Regular Article Evaluation of a new high power, wide separation laser Doppler probe: Potential measurement of deeper tissue blood flow Geraldine Clough a, ⁎, Andrew Chipperfield b , Christopher Byrne a,b , Frits de Mul c , Rodney Gush d a School of Medicine, University of Southampton, UK b School of Engineering Sciences, University of Southampton, UK c Biomedical Engineering University Medical Center, University of Groningen, UK d Moor Instruments, UK abstract article info Article history: Received 23 April 2009 Accepted 12 May 2009 Available online 19 May 2009 Keywords: Laser Doppler blood flow Skin Sub-dermal tissue Skin warming Monte Carlo modeling Objective: To compare the output from a novel high power, wide separation laser Doppler flow probe (DP1- V2-HP, 4 mm, with IRLD20) with that of a standard flow probe (DP1-V2, 0.5 mm, with DRT4) (Moor UK) and to explore its potential for use in the noninvasive measurement of blood flow in deeper tissues in humans. Methods: Monte Carlo modeling was used to predict depths of light scattering in skin with each probe, geometry. Experimentally, forearm blood flow was measured at rest and during local warming of the skin surface and post occlusion reactive hyperaemia (PORH). Laser Doppler blood flux (LDF) and the power spectral density of its component frequency intervals, were compared. Results: Monte Carlo modeling indicated that while the majority of wide probe LD signal derives from deeper tissue, a significant portion is from superficial (dermal) tissue (and vice versa for standard probe). Perturbation of local blood flow differentially increased LDF and spectral power as measured by the two probes, with the standard skin probe showing a significantly greater response to local skin warming (p b 0.01). Conclusions: These differences support our hypothesis that the wide probe is recording predominantly blood flux within the vasculature of sub-dermal tissue. This is in agreement with Monte Carlo simulation. © 2009 Elsevier Inc. All rights reserved. Introduction Laser Doppler flowmetry (LDF) is a noninvasive technique for monitoring tissue microcirculation. Since its inception (Stern, 1975), many thousands of studies have been published in which the technique has been used in basic and clinical research, and as a diagnostic screening tool. One of the major advantages of the technique is that it can be used to quantify relative changes in skin blood flow and by this a measure of the responsiveness of the skin microcirculation to a given stimulus. The stimuli most frequently used include brief arterial occlusion and the subsequent post occlusive hyperaemia (Rossi et al., 2007), local thermal hyperaemia (Gooding et al., 2006), and the response to vasoactive mediators such as the endothelium-dependent vasodilator acetylcholine (ACh) and the endothelium-independent nitric oxide donor sodium nitoprusside (SNP) delivered by iontophoresis (Morris and Shore, 1996; Khan et al., 1997; Rossi et al., 2002, 2008) or directly into the interstitial space by microdialysis (Boutsiouki et al., 2004; Kellogg et al., 2008) (for review see Cracowski et al., 2006). Changes in the cutaneous LDF response to these stimuli are taken as indicative of altered endothelial function or of dysfunction and hence it is frequently used as a non invasive biomarker of vascular disease. LDF relies on the detection of low power light from a mono- chromatic, stable laser incident on tissue. The laser light is brought to the skin (or other tissue) via an optical fibre (Graaff et al., 2003). In the tissue, some light is scattered and Doppler-shifted by moving blood and some light is scattered by static tissue without Doppler shift. A fraction of the back-scattered light is then collected by one or more optic fibres and conveyed to a photo-detector. Here the two types of scattered light mix and are processed to form a laser Doppler signal which reflects the blood flow in the tissue. The term ‘flux’ is used for the output of the measurement and is expressed in arbitrary ‘perfusion units’ (PU). This quantity is proportional to the product of the average speed of moving blood cells and their concentration (blood volume) (Bonner and Nossal, 1980; Nilsson et al., 1980; Humeau et al., 2007a,b). The depth of tissue from which the LD signal is derived depends on laser power and the separation of the emitting and collecting fibres (Gush et al., 1984; Jakobsson and Nilsson, 1993; Morales et al., 2003; Freccero et al., 2003). Recently, the use of the LDF technique has been extended to explore microvascular control mechanisms within the skin through analysis of the component frequencies of the laser Doppler signal (Humeau et al., 2007a,b). These periodic oscillations in the LDF blood Microvascular Research 78 (2009) 155–161 ⁎ Corresponding author. Institute of Developmental Sciences, School of Medicine, University of Southampton, Southampton General Hospital (MP 887), Southampton SO16 6YD, UK. Fax: +44 02380 704183. E-mail address: g.f.clough@soton.ac.uk (G. Clough). 0026-2862/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.mvr.2009.05.003 Contents lists available at ScienceDirect Microvascular Research journal homepage: www.elsevier.com/locate/ymvre