Experimental calibration of a two-stage prism-grating system for measuring cell velocity

Fiber optical spatial filter anemometry--intravital measurement of red blood flow velocity (RBCV) in the microcirculation

The fiberoptical spatial filter anemometry (SFA) is a common technique based on an optical grid to measure the velocity of corpuscular components in a multiphase flow, e.g. in the microvessels. The technical innovation is the analysis of flow velocities using an optical grid sensor and frequency analysis by Fast Fourier Transformation (FFT). This study describes a non-invasive, on-line technique to measure RBCV in the microcirculation. The sensor's validity was proven by in vitro measurements using a rotation disk of an exactly defined velocity with a correlation coefficient of 0.99967. For validation of RBCV measurements in the microcirculation in vivo, the setup was adapted to an intravital microscope. RBCV was measured in arterioles, capillaries, and postcapillary venules ranging from 8-140 microm diameter. As reference method for velocity measurements a computer assisted imaging system was used to measure the RBC-velocity in the identical vessels by frame to frame analysis. Both methods revealed a high significant correlation using transillumination technique for capillaries (r=0.986, p<0.001) and venules (r=0.952, p<0.001) as well as epiillumination technique (capillaries r=0.939, venules r=0.975, p<0.001).

Validation and reproducibility of bidirectional laser Doppler velocimetry for the measurement of retinal blood flow

Bidirectional laser Doppler velocimetry (BLDV) for the measurement of retinal blood flow was validated in six anaesthetised minipigs, by comparing BLDV derived results with those obtained using radioactively labelled microspheres (RLM). The mean velocity of blood (Vmean) was calculated from the maximum red blood cell velocity measured by BLDV. Volumetric flow rate was determined from Vmean and vessel diameter, measure from monochromatic fundus photographs. Total retinal blood flow (TRBF) was calculated by summating flow values obtained for each retinal vein draining into the optic disc. A significant correlation was found between the TRBF results obtained by the two techniques (r = 0.99, p less than 0.001). The BLDV results were between 3-35 microliters/min lower than the corresponding RLM results (p = 0.05). Values of 57 +/- 24 microliters/min and 76 +/- 34 microliters/min were obtained for TRBF using the BLDV and RLM techniques respectively. Reproducibility studies with BLDV were also performed in six anaesthetised pigs over three hours and in six normal human volunteers over two hours and two weeks. No significant difference between measurements was found with time. Ninety five percent confidence limits of +/- 9.8% for the six pigs and +/- 8.9% for the six human volunteers were found for measurements on the same day and at two weeks. We conclude that with a sample size of six, changes in flow of approximately 20% can be detected using BLDV and monochromatic fundus photography.

The time-space correlation method for measurement of erythrocyte velocity in microvessels using a CCD linear image sensor

A new method for measuring erythrocyte velocity in the microvascular system was developed. A correlation function varying both time and distance, "time-space correlation method," was used in order to improve the time correlation and the space correlation methods. A CCD linear image sensor installed at the center of the focal plane of a camera was used to convert the passage of erythrocytes through the microvascular system to electrical signals via intravital microscopy. The method was applied to small vessels in the mesentery of rats. Using the present method, the time required for the measurement and the length of the vessel necessary for the measurement have been reduced, as compared with other methods.

Determination of volumetric flow in capillary tubes using an optical Doppler velocimeter

The purpose of this study was to use the optical Doppler velocimeter of Borders and Granger [(1984), Microvasc. Res. 27, 117-127] to determine the ratio of centerline red cell velocity to mean velocity of blood flowing in small glass tubes. Red blood cell suspensions were perfused at different rates through capillary tubes (15-100 microns, id) while measuring centerline velocity (Vmeas). Mean red cell velocity (Vmean) was calculated from measurements of volume flow in the tubes. With the effective slit width (sensor size) of the velocimeter set at 9.0 microns, the ratio of Vmeas/Vmean averaged 1.54 +/- 0.03 (mean +/- SEM), and was essentially independent of tube size. When the slit width was increased, the ratio of Vmeas/Vmean was significantly lower and appeared to vary as a function of tube diameter. These results are compared with previous measurements using other velocimeters, and the relationship between relative sensor size and ratio of Vmeas/Vmean is discussed.

