相控诊断超声成像波束控制技术研究
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摘要
超声波在生物体中传播时，组织特性差异导致回波信号的幅度、相位、时间等参量发生改变。通过对回波信号的转换、重建或反演处理，诊断超声成像技术可获得待测生物组织结构信息。作为一种无创诊断手段，诊断超声成像技术已成为医学临床诊断不可或缺的常规诊断技术之一。
相控诊断超声成像技术采用电子聚焦与扫描技术，在整个视场范围内均可获得具有良好的时间特性与空间特性的聚焦波束，经相控波束形成重建声像图具有分辨率高、动态范围大、几何失真小等特点。因此，与传统超声成像技术相比，相控诊断超声成像技术可显著提高成像质量。本文从诊断超声数据处理前端入手，针对相控诊断超声成像中的波束设计及实现技术进行了深入研究，提出了三种不同复杂度的波束形成算法，提高成像空间分辨率、时间分辨率与均匀性，最后给出了一种48通道高集成度、可编程的、可满足波束形成算法研究与新兴超声临床应用研究的相控诊断超声成像系统设计方案，解决了其中的主要关键技术并进行了验证。本文主要内容包括以下几个方面：
(1)对超声成像理论基础进行了论述，根据波动方程给出了不同形状换能器单频连续波辐射声场与脉冲激励声场计算模型，并对影响辐射声场特性的因素及改善方法进行了分析。在此基础上，给出了相控脉冲回波成像模型，仿真分析表明根据接收回波信号可以完全重构由散射介质引起的声场变化，通过改善声场分布特征可显著提高成像质量。
(2)全面分析了相位控制精度、聚焦控制与波束指向性控制对于提高成像动态范围与空间分辨率的关系，并据此提出了一种可成倍提高成像时间分辨率的实时波束形成算法。该方法采用4并行接收波束使得成像帧频提高4倍，同时通过抑制发射波束旁瓣幅度与控制主瓣宽度获得高质量的图像。仿真结果表明采用该方法成像对比度明显优于常规延时累加波束形成算法。
(3)提出了一种基于子空间法的低复杂度稳健自适应波束形成算法。该方法首先采用Toeplitz矩阵预处理结合特征值重构法提高采样协方差矩阵估计精度，然后采用特征子空间波束形成法实现孔径权值的自适应计算。针对点散射目标与囊肿目标仿真实验表明，该方法不仅可有效解决回波信号相干干扰问题，避免了传统空域平滑算法导致的有效孔径损失，而且可实现侧向分辨率与对比度的同时提高。
(4)研究了非线性最小二乘支持向量回归算法在波束形成中的应用。该方法采用二次代价函数取代传统最小方差无失真响应波束形成算法中的线性约束条件，显著提高存在模型失配时波束形成器的稳健性与干扰抑制能力，采用高斯核函数则使得算法的泛化能力得以扩展，然后，推导了解的快速迭代求解方法，使得算法的实时运行得以实现。最后，基于奇异性准则提出了一种可有效削减解的规模的稀疏化算法。仿真结果表明该算法在导向矢量失配、复杂多干扰与样本快拍数受限等情况下，不仅可保持良好的主瓣指向性，而且旁瓣幅度明显低于传统自适应波束形成算法。
(5)采用模块化设计思想与可重构逻辑设计技术，设计了一套高性能、高集成度的、可满足波束形成算法研究与新兴超声临床应用技术研究需要的相控超声成像平台。该平台可产生48独立通道、脉冲宽度与脉冲数可调的发射脉冲与实现48路回波信号的12bit精度50MHz采样。在发射时，采用两级可变延时控制结构，在较低系统工作频率下使发射脉冲延时控制精度达1.25ns；在接收时，采用基于最小均方误差的分数时延多相滤波技术提高接收相位控制精度与实现逐点动态聚焦。此外，对于相控诊断超声波束形成中的发射脉冲设计技术、扫描控制技术、多通道同步数据采集技术、波束形成器设计与实现技术进行了详尽阐述。
作为相控阵列成像技术基础研究的一部分，本文研究成果可为多种新兴成像技术的研究提供技术支撑。此外，本文研究还可为开发商业化的高性能医学影像系统提供理论依据和实践参考。
For ultrasound propagating through the organism, the character divergence ofbiological tissues results in the parameter variation of returned sound waves, such asamplitude, phase, and time. The returned waves are recorded, transformed, or be reverseprocessed to retrieve the tissue information. Thus, the images of the biological tissuescan be obtained. Because of its noninvasive, medical ultrasound imaging technology hasalready been widely accepted as one of the most important diagnostic methods inclinical.
