The Variation Of Frequency Locations In Doppler Ultrasound Spectra For Maximum Blood Flow Velocities In Narrowed Vessels

In recent years, the sick rate and discovery rate of cardiac and cerebro vascular diseases have been increasing with the development of life level and medical treatment technology. These are important facts that vascular diseases, narrowed neck vascular and atherosclerosis that can lead to cardiac and cerebro vascular diseases. In clinic, it is a usual and important method that Ultrasound detection technology is used to detect arterial vascular. However, the Ultrasound detection technology used nowadays can’t correctly extract the blood flow velocity in narrowed vessel and rate of vascular diseases. Therefore, it is meaningful and useful to accurately estimate maximum blood flow velocity and research the relationship in the vessels, especially for those with different degrees of stenosisThe variation of maximum blood flow velocity is also called the maximum frequency shift of Doppler ultrasound. It is a widely used method in clinics and researches that the maximum frequency is determined according to the Doppler blood flow spectrum, and then the maximum blood flow velocity is calculated with the Doppler equation. Commonly used algorithms include simple threshold method (STM), percentile method (PM), threshold-crossing method (TCM), modified threshold-crossing method (MTCM), and hybrid method (HM). Although these methods mentioned above are simple and easy to realize, because of intrinsic spectral broadening and estimated spectral window broadening, it is difficult to objectively and accurately confirm the maximum frequency from the Doppler blood flow spectrum. Moreover, the Doppler blood flow signals are easy to be interfered by noise in practice, and the blood flow spectrum could be polluted by noise component with different levels. In this case, the validity and reliability of maximum blood flow velocity based on the maximum frequency extracted from the polluted spectrum could be limited.In order to overcome the deficiency of these traditional methods, David et al. proposed a novel approach to find the maximum blood flow velocity in normal (stenosis free) vessel from its Doppler spectrum. The results show that the maximum frequency location is at the middle position of the falling edge for the normal vessel. Additionally, the noise pollution in the spectrum can be offset in the calculation process because the position at the spectral falling-edge is confirmed by a substraction operation between the maximum and minimum power values. Therefore, noise interference can be avoided effectively, and the validity and reliability of this approach are higher than those based on the traditional methods. However, this research is aimed at the normal vessel, in which blood flow is assumed as parabolic distribution of steady flow conditions. In clinical practice, the diseased vessels may be narrowed, which leads to blood flow status become more complicated, and its spectrum differing with that from normal vessel is broadened. The result for the normal vessel mentioned above is no more applicable. Thus it is necessary to investigate the variation between the maximum velocities of blood flow and their frequency locations at the Doppler spectra in the stenotic vessels.In this paper, the relationship between the blood flow velocity and Doppler power spectrum in stenosed vessel is investigated to obtain the variation between the maximum velocities and their frequency locations at the spectra in the vessels with different degrees of stenosis. Firstly, a geometric vessel-model with stenosis of bilateral axial-symmetry is established to get the flow velocity distributions in an analytic form for different degrees of stenosis according to Navier-Stokes equations (NSEs). In computational analysis, the particles distributed in the vessel segment are moving according to the obtained velocity distributions. During observing time interval, the power spectrum of windowed signal for each particle is the square of line spectrum, whose frequency corresponds to moving velocity, multiplied by Sinc function, and then accumulated to yield the Doppler spectra from entire segment of the vessel covered by sound field. Finally, corresponding maximum frequency locations in the spectra are determined according to a cross point between the maximum values chosen from blood flow velocity distributions and their calculated spectra. In the theoretical calculation, the locations of normal,5%,10%,20%,30% and 40% degrees of stenosis are confirmed separately, and the variation between the maximum velocities and their frequency locations at the spectra in the vessels with different degrees of stenosis are acquired. In order to verify the theoretical analysis and computation results, the simulation experiments based on the Field II software platform are implemented. A dynamic scatterer phantom of vessel with different degrees of stenosis is established to simulate the Doppler ultrasound blood flow signals by the Field II. The corresponding spectra are calculated by using short time Fourier transform (STFT), and the locations of maximum frequencies in STFT based spectra are confirmed. The experimental results are compared with those of the theoretical calculation for validation.