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Tomographic reconstruction of sound velocity distribution in the Breast using linear arrays
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Experimental investigation of computed tomography sound velocity reconstruction using incomplete data
No full text[[abstract]]An approach for reconstructing the sound velocity distribution in the breast was previously proposed and verified by simulations, and the present study investigated the approach experimentally. The experimental setup comprised a 5-MHz, 128-channel linear array, a programmable digital array system, a phantom containing objects with differing physical properties, and a computer. The array system was used to collect channel data for simultaneous B-mode image formation and limited-angle tomographic sound velocity reconstruction. The phantom was constructed from materials mimicking the following tissues in the breast: glandular tissue, fat, cysts, high-attenuation tumors, and irregular tumors. The sound velocities in these materials matched those in the corresponding real tissues. The imaging setup is similar to that of x-ray mammography, in which a linear array is placed at the top of the breast and a metal plate is placed at the bottom for reflecting sound waves. Thus, both B-mode images and the sound velocity distribution can be acquired using the same setup. An algorithm based on a convex programming formulation was used to reconstruct the sound velocity images. By scanning the phantom at different positions, nine cases were evaluated. In each of the nine cases, the image object comprised a background (glandular tissue) and one or three regions of interest (fat, tumor, or cyst). The sound velocity was accurately estimated in the nine cases evaluated, with sound velocity errors being less than 5 m/s in 8 of 11 regions of interest. Thus, obtaining the sound velocity distribution is feasible with a B-mode imaging setup using linear arrays. Knowledge of the sound velocity distribution in the breast can be used to complement B-mode imaging and to enhance the detection of breast cancer
Arbitrary waveform coded excitation using bipolar square wave pulsers in medical ultrasound
No full text[[abstract]]This paper presents a new coded excitation scheme that efficiently synthesizes codes for arbitrary waveforms using a bipolar square wave pulser. In a coded excitation system, pulse compression is performed to restore the axial resolution. In order to maintain low range sidelobes, the system needs to transmit signals that have smooth spectra. However, such a transmitter requires the generation of arbitrary waveforms and, therefore, is more expensive. In other words, a trade-off is necessary between the compression performance and the transmitter cost. Here we propose a method that preserves the low-cost advantage of a bipolar pulser while achieving approximately the same compression performance as an arbitrary waveform generator. The key idea of the proposed method is the conversion of a nonbinary code (i.e., requiring an arbitrary waveform generator) with good compression performance into a binary code (i.e., requiring only a bipolar pulser) by code translation and code tuning. The code translation is implemented by sending the nonbinary code into a virtual one-bit, sigma-delta modulator, and the code tuning involves minimizing the root-mean-square error between the resultant binary code and the original nonbinary code by sequential and iterative tuning while taking the transducer response into account. Tukey-windowed chirps are known to have good compression performance. Such chirps of different durations (16, 20, and 24 ps), all with a taper ratio of 0.15, a center frequency of 2.5 MHz, and an equivalent bandwidth of 1.5 MHz, were converted into binary Tukey-windowed chirps that were compared with pseudochirps (i.e., direct binary approximations of the original chirp) over the same spectral band. The bit rate was 40 MHz. Simulation results show that the use of binary Tukey-winclowed chirps can reduce the code duration by 20.6% or the peak sidelobe level by 6 dB compared to the commonly used pseudochirps. Experimental results obtained under the same settings were in agreement with the simulations. Our results demonstrate that arbitrary waveform coded excitation can be realized using bipolar square wave pulsers for applications in medical ultrasound
Ultrasonic computed tomography reconstruction of the attenuation coefficient using a linear array
