The physical basis of using MRI to detect cancer

Nowadays, the most effective way of detecting cancer in a patient is using MRI (Magnetic Resonance Imaging). This technique is based on the measurement of hydrogen nuclear spin signal of water molecules inside the body. During the development of the MRI technique, it needed to overcome two major difficulties: (1) How to produce a contrast between water molecules inside the cell and the extra-cellular water? (2) How to differentiate the water signals contributed by cancer cells from those contributed by normal cells? These difficulties were resolved mainly through the discovery that the relaxation times of water protons inside the cells are very different from those of the extra-cellular water. Furthermore, the relaxation times of water molecules in normal cells are found to be significantly different from those in the cancerous cells. In this presentation, I will give a concise review of the evidence for these discoveries. Finally, I will discuss the possible physical basis that may account for the relaxation time difference between different cells.