Abstract
Photoacoustic microscopy and macroscopy (PAM) using focused detector scanning are emerging imaging methods for biological tissue, providing high resolution and high sensitivity for structures with optical absorption contrast. However, achieving a constant lateral resolution over a large depth of field for deeply penetrating photoacoustic macroscopy is still a challenge. In this work, a detector design for scanning photoacoustic macroscopy is presented. Based on simulation results, a sensor array geometry is developed and fabricated that consists of concentric ring elements made of polyvinylidene fluoride (PVDF) film in a geometry that combines a centered planar ring with several inclined outer ring elements. The reconstruction algorithm, which uses dynamic focusing and coherence weighting, is explained and its capability to reduce artefacts occurring for single element conical sensors is demonstrated. Several phantoms are manufactured to evaluate the performance of the array in experimental measurements. The sensor array provides a constant axial and lateral resolution of 95 µm and 285 µm, respectively, over a depth of field of 20 mm. The depth of field corresponds approximately to the maximum imaging depth in biological tissue, estimated from the sensitivity of the array. With its ability to achieve the maximum resolution even with a very small scanning range, the array is believed to have applications in the imaging of limited regions of interest buried in biological tissue.
Highlights
Photoacoustic microscopy provides an alternative to established imaging methods of biological tissue, offering high resolution and high contrast for light-absorbing structures [1]
Compared to photoacoustic imaging devices that use large arrays and tomographic reconstruction of absorbing structures distributed over extended volumes, the AR-Photoacoustic microscopy and macroscopy (PAM) approach is slower owing to the pointwise scanning, but it can use smaller lasers with higher pulse repetition rates due to the smaller irradiated volume
The results demonstrate a large depth of field (DOF), a reduction of artefacts caused by dynamic focusing and a good lateral resolution over a wide range of depths
Summary
Photoacoustic microscopy provides an alternative to established imaging methods of biological tissue, offering high resolution and high contrast for light-absorbing structures [1]. In optical resolution photoacoustic microscopy (OR-PAM), a resolution on the micrometer scale is achieved by focusing the excitation beam and scanning over the surface point by point. The strong scattering of light in biological tissue prevents high resolution by optical focusing beyond the diffusion limit [2]. Deeper imaging is possible with acoustical resolution photoacoustic microscopy (AR-PAM), which uses a wider illumination beam [3]. Penetrating photoacoustic imaging methods using focused detector scanning can reach an imaging depth of a few centimeters, with a lateral resolution of about 500 μm and are denoted as photoacoustic macroscopy [3,4]. Compared to photoacoustic imaging devices that use large arrays and tomographic reconstruction of absorbing structures distributed over extended volumes, the AR-PAM approach is slower owing to the pointwise scanning, but it can use smaller lasers with higher pulse repetition rates due to the smaller irradiated volume
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