Origin of the Doppler ultrasound spectrum from blood

Flexible Ultrasound Transducer With Embedded Optical Shape Sensing Fiber for Biomedical Imaging Applications

Flexible ultrasound transducers (FUTs), capable of conforming to irregular surfaces, have become a research hotspot in the field of medical imaging. With these transducers, high-quality ultrasound images can be obtained only if strict design criteria are fulfilled. Moreover, the relative positions of array elements must be determined, which are important for ultrasound beamforming and image reconstruction. These two major characteristics present great challenges to the design and fabrication of FUTs compared to that for traditional rigid probes. In this study, an optical shape-sensing fiber was embedded into a 128-element flexible linear array transducer to acquire the real-time relative positions of array elements to produce high-quality ultrasound images. Minimum concave and convex bend diameters of approximately 20 and 25 mm, respectively, were achieved. The transducer was flexed 2000 times, and yet no obvious damage was observed. Stable electrical and acoustic responses confirmed its mechanical integrity. The developed FUT exhibited an average center frequency of 6.35 MHz, and average -6-dB bandwidth of 69.2%. The array profile and element positions measured by the optic shape-sensing system were instantly transferred to the imaging system. Phantom experiments for both spatial resolution and contrast-to-noise ratio proved that FUTs can maintain satisfactory imaging capability despite bending to sophisticated geometries. Finally, color Doppler images and Doppler spectra of the peripheral arteries of healthy volunteers were obtained in real time.

Determining the relationship between three-dimensional power Doppler data and true blood flow characteristics: an in-vitro flow phantom experiment

OBJECTIVES

Three-dimensional (3D) ultrasound can be used to acquire power Doppler data which can be quantified to give an objective impression about blood flow within a tissue or organ. Proprietary software can be used to calculate three indices of vascularity: vascularization index (VI), flow index (FI) and vascularization flow index (VFI). Although these indices appear to have a predictive value in the clinical setting and can be shown to vary between different patient populations and over time within the same population, their relationship with true in-vivo blood flow characteristics has not been established. The objective was to examine the effect of flow rate, vessel number, attenuation and erythrocyte density on these indices.

METHODS

A computer-driven flow phantom was used to continuously pump a nylon particle-based blood mimic (Orgasol(trade mark)) around a closed system through three different ultrasound test tanks. These tanks were designed specifically for these experiments and contained C-Flex(trade mark) tubing, in a variety of arrangements, encased in an agar-based tissue mimic. The test tanks were insonated with a modified 3D transvaginal 4-8-MHz ultrasound transducer and 3D power Doppler data were then acquired over a graduated series of flow rates, depths and blood mimic concentrations. Regression analysis was used to determine the resulting relationships.

RESULTS

The VI increased linearly with an increase in flow rate (P < 0.05), whereas the FI increased in a cubic manner with a more rapid initial increase (P < 0.05). The VI demonstrated a similar linear increase with an increase in the erythrocyte mimic density (P < 0.05), whereas the FI increased markedly with a small change in erythrocyte mimic density and then plateaued (P < 0.01). There was a significant reduction in each index as the distance between the transducer and vessel increased (P < 0.05). Patterns similar to those seen in relation to the change in flow rate were evident, with a more linear relationship between depth and the VI and VFI than between depth and the FI, although the FI remained relatively constant and was not significantly affected by distance from the transducer until a depth of 55 mm was reached. Although a positive linear relationship was seen between vessel number and VI and VFI (P < 0.05) the FI demonstrated a very different and complex, cubic relationship (P < 0.001), increasing linearly until a maximum of three vessels were present when it decreased, and no overall correlation was seen (P > 0.05).

CONCLUSIONS

The VI, FI and VFI are all significantly affected by volume flow, attenuation, vessel number and erythrocyte density, but in different ways. The VI and VFI seem to have a more predictable relationship, whereas the FI often demonstrates a more complex cubic relationship that is not always logical. Further work is required to establish the effect of other confounding parameters before valid conclusions may be made and a better understanding of 3D power Doppler ultrasound imaging achieved.

