Native tissue imaging at superharmonic frequencies

For a few years now, second harmonic imaging has become an established technique for imaging ultrasound contrast agent. This imaging mode has been immediately adopted and adapted to image tissue only and showed considerable improvements in image quality in several applications compared to fundamental mode. The improvements were attributed to the effects of wave distortion due to nonlinear propagation in tissue. However, imaging tissue at the second harmonic frequency only is associated with various drawbacks. Because the energy in the second harmonic frequency band is much lower than that in the fundamental frequency band, there must be excellent sensitivity and dynamic range in the receiving system to achieve an acceptable amount of signal-to-noise ratio. To increase the sensitivity, the spectral overlap between the fundamental and the second harmonic has to be diminished, which in return deteriorates the imaging resolution. Consequently, a trade-off is mandatory between the resolution and the sensitivity. Using simulations and measurements, we show in this paper that, at appropriate scanning acoustic settings, higher harmonics are significantly generated in tissue. The higher harmonics represent an additional and relevant information for tissue imaging and characterization. An elegant way to take advantage of the higher harmonics and to bring all the information together is to combine and incorporate all the multiple higher harmonics into a single component that we call superharmonic component. Using a newly developed array transducer having a wide frequency band, B-mode images of a phantom were made in the superharmonic mode transmitting at 1.2 MHz. The images generated using superharmonic have exceptionally improved clarity and yielded a dramatically cleaner and sharper contrast between the different structures being imaged. In addition to increased signal-to-noise ratio, superharmonic imaging showed better contrast and axial resolution as well as acceptable penetration depth.