This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
The working of an MRI machine is based on the propeies of which of the follwoing? 
Select and explain. 
A. Protons, B. Electrons, C. CO2, D. O2
A: Protons
Answer: A: Protons 
Explanation: The patient is placed in a strong magnetic field. The protons (H-atoms) in the body align themselves in the body either in the same direction or in the opposite direction. A strong radio signal is sent at the level where an image needs to be taken. The vibration or spin of te atoms is used to record the tissue images. It is best suited for non-calcified tissue. It is contraindicated with pacemaker and chips. It has no ionising radiation. It is generally not useful for lung and bone pathology.