Laser Doppler imaging: a developing technique for application in the rheumatic diseases

Microcirculatory abnormalities contribute to the pathogenesis and pathophysiology of many of the rheumatic diseases. This is best recognized in systemic sclerosis (SSc), in which structural microvessel disease can be well demonstrated using the technique of capillary microscopy [1], and more recently by video and digital capillaroscopy [2, 3]. However, in many other conditions the microvasculature is more subtly involved. By the ‘microvasculature’ we mean the arterioles, the capillaries and the venules. Any inflammatory state is associated with profound microvascular perturbation. For example, in rheumatoid arthritis the synovial microvasculature undergoes major change with formation of new blood vessels (angiogenesis) in the hypertrophied synovium and with lymphocyte trafficking through high endothelial venules. These high endothelial venules are lined by specialized endothelial cells whose formation has been induced during the inflammatory process. [4]. In the study of disease, we must be concerned not only with understanding basic pathophysiology but also with the measurement of disease progression. If we cannot measure the disease, then we cannot assess its progression or responsiveness to treatment. In addition, the ability to measure disease processes can give us indirect insights into pathophysiology by allowing us to assess response to therapeutic interventions which are known to have specific mechanisms of action. Can we measure microvascular disease/involvement by disease, and apply this to the study of rheumatological disorders? As already mentioned, we can examine nailfold capillary structure in certain connective tissue diseases, such as SSc and dermatomyositis, using nailfold microscopy and video capillaroscopy, and one aspect of capillary function (permeability) can be examined by fluoroscopy [5, 6], which is, however, invasive in that it requires an intravenous dye injection. In this review we shall discuss the relatively new technique of laser Doppler imaging (otherwise termed ‘scanning laser Doppler’), which gives a direct measure of microcirculatory flow. We believe laser Doppler imaging affords significant potential in the study of microcirculatory involvement of the rheumatic diseases and it is non-invasive. The observed wavelength of electromagnetic radiation is affected by relative motion between the source and observer. This phenomenon (also applicable to sound waves, as in the technique of Doppler ultrasound) is known as the Doppler effect. When low-level laser light, of a few milliwatts, is directed onto the skin's surface a fraction of the light penetrates the skin and interacts with both static tissue and moving cells (primarily red blood cells). The penetration depth of light is dependent upon the tissue morphology, absorption and the wavelength used [7, 8]. The light that is reflected or randomly scattered from the static tissue remains unchanged in wavelength. In contrast the light that is scattered from the moving blood cells undergoes a small change in wavelength, proportional to the speed of the erythrocytes, due to the Doppler effect. Backscattered light from the tissue, incident on a detector, is processed to provide a signal that is proportional to the speed and density of the moving cells [9]. Stern [10] was the first to exploit the Doppler effect to monitor blood flow. He collected the backscattered light, Doppler-broadened according to the internal motion of circulation, on a photodetector. He demonstrated the difference in perfusion of a fingertip under normal flow conditions and those of brachial occlusion using a helium–neon (HeNe, 633 nm) laser. It was also noted that, after administration of ethanol, the vasodilatory effects were observed as an increase in blood flow at the fingertip, which increased over a 15-min period. From these initial observations came the method of laser Doppler flowmetry (LDF): fibre-delivered, single-point perfusion monitoring. LDF has been adapted and improved to remove many of the preliminary problems and has been widely used both in research and as a clinical tool over the past 20 yr in the measurement of cutaneous microcirculatory flow. The technique with which most clinicians are familiar is the single-probe technique. Single-probe laser Doppler has been used extensively by rheumatologists to quantify blood flow in studies of Raynaud's phenomenon [11–14]. The principles underlying this technique are demonstrated in Fig. 1a: the laser light is delivered to the tissue surface via an optical fibre and the backscattered and reflected light is collected by a second (or several) fibre(s). Delivery and collection fibres are housed in a single probe. (a) Schematic of LDF. (b) Schematic of LDI. The incident laser light beam has a depth of penetration of approximately 1 mm, depending upon the wavelength and configuration of the equipment used. Therefore all elements of the dermis may be included, from superficial nutritional to deeper thermoregulatory vessels. For a given wavelength of light, the absorption spectrum of components of the tissue determines the interaction that occurs [7]. The penetration of light is predetermined by the path that the light takes through the skin and is limited by the absorption of the tissue. Towards the red end of the visible spectrum haemoglobin and water absorption are lower, hence the dominance of the HeNe laser in earlier LDF. For this same reason, near-infrared diode lasers (670, 780 and 810–850 nm) are also popular; these long wavelengths exhibit a deeper penetration depth and, due to the low absorption of melanin in the near infrared, show less dependence on skin colour. where k is an arbitrary constant. Flux is expressed in terms of arbitrary perfusion units, which do not give absolute values for the blood flow speed. Therefore, although it is not meaningful to compare absolute values of perfusion between individuals, intra- and inter-individual comparisons of dynamic responses (to standard stimuli) can be made. The LDF instrument can be calibrated in perfusion units by measuring the Brownian motion of a standard suspension of polystyrene microparticles in water. LDF offers continuous perfusion measurement. However, cutaneous circulation is known to be heterogeneous, and examining a small area of perfusion does not necessarily give representative data for the surrounding perfusion [16]. This problem can be overcome by collection of blood flow data over a larger area. A relatively recent application of laser Doppler is the development of laser Doppler imaging (LDI). This technique has two major advantages over the single-probe technique. (i) The first is that blood flow is measured over an area rather than at a single site, obviating