1158 
MONITORING 
PULMONARY 
CIRCULATION 
Figure 7. — Residue Detection with Recirculation for the Brain. 
ally, the model accounts for interfering recircu- 
lation of tracer to adjacent perfused regions in 
the field of view of the detector. Central to the 
model is the use of two injections of tracer — 
one upstream (arterial) and one downstream 
(venous) — of the particular organ or region of 
interest. Thus, two residue curves are obtained. 
We have developed equations indicating how to 
employ the two residue curves in order to deter- 
mine the mean transit time of tracer through 
the vascular system of interest, as well as 
higher moments of the transit-time distribution 
if they are desired. These equations are natural 
generalizations of Zierler's residue-detection 
method in that they do not depend for their va- 
lidity on any model of mass transport within 
the system of interest. The numerical calcula- 
tions implied by our equations need not rely on 
curve-fitting of the data. In addition to mean 
flow per unit volume, our method can yield com- 
partmental parameters if these are appropriate 
and desired. 
We describe briefly various animal experi- 
ments designed on the basis of our dual-injec- 
tion theory, and give some indication of the 
agreement between blood-flow values predicted 
on our theory with those determined independ- 
ently. 
ACKNOWLEDGMENTS 
We thank Dr. Jerome R. Cox, Jr. for his 
support and encouragement of our work. Drs. 
Maxine L. Rockoff and Peter E. Peters called 
our attention to the problem of recirculating 
tracer in blood-flow studies, and we thank them 
for their illuminating discussions. Drs. John 0. 
Eichling, Judith M. Metzger, and Peter E. Pe- 
ters have obliged us by testing the usefulness of 
our model and results in practice through 
blood-flow studies in patients and animals. 
These will be reported in later papers. 
REFERENCES 
1. ZlERLER, K. L. Equations for measuring blood 
flow by external monitoring of radioisotopes. Circ. 
Res. 16:309-321, 1965. 
