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The Pri..-iplies and Applications of Radivuctive Tracers
To Tho Medical and Biological Scicnces

device, such as a Geiger counter tube, over it and measuring the intensity of the
gamma rays emitted from the radioactive atoms which have been stored selcctively
in the tissuee The application of this sccond major type of tracer technic is
illustrated in the study of the iodine metabolism of the thyroid gland in normal

and goitrous human subjectse

The selective deposition of the administered radio-

jodine by the thyroid is determined by placing a Geiger counter tubo over the
neck and measuring tho intensity of the gamma rays emittod from the radio-iodine
accumulated in the thyroid tissuee Sincc by this method the necessity of
removing tissue for detcrmination of its radioactivity is eliminated, it offers
two distinct advantages. First, it enables the observer to follow the cobb and
flow of the labolled olement or compound in the same human subject or animal
for a concidcrable period of timee The results thus obtained give a continuous
record of the fate of the accumulated radioactive atoms or moleculese
it makes possible in vivo tracer studies in normal human subjectse

Secondly,

The third technic mikes use of the photographic action of the radiations
from artificial radio-clements, by means of which the distribution of the
administered labelled elements or compounds in tissues may bo investigatede
Thin sections o° the radion. tive tissues arc olaccd against phoncgraphic filmse

After an inter7.1 of time sufficient for adequate exposure, the tilm is removed
and developeds
The sections ure stuined, ond exch section with its corresponding

picce of developed film (rudioeautograoh) is oxamined undur the microscopee

arcas of darkening in the film correspond to the regions of che tissues in
which the greatest deposition of the racio-elument has taken placce Thus a
correlation between the deposition of the labelled element or compound and

The

the histclogical strusture of the tissue can be established,

DO® ARC’

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