Doppler spectrum shape analysis based on the summation of flow-linespectra

A method of evaluating the shape of the Doppler spectrum in pulsed wave (PW) Doppler flowmeters in the presence of velocity gradients is presented and is demonstrated for a circular geometry transducer insonating a uniform distribution of isotropic scatterers moving in paraxial conditions. The method computes the spectrum shape by adding the power spectra of individual flow lines present in the sample volume of the moving fluid being analyzed and can be applied to any focused transducer aperture for which the flow-line spectra are known. With a parabolic velocity profile insonated by a focused circular transducer, several computed spectra are compared with experimental ones for different insonation angles; it is also shown how changes in the spectral shape, due to variations of sample volume size, can be evaluated. The results should help to provide a better understanding of PW Doppler spectrum broadening and its use in medical ultrasound diagnostics     

Modulation transfer function measurement of sparse-array sensors using a self-calibrating fringe pattern

A simple method for the measurement of the pixel modulation transfer function (MTF) of sparse-array (extended MTF) sensors has been developed. We use a phase-shifting Twyman-Green interferometer to generate a series of single spatial-frequency fringe patterns incident on the sensor The resulting signal modulation is measured. We achieve self-calibration by restricting the measured spatial frequencies to multiples of the Nyquist frequency. The aliased patterns at these frequencies are unique and easily identifiable. Spatial frequencies of 480 cycles/mm are generated and measured. This frequency value is more than ten times that of the sensor sampling frequency. The expected MTF shape is obtained at multiples of the sampling frequency. At odd multiples of the Nyquist frequency, the MTF's are affected by the electronic bandwidth and cross talk in the charge-injection device sensor.     

Target velocity estimation with FM and PW echo ranging Doppler systems I. Signal analysis

The theoretical foundation is presented for velocity estimation with a pulsed wave (PW) Doppler system transmitting linear FM signals. The Doppler system possesses echo ranging capabilities and is evaluated in the context of Doppler ultrasound for blood velocity measurement. The FM excitation signal is formulated and the received signal is derived for a single moving particle. This signal is similar to the transmitted signal, but with modified parameters due to Doppler effect and range. The demodulated received signal is subsequently derived and analyzed. It is shown that, due to the Doppler effect, this is a linear sweep signal as well. The velocity and range information obtainable from one and two consecutively received signals are described. The latter case establishes the basis for an FM Doppler system for blood velocity measurements.     

An extended autocorrelation method for estimation of blood velocity

The conventional autocorrelation method (AM) to estimate the blood velocity for color flow imaging (CFI) is based on the phase estimation of the autocorrelation function. In this paper, a new extended autocorrelation method (EAM) that uses both phase and magnitude of the two dimensional (depth and temporal direction) autocorrelation function for estimating the blood velocity is presented. It is shown that the EAM has similar performance as the cross-correlation method (CCM). Both of them have smaller estimation variance than the AM and have the ability to estimate velocities beyond the Nyquist velocity, but the EAM is more computationally efficient than the CCM. A 2-D blood flow signal with rectilinear velocity including the transit time effect has also been simulated and the results are presented in this paper. For comparison, the EAM and the CCM have been applied to the simulated signals in which the flow velocities are up to four times the Nyquist velocity. The EAM has been further verified by experimental RF data from the subclavian artery     

Super-resolution technique of microzooming in electro-optical imaging systems

Abstract

The main resolution limit for an electro-optical imaging system comprising a focal plane array (FPA) sensor obeys Nyquist theorem which states that an object must be sampled at more than twice the cut-off frequency to avoid aliasing. A novel super-resolution technique, microzooming (MZ), is presented. MZ provides electro-optical imagery with an extended unaliased bandwidth using multiple aliased images at different optical magnifications. A number of simulation and experiment results demonstrate that MZ is a practical advance for reducing aliasing and enhancing resolution as well as helping to suppress white noise.     

