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Motion Artifacts of Pulse Inversion-Based Tissue Harmonic Imaging
No full textMotion artifacts of the pulse inversion technique
were studied for finite amplitude distortion-based
harmonic imaging. Motion in both the axial and the lateral
directions was considered. Two performance issues were investigated.
One is the harmonic signal intensity relative to
the fundamental intensity and the other is the potential image
quality degradation resulting from spectral leakage. A
one-dimensional [ 1-D) correlation-based correction scheme
also ,was used to compensate for motion artifacts. Results
indidated that the tissue harmonic signal is significantly affected
by tissue motion. For axial motion, the tissue harmonic
intensity decreases much more rapidly than with lateral
,motion. The fundamental signal increases for both axial
and lateral motion. Thus, filtering is still required to remove
the fundamental signal, even if the pulse inversion technique
is aiplied. The motion also potentially decreases contrast
resolution because of the uncancelled spectral leakage. Also,
it was indicated that 1-D motion correction is not adequate
if nonaxial motion is present
Optimal Transmit Phasing for Harmonic-Background Suppression with Bipolar Square-Wave Pulser
No full textPulse Inversion Based Ultrasonic Nonlinear Imaging
No full text本論文主要針對脈衝反相技術(pulse inversion)在醫用超音波非線性影像(nonlinear imaging)上的應用與限制進行探討，脈衝反相技術的原理是發射一組相位相反的正負脈衝並將所收到的回音相加來擷取信號中的非線性成分，其優點是能夠大幅提升超音波非線性影像中的對比度。
本論文分成三大主題，第一部份是脈衝反相技術對組織諧波影像(tissue harmonic imaging)中諧波溢漏現象(spectral leakage)的影響。由於組織諧波影像一般是以濾波方式來擷取非線性信號，因此當超音波系統在諧波影像的接收頻帶因發射波型或系統非線性而有溢漏諧波存在時，會對組織諧波信號產生干擾而影響組織諧波影像的對比。我們的研究是第一個對溢漏諧波現象做了完整探討的文獻，包含發射信號的封包與頻寬對影像品質的影響。此外組織不均勻(tissue inhomogeneities)與影像系統本身的非線性度也是研究的重點。結果發現諧波溢漏現象對組織諧波影像品質有很大的影響，此時若使用脈衝反相技術不但可以提升影像對比度，同時也能保有足夠信號頻寬以及降低發射信號的設計複雜度。
但脈衝反相技術的限制在於影像物體可能在正負兩次發射中位移而產生移動雜訊干擾(motion artifacts)，此時所有線性信號會因相消不完全而大幅增加。在本論文第二部份中我們將移動雜訊的影響分成信號強度以及影像品質兩個方面來探討，結果顯示在移動雜訊嚴重時會造成原本應該消除的基頻信號大幅上升，故脈衝反相技術仍須適當的濾波才能進行組織諧波成像，此外無法相消的溢漏諧波也會造成影像對比度的下降。
在本論文的第三部份我們則率先提出了一個以脈衝反相為基礎的創新基頻影像技術(pulse inversion fundamental imaging)來增強超音波對比劑影像(contrast imaging)上對血液灌流區的偵測能力，結果發現脈衝反相基頻影像相較於傳統之基頻影像或二次諧波影像均能大幅加血液灌流區和周遭組織的對比度(contrast-to-tissue ratio)，此外不同特性的發射波形所誘發的震盪差異也可被運用於進一步增強此影像方法在對比劑偵測上的表現，但由於移動雜訊會大幅增加組織基頻信號的強度而降低血液和組織間的對比度，故適當修正移動雜訊對脈衝反相基頻影像的對比劑偵測能力十分重要。此方法不僅在臨床應用上極具價值，亦可延伸至超音波分子影像之領域。
本論文在附錄中也介紹了以van Cittert-Zernike理論為基礎的組織諧波影像的空間相關度(spatial covariance)分析，結果顯示諧波空間相關度受限於訊雜比而偏低，因此組織諧波影像應該結合線性信號來進行相位誤差修正(phase aberration correction)才能進一步提升影像品質。The purpose of this dissertation is to investigate various issues in pulse inversion (PI) based ultrasonic nonlinear imaging. In PI technique, two phase-inverted transmissions are required for each beam. The linear signal is cancelled by summing both echoes together and the nonlinear signal remains in the sum. Image contrast can be significantly improved by the PI technique in ultrasonic nonlinear imaging.
The first topic in the dissertation is spectral leakage in nonlinear tissue imaging. Conventional nonlinear imaging suffers from contrast degradation due to spectral leakage when non-negligible harmonic components are present prior to propagation. The leakage signal cannot be filtered out from the received signal due to spectral overlap with the nonlinear signal of interest. Our study is the first complete report in the literature to investigate issues related to spectral leakage in nonlinear tissue imaging. Effects of transmit signal, including envelope and bandwidth, are investigated. Nonlinearity of imaging system and tissue inhomogeneities are also discussed. It is found that the image contrast significantly decreases when spectral leakage is present. The PI technique can restore the image contrast by canceling the leakage signal in the sum, and the trade-off between axial resolution and image contrast is also avoided.
