K. B. LARSON AND D. L. SNYDER 
1157 
P.C. IN 
DETECTOR 
FIELD 
BLOOD 
IN 
L.H. 
SYSTEMIC 
CIRCULATION 
OUTSIDE 
DETECTOR FIELD 
© 
A 
MYO- 
CARDIUM 
h(t) 
S.C. IN 
DETECTOR 
FIELD 
DETECTOR 
FIELD, D 
© 
BLOOD 
IN 
R.H. 
P.C. IN 
DETECTOR 
FIELD 
PULMONARY 
CIRCULATION 
OUTSIDE 
DETECTOR FIELD 
Figure 6. — Residue Detection with Recirculation for the Heart. 
tion is obtained on our model is defined by the 
location of the sites for injection experiments A 
and V. In the Washington University dual- 
injection experiments, two radioisotopes have 
been employed: ^^0 and ^^^Xe. The first is used 
to label v^ater v^hich is then mixed with saline 
solution to form the injectate. This cyclotron- 
produced oxygen isotope is a positron-emitter 
whose short half-life of 124 seconds requires 
that it be prepared immediately prior to injec- 
tion. Five-day ^^^Xe, obtained commercially, is 
a gamma-emitter, and is dissolved in saline so- 
lution prior to injection. The annihilation radia- 
tion from and the gamma radiation from 
"^Xe are monitored continuously as a measure 
of the amounts of tracer within the organ and 
adjacent tissues following injection. The detec- 
tors employed are collimated Nal(Tl) scintilla- 
tion counters. Detector data are accumulated on 
magnetic tape using a LINC computer, which is 
also programmed to correct the data for physi- 
cal decay of the isotopes and for radioactive 
background. 
The responses (normalized to the injected 
doses) corresponding to the two injection stim- 
uli are denoted qa(t) and qv(t), respectively. 
When there is little or no loss of tracer during 
the course of an experiment, both normalized 
responses approach a common steady-state 
value, denoted q ^ . These raw data are then 
processed as specified by Equation (10) , yielding 
a value of blood flow relative to volume. For 
those experiments in which flow, volume, and 
the partition coeflScients have been measured in- 
dependently, agreement with the ratio predicted 
by Equation (10) has generally been to within 
10% or less. These results are encouraging and 
allow us to place confidence in the applicability 
of our theory. 
SUMMARY 
We have developed a mathematical model ap- 
plicable for external monitoring of radiotracer 
emissions when recirculation is not a late event 
and must be taken into consideration. Addition- 