A change of blood flow during strenuous physical exercises using cycle ergometer

Our purpose of the study is to measure the blood flow in the carotid artery during physical exercises. We have developed the blood flow velocimeter during physical exercises by using the non-invasive Doppler ultrasound method. We used it and measured blood flow in the carotid artery in which using a cycle ergometer and had two exercise tests in this report. The protocol exercise with transient response and ramp response were conducted. As a result, maximum blood flow velocities changed largely during physical exercises. In particular the maximum blood flow velocity was decreased during physical exercise in transient response. In addition, we could also detect arrhythmia that occurred in the subject immediately after exercise. For this reason, blood flow measurement during physical exercise maybe useful for the early detection of cardiovascular disease. Recently, the physical exercises are very popular for health maintenance.

Monitoring Carotid Blood Flow and ECG for Cardiovascular Disease in Elder Subjects

This report is to investigate the correlation between common carotid blood flow (CCBF) properties and the risk factors for cardiovascular disease. The measurement system was designed for simultaneously monitoring of CCBF and electrocardiogram (ECG) in order to get more valid information for detection and diagnosis of cardiovascular disease. In our study, blood flow velocimeter was developed to measure blood flow in carotid by using ultrasound Doppler technique. This measurement system is based on a continuous-wave (CW) Doppler ultrasound method with two semicircular piezoelectric (PZT) transducers, one continuously transmitting ultrasound, and the other continuously receiving the echoes. Eleven patients with cardiovascular disease underwent in the experiment. In the report, blood flow was compared with data from 25 healthy subjects which asymptomatic subjects putatively free of cardiovascular disease as controls. The maximum of CCBF velocity was 102.8 (SD 18.3) cm/s in the 25 healthy subjects. While, the maximum of CCBF velocities were 53.9 (SD 16.6) cm/s in the eleven cardiac patients. The data suggest that CCBF velocity decreased significantly in the cardiac patients and by simultaneously monitoring of CCBF and ECG were probably obtained more valid information to detect and diagnose cardiovascular disease at the early stage.

Variations of the maximum blood flow velocity in the carotid, brachial and femoral arteries in a passive postural changes by a Doppler ultrasound method

We have developed a blood flow velocimeter to measure maximum blood flow velocity (MBFV) in carotid, brachial and femoral arteries simultaneously by using a Doppler ultrasound technique. This measurement system is based on a continuous wave Doppler ultrasound method with two semicircular piezoelectric (PZT) transducers, one continuously transmitting ultrasound, and the other continuously receiving the echoes. This report is to investigate variations of MBFV in 3 arteries of 5 healthy normal male volunteers during passive postural changes in supine, 90 degrees head-down-tilt (HDT) and 90 degrees head-up-tilt (HUT) respectively. As a result of these measurements, we could confirm that MBFV in those arteries especially in brachial and femoral are increased by supine to HDT posture and are decreased by supine to HUT posture. But, it's just changed a little in carotid artery. An increasing and a decreasing in MBFV are expected to be an effect of venous pressure and a constant MBFV in carotid artery is caused a cerebral autoregulation.

Cyclic and radial variation of the Doppler power from porcine whole blood

The Doppler power from porcine blood was observed in a mock flow loop to have cyclic and radial variation during a pulsatile cycle. It was found to decrease with shear rate under steady flow, except near the center of the tube at which other mechanisms such as the effects of radial distribution on the rouleaux might be involved. Under pulsatile flow, the timing of the peak of the Doppler power measured at the center of the tube became closer to the peak systole from 20 to 60 beats/minute (BPM), and the power and velocity peaks coincided at 60 BPM. The overall radial variation of the Doppler power during a whole pulsatile cycle was prominent due to the increase of shear rate from the center to 4.5 mm radial position within a tube of 6.35 mm radius. The cyclic variation of the Doppler power varied with the radial position, being relatively large at the center, reaching a minimum at an intermediate radial position, and increasing again near the wall. The peak of the Doppler power occurred at early systole near the tube wall and lagged the flow closer to the center. The "black hole" phenomenon was observed only over portions of the flow cycle. All these complex variations of the Doppler power across the tube over a cycle are thought to be the result of red cell aggregation, which can be affected by shear rate and acceleration.

