Photoacoustic imaging (PAI) is an emerging modality that has generated increasing interest for its uses in clinical research and translation. To fully exploit its potential for various preclinical and clinical applications, it is necessary to develop systems that offer high imaging speed, reasonable cost, and manageable data flow. Currently, a significant challenge lies in the fabrication of ultrasound arrays, as many of them are not densely populated enough to fully sample the signals. Ideally, the pitch of the arrays should be half of the center ultrasonic wavelength to prevent spatial undersampling and the subsequent reconstruction artifacts, such as aliasing artifacts and structural deformation. Here, a novel photoacoustic sparse sampling Transformer-CNN coupling network (passFormer) is proposed to decouple target details and spatial under-sampling artifacts from high-frequency image information in a heterogeneous feature-aware manner. To be specific, we first decompose the sparse sampling (SS) photoacoustic (PA) images into 2 parts: high-frequency (HF) and low-frequency (LF) compositions. Our methodology incorporates two bridging modules, the LF modules and the HF modules. The LF coupling module extracts content features (Xc) and latent texture feature (Xtex), and the HF coupling module extracts high-frequency embedding (Xemb) containing the target details features (Xdetail) and under-sampling artifacts (Xart). We feed Xt and Xemb into a modified transformer with three encoders and decoders to obtain well-refined HF texture features. At last, we combine the refined HF texture features with pre-extracted Xc by pixel-wise summation reconstruction. Experimental results on publicly available full and sparse reconstruction datasets of mouse and phantom PA images highlight the superior performance of our method, particularly in live mouse imaging. This new approach enables accelerated data acquisition and image reconstruction, facilitating the development of practical and cost-effective imaging systems.
Significance: Photoacoustic computed tomography (PACT) is a fast-growing imaging modality. In PACT, the image quality is degraded due to the unknown distribution of the speed of sound (SoS). Emerging initial pressure (IP) and SoS joint-reconstruction methods promise reduced artifacts in PACT. However, previous joint-reconstruction methods have some deficiencies. A more effective method has promising prospects in preclinical applications.Aim: We propose a multi-segmented feature coupling (MSFC) method for SoS-IP joint reconstruction in PACT.Approach: In the proposed method, the ultrasound detectors were divided into multiple sub-arrays with each sub-array and its opposite counterpart considered to be a pair. The delay and sum algorithm was then used to reconstruct two images based on a subarray pair and estimated a direction-specific SoS, based on image correlation and the orientation of the subarrays. Once the data generated by all pairs of subarrays were processed, an image that was optimized in terms of minimal feature splitting in all directions was generated. Further, based on the direction-specific SoS, a model-based method was used to directly reconstruct the SoS distribution.Results: Both phantom and animal experiments demonstrated feasibility and showed promising results compared with conventional methods, with less splitting and blurring and fewer distortions.Conclusions: The developed MSFC method shows promising results for both IP and SoS reconstruction. The MSFC method will help to optimize the image quality of PACT in clinical applications.
Unmixing multispectral photoacoustic (PA) images is difficult because the excitation spectra in deep tissue are contaminated by absorption and scattering of the surrounding tissue in a highly unpredictable manner. In this work, we found a close relationship between the covariance matrix of a multispectral photoacoustic image and its average tissue oxygenation level. Based on the photon diffusion process, a spectral-domain model of multispectral photoacoustic imaging is established. Combined with the above two findings, accurate estimation of blood oxygen saturation (median error 2.7%) and accurate probe identification (detection rate 86%, false alarm rate 0.035%) were realized in realistic simulation test.
KEYWORDS: Photoacoustic imaging, Blood, Tissue optics, Blood oxygen saturation, In vivo imaging, Data modeling, Multispectral imaging, Diffusion, Biological research, Monte Carlo methods
In multispectral photoacoustic imaging (PAI), the illumination spectrum inside biological tissue varies spatially, leading to poor quantification accuracy of blood oxygen saturation (SO2). The key to solving this problem is to invert light diffusion, which is extremely complicated and inaccurate due to the limited information available in PAI. Despite the great effort devoted, to date, the few available methods are all limited in terms of in vivo performance and physical insights. Here, we introduce an analytical Monte Carlo method, with which we prove that the light spectrum in biological tissue mathematically lies in a high dimensional convex cone set. The model offers new insights into the origin of the spectral deterioration, and we find it possible to calculate blood oxygen saturation (SO2) accurately by using only the photoacoustic data at a single spatial location when signal to noise ratio is sufficient. The method was demonstrated numerically, and our preliminary phantom experiment results also confirmed its effectiveness.
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