Spectrum of Doppler ultrasound signals from nonstationary blood flow

Ultrasound Vector Flow Imaging-Part I: Sequential Systems

This paper gives a review of the most important methods for blood velocity vector flow imaging (VFI) for conventional sequential data acquisition. This includes multibeam methods, speckle tracking, transverse oscillation, color flow mapping derived VFI, directional beamforming, and variants of these. The review covers both 2-D and 3-D velocity estimation and gives a historical perspective on the development along with a summary of various vector flow visualization algorithms. The current state of the art is explained along with an overview of clinical studies conducted and methods for presenting and using VFI. A number of examples of VFI images are presented, and the current limitations and potential solutions are discussed.

Duplex Doppler ultrasound study of the temporomandibular joint

INTRODUCTION

The anatomy and physiology of the temporomandibular joint can be studied clinically and by diagnostic imaging. Magnetic resonance imaging (MRI), radiography (X-ray) and computed tomography (CT) have thus for many years contributed to the study of the kinetics in the mandibular condyle. However, also duplex Doppler ultrasound (US) examination is widely used in the study of structures during movement, particularly vascular structures.

MATERIALS AND METHODS

A total of 30 patients were referred by the Department of Orthodontics to the Department of Radiological, Oncological and Pathological Sciences, University of Rome "La Sapienza". All patients underwent duplex Doppler ultrasound (US) examination of the temporomandibular joint using Toshiba APLIO SSA-770A equipment and duplex Doppler multi-display technique, which allows simultaneous display of US images and color Doppler signals. A linear phased array probe with crystal elements was used operating at a basic frequency of 6 MHz during pulsed Doppler spectral analysis and 7.5 MHz during US imaging.

RESULTS

In normal patients a regular alternation in the spectral Doppler waveforms was obtained, while in patients with temporomandibular joint meniscus dysfunction there was no regularity in the sum of the Fourier series with an unsteady waveform pattern related to irregular movements of the temporomandibular joint.

CONCLUSIONS

In all cases duplex Doppler US examination proved able to differentiate between normal and pathological patients and among the latter this technique permitted identification of the most significant aspects of the dysfunctional diseases.

Computer aided diagnosis system for breast cancer based on color Doppler flow imaging

Color Doppler flow imaging takes a great value in diagnosing and classifying benign and malignant breast lesions. However, scanning of color Doppler sonography is operator-dependent and ineffective. In this paper, a novel breast classification system based on B-Mode ultrasound and color Doppler flow imaging is proposed. First, different feature extraction methods were used to obtain the texture and geometric features from B-Mode ultrasound images. In color Doppler feature extraction stage, several spectrum features are extracted by applying blood flow velocity analysis to Doppler signals. Moreover, a velocity coherent vector method is proposed based on color coherence vector, which is helpful for designing to the optimize detection of flow indices from different blood flow velocity fields automatically. Finally, a support vector machine classifier with selected feature vectors is used to classify breast tumors into benign and malignant. The experimental results demonstrate that the proposed computer-aided diagnosis system is useful for reducing the unnecessary biopsy and death rate.

Correlation functions and power spectra of Doppler response signals in ultrasonic medical applications

Ultrasound Doppler methods are widely used in clinical practice as prospective investigational tool to study the vascular system and soft biological tissues. Meanwhile, the most general relationship between the power Doppler spectra, spectral characteristics of the scattering fluctuations and the probing ultrasound field parameters for some clinical implementations are still unexplored. Based upon the continuum model of scattering inhomogeneities, a set of the closed-form expressions for the correlation functions and the spectra of Doppler response of soft tissues and blood have been derived. The influence of the correlation among inhomogeneities and the diffusion processes on the Doppler power spectra formed by stationary flows have been examined. Computer simulations of Doppler spectra were performed for different values of correlation radius and diffusion coefficient. With simulation results the effects of the correlation among inhomogeneities and the diffusion processes on the spectral width and mean frequency are established and discussed in respect to turbulent flows. Closed-form expressions for correlation functions and Doppler spectra for the vibrational sonoelastography technique for visualizing malignant tumors in tissues have been derived. Based on the peculiarities of the obtained Doppler spectra, it is shown that the differentiation of soft tissues with respect to the amplitude value of constrained oscillations is feasible. The expressions were derived for the cases of non-stationary accelerated blood movement. It has been found that the frequency dependence reveals solely at a finite time of observation and depends on the initial phase of the accelerated movement.

Accurate Doppler angle estimation for vector flow measurements

Traditional Doppler methods measure only the axial component of the velocity vector. The lack of information on the beam-flow angle creates an ambiguity that can lead to large errors in velocity magnitude estimates. Different triangulation techniques so far have been proposed, which basically perform multiple measurements of the Doppler frequency shift originating from the same region. In this work, an original approach is introduced, in which two ultrasound beams with known relative orientation are directed toward the same vessel, but only one of them is committed to perform a Doppler measurement; the second (reference) beam has the specific task of detecting the beam-flow angle. The latter goal is obtained by accurately identifying the achievement of the target 900 reference-beam-to-flow angle through the inspection of the backscattered Doppler signal spectrum. In transverse flow conditions, in fact, such spectrum is expected to be centered on the zero frequency, and even small deviations from the desired 900 orientation cause noticeable losses of spectral symmetry. Validation of the new method has been performed through experimental tests, which show that the beam-flow angle can be estimated with high accuracy (rms errors lower than 1 degree), and repeatable velocity magnitude measurements are possible. A procedure for automatically tracking the desired orientation by the reference beam is also introduced and shown suitable for implementation in steerable linear array transducers.

