraindications for intravenous contrast agents?
    • Your patient is referred to a medical imaging examination with intravenous contrast. What is necessary to check before the examination?

Translated by Zsuzsanna Lénárd

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