Abstract
Optical detection of ultrasound is attractive to photoacoustic imaging due to its high sensitivity per unit area, broad bandwidth, and electromagnetic immunity. To enhance the sensitivity, previous optical transducers commonly necessitate bulk acoustic lenses to achieve focused ultrasound detection. Here, we proposed and demonstrated a novel lens-free focused optical ultrasound sensor by mechanically bending a flexible fiber laser. At a curvature radius of 30 mm, the curved fiber laser well conformed to the spherical wavefront of ultrasound exhibiting ~5 times higher sensitivity compared with the straight one. The focused fiber laser ultrasound sensor (FUS) presented a minimum detectable pressure of ~36 Pa with a working distance equal to its curvature radius. The sensor was applied to circular scanning photoacoustic computed tomography (PACT), which showed a ~70 μm in-plane resolution and a ~500 μm elevational resolution. In vivo imaging of a zebrafish and mouse brain shows the potential of this focused FUS for photoacoustic imaging in biological/medical studies.
Highlights
Photoacoustic tomography (PAT) is a rapidly developing noninvasive imaging modality with potential applications in areas of vascular biology [1,2], dermatology [3,4], and neurology [5,6]
photoacoustic computed tomography (PACT) commonly uses state-of-the-art piezoelectric transducers (PZTs) to acquire PA signals emitted from the region of interest (ROI) by mechanically scanning a single transducer [10] or by using linear [11], ring-shaped [12] or spherical [13] array transducers
Summary In summary, a flexible lens-free focused fiber laser ultrasound sensor (FUS) was demonstrated for PACT imaging
Summary
Photoacoustic tomography (PAT) is a rapidly developing noninvasive imaging modality with potential applications in areas of vascular biology [1,2], dermatology [3,4], and neurology [5,6]. The acoustic scattering, in analogy to Mie scattering in optics, establishes a discrete set of mechanical modes over fiber cross section This acoustic interaction vibrates the fiber and induces a detectable response in optical phase change, which can be readily read out by fiber-optic Michelson, Mach-Zehnder interferometry or phase-shift fiber grating. The mouse ear needed to be placed extremely close to the fiber surface during the imaging process because the FUS sensitivity rapidly decayed with increasing working distance. This phenomenon was caused by the wavelength-scale equivalent interaction length of a straight fiber subjected to spherical ultrasound waves [27] and might limit the potential of FUS for deep penetration PACT imaging. In contrast to traditional PZTs, the proposed focused FUS is lens-free, optical transparent and focus-tunable, making it promising for PA imaging in biological/medical studies as well as clinical applications
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