Passive Acoustic Mapping (PAM) is an ultrasoundbased imaging method developed for monitoring therapeutic ultrasound. By using diagnostic transducers to passively record the acoustic signals that are emitted by cavitation bubbles, the origin of the bubbles can be reconstructed and displayed as intensity maps. In this study, two matrix arrays with different aperture sizes were used for the volumetric reconstruction of simulated and experimental data. In a second step, the number of elements being used for the reconstruction was reduced by more than the factor of eight in order to assess the influence on the imaging quality. In the numerical part of the study, the image quality was greatly improved by increasing the aperture size, while a high number of elements used for the reconstruction merely offers minor improvements. The experimentally obtained results were able to confirm the numerical findings regarding the achievable reconstruction quality.

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Arthritic diseases, including rheumatoid arthritis, psoriatic arthritis and osteoarthritis, have a prevalence between 2 and 3% and lead to joint destruction and deformation resulting in a loss of function. Current diagnostic methods rely on B-scan and Doppler ultrasound, x-ray or MRI, which have the drawbacks of low sensitivity and high user-dependency, involvement of ionizing radiation and high costs, respectively. We developed a combined US/OA imaging system for investigation of fingers allowing taking advantage of the high sensitivity of OA for imaging of vasculature and inflammation-related neovascularization. Our system consists of 4 arc-shapes transducer arrays based on high-bandwidth cMUTs, allowing imaging of all 3 finger joints (full tomographic view of DIP and PIP, top/bottom view of MCP). The pitch of the arrays is 150 µm and the center frequency is around 10 MHz. The total number of 768 elements is connected to a multichannel electronics platform. The system DiPhAS (Digital Phased Array System, Fraunhofer IBMT) has 128 channels for transmit and receive, each allowing the digitization with up to 80 MSamples/s, and has been equipped with a 1 to 8 multiplexer. Signals are generated with a pulsed OPO laser system with a PRF of 100 Hz and a pulse duration of 6 ns (NT232, EKSPLA). The system´s performance has been evaluated using different phantom structures. The measurements were made on wires directly immersed in the water bath. In addition, tissue phantoms made of PVA, in which bone material was integrated to mimic a real finger were developed. An isotropic resolution of approximately 150 µm was achieved in both modes. The system has furthermore been tested for compliance with MDD 93/42/EEC so that a clinical study can be performed soon.

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