A Primer on the Physical Principles of Tissue Harmonic Imaging

Abstract

Tissue harmonic imaging (THI) is a routinely used component of diagnostic ultrasonography (US). In this method, higher-frequency harmonic waves produced by nonlinear fundamental US wave propagation are used to generate images that contain fewer artifacts than those seen on conventional fundamental wave US tissue imaging. Harmonic frequencies are integer multiples of the fundamental frequency. The majority of current clinical US systems use second harmonic echoes for THI image formation. Image processing techniques (ie, bandwidth receive filtering, pulse inversion, side-by-side phase cancellation, and pulse-coded harmonics) are used to eliminate the fundamental frequency echoes, and the remaining harmonic frequency data are used to generate the diagnostic image. Advantages of THI include improved signal-to-noise ratio and reduced artifacts produced by side lobes, grating lobes, and reverberation. THI has been accepted in US practice, and variations of the technology are available on most US systems typically used for diagnostic imaging in radiologic practice. Differential THI is a further improvement that combines the advantages of THI, including superior tissue definition and reduced speckle artifact, with the greater penetration of lower frequency US, which permits high-quality harmonic imaging at greater depth than could previously be performed with conventional THI.

Reader agreement and accuracy of ultrasound features for hepatic steatosis

“…Importantly, the same feature was selected by the CART for each transducer, suggesting the feature may be robust to the choice of transducer. Nevertheless, vessel blurring as an imaging feature of hepatic steatosis has limitations, as it potentially also may be caused by inadequate penetration in obese patients, by thickness of overlying fat on beam focus and it can depend on the type of harmonic imaging used, which may vary with manufacturers [35–37]. …”

Economical Sponge Phantom for Teaching, Understanding, and Researching A‐ and B‐Line Reverberation Artifacts in Lung Ultrasound

“…Soldati et al examined wet synthetic partially aerated polyurethane sponges. In their opinion, “comets represent superficial, artifactual, density and geometry correlated phenomena due to the acoustic permeability of a broken (collapsed) specular reflector, normally present when the phantom is dry.” Anvari et al elaborated that B‐lines “represent a form of reverberation where the two reflective interfaces are closely spaced” and where “sequential echoes may be so close together that individual signals are not perceivable.” If 2 separate reflections can be resolved (as in the case of A‐lines; see Figure a), the cavity dimensions can be deduced. However, the smallest cavity that one can resolve is approximately given by the wavelength (Rayleigh limit).…”

“…With tissue harmonic imaging, a harmonic overtone is detected, and noise and some of the artifacts caused by reverberation and aberration can be reduced. On the other hand, other artifacts such as acoustic enhancement deep to fluid‐filled structures (like cysts), acoustic shadowing, and comet tail artifacts can be enhanced . Enabling Coded Harmonic Imaging led to smoothed contours of the B‐lines displayed in the phantom setup.…”

“…On the other hand, other artifacts such as acoustic enhancement deep to fluid-filled structures (like cysts), acoustic shadowing, and comet tail artifacts can be enhanced. 28 Enabling Coded Harmonic Imaging led to smoothed contours of the B-lines displayed in the phantom setup.…”

Contrast-Enhanced Tissue Harmonic Imaging versus Phase Inversion Harmonic Sonographic Imaging for the Delineation of Hepatocellular Carcinomas

“…The second factor is deeper penetration. Contrast THI could offer deeper ultrasound penetration than conventional CHI, and thus a wide viewing image could be obtained [19,[23][24][25] . The third is the higher frame rate of THI needed to create a smooth moving image compared with CHI.…”