Mitigation of off-axis specular reflection artifacts in diagnostic bone imaging with medical ultrasound using mechanically scanned piston transducers

Medical ultrasound is a potentially superior alternative to X-ray for bone imaging due to its portability, low cost, and avoidance of ionizing radiation. However, bone imaging, using conventional medical ultrasound systems, exhibits a higher level of artifact when compared with soft tissue imaging, and therefore, clinical efficacy is limited. Simulations in FIELD II were conducted to assess the effects of transducer geometry, imaging parameters, and orientations of specular reflecting surfaces on image artifact. The simulation model was experimentally validated using an Ultrasonix Sonix RP scanner with specular surfaces constructed from fused sheets of PMMA. Simulation and experimental results demonstrated that linear transducer arrays exhibit an increase in artifact signal at higher frequencies where the grating lobe angle, relative to main lobe angle, was reduced and when grating lobes were perpendicularly incident upon off-axis surfaces. The artifact signal was reduced with the linear array geometry when off-axis surfaces were simulated as diffusive scatterers rather than specular reflectors. The simulation model was validated experimentally achieving a correlation coefficient of 0.97 between experimental and simulated on-axis to off-axis peak reflection contrast. This work indicates that off-axis reflections are a major source of ultrasound image artifact particularly in environments comprising specular reflecting (i.e. bone or bone-like) objects. Transducer geometries with reduced sensitivity to off-axis surface reflections, such as a piston transducer geometry, yield reductions in image artifact.