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| The '''photoacoustic Doppler effect''', as its name implies, is one specific kind of Doppler effect, which occurs when an intensity modulated light wave induces a photoacoustic wave on moving particles with a specific frequency. The observed frequency shift is a good indicator of the velocity of the illuminated moving particles. A potential biomedical application is measuring blood flow.
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| Specifically, when an intensity modulated [[light wave]] is exerted on a localized medium, the resulting heat can induce an alternating and localized pressure change. This periodic pressure change generates an acoustic wave with a specific frequency. Among various factors that determine this frequency, the velocity of the heated area and thus the moving particles in this area can induce a frequency shift proportional to the relative motion. Thus, from the perspective of an observer, the observed frequency shift can be used to derive the velocity of illuminated moving particles.<ref name=bopi>{{cite book
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| | author=LV Wang and HI Wu
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| | title=Biomedical Optics: Principles and Imaging
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| | publisher=Wiley
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| | year=2007
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| | isbn =9780471743940}}</ref>
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| ==Theory==
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| To be simple, consider a clear medium firstly. The medium contains small optical absorbers moving with velocity vector <math>\vec{v}</math>. The absorbers are irradiated by a laser with intensity modulated at frequency <math>f_{0}</math>.
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| Thus, the intensity of the [[laser]] could be described by:
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| <math>I={I }_{0 } \left[ 1+cos \left ( 2 \pi f_{0}t \right ) \right ] /2</math><ref name=prl>H. Fang, K. Maslov, L.V. Wang. "Photoacoustic Doppler Effect from Flowing Small Light-Absorbing Particles." Physical Review Letters 99, 184501 (2007)</ref>
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| [[Image:schematic.jpg|frame|right|50px|'''Figure 1: Overview of PAD Effect'''<ref name=prl>H. Fang, K. Maslov, L.V. Wang. "Photoacoustic Doppler Effect from Flowing Small Light-Absorbing Particles." Physical Review Letters 99, 184501 (2007)</ref>
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| When <math>\vec{v}</math> is zero, an [[acoustic wave]] with the same frequency <math>f_{0}</math> as the light intensity wave is induced. Otherwise, there is a frequency shift in the induced acoustic wave. The magnitude of the frequency shift depends on the relative velocity <math>\vec{v}</math>, the angle <math>\alpha</math> between the velocity and the photon density wave propagation direction, and the angle <math>\theta</math> between the velocity and the ultrasonic wave propagation direction.
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| The frequency shift is given by:
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| <math>f_{PAD}=-f_{0} \frac{ v}{ c_{0}} cos \alpha +f_{0} \frac{v}{c_{a}} cos \theta</math><ref name="prl"/>
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| Where <math>c_{0}</math> is the speed of light in the medium and <math>c_{a}</math> is the speed of sound. The first term on the right side of the expression represents the frequency shift in the photon density wave observed by the absorber acting as a moving receiver. The second term represents the frequency shift in the photoacoustic wave due to the motion of the absorbers observed by the [[ultrasonic transducer]].<ref name="prl"/>
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| In practice, since <math>\frac{c_{0}}{c_{a}}\sim 10^{5}</math> and <math>v \ll c_{a}</math>, only the second term is detectable. Therefore, the above equation reduces to:
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| <math>f_{PAD}= f_{0} \frac{v}{c_{a}} cos \theta = \frac{v}{\lambda} cos \theta </math><ref name="prl"/><ref name=apl>H. Fang, K. Maslov, L.V. Wang. "Photoacoustic Doppler flow measurement in optically scattering media." Applied Physics Letters 91 (2007) 264103</ref>
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| In this approximation, the frequency shift is not affected by the direction of the optical radiation. It is only affected by the magnitude of velocity and the angle between the velocity and the acoustic wave propagation direction.<ref name="prl"/>
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| This equation also holds for a scattering medium. In this case, the photon density wave becomes diffusive due to light scattering. Although the diffusive photon density wave has a slower phase velocity than the speed of light, its wavelength is still much longer than the acoustic wave.<ref name="apl"/>
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| ==Experiment==
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| [[Image:Scat-data.JPG|frame|right|40px|'''Figure 2: Average Photoacoustic Doppler Shift vs. Velocity for a Scattering Medium ''' <ref name="apl"/>]]
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| In the first demonstration of the Photoacoustic Doppler effect, a continuous wave [[laser diode|diode laser]] was used in a [[photoacoustic imaging in biomedicine|photoacoustic microscopy]] setup with an [[ultrasonic transducer]] as the detector. The sample was a solution of absorbing particles moving through a tube. The tube was in a water bath containing scattering particles<ref name="prl"/>
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| Figure 2 shows a relationship between average flow velocity and the experimental photoacoustic Doppler frequency shift. In a scattering medium, such as the experimental phantom, fewer photons reach the absorbers than in an optically clear medium. This affects the signal intensity but not the magnitude of the frequency shift. Another demonstrated feature of this technique is that it is capable of measuring flow direction relative to the detector based on the sign of the frequency shift.<ref name="prl"/> The reported minimum detected flow rate is 0.027 mm/s in the scattering medium.<ref name="apl"/>
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| ==Application==
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| One promising application is the non-invasive measurement of flow. This is related to an important problem in medicine: the measurement of blood flow through [[arteries]], [[capillaries]], and [[veins]].<ref name="apl"/> Measuring blood velocity in capillaries is an important component to clinically determining how much oxygen is delivered to tissues and is potentially important to the diagnosis of a variety of diseases including [[diabetes]] and [[cancer]].However, a particular difficulty of measuring flow velocity in [[capillaries]] is caused by the low blood flow rate and micrometre-scale diameter. Photoacoustic Doppler effect based imaging is a promising method for blood flow measurement in capillaries.
