A real-time autoregressive spectrum analyzer for Doppler ultrasound signals

A system based on a digital signal processor and a microcomputer has been programmed to estimate the maximum entropy autoregressive (AR) power spectrum of ultrasonic Doppler shift signals and display the results in the form of a sonogram in real-time on a computer screen. The system, which is based on a TMS 320C25 digital signal processor chip, calculates spectra with 128 frequency components from 64 samples of the Doppler signal. The samples are collected at a programmable rate of up to 40.96 kHz, and the computation of each spectrum takes typically 3.2 ms. The feasibility of on-line AR spectral estimation makes this type of analysis an attractive alternative to the more conventional fast Fourier transform approach to the analysis of Doppler ultrasound signals.     

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.     

Doppler ultrasound tracking instrument for monitoring blood flow velocity

Doppler ultrasound (US) is potentially a valuable method for monitoring changes of blood flow velocity over a period of many minutes or even hours, but is seldom used in this way. One difficulty that may have contributed to this is the problem of maintaining a fixed geometry between the US beam and the blood vessel. A method of improving the success of monitoring might be to actively steer the US beam so as to maintain an adequate signal even when small displacements of the transducer occur. We have designed and built a prototype system for this purpose. The system comprises a continuous-wave phased-array transducer controlled by a purpose-built Doppler unit. The system constantly evaluates the quality of the returning Doppler signal in terms of total power and signal-to-noise ratio (SNR) (evaluated by assessing the quality of derived envelope signals), and steers the ultrasonic beam in a manner so as to improve the signal, should this be necessary. The system was tested in vitro, where the automatic tracking of the Doppler signal doubled the effective beam width of the transducer. Further developments that increase sensitivity and steering range should result in US Doppler systems that are better suited to long-term monitoring.     

Sources of error in maximum velocity estimation using linear phased-array Doppler systems with steady flow

Using linear-array Doppler ultrasound (US) transducers, the measured maximum velocity may be in error and lead to incorrect clinical diagnosis. This study investigates the existence and cause of maximum velocity estimation errors for steady flow of a blood-mimicking fluid in a tissue-mimicking phantom. A specially designed system was used that enabled fine control of flow rate, transducer positioning and transducer angle relative to the flow phantom. Doppler machine settings (transducer aperture size, focal depth, beam-steering, gain) were varied to investigate a wide range of clinical applications. To estimate the maximum velocity, a new signal-to-noise ratio (SNR) independent method was developed to calculate the maximum frequency from an ensemble averaged Doppler power spectrum. This enabled the impact of each factor on the total Doppler error to be determined. When using the new maximum frequency estimator, it was found that the effect of transducer focal depth, intratransducer, intramachine, intermachine (that was tested) and beam-steering did not significantly contribute to maximum velocity estimation errors. Instead, it was the dependence of the maximum velocity on the Doppler angle that made, by far, the greatest contribution to the estimation error. Because our maximum frequency estimator took into account the effect of intrinsic spectral broadening, the degree of overestimation error was not as great as that previously published. Thus, the effects of Doppler angle and intrinsic spectral broadening are the chief sources of Doppler US error and should be the focus of future efforts to improve the accuracy.     

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.     

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.     

Do ultrasonic contrast agents artificially increase maximum Doppler shift? In vivo study of human common carotid arteries.

The aim of this study was to establish whether an increase of maximum Doppler shift occurs in the human common carotid artery after the administration of Levovist, an ultrasonographic echo enhancer. Twenty common carotid arteries of 10 patients were examined. Spectral Doppler waveform examinations were performed before and after administration of Levovist using an Acuson 128 XP 10 and a 7.0 MHz transducer probe. Time averaged mean velocity, peak velocity, maximum Doppler shift, and spectral Doppler indices (pulsatility index, resistive index, systolic-diastolic ratio) were assessed. No significant changes in any of the measured parameters, including maximum Doppler shifts, peak velocity (P = 0.35, Wilcoxon rank sum test), pulsatility indices (P = 0.70), resistive indices (P = 0.98), or other spectral indices, were found. We conclude that an increase in Doppler shift does not inevitably occur after the administration of a signal enhancer when examining the human common carotid artery.     

