Photoacoustic Spatial Coherence Theory and Applications to Coherence-Based Image Contrast and Resolution

Abstract

The photoacoustic effect relies on optical transmission, which causes thermal expansion and generates acoustic signals. Coherence-based photoacoustic signal processing is often preferred over more traditional signal processing methods due to improved signal-to-noise ratios, imaging depth, and resolution in applications such as cell tracking, blood flow estimation, and imaging. However, these applications lack a theoretical spatial coherence model to support their implementation. In this article, the photoacoustic spatial coherence theory is derived to generate theoretical spatial coherence functions. These theoretical spatial coherence functions are compared with k-Wave simulated data and experimental data from point and circular targets (0.1-12 mm in diameter) with generally good agreement, particularly in the shorter spatial lag region. The derived theory was used to hypothesize and test previously unexplored principles for optimizing photoacoustic short-lag spatial coherence (SLSC) images, including the influence of the incident light profile on photoacoustic spatial coherence functions and associated SLSC image contrast and resolution. Results also confirm previous trends from experimental observations, including changes in SLSC image resolution and contrast as a function of the first M lags summed to create SLSC images. For example, small targets (e.g., <1-4 mm in diameter) can be imaged with larger M values to boost target contrast and resolution, and contrast can be further improved by reducing the illuminating beam to a size that is smaller than the target size. Overall, the presented theory provides a promising foundation to support a variety of coherence-based photoacoustic signal processing methods, and the associated theory-based simulation methods are more straightforward than the existing k-Wave simulation methods for SLSC images.

GPU implementation of photoacoustic short-lag spatial coherence imaging for improved image-guided interventions

“…5(b) show decreased gCNR, and gCNR is otherwise constant as M increases. Figures 5(c) and 5(d) show that lateral resolution improves as Mincreases, which is expected 9,45,48.…”

“…Figures 5(c) and 5(d) show that lateral resolution improves as increases, which is expected. 9 , 45 , 48 …”

“…Short-lag spatial coherence (SLSC) beamforming 45 – 48 is another option that has shown substantial promise in multiple interventional tasks. 9 , 15 , 16 , 29 , 49 Therefore, SLSC is considered to be one of the more suitable beamforming options available to improve photoacoustic-based visual servoing.…”

“…SLSC beamforming requires multiple normalized cross-correlations of delayed data to directly display measurements of aperture-domain spatial coherence rather than amplitude. Although SLSC beamforming is known to be insensitive to signal amplitude, 48 , 50 the proposed application of visual servoing and surgical tool tracking does not require this sensitivity. The benefits of using SLSC beamforming for the proposed application are that it enhances the contrast of single-frame photoacoustic images (i.e., no frame averaging required) 15 , 46 , 51 and triples effective penetration depths when compared with DAS beamforming.…”

Generalized spatial coherence reconstruction for photoacoustic computed tomography

“…Owing to the similarity of ultrasound and PACT reconstruction, these coherence factors have also been translated to PACT; despite that, a rigorous theory such as the van Cittert-Zernike (VCZ) theorem in ultrasound imaging was not established for photoacoustic imaging until recently. 15 This is largely because the idea of exploiting spatial coherence between signals recorded at different sensors to discriminate against random noises is general. The recent linkage of the spatial coherence of photoacoustic signals to the VCZ theorem further solidifies its theoretical basis.…”