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Lasers, Tomatoes and Blood

September 01, 1998

The monitoring of blood flow in the skin is an important diagnostic tool in many areas of medicine. These include the diagnosis of diabetes and of various dermatological conditions, the assessment of burn and other wound damage, and the response of the vascular system to smoking and other drugs.

The method depends on the fluctuations seen in laser light scattered from the skin. Other non-contact techniques have been developed to monitor these fluctuations, but they can only measure at one point at a time. If a map of blood flow is required, some form of scanning is necessary. This precludes real-time operation for these techniques. Our method (LASCA = LAser Speckle Contrast Analysis) provides a map of blood flow in less than one second - effectively real-time operation.
Non-invasive diagnostic techniques improve patient experience and provide an important medical tool.
Laser light is the purest form of light presently available. It consists of a single colour - a single wavelength. This leads to properties that allow laser light to be used in ways that ""ordinary"" light cannot be.

One such property is that of ""interference"" - the production of patterns of light and dark areas where the waves of the light either re-inforce one another or cancel out to give darkness. Shine laser light onto a rough surface and you see a random ""interference pattern"" known as ""laser speckle."" This is the granular effect that is characteristic of surfaces illuminated with laser light.

When the surface is a living object - a tomato, an apple, a carrot, or human skin - the ""speckle pattern"" twinkles. Each speckle fluctuates in intensity. These fluctuations are caused by movement inside the object - cellular activity or fluid movement in plants, blood flow in human skin. The speed of the fluctuations is determined by the speed of the moving particles (blood cells in the case of human skin).

Several techniques have been developed to monitor these speckle fluctuations and hence to monitor blood flow. Some are based on the theory of the Doppler phenomenon. (This is the frequency shift produced when light (or sound) is reflected from moving objects - the most common manifestation of this phenomenon is the change in pitch of the siren of an ambulance as it passes by.) Others use the theory of speckle formation. Almost all, however, rely on the measurement of the light intensity fluctuations at a single point. This means that if a map of velocity is required, it is necessary to scan the area of interest. Due to the vast amount of data that has to be analysed, this scanning usually takes several minutes.

Our technique uses the fact that if a time-exposure picture is taken of the fluctuating speckle pattern, then the contrast of the speckle is reduced. We are thus using the spatial statistics of the speckle pattern rather than the temporal statistics of the intensity fluctuations. The reduction in speckle contrast depends on the speed of the fluctuations, and hence on the speed of the moving particles (blood cells). Hence blood flow is mapped as variations in speckle contrast.

The main advantage of our technique is that the much-reduced data analysis needed to compute speckle contrast allows us to produce a map of speckle contrast, and hence blood velocity, in less than one second. It is thus very close to being an instantaneous, real-time technique for monitoring blood flow. The lack of the complex data analysis, and the fact that scanning is not necessary, also makes the process much cheaper than the corresponding Doppler techniques. The disadvantage is that the calculation of spatial contrast requires a finite number of pixels: this reduces the spatial resolution achievable compared with those techniques that monitor the intensity fluctuations of single pixels.
We have shown the feasibility of this technique in the laboratory. The next step is the industrialization of the method. This will require the collaboration of a suitable company.
We are not aware of any other groups working on this exact technique. However, there are several groups working on the complementary techniques that monitor the time-fluctuations of individual speckles (whether on the Doppler principle or not). Indeed, commercial ""scanning laser Doppler"" systems are already on the market and in clinical use. The advantage we offer is that of real-time operation.


Aberystwyth, University of Wales