Contrast-agent detection using orthogonal decoding with pulse-inversion harmonic suppression in Hadamard-encoded multi-pulses (HEM) transmission

Hadamard-encoded multi-pulses (HEM) transmit has recently been utilized for tissue background suppression in ultrasound contrast imaging to enhance contrast-to-tissue ratio (CTR). Nonetheless, second harmonic component in HEM transmit results in residual tissue background after decoding and thus compromises detection of contrast microbubbles. Theoretically, high-order HEM transmit can produce harmonic-free background but the memory effect which considers the nonlinear contribution of previous bit waveform into next one in the progress of harmonic generation may limit the achievable tissue suppression. In this study, three possible harmonic-free pairs using time-shifted subtraction (SH1, SH2 and SH3) in the fourth-order Hadamard decoding are analyzed and experimentally compared using hydrophone measurement and B-mode imaging. Moreover, orthogonal decoding of HEM transmit is also proposed with pulse-inversion harmonic suppression (PIHS) to remedy memory effect on tissue background. Results shows that SH3, which utilizes the third and the fourth rows for decoding, provides the lowest magnitude of tissue background among all possible decoding pairs and performs comparably to the reference pulse inversion and amplitude modulation (PIAM) sequence in terms of CTR. For PIHS orthogonal decoding, the pulse subtraction effectively removes the harmonic interferences from memory effect and thus further improves the CTR by 5.4 dB compared to SH3. For high-order HEM transmit, PIHS orthogonal decoding can help to eliminate residual tissue background due to memory effect and is comparable to Hadamard decoding in temporal resolution and possible motion artifacts.