Nuclear magnetic resonance imaging. Basic principles.

The physical principles underlying nuclear magnetic resonance (NMR) imaging (also known as MRI) are described. NMR is an important new non-invasive imaging modality, which does not use ionizing radiation. Its ability to map hydrogen ion distribution, and to detect two intrinsic parameters ("relaxation times") which are indicative of the immediate chemical environment of the hydrogen nuclei, results in images of superior spatial detail in the brain and spinal cord. The potential of this technique for quantitating blood flow and for the exact identification of tissues is discussed.