Phase aberration of focused ultrasound by tissue structure causes focus degradation and reduces the quality of B-mode images. Refraction at the boundary between subcutaneous fat and muscle is one of the dominant factors behind such degradation. To correct this, we propose a refraction compensation method in which ultrasound is transmitted and received twice. The boundary shape between different tissues is detected by the first ultrasound transmission. Next, ultrasound rays from probe elements to the target are calculated taking refraction into account. Corrected delay times are calculated from the length of the rays and the sound velocity of the medium. Finally, ultrasound is transmitted a second time using the corrected delay time and a B-mode image is created. We evaluate the correction effect of the proposed method by numerical simulation and experiments with non-compensated and refraction-compensated cases of intensity distribution of the focused ultrasound. Results show that focus degradation is effectively corrected by the proposed method.
Dental implants are in 96% positioned free hand including a risk of suboptimal placement. A miniaturized navigation system has been developed to overcome these limitations by fixing two stereo cameras to the drill to guide the surgeon during implant insertion. The aim of the present study was to develop and analyse a dual-mode marker with a highly precise, high contrast optical pattern, visible by the camera system, on a precise substrate clearly identifiable on Cone Beam CT (CBCT) images and sterilisable. The marker was developed with a laser engraved optical pattern brought to a ceramic substrate containing two holes to ensure clear identification of the position of the marker within CBCT images.The substrate dimensions were verified with a micrometer gauge and compared to the expected tolerances. The position and angular error of the optical pattern on the substrate was analysed on optical microscope images. Six markers were exposed to 20 cleaning and steam sterilization cycles and changes were analysed on optical and scanning electron microscope (SEM) images. Energy dispersive X-ray spectroscopy (EDX) analysis was performed to study the percentage composition of the different elements.Substrate dimensions and angular errors remained always within the defined tolerances. Positioning errors were higher than the tolerances for some of the measurements. The error distribution indicates a systematic error. Results were reproducible over the samples. No changes could be observed visually on the optical microscope images between the initial marker and the 20th sterilization cycle. EDX results show slight percentage variations of the different elements between the different cycles. Fluctuations were most important for the carbon-content.A dual mode marker could be developed with a very accurate optical pattern very precisely positioned on a ceramic substrate making any marker calibration procedure un- necessary. Precision analysis confirmed a reproducible position of the pattern on the substrate, indicating a stable and robust process. Systematic errors can be corrected directly at the used Laser system. Cleaning and sterilization tests confirmed the stability of the dual-mode marker after cleaning and sterilization. The present study has shown that highly accurate, high contrast and robust markers can be manufactured using laser- engraving technology on ceramic substrates.
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