https://core.ac.uk/download/pdf/53261125.pdf
coded harmonic angio—A pulse inversion technology
in which pulse inversion harmonic imaging
is combined with a so-called coded excitation
technique, transmitting coded pulse sequences
and decoding them on receipt.
https://sonoscape.ru/articles/rezhim-tkanevoj-garmoniki.html
https://pubs.rsna.org/doi/pdf/10.1148/rg.2015140338
Pulse Inversion.—Pulse inversion is a technique
in which two pulses with a 180° phase difference
(ie, opposite phase) are emitted sequentially into
the tissue along the same line.
Various techniques are used
to remove the fundamental wave in THI, including
bandwidth receive filtering, pulse inversion,
side-by-side phase cancellation, and pulse-coded
harmonics

Pulse-coded Harmonics.—The pulse-encoding
technique transmits relatively complex pulse sequences
into the body with a unique and recognizable
code imprinted on each pulse. The unique
code is then recognized in the echoes (21). Because
the fundamental echoes have a specific code, they
can be identified and canceled. The remaining harmonic
echo is then processed to form the image.

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Note added at 7 hrs (2018-11-15 13:36:08 GMT)
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Park J, Huang Y, Chen R, et al. Pulse inversion chirp coded
tissue harmonic imaging (PI-CTHI) of zebrafish heart using
high frame rate ultrasound biomicroscopy.