Evaluation of enhanced high-resolution laser Doppler imaging in an in vitro tube model with the aim of assessing blood flow in separate microvessels

Evaluating medical device and material thrombosis under flow: current and emerging technologies

This review highlights the importance of flow in medical device thrombosis and explores current and emerging technologies to evaluate dynamic biomaterial Thrombosis in vitro.

A polymeric micro-optical interface for flow monitoring in biomicrofluidics

We describe design and miniaturization of a polymeric optical interface for flow monitoring in biomicrofluidics applications based on polydimethylsiloxane technology, providing optical transparency and compatibility with biological tissues. Design and ray tracing simulation are presented as well as device realization and optical analysis of flow dynamics in microscopic blood vessels. Optics characterization of this polymeric microinterface in dynamic experimental conditions provides a proof of concept for the application of the device to two-phase flow monitoring in both in vitro experiments and in vivo microcirculation investigations. This technology supports the study of in vitro and in vivo microfluidic systems. It yields simultaneous optical measurements, allowing for continuous monitoring of flow. This development, integrating a well-known and widely used optical flow monitoring systems, provides a disposable interface between live mammalian tissues and microfluidic devices making them accessible to detectionprocessing technology, in support or replacing standard intravital microscopy.

Comparison of instruments for investigation of microcirculatory blood flow and red blood cell concentration

The use of laser Doppler perfusion imaging (LDPI) and laser speckle perfusion imaging (LSPI) is well known in the noninvasive investigation of microcirculatory blood flow. This work compares the two techniques with the recently developed tissue viability (TiVi) imaging system, which is proposed as a useful tool to quantify red blood cell concentration in microcirculation. Three systems are evaluated with common skin tests such as the use of vasodilating and vasoconstricting drugs (methlynicotinate and clobetasol, respectively) and a reactive hyperaemia maneuver (using a sphygmomanometer). The devices investigated are the laser Doppler line scanner (LDLS), the laser speckle perfusion imager (FLPI)-both from Moor Instruments (Axminster, United Kingdom)-and the TiVi imaging system (WheelsBridge AB, Linkoping, Sweden). Both imaging and point scanning by the devices are used to quantify the provoked reactions. Perfusion images of vasodilatation and vasoconstriction are acquired with both LDLS and FLPI, while TiVi images are acquired with the TiVi imager. Time acquisitions of an averaged region of interest are acquired for temporal studies such as the reactive hyperaemia. In contrast to the change in perfusion over time with pressure, the TiVi imager shows a different response due its measurement of blood concentration rather than perfusion. The responses can be explained by physiological understanding. Although the three devices sample different compartments of tissue, and output essentially different variables, comparisons can be seen between the three systems. The LDLS system proves to be suited to measurement of perfusion in deeper vessels, while FLPI and TiVi showed sensitivity to more superficial nutritional supply. LDLS and FLPI are insensitive to the action of the vasoconstrictor, while TiVi shows the clear boundaries of the reaction. Assessment of the resolution, penetration depth, and acquisition rate of each instrument show complimentary features that should be taken into account when choosing a system for a particular clinical measurement.

Bio-Microfluidics Real-Time Monitoring Using CNN Technology

A new non-invasive real-time system for the monitoring and control of microfluidodynamic phenomena involving transport of particles and two phase fluids is proposed. The general purpose design of such system is suitable for in vitro and in vivo experimental setup and, therefore, for microfluidic applications in the biomedical field, such as lab-on-chip and for research studies in the field of microcirculation. The system consists of an ad hoc optical setup for image magnification providing images suitable for acquisition and processing. The main feature of the optical system is the accessibility of the information at any point of the optical path. It was designed and developed using discrete opto-mechanic components mounted on a breadboard. The optical sensing, acquisition, and processing were all performed using an integrated vision system based on cellular nonlinear networks (CNNs) analogic (analog plus logic) technology called focal plane processor (FPP, Eye-RIS, Anafocus) that was inserted in the optical path. Ad hoc algorithms were implemented for the real-time analysis and extraction of fluidodynamic parameters in micro-channels. They were firstly tested on sequences of images recorded during in vivo microcirculation experiments on hamsters and then applied on images acquired and processed in real-time during in vitro experiments on two-phase fluid flow in a continuous microfluidic device (serpentine mixer, ThinXXS).

