Core Messages

96

S. Wolf

Þndings and alterations of arteriovenous passage

laser ophthalmoscope is converted into digital

time and mean dye velocity [11, 31].

information and recorded digitally.

Beside dye dilution techniques, direct visualiza-

For the measurement of retinal macrocircula-

tion of retinal capillary microcirculation in perifo-

tion, density variations in the ßuorescein angio-

veal capillaries is possible using high-speed

grams are analyzed by means of digital image

scanning laser techniques [8, 12, 20, 21, 25, 26, 32].

processing system. After correction for eye move-

Perifoveal capillary ßow velocities give data on the

ments, the digital image processing system mea-

macular microcirculation. By means of the scan-

sures the entire angiogram sequence, recording the

ning laser technique, ßow velocities of segments of

intensity of ßuorescein at various locations. Six

low ßuorescence were quantiÞed in perifoveal cap-

points are interactively selected for measurement.

illaries. Biomicroscopic recordings of conjunctival

The computer then analyzes the entire angiogram

and periungual capillaries [4] clearly show seg-

frame by frame. Fifty frames per second are evalu-

mentation corresponding to erythrocytes (rouleaux

ated. For each image, the program recorded the

formations) versus plasma. From these Þndings, it

mean intensity levels at each of the six selected

is clear that segmentation in the ßuorescence inten-

locations. Intensity curves are obtained by plotting

sity corresponds to segments of erythrocytes and

the collected data against the time axis. The time

cell-free plasma [3]. Measurements of capillary

of the Þrst appearance of ßuorescein is evaluated

ßow velocities with ßuorescein techniques corre-

from the intensity curves. According to the loca-

spond to the velocity of segments of cell-free

tion of the measuring points, several parameters of

plasma in the bloodstream.

retinal circulation are assessed.

In the following, the measuring techniques as

Two points are selected for measurement on

well as clinical data from blood ßow measure-

the superiotemporal and inferiotemporal arteries

ments using angiographic techniques will be

(0.5 disc diameter) from the disc margin. Two

described.

more distally located points, two disc diameters

from the disc margin on each artery are similarly

monitored. The differences in appearance time

5.1

Measuring Technique

and the actual distance between the proximal and

distal marking point at each artery are used for

High-speed digital ßuorescein angiography is

calculation of the mean arterial dye velocity. The

performed using a scanning laser ophthalmoscope

time elapsed between the appearance of dye at

after intravenous injection of 5 ml sodium ßuores-

the proximal reference point at the temporal

cein (10%) with a 10 ml saline ßush. Digital pic-

arteries and an adjacent point at its corresponding

ture analysis of high-speed digital ßuorescein

vein is used to determine the arteriovenous pas-

angiograms allows for quantitative assessment of

sage times.

retinal circulation. Retinal macrocirculation can

Measurement of retinal microcirculation of

be characterized by the arm-retina time, the arte-

the 15Ð20¡ Þeld of the scanning laser ophthalmo-

riovenous passage time, arterial mean dye veloc-

scope is used. In these digital high-deÞnition

ity, and the arterial vessel diameters. In contrast,

angiograms, segments of low and high ßuores-

the measurement of capillary ßow velocity pro-

cence can be observed moving through the peri-

vides data on retinal microcirculation. Macro-

foveal network. The sequences are processed

circulatory measurements are carried out in the

off-line to evaluate the mean capillary ßow veloc-

40¡ observation Þeld. This mode allows for imag-

ity and coefÞcient of variation of mean capillary

ing the entire posterior pole. For the assessment of

ßow velocity. The measurement of ßow velocity

capillary ßow velocities, high-deÞnition angio-

is based on the determination of transit time ÒDtÓ

grams of the perifoveal capillary network are nec-

between two measuring points, separated by a

essary. Therefore, the 15Ð20¡ Þeld of the scanning

known distance ÒDsÓ. The actual distance ÒDsÓ

laser ophthalmoscope is used. For all measure-

were measured by a digital image processing sys-

ments, the video signal generated by the scanning

tem counting all pixels on the capillary between