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Læknablaðið - 15.03.1984, Side 41

Læknablaðið - 15.03.1984, Side 41
LÆKNABLADID 70,107-117,1984 107 Brian A. Worthington THE CLINICAL POTENTIAL OF NMRIMAGING SUMMARY Nuclear Magnetic Resonance (NMR) is a new non invasive method of studying the internal structure of the body which completely avoids the use of ionizing radiation and appears to be unassociated with any adverse effects. The principles underlying the technique are ex- plained and an indication is given of its great versatility in studying pathology by reference to the preliminary findings in a study of over 700 patients. Much work i^now being directed towards both defining the'optimal technique for a given pathological condition which requires a better understanding of the factors controlling tissue contrast, and exploring the specificity of the information which can be derived from the images so as to determine the extent to which precise tissue identifica- tion will be possible. INTRODUCTION Within the electromagnetic spectrum there are several windows available for imaging the body consisting of bands of radiation which can penetrate human tissue. Nuclear Magnetic Resonance (NMR) imaging utilises radio-fre- quency radiation in the presence of a magne- tic field to produce cross-sectional images of the body which portray the distribution den- sity of hydrogen nuclei and parameters relat- ing to their motion in cellular water and lipids. As with CT, the display of soft tissue detail is at a premium and with NMR there is the added advantage of being able to manipulate the contrast between different tissues in order to highlight pathological changes by altering the pattern of radio-frequency pulses which is applied. In NMR imaging there are no specifi- cally directed photons or specially aligned This article is based on a paper read to the Icelandic Radiological Society on December, 6th, 1982 by professor B. S. Worthington, B.Sc., M.B.B.S., D.M.R.D., F.R.C.R. All enquiries and correspondence should be addressed to: Division of Radiology, Queen’s Medical Centre, Notting- ham, NG7 2UH, England. detectors. Choice of imaging plane is achieved by methods which restrict data collection to the desired region; this means that choice of slice thickness and position can be achieved without patient movement and that the addi- tional perspective of direct sagittal and coro- nal views is possible as well as the more conventional transverse sections (1). This avo- ids the time and radiation dose penalty associ- ated with reformatted projections in compu- ted tomography. NMR has the further advan- tages of being non-invasive; avoiding the use of ionizing radiation; penetrating bone witho- ut significant attenuation and it appears to be without hazard. The phenomenon of nuclear magnetic resonance (NMR) was discovered in 1945 and from it derives the discipline of NMR spectroscopy which is now widely used in the study of matter at the molecular level. Only relatively recently has it been applied as a method for imaging the body, but there has now been progression from a period where work was only carried out on laboratory prototypes to the present widespread installa- tion of commercial machines for clinical evalu- ation (2, 3, 4). PRODUCTION OF THE NMR SIGNAL Almost all of the images to date have been of the nuclear magnetism of the hydrogen nucle- us, or proton, which is a particularly favour- able nucleus from the NMR standpoint becau- se it gives a relatively high signal and it has a high abundance in biological tissues. Protons have associated with them both a spin and a magnetic field and can be regarded as tiny bar magnets spinning about the vertical axis. When placed in a uniform magnetic field this axis is both tilted and caused to rotate with a precessionel movement like a tiny gyroscope. The frequency of this precession is directly proportional to the applied magnetic field and for protons in a field of 0.15 tesla is 6.39 MHz. If a pulse of oscillating radiofrequency (RF)

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