- Positive X-ray contrast agents -
absorb X-ray radiation more than the surrounding area
A positive contrast agent is a substance that increases the absorption, therefore appearing whiter on the image.
They usually have a higher proton number.
We have two main types of positive contrast agents – barium and iodine.
- Negative X-ray contrast media -
absorbs X-rays less than the surrounding area
Negative contrast agents decrease the absorption, therefore, they appear darker on images.
The most common example for negative contrast agent is air.
We can also use CO2, methylcellulose or mannitol. In XR imaging we use negative agents usually for intestinal imaging.
- Positive X-ray contrast agents include -
Barium sulfate
The Barium contrast agent contains an undissolvable compound of barium – barium sulfate. It is used strictly for gastrointestinal imaging, because it can cause very severe inflammation in tissues or in the peritoneal cavity. Thus an absolute contraindication of barium imaging is when there is a GI tract perforation. We can administer it per os or per rectum. A great advantage of barium is its adhesiveness – it adheres to the mucosas, thus it nicely outlines the GI tract. Barium is not used for CT because it is very dense and can make an artefacts on CT, the only exception is barium tracking in CT colography.
- Negative X-ray contrast media include -
Air
The most common example for negative contrast agent is air. We can also use CO2, methylcellulose or mannitol. In XR imaging we use negative agents usually for intestinal imaging.
- Contrast agents in X-ray examination –
are both white and black, depending on the type of substance
- A double contrast image refers to -
an examination where we use both positive and negative contrast agents
- Barium contrast agent -
can cause serious complications when in contact with the peritoneum.
The Barium contrast agent contains an undissolvable compound of barium – barium sulfate. It is used strictly for gastrointestinal imaging, because it can cause very severe inflammation in tissues or in the peritoneal cavity. Thus an absolute contraindication of barium imaging is when there is a GI tract perforation. We can administer it per os or per rectum. A great advantage of barium is its adhesiveness – it adheres to the mucosas, thus it nicely outlines the GI tract. Barium is not used for CT because it is very dense and can make an artefacts on CT, the only exception is barium tracking in CT colography.
- Barium contrast agent is used -
essentially only to view the Gastrointestinal tract
- Iodine contrast agent can be used -
intravenously, into cavities or the Gastrointestinal tract
Iodine contrast agents are the most commonly used positive XR contrasts. They contain iodine salts. They are water soluble and can be administred generally anyway you need – mostly intravenously or into the GI tract or cavities. When it gets into the bloodstream, it is excreted by the kidneys. We have two main types of iodine contrast agents.
Ionic (or high molecular contrast) agents, but we don’t use them anymore (there are cheaper but they have more adverse effects when administered into the bloodstream. We use them for all the other indications.
Non-ionic (or low molecular contrasts) are more expensive, but they have less adverse effects, so they are used for i.v. administration.
**There is another type of iodine contrast agent –called an oil contrast, but it is rarely used these days. We can use it for lymphography or for sialography (imaging of salivary glands ducts and lymph nodes).
- Allergic reaction to iodine contrast agent is -
Relatively common
There are two main adverse effects of iodine contrast agent, that happen usually only after i.v. adminstration, other methods of administration are usually safe.
**Allergic reactions, which do not depend on the dosage of contrast agent.
The allergic reaction after iodine contrast agent can be mild, intermediate or severe. Mild reaction includes rashes, nausea, uticaria. Intermediate reactions include tachycardia with hypotension, laryngospasm and difficulty breathing. Severe reaction is generalised and looks like anaphylactic shock.
**Chemo-toxicity, especially nephrotoxicity. This reaction is dependent on the dosage of the agent. Chemo-toxic reaction includes mainly damage to the kidneys, but there has also been instances documented of neuro and cardiac toxicity. The main principle of renal protection is optimalisation of contrast agent usage – that means use as low amount of contrast agent as possible and also hydrate the patient well both before and after the examination.
A special problem is during paravasal application – when the vein in which the contrast is administered bursts and the contrast agent spreads subcutaneously instead.
- A mild allergic reaction to iodinated contrast agents will manifest as -
Urticaria, redness and nausea
- A moderate allergic reaction to iodinated contrast agents will manifest as -
Tachycardia, hypotension and bronchospasm
- A severe allergic reaction to the iodinated contrast agent will manifest as -
Anaphylactic shock
There are two main adverse effects of iodine contrast agent, that happen usually only after i.v. adminstration, other methods of administration are usually safe.
**Allergic reactions, which do not depend on the dosage of contrast agent.
The allergic reaction after iodine contrast agent can be mild, intermediate or severe. Mild reaction includes rashes, nausea, uticaria. Intermediate reactions include tachycardia with hypotension, laryngospasm and difficulty breathing.
*Severe reaction is generalised and looks like anaphylactic shock.
***Chemo-toxicity, especially nephrotoxicity. This reaction is dependent on the dosage of the agent. Chemo-toxic reaction includes mainly damage to the kidneys, but there has also been instances documented of neuro and cardiac toxicity. The main principle of renal protection is optimalisation of contrast agent usage – that means use as low amount of contrast agent as possible and also hydrate the patient well both before and after the examination.
A special problem is during paravasal application – when the vein in which the contrast is administered bursts and the contrast agent spreads subcutaneously instead.
- Which of the following organs are most affected by intravenous iodine contrast media? -
Kidneys
- The nephrotoxicity of iodinated contrast media can be reduced -
by decreasing the amount of iodine contrast agent to a minimum and by sufficient hydration of the patient
- Nephrotoxicity of iodinated contrast agent -
is dose-dependent
- For CT scanning with iodine contrast agent -
the patient should fast 4 hours before examination
- What substrate is used as a contrast medium for ultrasound examination? –
Microbubbles
Ultrasound contrast agents rely on the different ways in which sound waves are reflected from interfaces between substances. Commercially available contrast media are gas-filled microbubbles that are administered intravenously to the systemic circulation. Microbubbles have a high degree of echogenicity that creating great difference in echogenicity between the gas in the microbubbles and the soft tissue surroundings of the body. Thus, ultrasound imaging using microbubble contrast agents enhances the reflection of the ultrasound waves, to produce a sonogram with increased contrast due to the high echogenicity difference.
We use Contrast-enhanced ultrasound to visualize blood perfusion of organs, measure blood flow rate in the heart and other organs, plus many more things. The most common indication is differential diagnosis of liver lesions.
- What substrate is used as a contrast medium for MRI examination? -
Gadolinium chelates
Contrast agents in MRI change the relaxation times of tissues, thus changing the signal intensity. The agents are substances with paramagnetic or superparamagnetic properties, which shorten T1 and T2 relaxation times. The most common substance is gadolinium – it has 7 unpaired electrons making it very paramagnetic.
Most of the MRI contrast agents are extracellular, which means that they are in bloodstream and in the interstitium. Gadolinium needs to be chelated to different ligands since the atom itself is quite toxic. It is administered intravenously and it is excreted through the kidneys with a 90 minute half-life.
Gadolinium is generally much safer than iodine contrast agents. However it can still cause adverse effects such as; nausea, vomiting, headache and paresthesia. Allergies are rare and anaphylactic reactions even rarer.
- How do MRI contrast agents work? -
Paramagnetic substances extend the T1 and T2 weighted times
- Intravenous gadolinium chelating agents (contrast agents for MRI) are excreted from the body mainly -
via the kidneys into the urine
- The administration of gadolinium in pregnancy -
is contraindicated because gadolinium passes across the placenta and can affect fetus