No full text[[abstract]]The attenuation coefficient distribution and sound velocity distribution in the breast can be used to complement B-mode ultrasound imaging in the detection of breast cancer. This study investigated an approach for reconstructing the attenuation coefficient distribution in the breast using a linear array. The imaging setup was identical to that for conventional B-mode breast imaging, and the same setup has been used for reconstruction of sound velocity distributions in previous studies. In this study, we further developed a reconstruction method for the attenuation coefficient distribution. In particular, the proposed method incorporates the segmentation information from B-mode images and uses the sound velocity distribution to compensate for refraction effects. Experiments were conducted with a setup consisting of a 5-MHz, 128-channel linear array, a programmable digital array system, a phantom, and a computer. The constructed phantom contained materials mimicking the following breast tissues: glandular tissue, fat, cysts, high-attenuation tumors, and irregular tumors. Application of the proposed technique resulted in all the cysts and tumors (including high-attenuation and irregular tumors) being distinguished by thresholding the reconstructed attenuation coefficients. We have demonstrated that it is possible to use the same imaging setup to acquire data for B-mode image, sound velocity distribution, and attenuation coefficient distribution simultaneously. Moreover, the experimental data indicate its potential in improving the detection of breast cancer
Computed tomography sound velocity reconstruction using incomplete data
No full text[[abstract]]An approach based on limited-angle transmission tomography for reconstruction of the sound velocity distribution in the breast is proposed. The imaging setup is similar to that of x-ray mammography. With this setup, the time-of-flight data are acquired by a linear array positioned at the top of the compressed breast that both transmits and receives, and a metal plate is placed at the bottom as a reflector. The setup allows acoustic data acquisition for simultaneous B-mode image formation and the tomographic sound velocity reconstruction. In order to improve the sound velocity estimation accuracy, a new reconstruction algorithm based on a convex programming formulation has been developed. Extensive simulations for both imaging and time-of-flight data based on a 5-MHz linear array were performed on tissues with different geometries and acoustic parameters. Results show that the sound velocity error was generally 1-3 m/s, with a maximum of 5.8 m/s. The radii of the objects under investigation varied from 2 to 6 mm, and all of them were detected successfully. Thus, the proposed approach has been shown to be both feasible and accurate. The approach can be used to complement conventional B-mode imaging to further enhance the detection of breast cancer
Full-Bridge Phase-Shifted Driving Circuit with Current Balancing for Cold Cathode Fluorescent Lamps
No full text本論文主要在討論具均流全橋相移式冷陰極螢光燈管驅動電路，由於傳統多燈管系統中，燈管阻抗不一定相同，使流經各燈管的電流差異很大，造成亮度不均勻。本文的目的就是使用平衡變壓器，讓流經各燈管的電流差異變小，使各燈管亮度均勻。本文使用之電路架構為全橋相移式換流器，使用全橋相移式脈波寬度調變控制，使換流器在切換過程達到零電壓切換，降低切換過程的損耗，並詳細說明其動作原理、最後綜合以上的方式實作一台冷陰極螢光燈管驅動電路，以實驗結果驗證理論分析。The aim of this thesis is to study and implement a full-bridge phase-shifted driving circuit with current balancing for cold cathode fluorescent lamps. Because the impedance of each lamp in a traditional multi-lamp system is not necessarily the same, the current through each lamp would be dramatically different. Therefore, the brightness of the system is not uniformly distributed. The aim of this thesis is to utilize balancing transformers to reduce the current differences between each lamp such that the brightness of each lamp maintains uniform. The circuit architecture in the thesis is full-bridge phase-shifted inverter. It utilizes full-bridge phase-shifted pulse width modulation to achieve zero voltage switching and reduce the switching loss. In the end, a driving circuit for cold cathode fluorescent lamp is demonstrated and the experimental results can verify the theoretical analysis.第一章 緒論
1.1 簡介............................................1
1.2 內容介紹........................................2
第二章 螢光燈介紹
2.1 螢光燈的構造....................................3
2.2 螢光燈的發光原理................................5
2.3 螢光燈的電氣特性................................7
第三章 換流器及諧振網路
3.1 換流器..........................................9
3.1.1 Class-E諧振換流器............................9
3.1.2 半橋式換流器................................10
3.1.3 推挽式換流器................................10
3.1.4 全橋式換流器................................11
3.2 諧振網路.......................................12
3.2.1 串聯諧振串聯負載............................12
3.2.2 串聯諧振並聯負載............................14
3.2.3 串聯諧振串並聯負載..........................15
第四章 全橋相移式換流器
4.1 全橋相移式換流器電路架構.......................18
4.2 柔性切換.......................................21
4.3 全橋相移式換流器動作原理.......................23
第五章 電路設計及實作
5.1 控制電路.......................................33
5.2 燈管電流平衡電路...............................39
5.3 零件選擇.......................................42
5.4 實驗波形.......................................43
第六章 結論與未來展望
6.1 結論...........................................51
6.2 未來展望.......................................51
參考文獻..............................................5
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