Study On Photoacoustic Doppler Of Micron-nano-sized Particles For The Flow Measurement In Microcirculatory

The microcirculatory system in human body is mainly micro vascular which isalso called capillaries. It is a material exchange place for blood and surroundingtissues whose pathological process can lead to the blood flow change, and flowvelocity is an important parameter representing vascular performance. Micro vascularflow measurement and imaging for diseases’ prevention and treatment such ascardiovascular and cerebrovascular diseases plays an important role.The blood flow velocity measurement is mainly using Doppler technology,mainly including ultrasonic Doppler velocity measurement and laser Doppler velocitymeasurement. The principle of ultrasonic Doppler velocity measurement is based onparticles moving in fluid, through scattering sound wave’s Doppler shift to obtainliquid velocity information. Because of the slow motion of body organization, whenmeasuring low velocity, the Doppler signals obtained is not only from the particlesbut also from the influence of the vascular wall, so ultrasonic Doppler flowmeasurement in the low velocity are greatly restricted. The principle of laser Doppleris similar to ultrasonic Doppler’s, also using the light scattering of moving particles,even though it can measure low velocity, the measuring depth is limited and cannotobtain the direction of the flow of information. Photoacoustic Doppler technology cansolve the problem which has advantages of both acoustic measurement and opticalmeasurement, having high optical contrast and low background noise.In this project, we design a set of lock-in amplifier as the core in the low velocitymeasuring system, according to the linear relationship between photoacoustic Dopplershift and the velocity. Through mixing operation of the photoacoustic Doppler signaland reference signal, we can get photoacoustic Doppler shift signal, and then carry onthe fast Fourier transform to get Doppler shift, and then calculate the average flowvelocity. In order to study effects generated by the angle between velocity directionand ultrasonic probe (we call it Doppler Angle) on the Doppler shift, and enrich andsupplement the photoacoustic Doppler theory, we improved the original experimentsystem, using stepping motor to drive the rotation of ultrasonic probe, completing ofthe Doppler Angle’s scanning. The experimental results show that the relationship between Doppler Angle’scosine value and Doppler shift is linear. The measurement method in this paper issuitable for low velocity measurement, compared with the other Doppler velocitymeasurement technology, and it not only has good measurement accuracy, but alsocan acquire the direction of flow velocity. Besides, we discussed the relationship ofDoppler Angle and Doppler shift, and as a new technology, it may provide a newnoninvasive measurement method in the aspects of micro vascular low velocitymeasurement.