Correction for broadening in Doppler blood flow spectrum estimated using wavelet transform

The conventionally used spectral estimation technique for Doppler blood flow signal analysis is short-time Fourier transform (STFT). But this method requires stationarity of the signal during the window interval. Wavelet transform (WT), which has a flexible time-frequency window, is particularly suitable for nonstationary signals. In recently years, the WT has been used to investigate its advantages and limitations for the analysis of Doppler blood flow signals. In these studies, the estimated spectral width of Doppler blood flow signals using the WT might include significant window and nonstationarity broadening errors. These broadening errors of the time-varying spectrum were clearly undesirable since it would tend to mask the effect of flow disturbance on the spectra width. In this paper, a closed form expression for window and nonstationary root-mean-squared (rms) spectral width is given when using the WT to estimate the Doppler blood flow spectrum. The increases in the rms spectral width can be calculated and then the spectral width estimation based on the WT can be corrected.

Doppler ultrasound spectral enhancement using the Gabor transform-based spectral subtraction

Most of the important clinical indices of blood flow are estimated from the spectrograms of Doppler ultrasound (US) signals. Any noise may degrade the readability of the spectrogram and the precision of the clinical indiCes, so the spectral enhancement plays an important role in Doppler US signal processing. A new Doppler US spectral enhancement method is proposed in this paper and implemented in three main steps: the Gabor transform is used to compute the Gabor coefficients of a Doppler US signal, the spectral subtraction is performed on the magnitude of the Gabor coefficients, and the Gabor expansion with the spectral subtracted Gabor coefficients is used to reconstruct the denoised Doppler US signal. The different analysis and synthesis windows are examined in the Gabor transform and expansion. The signal-to-noise ratio (SNR) improvement together with the overall enhancement of spectrograms are examined on the simulated Doppler US signals from a femoral artery. The results show the denoising method based on the orthogonal-like Gabor expansion achieves the best denoising performance. The experiments on some clinical Doppler US signals from umbilical arteries confirm the superior denoising performance of the new method.

Microembolic signal characterization using adaptive chirplet expansion

The adaptive chirplet expansion (ACE) is proposed to characterize high-intensity, transient signals from circulating microemboli. The nonnegative adaptive spectrogram based on the ACE gives a compact representation of the microembolic signal (MES) in joint-time, frequency domain. The mean instantaneous power (MIP) and mean instantaneous frequency (MIF) of MES are estimated from the adaptive spectrogram. Then, several important characteristics of MES, such as embolus-to-blood ratio (EBR), half width maximum (HWM), and embolic signal onset (ESO), are computed from the MIP, and the frequency modulation is examined in the MIF. To validate the new method, we improved the simulation model of the audio Doppler ultrasound signal. Some MESs together with a Doppler ultrasound signal from carotid blood flow are simulated in the simulation study. As a comparison, the adaptive Gabor expansion (AGE) also is implemented on these simulated signals. The experimental results of the simulation study show that the new method, based on the ACE, outperforms the AGE-based method in MES characterization. The consistent conclusion has been confirmed by the clinical study on some clinical MESs.

Angle-dependence and reproducibility of dual-beam vector doppler ultrasound in the common carotid arteries of normal volunteers

Dual-beam vector Doppler has the potential to improve peak systolic blood velocity measurement accuracy by automatically correcting for the beam-flow Doppler angle. Using a modified linear-array system with a split receive aperture, we have assessed the angle-dependence over Doppler angles of 40 degrees -70 degrees and the reproducibility of the dual-beam blood maximum velocity estimate measured in the common carotid arteries (CCA) 1 to 2 cm prior to the bifurcation of 9 presumed-healthy volunteers. The velocity magnitude estimate was reduced by approximately 7.9% as the angle between the transmit beam and the vessel axis was increased from 40 degrees to 70 degrees. With repeat measurements made, on average, approximately 6 weeks apart, the 95% velocity magnitude limits of agreement were as follows: Intraobserver -41.3 to +45.2 cm/s; interobserver -29.6 to +46.8 cm/s. There was an 8.6 cm/s interobserver bias in velocity magnitude. We conclude that the dual-beam vector Doppler system can measure blood velocity within its scan plane with low dependence on angle and with similar reproducibility to that of single-beam systems.

Angle-independent estimation of maximum velocity through stenoses using vector Doppler ultrasound

Categorisation for arterial stenoses treatment is determined primarily by the degree of occlusion, which is often estimated ultrasonically from blood velocity measurements. In current single-beam ultrasound (US) systems, this estimate can suffer from gross errors due to angle-dependence. The purpose of this study was to find out if an experimental dual-beam US system could reduce the angle-dependence of the velocity estimates. We compared four dual-beam velocity estimation algorithms on both a string phantom and straight tube wall-less flow phantoms incorporating symmetrical and asymmetrical stenoses from 0% to 91% by area. The estimated maximum velocity varied, on average, by 7.6% for beam-vessel angles from 40 degrees to 80 degrees. The fluctuation in the magnitude estimate was reduced by a factor of 2.6 using a hybrid single-dual-beam algorithm. We conclude that, when the true velocity lies in the scan plane, the dual-beam system reduces the angle-dependence and, thus, has the potential to improve categorisation of patients with arterial stenoses.

Velocity fluctuation reduction in vector Doppler ultrasound using a hybrid single/dual-beam algorithm

In order to reduce the fluctuations in the velocity magnitude estimate, we propose a modification to the standard algorithm for reconstructing the (two component) vector velocity from the measured Doppler shifts in two directions. This uses the standard dual-beam algorithm, combined with temporal smoothing, to find only the velocity angle, then uses the single-beam algorithm to estimate the velocity magnitude. We present initial data showing the significant reduction in velocity estimate fluctuation that this hybrid method achieves compared to the standard algorithm.