A new imaging technique based on the nonlinear properties of tissues

Finite amplitude sound propagating in a medium undergoes distortion due to the nonlinear properties of the medium. The nonlinear distortion produces harmonic (and subharmonic) energy in the propagating signal. The amplitudes used by commercial medical scanners during routine diagnostic scanning are in most cases finite and thus within the range that produces nonlinear distortion. Thermoviscous absorption of tissue which is frequency dependent rapidly dissipates this harmonic energy. This has led to the widely held assumption that nonlinear distortion was not a significant factor in medical diagnostic imaging. However, the wide dynamic range, digital architecture, and the signal processing capabilities of modern diagnostic ultrasound systems make it possible to utilize this tissue generated harmonic energy for image formation. These images often demonstrate reduced nearfield artifacts and improved tissue structure visualization. Previously, those images were believed to be the result of transmitted second harmonic energy. It is shown that the nonlinear properties of tissue are the major contribution of harmonic images.