Color Doppler US pulmonary artery vessel signal: a sign for predicting the benign lesions

The lung cancer blood supply originates from the bronchial artery. If vessel signals within pulmonary lesions can be confirmed to be those of the pulmonary artery, color Doppler ultrasound (US) should be able to predict and differentiate benign lesions from lung cancers. Two hundred sixty-four patients with abutting thoracic lesions (including 125 lung cancers and 139 benign lesions) underwent color Doppler US examinations. A pulsatile flow, with the vessel signal length on sonographic appearance > or =1 cm demonstrated by color Doppler US, was arbitrarily defined as a pulmonary artery vessel signal. Of the 264 thoracic lesions, 73 (58%) lung cancers and 107 (77%) benign lesions had detectable color Doppler US pulsatile flow vessel signals. Analyzing the pulsatile flow vessel signals, the color Doppler US pulmonary artery vessel signal was present in 74 (53%) benign lesions, but was found in only two (2%) lung cancers of a specific alveolar cell carcinoma with lobar consolidation. Using the pulmonary artery vessel signal, color Doppler US can be valuable in predicting and differentiating benign lesions from lung cancers (p < 0.0001, sensitivity = 0.53, specificity = 0.98 and positive likelihood ratio 26.5). In conclusion, color Doppler US pulmonary artery vessel signal sign is useful in predicting and differentiating benign lesions from lung cancers.     

Quantitative Doppler measures in coiled vessels: investigation on excised umbilical veins

Quantitative assessment of umbilical venous blood velocity with Doppler ultrasound (US) must cope with the coiled structure of the vein inside the cord. Both an experimental and a theoretical approach showed remarkable variations in the insonation angle when the probe was moved along the vein, provided the inclination between the Doppler probe and the cord was kept constant. Inaccurate signal processing, stochastic variability and flow disturbances could, however, mask the influence of the geometry. The above hypotheses were assessed by investigating five cords in vitro a few hours after delivery from normal pregnancies at term. The Doppler signal was sampled at different sites along each cord and the mean Doppler shift estimated by FFT spectral analysis, both directly and through the noise rejection D’Alessio’s algorithm, which proved effective in improving the Doppler shift estimate in condition of low signal-to–noise ratio (SNR).     

Assessment of blood flow velocity waveforms of the pulmonary circulation by multigate spectral Doppler scanning and traditional pulsed Doppler ultrasonography.

OBJECTIVE: To assess (1) pulmonary artery and peripheral pulmonary blood flow by using a multigate spectral Doppler system and standard pulsed color Doppler ultrasonography and (2) the reproducibility of peripheral pulmonary artery blood flow velocity waveforms. METHODS: Thirty-three women were enrolled in the study Fetal pulmonary artery flow velocity waveforms were investigated by multigate spectral Doppler scanning and traditional pulsed Doppler ultrasonography plus color flow mapping. To establish the relationship between these 2 methods, we calculated the correlation coefficient, coefficient of variation, and limits of agreement. RESULTS: The 2 methods were not significantly correlated. The range of measurement agreement for these 2 methods suggests that there is a significant difference between the main and peripheral pulmonary artery blood flow, and the difference should be taken into account in the interpretation of Doppler flow velocity studies of the pulmonary artery. CONCLUSIONS: Multigate spectral Doppler scanning with power Doppler imaging may be a better way to assess the average flow impedance in a highly vascularized organ. This technique supplies a large number of Doppler signals in the region of interest and offers a quantified range of Doppler measurements, improving our understanding of fetal hemodynamics.     

肝脏局限性结节性增生的超声与病理对照研究

目的：讨论肝脏局限性结节性增生（FNH）的超声表现及病理基础。方法：回顾性分析13个病理证实的FNH的超声特点并与病理对照。结果：结节大小为2．4－13．7cm,低回声8例,等回声4例,中高回声1例,5例病灶内见星状强回声区,B超显示4例结节内血管图像,多普勒超声于病灶内记录到丰富的低阻动脉频谱（能量多普勒显示全部病灶,彩色多普勒示11／13呈富血供）,7例病灶内血管呈放射状排列。结论 ;彩色多普勒和能量多普勒超声能够显示FNH的特征性血流表现,提高对FNH的诊断率。     