Phased array ultrasound imaging technology uses electronic focusing and scanningtechnologies to form beams with improved spatial and time characteristics in the wholemeasure area, which results reconstructed images with enhanced spatial resolution,extended detection dynamic range, and reduced geometry distortion. Thus, the imagingquality can be greatly improved compared with traditional ultrasound imaging methods.In this dissertation, the beamforming algorithms and its implementation details arethoroughly studied and analyzed. We also proposed three beamforming approaches，with different computation complexity, to improve the spatial resolution, time resolutionand uniformity of the images. Finally, a highly integrated, programmable array medicalultrasound imaging architecture consisting48independent channels, which can meet therequirements of medium-level ultrasound machines while providing a flexible platformfor supporting the development of new algorithms and emerging clinical applications ispresented. In summary, the main contributions of this dissertation are as follows:
(1) The theory of ultrasound imaging and the model for calculating the field profilefrom arbitrarily shaped transducers under continuous wave and pulsed excitation arediscussed. The factors affecting the pressure fields and the improving methods are alsopresented. Finally, the imaging model based on phased pulse-echo fields is given. Thesimulation results show that the filed profile changes that were induced by scatterslocated in the front of transducer can be reconstructed by the returned waves, andenhanced image quality can be obtained by improving the directivity of the pulse-echofields.
(2) The relationship between imaging dynamic range, spatial resolution and phasecontrol accuracy, focusing methods, beam directivity are discussed. A real-timebeamforming approach which can increase the fame-rate without scarifying imagingquality is proposed. This method uses1/4th transmit beams combined with four parallelreceive beams to increase the frame-rate by a factor of four. Mainlobe width controllingand sidelobes suppressing are also used to improve spatial resolution. Simulation resultsdemonstrate that improved imaging quality can be obtained comparing with traditionaldelay-and-sum bamforming methods.
(3) A low computation complexity subspace-based adaptive bemaformingalgorithm is presented. In this method, Toeplitz matrix preprocessing and eigenvaluesreconstruction are employed to get a good estimation of array covariance matrix, whichis then employed in minimum variance (MV) weights calculation. Simulations on pointtargets and cyst demonstrate that the proposed bemaforming method can effectivelyimprove the performance of the beamformer in coherent interference environment. Theaperture loss results from spatial smoothing used by traditional beamformer can beavoided. Thus, the lateral resolution and contrast of beamforming images can besimultaneously improved.
(4) The LS-SVM algorithm is extended for solving robust beamforming problems.In this approach, a squared-loss function is used to replace the conventional linearlyconstrained minimum variance cost function, which could significantly increaserobustness against mismatch problems and provide additional control over the sidelobelevel. Then, Gaussian kernels function is applied to the array observations to improvethe generalization capacity. Finally, a recursive regression procedure is presented to findthe solutions on real-time. Model reduction to reduce the final size of the beamformerwas also presented in the proposed approach. The test results show that the proposedbeamforming method significantly outperforms many other recently proposed linearrobust beamforming techniques in terms of signal distortion in the desired signal andnoise reduction in scenarios with DOA mismatch, limited observation samples, andnumerous interferences.
(5) A programmable, highly integrated, phased array medical imaging architecture，which can support the development of new algorithms and emerging clinicalapplications is presented. This system can provide48independent transmit pulses. The pulse width and the number of transmit pulse can also be adjusted. The sample accuracyof the system is12bit and the sample rate can reach to50MHz. A two level variabletime delay technology is used to increase phase control accuracy of the transmit pulse to1.25ns at low system clock frequency. A fractional time delay polyphase filter based onleast square error is also provided to improve receving time resolution and realizedynamic focus. In addition, the plan of transmit pulse, the technology of scancontrolling, the method of mult-channel data synchronous sampling, and the design ofbeamformer are also discaussed with detail.
