CAMRT Radiography Practice Exam

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What would be the CT number for blood?

-100
20
0
400

The correct answer is: 20

The CT number, also known as the Hounsfield unit, is a measure of the radiodensity of a substance compared to that of water, which has a CT number of 0. Blood, being composed of water, plasma, and cells, typically has a CT number that is close to that of water but slightly higher due to the presence of cells and proteins. In radiology, the Hounsfield unit ranges from negative values (for substances less dense than water, like air) to positive values (for dense substances like bone). The CT number for blood is commonly accepted to be around 20, which indicates that it is denser than water but not nearly as dense as structures like bone, which would have significantly higher values. This understanding is important as it helps radiologists distinguish between different types of tissues and fluids within the body based on their CT values, aiding in diagnosis and treatment planning. For comparison, substances such as fat usually have lower CT numbers (around -100), water is at 0, and bone can exceed 400, making the CT number of blood substantially distinguishable within the context of these values.