Movement artefact suppression in blood perfusion measurements using a multifrequency technique

The standard way of suppressing movement artefacts in Doppler measurements is by means of a high-pass filter. This is because artefacts usually are of high amplitude, but have low frequencies. The immediate drawback is, then, that low-velocity blood flow is also filtered out. In this paper, a method to reduce movement artefacts in blood perfusion measurements is proposed, using simultaneous transmission and reception of multiple frequencies in a continuous-wave Doppler system. It is shown that Doppler signals originating from blood may be considered uncorrelated for a large enough frequency separation between channels, and tissue movements are more correlated. By subtracting perfusion estimates obtained by time-domain processing, correlated signals can be suppressed. The subtraction algorithm is shown to produce a linear perfusion estimate, but with twice the standard deviation compared to an estimate obtained by simply averaging channels. Movement artefacts in both in vitro and in vivo models are shown to be reduced by the algorithm. Imbalance between channels does, however, cause the artefacts to be only partly reduced. The problem can be alleviated by filtering the signals prior to subtraction, but this results in a nonlinear estimate, especially for large time constants in the filter. Some amount of filtering can still be desirable to suppress partly correlated artefacts, even if identical time-domain processing units are implemented, as could be done digitally.     

Detection of Doppler embolic signals: psychoacoustic considerations

The purpose of this study was to improve reliability in the identification of Doppler embolic signals by determining the decibel threshold for reproducible detection of simulated "emboli" as a function of signal duration, frequency and cardiac-cycle position. The auditory sensitivity of 16 participants to 574 simulated "emboli" was examined using psychoacoustic techniques to assess how the probability of detection varies with embolic signal parameters. Detailed measurements of the threshold for detection of simulated embolic signals are presented. These provide evidence that the measured embolus-to-blood threshold ranges between 2 dB and 14 dB as a continuous function of signal duration and frequency. The level of the threshold is closely linked to both embolic signal parameters and the properties of the blood flow signal. We conclude that the current fixed choice of threshold does not provide a good approximation to the true threshold of detection across the full range of embolic signal parameters.     

In vivo potentiostatic studies at the electrode tissue interface: filter properties of the monophasic action potential (Ag/AgCl) electrode in living rat heart

The monophasic action potential (Franz) catheter is regarded as the criterion standard for high fidelity recording of a class of physiological signals. However, its signal modulation characteristics have never been reported. Broadband impedance spectroscopy was performed in perfused living rat heart in a three-electrode potentiostatic configuration to determine the filtering characteristics of the MAP and model Ag/AgCl electrode-tissue interfaces. The filter transfer function H(f) (attenuation [dB] vs log(f) [log(Hz)]) was derived for the frequency range 10 Hz-10(6) Hz. As a filter, the MAP interface is characterized by two ranges of filtering behavior. At high frequency the MAP interface is a high-pass filter with passband frequency 54 kHz-549 kHz (median 321 kHz) and with -3 dB cutoff points ranging from 10 kHz to 302 kHz. In this high frequency range the transfer function is characterized by decreasing attenuation per decade. However, in the lower frequency range relevant to physiological signals (the monophasic action potential, 0.1-40 Hz), it is a severely attenuating nondiodic high-pass filter element with an average attenuation of 16.87 dB relative to passband. In this physiological range, rolloff is nonlinear with increasing attenuation per decade. While the MAP electrode and model Ag/AgCl electrodes are high-pass filters with robust transfer functions for high frequency signals in the living heart, the attenuation of signals in a frequency range relevant to in vivo physiological recording imparts extreme attenuation that may distort physiological signals unpredictably. This disadvantage may be mitigated by amplitude scaling to a calibrated pure tone signal within the physiological frequency band to recover a reproducible signal.     

Bias and variance in the estimate of the Doppler frequency induced by a wall motion filter

In pulsed Doppler flowmeters, processing of the Doppler signals is often done digitally. The first step in the analysis of the echoes is the filtering which is needed to remove stationary components and low frequency shifts induced by wall motion. This preliminary step is of utmost importance. The influence of uncorrelated noise on the measurement of Doppler signals at the input of this filter is analysed. The frequencies of the Doppler signals are extracted by an algorithm based on correlation techniques. We observed that the filter induces a correlated noise term, which results in an overestimation of the frequency. An effect similar to frequency aliasing may appear. The level of the bias is dependent on filter characteristics and noise level. Our study was carried out on simulated Doppler signals using first and second order filters. An especially desirable solution in flow mapping is proposed in order to decrease this error.     