As one part of phased array imaging technology, the research results in thisdissertation may lay a foundation for the development of beamforming algorithms andproviding some suggestions for the design of clinical applications from theoretical andpractical angle.
相控诊断超声成像技术采用电子聚焦与扫描技术，在整个视场范围内均可获得具有良好的时间特性与空间特性的聚焦波束，经相控波束形成重建声像图具有分辨率高、动态范围大、几何失真小等特点。因此，与传统超声成像技术相比，相控诊断超声成像技术可显著提高成像质量。本文从诊断超声数据处理前端入手，针对相控诊断超声成像中的波束设计及实现技术进行了深入研究，提出了三种不同复杂度的波束形成算法，提高成像空间分辨率、时间分辨率与均匀性，最后给出了一种48通道高集成度、可编程的、可满足波束形成算法研究与新兴超声临床应用研究的相控诊断超声成像系统设计方案，解决了其中的主要关键技术并进行了验证。本文主要内容包括以下几个方面：
(1)对超声成像理论基础进行了论述，根据波动方程给出了不同形状换能器单频连续波辐射声场与脉冲激励声场计算模型，并对影响辐射声场特性的因素及改善方法进行了分析。在此基础上，给出了相控脉冲回波成像模型，仿真分析表明根据接收回波信号可以完全重构由散射介质引起的声场变化，通过改善声场分布特征可显著提高成像质量。
(2)全面分析了相位控制精度、聚焦控制与波束指向性控制对于提高成像动态范围与空间分辨率的关系，并据此提出了一种可成倍提高成像时间分辨率的实时波束形成算法。该方法采用4并行接收波束使得成像帧频提高4倍，同时通过抑制发射波束旁瓣幅度与控制主瓣宽度获得高质量的图像。仿真结果表明采用该方法成像对比度明显优于常规延时累加波束形成算法。
(3)提出了一种基于子空间法的低复杂度稳健自适应波束形成算法。该方法首先采用Toeplitz矩阵预处理结合特征值重构法提高采样协方差矩阵估计精度，然后采用特征子空间波束形成法实现孔径权值的自适应计算。针对点散射目标与囊肿目标仿真实验表明，该方法不仅可有效解决回波信号相干干扰问题，避免了传统空域平滑算法导致的有效孔径损失，而且可实现侧向分辨率与对比度的同时提高。
(4)研究了非线性最小二乘支持向量回归算法在波束形成中的应用。该方法采用二次代价函数取代传统最小方差无失真响应波束形成算法中的线性约束条件，显著提高存在模型失配时波束形成器的稳健性与干扰抑制能力，采用高斯核函数则使得算法的泛化能力得以扩展，然后，推导了解的快速迭代求解方法，使得算法的实时运行得以实现。最后，基于奇异性准则提出了一种可有效削减解的规模的稀疏化算法。仿真结果表明该算法在导向矢量失配、复杂多干扰与样本快拍数受限等情况下，不仅可保持良好的主瓣指向性，而且旁瓣幅度明显低于传统自适应波束形成算法。
(5)采用模块化设计思想与可重构逻辑设计技术，设计了一套高性能、高集成度的、可满足波束形成算法研究与新兴超声临床应用技术研究需要的相控超声成像平台。该平台可产生48独立通道、脉冲宽度与脉冲数可调的发射脉冲与实现48路回波信号的12bit精度50MHz采样。在发射时，采用两级可变延时控制结构，在较低系统工作频率下使发射脉冲延时控制精度达1.25ns；在接收时，采用基于最小均方误差的分数时延多相滤波技术提高接收相位控制精度与实现逐点动态聚焦。此外，对于相控诊断超声波束形成中的发射脉冲设计技术、扫描控制技术、多通道同步数据采集技术、波束形成器设计与实现技术进行了详尽阐述。
作为相控阵列成像技术基础研究的一部分，本文研究成果可为多种新兴成像技术的研究提供技术支撑。此外，本文研究还可为开发商业化的高性能医学影像系统提供理论依据和实践参考。
For ultrasound propagating through the organism, the character divergence ofbiological tissues results in the parameter variation of returned sound waves, such asamplitude, phase, and time. The returned waves are recorded, transformed, or be reverseprocessed to retrieve the tissue information. Thus, the images of the biological tissuescan be obtained. Because of its noninvasive, medical ultrasound imaging technology hasalready been widely accepted as one of the most important diagnostic methods inclinical.
