Photoacoustic Doppler effect
The photoacoustic Doppler effect is a type of Doppler effect that 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.
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.
The medium contains small optical absorbers moving with velocity vector
between the velocity and the photon density wave propagation direction, and the angle
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.
[2][3] In this approximation, the frequency shift is not affected by the direction of the optical radiation.
In this case, the photon density wave becomes diffusive due to light scattering.
[3] In the first demonstration of the Photoacoustic Doppler effect, a continuous wave diode laser was used in a photoacoustic microscopy setup with an ultrasonic transducer as the detector.
The tube was in a water bath containing scattering particles[2] Figure 2 shows a relationship between average flow velocity and the experimental photoacoustic Doppler 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.
[2] The reported minimum detected flow rate is 0.027 mm/s in the scattering medium.
This is related to an important problem in medicine: the measurement of blood flow through arteries, capillaries, and veins.
[3] 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.
Photoacoustic Doppler effect based imaging is a promising method for blood flow measurement in capillaries.
This technique is currently used in biomedicine to measure blood flow in arteries and veins.
cm/s) generally found in large vessels due to the high background ultrasound signal from biological tissue.
[3] 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 1mm3 without information about flow direction.
[3] Wideband laser Doppler imaging 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.
Doppler Optical coherence tomography is an optical flow measurement technique that improves on the spatial resolution of laser Doppler flowmetry by rejecting multiple scattering light with coherent gating.
The detection depth is usually limited by the high optical scattering coefficient of biological tissue to
Conversely, optical imaging is able to achieve high 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.
The high spatial resolution could make it possible to pinpoint only a few absorbing particles localized to a single capillary.
