Both MRI and CT scanners are both non-invasive medical imaging technologies that provide two-dimensional cross-sections from which three-dimensional information about the body can be reconstructed. MRI stands for Magnetic Resonance Imaging whilst CT (also known as CAT) stands for Computed Axial Tomography.
CT scans were developed in the early 1970s by Allan Macleod Cormack and Godfrey N. Hounsfield. For this they were to win the Nobel Prize for Physiology or Medicine in 1979. They involve X-ray sources and detectors that move around the body, recording X-ray transmission information through 360 degrees. This allows cross-sectional images of basic organ anatomy to be reconstructed into three dimensions using a computer.
MRI scans are the result of several decades of technological development that ultimately led to a Nobel Prize for Paul Lauterbur and Peter Mansfield in 2003. It is an improvement on CT scans allowing access to information about organ chemistry and metabolism as well as more detailed organ anatomy. They produce images of the human body using the principle of nuclear magnetic resonance of protons.
This relies on the discovery that some nuclei only take up certain orientations when placed in a magnetic field. By aligning the protons of, typically, hydrogen atoms in the body in this way and then applying a radio wave pulse to the tissue, radio emissions can then be detected. This allows the construction of a proton density map of the body.
In overall comparison between the two techniques CT scans tend to be used where there is high contrast in atom density of a structure to its surroundings as with calcified tissue such as bone. MRI is better suited for non-calcified tissue. MRI is also better in that it can be used in any plane, not just the axial plane like in the CT scanner. MRI also has greater functionality for achieving image contrast through changing scanning parameters.