Precision velocimetry with digital cross-correlation for flow measurements during thrombus growth

A precise velocimetry with good frequency response is an indispensable prerequisite to studying hemodynamic effects. The need to monitor minute changes of velocity prompted the development of a software-driven velocimeter that uses a laboratory computer without implementation of specialized arithmetic hardware. Without an on-line output, this setup offers high precision (6000 time shifts), extension of the measuring range to lower velocities by nearly two orders of magnitude, an excellent temporal resolution, and resistance to bad signal-to-noise ratios. The system is described and its characteristic parameters are demonstrated. Velocimetry was performed during experiments on thrombogenesis with and without pretreatment with antithrombotic agents and was found to be valuable in assessing thrombus growth and thrombus stability in vivo.

Responses of sequentially branching macro- and microvessels during reactive hyperemia in skeletal muscle

Small artery and microvascular responses during reactive hyperemia were compared to determine which resistance-bearing vessels played a role in controlling blood flow and resistance for the cremaster skeletal muscle. Using an intravital video microscopy system, measurements of microvessel pressure, flow velocity, and diameter were obtained from cremaster muscles in anesthetized rats. These were compared with measurements of diameter that were obtained from the small arteries feeding the cremaster muscle. After a 60-sec occlusion of the sacral aorta, total cremaster blood flow increased approximately 28% and calculated microvascular resistance for the cremaster muscle fell 50%. During the period of occlusion, diameters of small arteries (159-292 micron) decreased despite the presence of smooth muscle tone. Likewise, the diameters of large arterioles (65-117 micron) decreased whereas small arterioles (16-30 micron) dilated. The decrease in diameter of the small arteries and large arterioles was accompanied by a significant fall in intravascular pressure, suggesting that the behavior of these vessels was largely passive. Immediately following the release of occlusion, small arteries and large arterioles returned to their control diameters while small arterioles remained in a dilated state for approximately 2 min. These results indicate that for the cremaster muscle, vascular responses vary along the length of the arterial tree during reactive hyperemia, small but not large arterioles are primarily responsible for the decrease in network resistance and subsequent hyperemia following occlusion, and the small feeder arteries did not dilate during reactive hyperemia but instead acted to set a limit on the decrease in network resistance and the increase in blood flow.

Methods to measure blood flow velocity of red blood cells in vivo at the microscopic level

Several methods to measure red blood cell velocity in microvessels by electronic means are discussed. Signals are generated by the red blood cells present in the microscopic image of the microvessels. These signals can be converted to obtain an output signal proportional to the actual red blood cell velocity. The method of spatial filtering by interlacing gratings is discussed in terms of a filter with an input signal. Adaptation of optical factors that might improve the velocity measurement is obtained by a mathematical analysis. Different methods of correlation are presented. The temporal correlation (dual slit and video window) and spatial correlation methods are discussed in relation to factors influencing the quality of the correlogram, the peak of which is proportional to red blood cell velocity. The conversion of red blood cell velocity to volume flow is put in perspective.

Use of digital cross-correlation for on-line determination of single-vessel blood flow in the mammalian kidney

The empirical relationship between erythrocyte velocity (Vrbc) and mean blood velocity (Vblood) was studied in quartz capillaries by television microscopy using the dual-slit technique. A newly designed desktop digital on-line cross-correlator was combined with a computer to determine Vrbc. The accuracy of the digital correlator was tested for velocities ranging from 0 to 3 mm/sec and compared with values determined using an analog tracking correlation device. There was good agreement. Small-bore glass tubes with diameters ranging from 12 to 26 micron were perfused with suspensions of erythrocytes having hematocrits between 10 and 37%. The relationship between mean blood velocity and erythrocyte velocity in these quartz tubes was found to be Vblood = 0.88 Vrbc - 0.11, and was independent of diameter and hematocrit within the range investigated. The mean ratio for Vrbc/Vblood was 1.42 +/- 0.06.

