A NEW SIMPLE METHOD FOR ACCURATE MEASUREMENT OF URINARY I131AFTER TRACER AND THERAPEUTIC DOSES*

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Abstract
A new method for the measurement of urinary I131 is described. Four platinum cathode Geiger-Mueller tubes are connected in a parallel electrical circuit and arranged in a horizontal plane about the source. In the expts. described, a circle diam. of 120 cm. was used. The urinary I131 content is measured by determining the radiation from the entire 24-hr. collection of urine placed in the center of this circle. The standard of reference is a 5-10 cc. aliquot of the I131 stock soln. from which the patient''s dose is taken, diluted to approx. 1500 cc. in a gallon jug. The % urinary excretion was detd. by this method in 38 subjects and compared to a gamma counting method (Marinelli beaker) requiring dilution and measurement of urinary volume. The avg. difference between the 2 methods was 3.4%, standard deviation 1.8. The urine volumes varied from 430-3730 cc. No relationship was found between urine volume and the % difference in urinary I131 content. The results obtained provide exptl. confirmation of the theoretic considerations which indicated the virtual freedom from absorption and geometry of the platinum cathode Geiger-Mueller tube arranged in a circle and connected in a parallel electrical circuit. The adaptation of this method, utilizing Cu screen cathode Geiger-Mueller tubes, for measurement of urinary excretion following therapeutic doses is described. The advantages and limitations are indicated. The minimum handling of the urine sample and limited exposure to radiation have been noted.

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