Contrast agents

Contrast agents improve the imaging of an organ or vessel. The contrast agent is injected intravenously (usually through a vein at the front of the elbow) and then spreads through the circulation to the entire body. When using an intravenous contrast agent, the question to be answered is important. For adequate assessment, the contrast agent should be located in the target organ/vessel. 

Example: when asked to confirm pulmonary embolisms, the scan is made at the moment the contrast agent is in the pulmonary artery (9-15 sec following injection, fig. 8). If you are more interested in the status of the carotids, you will need to scan 16-24 seconds after injection.

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CTA of the thorax; high contrast in the pulmonary artery.
CTA of the thorax; high contrast in the pulmonary artery.

Figure 8. High contrast in the pulmonary artery (through elbow – superior caval vein – right side of the heart). Note the contrast agent has not yet reached the aorta.

Oral and rectal barium contrast can be used to evaluate the intestines and help distinguish intestines from surrounding tissues. Iodinated contrast fluid may damage the kidneys. Hydration is essential to prevent contrast nephropathy. Refer to the protocol of the hospital where you are working for more details on preventive measures (including pre/posthydration) and the risk factors.