CAMRT Radiography Practice Exam

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What is the CT number for fat?

-200
-100
20
100

The correct answer is: -100

The CT number, also known as the Hounsfield unit, is a quantitative measure that indicates the radiodensity of a particular substance in a computed tomography (CT) scan. For fat, the Hounsfield unit is generally around -100. This negative value reflects that fat is less dense than water, which has a CT number of approximately 0. The significance of fat's negative CT number is particularly useful in distinguishing various tissues in the body during imaging. It helps radiologists and technologists to differentiate between fat, muscle, and water, aiding in accurate diagnoses and analysis. In contrast, other substances like air have a much lower CT number (around -1000), and various types of soft tissues, as well as fluids, typically have values closer to zero or even positive values, depending on their density. Understanding the CT numbers of different tissues plays a crucial role in interpreting CT images accurately and effectively.