Research On Blood Flow Velocity Measurement Method Based On Nonlinear Photoacoustic

Blood flow velocity is one of the important functional parameters of the human body,and accurate measurement of blood flow velocity provides a powerful tool for early diagnosis and treatment of many different diseases.For example,a lesion in a microvessel or a certain tissue may cause changes in the blood flow velocity in this area.Therefore,the measurement and imaging of blood flow velocity plays a very important role in the prevention and treatment of some diseases such as atherosclerosis,stroke and cardiovascular and cerebrovascular diseases.So far,multiple methods have been reported to measure blood flow velocity.Among the most commonly used technologies include ultrasonic Doppler velocimetry,Doppler OCT(Optical Coherence Tomography)velocimetry technology and laser Doppler velocimetry technology,which are based on the Doppler effect.However,in the process of measuring the velocity of live blood flow,the movement of some organs in the complex environment such as human tissues will make the measured ultrasonic Doppler signal contain the influence of noise.Doppler OCT,especially phase-resolved Doppler OCT technology,is very fast and can be used for live tissue imaging,but the interference of environmental noise has a significant impact on the results of this technology,and the angle between the flow rate and the detection light needs to be known in advance during the measurement.Laser Doppler technology uses the light scattering of moving particles to obtain the Doppler shift,although the flow rate that can be measured is low,but it is limited by the light scattering of human tissue,and its measurement depth is limited.The photoacoustic-based velocimetry method combines the advantages of sound penetration and optical contrast,and uses flowing hemoglobin as an absorber to reduce the interference of background noise,which has become an emerging direction for studying blood flow measurement in vivo.In this paper,a method for measuring blood flow velocity based on nonlinear photoacoustics is proposed,and a low-power CW laser is used to provide nonlinear thermal labeling for the first time.Compared to the pulsed laser thermal tagging method,this method can achieve high-speed blood flow velocity measurement of more than 100 mm/s while greatly reducing the experimental cost and optical fluence at the sample surface.First,the relationship between the rate of change of photoacoustic signal and the blood flow velocity before and after heating is theoretically derived.Then,in the experiment,defibrogized bovine blood is injected into the capillary glass tube through the syringe pump to simulate the human in vivo environment,and the calibration experiment and test experiment prove the feasibility of the proposed method in vivo blood flow measurement.Finally,the relationship between the maximum measurable blood flow velocity and the continuous laser heating power of the method is discussed according to the theoretical derivation.