Medical Physics II

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The close relation of physics and medicine is illustrated in the development of x-ray and radium therapy. Twenty-five years ago roentgen rays were produced by temperamental gas tubes connected to transformers with mechanical rectification. The maximum potential was about 130,000 volts. Roentgen ray output was erratic and not easily reproduced; only with great difficulty could the amount and the kind of radiation be estimated. Transfer of a given dosage from one machine to another was a problem since the outputs of two machines might vary widely, even though such technical factors as voltage, amperage, distance, and filter were identical. The common methods of calculating dosage from these factors alone were therefore often condemned to failure.

Physicists and radiologists working together steadily improved x-ray apparatus and methods of treatment.1, 2, 3, 4 Modern generators produce potentials up to several million volts, and hot filament x-ray tubes operate on these high potentials. Radiation output is constant and reproducible. Today both quantity and quality of radiation can be readily and accurately determined.

Roentgen ray quantities are expressed in an internationally accepted dosage unit, the roentgen; the unit of quality is the half value layer, which classifies x-rays according to their penetrating power. Absorption and scattering of x-rays within the body have been studied for many radiation qualities and for numerous therapeutic technics. The results of these studies have been arranged in convenient tables from which surface doses, depth doses, and exit doses for the patient are quickly ascertained. Numerous clinical and biological dosages. . .



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