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| === Existing techniques ===
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| Based on either [[ultrasound]] or light there are several techniques currently being used to measure [[blood]] velocity in a clinical setting or other types of flow velocities.
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| ====Doppler ultrasound====
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| The [[Doppler ultrasound]] technique uses Doppler frequency shifts in ultrasound wave. This technique is currently used in biomedicine to measure blood flow in [[arteries]] and [[veins]]. It is limited to high flow rates (<math>>1 </math>cm/s) generally found in large vessels due to the high background ultrasound signal from biological tissue.<ref name="apl"/>
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| ====Laser doppler flowmetry====
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| Laser Doppler Flowmetry utilizes light instead of [[ultrasound]] to detect flow velocity. The much shorter optical wavelength means this technology is able to detect low flow velocities out of the range of [[Doppler ultrasound]]. But this technique is limited by high background noise and low signal due to [[multiple scattering]]. Laser Doppler flowmetry can measure only the averaged blood speed within 1mm<sup>3</sup> without information about flow direction.<ref name="apl"/>
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| ====Doppler optical coherence tomography====
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| Doppler [[Optical coherence tomography]] is an optical flow measurement technique that improves on the spatial resolution of laser Doppler flowmetry by rejecting [[scattering|multiple scattering light]] with coherent gating. This technique is able to detect flow velocity as low as <math>100 \mu </math>m/s with the spatial resolution of <math>5\times 5\times 15\mu </math>m<math>^{3}</math>. The detection depth is usually limited by the high optical scattering coefficient of biological tissue to <math><1</math>mm.<ref name="apl"/>
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| ===Photoacoustic doppler flowmetry===
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| Photoacoustic Doppler effect can be used to measure the blood flow velocity with the advantages of [[Photoacoustic imaging in biomedicine|Photoacoustic imaging]]. [[Photoacoustic imaging in biomedicine|Photoacoustic imaging]] combines the spatial resolution of [[ultrasound]] imaging with the contrast of optical absorption in deep biological tissue.<ref name="bopi"/> [[Ultrasound]] has good spatial [[Image resolution|resolution]] in deep biological tissue since ultrasonic scattering is much weaker than optical scattering, but it is insensitive to biochemical properties. Conversely, optical imaging is able to achieve high [[contrast (vision)|contrast]] in biological tissue via high sensitivity to small molecular optical absorbers, such as [[hemoglobin]] found in [[red blood cells]], but its spatial resolution is compromised by the strong [[scattering]] of light in biological tissue. By combining the optical imaging with ultrasound, it is possible to achieve both high contrast and spatial resolution.<ref name="bopi"/>
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| The photoacoustic Doppler flowmetry could use the power of photoacoustics to measure flow velocities that are usually inaccessible to pure light-based or ultrasound techniques. The high spatial resolution could make it possible to pinpoint only a few absorbing particles localized to a single capillary. High contrast from the strong optical absorbers make it possible to clearly resolve the signal from the absorbers over the background.
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| ==See also==
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| * [[Photoacoustic spectroscopy]]
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| * [[Photoacoustic imaging in biomedicine]]
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| * [[Photoacoustic tomography]]
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| * [[Doppler effect]]
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| ==References==
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| {{reflist}}
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| {{DEFAULTSORT:Photoacoustic Doppler Effect}}
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| [[Category:Doppler effects]]
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| [[Category:Radio frequency propagation]]
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| [[Category:Wave mechanics]]
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| [[Category:Radar signal processing]]
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Hello dear visitor. I'm Kittie Camara and I like it. I am really fond of to collect kites nevertheless haven't developed a dime destinations. Procuring is what he does for a living and he's doing very good financially. Her house is now in Alaska. If you wish to find uot more away his website: http://www.vimeo.com/104501099