Fig. 1. Design and detection performance characteristics of the miniature needle US sensor. (a) Schematic and external dimensions of the sensor. Preamp, preamplifier. (b) The detected OA signal from a thin black tape and its frequency spectrum.

Fig. 2. Sensitivity characterization of the developed needle US sensor and evaluation of the effective US detection area. (a) Schematic of the experimental setup for evaluating the US detection performance. (b) Frequency-dependent normalized signal measurements of the needle US sensor. (c) Corresponding results for a commercial NH0500 hydrophone. (d) Ratio between the curves in (b) and (c). (e) Absolute frequency-dependent sensitivity in $\mu \mathrm{V}/\mathrm{Pa}$. (f) Result of needle US sensor directivity measurements. (g) Schematic of US detection/light scanning area and incident angles for scan points located at the edge of US detection area. The US detection SNR is found to be consistent within this area.

Fig. 3. The customized optical-resolution light scanning OAM system and image postprocessing measures. (a) Schematic of the system. SL, scan lens; TL, tube lens; M, mirror; DAQ, data acquisition card; Obj, objective lens; and NS, needle US sensor. (b) Image postprocessing pipeline; (c) raw OA image; and (d) processed OA image via the optimized image postprocessing pipeline.

Fig. 4. Images acquired by the light scanning-based OAM system integrated with the needle US sensor. (a) OAM image of a surgical blade; (b) OAM image of $7\mu \mathrm{m}$ diameter carbon fibers; (c) in vivo image acquired from a mouse ear; and (d) in vivo mouse brain vasculature image. The green dashed arrows in (c) and (d) denote large ($\sim 29$–$46\mu \mathrm{m}$ diameter) vessels, whereas blue solid arrows denote $\sim 3$–$7\mu \mathrm{m}$ diameter microcapillaries. The yellow arrowheads indicate individual red blood cells within a microvessel. Scale bars: $50\mu \mathrm{m}$.

Table 1. Comparison of material properties and measured parameters of two needle US sensors with different piezo thin films.^a