Fig. 2: Direct comparison between the newly developed PVDF- and conventional piezocomposite (PZT)-based spherical arrays34 with similar geometry, number of elements and frequency range.
a Normalized spectral sensitivity of the individual PVDF versus PZT array elements, as experimentally measured by etalon comparison using a calibrated needle hydrophone. b The PVDF array attains a higher SNR when recording OA responses from a 20 µm-diameter absorbing microsphere. c Imaging of a large uniformly illuminated bulk cylindrical phantom comprising a 1.5 mm thick 10 mm diameter scattering and absorbing agar slab. The PVDF attains an order of magnitude better SNR for low frequencies, which remains significantly higher in the medium frequency range. d When imaging a 3 mm diameter tube (mimicking a large blood vessel) placed along the Y axis, the PVDF-based detection results in slow (theoretically anticipated) exponentially decaying responses whereas the PZT array fails to detect the expected OA waveforms, primarily due to lack of sensitivity in the lower frequency band. e Simulated versus experimental signal profiles through the vessel demonstrating the good correspondence between the theory and experimental results obtained with the PVDF array. f In vivo imaging of a human radial artery further demonstrates the importance of ultrawideband detection. g In the X-Z projection of the volumetric image, the low frequency OA signal content (0.3–5 MHz) clearly reveals the smooth light fluence decay across the artery in the depth (Z) direction whereas the large artery is undetectable when performing image reconstruction with the high frequency signal components (5–40 MHz). h The corresponding low frequency signal profile through the artery corresponds to an effective optical attenuation coefficient of ~1.6 mm-1 (assuming an exponential decay), closely resembling the known blood optical parameters at the 850 nm illumination wavelength
