Assessment of Blood–Brain Barrier Breakdown - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 390

the length constant ( 50 ) of the vessel and is a function of its ionic
permeability. While the approach is frequently used in in vitro
models of the BBB, in animals it is invasive, technically challenging
and is limited in its ability for repeated measurements at different
time points and brain regions.
Near infrared spectroscopy (NIRS)— NIRS is a spectroscopic
method which uses the near infrared region of the electromagnetic
spectrum. This method is noninvasive, offers several advantages,
including high sensitivity, the use of nonionizing radiation, as well
as its demand for relatively simple and inexpensive instrumentation
( 51, 52 ). The primary application of NIRS to the human body
uses the fact that the transmission and absorption of NIR light in
human body tissues contains information about hemoglobin con-
centration changes. When a specifi c area of the brain is activated,
the localized blood volume in that area changes quickly. Optical
imaging can measure the location and activity of specifi c regions of
the brain by continuously monitoring blood hemoglobin levels
through the determination of optical absorption coeffi cients. Using
fl uorescent probes, near infrared fl uorescent (NIRF) imaging
becomes a useful method to visualize BBB disruption ( 53, 54 ). A
clear limitation of planar NIRF imaging is that it does not allow
absolute quantifi cation. In addition, since the depth penetration of
the excitation and emission light is limited, fl uorescence can only
be detected from cortical structures of the brain.
Other imaging techniques —Estimating BBB permeability using
in vivo imaging has been also studied in methods often applied in
the clinical settings, including computerized tomography (CT)
and magnetic resonance imaging (MRI). The latter, which uses
powerful magnetic fi eld to study brain anatomical lesions is being
rapidly developed in recent years for the use of animal studies to
follow-up in vivo the development of brain diseases. Measuring
BBB breakdown has been done using semiquantitative approaches
(mainly by measuring changes in signal intensity following the
injection of non-permeable tracer—see refs. ( 55, 56 )) and more
recently using more quantitative and dynamic approaches (“dynamic
contrast enhanced imaging”—DCE-MRI, see ref. ( 57 )). These
method take advantage of the relatively high temporal resolution
obtained where contrast agent—such as gadolinium diethylene tri-
amine pentaacetic acid (Gd-DTPA) is injected followed by dynamic
acquisition of the signal, extraction of blood fl ow parameters and
estimating permeability values based on a two-compartmental
model ( 10, 56, 58, 59 ).
To summarize, several methods were developed trying to
detect and evaluate BBB permeability: the common, more tradi-
tional methods are based on the detection of BBB non-permeable
proteins (e.g., Evans blue-albumin complex and immunostaining)
within the brain tissue using microscopic visualization in fi xed tis-
sue. Others tried to detect brain's biomarkers (e.g., S100B) in the