Phased array ultrasound imaging technology uses electronic focusing and scanningtechnologies to form beams with improved spatial and time characteristics in the wholemeasure area, which results reconstructed images with enhanced spatial resolution,extended detection dynamic range, and reduced geometry distortion. Thus, the imagingquality can be greatly improved compared with traditional ultrasound imaging methods.In this dissertation, the beamforming algorithms and its implementation details arethoroughly studied and analyzed. We also proposed three beamforming approaches，with different computation complexity, to improve the spatial resolution, time resolutionand uniformity of the images. Finally, a highly integrated, programmable array medicalultrasound imaging architecture consisting48independent channels, which can meet therequirements of medium-level ultrasound machines while providing a flexible platformfor supporting the development of new algorithms and emerging clinical applications ispresented. In summary, the main contributions of this dissertation are as follows:
(1) The theory of ultrasound imaging and the model for calculating the field profilefrom arbitrarily shaped transducers under continuous wave and pulsed excitation arediscussed. The factors affecting the pressure fields and the improving methods are alsopresented. Finally, the imaging model based on phased pulse-echo fields is given. Thesimulation results show that the filed profile changes that were induced by scatterslocated in the front of transducer can be reconstructed by the returned waves, andenhanced image quality can be obtained by improving the directivity of the pulse-echofields.
(2) The relationship between imaging dynamic range, spatial resolution and phasecontrol accuracy, focusing methods, beam directivity are discussed. A real-timebeamforming approach which can increase the fame-rate without scarifying imagingquality is proposed. This method uses1/4th transmit beams combined with four parallelreceive beams to increase the frame-rate by a factor of four. Mainlobe width controllingand sidelobes suppressing are also used to improve spatial resolution. Simulation resultsdemonstrate that improved imaging quality can be obtained comparing with traditionaldelay-and-sum bamforming methods.
(3) A low computation complexity subspace-based adaptive bemaformingalgorithm is presented. In this method, Toeplitz matrix preprocessing and eigenvaluesreconstruction are employed to get a good estimation of array covariance matrix, whichis then employed in minimum variance (MV) weights calculation. Simulations on pointtargets and cyst demonstrate that the proposed bemaforming method can effectivelyimprove the performance of the beamformer in coherent interference environment. Theaperture loss results from spatial smoothing used by traditional beamformer can beavoided. Thus, the lateral resolution and contrast of beamforming images can besimultaneously improved.
(4) The LS-SVM algorithm is extended for solving robust beamforming problems.In this approach, a squared-loss function is used to replace the conventional linearlyconstrained minimum variance cost function, which could significantly increaserobustness against mismatch problems and provide additional control over the sidelobelevel. Then, Gaussian kernels function is applied to the array observations to improvethe generalization capacity. Finally, a recursive regression procedure is presented to findthe solutions on real-time. Model reduction to reduce the final size of the beamformerwas also presented in the proposed approach. The test results show that the proposedbeamforming method significantly outperforms many other recently proposed linearrobust beamforming techniques in terms of signal distortion in the desired signal andnoise reduction in scenarios with DOA mismatch, limited observation samples, andnumerous interferences.
(5) A programmable, highly integrated, phased array medical imaging architecture，which can support the development of new algorithms and emerging clinicalapplications is presented. This system can provide48independent transmit pulses. The pulse width and the number of transmit pulse can also be adjusted. The sample accuracyof the system is12bit and the sample rate can reach to50MHz. A two level variabletime delay technology is used to increase phase control accuracy of the transmit pulse to1.25ns at low system clock frequency. A fractional time delay polyphase filter based onleast square error is also provided to improve receving time resolution and realizedynamic focus. In addition, the plan of transmit pulse, the technology of scancontrolling, the method of mult-channel data synchronous sampling, and the design ofbeamformer are also discaussed with detail.
As one part of phased array imaging technology, the research results in thisdissertation may lay a foundation for the development of beamforming algorithms andproviding some suggestions for the design of clinical applications from theoretical andpractical angle.
引文
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