Velocity magnitude estimation with linear arrays using Doppler bandwidth

The dependence of pulsed wave Doppler bandwidth on parameters typical of linear transducer arrays used in commercial Duplex and color flow mapping systems is investigated experimentally. For a single flow line it is observed that this bandwidth generally depends not only on the scatterer velocity and the beam-to-flow angle, but also on the flow line range and orientation. This is due to the fact that in Duplex and color flow systems the transducer is differently focused in the scan and elevation planes and its aperture and focal lengths are often made to vary, depending on the distance of the flow line from the transducer. It is however experimentally demonstrated that, at points where the ultrasound beamwidths in the scan and elevation planes are both comparable to the sample volume length, the Doppler bandwidth is independent of the beam-to-flow angle. It is also shown that this invariance can be extended to other ranges by appropriately modifying the array aperture. Finally, as an application of this independence, the flow-line velocity magnitude in these beam regions is estimated with better than 5% uncertainty through a simple bandwidth measurement.

In vitro validation of volumetric blood flow measurement using Doppler flow wire

Determination of any volumetric blood flow requires assessment of mean blood flow velocity and vessel cross-sectional area. For evaluation of coronary blood flow and flow reserve, however, assessment of average peak velocity alone is widely used, but changes in velocity profile and vessel area are not taken into account. We studied the feasibility of a new method for calculation of volumetric blood flow by Doppler power using a Doppler flow wire. An in vitro model with serially connected silicone tubes of known lumen diameters (1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mm) and pulsatile blood flow ranging from 10 to 200 mL/min was used. A Doppler flow wire was connected to a commercially available Doppler system (FloMap(R), Cardiometrics) for online calculation of the zeroth (M(0)) and the first (M(1)) Doppler moment, as well as mean flow velocity (V(m)). Two different groups of sample volumes (at different gate depths) were used: 1. two proximal sample volumes lying completely within the vessel were required to evaluate the effect of scattering and attenuation on Doppler power, and 2. distal sample volumes intersecting completely the vessel lumen to assess the vessel cross-sectional area. Area (using M(0)) and V(m) (using M(1)/M(0)) obtained from the distal gates were corrected for scattering and attenuation by the data obtained from the proximal gates, allowing calculation of absolute volumetric flow. These results were compared to the respective time collected flow. Correlation between time collected and Doppler-derived flow measurements was 0.98 (p < 0.0001), with a regression line close to the line of equality indicating an excellent agreement of the two measurements in each individual tube. The mean paired flow difference between the two techniques was 1.5 +/- 9.0 mL/min (ns). Direct volumetric blood flow measurement from received Doppler power using a Doppler flow wire system is feasible. This technique may potentially be of great clinical value because it allows an accurate assessment of coronary flow and flow reserve with a commercially available flow wire system.

Doppler power spectrum from a Gaussian sample volume

A closed-form expression for the Doppler power spectrum due solely to the range of blood velocities passing through a Gaussian sample volume placed anywhere in a vessel under conditions of axisymmetric flow, uniform backscatter and negligible intrinsic spectral broadening has been derived. The formulation presented here allows the independent specification of the sample volume position and width, in the three dimensions, and enables simple estimations of spectral shape for pulsed wave Doppler systems. Simpler expressions were derived for the cases of symmetric sample volume projections onto the vessel cross-section and/or sample volumes centred in the vessel. Closed form expressions were derived for mean frequency and spectral width in the case of a symmetric sample volume projection centred in the vessel. The effects of sample volume size and position on the Doppler spectral width and mean frequency are shown for a range of velocity profiles.

