Contrast Media (week 12 and 13)

Attenuation

The reduction in the total number of x-ray photons in the beam after passing through a given thickness of material.

Absorption

The process where atoms actually absorb radiation in photoelectric interactions.

Purpose of using contrast media

To increase the radiographic contrast in an area where it was absent or low before.

What affects subject attenuation?

Atomic number, mass density, thickness, and electron density of the absorber.

Positive contrast agents

Contrast agents that enhance the visibility of structures on a radiograph.

Negative contrast agents

Low atomic number agents that allow x-rays to pass through, making structures appear black.

Examples of negative contrast agents

Room air, CO2, N2O, O2.

Contraindication for negative contrast agents

They must never be injected intra-arterially or intravenously, as this can have serious consequences.

Effects of negative agents on x-ray imaging

They decrease the atomic number of the area, decreasing attenuation and increasing optical density.

Double contrast study

A study using both a negative and a positive contrast agent.

Barium-based positive contrast agent

Barium has an atomic number of 56 and is used for the digestive system and alimentary canal.

Iodine-based positive contrast agent

Iodine has an atomic number of 53; it is water soluble and absorbed, used for urinary and cardiovascular systems.

When can’t barium be used?

Barium cannot be administered intravascularly or intrathecally due to its non-water soluble properties.

Ionic vs non-ionic iodine media

Non-ionic is used in intravascular studies with fewer reactions; ionic is used for biliary and GI tract studies.

Benefit of non-ionic contrast agents

They have less chance of causing adverse reactions.

Radiolucent definition

Materials that x-rays transmit through the body.

Radiopaque definition

Materials that x-rays are absorbed by the body.