Laser Speckle Imaging - Animal Models of Acute Neurological Injuries: Injury and Mechanistic Assessments - page 492

The quality of these fl ux measurements are termed “relative,” since
it is neither possible to deduct or quantify the absolute CBF
(measured in ml/100 mg/min), nor compare regional blood
fl ow velocities over a large surface area.
The concept of Laser Speckle Contrast Analysis (LASCA) was
initially investigated in 1982 as a method for noninvasive measure-
ment of retinal blood fl ow ( 3 ). The “Speckle” is a random interfer-
ence effect that occurs after an object is illuminated by laser light.
LASCA exploits the fact that a random speckle pattern - generated
when tissue is illuminated by laser light - changes, when blood cells
move within this illuminated area. When there is a high level of
movement (fast fl ow) the speckle pattern becomes more blurred
and the contrast in that region reduces accordingly. Therefore, low
contrast is related to high fl ow, high contrast to low fl ow. These
real-time contrast images are then recorded by a CCD camera,
processed on a personal computer and mapped as color-coded live
images (flux image: red = high flow, blue = low flow) of blood
fl ow velocity in the tissue to provide a continuous map of blood
fl ow velocities/tissue perfusion changes in real-time over a variable
scan area (=full-fi eld laser speckle) ( 2 ).

2. Methods

2.1. Technical
Requirements
for Laser Speckle
Contrast Analysis

The general set-up of the system is straightforward ( 6 ):

1. Laser light is diverged by a lens to illuminate a selected tissue area.
2. The illuminated tissue area and refl ection pattern is imaged by
a CCD camera and processed by a personal computer.
3. With purpose-designed software, the data are simultaneously
integrated and displayed as a color-coded image sequence of
blood fl ow (=fl ux) velocities.

The LASCA imager described in the following chapter is a
22 × 8 × 23 cm portable device (MoorFLPI, Moor Instruments,
Devon, England), mounted on an adjustable tripod and connected
to a standard laptop computer equipped with real-time data acqui-
sition software (MoorFLPI Measurement Software Version 2.01,
Moor Instruments, Devon, England). Video frame rates of fl ow
within the microcirculation are provided up to 25 images per sec-
ond at a maximum resolution of 49,000 pixels/cm 2 . For visual
reference, the system simultaneously records a corresponding gray
scale image with its integrated CCD video camera. The manual
zoom allows a variable full-fi eld scan area of 5 × 7 mm to 8 × 12 cm
with a working distance of 15-45 cm between the scan head and
the measurement site (Fig. 1 ).

2.1.1. System