Generation of wide-directivity broadband ultrasound by shortlaser pulses

ABSTRACT A three-dimensional mouse imaging system combining optoacoustic tomography and laser ultrasound (LOUIS-3DM) has been developed. It features broadband laser ultrasound emitters positioned opposite an array of transducers. This imaging geometry allows reconstruction of images that either depicts the speed of sound distribution from measured time of flight data, or acoustic attenuation from the measured signal amplitude. We have investigated the performance of two laser ultrasound source designs, both easily adapta ble to a commercial imaging system: small diameter source (600 µm) generated off a flat surface, and larger diameter (3 mm) spherical source. Laser energy requirements are modest, well below 1 mJ per pulse for either design. Their performance at normal incidence is comparable both in amplitude and frequency response. However, off-axis generation differs dramatically and the shortcomings of th e simple flat emitter design are eviden t. We show that, in order to achieve optimal performance through proper illumination of the detector array, spherical wave front characteristics are desired. Keywords: Broadband ultrasound, laser ultrasonics, wave front propagation, delta-source, spherical wave generation, wide-directivity source. INTRODUCTION We have developed a three-dimensional mouse imaging system (LOUIS-3DM) combining optoacoustic (OA) tomography