42:
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,
529:
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
766:
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
1056:
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
732:
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.
767:
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. The reported minimum detected flow rate is 0.027 mm/s in the scattering medium.
43:
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.
471:
724:
728:
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.
787:. 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
885:
by digital holography with a high-speed camera can overcome some of the limitations of laser
Doppler flowmetry and achieve blood flow measurements in superficial vessels at higher spatial and temporal resolution.
1037:
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
589:
958:
282:
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
799:
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.
208:
622:
309:
249:
78:
923:
349:
329:
1007:
856:
527:
500:
280:
105:
39:. The observed frequency shift is a good indicator of the velocity of the illuminated moving particles. A potential biomedical application is measuring blood flow.
984:
356:
629:
775:
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
763:
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
1155:
H. Fang, K. Maslov, L.V. Wang. "Photoacoustic
Doppler Effect from Flowing Small Light-Absorbing Particles." Physical Review Letters 99, 184501 (2007)
1053:
of light in biological tissue. By combining the optical imaging with ultrasound, it is possible to achieve both high contrast and spatial resolution.
1057:
capillary. High contrast from the strong optical absorbers make it possible to clearly resolve the signal from the absorbers over the background.
1179:
H. Fang, K. Maslov, L.V. Wang. "Photoacoustic
Doppler flow measurement in optically scattering media." Applied Physics Letters 91 (2007) 264103
873:
to detect flow velocity. The much shorter optical wavelength means this technology is able to detect low flow velocities out of the range of
1125:
1200:
1091:
1071:
1022:
1018:
756:
827:
technique uses
Doppler frequency shifts in ultrasound wave. This technique is currently used in biomedicine to measure blood flow in
881:. Laser Doppler flowmetry can measure only the averaged blood speed within 1mm without information about flow direction. Wideband
1210:
742:
213:
898:
is an optical flow measurement technique that improves on the spatial resolution of laser
Doppler flowmetry by rejecting
895:
540:
51:
To be simple, consider a clear medium firstly. The medium contains small optical absorbers moving with velocity vector
1066:
866:
1195:
1076:
986:. The detection depth is usually limited by the high optical scattering coefficient of biological tissue to
1205:
928:
858:
cm/s) generally found in large vessels due to the high background ultrasound signal from biological tissue.
1086:
882:
760:
531:
351:
between the velocity and the ultrasonic wave propagation direction. The frequency shift is given by:
116:
32:
25:
1017:
Photoacoustic
Doppler effect can be used to measure the blood flow velocity with the advantages of
878:
874:
824:
594:
285:
225:
54:
1121:
1038:
905:
334:
314:
1034:
466:{\displaystyle f_{PAD}=-f_{0}{\frac {v}{c_{0}}}cos\alpha +f_{0}{\frac {v}{c_{a}}}cos\theta }
989:
838:
505:
478:
258:
83:
1046:
963:
719:{\displaystyle f_{PAD}=f_{0}{\frac {v}{c_{a}}}cos\theta ={\frac {v}{\lambda }}cos\theta }
1041:
in biological tissue via high sensitivity to small molecular optical absorbers, such as
1081:
21:
331:
between the velocity and the photon density wave propagation direction, and the angle
1189:
252:
746:
Figure 2: Average
Photoacoustic Doppler Shift vs. Velocity for a Scattering Medium
751:
In the first demonstration of the Photoacoustic Doppler effect, a continuous wave
624:, only the second term is detectable. Therefore, the above equation reduces to:
80:. The absorbers are irradiated by a laser with intensity modulated at frequency
902:
with coherent gating. This technique is able to detect flow velocity as low as
796:
780:
752:
877:. But this technique is limited by high background noise and low signal due to
1050:
1042:
1030:
1026:
899:
870:
808:
28:
1029:
imaging with the contrast of optical absorption in deep biological tissue.
36:
828:
788:
776:
811:
or light there are several techniques currently being used to measure
741:
212:
792:
832:
815:
velocity in a clinical setting or other types of flow velocities.
