光學解析度光聲顯微鏡(OR-PAM)雖然藉由光學聚焦而具有高橫
向解析度，但卻因為其較差的軸向解析度而受到限制。OR-PAM 的軸
向解析度主要取決於超音波感測器的頻寬。為了取得具有等距空間解
析度的影像，使用超過100 MHz 的超高頻超音波探頭並搭配維納
(Wiener)反折積演算法已經被採用，然而卻因為嚴重的高頻衰減而限制
了成像深度和工作距離。在這項研究中，我們改寫最小變異波束成形
以及同調權重等技術，並在不實際增加超音波感測器之頻寬的條件下
使其用於改善OR-PAM 之軸向解析度。這些方法最初被應用於陣列數
據中以改善超音波及光聲陣列成像之橫向解析度。藉由頻譜白化之技
術，我們將最小變異法改寫為在所需深度合成OR-PAM 訊號之各頻率
成分，同時最小化來自其他深度的干擾，從而提高軸向解析度。此外，
我們使用同調權重於計算期望訊號與總訊號能量之比例，並以此縮減
軸向主瓣寬度及抑制軸向旁瓣訊號。我們提出的方法已經在仿體與微
血管成像實驗中被證實優於傳統的維納反折積演算法，並能將中心頻
率為25 MHz 的超音波探頭之軸向解析度由86 m 提高至約61 m 。

Optical resolution photoacoustic microscopy (OR-PAM), though possessing high lateral resolution via optical focusing, has been limited by its poor axial resolution. Its axial resolution mainly depends on the bandwidth of the acoustic detector. To form images with isometric spatial resolution, larger than 100-MHz ultrahigh frequency detectors along with a Wiener deconvolution method has been employed, yet suffering severe high-frequency attenuation and thus limited imaging depth and working distance. In this study, we adapt the minimum-variance (MV) beamforming and coherence factor (CF) weighting techniques to improve axial resolution of OR-PAM without physically increasing the detection bandwidth. These methods are originally proposed to apply over array data in both ultrasound and photoacoustic array imaging to improve lateral resolution. Via the help of spectral whitening, we adapt the MV method to the frequency components which synthesize the OR-PAM signal at a desired depth, minimizing the interference from other depths and thus improving the axial resolution. Moreover, the CF weighting is utilized to indicate the ratio of the desired signal over total energy, which can reduce the range mainlobe width and suppress range sidelobes. The proposed method experimentally outperformed the conventional Wiener deconvolution algorithm, improving the axial resolution from 86 μm to ~61 μm in imaging a polymethylpentene (TPX) plastic phantom and in vivo microvasculature for a 25-MHz detector.

摘要 ................................ ................................ .............................. I
Abstract ................................ ................................ ...................... II
Table of Contents ................................ ................................ .... III
List of Figures ................................ ................................ ........... VI
Chapter 1 Introduction................................ .............................. 1
1.1 Photoacoustic Microscopy ................................ ................................ ................. 1
1.2 Optical-Resolution Photoacoustic Microscopy (OR-PAM) ............................ 2
1.3 Adaptive signal processing ................................ ................................ ................ 3
1.4 Motivation and Purpose ................................ ................................ .................... 4
1.5 Composition of the Thesis ................................ ................................ ................. 5
Chapter 2 Materials and Methods ................................ ........... 7
2.1 Imaging Using OR-PAM ................................ ................................ ................... 7
2.1.1 Laser Pulse Length ................................ ................................ ..................... 7
2.1.2 Electrical Impulse Response ................................ ................................ ...... 8
2.1.3 Spatial Impulse Response ................................ ................................ ........... 9
2.1.4 A-Line Signal Model of an OR-PAM System ................................ ......... 10
2.2 Axial Resolution and Problem Description ................................ .................... 11
2.3 Spectral Domain Minimum-Variance Apodization ................................ ...... 14
2.3.1 Signal Model ................................ ................................ .............................. 14
2.3.2 Spectral Domain Minimum-Variance Apodization ............................... 15
2.3.3 Spectral Whitening ................................ ................................ ................... 17
2.3.4 Estimation of the Covariance Matrix ................................ ...................... 19
2.3.4.1 Spectral Averaging ................................ ................................ ............. 20
2.3.4.2 Forward-Backward Spectral Averaging ................................ .......... 21
2.3.4.3 Diagonal Loading ................................ ................................ ............... 22
2.4 Coherence Weighting ................................ ................................ ....................... 23
2.5 Signal Processing Flow Chart ................................ ................................ ......... 24
Chapter 3 Experimental Results and Discussion .................... 27
3.1 Simulations ................................ ................................ ................................ ....... 27
3.1.1 Zero-Padding ................................ ................................ ............................. 28
3.1.2 Size of the Covariance Matrix ................................ ................................ . 29
3.1.3 Diagonal Loading ................................ ................................ ...................... 31
3.1.4 Comparison Between MV and MVCF ................................ .................... 34
3.2 Experiments ................................ ................................ ................................ ...... 35
3.2.1 TPX Phantom Experiment ................................ ................................ ....... 36
3.2.2 In Vivo Microvasculature Imaging ................................ .......................... 40
Chapter 4 Conclusions and Future Work ............................. 42
4.1 Conclusions ................................ ................................ ................................ ....... 42
4.2 Future Work ................................ ................................ ................................ ..... 43
References ................................ ................................ ................. 44

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