Speckle formation in acoustic-resolution photoacoustic imaging (Conference Presentation)

In this work, speckle in acoustic-resolution photoacoustic (PA) imaging systems is discussed. Simulations and experiments were used to demonstrate that PA speckle carries structural information related to sub-resolution absorbers. Numerical simulations of phantoms containing spherical absorbers were performed using Green’s function solutions to the PA wave equation. A 21 MHz linear array was simulated (256 elements, 75×165 µm resolution, bandwidth 9-33 MHz) and used to record, bandlimit and beamform the generated PA signals. The effects of absorber size (10-270 µm) and concentration (10-1000/mm3) on PA speckle were examined using envelope statistics and radiofrequency spectroscopy techniques. To examine PA speckle experimentally, a VevoLAZR system was used to image gelatin phantoms containing 3 and 15µm polystyrene beads, a tissue mimicking radial artery phantom, and murine tumour vasculature in vivo. Fully developed speckle, as assessed by Rayleigh distribution fits to PA signal envelopes, was present in all images (simulated and experimental) containing at least 10 absorbers per resolution volume, irrespective of absorber size. Changes in absorber size could be detected using the spectral slope of the normalized power spectrum (4.5x decrease for an 80 µm increase in size). PA images of flowing blood in the radial artery phantom also revealed the presence of speckle with intensity that fluctuated periodically with beat rate (4 dB per cycle). Speckle was ubiquitous to all murine tumor vasculature images. During treatment-induced vascular hemorrhaging, the spectral slope decreases by 80% compared to untreated mice. These results demonstrate that photoacoustic speckle encodes information about the underlying absorber distribution.