Simulation of real-time frequency estimators for pulsed Doppler systems

Four time-domain oriented, real-time frequency estimators, based on the detection of phase, zero-crossings, instantaneous frequency or autocorrelation, were simulated on a digital computer and subjected to computer generated Doppler signals, enabling the investigation of the influence of spectral shape, filtering, frequency shift, noise and quantitation. Three estimators, the autocorrelator as well as the instantaneous frequency detector and the autocorrelator, both with extended frequency range, appeared to be very accurate. They exhibit a bias in the estimator output of less than 2 percent over a wide frequency range, the former up to nearly the Nyquist frequency, the latter two beyond, even for skew spectra and under poor signal conditions regarding bandwidth and noise.     

Reducing CPR artefacts in ventricular fibrillation in vitro

CPR creates artefacts on the ECG, and a pause in CPR is therefore mandatory during rhythm analysis. This hands-off interval is harmful to the already marginally circulated tissues during CPR, and if the artefacts could be removed by filtering, the rhythm could be analyzed during ongoing CPR. Fixed coefficient filters used in animals cannot solve this problem in humans, due to overlapping frequency spectra for artefacts and VF signals.In the present study, we established a method for mixing CPR-artefacts (noise) from a pig with human VF (signal) at various signal-to-noise ratios (SNR) from -10 dB to +10 dB. We then developed a new methodology for removing CPR artefacts by applying a digital adaptive filter, and compared the results with this filter to that of a fixed coefficient filter. The results with the adaptive filter clearly outperformed the fixed coefficient filter for all SNR levels. At an original SNR of 0 dB, the restored SNRs were 9.0+/-0.7 dB versus 0.9+/-0.7 dB respectively (P<0.0001).     

Evaluation of a 1 MHz transducer for transcranial Doppler ultrasound including embolic signal detection

A 1 MHz transducer for use with transcranial Doppler ultrasound may improve the intensity and therefore the detection of embolic signals (ES) and may also reduce the number of absent acoustic windows. A series of studies was performed to investigate its potential benefits. Firstly, ES were detected using a 1 MHz and a 2 MHz transducer both in vitro and in vivo. Secondly, the time taken to identify 100 middle cerebral arteries (MCA) was recorded for both transducers and the best Doppler signal obtained was reviewed off-line and graded for quality. ES were more intense when detected with the 1 MHz compared with the 2 MHz transducer, both in vitro (p <.0001) and in vivo (p <.0001). Of the 100 MCAs studied, 81 had acoustic windows identified with both transducers. The number of acoustic windows detected with one transducer but not the other was the same for both transducers (n = 3). The time taken to identify the MCA was longer with the 1 MHz transducer (p <.0001) and the quality of the signal achieved was poorer (p <.0001). In conclusion, the 1 MHz transducer improved embolic signal intensity but the overall quality of the flow spectrum obtained was poorer with the 1 MHz than with the 2 MHz transducer. A lower frequency transducer of 1 MHz or possibly 1.5 MHz with transcranial Doppler ultrasound may improve the application of embolic signal detection but may not improve the signal for routine measurement of flow velocities.     

Automated embolus identification using a rule-based expert system

Transcranial Doppler ultrasound (US) can be used to detect microemboli in the cerebral circulation, but is still limited because it usually relies on "human experts" (HEs) to identify signals corresponding to embolic events. The purpose of this study was to develop an automatic system that could replace the HE and, thus, make the technique more widely applicable and, potentially, more reliable. An expert system, based around a digital signal-processing board, analysed Doppler signal patterns in both the time domain and frequency domain. The system was trained and tested on Doppler signals recorded during the dissection and recovery phases of carotid endarterectomy. It was tested with 74 separate 2.5-min recordings that contained at least 575 artefacts in addition to 253 s of diathermy interference. The results were compared with the results obtained by three HEs. Using a "gold-standard" that classified any event detected by the majority of HEs as an embolus, the automatic system displayed a sensitivity of 94.7% and a specificity of 95.1% for 1151 candidate events 7 dB or more above the clutter (signal-to-clutter ratio, SCR, > or = 7 dB), and 89.6% and 95.3%, respectively, for 2098 candidate events with SCR > or = 5 dB. The system had a very similar performance to individual HEs for SCR > or = 7dB, and was only marginally worse for SCR > or = 5 dB.     

