Thomas and Rubin correctly predicted in 1998 that tissue harmonic imaging would soon become the standard for cardiac ultrasound. In fact, tissue harmonic imaging (THI) has increasingly become the standard for a wide variety of ultrasound imaging, from abdominal through to vascular. The technology has also permeated the market to such a degree that it is no longer only to be found in top of the range £100k+ machines: it’s even present on starter systems like the Sonoscape E1.
What is it?
THI allows us to break all the rules. Traditionally, if we wanted to achieve better penetration, we’d have to use lower frequency ultrasound – at the expense of spatial resolution due to a longer ultrasound pulse length (Stowell, 2018). Using THI, we send at lower frequencies in order to achieve the penetration required in larger patients or deeper organs, but the ultrasound machine only pays attention to the higher frequency echoes coming back. These higher frequency echoes are multiples of the original (fundamental) frequency, and occur naturally as the sound travels through tissue: we just never knew to look for them until people began experimenting with contrast agents in the 1980s and configuring their systems to receive these harmonic frequencies.
To make things even better, because harmonics are generated over distance, the area of the body where most artifacts originate – the superficial layers (where there is often fat, for example) – does not generate harmonics. Turning THI on can therefore often instantly remove the worst offending artifacts and clear up the image.
Since the early days of discovery, there have been many developments in how harmonics are generated and received. Higher frequency harmonics obviously deliver higher image resolution, but are much weaker, confining most applications to the second harmonic only. One of the latest developments is fusion tissue harmonics, and this new technology has been built into the Siui Apogee 2300. With fusion THI, the first and second harmonic are summed, allowing for further noise and speckle reduction. The basis for this is that if something is present in one harmonic, but not the other, it must be artefactual. If a structure is present in both, then it is a real reflector.
In addition, it allows the ultrasound machine to take the ‘best of both worlds,’ giving better penetration from using the combined first and second harmonics than one would obtain from the second harmonic alone (Turek et al., 2005).
Stowell, C. (2018). Ultrasound for Canine Pregnancy Scanning.
Thomas, J., Rubin, D. (1998). Tissue Harmonic Imaging. Why does it work? In Journal of the American Society of Echocardiography 11(8):803-8.
Turek, J., Sulam, J., Elad, M., Yavneh, I. (2005). Simultaneous Ultrasound Harmonic Imaging Fusion and Clutter Removal with Sparse Signal Separation. Conference Paper, available online.