Osteoarthritis is the most common degenerative joint disease and a major cause of reduced physical function and the quality of life. Proper application of ultrasound has been proven to be effective for non-invasive osteoarthritis treatment. With a 2-D array transducer, spatial focusing of treatment pulses and three-dimensional (3-D) imaging of cartilage structures and intra-articular soft tissue are feasible for more effective treatment and diagnosis. However, supporting both imaging and therapy with a single 2-D ultrasound transducer is challenging due to the physical limitations caused by the array geometry. Given the number of active channels, increasing the element pitch can improve the lateral or elevational resolution and treatment efficacy, but introduces the grating lobe artifacts, degrading the overall image quality. To utilize a 2-D array configured with relatively large pitch elements for both 3-D imaging and low-frequency treatment, this study proposes a 3-D sub-pitch plane-wave imaging method. This method acquires channel RF data by physically translating the 2-D array transducer in the elevational and lateral directions and synthesizes all acquired RF data to reconstruct the single image, effectively maintaining the resolution while reducing grating lobe artifacts. We have demonstrated effective reduction in grating lobes through beam pattern analysis and quantitatively evaluated the imaging capabilities by Field II simulations and in-vitro experiments using a 2-D array with 8 x 8 elements centered at 2 MHz with 55% fractional bandwidth. These results could suggest that our approach may be useful in a theranostic ultrasound system supporting both treatment and diagnosis of osteoarthritic diseases.