Visualisation of morphological changes in living intact human microvessels using confocal microscopy

Conventional techniques to visualise microvascular structure often involve fixed tissue slices that provide two-dimensional images. A previous study using diffusive labelling of fresh, dissected tissue samples with fluorescently-tagged endothelial markers demonstrated the possibility of examining the three-dimensional architecture of the microvasculature using confocal microscopy. The present study extends the use of this quick and simple method of diffusive labelling to examine the possibility of repeatedly measuring changes in the morphology of intact microvessel in response to pharmacological stimuli. Initially, three-dimensional surface-rendered images of the same microvessel derived from the placenta and subcutaneous biopsies demonstrated morphological and topological changes in response to temperature and increasing potassium changes of physiological salt solutions, respectively. Furthermore, a dose-response study was performed with subcutaneous microvessels using the potent vasodilator, adrenomedullin. Analysis of a series of z-stack, superimposed to form a single maximum brightness image, demonstrated an inverse dose-response relationship, with responses to increasing adrenomedullin concentrations (10(-12) to 10(-8) M). In vessels that had constricted in response to noradrenaline (diameters: 22.4 to 58.0 microm), physiological concentrations of 10(-12) M increased vessel diameter by 108% above baseline conditions. Control treatment using physiological salt solution did not demonstrate any changes. The technique described suggest that diffusive labelling with vascular endothelial markers such as ulex europeaus agglutinin I in live tissue samples may be used in conjunction with confocal microscopy to demonstrate heterogeneous morphological and topological changes in intact segments of the microvasculature.

Effect of haemodialysis on retinal circulation in patients with end stage renal disease

AIMS

To investigate the effect of haemodialysis on retinal circulation in patients with end stage renal disease (ESRD).

METHOD

Seventeen consecutive patients with ESRD were recruited into the study. The authors simultaneously measured changes in vessel diameter and blood velocity and calculated the retinal blood flow (RBF) in the retinal veins in patients with ESRD before and after haemodialysis using a laser Doppler velocimetry system. In addition, the relations between the changes in systemic and retinal circulatory parameters were examined.

RESULTS

There was a group averaged increase in vessel diameter (p = 0.003) after haemodialysis. However, the blood velocity and RBF values obtained after haemodialysis were not significantly different from those before haemodialysis (p = 0.66 and p = 0.63, respectively). The changes in vessel diameter were negatively (r = -0.549, p = 0.02) correlated with the change in MABP, but the changes in blood velocity and RBF were positively correlated with the change in MABP (r = 0.683, p<0.002 and r = 0.589, p<0.01, respectively). The change in RBF was also inversely correlated with the increase in haematocrit (r = -0.693, p<0.002) and the amount of fluid removed (r = -0.597, p<0.01).

CONCLUSION

The results indicate that haemodialysis and the associated changes in systemic circulatory parameters may affect the retinal circulation in patients with ESRD.

Modified laser speckle imaging method with improved spatial resolution

A two-dimensional map of blood flow is crucial for physiological studies. We present a modified laser speckle imaging method (LSI) that is based on the temporal statistics of a time-integrated speckle. A model experiment was performed for the validation of this technique. The spatial and temporal resolutions of this method were studied in theory and compared with current laser speckle contrast analysis (LASCA); the comparison indicates that the spatial resolution of the modified LSI is five times higher than that of current LASCA. Cerebral blood flow under different temperatures was investigated by our modified LSI. Compared with the results obtained by LASCA, the blood flow map obtained by the modified LSI possessed higher spatial resolution and provided additional information about changes in blood perfusion in small blood vessels. These results suggest that this is a suitable method for imaging the full field of blood flow without scanning and provides much higher spatial resolution than that of current LASCA and other laser Doppler perfusion imaging methods.

Cortical blood flow autoregulation revisited using laser Doppler perfusion imaging

Methods of laser Doppler perfusion monitoring (LDPM) and imaging (LDPI) have been validated and found useful for measurements of brain blood flow in several studies. The present work was undertaken to examine the cortical blood flow autoregulatory phenomenon as it has lately been questioned and claimed to be method-dependent and related to sample volume. Spatial variations in cerebral cortical blood flow (CBF(cortex)) in the pressure range 20-140 mmHg (static cerebral autoregulation; caval block/angiotensin infusion) were studied in six mechanically ventilated (hypocapnic, normocapnic and hypercapnic) pigs anaesthetized with propofol and fentanyl. Although the cortical blood flow values sampled were highly heterogeneously distributed, they were strongly pressure-dependent as well as CO2-dependent (P < 0.001). A cumulative cerebral blood flow (CBF)-pressure (MAP) plot comprising all values obtained indicated a pressure range between 70 and 120 mmHg where CBF remained almost constant. However, at the local level in the cortex (mm2) the same type of 'classic' autoregulatory flow : pressure graphs (FPG) were found in only a few of the cases of the cortical areas examined (n = 96). Alterations in blood P(a)CO2 saturation did not affect the pressure : flow relationship at low perfusion pressures, whereas at normal or above normal values, and as anticipated, hypercapnia considerably increased CBF (P < 0.001). 'Classic' autoregulatory FPGs were found only when all values sampled were clustered together, whereas, as a new finding, data are presented indicating that autoregulatory capacity is lacking at the local level at some cortical surface areas.