Microembolic signal description: a reappraisal based on a customized digital postprocessing system

The high variability in presence and signature of microembolic signals (MES), detected with transcranial Doppler sonography (TCD) in the middle cerebral artery (MCA), cannot be explained with the currently available published data. We applied customized postprocessing on the radiofrequency (RF) signal of a standard TCD system. The spatial resolution was on the order of 2 mm, depending only on the length of the ultrasound (US) burst emitted. The amplitude of clutter-filtered RF signals was color-coded and plotted as a function of time and depth (range 30 mm). Additionally, 128 point fast Fourier transforms (FFTs) (50% temporal overlap) were calculated, visualizing both the background Doppler spectrum and the MES. We evaluated 122 gaseous MES from two patients during cardiac surgery and 52 particulate MES from four patients after carotid endarterectomy. Both MES categories showed comparable properties: they appeared in the RF amplitude plot as rather straight lines of increased intensity, indicating that the velocity remained approximately the same while they passed the US beam. The velocity calculated from the amplitude plot never exceeded that of the background Doppler spectrum. Various "MES patterns" could be identified with respect to the depth range at which the MES were visible. A quarter of the gaseous MES changed their direction at a specific depth, suggesting that the MES entered a branch (e.g., an M2 artery or the anterior cerebral artery). In the FFT analysis, these MES contained both positive and negative frequencies. It is concluded that MES show consistent signature patterns in the amplitude-time plots and that the previously reported variability of MES appearance in conventional Doppler systems is an artefact caused by relatively large signal amplitudes and sample volumes.     

Carotid artery stenosis: grayscale and Doppler ultrasound diagnosis--Society of Radiologists in Ultrasound consensus conference

The Society of Radiologists in Ultrasound convened a multidisciplinary panel of experts in the field of vascular ultrasonography (US) to come to a consensus regarding Doppler US for assistance in the diagnosis of carotid artery stenosis. The panel's consensus statement is believed to represent a reasonable position on the basis of analysis of available literature and panelists' experience. Key elements of the statement include the following: First, all internal carotid artery (ICA) examinations should be performed with grayscale, color Doppler, and spectral Doppler US. Second, the degree of stenosis determined at grayscale and Doppler US should be stratified into the categories of normal (no stenosis), less than 50% stenosis, 50 to 69% stenosis, > or =70% stenosis to near occlusion, near occlusion, and total occlusion. Third, ICA peak systolic velocity (PSV) and the presence of plaque on grayscale and/or color Doppler images are primarily used in the diagnosis and grading of ICA stenosis. Two additional parameters (the ICA-to-common carotid artery PSV ratio and ICA end diastolic velocity) may also be used when clinical or technical factors raise concern that ICA PSV may not be representative of the extent of disease. Fourth, ICA should be diagnosed as normal when ICA PSV is less than 125 cm/second and no plaque or intimal thickening is visible, less than 50% stenosis when ICA PSV is less than 125 cm/second and plaque or intimal thickening is visible, 50 to 69% stenosis when ICA PSV is 125 to 230 cm/second and plaque is visible, > or =70% stenosis to near occlusion when ICA PSV is more than 230 cm/second and visible plaque and lumen narrowing are seen, near occlusion when there is a markedly narrowed lumen on color Doppler US, and total occlusion when there is no detectable patent lumen on grayscale US and no flow on spectral, power, and color Doppler US. Fifth, the final report should discuss velocity measurements and grayscale and color Doppler findings. Study limitations should be noted when they exist. The conclusion should state an estimated degree of ICA stenosis as reflected in these categories. The panel also considered various technical aspects of carotid US and methods for quality assessment, and identified several important unanswered questions meriting future research.     

Assessment of Doppler velocimetry of the fetal umbilical artery by multigate spectral Doppler scanning and traditional pulsed Doppler ultrasonography plus color flow mapping.

