New developments in the application of radiation to medicine are occurring so rapidly that this is possibly the fastest growing branch of medicine today. In the past decade alone, we have seen enormous progress made in techniques used both for the diagnosis of disease, such as computerized tomography, digital radiography, ultrasonography, computerized nuclear medicine scanning, and nuclear magnetic resonance imaging, and for its treatment, such as the radiotherapeutic utilization of high-LET radiations, and the widespread application of computers to perform elegant dosimetry calculations for 3-D treatment planning and imaging. This series will provide in-depth reviews of the many spectacular technical advances and sophisticated concepts, which are developing in medical radiation physics at such an alarming rate that it has become increasingly difficult to keep one's knowledge up-to-date. These comprehensive review articles will help to bridge the communications gap between the international research community, and the medical physicists and phy­ sicians whose responsibility it is to put these advances into clinical use. These articles should also be of value to the increasing number of physical scientists and engineers who are interested in the application of their knowledge and talents to the field of medicine.