超声多普勒功率及其在检测颅内血流分布上的应用

本文结合背向散射超声多普勒信号模型对超声多普勒功率的概念及其特性进行了阐述，并提出了一种用超声多普勒功率来检测颅内血管血流分布的方法。实验结果证明，该方法能在噪声背景下灵敏准确地反应沿声束指向一维空间各个位置是否存在血流以及血流强度的大小。这一方法已被用于经颅多普勒（ＴＣＤ）脑血流分析仪中，解决了现有经颅多普勒检查时只能凭经验盲目搜寻颅内血管的问题，并提供了颅内血管分布的相对位置信息以帮助临床医生更方便准确地判定所检测的是哪一根血管。     

A new approach to the noninvasive measurement of cardiac output using an annular array Doppler technique--I. Theoretical considerations and ultrasonic fields

This paper describes the development of a Doppler flowmeter capable of measuring blood volume flow rate without the need to measure the vessel lumen area or beam-vessel angle. It requires the production of a uniform wide ultrasound beam to encompass the whole vessel and thus to produce a Doppler spectrum which corresponds to all the flowing blood, and a narrow reference beam placed within the lumen to compensate for various unknown quantities, such as tissue attenuation. The general definition of volume flow rate is described and applied to a new flowmeter, which allows an absolute value of volume flow rate to be measured independently of vessel size, beam-vessel angle, and tissue attenuation. By electronically apodising an annular array transducer in transmission and reception, a uniform wide beam and a narrow reference reception beam are produced. Theory to predict these beam patterns is developed and a computer simulation is made. The ultrasonic fields obtained from an annular array transducer in water are compared with the theoretical fields.     

A wall-less vessel phantom for Doppler ultrasound studies

Doppler ultrasound flow measurement techniques are often validated using phantoms that simulate the vasculature, surrounding tissue and blood. Many researchers use rubber tubing to mimic blood vessels because of the realistic acoustic impedance, robust physical properties and wide range of available sizes. However, rubber tubing has a very high acoustic attenuation, which may introduce artefacts into the Doppler measurements. We describe the construction of a wall-less vessel phantom that eliminates the highly attenuating wall and reduces impedance mismatches between the vessel lumen and tissue mimic. An agar-based tissue mimic and a blood mimic are described and their acoustic attenuation coefficients and velocities are characterised. The high attenuation of the latex rubber tubing resulted in pronounced shadowing in B-mode images; however, an image of a wall-less vessel phantom did not show any shadowing. We show that the effects of the highly attenuating latex rubber vessels on Doppler amplitude spectra depend on the vessel diameter and ultrasound beam width. In this study, only small differences were observed in spectra obtained from 0.6 cm inside diameter thin-wall latex, thick-wall latex and wall-less vessel phantoms. However, a computer model predicted that the spectrum obtained from a 0.3-cm inside diameter latex-wall vessel would be significantly different than the spectrum obtained from a wall-less vessel phantom, thus resulting in an overestimation of the average fluid velocity. These results suggest that care must be taken to ensure that the Doppler measurements are not distorted by the highly attenuating wall material. In addition, the results show that a wall-less vessel phantom is preferable when measuring flow in small vessels.     

Intraluminal ultrasound intensity distribution and backscattered Doppler power

Ultrasound (US) incident obliquely on a cylindrical vessel is redistributed in space when the propagation path includes walls with acoustic impedance different from that of the surrounding media. We investigated this using low-density polyethylene (PE) as the vessel wall material. Both simulations and experiments were carried out. Direct hydrophone measurements of the acoustic field were made within a half section of the PE tube, and the distribution of backscattered Doppler power along a scan line was obtained using a range-Doppler instrument. Both simulation and hydrophone results demonstrate lateral shadow regions within the lumen. In every one of various Doppler flow experiments conducted, the backscattered Doppler power, compensated for on-axis transducer behaviour, increased with depth. Simulation results for an incident continuous-wave (CW) plane wave show that it tends to be focused by the curvature of the PE tube walls. The wall interactions are, however, angle-dependent and so the behaviour of a focused US beam depends on the beam as well as the walls. This study demonstrates alterations in the spatial distribution of US within a cylindrical vessel as a result of known vessel wall properties. It also provides evidence that local intensity variations within the lumen affect the relative Doppler power backscattered from small sample volumes.     

Comparison of methods of recording and analysis of Doppler blood velocity signals in normal subjects

Doppler blood velocity signals were recorded from the aorta and pulmonary artery in normal adults, children and premature infants, using three different pulsed, range-gated instruments. The tracings analyzed by three independent methods of spectral analysis with axially aimed transducers showed velocity patterns with a narrow range of frequencies (blunt velocity profile) with a constant acceleration of blood flow in early systole. A similar velocity pattern was seen in the premature infant's aorta in which the transducer beam was larger than the vessel insonated. We conclude that the normal velocity pattern in the central circulation is close to blunt, and that tracings obtained with transducers that insonate varying proportions of the vessel give similar signals.     