In vivo Doppler Ultrasound Quantification of Turbulence Intensity Using A High-Pass Frequency Filter Method

The objective of this investigation was to implement a high-pass frequency filter method to analyze Doppler ultrasound velocity waveforms and quantify turbulence intensity (TI) in vivo. Doppler velocity data were analyzed using two techniques, based on either ensemble averaging or high-pass frequency domain filtering of the periodic waveforms. The accuracy and precision of TI measurements were determined with controlled in vitro experiments, using a pulsatile-flow model of a stenosed carotid bifurcation. The high-pass filter technique was also applied in vivo to determine whether this technique could successfully distinguish between pertinent hemodynamic sites within the carotid artery bifurcation. Twenty-five seconds of Doppler audio data were acquired at three sites (common carotid artery [CCA], internal carotid artery [ICA] stenosis and distal ICA) within 10 human carotid arteries, and repeated three times. Doppler velocity data were analyzed using a ninth-order high-pass Butterworth filter with a 12-Hz inflection point. TI measured within the CCA and distal ICA was found to be significantly different (p < 0.0001) for moderate to nearly occluded carotid artery classifications. Also, TI measured within the distal ICA increased with stenosis severity, with the ability to distinguish between each stenosis class (p < 0.05). This investigation demonstrated the ability to precisely quantify TI using a conventional Doppler ultrasound machine in human subjects, without interfering with normal clinical protocols. (E-mail: david.holdsworth@imaging.robarts.ca)     

Online automated detection of cerebral embolic signals from a variety of embolic sources

A major limitation of embolic signal (ES) detection by transcranial Doppler ultrasound is the lack of a reliable automated system. The performance of an automated system needs to be evaluated for different embolic sources on consecutively acquired typical data. We evaluated a new online frequency filtering approach in a total of 565 h of data containing 925 ES from four groups of patients: post carotid endarterectomy (postCEA), symptomatic carotid stenosis (SCS), asymptomatic carotid stenosis (ACS) and atrial fibrillation (AF). The following sensitivities and specificities were achieved: postCEA = sensitivity 95.8%, specificity 88.2%; SCS = sensitivity 98.4%, specificity 88.6%; ACS = sensitivity 85.7%, specificity 13.0%; AF = sensitivity 54.8%, specificity 7.0%. This online automated system performed similarly to the human expert in the postCEA and SCS groups, but less well in patients with AF and ACS. The low ratio of ES to normal data in patients with ACS may have contributed to the lower specificity; further evaluation with a higher number of ES is required. Refinement of the algorithm is required to improve its sensitivity for AF data.     

自制包裹氟烷气体的表面活性剂类超声造影剂观察肝肿瘤血流实验研究

目的研究含氟烷气体的表面活性剂类超声造影剂对肿瘤血流信号增强的情况。方法5只荷肝VX2肿瘤的新西兰大白兔麻醉后，经腹部探测肝脏，显示肝肿瘤的血流信号。经耳缘静脉注射表面活性剂类超声造影剂，剂量0．1mg／kg，观察肿瘤周围血管及内部血流信号增强情况及持续时间。结果肝肿瘤周边及内部能量多普勒血流信号明显增强，且增强时间较长。结论表面活性剂类超声造影剂经外周静脉注射后血管二维及血流信号增强显著，值得进一步研究。     

Online automated detection of cerebral embolic signals using a wavelet-based system

Transcranial Doppler ultrasound (US) can be used to detect emboli in the cerebral circulation. We have implemented and evaluated the first online wavelet-based automatic embolic signal-detection system, based on a fast discrete wavelet transform algorithm using the Daubechies 8th order wavelet. It was evaluated using a group of middle cerebral artery recordings from 10 carotid stenosis patients, and a 1-h compilation tape from patients with particularly small embolic signals, and compared with the most sensitive commercially available software package (FS-1), which is based on a frequency-filtering approach using the Fourier transform. An optimal combination of a sensitivity of 78.4% with a specificity of 77.5% was obtained. Its overall performance was slightly below that of FS-1 (sensitivity 86.4% with specificity 85.2%), although it was superior to FS-1 for embolic signals of short duration or low energy (sensitivity 75.2% with specificity 50.5%, compared to a sensitivity of 55.6% and specificity of 55.0% for FS-1). The study has demonstrated that the fast wavelet transform can be computed online using a standard personal computer (PC), and used in a practical system to detect embolic signals. It may be particularly good for detecting short-duration low-energy signals, although a frequency filtering-based approach currently offers a higher sensitivity on an unselected data set.     

The effect of echo contrast agent on Doppler velocity measurements

The purpose of this investigation was to determine the effect of echo contrast agents on spectral Doppler velocity measurements. SH U 508A was administered by IV injection in 15 patients. The transmitral flow velocity was measured at the E- and A-wave peaks before the start and at the peak of the contrast effect. The Doppler velocity was determined from the Doppler video spectral display and from power spectral analysis of the audio Doppler signal. The Doppler signal intensity was also measured. The Doppler signal intensity increased 17.4 +/- 3.5 dB (p < 0.0001) following echo contrast injection. This was associated with a significant increase in the spectral peak velocity as determined from either the video display or audio analysis. (p < 0.0001). The velocity corresponding to the audio power peak frequency (the modal velocity) did not change significantly (p = NS) and was independent of Doppler signal strength.     

