Optoacoustic imaging is a rapidly developing area of biomedical imaging due its combination of rich optical
contrast and ultrasound depth penetration. Just like conventional pulse-echo ultrasound imaging, optoacoustic
tomography relies on the use of ultrasound detector arrays with a large number of elements. The precise
knowledge of the transducer’s sensitivity is crucial for the prediction of its performance for a given imaging
task. Sensitivity characteristics such as the central frequency and bandwidth are routinely characterized.
However, this characterization is typically performed solely under normal incidence since the measurement of
the angle and frequency depended sensitivity (directivity) is difficult and time consuming with existing
ultrasound characterization methods. We present a simple and fast characterization method for broadband
directivity measurements of the angular transducer sensitivity based on the optoacoustic effect. The method
utilizes a thin absorbing suture in order to generate omnidirectional and broadband optoacoustic signals,
which are calibrated using a needle hydrophone. We applied this method to characterize and compare the
directivity of a conventional piezoelectric (PZT) transducer to the directivity of a capacitive micromachined
ultrasonic (cMUT) transducer. Both technologies showed a similar broadband response at normal incidence
and the PZT transducer displayed a more than two times larger signal to noise ratio at normal incidence.
However, the cMUT transducer’s sensitivity was significantly less angle-depended and outperformed the
PZT’s sensitivity for angles larger than 20°.
Frequency characteristics of ultrasound detectors used in optoacoustic tomography have a major impact on imaging performance. It is common practice to select transducers based on their sensitivity at the central frequency and under normal incidence. However, the bandwidth and angular sensitivity play an equally important role in establishing the quality and accuracy of the reconstructed images. Here, we developed a calibrated optoacoustic characterization method specifically tailored for broadband measurements of the angular transducer sensitivity (directivity). Ultrawideband omnidirectional optoacoustic responses were generated by uniformly illuminating thin absorbing sutures with nanosecond laser pulses and characterized with a needle hydrophone. This calibrated optoacoustic source was used to characterize the frequency dependence of the angular response by a conventional piezoelectric transducer (PZT) and a capacitive micromachined ultrasonic transducer (cMUT) with similar size and central frequency. Furthermore, both transducers had no preamplification electronics directly attached to the detection elements. While the PZT presented a 7.8 dB sensitivity advantage at normal incidence, it was able to provide detectable signal-to-noise levels only at incidence angles of up to 20 deg whereas the cMUT maintained reasonable sensitivity levels and broadband response at incidence angles of 40 deg and beyond. We further experimentally showcase a reduction in the limited-view image artifacts resulting from the broader acceptance angle of the cMUT.
In photoacoustic imaging, the angular reception performance of ultrasonic transducers is a critical parameter to be considered for system designers. The quantitative comparison between cMUT and PZT emphasizes the difference between the transducer requirements and specifications between conventional ultrasound and photoacoustic imaging. In this present work, we show significant benefits of cMUT based array transducers over conventional PZT arrays for the improvement of quality in photoacoustic imaging systems.