单边狭窄血管中超声多普勒信号的仿真研究

超声多普勒技术作为一种无损检测手段被广泛应用于血管狭窄的检测。以往的血管狭窄仿真信号的研究仅限于双边狭窄的对称情况,文章提出了一种单边狭窄血管中超声多普勒信号的仿真方法。首先用有限元分析方法(FEM)计算出狭窄血管中血流流速场分布情况,然后用总体分布非参数估计法计算出超声多普勒信号的功率谱密度(PSD),再用余弦叠加法获取仿真的超声多普勒时域信号。用快速傅里叶变换(FFT)计算仿真超声多普勒信号的频谱,从中计算最大频率、平均频率和频谱宽度等参数,分析它们在不同流速和狭窄程度下的特征,为血管疾病的诊断提供敏感的参数。     

Design and implementation of a smartphone-based portable ultrasound pulsed-wave Doppler device for blood flow measurement

Blood flow measurement using Doppler ultrasound has become a useful tool for diagnosing cardiovascular diseases and as a physiological monitor. Recently, pocket-sized ultrasound scanners have been introduced for portable diagnosis. The present paper reports the implementation of a portable ultrasound pulsed-wave (PW) Doppler flowmeter using a smartphone. A 10-MHz ultrasonic surface transducer was designed for the dynamic monitoring of blood flow velocity. The directional baseband Doppler shift signals were obtained using a portable analog circuit system. After hardware processing, the Doppler signals were fed directly to a smartphone for Doppler spectrogram analysis and display in real time. To the best of our knowledge, this is the first report of the use of this system for medical ultrasound Doppler signal processing. A Couette flow phantom, consisting of two parallel disks with a 2-mm gap, was used to evaluate and calibrate the device. Doppler spectrograms of porcine blood flow were measured using this stand-alone portable device under the pulsatile condition. Subsequently, in vivo portable system verification was performed by measuring the arterial blood flow of a rat and comparing the results with the measurement from a commercial ultrasound duplex scanner. All of the results demonstrated the potential for using a smartphone as a novel embedded system for portable medical ultrasound applications.     

Velocity measurement by vibro-acoustic Doppler

We describe the theoretical principles of a new Doppler method, which uses the acoustic response of a moving object to a highly localized dynamic radiation force of the ultrasound field to calculate the velocity of the moving object according to Doppler frequency shift. This method, named vibro-acoustic Doppler (VAD), employs two ultrasound beams separated by a slight frequency difference, Δf, transmitting in an X-focal configuration. Both ultrasound beams experience a frequency shift because of the moving objects and their interaction at the joint focal zone produces an acoustic frequency shift occurring around the low-frequency (Δf) acoustic emission signal. The acoustic emission field resulting from the vibration of the moving object is detected and used to calculate its velocity. We report the formula that describes the relation between Doppler frequency shift of the emitted acoustic field and the velocity of the moving object. To verify the theory, we used a string phantom. We also tested our method by measuring fluid velocity in a tube. The results show that the error calculated for both string and fluid velocities is less than 9.1%. Our theory shows that in the worst case, the error is 0.54% for a 25° angle variation for the VAD method compared with an error of -82.6% for a 25° angle variation for a conventional continuous wave Doppler method. An advantage of this method is that, unlike conventional Doppler, it is not sensitive to angles between the ultrasound beams and direction of motion.     

Ultrasound simulation of complex flow velocity fields based on computational fluid dynamics

In this work, a simulation environment for the development of flow-related ultrasound algorithms is presented. Ultrasound simulations of realistic Doppler signals require accurate modeling of blood flow. Instead of using analytically described flow behavior, complex blood movement can be derived from velocity fields obtained with computational fluid dynamics (CFD). By further modeling blood as a collection of point scatterers, resulting RF-signals can be efficiently retrieved using an existing ultrasound simulation model. The main aim of this paper is to elaborate on creating CFD-based phantoms for ultrasound simulations. The coupling of a computed flow field with an ultrasound model offers flexible control of flow and ultrasound imaging parameters, beneficial for improving and developing imaging algorithms. The proposed method was validated in a straight tube with a stationary parabolic velocity profile and further demonstrated by an eccentrically stenosis carotid bifurcation. The estimated flow velocities are in good agreement with the CFD reference, both for color flow imaging and pulsed-wave doppler simulations. The presented method can also be extended to include wall mechanics simulations in future work.     

Sub-Nyquist interferometry using a computer-stored reference

Abstract

In the testing of deep aspheric wavefronts using the phase-shifting technique a CCD video camera and a digital computer is limited by the so-called Nyquist frequency. The Nyquist frequency limits the maximum wavefront slope that can be measured over the CCD array to π radians per pixel. A wavefront containing spatial frequencies beyond this limit gives a sub-sampled or aliased interferogram. If the wavefront under test is assumed to be smooth up to its first or second derivative, then it is possible to recover the deep aspheric wavefront. Here we present a new and simple technique to recover a deep aspheric wavefront under test whenever a close estimation of the wavefront being tested is available.     

