Simultaneous Photoacoustic and Ultrasound Imaging Using a Hemispherical Sensor Array

A hemispherical sensor array enables photoacoustic (PA) imaging with more spatially isotropic and finer vasculature than those of a planar sensor array. Combining ultrasound (US) and PA images can clarify structural relations of tissues and vessels. Although a piezoelectric element of the hemispherical sensor array can perform pulse-echo, the array is sparse because of the few channels on data acquisition systems. Therefore, beamforming cannot be applied properly: adequate US image quality is obtainable. For this study, we used mechanical scanning to increase the number of virtual elements. We also used synthetic aperture (SA) imaging to improve US image quality. They enable simultaneous PA and US measurements using a hemispherical sensor array. Measurements taken using PA and US signals from a tissue-mimicking agar phantom demonstrate that scanning sensors improved the US image contrast and the field of view compared to results obtained when a sensor is not scanned. Moreover, a PA image visualized only optical absorbers. Furthermore, combining PA and US images facilitates identification of where optical absorbers are located.