AutoSAFT: Autofocusing for extended depth of imaging in scanning acoustic microscopy.

Abstract

In the scanning acoustic microscopy (SAM) imaging, it is essential to address the diminished lateral resolution in the out-of-focus region, as image quality correlates with the ultrasound propagation distance either above or below the focal plane. To focus the scanned image, a refocusing technique called synthetic aperture focusing technique (SAFT) is widely used, which improves the resolution by extending the depth of focus manually. In SAM, refocusing the image accurately is challenging without prior defocusing information. This paper introduces AutoSAFT, an automated version of SAFT. It employs the reference-less image-quality index (IQI) called Blind/Referenceless Image Spatial Quality Evaluator (BRISQUE) score to evaluate and optimise the defocusing distance automatically. The focused images were qualitatively analysed using metrics such as structural similarity index matrix (SSIM) and peak signal-to-noise ratio (PSNR). The combined qualitative and quantitative analysis demonstrates that the AutoSAFT technique is the most suitable method for the automatic focusing of acoustic imaging from the SAM. The proposed AutoSAFT opens a new avenue in photoacoustic image focusing methodologies.