动脉粥样硬化经高强度聚焦超声作用后血管的结构和功能变化

目的 观察高强度聚焦超声（HIFU）在体辐照兔粥样硬化腹主动脉后血管的结构和功能变化，探讨HIFU对病理性血管的影响。方法建立粥样硬化兔动物模型，经HIFU辐照后，通过光镜、透射电镜、彩色多普勒血流显像（CDFI）和数字减影血管造影（DSA）测定血管组织形态学和血流动力学变化。结果 经HIFU辐照后，兔动物模型粥样硬化腹主动脉出现一定程度损伤，但无血管栓塞和破裂，实验组和对照组血管损伤程度差异无统计学意义；CDFI系列血流参数均下降（P〈0．05）；DSA显示动脉血流阻滞，充盈延迟，无血管栓塞和破裂。辐照后5d及10d观察损伤血管呈逐渐恢复趋势，动脉血流参数呈恢复趋势，对照组恢复更好。结论 HIFU辐照兔粥样硬化腹主动脉将导致可逆性损伤和血流动力学改变。     

Pulsatile flow phantom for ultrasound image-guided HIFU treatment of vascular injuries

A pulsatile flow phantom was developed for studies of ultrasound image-guided high intensity focused ultrasound (HIFU) application in transcutaneous hemostasis of injured blood vessels. The flow phantom consisted of a pulsatile pump system with instrumented excised porcine carotid artery, which was imbedded in a transparent agarose gel to model structural configuration of in vivo tissues. Heparinized porcine blood was circulated through the phantom. The artery was injured using an 18-gauge needle to model a penetrating injury in human peripheral vasculature. A HIFU transducer with the diameter of 7 cm, focal length of 6.3 cm and frequency of 3.4 MHz was used to seal the puncture. Ultrasound imaging was used to localize and target the puncture site and to monitor the HIFU treatment. Triphasic blood flows present in the human arteries were reproduced, with flow rates of 50 to 500 mL/min, pulse rates of 62 to 138 beats/min and peak pressures of 100 to 250 mm Hg. The penetrating injury of an artery was mimicked successfully in the flow phantom setting and was easily visualized both optically through the transparent gel and with power Doppler ultrasound imaging. Hemostasis was achieved in 55 +/- 31 s (n = 9) of HIFU application. Histologic observations showed that a HIFU-sealed puncture was filled with clotted blood and covered with a fibrin cap. The pulsatile flow phantom provides a controlled and repeatable environment for studies of transcutaneous image-guided HIFU application in hemostasis of a variety of blood vessel injuries.     

Precision and accuracy of Doppler flow measurements. In vitro and in vivo study of the applicability of the method in human fetuses

The precision of the Doppler method for quantitative blood flow measurement in the fetal descending thoracic aorta and in the umbilical vein and for estimation of the Pulsatility Index from the velocity curve from fetal aorta was tested in vivo by examination of six pregnant women eight times. Two investigators examined each patient twice in random order upon two successive days. The diameter of the vessel was measured using planimetry on a magnified time-motion image of the diameter variations during the heart cycle, while the angle between the ultrasound Doppler beam and the vessel of interest was measured on the hard copy image. No systematic variation was found between observers, days, repeated observations or repeated readings of curves and images. The mean coefficient of variations was 5.6% for the quantitative flow per kilogram estimated fetal weight measured in the fetal descending aorta, 6.8% for the quantitative flow per kilogram estimated fetal weight measured in the umbilical vein and 9.8% for the Pulsatility Index. When the diameter of aorta was calculated as the mean of the maximal and the minimal diameter measured on the hard copy image, the mean coefficient of variation for the flow increased to 9.4%. In vitro tests of the Doppler instrument and the real-time scanner revealed a systematic overestimation of Doppler measured flow of only 4.4% compared with the true flow, and a real-time scanner underestimation of vessel diameter of only 1.1%.     

Using doppler signal power to detect changes in vessel size: a feasibility study using a wall-less flow phantom

The power of a Doppler signal is theoretically proportional to the volume of blood within the sample volume of an ultrasound (US) beam and, hence, may provide a means of detecting in vivo changes in the cross-sectional area of cerebral vessels, such as the middle cerebral artery. The purpose of this study was to examine the relationship between power and vessel size for signals recorded from a wall-less flow phantom. The results demonstrate the importance for the in vitro case of maximising the received signal power for each channel to obtain the true relationship between power and size, and show that a nonproportional relationship observed between the two parameters is primarily caused by high-pass filtering and nonuniform insonation. In addition, an investigation of the reproducibility of power values after transducer repositioning has shown that variation occurs even when extreme care is taken to maximise the received signal intensity. The implications of these results for the in vivo use of the Doppler signal power method are discussed.     

