Q: How Magnetic Resonance Imaging (MRI) works? 


Answer: MRI uses a magnetic field to align rotating hydrogen protons within the tissue being imaged. During realignment of the protons, energy is released and sampled at different time intervals. The measured signal intensity from this energy depends upon the degree and rate of realignment within a very specific time period, which in turn depends upon the water and fat content of the different tissues. These signals are then converted into gray-scale cross-sectional images that can be depicted in multiple planes or in three dimensions.


Clinical implication: Most diseases manifest themselves by an increase in water content, so MRI is a sensitive test for the detection of disease.


Reference: 

Abi Berger - How does it work? Magnetic resonance imaging -,BMJ. 2002 Jan 5; 324(7328): 35.