Design and simulation of a high-frequency ring-shaped linear array for capsule ultrasound endoscopy

Current research into endoscopy and colonoscopy has significantly advanced visualization of the gastrointestinal tract (GIT). The Sonopill project seeks to combine the imaging capabilities of endoscopic ultrasound with the full GIT transit of capsule endoscopy through the development of a capsule capable of ultrasonic imaging of the GIT, focusing on the small intestine. However, due to the small volume of the proposed capsule and the need to transmit received data wirelessly, the Sonopill system is limited both in data bandwidth and power. This paper presents a MATLAB-based simulation to allow testing of transducer topologies and imaging methodologies to achieve optimum results within the physical limitations of the system. To allow rapid evaluation of possible transducer configurations and circuit elements, a hybrid MATLAB simulation was created, incorporating both KLM circuit elements for analog analysis and digitizing and beamforming elements to render a final grey-scale image for imaging quality analysis. This was used in conjunction with a theoretical acoustic propagation model to image ideal point scatterers. The proposed transducers consist of a single, unfocused transmit ring of radius 5 mm separated into eight segments for impedance control, and a 512-element receive linear array curved into a matching ring. Because of the high element count and pad limitations on the intended electronics, the design requires the use of 32 integrated 16:1 multiplexers which will be bonded directly to the connecting flex circuit before the ASIC. Simulating the loading effects of these multiplexers as well as the proposed transducer configuration was critical to the analysis of the design. The MATLAB model was used to simulate a standard pulser transmitting over a 2.5 m cable to a 0.25 mm × 8 mm × 85 μm PMN-PT piezocrystal transmit transducer with a centre frequency of 25 MHz. B-scan images were then modelled for three imaging phantoms, one containing three point target resolution phantoms, a resolution phantom containing two virtual walls, and a tissue mimicking phantom containing particles with two levels of reflectivity to represent a three layer gut phantom with a high-reflectivity front surface.