Difference-Frequency Ultrasound Imaging With Non-Linear Contrast

Conventional ultrasound imaging is based on the scattering of sound from inhomogeneities in the density and the speed of sound and is often used in medicine to resolve pathologic compared to normal tissue. Here we demonstrate a difference-frequency ultrasound (dfUS) imaging method that is based on the interaction of two sound pulses that propagate non-collinearly and intersect in space and time. The dfUS signal arises primarily from the second-order non-linear coefficient, a contrast mechanism that differs from linear and harmonic US imaging. The distinct contrast mechanism allows dfUS to image anatomic features that are not identifiable in conventional US images of salmon and pig kidney tissue. Further, dfUS produces enhanced contrast of glioblastoma tumor implanted in the mouse brain, revealing its potential for improving medical diagnosis. Progress towards a real-time system is discussed.

I. Introduction

Ultrasound (US) imaging is widely used in medical diagnosis and treatment, with excellent real-time performance, portability, lack of ionizing radiation, and low expense [1]. It is also used in conjunction with other non-invasive medical imaging methods to improve diagnostic capabilities. The most frequently used ultrasound imaging mode is the B-mode that uses the brightness of an imaging pixel to represent the strength of the echo from the corresponding voxel (imaging volume) inside the tissue, where sound scatters off inhomogeneities in the speed of sound and density, two quantities that characterize sound propagation in the linear approximation.