A Method for Continuously Recording the
Disappearance of Radioactive Tracers
from Circulating Blood
By WILLIAM J. MACINTYRE, P H . D . , AND JACK R. LEONARDS, P H . D .
A method of continuous recording of the disappearance of radioactive materials from the circulating blood is described. By cannulation of the right femoral artery of dogs and leading the tubing
past a scintillation counter into the right femoral vein a complete circuit is established in which
the concentration of radioactivity may be continuously monitored. Curves showing concentration
of the radioactive material versus time have been obtained following injection of iodinated (I111)
plasma, Xa", I"1, PM, R°, and colloidal Au198. Xo errors are introduced by sample withdrawal,
timing of samples, or geometric variations.
I
in time of withdrawal, volume of sample, and
geometrical variations in measurement. For
radioactive materials disappearing at a fast
rate from circulating blood (such as Na24
or Iv12), a measurement of the concentration
can be recorded as fast as once per second. In
investigations of the intravascular distribution
of Na24 such as done by Bakay, Selverstone,
and Sweet1, this would result in perhaps a
twenty-fold increase in the number of points
recorded during the fast portion of the continuous disappearance curve with an accuracy
dependent entirely on the statistical recording
of the counting rate. This statistical accuracy
depends on the total number of counts collected before activating the counting rate
computer, and is therefore dependent on concentration of radioactivity and the time interval at which the computer is set to record. For
the following curves the standard deviation of
the data is normally 1 per cent to 3 per cent.
N a previously described continuous recording system for the determination of
cardiac output 1 ' 2 , the dilution of a radioactive material, iodinated (I111) human serum
albumin, was measured immediately following
injection. The primary interest centered about
the first minute of flow in addition to the ten
minute equilibrium level. By establishing a
complete external circuit through which blood
could continuously flow back into the distal
portion of the proximally cannulated femoral
artery, continuous dilution could be followed
during the first ten minutes after injection
without the loss of blood or without withdrawing samples.
Because of the simplicity of monitoring this
external circuit, it seemed advisable to extend
this system to a measurement of the disappearance of various radioactive substances from the
circulating blood.
The advantages of this system appear to be
twofold: fast changes in concentration could
be accurately followed without the necessity of
withdrawing many samples; and each point
recording would be independent of any error
METHOD
The right femoral artery of dogs weighing 10 to
25 kg. was cannulated through rubber tubing to a
thin stainless steel tube of 3 mm. diameter and the
complete flow circuit established by returning the
tubing to the right femoral vein. The stainless steel
tube was held rigidly in a lead shield and mounted
flush against the crystal of a scintillation counter.
The length of the tube exposed to the counter
measured S cm., resulting in a volume of less than
0.6 cc. exposed to the counter at any one time.
As the concentration of radioactivity is detected by
the counter, the counts are collected by a sealer and
From the Atomic Energy Medical Research Project
and the Department of Biochemistry, Western Reserve University School of Medicine, Cleveland,
Ohio. This work was performed under A.E.C. Contract No. W31-109-eng-7S with Western Reserve
University and supported in part by a grant from the
National Heart Institute Public Health Service.
Received for Publication: July 29, 1954.
14
Circulation Research, Volume III, January 196S
WILLIAM J. MACINTYRE AND JACK R. LEONARDS
FEMORAL
ARTERY"^
CAPILLARY
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FEMORAL
VEIN -
ESTERLINE
ANGUS
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METER
COUNTING
RATE
COMPUTER
SCALER
Fia. 1. Schematic diagram showing external cannulation path and relation of apparatus.
fed into a counting rate computer and recording
milliammeter. The injection was made on the venous
side of the external loop so that the first concentration recorded will be diluted by cardiac mixing.
A schematic diagram of the experimental setup is
shown in Fig. 1.
For gamma ray detection a sodium iodide
(thallium-activated)* crystal scintillation counter
was used. The diameter of the crystal measured
VA inches with a thickness of H inch and exhibited
an efficiency to the gamma radiation of I131 of about
50 per cent. AVith the geometrical placement of the
stainless steel tubing about 20 per cent of the gamma
ray emission was incident upon the crystal. Thus
for a concentration of 0.01 MC/CC. of I"1 in the circulating blood a counting rate in the mnge of 10001500 counts/min. was recorded from the 0.5 to 0.6
cc. volume. This counting rate could be increased
perhaps by a factor of four or five by passing the
tubing through the crystal itself. A larger diameter
of tubing could also increase the counting rate by
presenting a larger volume to the ciystal. It was felt,
however, that the 3 mm. diameter tubing would
prevent |X>oling and would be an actual representation of concentration in the actual arterial blood
circulating generally at that specific time. How
much larger the diameter could be increased without
pooling has not been detemiined. For beta detection
a conventional thin window (1.4 rag/cm1) Geiger
counterf was used with thin rubber tubing substituted for the stainless steel.