Laser Doppler, speckle and related techniques for blood perfusion mapping and imaging

Laser Doppler velocimetry uses the frequency shift produced by the Doppler effect to measure velocity. It can be used to monitor blood flow or other tissue movement in the body. Laser speckle is a random interference effect that gives a grainy appearance to objects illuminated by laser light. If the object consists of individual moving scatterers (such as blood cells), the speckle pattern fluctuates. These fluctuations provide information about the velocity distribution of the scatterers. It can be shown that the speckle and Doppler approaches are different ways of looking at the same phenomenon. Both these techniques measure at a single point. If a map of the velocity distribution is required, some form of scanning must be introduced. This has been done for both time-varying speckle and laser Doppler. However, with the speckle technique it is also possible to devise a full-field technique that gives an instantaneous map of velocities in real time. This review article presents the theory and practice of these techniques using a tutorial approach and compares the relative merits of the scanning and full-field approaches to velocity map imaging. The article concludes with a review of reported applications of these techniques to blood perfusion mapping and imaging.

Image correlation method for measuring blood flow velocity in microcirculation: correlation 'window' simulation and in vivo image analysis

To elucidate the function of the microcirculation system, it is very important to know the blood flow velocity and its distribution in the microvessels. We have developed an automated system for measuring blood flow velocity in microcirculation by image correlation. The 'window' in the image correlation method is equivalent to the sensors in various other measurement methods. We performed simulations with virtual blood flow images consisting of random dots before measuring actual ones, and examined the optimum window shape and size. We found that by reducing the size of a circular window to the size of erythrocytes we could measure in vivo blood flow images with high accuracy. We recorded them with a high-speed video camera system at high temporal resolution, and measured the velocity in microvessels of normal Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). SHR had higher blood velocity than WKY even though the vessel diameters were the same. Using this method to measure the blood flow velocity profile at the bent corner of SHR's arteriole at the heart systole, we found that erythrocytes flow faster at the inner side of the bend, so the vessel wall was exposed locally to higher shear stress in the hypertensive condition.

Failure of laser Doppler signal to correlate with total flow in muscle: is this a question of vessel architecture?

The signal strength from LDF probes positioned in perfused muscle can be altered by vasoconstrictors despite total flow being maintained constant. Apart from redistribution of flow via collateral channels outside the region of measurement, the change in LDF signal may arise because the vasoconstrictors have switched flow to vessels of different architecture or altered the architecture of the blood vessels being perfused. Thus we have examined the effect of tube architecture on LDF signal using polymer tubes of 250, 100, and 50 microm internal diameter. At 3% hematocrit the LDF signal was linear for each of the three tube sizes from 10 to 80 microl/h. The signal strength was greatest from the smallest tube and least from the largest tube. For a single tube (100 microm) that doubled back on itself twice to cross the field of measurement three times, the LDF signal at any flow (10-80 microl/h, hematocrit 3%) was approx threefold greater than that for the same tube crossing the field of measurement once. The effect of progressively switching flow (constant at 120 microl/h, hematocrit 9%) from five to one tube in a manifold of five tubes (100 microm) gave rise to a progressive increase in signal. It is concluded that LDF signal derives predominantly from nonvectorial cell speed and less from cell number. Thus any agent that alters the architecture has the potential to alter the LDF signal.

Red blood cell velocity and volumetric flow assessment by enhanced high-resolution laser Doppler imaging in separate vessels of the hamster cheek pouch microcirculation

An enhanced high-resolution laser Doppler imager (EHR-LDI), configured to fit the demands of a measurement area containing separate microvessels, was evaluated for perfusion measurements in hamster cheek pouch preparations during ischemia, reperfusion, and pharmacologically induced vasodilation and vasoconstriction. Measurements in separate microvessels where the laser beam was smaller than the vessel diameter were referred to as red blood cell (RBC) velocity estimates, as previously validated in vitro, whereas a relative flow index, RFI (mean RBC velocity/tissue area), was introduced as a volumetric flow measure. Microvessel diameter and RBC velocity changes during ischemia, reperfusion, as well as during vasoconstriction and vasodilation correlated to the data obtained from the microscope. Correspondingly, during the described provocations anticipated volumetric flow changes were registered as changes in the RFI. When data on intravessel RBC velocity profiles are presented they reflect a parabolic flow profile usually seen in this size microvessel. The EHR-LDI appears a promising tool for investigation of the microvasculature, as it almost simultaneously provides information on relative changes of both in vivo RBC velocity and volumetric flow (RFI), although the latter estimate needs to be further refined.