An optical doppler intravital velocimeter

A system has been implemented for measurement of red blood cell velocity in microvessels by using an optical Doppler technique. Ronchi rulings are used to stimulate a differential grating to translate red blood cell movement to light intensity variations. These variations are sensed by two photodiodes coupled in a resistive subtraction mode. The nonelectronic subtraction allows high transimpedance gains (2 X 10(9) V/I) while noise is held to a minimum (4.5 mv RMS in a 5-kHz bandwidth). To derive average velocity the average frequency determination of the amplified signal is performed with a thresholding frequency-to-voltage functional block. The velocimeter provides the typical performance features of an optical Doppler system, including high-frequency response, without the need for the complications of the laser Doppler technique or the requirement of custom micro-prism gratings. The device represents a cost-effective approach to intravital work, and offers significant improvements in performance over standard techniques.

The application of an improved dual-slit photometric analyzer for volumetric flow rate measurements in microvessels

The dual-window photometric analyzer is currently a widely used instrument to estimate volumetric flow rates in microvessels. This instrument provides an analog voltage that is proportional to cell velocity, but the question arises how this voltage is related to the mean velocity in the blood vessel. It requires a separate investigation and in this report a calibration of an improved version of a dual-window photometric analyzer for volumetric flow rate measurements in microvessels as large as 470 microns in diameter is presented. The relationship between correlator velocity and blood mean velocity is presented in a closed empirical form. Within a 5% error of the mean it is found to be dependent on tube diameter but independent of hematocrit (between 3 and 40%). A rotating transparent disc plated with dried red blood cells was used to test the dynamic response of the instrument for frequencies up to 10 Hz. The amplitude reduction was less than 10% up to 3 Hz. Thereafter it fell off at 6 db/octave.

Measuring the microcirculation in the human conjunctiva bulbi under normal and hyperperfusion conditions

We present a system consisting of a slit lamp stereomicroscope and an adapted video system for the examination of the conjunctival microcirculation. This system permits measurement of the flow of erythrocytes in the vessels of the bulbar conjunctiva. We present values obtained from clinically normal individuals under normal conditions and under conditions of hyperperfusion.

In vitro and in vivo measurement of red cell velocity with epi- and transillumination

Measurement of red cell velocity with the dual-slit cross-correlation method in glass capillary tubes during transillumination indicates that the measured velocity must be divided by a correction factor of approximately 1.6 to equal the average velocity calculated from a known flow and inner diameter. Whether the same correction factor exists when red cell velocity is measured during epiillumination is questionable. Red cell velocity was measured with the dual-slit correlation method nearly simultaneously using epi- (EL) and transillumination (TL) while glass tubes (40-100 microns, i.d.) were pump perfused with whole human blood (hematocrit 39-42%). With TL, the measured velocity is 1.58 +/- 0.07 (SEM) times the calculated average velocity, whereas a factor of 2.04 +/- 0.04 (SEM) was obtained with epiillumination. When intestinal arterioles with approximately the same inner diameters and flow velocities as the glass tubes were used, the ratio of velocities measured with TL to EL was 1.21 +/- 0.02 (SEM) as compared to 1.31 +/- 0.09 (SEM) for glass tubes using TL and EL of the tube at the same pump flow. This similarity of TL to EL velocity ratios for glass tubes and microvessels may be fortuitous or indicate that comparable flow properties and measurement conditions exist for in vitro and in vivo situations. The major finding of the study is, however, that different velocity correction factors exist for EL and TL measurements when the dual-slit correlation method is used to estimate red cell velocities in tubes of an internal diameter of 40-100 microns at normal hematocrits.