View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Universiteit Twente Repository Lasers Med Sci (2009) 24:269–283 DOI 10.1007/s10103-007-0524-0 REVIEW ARTICLE Review of methodological developments in laser Doppler flowmetry Vinayakrishnan Rajan & Babu Varghese & Ton G. van Leeuwen & Wiendelt Steenbergen Received: 24 July 2007 /Accepted: 24 October 2007 /Published online: 31 January 2008 # Author(s) 2007 Abstract Laser Doppler flowmetry is a non-invasive red blood cells to the illuminating coherent light. A laser method of measuring microcirculatory blood flow in tissue. Doppler instrument output often gives flux, velocity and In this review the technique is discussed in detail. The concentration of the moving blood cells. These parameters theoretical and experimental developments to improve the are extracted from the power spectrum of the photocurrent technique are reviewed. The limitations of the method are fluctuations produced by reflected light illuminating a elaborated upon, and the research done so far to overcome photodetector. There are two types of perfusion measure- these limitations is critically assessed. ments in practice, laser Doppler perfusion monitoring (LDPM) and laser Doppler perfusion imaging (LDPI). Keywords Laser Doppler flowmetry. Microcirculation . Since the commercialization of the technique in the early Diagnostic imaging . Doppler effect 1980s, LDF has had a modest but stable and steadily growing commercial market. Figure 1 illustrates the growth of the use Abbreviations of LDF in research by the number of peer-reviewed articles LDF Laser Doppler flowmetry published each year1 in which laser Doppler flowmetry LDPM Laser Doppler perfusion monitoring techniques have been used (LDPM and LDPI). LDPI Laser Doppler perfusion imaging Other techniques that were used for blood flow measure- LASCA Laser speckle contrast analysis ments before the evolution of the laser Doppler technique VCSEL Vertical cavity surface emitting laser were the xenon washout technique [2] and the radioactively CMOS Complementary metal oxide semiconductor labeled microsphere technique [3, 4]. These techniques CCD Charge-coupled device require an injection of a radioactive substance into the BZ Biological zero blood stream. Doppler ultrasound methods to measure blood flow are non-invasive but incapable of measuring the microcircula- tion. In Table 1 various techniques to measure skin blood Introduction flow are compared. The major advantage of the laser Doppler techniques Laser Doppler flowmetry (LDF) is a non-invasive diagnostic in general is their non-invasiveness and their ability to method of measuring blood flow in tissue. The technique is measure the microcirculatory flux of the tissue and fast based on measuring the Doppler shift [1] induced by moving changes of perfusion during provocations. The technique can measure perfusion quantitatively (although relative) V. Rajan (*) : B. Varghese : T. G. van Leeuwen : W. Steenbergen in real time. Institute of Biomedical Technology, Biophysical Engineering Group, University of Twente, P.O. Box 217, 1 Web of Science (ISI Web of Knowledge), search terms: [(laser 7500AE Enschede, The Netherlands and Doppler) or (laser-Doppler)] and (perfusion or microcirculation e-mail: [email protected] or flowmetry). 270 Lasers Med Sci (2009) 24:269–283 of quantitative units for perfusion, lack of knowledge of the depth of measurement and the biological zero signal (perfusion measured at no flow condition). These limita- tions motivate ongoing research in both the instrumentation and theoretical aspects of the technique. Laser Doppler flowmetry techniques for measuring blood flow have been reviewed by many authors consider- ing different aspects of the technique. Early developments in laser Doppler flowmetry and its clinical applications were reviewed by Shepherd and Öberg [10]. Obeid et al. [12] reviewed the limitations of the technique. In a later review Vongsavan and Matthews [13] compared the perfor- mance of two instruments which were then widely used in clinics: the Periflux model PF3 (Perimed, Stockholm, Sweden) and the Moor blood flow monitor model MBF3D/ 42 (Moor Instruments, Axminster, Devon, UK). Leahy et al. Fig. 1 Statistics of publications using LDF techniques from 1980 till 2006 [14] reviewed the principles and use of laser Doppler instruments and discussed different aspects of the method. There are some limitations of the technique. The major In a recent review Briers [9] compared laser Doppler tech- limitations are the influence of the tissues’ optical proper- niques with speckle techniques, which progressed in ties on the perfusion signal, motion artifact noise, lack parallel. A review of the techniques of whole-field blood Table 1 Methods of measuring skin blood flow Technique Non- Principle Measured quantity Disadvantages invasive? Fluorescent No A fluorescent dye is injected into the blood Blood flow No quantitative measurement. tracers [5] stream. Tissue fluorescence is taken as an distribution Cannot study fast changes in blood indication of blood flow flow Radioactive No Radioactive