Radioactive tracer atoms, either natural or purposefully built into
molecules of vital materials like proteins, are revealing how these
substances function in the body to produce energy or to form new
tissues. When we know accurately the normal totals and kinds of
radioactive substances in the body, we can undertake new kinds of tracer
studies without using large amounts of additional radiation. Small
instruments called scanners (see Figure 18) usually are used to track
tracer isotopes, but whole body counters are useful in special
circumstances.
[Illustration: Figure 18 _A multidetector positron scanner to record
radiations with opposed pairs of detection crystals. Scanning devices
are commonly used for noting the fate of tracer isotopes in medical
diagnosis._]
Two types of adaptations enable whole body counters to locate
accumulations of radioactive materials in specific organs or small
portions of the body. At the National Institutes of Health, Bethesda,
Md., one counter is fitted with three rows of six 12-by-12-inch plastic
blocks. Each block has four photomultiplier tubes to collect the
scintillations from the crystal. The rows are curved so as to be equally
distant from the patient’s body.
The current pulses from each of the 18 blocks can be fed individually
into the pulse sorter, counter, and recorder. Thus the kinds and numbers
of gamma rays from the sector of the patient adjacent to any block can
be studied individually. Similarly, the path and speed of the
administered materials can be followed by taking recordings from the
blocks sequentially.
At the U. S. Naval Hospital in Bethesda, Md., a whole body counter is
fitted with a crystal that can be moved at controlled speed past the
body of the person being studied (Figure 19). To increase its scanning
efficiency, the crystal is fitted with a slit and a focusing device.
Instruments record the body radioactivity visibly at timed intervals as
the crystal moves along the patient’s body. A television screen enables
the operator to observe the patient during the counting.
[Illustration: Figure 19 _The U. S. Naval Hospital whole body counter,
showing the moving crystal, left, and instruments, including the
television screen used by the operator. The moving crystal makes it
possible to use this whole body counter for scanning._]
The role of iron in preventing one form of anemia has been clarified by
using iron-59 as a tracer. Persons suffering from chronic infections or
such blood diseases as leukemia and polycythemia vera have been checked
for the amount of iron carried by their red blood cells. Cobalt-60 atoms
have been substituted for stable cobalt in molecules of vitamin B-12 so
that the way the body makes use of this vitamin can be studied.
Similarly, the body’s use of sodium can be studied by labeling sodium
chloride with sodium-22 and then administering solutions of the tagged
salt orally or by injection.
Whole body counters used in tracer studies cause a minimum of
inconvenience for the patient. Their sensitivity permits use of smaller
quantities of radioactive material than is required with small scanning
instruments.
Those are unusual jobs for whole body counters, however. Scanners or
other types of instruments are used more typically in following tracer
isotopes.