近年來哈達瑪( Hadamard )多重編碼發射序列已經被用來提升對比劑影像之contrast-to-tissue ratio (CTR)，其中二次諧波的干擾所造成的背景殘餘信號是影響對比劑影像偵測的主因，雖然理論上高階哈達瑪多重編碼發射序列在解碼過程中能有條件地完整移除二階諧波信號，但發射編碼波形若受記憶效應影響，當前位元波形會受前級位元波形影響，導致發射波形拉長及失真，而該受記憶效應影響之失真波形經非線性傳遞後，其二次諧波成分將無法完整地被移除，導致其組織抑制能力受限。本篇論文採用四階哈達瑪矩陣構建發射序列，在解碼後根據選擇的解碼序列進行平移相消法可得到SH1、SH1、SH3三種組織抑制方法，其中以SH3之效果最佳；哈達瑪矩陣同時也可構建格雷正交式編碼對，而為了抑制二次諧波及記憶效應的干擾我們將正交解碼結合脈衝反相並將此法法稱做PIHS-OD，其中PIHS-OD能有效地抑制記憶效應。實驗採用水聽筒及B-mode影像進行驗證。結果方面以單一脈衝及脈衝反相之振幅調變方法( PIAM )做為本論文之CTR比較基準，其中正交解碼中最好的方法PIHS-OD相對於SH3其CTR高出約5.4 dB。

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.

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