Technical progress of medical ultrasonic harmonic imaging

Medical ultrasound scanners are widely used in hospitals all over the world for diagnostic purposes. While many technological improvements have been achieved over the years that resulted in good images, a large number of patients are still difficult to image due to inhomogeneous skin layers and limited penetration. In recent years, harmonic frequency is adopted in tissue imaging and contrast agents imaging, which improves the image quality. Harmonics in ultrasound are generated by tissue or by contrast agents. Contrast-agent harmonics are generated by reflections from the injected contrast agent and not from reflections of tissue. When no contrast is employed, harmonics are generated by tissue itself as a result of nonlinear propagation. Harmonic imaging of tissues or contrast agent forces an inherent compromise between image resolution and contrast that limits its sensitivity to nonlinear signals. This paper describes the technical progress of harmonic imaging. Its clinical application prospects are discussed also.