High-Order Hadamard-Encoded Transmission for Tissue Background Suppression in Ultrasound Contrast Imaging: Memory Effect and Decoding Schemes.

Hadamard-encoded multipulses (HEM) transmit has recently been utilized for tissue background suppression in ultrasound contrast imaging to enhance the contrast-to-tissue ratio (CTR). Nonetheless, the second-harmonic component in HEM transmit results in residual tissue background after decoding and, thus, compromises the 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 the next one in the progress of harmonic generation, may limit the achievable tissue suppression. In this paper, 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, the orthogonal decoding (OD) of HEM transmit is also proposed with pulse-inversion harmonic suppression (PIHS) to remedy memory effect on the tissue background. Results show that SH3, which utilizes the third and fourth rows for decoding, provides the lowest magnitude of tissue background among all possible decoding pairs and performs comparably to the reference PI and amplitude-modulation sequence in terms of CTR. For PIHS-OD, 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-OD can help to eliminate the residual tissue background due to memory effect and is comparable to Hadamard decoding in temporal resolution and possible motion artifacts.