What is the expected difference in Hounsfield Units (HU) during an equilibrium phase after contrast media injection?

In the context of imaging, particularly in computed tomography (CT) scans, Hounsfield Units (HU) are a measure of radiodensity. When a contrast medium is injected, it enhances the visibility of certain tissues or vascular structures due to the increased atomic number of the contrast agent compared to the surrounding tissues.

During the equilibrium phase following contrast media injection, there may be a small increase in the average Hounsfield Units measured in the area where the contrast was distributed. In this phase, the contrast is evenly dispersed throughout the intravascular and extravascular compartments, leading to a relatively minor elevation in HU values.

An expected difference of less than 10 HU is typical after contrast administration during this phase, as the tissue contrasts primarily reflect the diluted concentration of the contrast medium. This relatively modest increase is due to the balance achieved as the contrast moves into the extracellular space. Therefore, the correct answer indicates that the change in HU is not drastic—which aligns with the physiological behavior of contrast in tissues post-injection—reiterating that a change of less than 10 HU is a reasonable expectation in this clinical scenario.