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Photoacoustic imaging instruments based on the Fabry Perot ultrasound sensing concept have been used extensively for the preclinical assessment of mouse models and shown to provide high fidelity images to sub-cm depths. In a new development, a 3D high resolution scanner based on the same technology has now been specifically engineered into a format comprising a mobile platform and a convenient hand-held imaging probe for clinical use. A number of key engineering developments designed to advance the clinical translation of the technology have been implemented. The system now employs a novel 32-channel optical scanning architecture and a 1kHz PRF excitation laser providing an order-of-magnitude faster acquisition than previous pre-clinical embodiments. 3D images can now be acquired within 1 second, and video rate 2D synthetic aperture imaging is achievable. Image acquisition speed can be further accelerated by employing sub-sampling techniques based on total variation and deep learning image reconstruction, e.g. 3D images can be obtained at the rate of 4Hz with a typical 25% sub-sampling factor. To further aid clinical utility, the scanner allows rapid switching between the two imaging modes. This enables the ROI to be searched for and located in real-time using the 2D video rate mode prior to 3D image acquisition. Additional recent technical developments include bias wavelength tracking for temperature compensation, synthetic 1.5D array based receive beam forming for out-of-plane signal rejection, fast image reconstruction and visualisation and the implementation of an intuitive user-friendly interface.
To confirm clinical applicability, proof-of-concept studies both in healthy volunteers and patients have been conducted using the system. Following ethical and local regulatory approval, consenting patients were recruited from a single tertiary care hospital. Participants had previously been diagnosed with peripheral vascular disease (PVD), head and neck malignant tumours (including nodal deposits), inflammatory arthritis, or were under active clinical investigation for these conditions. We obtained mutliwavelength 3D images of the superficial vasculature in critically-ischaemic and normally perfused regions in patients with PVD. In both cases, the photoacoustic images were compared to clinical B-mode and Doppler ultrasound scans. The results show that the scanner is able to visualise the spatial-temporal changes in human microvasculature and thus may be able to identify regions of ischaemia otherwise undetectable using existing modalities. Images of small joint arthopathies, and malignant lymph nodes were also obtained, and compared with contemporaneous high resolution ultrasound. Patients found the use of the scanner highly acceptable, both in degree of comfort and the duration of the scan procedure. This exploratory phase clinical study represents an initial step towards establishing the clinical utility of photoacoustic imaging in a range of clinical conditions.
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