The sealer collecting the counts from the detector
head was set for a varying numlDer of collected
counts before activation of the counting rate com• The Harshaw Chemical Company, 1945 East 97th
Street, Cleveland, Ohio.
t Amperex 200 CB, Amperex Electronic Corporation, 25 Washington St., Brooklyn 1, N. Y.
15
puter. The selection of scaling factors is a function of
accuracy desired and the frequency with which
concentration points are to be recorded. At a
counting rate of 10,000 counts/min., for example, a
sealer factor of 1000 would indicate that a concentration point could be I'ecoitled every six seconds with
each point exhibiting a standard deviation of
±3 per cent. If a point recording should not be
required that often a sealer factor of 10,000 would
give a concentration reading once per minute with a
standard deviation of ±.\ per cent. In the cases
where a relatively high concentration is obtainable
both high statistical accuracy and frequent recording is possible limited only by coincidence loss of
the sealer.
The counting rate computer, Berkeley Model
1600, then records the counting rate on an Esterline
Angus Graphic Ammeter. Although a conventional
counting rate meter would also be applicable to this
recording problem, the advantages of the counting
rate computer are twofold: the statistical accuracy
of each point is dependent solely on the number of
points collected without additional effect of the
electronic circuit; and if any fast responses occur,
the counting rate will be recorded with no memory
effect, and thus the fall will not be obscured because
of the influence of previous high counting rates.
RESULTS
m
A. Iodinated (/ ) serum albumin.—In Fig.
2 the disappearance of iodinated (I1S1) serum
albumin over the first six hours after injection
is shown. Curve A was obtained by injection
of 176 y.c of iodinated (I111) canine plasma,
curve B by injection of 147 MC of the same material, and curve C by injection of 295 nc of
iodinated (I111) human serum albumin. Circulating blood volumes in all three cases were
between 850 and 1050 cc. A scaling factor of
10,000 was used and concentration recording
was made every ten to forty seconds dependent upon the counting rate. Initial counting rates, dependent on the dosage and blood
volume, ran from 30,000 to 60,000 counts per
min. As points are plotted every 2% minutes
each point is an estimated average of 75,000 to
150,000 counts. While this would place the
statistical error very low, it is impossible to
read the chart to more than an estimated third
figure. Each plotted point is then no more than
2% accurate due to recording limitations.
These points have been plotted on semilogarithmic paper through the first two hours
disappearance and the linear extrapolation
DISAPPEARANCE OF RADIOACTIVE TRACERS
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FIG. 2. Disappearance of iod'vnated (I111) servim albumin for the first six hours following i.v. injection. The injection of 25 cc. of saline illustrates the variation in volume that can be detected by
this method (curve c).
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Fia. 3. Disappearance of electrolytes for the first 17 minutes following i.v. injection.
taken through zero time for extrapolated blood
volume. In Eig. 2 curves A and C are plotted
so that the relative concentration of radioactivity at zero time is 100% in each case.
Each succeeding point can then be expressed
as the percentage of the concentration at time
zero. Curve C is plotted as relative counting
rate.
WILLIAM J. MACLNTYRE AND JACK R. LEONARDS
From construction of tangents to the disappearance curves plotted on linear coordinates
at various times the rates of disappearance are:
at one hour, 5.2%/hr for curve A, 11%/hr for
curve B, and 8 %/hr for curve C; at two hours,
4.3%/hr for curve A, 6.4 %/hr for curve B
and 4.5 %/hr for curve C; at 6 hours 1.0 %/hr
for curve A, y2 %/hr for curve B and 2 %/hr
for curve C. Storey, Moshman, and Furth4
have reported somewhat similar rates of disappearance with the following average values:
19 %/hr during second half hour, 6 %/hr during
the second hour and slower thereafter.
The accuracy of this method is illustrated in
curve C of Fig. 2. At the end of 107 minutes a
saline injection of 25 cc. or about 2 ^ % of
blood volume was given intravenously. The
subsequent decrease in concentration is detectable as is the time for the circulating blood
volume to adjust to its normal value. The
dotted line would then be the expected disappearance if no saline were administered.
B. Na24, P82, ami I131.—The disappearance
of typical electrolytes is shown in Fig. 3. Curve
A results from the injection of 270 nc of Na24
in the form of Na2CO3 as obtained from Oak
Ridge. The scaling factor used here was 400,
and with a counting rate during most of the
curve of 20,000 to 8,000 counts per minute, a
point was recorded every one to three seconds
apart.
During the first minutes of the curve each
point is plotted as often as recorded (every
one or two seconds); during the second minute
each point is plotted as the average count over
a ten second interval and thus forms a total of
4000 to 2000 counts. During the next three
minutes each point is averaged over a twenty
second interval (about 4000 counts) and from
five to sixteen minutes averaged over one
minute (about 8000 counts). Even during this
latter part of the curve the concentration was
recorded every three seconds by the computer
so that if faster changes did occur they could
be observed. The statistical error from the
total counts on each point show a standard
deviation of 2% or less for each point. This
again is no better than the accuracy of reading
each point from the chart.