The Doppler signal of blood flow originates from the sonographic scattering from the circulating red blood cells. However, the physics of blood flow is complex as expressed by the Bernoulli equation, and the flow velocity at different positions in the laminar flow of the same vessel is variable. Using multigate spectral Doppler scanning, we recorded multiple Doppler flow signals over a segment of the umbilical artery and compared the results with traditional pulsed Doppler ultrasonography. The intraobserver variations of the pulsatility index, the resistive index, and the systolic-to-diastolic ratio were evaluated in 30 human fetuses between 29 and 42 weeks of gestation. The correlation coefficient was calculated to establish the relationship between the results of multigate spectral Doppler scanning and the traditional pulsed Doppler ultrasonographic method. The Doppler indices of these two measurements are all significantly correlated. However, since a significant difference exists between the Doppler flow measurements of multigate spectral Doppler scanning and the traditional pulsed Doppler ultrasonographic method, the range of measurement agreement for these two methods suggests that this difference should be taken into account in the interpretation of Doppler flow velocity measurements.     

Spectral composition of Doppler signals.

The spectral content of the Doppler signal (as revealed by spectral analysis) provides useful diagnostic information about local hemodynamic conditions. Changes in these conditions are used to diagnose atherosclerotic lesions in arteries accessible to ultrasound. Extremely high frequencies, as found in and distal to a tight stenosis, indicate the presence of a jet, while Doppler signals with a wide bandwidth are related to regions with wide velocity ranges and/or disturbed flow patterns. However, the random interactions of the scatterers, the dimensions of the sample volume, the velocity of the scatterers and the method of signal processing used affect the displayed spectral composition of the Doppler signal. This article reviews the basic mechanisms and cautions the investigator against a quick and superficial interpretation of the results obtained without a proper appreciation of the random fluctuations due to the methods used to record and process the signals.     

The wall signal removal in Doppler ultrasound systems based on recursive PCA

In Doppler ultrasound (US) systems, a high-pass filter is usually employed to remove the wall component from the blood flow signal. However, this will lead to the loss of information from the low velocity flow. In this paper, an algorithm based on the principal components analysis (PCA) is proposed, in which singular value decomposition (SVD) is used to extract the main component from the mixed signals. Furthermore, the recursive process is incorporated into the PCA method to improve the performance of wall signal removal. This approach and the traditional high-pass filtering one are, respectively, applied to analyze the computer-simulated in vitro and in vivo Doppler US signals. With the proposed method, the wall signal can be removed while a large portion of low-velocity blood signal remains. Comparison experiments show that this novel approach can satisfy the requirements of Doppler US system and is practicable under a broad range of measurement conditions. Because this algorithm is based on real data, it is currently applied to unidirectional signals.     

Acoustically stimulated transient power scattering explains enhanced detection of the very low velocities in myocardial capillaries by power Doppler imaging: an in vitro study.

BACKGROUND: Although enhanced detection of myocardial perfusion signals by power Doppler imaging during contrast echocardiography has been noted, flow velocities in the coronary microvasculature should generally be below the threshold for Doppler motion detection. It has been suggested that in this situation nonlinear scattering related to acoustically stimulated microsphere oscillation or destruction may be responsible for the detected Doppler shift. METHODS AND RESULTS: This study examined the behavior of MRX 115 (ImaRx Pharmaceuticals) microbubbles during harmonic and nonharmonic power Doppler imaging at varying power outputs (mechanical indexes 0. 3, 0.5, 0.7, and 0.9) in a perfusion tube model under zero-flow conditions. Boluses of MRX 115 0.5-mL suspension were introduced into the model, and flow was halted during each imaging period. Once power Doppler imaging was implemented, a signal was detected as unique sparkling color pixels corresponding to individual bubble destruction events, even in the absence of contrast movement. This phenomenon continued until all contrast bubbles disappeared from the region subjected to power Doppler imaging, usually within 35 to 40 seconds. Off-line videointensity measurements showed that initial power Doppler signal intensity and maximum signal decay rates increased parallel to increasing power output and were substantially greater for nonharmonic than for harmonic imaging modes. CONCLUSION: This relationship between signal intensity and decay rate and acoustic power output suggests that transient scattering related to bubble destruction is responsible for generation of the power Doppler signal in the absence of flow. This would explain the enhanced detection of the very low velocity flows in the myocardial capillaries by power Doppler contrast imaging.     