The practical significance of two-dimensional deconvolution in echography

This paper evaluates deconvolution (inverse filtering) as applied to ultrasonic imaging systems, and discusses the obstacles which are encountered employing the technique in practice. A minicomputer is used to generate artificial echo signals, simulating rf signals resulting from a set of point reflectors in a homogeneous medium, as recorded by an electronically focused group-steered linear array scanner. Two-dimensional deconvolution in combination with a Wiener noise reduction filter (i.e., a Wiener-Inverse filter) is applied to these simulated rf signals, which were contaminated with white noise. The efficacy of the Wiener-Inverse filter is defined in terms of its ability to resolve two point reflectors with a lateral spacing equal to the local -6 dB width of the ultrasonic beam. In favorable circumstances, the targets are resolved at signal-to-noise ratios (SNR) better than 20 dB, where SNR is defined as the maximum signal power divided by the average noise power level. Nonlinear effects due to quantization or signal clipping are investigated. In order to improve the resolution of an rf signal with a dynamic range of 40 dB, the input signal should be digitized at a minimum of 12 bits. The problem of signal clipping can be circumvented by oversampling. The two-dimensional Wiener-Inverse filter is defined in terms of both temporal and spatial properties of the insonification. Effects of wave diffraction give rise to a depth-dependent ultrasonic beam. As a result of a misfit of the Wiener-Inverse filter and the local properties of the ultrasonic beam, erroneous noisy texture arises in the image. Adaptation of the Wiener-Inverse filter with respect to the beam properties gives acceptable results, at the expense of a rather large computational effort.     

Vascular flow and perfusion imaging with ultrasound contrast agents

Current techniques for imaging ultrasound (US) contrast agents (UCA) make no distinction between low-velocity microbubbles in the microcirculation and higher-velocity microbubbles in the larger vasculature. A combination of radiofrequency (RF) and Doppler filtering on a low mechanical index (MI) pulse inversion acquisition is presented that differentiates low-velocity microbubbles (on the order of mm/s) associated with perfusion, from the higher-velocity microbubbles (on the order of cm/s) in larger vessels. In vitro experiments demonstrate the ability to separate vascular flow using both harmonic and fundamental Doppler signals. Fundamental and harmonic Doppler signals from microbubbles using a low-MI pulse-inversion acquisition are compared with conventional color Doppler signals in vivo. Due to the lower transmit amplitude and enhanced backscatter from microbubbles, the in vivo signal to clutter ratios for both the fundamental (-11 dB) and harmonic (-4 dB) vascular flow signals were greater than with conventional power Doppler (-51 dB) without contrast agent. The processing investigated here, in parallel with conventional pulse-inversion processing, enables the simultaneous display of both perfusion and vascular flow. In vivo results demonstrating the feasibility and potential utility of the real-time display of both perfusion and vascular flow using US contrast agents are presented and discussed.     

Application of autoregressive methods to multigate spectral analysis

Multigate analysis is known to be capable of detecting accurate blood velocity profiles from human vessels. Experimental systems so far presented in the literature use time-domain frequency estimations and, more recently, the fast Fourier transform (FFT) for real-time analysis of Doppler signals from multiple range cells. This experimental study is aimed at evaluating the application of an autoregressive (AR) method (Burg algorithm) to multigate Doppler analysis. Both in vitro and in vivo results were collected with a commercial Duplex scanner coupled with a prototype multigate unit developed in our laboratory. The same multigate signals are, thus, processed according to both the FFT and the Burg algorithms. The related spectral and maximum frequency profiles are reported and statistically compared. AR, implemented with the Burg algorithm, is demonstrated to be a way to perform multigate spectral analysis with reduced spectral variance, suitable for maximum velocity profile extraction through a simple threshold.     

