The new SonoScape S8Exp boasts pulse inversion imaging over the standard tissue harmonic imaging of the S8v. So what is it, and how does it work?

Tissue harmonic imaging has, for most patients, improved image quality dramatically, due primarily to reductions in noise and side lobe artefacts. This is because only the centre of the beam has sufficient amplitude to generate harmonics. However, it traditionally suffers from a low signal-to-noise ratio (Ma et al., 2005).

Also, to prevent overlap between the transmitted fundamental and the received second harmonic, a narrow bandwidth has to be used, resulting in lower axial resolution than with fundamental imaging. This is because a broader bandwidth results in a shorter pulse length, upon which axial resolution depends (being approximately equal to half the spatial pulse length).

Although the greater reproducibility of measurements taken using THI (due to clearer border definition) tends to counteract the negative effects of the reduced axial resolution (Turner & Monaghan, 2006), clearly, any improvement to axial resolution would be highly desirable.

Pulse inversion involves sending every second pulse in antiphase to the proceeding pulse. Rather than filtering, the returning signal is summed. Once summed together, the linear echoes cancel (as do the odd harmonics), and only the even harmonics remain and are reinforced. This allows a broad bandwidth to be used, at the expense of frame rate.

References

Ma, Q., Ma, Y., Gong, X., Zhang, D. (2005). Improvement of tissue harmonic imaging using the pulse-inversion technique. Ultrasound in Medicine & Biology 31(7):889-894.

Turner, S. & Monaghan, M. Tissue Harmonic imaging for standard left ventricular measurements; fundamentally flawed? European Journal of Echocardiography (7):9-15.