Role of color and power doppler imaging in differentiating between malignant and benign solid breast masses

PURPOSE: This study analyzed the color and power Doppler signals in solid breast masses and assessed their value in differentiating malignant from benign lesions. METHODS: One hundred twenty-nine biopsy-proven solid breast masses (54 malignant and 75 benign) were evaluated with color and/or power Doppler sonography using a 7-MHz linear-array transducer. We retrospectively analyzed the location, shape, and penetration of the Doppler vascular signals in the breast masses. The location of the vascular signals was categorized as central, peripheral, or both. The shape of the signals was categorized as linear, irregular, branching, or a single dot. A penetrating vessel was defined as a continuous vascular signal extending from outside the lesion to inside it. In 43 cases, power and color Doppler sonograms were compared. RESULTS: Doppler features suggestive of malignant lesions were the presence of both peripheral and central vascularity (odds ratio, 6.0), presence of penetrating vessels (odds ratio, 5.4), and presence of branching vessels (odds ratio, 13.7). Power Doppler sonography was more sensitive than color Doppler sonography in detecting vascular signals in 49% of cases. CONCLUSIONS: Color (power) Doppler imaging is a valuable adjunct to conventional sonography in differentiating between malignant and benign breast lesions.     

能量多普勒和灰阶超声对肾实体占位的诊断及鉴别诊断

目的　探讨肾实体肿瘤的灰阶超声特征及其能量多普勒血供模式 ,并评价二者联合应用的诊断及鉴别诊断价值。方法　用灰阶超声及能量多普勒超声评价 4 2例肾实体肿瘤。在灰阶超声图上 ,观察肿块回声、均匀性、无回声边、肿瘤内无回声区及后方衰减。在能量多普勒超声上 ,肿块的血管分布被分成 5种模式。结果　 4 2例肾实体肿瘤 ,肾细胞癌 (RCC) 33例 ,肾血管平滑肌脂肪瘤 (AML ) 9例。在灰阶超声上 ,2 4例 (73% ) RCC可见无回声边和 /或肿瘤内无回声区 ;2 6例 (79% ) RCC肿块回声不均匀。 1例 AML (11% )出现声衰减。肿块回声强度不能提示病理诊断。在能量多普勒超声上 ,33例 RCC中 ,模式 3(边缘血流型 ) 4例 ,模式 4 (混合性边缘及穿入血流型 ) 2 9例。 9例 AML中 ,模式 4血流 1例 ,模式 0 (无血流信号型 ) 5例 ,模式 2 (穿入性血流型 ) 3例。在本组 RCC及 AML中均未见模式 1(肿瘤内局限性血流信号型 )。模式 0、模式 2对 AML有特异性。与单独灰阶超声 (6 0 % )及单独能量多普勒超声 (19% )的诊断率比较 ,二者联合诊断率 (79% )明显提高。结论　无回声边及肿瘤内无回声区提示 RCC的诊断。在灰阶超声发现的基础上 ,能量多普勒超声血管分布为 RCC及 AML的鉴别诊断提供了重要的依据     

Fetal heart rate and blood flow velocity variability in the presence of increased nuchal translucency: a preliminary study.

OBJECTIVE: To study the power spectrum distribution of heart rate and umbilical artery flow velocity variability in fetuses with increased nuchal translucency thickness (NT). METHODS: Doppler velocity waveforms were collected from long-lasting (>20 s) umbilical artery recordings in 18 fetuses with increased NT (>3 mm) and 18 normal controls matched for gestational age at 11-14 (median, 12) weeks. The NT group included 11 abnormal karyotypes: trisomy 18 (n = 3), 45,X (n = 4), trisomy 21 (n = 3) and a balanced translocation. Absolute heart rate as well as the coefficient of variation for both beat-to-beat heart rate variability and umbilical artery blood flow velocity variability were determined. The ratios of the integrated low-frequency components (0.05-0.2 Hz) and the integrated high-frequency ones (0.25-1.6 Hz; LH ratio) from normalized power spectrum distributions were established to reflect sympathovagal balance. RESULTS: The mean heart rate was not significantly different between the two groups. However, mean heart rate variability and time-averaged flow velocity variability were significantly increased in the NT group, while there was no significant difference in the LH ratios between the two groups. The mean umbilical artery pulsatility index was significantly higher in the NT group. CONCLUSION: The autonomic nervous system does not seem to play a role in the altered cardiovascular homeostasis in the presence of increased fetal NT.