812:
784:
740:
211:
108:
795:. However, a particular difficulty of measuring flow velocity in
1049:, but its spatial resolution is compromised by the strong
992:
966:
931:
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57:
1001:
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494:
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343:
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303:
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202:
99:
72:
584:{\displaystyle {\frac {c_{0}}{c_{a}}}\sim 10^{5}}
8:
1118:Biomedical Optics: Principles and Imaging
991:
970:
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35:wave on moving particles with a specific
502:is the speed of light in the medium and
1103:
1111:
1109:
1107:
953:{\displaystyle 5\times 5\times 15\mu }
1175:
1173:
1171:
1169:
1167:
1165:
1163:
1161:
1151:
1149:
1147:
1145:
1143:
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1139:
1137:
835:. It is limited to high flow rates (
7:
1092:Doppler optical coherence tomography
1072:Photoacoustic imaging in biomedicine
890:Doppler optical coherence tomography
1025:combines the spatial resolution of
925:m/s with the spatial resolution of
14:
217:Figure 1: Overview of PAD Effect
203:{\displaystyle I={I}_{0}\left/2}
1013:Photoacoustic doppler flowmetry
295:
235:
107:. Thus, the intensity of the
64:
1:
1116:LV Wang & HI Wu (2007).
896:Optical coherence tomography
18:photoacoustic Doppler effect
1201:Radio frequency propagation
1227:
1067:Photoacoustic spectroscopy
869:utilizes light instead of
617:{\displaystyle v\ll c_{a}}
304:{\displaystyle {\vec {v}}}
244:{\displaystyle {\vec {v}}}
73:{\displaystyle {\vec {v}}}
900:multiple scattering light
1077:Photoacoustic tomography
757:photoacoustic microscopy
255:with the same frequency
1211:Radar signal processing
918:{\displaystyle 100\mu }
867:Laser Doppler Flowmetry
862:Laser doppler flowmetry
344:{\displaystyle \theta }
324:{\displaystyle \alpha }
111:could be described by:
1003:
980:
954:
919:
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748:
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585:
523:
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467:
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101:
74:
1087:laser Doppler imaging
1023:Photoacoustic imaging
1019:Photoacoustic imaging
1004:
1002:{\displaystyle <1}
981:
955:
920:
883:laser Doppler imaging
853:
851:{\displaystyle >1}
761:ultrasonic transducer
744:
721:
619:
586:
532:ultrasonic transducer
524:
522:{\displaystyle c_{a}}
497:
495:{\displaystyle c_{0}}
468:
346:
326:
306:
277:
275:{\displaystyle f_{0}}
246:
215:
205:
102:
100:{\displaystyle f_{0}}
75:
990:
979:{\displaystyle ^{3}}
964:
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906:
839:
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357:
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117:
84:
55:
24:that occurs when an
879:multiple scattering
803:Existing techniques
537:In practice, since
26:intensity modulated
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976:
950:
915:
875:Doppler ultrasound
848:
825:Doppler ultrasound
819:Doppler ultrasound
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519:
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1127:978-0-471-74304-0
1033:has good spatial
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807:Based on either
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530:observed by the
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1196:Doppler effects
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1135:
1128:
1115:
1114:
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1047:red blood cells
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1206:Wave mechanics
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1082:Doppler effect
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22:Doppler effect
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253:acoustic wave
232:
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88:
61:
46:
44:
40:
38:
34:
33:photoacoustic
30:
27:
23:
20:is a type of
19:
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1055:
1016:
893:
865:
822:
806:
774:
765:
750:
745:
731:
727:
626:
536:
474:
353:
311:, the angle
251:is zero, an
221:
216:
113:
50:
41:
17:
15:
797:capillaries
781:capillaries
771:Application
753:diode laser
1190:Categories
1098:References
1051:scattering
1043:hemoglobin
1035:resolution
1031:Ultrasound
1027:ultrasound
871:ultrasound
809:ultrasound
737:Experiment
31:induces a
29:light wave
1120:. Wiley.
1045:found in
948:μ
942:×
936:×
913:μ
714:θ
700:λ
689:θ
602:≪
569:∼
461:θ
419:α
380:−
339:θ
319:α
296:→
236:→
167:π
65:→
37:frequency
1061:See also
1039:contrast
894:Doppler
829:arteries
789:diabetes
777:arteries
1124:
793:cancer
783:, and
475:Where
47:Theory
833:veins
813:blood
785:veins
222:When
109:laser
1122:ISBN
1009:mm.
994:<
843:>
831:and
823:The
791:and
591:and
16:The
910:100
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1160:^
1136:^
1106:^
1021:.
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645:D
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