Paul Lauterbur and the Invention of MRI

Appendix C: Draft Disclosure, August 1972

Appendix C: Draft Disclosure, August 1972 Appendix C 235 Paul C. Lauterbur August 8, 1972 Magnetography Abstract of the Disclosure There is disclosed a technique and apparatus for the analysis of matter which depends upon the local fields to which the matter is subjected, whereby it is possible to selectively alter the fields in a known way to locate components or constituents in an object. Specification This invention relates to those fields of magnetic resonance spectroscopy in which the properties of substances dispose in a magnetic field are investigated by the application of energy in the radio frequ3ncy or microwave frequency range: such as nuclear magnetic resonance (NMR) and electron spin resonance (ESR) respectively (Marginal note: ESR can be radio frequency at low fields.) These techniques and apparatus are described in numerous texts, articles and other publications. In the conventional techniques here to fore practiced the observed signal, for example, the nuclear induction signal in pulsed NMR spectroscopy, represents the resultant of the signals derived from the signals derived from the excited nuclei of the sample under investigation which have been subjected to the applied magnetic and r.f. fields and subsequently detected without regard otherwise to the location or distribution within the sample of the nuclei contributing to the signal response. For convenience in expression, the present invention will be described in connection with NMR but it should be understood that it is applicable to other forms of magnetic resonance techniques. The present invention provides a technique and means whereby these limitations are overcome so that the derived signals are indicative, for example, of the location and distribution of the excited nuclei and moreover may be utilized to construct the size and shape of objects which are hidden from view, which can not be done with conventional techniques, but the utility of which is apparent in a variety of applications such as medical diagnosis. In this latter sense the invention is somewhat analogous to X-ray and ultrasonic techniques. 236 Appendix C Because the invention provides a means for graphic display of magnetic properties of objects the term “magnetography” has been coined and adopted as the most accurate general description of the technique. (Marginal note: “Magnetograph” has been used as a name for a recording magnetometer. Is that a problem, patentwise?) The basic principle of magnetography can best be described if one first understands the basic phenomena of magnetic resonance. The nuclei of some isotopes of most elements will give nuclear magnetic resonance (NMR) signals if placed in a magnetic field and exposed to radio-frequency radiation. The frequency at which the phenomenon occurs is directly proportional to the strength of the magnetic field. For example, protons, the nuclei of ordinary hydrogen atoms, give a magnetic resonance signal at 100 MHz (in the FM radio band) in a magnetic field of 23.487 gauss, a field strength readily reached by laboratory electromagnets or permanent magnets. In a field half as strong, 11,744 gauss, the resonance frequency would be 50 MHz, and in the earth’s field of about 0.5 gauss, resonance occurs at about 2000 Hz. This proportionality between the NMR frequency and the magnetic field provides the basis for the various forms of magnetography and magnetoscopy. If a magnetically homogeneous object is placed in a non-uniform magnetic field, one, for example that decreases linearly with distance across the object, the single resonance ordinarily observed is replaced by a band of resonances, each representing a particular magnetic field and therefore a particular portion of the sample. The intensity of the signal at each frequency is simply proportional to the number of nuclei in the corresponding magnetic field region. Repetition of the experiment in several differently oriented field gradients provides enough information to construct a twoor three-dimensional projection of the shape and interior structure of the object. In three dimensions, the object may be considered to contain planes of constant magnetic field. The intensity of the resonance at each frequency is proportional to the number of nuclei in a plane of constant magnetic field. A threedimensional image may be constructed from spectra with differentlyoriented magnetic field gradients. Two-dimensional images may be formed more directly from data collected by special pulse sequences. A strip (in two dimensions) or “slice” (in three dimensions) may be selectively excited in one field gradient, and the resonances of the excited nuclei analyzed in another (usually Appendix C 237...