Super-resolution photoacoustic and ultrasound imaging with sparse arrays

It has previously been demonstrated that model-based reconstruction methods relying on a priori knowledge of the imaging point spread function (PSF) coupled to sparsity priors on the object to image can provide super-resolution in photoacoustic (PA) or in ultrasound (US) imaging. Here, we experimentally show that such reconstruction also leads to super-resolution in both PA and US imaging with arrays having much less elements than used conventionally (sparse arrays). As a proof of concept, we obtained super-resolution PA and US cross-sectional images of microfluidic channels with only 8 elements of a 128-elements linear array using a reconstruction approach based on a linear propagation forward model and assuming sparsity of the imaged structure. Although the microchannels appear indistinguishable in the conventional delay-and-sum images obtained with all the 128 transducer elements, the applied sparsity-constrained model-based reconstruction provides super-resolution with down to only 8 elements. We also report simulation results showing that the minimal number of transducer elements required to obtain a correct reconstruction is fundamentally limited by the signal-to-noise ratio. The proposed method can be straigthforwardly applied to any transducer geometry, including 2D sparse arrays for 3D super-resolution PA and US imaging

Evaluation of precision in optoacoustic tomography for preclinical imaging in living subjects.

Optoacoustic Tomography (OT) is now widely used in preclinical imaging, however, precision (repeatability and reproducibility) of OT has yet to be determined. METHODS: We used a commercial small animal OT system. Measurements in stable phantoms were used to independently assess the impact of system variables on precision (using coefficient of variation, COV), including acquisition wavelength, rotational position, frame averaging. Variables due to animal handling and physiology, such as anatomical placement and anesthesia conditions were then assessed in healthy nude mice using the left kidney and spleen as reference organs. Temporal variation was assessed by repeated measurements over hours and days both in phantoms and $\textit{in vivo}$. Sensitivity to small molecule dyes was determined in phantoms and $\textit{in vivo}$; precision was assessed $\textit{in vivo}$ using IRDye800CW. RESULTS: OT COV in a stable phantom was less than 2% across all wavelengths over 30 days. The factors with greatest impact on the signal repeatability in phantoms were rotational position and user experience, both of which still resulted in a COV of less than 4%. Anatomical ROI size showed the highest variation at 12% and 18% COV in the kidney and spleen respectively, however, functional SO₂ measurements based on a standard operating procedure showed exceptional reproducibility of <4% COV. COV for repeated injections of IRDye800CW was 6.6%. Sources of variability for $\textit{in vivo}$ data included respiration rate, user experience and animal placement. CONCLUSION: Data acquired with our small animal OT system was highly repeatable and reproducible across subjects and over time. Therefore, longitudinal OT studies may be performed with high confidence when our standard operating procedure is followed.This work was funded by: the EPSRC-CRUK Cancer Imaging Centre in Cambridge and Manchester (C197/A16465); CRUK (C14303/A17197, C47594/A16267); EU-FP7-agreement FP7-PEOPLE-2013-CIG-630729; and the University of Cambridge EPSRC Impact Acceleration Account