This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fundamentals in Radiology, Subject name: Radiology
Hounsfield units depends on - 
Select and explain. 
A. Electron density, B. Mass density, C. Effective atomic number, D. Attenuation coefficient
D: Attenuation coefficient
Answer: D: Attenuation coefficient 
Explanation: Hounsfield units are used to measure the X ray attenuation in CT scan. Depends on absorption/attenuation coefficient. Water is assigned a HU value of 0.