Photoacoustic Computed Tomography (PACT) With A Multi-Angle Scanning Fiber-Optic Ultrasound Sensor

Photoacoustic imaging is a rapidly growing biomedical imaging method.It images optical-absorption contrast(such as blood vessels)over a range of spatial scales,by illuminating the biological tissue with nanosecond laser pulses and detecting the optically induced ultrasound waves.In photoacoustic imaging,biological tissue can be imaged by using the photoacoustic computed tomography(PACT)system in order to obtain a deeper penetration.The traditional photoacoustic imaging equipment usually uses the linear piezoelectric sensor array to detect the induced ultrasonic signal,but some features can not be visualized as a result of the limited acceptance angle.The optical ultrasound sensors for photoacoustic imaging have received great interests,because of their compact sizes,comparable sensitivities to their electric counterparts,as well as the extended field/angle-of-view.We have developed a highly sensitive ultrasound sensor by using a distributed-Bragg-reflector(DBR)laser,by detecting the acoustically induced variations in lasing frequencies.The sensor has smaller size and larger acceptance angle.In this work,we have developed a PACT system with a multi-angle scanning fiber optic sensor.Two-dimensional imaging was performed by horizontally scanning the sensor and image reconstruction via back projection,and three-dimensional imaging was further achieved by repeating such scanning process at multiple angles,based on inverse Radon transformation.The axial and lateral resolutions are 93 and 220 ?m in three-dimensional imaging.The fiberbased PACT can resolve more features than that with a piezoelectric transducer array,taking advantage of the dual-60-degree vision angles of the sensor.The main research of this work is as follows:(1).Frequency response characteristics of optical fiber laser ultrasonic sensor.We studied the response of the orthogonal dual-polarization fiber laser to the ultrasonic wave along the axial and azimuthal,verified the sensitive region of the fiber to the ultrasonic wave,and obtained the NEP of the fiber sensor is 46 Pa.(2).For two-dimensional photoacoustic computed tomography,a 2D image of the optical absorption source can be reconstructed based on the back-projection algorithm by linearly scanning the fiber optic sensor and detecting the ultrasound signals at multiple positions.The axial and the lateral resolution are 86 and 113 ?m,imaging depth is greater than 14 mm.(3).By performing the linear scanning at multiple angles,the sample is rotated by a certain angle ? before the linear scanning is repeated.As a result,2D reconstruction can be performed at every angle,and a 3D image can be obtained by superimposing the 2D images by using the inverse “Radon transform algorithm”.Finally,we demonstrated 3D image of a chick chorioallantoic membrane(CAM)in vitro,which shows spatially resolved vasculature,with a lateral resolution of 220 ?m and an axial resolution of 93 ?m,respectively.The penetration depth in CAM is over 6 mm.