Josenh G. Hamilton, E.D.

The fundamental postulate upon which rests the value of the artificial

radioelements as tracers is the fact that the radioactive isotope of a stable
element differs only in its property of radioactivity. The chemical and physio-

logical properties of the two forms of the element, or compound, into which a
radioactive element may be incorporated, are identical so long as the radiations

from the radioactive isotopes are not sufficiently intense to produce physiological
changes. In general, this limiting factor can be avoided in most tracer studies
since the amounts of radioactive material required for studies of this character
are too small to produce any chemical or physiclogical changes by the action of
their radiations.
Three general technics have been developed for the use of radioactive
isotopes as tracers in the biological and medical sciences: First, the assimila-

tion of the administered radioelement, or labelled compound, its distribution in

the tissues, conversion into other compound by the body, and finally its elimination may be followed quantitatively by direct measurement of the radioactivity of
the samples of tissues after the removal from the body, Second, the selective
accumulation of the radioactive isotopes of a number of elements and compounds may

be observed in the organs and tissues of the intact animal or human subject by

measurement of the radioactivity of these structures in situ.

Third, the correla-

tion between the distribution of the radioactive clement or compound in the tissues
to their microscopic anatomy may .e studied by the aid of photogranhic films.
This last mentioned technic is usually defined as radioautography,.
The first named technic is the most widely employed and is applicable to
all the radio-elements, The radioactive material may be given either as a simple
inorganic compound, or may be incorporated into a complex organic molecule. For
examole, radio-sodium is usually administered as sodium chloride, radio-phosphorous
as disodium phosphate, radioc-iodine as sodium iodide, etc. The distribution of
labelled elements or compounds in the body may be followed as the total content
of the administered radioactive material in the various organs, tissues and excreta.

For example, the rates of conversion of administered radioelements to various complex
compounds by the physiological processes of the body may be observed.


of this point is the synthesis of hemoglobin from iron, of phospholipids, nucleic

acids, and nucleo-vroteins from inorganic phosphates, and of thyroxin from iodine.
In studies of this character, the compound is isolated from the tissues and its
radioactivity is compared with the total amount of radio-element originally administered. Complex organic compounds may be labelled for traccr studies by the inclusion

of radioactive atoms in the molecules,

For example, thiamin (vitamin B,) has been

tagged by synthesizing it from radio-sulfur. ‘The labelled thiamin is then admrinistered and its fate in the body followed by measuring the distribution of radioactivity from the radio-sulfur in the tissues, body fluids and excreta. By this
procedure its conversion in the body into other compounds may be observed. The),
comparatively recent availability of the 5,000 year radioisotopes of carbon, C-",

has made 1t possible to label amino acids, carbohydrates, drugs, hormones, vitemins,

etc., to study their metabolic nathways and fate in the body,

A second general technic makes use of the ability of many artificial radioelements to emit nenetratinge gamma rays which can pass through many centimeters of
tissue without serious attenuation and which can thus be measured at some distance
from the site of origin. The presence of the accumulated radio-clement in the
particular organ or tissue under study is detected by placing a suitable measuring



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