Effect of spatial filtering of ultrasound transducers on photoacoustic measurements

It is intuitively conjectured that the geometrical shape of an ultrasound transducer as well as ultrasound transducer’s transfer function affect the characteristics of measured photoacoustic signals. Previously, we theoretically demonstrated that a photoacoustic spectrum measured by a spherically focused ultrasound transducer exhibits resonance frequency peaks by combining the virtual detector concept with a Green function approach. With experimental demonstrations for photoacoustic resonance peaks, it was discussed that the origin of the photoacoustic resonance is spatial filtering of a limited measurement field of view of a focused ultrasound transducer to generated photoacoustic waves. Also, it was analytically confirmed that a time-domain photoacoustic signal derived from the resonant photoacoustic spectrum shows a temporal bipolar nature. In this Proceeding, we investigate photoacoustic magnitude variation to an absorption coefficient considering photoacoustic resonant spectra and simulated ultrasound transducer transfer functions simultaneously. The results show that a photoacoustic signal magnitude decreases as an absorption coefficient of a photoacoustic object increases after reaching its maximum value. This phenomenon is totally distinct from the commonly accepted sense that a photoacoustic signal magnitude is saturated to an absorption coefficient increase. We analyze the origin of this phenomenon by investigating the relationship between a central frequency of an ultrasound transducer transfer function and photoacoustic resonance frequency.