Physikalische Grundlagen : Kernmagnetische Resonanz

Die physikalischen Hintergründe der Kernmagnetischen Resonanz schildert Wiebren Veeman in seinem Artikel. Dabei geht er auch auf die historischen Entwicklungen ein.

Nuclear Magnetic Resonance: The Basic Principles: Nuclear magnetic resonance, NMR, is an important analytical technique for many branches of science (chemistry, molecular biology, medical sciences, physics). The technique relies on the fact that the nuclei of most atomic elements behave as tiny magnets (usually called spins) which orient in an external magnetic field. Unlike a compass needle in the earth magnetic field nuclear magnets can orient in a magnetic field in a discrete number of orientations. For instance, for the nuclei of the common hydrogen isotope, the proton, two orientations of the magnetic moment in the magnetic field occur, either parallel or antiparallel to the field. These two orientations correspond to a slightly different energy. A spin can make a transition between the two orientations when it is irradiated with electromagnetic radiation of the right frequency, the resonance frequency of the spins. For typical experiments the irradiation frequency is in the range of radio waves with wavelengths of the order of 1 m. A sample of macroscopical size contains a huge number of identical spins (for instance a gram of water contains ca. 1023 protons) and when these spins are irradiated with radiation of the correct frequency, the sample as a whole absorbs a small amount of energy. This phenomenon was detected for the first time in 1945 by the two physicists Bloch and Purcell. Basically, two types of NMR experiments exist: NMR spectroscopy and magnetic resonance imaging (MRI). NMR spectroscopy: For NMR spectroscopy the external magnetic field is homogeneous over the entire sample, each part of the sample experiences the same magnetic field strength. An NMR spectrum of the sample can be recorded by monitoring the energy absorption by the spins as a function of the frequency of the applied radiowaves. This spectrum is very characteristic of the atomic composition of the investigated matter, since the resonance frequency of the spins not only depends on the strength of the external magnetic field but also on the local chemical structure around the involved atoms. One reason for that is that the electrons around the atoms also respond to the external magnetic field which produces small electrical currents in the electron cloud. These currents cause small local magnetic fields, usually oppos-ing the external field. The NMR spectrum forms a fingerprint of the sample and can be used to determine the chemical composition of the sample in great detail. Magnetic resonance imaging: For the second category of NMR experiments the external magnetic field is not homogeneous over the entire sample. The magnet system is constructed in such a way that the field strength is a known function of the spatial coordinates x, y and z, or, alternatively, the basically homogeneous magnet is supplied with electrical coils that can produce magnetic field gradients in the x, y or z direction, either constant in time or in a pulsed fashion. As a result, the resonance frequencies of the atomic nuclei become dependent on their position in the field. The MRI technique, frequently used in hospitals, is based on this idea. The patient is positioned in a horizontal homogeneous magnet and surrounded by coils which produce the (pulsed) field gradients. A three-dimensional image can be made representing the density of hydrogen atoms. An example is given of a two-dimensional cross-section of a human head. Just as in the case of NMR spectros-copy, numerous variations on the basic technique exist. An exciting application is the imaging of brain functions. Active parts of the brain are rich in oxygen and the NMR imaging technique can distinguish the NMR signal of oxygen-rich from oxygen-poor blood. Therefore correlations can be made between physical or mental activities and the brain area that stimulates these activities.

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