Serbes Gorkem, Gulcur Halil Ozcan, Aydin Nizamettin
Biomedical Engineering Department, Bahcesehir University, Besiktas, Istanbul, Turkey.
Biomedical Engineering Institute, Bogazici University, Kandilli, Istanbul, Turkey.
Med Biol Eng Comput. 2016 Mar;54(2-3):295-313. doi: 10.1007/s11517-014-1224-0. Epub 2014 Nov 12.
Quadrature signals containing in-phase and quadrature-phase components are used in many signal processing applications in every field of science and engineering. Specifically, Doppler ultrasound systems used to evaluate cardiovascular disorders noninvasively also result in quadrature format signals. In order to obtain directional blood flow information, the quadrature outputs have to be preprocessed using methods such as asymmetrical and symmetrical phasing filter techniques. These resultant directional signals can be employed in order to detect asymptomatic embolic signals caused by small emboli, which are indicators of a possible future stroke, in the cerebral circulation. Various transform-based methods such as Fourier and wavelet were frequently used in processing embolic signals. However, most of the times, the Fourier and discrete wavelet transforms are not appropriate for the analysis of embolic signals due to their non-stationary time-frequency behavior. Alternatively, discrete wavelet packet transform can perform an adaptive decomposition of the time-frequency axis. In this study, directional discrete wavelet packet transforms, which have the ability to map directional information while processing quadrature signals and have less computational complexity than the existing wavelet packet-based methods, are introduced. The performances of proposed methods are examined in detail by using single-frequency, synthetic narrow-band, and embolic quadrature signals.
包含同相和正交相位分量的正交信号在科学和工程的各个领域的许多信号处理应用中都有使用。具体而言，用于无创评估心血管疾病的多普勒超声系统也会产生正交格式的信号。为了获得血流方向信息，必须使用非对称和对称相位滤波技术等方法对正交输出进行预处理。这些所得的方向信号可用于检测由小栓子引起的无症状栓塞信号，这些栓子是脑循环中未来可能发生中风的指标。各种基于变换的方法，如傅里叶变换和小波变换，经常用于处理栓塞信号。然而，大多数情况下，由于其非平稳时频特性，傅里叶变换和离散小波变换不适用于栓塞信号的分析。另外，离散小波包变换可以对时频轴进行自适应分解。在本研究中，引入了方向离散小波包变换，它能够在处理正交信号时映射方向信息，并且计算复杂度比现有的基于小波包的方法更低。通过使用单频、合成窄带和栓塞正交信号详细研究了所提出方法的性能。
