CAMRT Radiography Practice Exam

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What is the CT number for air?

-200
-1000
0
1000

The correct answer is: -1000

The CT number for air is typically represented as -1000. This value reflects the density of air relative to water, which is given a CT number of 0. In the Hounsfield scale used for computed tomography imaging, the CT number is a measure of radiodensity. Air is much less dense than water, leading to a significantly negative CT number. The standard reference points on the Hounsfield scale include water at 0, fat at around -100 to -120, and a variety of + values for denser substances like bone. Since air is the least dense substance commonly encountered in CT imaging, it has the lowest CT number among these choices, reinforcing its designation as -1000. This range is important for accurately interpreting CT images, as the densities help differentiate anatomical structures and types of tissue effectively.