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Speckles in ultrasound/photoacoustic (PA) images arise due to incoherent addition of waves from randomly distributed sources. Unlike ultrasound, PA images are almost speckle free. Texture or pattern of speckle carries the information of number density of randomly distributed sources and depends on the resolution of imaging transducer and presence of intrinsic electronic noise. In this simulation study we examine how speckle pattern changes with noise. We considered two cylindrical phantoms (Phantom 1 and Phantom 2). The central region in Phantom1 was filled with cancer cells with endocytosed gold nanoparticles. However, inside and outside regions were occupied with cells for Phantom 2. Zero mean Gaussian (with σ as its width) noise was added with the simulated PA signals. It is hard to find any perceptual difference between the B-mode images of the phantoms at noiseless and low noise (i.e. σ=0.01) conditions. The outside region appears more noisy than the inside as the noise level is raised (i.e. σ=0.1). Consequently, inside-outside contrast becomes low and speckle visibility (incoherent power/coherent power) increases. At very high noise, boundary becomes indistinguishable from the background. It can be concluded that the boundary effect in PA imaging gets reduced in presence of noise.
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