Assessment of Global Ischemia - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 356

2. Intravascular
Tracer Injection
for Assessing
Vascular Perfusion

This technique studies vascular fi lling to examine state of cerebral
perfusion. A tracer is introduced into the systemic circulation for a
short period of time so that it fi lls unobstructed, perfused vessels
completely and obstructed/constructed, nonperfused vessels only
partially. The difference in the number of tracer labeled and nonla-
beled vessels refl ects the perfusion status. This technique has been
used to examine perfusion defi cits following SAH. For example,
Asano and Sano in 1977, used this technique to detect foci of non-
fi lled vessels in the gray matter of dogs ( 15 ), and Trojanowski in
1984, to study reduction in cerebral perfusion in cat brain after
SAH ( 16 ). More recently, we have used intravascular tracer injec-
tion technique to study perfusion defi cits in rat after SAH ( 17 ).
The main advantage of this technique besides its simplicity and
cost-effectiveness is that it does not require perfusion fi xation.
Hence, vasculature remains preserved and areas constricted or
obstructed by platelet emboli remain undisturbed and can be
detected. The main disadvantage is that the tracer can move out of
the vessels during the fi xation process giving faulty results. However,
this problem can be easily avoided (see below).

2.1. Introduction

2.2. Main
Considerations

A number of tracers have been used to study cerebral perfusion
and include; India ink, Evans blue, carbon black, and fl uorescent
dextrans ( 15-18 ). High molecular weight fl uorescein conjugated
dextran (FITC-dextran; molecular weight 2 × 10 6 kDa, Sigma)
appears to be the best among these choices. The high molecular
weight of this compound ensures that it stays in the vessel and not
diffuses out; a problem associated with low molecular weight
(40,000-75,000 kDa) dextrans ( 19, 20 ). Moreover, its lack of pro-
tein binding ensures that it does not escape into the brain paren-
chyma with a modest opening of the BBB; a problem associated
with Evans blue ( 21 ). Hence, the problems of high back ground
fl uorescence and nonspecifi c binding that are common with other
intravascular tracers can be avoided by the use of high molecular
weight dextran (Fig. 1 ).

2.2.1. Choice of Tracer

Time requirement for fi lling a perfused and nonperfused vessel is
different. Given enough time both vessels will be fi lled, however, a
perfuse vessel will be fi lled before the nonperfuse vessel. Hence,
intravascular tracer injection technique can only distinguish
between perfused and nonperfused vessels if the time allowed for
tracer circulation is short enough to allow this distinction to be
achieved. We have found that 10 s circulation is plenty to achieve
this distinction (Fig. 1 ) ( 17 ).

2.2.2. Time of Tracer
Circulation