Influence of skin blood flow on near-infrared spectroscopy signals measured on the forehead during a verbal fluency task

Brain activity during a verbal fluency task (VFT) has been the target of many functional imaging studies. Most studies using near-infrared spectroscopy (NIRS) have reported major activation in the frontal pole, but those using PET or fMRI have not. This led us to hypothesize that changes in the NIRS signals measured in the forehead during VFT were due to changes in skin blood flow. To test this hypothesis, we measured NIRS signals and the Doppler tissue blood flow signals in the foreheads of 50 participants. The measurements were performed while each participant produced words during two 60-s periods with an interval of 100 s. In addition to a conventional optode separation distance of 30 mm (FAR channels), we used a short distance--5mm (NEAR channels)--to measure NIRS signals that originated exclusively from surface tissues. The oxygenated hemoglobin (oxyHb) concentration in the FAR and NEAR channels, as well as the Doppler blood flow signal, increased in a similar manner during the two periods of word production; the signal increase in the first period was twice as high as that in the second period. Accordingly, the mean changes in oxyHb concentration in the FAR channels were correlated closely with the changes in the NEAR channels (R(2) = 0.91) and with the integrated Doppler skin blood flow signal (R(2) = 0.94). Furthermore, task-related NIRS responses disappeared when we blocked skin blood flows by pressing a small area that covered a pair of optodes. Additionally, changes in the FAR channel signals were correlated closely with the magnitude of pulsatile waves in the Doppler signal (R(2) = 0.92), but these signals were not highly correlated with the pulse rate (R(2) = 0.43). These results suggest that a major part of the task-related changes in the oxyHb concentration in the forehead is due to task-related changes in the skin blood flow, which is under different autonomic control than heart rate.     

The use of the wavelet transform to describe embolic signals.

A number of methods to detect cerebral emboli and differentiate them from artefacts using Doppler ultrasound have been described in the literature. In most, Fourier transform-based (FT) spectral analysis has been used. The FT is not ideally suited to analysis of short-duration embolic signals due to an inherent trade-off between temporal and frequency resolution. An alternative approach that might be expected to describe embolic signals well is the wavelet transform. Wavelets are ideally suited for the analysis of sudden short-duration signal changes. Therefore, we have implemented a wavelet-based analysis and compared the results of this with a conventional FFT-based analysis. The temporal resolution, as measured by the half-width maximum, was significantly better for the continuous wavelet transform (CWT), mean (SD) 8.40 (8.82) ms, compared with the 128-point FFT, 12.92 (9.70) ms, and 64-point FFT, 10.80 (5.69) ms. Time localization of the CWT for the embolic signal was also significantly better than the FFT. The wavelet transform appears well suited to the analysis of embolic signals offering superior time resolution and time localization to the FFT.     

Transcutaneous detection of subcritical arterial stenoses by Doppler signal spectrum analysis.

Early clinical studies showed significant data overlap when Doppler signal spectral analysis was used to differentiate normal carotid vessels from those with slight stenoses. A canine common carotid model has been used to study the ability of spectral analysis to detect subcritical stenoses, i.e., those with 20 to 50 per cent diameter reductions. Using the ratio of peak systolic frequency at the stenosis site to that proximal to the stenosis, significant differences from controls were found for all degrees of stenosis. The mean peak frequency ratio was 1.15 +/- 0.05 for 20 per cent stenoses, increasing to 1.68 +/- 0.50 for 50 per cent stenoses. The peak frequency ratios for Doppler signal spectra recorded from a site 1 cm distal to the stenosis exceeded control values only for 40 and 50 per cent stenoses; at a site 3 cm from the stenosis no spectra were significantly different from controls. Spectral analysis can be a direct, sensitive method to evaluate slight carotid arterial stenoses, but careful examination technique is necessary and Doppler signals must be taken from the stenotic site itself.     

A new and simple Doppler method for measurement of fetal cardiac isovolumetric contraction time.

OBJECTIVES: The fetal cardiac isovolumetric contraction time is defined as the interval between mitral valve closure and aortic valve opening. The objective of this study was to develop a simple and reliable Doppler method for measuring fetal isovolumetric contraction time using a digital filtering and processing system. METHODS: Cardiac Doppler signals were recorded from 40 fetuses at 18-40 weeks' gestation using a continuous-wave ultrasound transducer. The raw signal was digitized, filtered and divided into five different frequency ranges: 250-375, 375-500, 500-750, 750-1000 and 1000-1500 Hz. To determine the most suitable filter setting for detecting mitral valve closure and aortic valve opening signals, we examined whether they were detected clearly in each filter range. RESULTS: Both mitral valve closure and aortic valve opening signals were detected clearly in the 500-1000 Hz range. The atrioventricular flow and outflow noises in the 250-500 and 1000-1500 Hz ranges helped us to identify the signals. It was found that dividing the raw signals into three ranges of 250-500, 500-1000 and 1000-1500 Hz was the most suitable digital-filter setting for measuring isovolumetric contraction time. CONCLUSIONS: We have developed a simple Doppler method for measuring fetal isovolumetric contraction time. The advent of digital processing has simplified the equipment and the simultaneous multidisplay of three different filtered signals enables easy and accurate measurement.