利用超声多普勒技术测量低速血流

利用常规的多普勒系统较难进行低速血流的测量，本文在讨论了常规的多普勒系统的局限后，就一种可测量低速血流的超声多普勒技术的原理，方法进行讨论，并通过计算机模拟实验实现了这种方法。     

Spectral Doppler estimation utilizing 2-D spatial information and adaptive signal processing

The trade-off between temporal and spectral resolution in conventional pulsed wave (PW) Doppler may limit duplex/triplex quality and the depiction of rapid flow events. It is therefore desirable to reduce the required observation window (OW) of the Doppler signal while preserving the frequency resolution. This work investigates how the required observation time can be reduced by adaptive spectral estimation utilizing 2-D spatial information obtained by parallel receive beamforming. Four adaptive estimation techniques were investigated, the power spectral Capon (PSC) method, the amplitude and phase estimation (APES) technique, multiple signal classification (MUSIC), and a projection-based version of the Capon technique. By averaging radially and laterally, the required covariance matrix could successfully be estimated without temporal averaging. Useful PW spectra of high resolution and contrast could be generated from ensembles corresponding to those used in color flow imaging (CFI; OW = 10). For a given OW, the frequency resolution could be increased compared with the Welch approach, in cases in which the transit time was higher or comparable to the observation time. In such cases, using short or long pulses with unfocused or focused transmit, an increase in temporal resolution of up to 4 to 6 times could be obtained in in vivo examples. It was further shown that by using adaptive signal processing, velocity spectra may be generated without high-pass filtering the Doppler signal. With the proposed approach, spectra retrospectively calculated from CFI may become useful for unfocused as well as focused imaging. This application may provide new clinical information by inspection of velocity spectra simultaneously from several spatial locations.     

Doppler angle and flow velocity estimations using the classic and transverse Doppler effects

Current clinical Doppler ultrasound systems could only measure the flow vector parallel to the ultrasound beam axis, and the knowledge of the Doppler angle (beam-to-flow angle) is needed to calculate the real flow velocity. Currently, the Doppler angle is determined visually by manually aligning a vessel axis marker along the blood vessel on the duplex scan image of the ultrasound. The application of this procedure is often limited by practical constraints; therefore, measurements are not reliable. In order to overcome this problem, the authors developed a simple Doppler angle and flow velocity estimation method using a combination of the classic and transverse Doppler effects. This method uses only a single focused annular array transducer to estimate the Doppler angle and the flow velocity. The authors have verified experimentally that this method is successful for measuring constant flow in a flow phantom between 45 degrees and 80 degrees Doppler angle. The standard deviation of the estimated Doppler angles is less than 4.5 degrees . This method could be implemented easily in medical Doppler ultrasound systems to automatically estimate the Doppler angle and the flow velocity.     

Novel instrument for high-resolution ultrasound flow imaging

In multigate systems employed in Doppler flow-imaging equipment, the velocities of targets located at different ranges along the ultrasound beam axis are simultaneously detected. Typically, to reduce the computation efforts, this processing is aimed at extracting a single parameter (e.g., the mean velocity) from each signal. In this article, a novel ultrasound multigate instrument is presented capable of providing a detailed analysis of Doppler signals originated from 64 different range cells. By means of a dedicated high-speed fast-Fourier transform processor, these signals are transformed into the frequency domain without loss of information. The implementation of the new multigate system is described here, and the pieces of information which it can provide are discussed. Significant results obtained in vitro and in vivo demonstrate that the new instrument is capable of accurately reproducing the actual velocity profiles of the interrogated flow. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 438–443, 1997     

Target velocity estimation with FM and PW echo ranging Doppler systems. II. Systems analysis

For part I see ibid., vol.40, no.4, pp.366-372 (1993). In Part I, the encoding of the velocity and range information into the received and demodulated signals based on transmission of coherent repetitive linear sweep signals, was discussed. In the present work, two different implementations of FM Doppler systems that can be used to obtain velocity profiles are presented. The first implementation is similar to the implementation of a conventional pulsed wave (PW) Doppler system, based on measurement of phase shift (correlation based system): the second implementation is a frequency-domain analog to the PW Doppler system, based on time shift measurements (cross correlation-based system).     

Analog sensor design proposal for laser Doppler velocimetry

Laser Doppler velocimetry (LDV) has been widely used for many years in fluid mechanics to measure particle velocity. However, in most applications, i.e., in industrial processes, such a system is often too expensive. This paper discusses a technique based on the use of an analog phase-locked loop and an analog integrator system for processing laser Doppler velocimeter data to infer particle velocity. This method appears to be suitable for designing low-cost integrated LDV sensors. A SIMULINK program has been written in order to validate the method for velocities in the 10-80 mm/s range. Finally, the performance of the estimator is illustrated by Monte-Carlo simulations obtained from synthesized Doppler signal.     

