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In terms of CT numbers, what is the value typically assigned to blood?

  1. -100

  2. 20

  3. 0

  4. 300

The correct answer is: 20

The typical CT value assigned to blood is indeed around +20 Hounsfield units, which corresponds to option B. Hounsfield units (HU) are used in computed tomography (CT) to express radiodensity, where air is assigned a value of approximately -1000 HU, water is at 0 HU, and various tissues and fluids are assigned values based on their densities relative to water. In this context, blood, being a fluid with some level of density, has a positive CT number. This reflects its composition, which includes water and suspended cells, contributing to its radiographic appearance. The value of +20 HU indicates that blood is denser than water but not as dense as many solid tissues, which have higher positive values. The values assigned to air, water, and dense tissue create a framework that helps in the interpretation and differentiation of various types of tissues in a CT scan. Understanding these values is essential for radiographers when assessing images and diagnosing conditions.