Medical nuclear magnetic resonance imaging: I. Physical principles.

For over 30 years, nuclear magnetic resonance (NMR) spectroscopy has been used for chemical analysis and for studying molecular behavior, but NMR imaging is a recent addition to the methods available to radiologists for investigating the interior of the body. It uses radiofrequency radiation in the presence of a magnetic field to produce anatomical cross sections. The images may be simple maps of the concentration of the hydrogen nucleus, or they may depend on tissue relaxation times, which describe how rapidly hydrogen nuclei exchange energy with their surroundings. In this article, the basic concepts and physical principles of conventional NMR spectroscopy are introduced. In subsequent articles, the various approaches to producing NMR images will be outlined, and the types of information obtainable from NMR scanners will be discussed.

Cell survival at low oxygen tension and dose build-up in argon.

Mammalian cells were exposed to 250 kVp X-irradiation in air, argon and nitrogen to determine whether cells irradiated when severely hypoxic have survival curves with lower extrapolation numbers (n) than their aerobic counterparts. Cells irradiated suspended in liquid showed no significant differences between values of 'n' irrespective of the gas used, neither was the sensitivity of cells irradiated in argon any greater than that of cells irradiated in nitrogen. In contrast, cells attached to glass dishes irradiated with the medium withdrawn were apparently much more sensitive in argon than in nitrogen. It has been demonstrated that the lower survival of cells irradiated in argon could have been caused by the greater photoelectric absorption in argon compared with nitrogen. When the dosimetric discrepancy was removed either by absorption of photoelectrons in liquid or by use of high energy radiations, there was no evidence that severe hypoxia during irradiation could lead to reduced values of 'n'.