some of the difficulties with site-to-site variability inherent in the signal-probe technique. Thus, reproducibility might be improved. (ii) Secondly, the laser beam is non-contact, as opposed to the single probe, which involves direct contact with the skin and which could, therefore, through this contact, influence blood flow via pressure and movement artefacts [17]. Figure 1b illustrates the principles underlying the LDI technique. The laser beam, approximately 1 mm in diameter, is scanned across tissue in two dimensions using a moving mirror. The scattered light signal is analysed to provide a two-dimensional image of blood flow. The bandwidth of the collected signal varies according to the scan speed (ms/pixel). A high-frequency cut-off (∼3–22 kHz) improves the signal-to-noise ratio and a low-frequency cut-off (∼20–250 Hz) eliminates movement artefacts. This low-frequency cut-off has some effect on the ability to measure low-speed blood flows; for low perfusion a slower scan rate can be used. Both large (e.g. torso) and small (e.g. finger, hand) areas can be scanned, larger areas by increasing the imaging height. Figure 2 shows a laser Doppler imager in use. LDI system in use. As the laser beam scans across the hand, the image appears on the monitor. Already a number of investigators have recognized the potential of the technique for clinical application and some examples of these clinical applications in specialities other than rheumatology are as follows. Several groups have described the measurement of burn depth with LDI [18–20]. Brown et al. [21] demonstrated the use of LDI in the evaluation of the clinical management of vesicant burns and Jeng et al. [22] described LDI confirmation of clinical judgement in the requirement for excision of burns of indeterminate depth. Dermatologists have drawn attention to the fact that, because of its accessibility, the skin provides the opportunity of studying mechanisms of inflammation. Harrison et al. [23] described the increase in cutaneous blood flow imaged during the tuberculin reaction. Clough [24] reported how LDI allows us to build upon pathophysiological studies (in this particular example investigating the role of nitric oxide in dermal inflammation) by including direct measurement of microvascular response. Eichhorn et al. [25] used LDI to monitor the healing of flaps in the maxillofacial area, identifying necrotic areas, venous stasis and normal wound healing. Ljung et al. [26] examined postoperative wound healing. Responses to heating and iontophoresis have been assessed in both patients with arterial disease and patients with diabetes [27]. Dermal replacement therapy for foot ulceration has been monitored in diabetic patients [28] and diabetologists have applied LDI in the study of other aspects of disease, including autonomic responses [29]. In rheumatology, an obvious clinical application is in quantitation of dermal blood flow in patients with primary and secondary Raynaud's phenomenon. To date a small number of cross-sectional studies have explored the potential of LDI to measure blood flow in patients with Raynaud's and SSc. Seifalian et al. [30] recommend the application of LDI in patients with vascular disease, including Raynaud's phenomenon, citing its advantages over single-point LDF, as a method to monitor perfusion in the hands of patients with SSc. We know that dermal microcirculatory flow is reduced, at least during vasospastic attacks, as evidenced by the classic colour changes which occur in the skin of the digits during these episodic attacks of ischaemia, which characterize the condition. Thus, in patients with Raynaud's is in flow at least in response to a dynamic and it is that in many patients with Raynaud's secondary to structural blood disease, as occurs in SSc is flow under Thus, blood flow studies to both conditions and dynamic responses to a a or et al. have laser Doppler in patients with primary Raynaud's phenomenon, patients with SSc and examining effects of a reported that patients with SSc demonstrated perfusion than patients with primary Raynaud's and and that between groups more during the and et al. the changes in perfusion due to and in patients with vasospastic disease and the were to to then induced a in changes in blood flow monitored by LDI were to be to patients from A small study to in the first between patients with patients with primary Raynaud's phenomenon and was reported by were of the of the at of and at and it was that two using the imaging technique in the and the difference in between the of the same hand) were to across the However, were across groups using Figure (i) the of the how are (ii) the fingertip in patients with and the of the between in patients with the between the of the structural vascular of the perfusion of hands at to and and with SSc. The is image high low are at the same for is how LDI to as a measurement well in the of Raynaud's phenomenon Seifalian et al. comparisons between and identifying a of between the two that such as the of in tissue by radiation and may for the between direct blood flow and A of the of LDI and was also by et al. examining the difference between the of the and the of the were at both and for both The study that LDI and In contrast to which blood flow on a tissue which other internal and may influence and which is offers which the given by the LDI has also been with in the evaluation of digital patients were for of one and one LDI perfusion well with and function and the investigators its use to assess the of LDI was not to be a replacement but to provide to The cross-sectional studies described that LDI evaluation in patients with Raynaud's phenomenon, including studies of reproducibility and has been in the of response to However, we have demonstrated the potential of LDI in change in microcirculatory flow in response to with SSc and primary Raynaud's phenomenon and all demonstrated a in perfusion after administration of in with after and with treatment. the of digital microcirculatory flow in patients with Raynaud's phenomenon is a obvious application of are many other potential applications in disease, as described et al. used LDI to measure over in patients with rheumatoid arthritis using red and near increase in perfusion was observed in the of patients in with which was more at than at 633 blood flow over the small of the as in this might quantify the of inflammation. In this it is that absolute are to be of but rather the ratio of the to over to quantify dermal blood flow in this might be in measuring the and this also be for tissue inflammation. 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and LDF, LDI is in the research LDI provides arbitrary perfusion its as an tool in data such as and or response. 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