Ultrasonic phased-array scanner with digital echo synthesis for Doppler echocardiography

For the purpose of the quantitative assessment of subtle disease processes in the cardiovascular system an electronically steered sector scanner that combines echographic imaging and Doppler blood velocity measurements has been developed. The integrated operation of a fast Fourier transform (FFT) Doppler signal processor for the simultaneous blood velocity evaluation of 64 individual gates is among the specific design goals. The instrument incorporates an unusually high degree of digital signal processing, which allows for high integration density, easy manufacturing and high reliability in future designs. The complex Doppler spectra are determined for each of the 64 Doppler gates in real time, and the subsequent computation of the first moment provides a reliable estimate of the mean blood flow velocities at the respective locations. The instantaneous velocity profile along the Doppler beam is displayed together with the calculated volume flow rate and a range-selected complete frequency spectrum. Results of both in vitro and in vivo tests indicate that in the future, a higher degree of digital signal processing could be implemented in complex ultrasonic systems.     

Pulse inversion Doppler: a new method for detecting nonlinear echoes from microbubble contrast agents

A novel technique for the selective detection of ultrasound contrast agents, called pulse inversion Doppler, has been developed. In this technique, a conventional Doppler or color Doppler pulse sequence is modified by inverting every second transmit pulse. Either conventional or harmonic Doppler processing is then performed on the received echoes. In the resulting Doppler spectra, Doppler shifts from linear and nonlinear scattering are separated into two distinct regions that can be analyzed separately or combined to estimate the ratio of nonlinear to linear scattering from a region of tissue. The maximum Doppler shift that can be detected is 1/2 the normal Nyquist limit. This has the advantage over conventional harmonic Doppler that it can function over the entire bandwidth of the echo signal, thus achieving superior spatial resolution in the Doppler image. In vitro measurements comparing flowing agent and cellulose particles suggest that pulse inversion Doppler can provide 3 to 10 dB more agent to tissue contrast than harmonic imaging with similar pulses. Similar measurements suggest that broadband pulse inversion Doppler can provide up to 16 dB more contrast than broadband conventional Doppler. Nonlinear propagation effects limit the maximum contrast obtainable with both harmonic and pulse inversion Doppler techniques.     

Single-pulse tissue doppler using synthetic transmit beams

Tissue Doppler imaging (TDI) is a common technique for investigating myocardial function. Typically, B-mode data and TDI data are recorded using separate acquisitions and combined into a single, color overlaid image. In this work, we present a novel method for TDI imaging, where both TDI and B-mode are created from the same acquisition. Velocities are calculated from the phase shift between neighboring transmit events in the B-mode scan; hence the name singlepulse tissue Doppler (SPTD). Using a novel transmit beam interleaving pattern, this method provides TDI and B-mode at the same high frame rate with an adjustable Nyquist velocity limit. Through simulations and measurements, this work investigates the bias and variance of the SPTD velocities and compares the estimates to those of the conventional TDI autocorrelation estimation method. The results showed that the method introduces an additional bias and variance in the velocity estimates compared with conventional TDI. However, by applying bias compensation, the SPTD velocity estimates were close to those of regular TDI. Using SPTD, the whole left ventricle was imaged within a 65-degree sector at a frame rate of 110 frames per second (43 transmissions per frame).     

A novel laser Doppler velocimeter

The idea of using a reference-beam laser Doppler velocimeter (LDV) to measure the velocity of a vehicle for an inertial navigation system is proposed. In order to reduce the measurement error produced by the vehicle's jolt, a novel laser Doppler velocimeter (LDV) based on the Janus configuration is presented. The system mounted at the bottom of the vehicle is composed of two single reference-beam subsystems, one of the laser probes looks forward and the other looks backward. Each of the subsystems transmits a laser at 532?nm with the same inclination angle and detects its own Doppler frequency from the scattered light. The pitch angle of the vehicle is calculated by the two detected Doppler frequencies and the inclination angle, and then the measured velocity of the vehicle is compensated. The results of the theory analysis and experiments show that the vehicle's jolt affects conventional reference-beam LDV strongly. LDV with a Janus configuration is insensitive to the inclination angle, and its measurement accuracy is much better than that of a conventional reference-beam LDV. Comparing with the DZL-1 electronic speedometer, the measurement mean error is less than 0.9%, so it is suitable to offer the parameter of velocity for a vehicle self-contained inertial navigation system.