Graph of Uptake in 1 hour (Percent of administered dose) versus Uptake
in 24 hours (Percent of administered dose)]
[Illustration: _It is especially important in isotope studies on infants
and small children that the radiation exposure be low. By carrying out
studies in the whole body counter room, the administered dose can be
greatly reduced. The photographs illustrate a technique of measuring
radioiodine uptake in the thyroid gland with extremely small amounts of
a mixture of iodine-131 and iodine-125. A shows a small television set
that is mounted above the crystal in such a way that good viewing
requires that the head be kept in the desired position. This helps solve
the problem of keeping small children still during a 15-minute counting
period. B shows a child in position for a thyroid uptake study._]
This simple procedure has been used widely. One difficulty in using it
is that its success is dependent upon the time interval between
injection and measurement. An overactive gland both concentrates iodine
rapidly and also discharges it back to the blood stream as PBI more
rapidly than normal. Modifications of the test have been made to compare
the amount of iodine-131 that was administered with the amount
circulating in the blood as PBI. The system acquires chemical separation
of the two forms of iodine from a sample of blood removed from a vein,
followed by separate counting. This computation of the “conversion
ratio” of radioactive plasma PBI to plasma-total ¹³¹I gives results that
are less subject to misinterpretation.
To determine local activity in small portions of the thyroid, an
automatic scanner is used. A collimator[9] shields the detector (a
Geiger-Müller tube or scintillating crystal) so that only those impulses
originating within a very small area are accepted by the instrument. The
detector is then moved back and forth slowly over the entire area and
the radiation is automatically recorded at definite intervals, creating
a “map” of the active area. In cases where lumps, or nodules, have been
discovered in the thyroid, the map is quite helpful in distinguishing
between cancerous and benign nodules. The former are almost always less
radioactive than surrounding tissues.
[Illustration: _Seven serial scans made with the whole body scanner were
put together to provide a whole body scan of this patient with thyroid
cancer that had spread to the lung. One millicurie of iodine-131 was
administered and the scan made 72 hours later. Note the uptake in the
lung. This patient was successfully treated with large doses of
iodine-131._]
Fragments of cancerous thyroid tissue may migrate to other parts of the
body and grow there. These new cancers are known as metastatic cancers
and are a signal of an advanced state of disease. In such a situation
even complete surgical removal of the original cancer may not save the
patient. If these metastases are capable of concentrating iodine (less
than 10% of them are), they can be located by scanning the whole body in
the manner that was just described. When a thyroid cancer is discovered,
therefore, a doctor may look for metastases before deciding to operate.
Human blood serum albumin labeled with ¹³¹I is used for measurement of
the volume of circulating plasma. The procedure is quite similar to that
used with radioactive chromium. Iodinated human serum albumin labeled
with ¹³¹I is injected into a vein. Then, after allowing time for
complete mixing of the sample with the blood, a second sample is counted
using a scintillation counter.
[Illustration: _Time-lapse motion pictures of the liver of a 3-year-old
girl were made with the scintillation camera 1 hour after injection of
50 microcuries of iodine-131-labeled rose bengal dye. This child was
born without a bile-duct system and an artificial bile duct had been
created surgically. She developed symptoms that caused concern that the
duct had closed. These scans show the mass of material containing the
radioactive material (small light area) moving downward and to the
right, indicating that the duct was still open._]
For many years, a dye known as _rose bengal_ has been used in testing
liver function. About 10 years ago this procedure was improved by
labeling the dye with ¹³¹I. When this dye is injected into a vein it
goes to the liver, which removes it from the blood stream and transfers
it to the intestines to be excreted. The rate of disappearance of the
dye from the blood stream is therefore a measure of the liver activity.
Immediately after administration of the radioactive dye, counts are
recorded, preferably continuously from several sites with shielded,
collimated detectors. One counter is placed over the side of the head or
the thigh to record the clearance of the dye from the blood stream. A
second is placed over the liver, and a third over the abdomen to record
the passage of the dye into the small intestine.
Human serum albumin labeled with ¹³¹I is sometimes used for location of
brain tumors. It appears that tumors alter a normal “barrier” between
the brain and blood in such a manner that the labeled albumin can
penetrate tumorous tissues although it would be excluded from healthy
brain tissue.
[Illustration: Showing:]
1 Human serum albumin labeled with ¹³¹I
2 Tumor selectively localizes labeled albumin
3 Survey of gamma radiation (Standard points)
The brain behaves almost uniquely among body tissues in that a
“blood-brain barrier” exists, so that substances injected into the blood
stream will not pass into brain cells although they will pass readily
into muscular tissue. This blood-brain barrier does not exist in brain
tumors. A systematic scanning of the skull then permits location of
these cancerous “hot spots”.