Contrast Agents in CT Scans

Study Notes

Contrast agents are used to differentiate adjacent tissues in a computed tomography (CT) scan. This differentiation is done by changing the density of target tissues.
Contrast agents create artificial differences in density between objects. These differences are temporary and made possible by injecting the contrast agent intravenously or orally.
There are two types of contrast agents:
- Positive contrast agents are materials that are radiopaque. This means that they block x-rays. These agents have a higher density than the surrounding structures. Examples include barium and iodine.
- Negative contrast agents are materials that are radiolucent. This means that they allow x-rays to pass through. These agents have a lower density than the surrounding structures. Examples include air and carbon dioxide.

Iodine-based contrast agents are frequently used because they are:
- Water-soluble
- Easy to administer into blood vessels
- Safe
- Enhance different tissues in various ways
Iodine-based contrast agents are metabolized differently by normal tissues versus abnormal tissues.

Osmolality: The number of particles in a solution per unit of fluid, compared to the blood.
- High Osmolality Contrast Media (HOCM): May have 7 times the concentration of blood.
- Low Osmolality Contrast Media (LOCK): Has about twice the concentration of blood.
- Iso-osmolar Contrast Media (IOCM): Has the same osmolality as blood.
Viscosity: The thickness of the fluid or its resistance to flow.
- The viscosity of contrast media is influenced by brand, temperature, and concentration.
- Viscosity can affect injectable contrast agents. The smaller needle and catheter size can make injection more difficult.
Ionicity: Ionized contrast agents contain molecules that form ions in a solution.
Clearance: Iodine-based contrast agents are not broken down by the body. They are primarily excreted from the body through the kidneys.
- The half-life in patients with normal kidney function is approximately 2 hours.
Dosage:
- Determined using the concentration of iodine and the volume administered.
- Children's dosages are often based on weight. The most common formula is 2 mL/kg.
- Adult dosages are usually uniform for each protocol. However, weight-based dosage for adults has several advantages.

CT scans are rarely performed during pregnancy due to concerns about ionizing radiation exposure to the fetus. However, CT scans can be essential in some cases to protect the health of the mother.
There is no evidence that contrast agents pose a risk to the fetus.
Small amounts of iodine-based contrast agents administered to the mother are excreted in breast milk.
For example, if a mother received 150 mL of contrast, her breastfed infant would only ingest 0.015 mL.
Therefore, it is considered safe for both the mother and the infant to continue breastfeeding after receiving contrast media.

Iodine-based contrast media is one of the most widely used medications.
Potential side effects can be categorized based on frequency:
- Common: Nausea, vomiting, flushing, and a metallic taste in the mouth.
- Less Common: Headache, rash, itching, and feeling warm.
- Rare: Seizures, anaphylaxis, and kidney failure.
While serious side effects are rare, it's important to be aware of them and to seek medical attention if they occur.