What Is a Contrast Medium?
Diagnostic imaging is advancing day to day. There are various imaging techniques such as ultrasound, computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography (PET) that help to create images of the organs and tissues within the body. However, to gain a clearer picture and precise diagnosis, certain chemicals can be used to delineate the desired structure. This is done by introducing some substances known as dyes or contrast agents. These contrast agents do not dye the structures permanently. Instead, once injected into the body, they impart a temporary color change, thus improving the diagnostic value of the imaging tool.
How Do Contrast Materials Work?
Once the contrast material is injected into the body, they change the appearance of specific tissues for a brief period. They do not cause any structural change in the organs or tissues; they simply enhance the appearance of these structures on the image. Contrast agents interact with the imaging tools differently than normal body structures. When a CT or X-ray examination is done using a contrast medium, body parts with these chemicals will absorb less radiation than the adjacent structures. In MRI, the contrast medium works by altering the magnetic field. Thus structures within the body that have absorbed the contrast medium will appear different than those which do not have the dye. This helps differentiate abnormal tissue from normal, thus making a more precise diagnosis.
Where Is the Contrast Medium Used?
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A contrast medium is used to assess various organs such as the liver, stomach, thyroid, heart, rectum, small intestine, large intestine, uterus, and brain.
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Contrast medium is used along with diagnostic procedures such as CT, X-ray, angiogram, MRI, and PET scanning.
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It can be used to view arteries and veins.
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To delineate any soft tissue masses.
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To check for blood clots.
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To detect any congenital or acquired abnormalities in the organs.
What Are the Routes in Which Contrast Medium Is Administered?
The contrast materials can be administered in the following three ways:
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Oral Administration: The patient may be asked to drink the contrast medium or swallow it in pill form.
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Intravenous Administration: The contrast medium is injected through an I.V (intravenous) line into the blood vessels.
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Enema: The contrast medium is introduced in the rectum.
What Are the Types of Contrast Medium Available?
Several contrast media are available. A few of them are discussed below:
Iodine and Barium Sulfate:
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Used in X-ray and CT imaging.
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Iodine materials are usually used in I.V administration.
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Barium sulfate is used either orally or rectally.
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Barium sulfate is available in powder, liquid, tablet, or paste form. The paste form is used for rectal application.
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Barium sulfate is contraindicated in patients with cystic fibrosis as they have an increased risk of small intestine blockages. In addition, these patients will have an increased risk for adverse reactions to barium sulfate.
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These compounds do not allow the X-rays to pass through them and appear distinctly on the images.
Gadolinium:
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Mostly used in MRI.
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MRI scanning works on the principle of magnetic fields. Gadolinium alters the magnetic field and gives a distinct appearance to the structures containing them.
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It is usually injected into the blood vessels.
Microbubbles:
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Usually used as a contrast medium for an ultrasound examination.
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Microbubbles are biocompatible synthetic polymer shells filled with gas.
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Used to detect any masses within the organs or tissues.
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Used to detect blood clots in arteries and veins.
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Ultrasound works on the principle of sound waves. So these microbubbles, once administered, enhance the echo of sound waves, thus increasing the image quality.
Saline Water:
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It is used as an intravascular (injected into the veins) contrast medium.
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Most commonly indicated in MRI (magnetic resonance imaging) of the brain.
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It is safe and causes no toxic reactions.
Air:
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Air is used as a contrast medium in abdomen imaging procedures.
What Are the Risk Factors Associated With Contrast Medium Reaction?
There are certain risk factors associated with contrast medium adverse reactions. They are as mentioned below:
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Previous history of allergic reaction to contrast material.
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History of renal diseases.
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Suffering from allergic diseases like asthma.
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History of food or medicine allergy.
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Hay fever (allergic disorder).
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Hyperthyroidism while using iodinated contrast medium.
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Family history of allergic reaction to contrast material during any procedures.
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Underlying cardiac or lung diseases.
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Elderly individuals.
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Patients who are being treated with Nonsteroidal anti-inflammatory drugs or Aminoglycosides, as they are nephrotoxic drugs (toxic to the kidney).
What Are the Different Types of Contrast Medium Reactions?
There are two significant types of contrast medium-induced adverse reactions:
Allergic or Hypersensitivity Reaction:
Allergic reactions could prove potentially fatal if not treated early. Therefore, they can be again classified as immediate reactions and delayed reactions.
1. Immediate Reaction:
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It usually occurs within an hour of administering the contrast medium.
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The reaction can be mild with just dizziness, vomiting, mild rashes with itching, or a pale appearance.
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The patient could also feel slightly agitated and confused with a rapid heartbeat.
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However, it could also lead to severe reactions such as extensive rash, inflammation of the throat resulting in difficulty breathing, and seizures.
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If not treated early, it can lead to cardiac arrest and circulatory failure.
2. Delayed Reaction:
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This reaction occurs within a few hours or even weeks after receiving the contrast medium.
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However, this reaction usually resolves on its own and mostly appears as skin lesions (rashes, redness, or swelling).
Contrast-Induced Nephropathy:
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Patients with kidney diseases should be scrutinized before administering a contrast agent.
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Contrast-induced nephropathy is a condition in which kidney functions worsen in individuals with pre-existing kidney disease after administering contrast material.
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It is considered to be a fatal reaction.
Extravasation Reaction:
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If the contrast medium extrudes into the local soft tissue while injecting, it could result in tissue damage.
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However, it could be managed easily with over-the-counter drugs or ice packs and heating pads. In addition, the damage induced is mostly reversible.
How Can You Prevent a Contrast Medium Reaction?
General Instructions:
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Use the smallest quantity of contrast medium.
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Give a gap of 48 hours in between procedures involving a contrast medium.
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Patients should be advised to stop taking nephrotoxic medications before the procedure.
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Patients should be asked to drink plenty of water after the procedure, as it facilitates the excretion of the contrast medium via urine and stool.
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This has been found to decrease renal adverse events to a great extent.
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The physician should choose the contrast agent carefully, keeping in mind the medical history of the patient and any associated risk factors.
Premedication:
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Patients with risk factors such as a previous history of allergy to contrast material can be premedicated with Corticosteroids.
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A steroid drug can be combined with a histamine blocker to reduce the adverse reactions further.
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Calcium channel blockers can also be given to the patient for three days starting the day before the procedure.
How Can You Treat a Contrast Medium Reaction?
Immediate Reactions:
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Subcutaneous injection of Epinephrine 0.3 to 0.5 mg every 20 minutes.
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Steroid injection.
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Radiology personnel should be well aware of cardiopulmonary resuscitation (CPR) in patients with circulatory collapse.
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Stabilize the airway, breathing, and circulation in severe cases.
Delayed Reactions:
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Usually, they resolve on their own.
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May require only supportive care.
Conclusion:
Diagnostic imaging procedures are seeing novel advancements periodically. The need for precision diagnosis is vital to provide prompt treatment and a better prognosis for the patient. Using contrast material helps achieve accurate diagnosis more explicitly. Though the possibility of adverse reactions cannot be ruled out, judicious use of contrast material with various imaging procedures will help deliver a spot-on diagnosis.