Despite of its advantages, the PI technique suffers from frame-rate reduction and potential motion artifacts. Motion artifacts may be critical when there is relative movement between the probe and the imaged tissue during the two firings. In this case, the linear signal increases because of incomplete cancellation in the sum. Effects of motion artifacts on signal intensity and image quality in nonlinear tissue imaging are also studied in the dissertation. It is shown that filtering is still necessary in PI imaging to suppress the uncancelled fundamental signal. Residue spectral leakage also degrades image contrast in the presence of motion artifacts.
In the third part of the dissertation, we propose a new PI-based imaging method for contrast nonlinear imaging by using signals in the fundamental band. This method is also known as the PI fundamental imaging. Compared to that obtained using either conventional fundamental imaging or second-harmonic imaging, the contrast-to-tissue ratio (CTR) is significantly improved in PI fundamental imaging. However, motion compensation is essential because the CTR decreases rapidly due to uncancelled tissue fundamental signal when the tissue moves. In addition to its clinical applications, PI fundamental imaging can also be extended to molecular imaging when microbubbles are used as molecular probes.
In the appendix, additionally spatial covariance analysis based on the van Cittert-Zernike theorem is also provided for tissue harmonic imaging. It is suggested that the linear signal at lower frequency should be used for effective correction of phase aberration in tissue harmonic imaging.摘要 ……………………………………………………………I
ABSTRACT …………………………………………………..II
CONTENTS …………………………………………………IV
LIST OF FIGURES .………………………………………..VI
LIST OF TABLES .……………………………………….. XII
CH 1. INTRODUCTION …………………………………… 1
1-1 Basics of Ultrasonic Tissue Harmonic Imaging …….……..2
1-2 Issue of Low SNR …………………………………………6
1-3 Spectral Leakage and Pulse Inversion Technique ……….. 10
1-4 Scope and Structure of the Dissertation…………………...13
CH 2. SPECTRAL LEAKAGE IN TISSUE HARMONIC IMAGING ……………………………………………14
2-1 Transmit Waveforms and Harmonic Beam Patterns ……..14
2-2 Effect of Harmonic leakage in Inhomogeneous Tissue …..20
2-3 Hydrophone Measurements ………………………………22
2-4 Concluding Remarks ……………………………………..27
CH 3. MOTION ARTIFACTS OF PULSE INVERSION BASED TISSUE HARMONIC IMAGING ……….. 29
3-1 Motion Effects on Signal Intensity ……………………… 29
3-2 Motion Effects on Spectral Leakage …………………….. 36
3-3 Discussions and Conclusions ……………………………..41
CH 4. PULSE-INVERSION-BASED FUNDAMENTAL IMAGING FOR CONTRAST DETECTION …….. 46
4-1 Introduction to Ultrasound Contrast Agents ……………… 46
4-2 Basics of PI-based Contrast Detection …………………… 48
4-3 Methods …………………………………………………. 50
4-4 Experimental Verification of PI Fundamental Imaging …. 52
4-5 Performance Issues of PI Fundamental Imaging ………... 54
A. Effects of third-order nonlinearity of tissue ………………55
B. Effects of the transmit pulse length ……………………… 57
C. Motion artifacts……………………………………………59
D. Effects of the driving waveform…………………………. 61
4-6 Discussion and Concluding Remarks …………………….62
CH 5. CONCLUSIONS AND FUTURE WORKS ……….. 64
APPENDIX …………………………………………………. 67
TISSUE HARMONIC IMAGES ANALYSIS BASED ON SPATIAL COVARIANCE
A-1 Introduction to Spatial Covariance ………………………67
A-2 Simulation Method and Experimental Setup …………… 68
A-3 Effect of SNR on Spatial Covariance ……………………71
A-4 Effect of Sound Velocity Inhomogeneities ………………77
A-5 Conclusions and Discussions …………………………….81
REFERENCES …………………………………………….. 83
PUBLICATION LIST..…………………………………….. 8
The Effect of Third Harmonic Transmit Phasing on Contrast Agent Responses for Ctr Improvement
No full textHarmonic detection of ultrasonic contrast agents provides a limited contrast-to-tissue ratio (CTR) mainly because of background tissue harmonic signals. To suppress tissue harmonic amplitude, the method of third harmonic (3f(0)) transmit phasing can be utilized by phasing an additional 3f (0) transmit signal to achieve effective cancellation between the frequency-sum component and the frequency- difference component of the tissue harmonic signal. For contrast agents, on the other hand, our results indicate that the nonlinear responses from microbubbles behave very differently in 3f(0) transmit phasing. Specifically, the suppression phase for the contrast microbubble deviates from that for tissue. Moreover, the achievable suppression of the contrast harmonic signal is markedly reduced. Consequently, with the selection of 3f(0) transmit phase that leads to maximal tissue suppression, the CTR can be optimally improved in harmonic imaging. Nevertheless, the CTR improvement in 3f(0) transmit phasing can be compromised when the tissue suppression decreases in the presence of spectral leakage. Consequently, multi-pulse sequences such as pulse inversion are suggested for leakage removal in 3f( 0 ) transmit phasing
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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