Advantages and limitations of laser Doppler flow meters

B. Matthews & N. Vongsavan

Department of Physiology. School of Medical Sciences. University of Bristol. Bristol. UK

Laser Doppler flow meters are used extensively to monitor changes in blood now in skin and other organs. Their advantages are that they are non-invasive. Simple to apply. and provide a continuous or near continuous record. Their principal disadvantage is that it is impossible to calibrate them in absolute units and their outputs may not be linearly related to blood now. So, for example, if the output signal increases by 100 %, it cannot be assumed that the blood now rate has increased by 100%.

The fact that they cannot be calibrated in absolute units stems from the fact that the signal derived from any one moving cell will depend upon the distance of that cell from the recording probe. and that distance is not known. The non-linearity arises from the effects of multiple collisions of photons with moving cells, the probability of which increases with increasing red cell concentration in the tissue. Non-linearity becomes a problem when the red cell volume fraction of a tissue exceeds 1%. This figure is likely to be exceeded in dental pulp and most other tissues (Vongsavan & Matthews 1992a.b).

Both the main types of instrument available in the UK (Perimed, Stockholm, Sweden; Moor. Axminster UK) operate on similar principles. They are capable of detecting blood now throughout the diameter of the pulp of a tooth. This has been demonstrated by recording from extracted teeth in which blood has been pumped at different rates and concentrations through cannulea implanted into the pulp chamber (Vongsavan & Matthews 1992c). With any one dilution of blood (range 0.5-45% v/v red cells) there was a near linear relationship between the output signal and the flow rate of blood through the tooth. However. With different red cell concentrations. a good correlation between the blood flow signal and red cell flux (product of concentration and mean velocity) was obtained only with red cell concentrations up to 1% v/v.

The ability of these instruments to detect blood flow in pulp within an intact tooth may in part be due to the dentinal tubules acting as light guides (Vongsavan & Matthews 1994c). The effect of this would be for a probe applied to the enamel surface on the crown of a tooth to pick up light scattered by red cells in pulp at a lower level in the tooth.

It is essential to avoid movements between the probe tip and the underlying tissue otherwise structures other than blood cells generate Doppler shifts. This source of artefact is much more easily avoided when recording from teeth than from soft tissues because the probe can be fixed rigidly to the tooth surface with some form of splint.

Because it is rarely possible to arrest the now of blood through a tissue to obtain a control record. alternative methods are required to estimate the signal that corresponds with zero now. The most satisfactory way of doing this is to record from a stationary reflector under conditions which are otherwise as near as possible to those present in vivo (Vongsavan & Matthews 1992a). It is particularly important to ensure that the light intensities are the same. The instrument cannot be zeroed in this sense by placing the probe against a non-reflective surface and it cannot be assumed that zero now is equivalent to an output of 0V.

Although they cannot be calibrated in absolute units. their sensitivity can be standardised. It is suggested that the method recommended by Perimed is adopted by which the signal produced by the Brownian motion of a 0.5% suspension of 0.48 µm diameter, polystyrene microspheres at 20°C is defined as 250 perfusion units.

Part of the signal recorded from the crown of an intact tooth is derived from blood flow in tissues outside the pulp. In experiments on pig incisors. we round that about 10% of the signal recorded with a probe centred 2 mm from the gingival margin was of non-pulpal origin. This was established by recording before and after section of the pulp.

When recordings are made from exposed dentine instead of the tooth surface, the signal from the pulp is increased. and the proportion of this signal which is due to non-pulpal tissues is decreased. However. For reasons which are not fully understood, the increase in the signal which occurs when vasodilator nerves are stimulated, is reduced in relation to the resting blood flow signal when recordings are made near the pulp surface.

In conclusion, while these instruments provide a very valuable method for studying pulpal blood flow, the data they provide must be interpreted with care.