Fundamentals of NMR imaging.

NMR imaging is a new kind of emission tomography that utilizes the resonance frequency radiation emitted by atomic nuclei, in particular the hydrogen nuclei, present in the human body. This radiation is emitted when these nuclei are in a state of vibration under the influence of a static magnetic field and after excitation by an external RF radiation pulse. By introducing well-known field inhomogeneities the frequency of the emitted radiation can be made spatially dependent. Through spectroscopic analyses and mathematical procedures the spatial distribution of the emission intensity, the NMR image, can be reconstructed. Characteristic differences in concentration and resonance properties of the hydrogen nuclei result in strong contrasts in the image between the various tissues, as well as between lesions and normal tissue. Due to a choice of imaging techniques and time parameter settings the NMR image contrasts can be varied widely. Shifting or rotation of the image plane is performed electronically without changing the patient's position. Multiple parallel slices as well as a 3-dimensional volume can be imaged in one measuring procedure.

Interpretation of nuclear magnetic resonance tomograms of the brain.

Nuclear magnetic resonance tomography is an imaging method based on the magnetic resonance behavior of protons, as they occur predominantly in tissue water or in fatty acids. This behavior, as characterized by the relaxation times T1 and T2, is determined by the chemical and physical environment of the protons. The relaxation properties of the tissue can be accentuated by selection of the appropriate pulse sequence. Interpretation of the images requires an understanding of the principles of the technique with which the relaxation times are expressed in the images, and acquaintance with the composition of the tissue in terms of relaxation properties (as in computerized tomography, tissue composition is expressed as attenuation coefficients).