Curve B in Fig. 3 shows a similar curve
17
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Fio. 4. Actual chart recording of the concentration
of K41 in the circulating blood for the first eighty
seconds following injection.
following the injection of 216 fie of I131 in
form of sodium iodide as obtained from Oak
Ridge. With P12 injected in the form of phosphate the plot of curve C is obtained. An
accurate representation of the concentration of
injected material during the first minute of
dilution is difficult to obtain because of the
primar}' recirculation. The relative times for
the appearance of the various peaks are not
necessarily representative of the injected
material since the peaks are dependent also
on the circulation times of the various animals,
variation in cardiac output and length of time
for the material to be injected. These conditions were not held constant in all the experiments and the first minute determinations in
curves of Fig. 3 are included merely to show the
factors tending to obscure the disappearance
during this time.
C. K*2 and colloidal Aum— The value of this
continuous recording method is perhaps best
illustrated by the disappearance of such substances that rapidly leave the circulating blood.
An actual chart recording of the concentration
of K42 in circulating blood for the first eighty
seconds following injection is shown in Fig. 4.
The concentration of both K4i and colloidal
Au198 during a longer period of time (five
minutes) is shown in Fig. 5.
After one minute the disappearance of the
colloidal Au19S appears to be reasonably
linear in the semi-logarithmic plot indicating
one effective exponential process of removal.
During this period, the time for the concentration to fall to one-half of its former value
occurs in the range of 90 seconds. This is con-
IS
DISAPPEARANCE OF RADIOACTIVE TRACERS
In the colloidal Au198 disappearance curve a
point was plotted every five seconds for one
minute to two minutes with a total of 800 to
1200 counts per point, and from two minutes
to five minutes plotted every ten seconds with
500 to 2000 accumulated counts per point.
Obviously the problem of withdrawing samples
at this rate and for this period of time would be
very difficult, although in the investigation by
Walker and Wilde6 samples as often as every
fifteen seconds were taken.
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Fio. 5. Disappearance of K41 and colloidal Au18*
from circulating blood for the first five minutes following i.v. injection.
sistent with the measurement of the disappearance time of colloidal Au19S by others'.
The disappearance of K42 from circulating
blood is seen to be appreciably faster in the
early portion of the curve. Thus this curve
initially falls from 4000 to 2000 in about 8
seconds, from 2000 to 1000 in about 18 seconds,
from 1000 to 500 in 35 seconds, and from 500
to 250 in 85 seconds. The appearance of this
curve is much like the curves of Walker and
Wilde8 in their investigation of the disappearance of K42 from the circulating blood of rabbits. Their analysis could be applied to this
curve if desired. The time for total injection
amounted to five seconds, with zero time for
the curve placed at the injection midpoint.
Approximately 25 rng. of carrier KI was injected.
In the Iv42 curve of Fig. 5 there are over
twenty points plotted during the first minute;
each point from one minute to two minutes is
plotted every ten seconds for a total of about
500 counts; and for two minutes to three minutes for a total of about 600 counts per point.
The method as outlined has been found to be
an accurate sampling technique for obtaining
curves of the disappearance of various radioactive substances from circulating blood.
Various curves for disappearance of iodinated (I131) plasma, Na24, I 1 ", P*2, K«, and
colloidal Au198 have been obtained by this
procedure and are illustrated along with a
discussion of the average interval of recording
of the concentration level and the accuracy of
the plotted points.
REFERENCES
1
MACINTYRE, W. J., PRITCHARD, W. H., ECKSTEIN,
R. W. AND FRIEDELL, H. L.: determination of
cardiac output by a continuous recording
system utilizing iodinated (I131) human serum
albumin. I. Animal Studies. Circulation 4: 552556, 1951.
'PRITCHARD, W. H., MACINTYRE, W. J., SCHMIDT,
W., BROFMAN, B. L. AND MOORE, D. J.: The
determination of cardiac output by a continuous recording system utilizing iodinated
(I111) human serum albumin. II. Clinical
studies. Circulation 6: 572-577, 1952.
'BAKAY, L., SELVERSTONE, B. AND SWEET, W. H.:
Intravascular distribution of Na14 injected
intravenously in man. J. Lab. & Clin. Med.
38: 893-903, 1951.
* STOREY, R. H., MOSHMAN, J., AND FURTH, J.:
A simple procedure for determination of the
approximate lymph space. Science 114: 665667, 1951.
'WISH, L., FURTH, J., SHEPPARD, C. W., AND
STOREV, R. H.: Disappearance rate of tagged
substances from the circulation of roentgen
irradiated animals. Am. J. Roentgenol. 67: No.
4, 62S-640, 1952.
'WALKER, W. G., AND WILDE, W. S.: Kinetics of
radiopotassium in the circulation. Am. J.
Physiol. 170: 401-413, 1952.