Ultrasound elastomicroscopy using water jet and osmosis loading: potentials for assessment for articular cartilage

Research in elasticity imaging typically relies on 1-10 MHz ultrasound. Elasticity imaging at these frequencies can provide strain maps with a resolution in the order of millimeters, but this is not sufficient for applications to skin, articular cartilage, or other fine structures. In this paper, we introduced two methods of ultrasound elastomicroscopy using water jet and osmosis loading for imaging the elasticity of biological soft tissues with high resolutions. In the first system, the specimens were compressed using water jet compression. A water jet was used to couple a focused 20 MHz ultrasound beam into the specimen and meanwhile served as a "soft" indenter. Because there was no additional attenuation when propagating from the ultrasound transducer to the specimen, the ultrasound signal with high signal-to-noise ratio could be collected from the specimens simultaneously with compressing process. The compression was achieved by adjusting the water flow. The pressure measured inside the water pipe and that on the specimen surface was calibrated. This system was easily to apply C-scan over sample surfaces. Experiments on the phantoms showed that this water jet indentation method was reliable to map the tissue stiffness distribution. Results of 1D and 2D scanning on phantoms with different stiffness are reported. In the second system, we used osmotic pressure caused by the ion concentration change in the bathing solutions for the articular cartilage to deform them. When bovine articular cartilage specimens were immerged in solutions with different salt concentration, a 50 MHz focused ultrasound beam was used to monitor the dynamic swelling or shrinkage process. Results showed that the system could reliably map the strain distribution induced by the osmotic loading. We extract intrinsic layered material parameters of the articular cartilage using a triphasic model. In addition to biological tissues, these systems have potential applications for the assessment of bioengineered tissues, biomaterials with fine structures, or some engineering materials. Further studies are necessary to fully realize the potentials of these two new methods.