Ultrasonic pulsed Doppler blood flowmeter for use in extracorporeal circulation

In cardiac surgeries, it is frequently necessary to make the external blood circulation and oxygenation artificially. This procedure is called extracorporeal circulation (EC) or heart-lung bypass. During EC, one of the most important parameters, which demands continuous monitoring, is the blood flow. In many cases, the blood flow is estimated by the pump velocity (in roller pumps) or measured with transducers based on electromagnetic methods (in centrifugal pumps). This article presents an ultrasonic Doppler blood flowmeter to be used in the arterial line of an extracorporeal circulation system. The ultrasonic probe is coupled to a half-inch tube connector of the extracorporeal arterial line, is not disposable, and does not need sterilization. The developed flowmeter revealed itself to be efficient and reliable and can be inserted in a closed-loop pump controller system or be used as stand-alone equipment.

Power Doppler-derived speckle tracking image of intraventricular flow in patients with anterior myocardial infarction: correlation with left ventricular thrombosis

The abnormal spatial distribution of intraventricular flow is superior to clinical and two-dimensional (2-D) echocardiographic variables in predicting left ventricular thrombosis after myocardial infarction. Echocardiography was prospectively performed in 79 patients within 72 h after anterior wall myocardial infarction onset and repeated before discharge. The apical rotating flow pattern in color flow map was recognized as abnormal. By power Doppler echocardiography, the moving blood could generate speckle tracking images to delineate the intraventricular flow. A swirling flow pattern indicating the compartmentalization of left ventricular blood flow with some blood stagnant in the apical dyssynergic area was identified. The flow pattern shown by the speckle tracking image was superior to the color-flow map in correlating with left ventricular thrombosis. It implicated that the more the detail in which we can describe the blood flow pathway, the more information we can realize.

The effect of hemodynamics, vessel wall compliance and hematocrit on ultrasonic Doppler power: an in vitro study

Previous in vitro studies in rigid tubes under pulsatile flow conditions have reported a lack of a cyclic variation in blood echogenicity that contradicts in vivo results. To investigate whether or not these variations can be attributed to the compliance of the vessel wall, a series of in vitro experiments with compliant tubes, under pulsatile flow conditions, was performed. Two important factors that may affect the Doppler power were investigated: 1. the dependence on hematocrit and 2. the effect of the vessel wall elasticity. In the present study, it is shown that, at the low beat rates, the peak of the mean Doppler power within the flow cycle depends on the vessel wall compliance. When the vessel becomes more compliant, the peak is shifted from the early to the late systole. Additionally, there is a correlation between the power peak and hematocrit that is more evident in compliant vessels. At a higher pulsation rate of 37 beats/min, a different variation is observed. A drop in the power occurs near peak systole in compliant tube experiments and is more pronounced as the vessel becomes more constricted. The observed power drop agrees with previously reported in vivo results, but is not seen in rigid tube experiments. The results of this study suggest that proper interpretation of cyclic variations in Doppler power requires a knowledge of hemodynamic parameters, such as the modulus of elasticity of the vessel wall, propagation velocity or, possibly, the phase angle of input impedance.

Relation of the flow field distal to a moderate stenosis to the Doppler power

An experimental investigation was undertaken to establish how different flow regimes affect the Doppler signal. A rigid tube model consisting of a 70% asymmetric area stenosis was used with steady and pulsatile flow conditions. The characteristics of the flow field at various sites was determined using a photochromic flow visualization method. Continuous-wave Doppler measurements were made using a 41% suspension of human red blood cells (RBCs) in saline as well as a dilute suspension of 4% fixed RBCs. For steady flow, the photochromic results indicated that for Reynolds numbers (Re) of 545 and 1410, turbulence was generated and the length of the turbulent region was found to increase with increasing Re. Under pulsatile flow conditions, turbulence was triggered around peak systole and began to dissipate in late deceleration, and by the end of diastole the flow field almost relaminarized. During the turbulent phase of the flow cycle, the poststenotic flow field was seen to consist of four distinct flow regimes similar to those observed for steady flow. For higher Womersley parameters and Reynolds numbers the turbulent zone was found to be larger and to occupy a greater fraction of the flow cycle. These flow visualization results were compared with the Doppler power measurements made at the same locations and under similar flow conditions. At physiological hematocrits (41%) the onset of turbulence for both steady and pulsatile flow increased the backscattered Doppler power. The location of the peak Doppler power coincided with the region of maximum turbulence observed using the photochromic technique.