microspheres are injected inti the Mean blood flow Tissue needs to be excised for the microsphere blood stream; mean blood flow is estimated measurement; no real time technique [3] from the recovered isotope count from measurement possible excised tissue 133Xe wash out No 133Xe is injected subdermally. The rate of Absolute blood flow Point measurement. Does not give technique [2] clearance is a measure of blood flow information about regional blood flow. Measures only one or few capillaries Capillary Yes The distance traveled by erythrocytes in Capillary Capillary perpendicular to the skin microscopy [6] successive frames gives a measure of blood distribution and surface cannot be visualized velocity blood velocity Thermography [7] Yes Skin temperature radiation is imaged and is Temperature map of Skin temperature and blood flow are related to the blood flow the skin not directly related Doppler Yes Based on the Doppler shift imparted by Spatially resolved Resolution insufficient for smallest ultrasound [8] moving blood cells to the probing sound velocity vessels; sensitive to only one waves velocity component Speckle Yes Speckles generated by back-scattered photons De-correlation time No spectral information; techniques [9] from the tissue analyzed in spatial and and speckle quantitative measurement is not temporal domain to estimate blood flow. contrast, related to possible blood flow Laser Doppler Yes Based on the Doppler shift imparted by the Relative No absolute measurement; no depth flowmetry [10] moving red blood cells to the probing light. concentration and information velocity of moving red blood cells Doppler Yes Combination of Doppler velocimetry with Spatially resolved Limited probing depth 1–2mm; OCT [11] low-coherence optical interferometry velocity of moving sensitive to only one velocity red blood cells component Lasers Med Sci (2009) 24:269–283 271 velocity measurement can be found in Vennemann et al. [15]. This article reviews laser Doppler velocimetry, laser speckle methods, and magnetic resonance and ultrasound techniques for particle image velocimetry, used for perfu- sion monitoring and velocity mapping. In our article we review the developments in laser Doppler perfusion monitoring and imaging, with emphasis on recent advances. The methodological progress of the technique is followed in detail, and special emphasis is given to the limitations of techniques and the research in Fig. 3 Speckles generated by light back-scattered from a particle this regard. This review does not cover imaging methods suspension. On the right is an illustration of the fluctuations in the photodetector intensity resulting from the dynamic speckle pattern. fD based on Laser speckle contrast analysis (LASCA). is the Doppler-shifted fraction of all detected photons Working principle of LDF techniques Light scattering in tissue Extraction of laser Doppler signal When a beam of laser light illuminates a small area of In laser Doppler flowmetry the interference of Doppler- tissue, photons will be scattered by static and dynamic shifted light with non-Doppler shifted light on the photo- particles. The moving red blood cells will impart a Doppler detector generates a dynamic speckle pattern. As a result ’ shift to the photon, depending on the scattering angle, the of these patterns, the detector s current signal will fluctuate. wavelength and the velocity vector of the scatterer. If a If the signal generated by the photodetector is from only wave with frequency ω is scattered from a moving particle Doppler-shifted light, it is called a homodyne signal in with velocity v (Fig. 2), the Doppler shift can be written laser Doppler terminology. The scattering from a tissue matrix with a sufficiently small volume of blood (dynamic as: Δ5 ¼ jjv jjkI À ks cos " , where kI is the incident wave scatterers) will result in a signal that is produced mainly vector, ks is the wave vector of the scattered wave, and β is the angle between the velocity vector and the scattering by interference of Doppler-shifted light with non-Doppler shifted light, a situation called heterodyne. It can be shown vector, which is defined as (kI-ks). With α the scattering angle and 1 the wavelength of the light in the medium, [16] that the alternating current (AC) signal generated by Δ5 ¼ ðÞ:=1 jj a fluctuating speckle pattern, normalized by the squared the Doppler shift can be written as 22 v 2 ðÞ!= " value of the direct current (DC ), is equal to sin 2 cos . In tissue with a large number of moving particles, and for sufficiently long photon path lengths, i2 AC ¼ 1 ðÞÀ ðÞ photons will undergo more than one Doppler shift, which 2 fD 2 fD 1 hiiDC N will result in a range of Doppler-shifted frequencies. More- 2 over, the random orientation of microcirculatory blood with iAC the mean square of the photocurrent fluc- vessels and randomization of the photons with different tuations, hiiDC the mean photocurrent, N the number scattering events give rise to a range of Doppler shifts, even of speckles on the detector and fD the Doppler-shifted when all particles move at the same speed.