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.     

Implementation of spectral width Doppler in pulsatile flow measurements

In this paper, we present an automatic beam-vector (Doppler) angle and flow velocity measurement method and implement it in pulsatile flow measurements using a clinical Doppler ultrasound system. In current clinical Doppler ultrasound flow velocity measurements, the axis of the blood vessel needs to be set manually on the B-scan image to enable the estimation of the beam-vector angle and the beam-vector angle corrected flow velocity (the actual flow velocity). In this study, an annular array transducer was used to generate a conical-shaped and symmetrically focused ultrasound beam to measure the flow velocity vectors parallel and perpendicular to the ultrasound beam axis. The beam-vector angle and flow velocity is calculated from the mode frequency (f(d)) and the maximum Doppler frequency (f(max)) of the Doppler spectrum. We develop a spectrum normalization algorithm to enable the Doppler spectrum averaging using the spectra obtained within a single cardiac cycle. The Doppler spectrum averaging process reduces the noise level in the Doppler spectrum and also enables the calculation of the beam-vector angle and flow velocity for pulsatile flows to be measured. We have verified the measurement method in vivo over a wide range of angles, from 52 degrees to 80 degrees, and the standard deviations of the measured beam-vector angles and flow velocities in the carotid artery are lower than 2.2 degrees and 12 cm/s (about 13.3%), respectively.     

A feasability study of color flow doppler vectorization for automated blood flow monitoring

An ongoing issue in vascular medicine is the measure of the blood flow. Catheterization remains the gold standard measurement method, although non-invasive techniques are an area of intense research. We hereby present a computational method for real-time measurement of the blood flow from color flow Doppler data, with a focus on simplicity and monitoring instead of diagnostics. We then analyze the performance of a proof-of-principle software implementation. We imagined a geometrical model geared towards blood flow computation from a color flow Doppler signal, and we developed a software implementation requiring only a standard diagnostic ultrasound device. Detection performance was evaluated by computing flow and its determinants (flow speed, vessel area, and ultrasound beam angle of incidence) on purposely designed synthetic and phantom-based arterial flow simulations. Flow was appropriately detected in all cases. Errors on synthetic images ranged from nonexistent to substantial depending on experimental conditions. Mean errors on measurements from our phantom flow simulation ranged from 1.2 to 40.2% for angle estimation, and from 3.2 to 25.3% for real-time flow estimation. This study is a proof of concept showing that accurate measurement can be done from automated color flow Doppler signal extraction, providing the industry the opportunity for further optimization using raw ultrasound data.     

Validation of a sensitivity performance index test protocol and evaluation of colour Doppler sensitivity for a range of ultrasound scanners

The ability to detect flow is the most crucial aspect of an ultrasound (US) system because, if flow cannot be detected, no other aspect of performance matters. The objectives of this study were to validate a Doppler "sensitivity performance index," a figure of merit, and to determine if it could be used to differentiate colour Doppler sensitivity performance in scanners of varying complexity. The sensitivity performance index was developed to give a combined measure of related aspects of sensitivity, such as the lowest detectable velocity, the vessel size and the penetration depth. The colour Doppler sensitivity was evaluated objectively as the lowest detectable velocity signal from the deepest achievable point within the Doppler sensitivity phantom free from extraneous noise in a small diameter vessel (3.2 mm inner diameter). The effect of vessel size and mean velocity on the sensitivity performance index were investigated and it was found that the index was not proportional to vessel size, but this may be accounted for by considering the effect of the acoustic properties of the vessel material, the clutter filter and beam shape. The results obtained using flow phantoms with vessel sizes different from those used in this study are, therefore, not directly comparable to the results found in this study; however, a similar trend should be found in the results for the effect of control settings and a similar range of US scanners. It was found that the Doppler sensitivity performance index was a robust challenging test because none of the US scanners evaluated was capable of achieving the highest sensitivity performance index score, which would be limited by the lowest pump velocity and the deepest point of the vessel within the flow phantom. Therefore, this suggests that this method of determining Doppler sensitivity performance is valuable in the absence of other suitable methods, despite the fact that the relationship between the sensitivity performance index and vessel size is not proportional. Furthermore, use of the Doppler sensitivity performance index for the evaluation of a range of scanners demonstrated that curvilinear transducers have higher sensitivity performance indices than higher-frequency linear transducers, due to the higher achievable penetration depth. The effect of instrument settings was assessed for two transducers, the 4C3 curvilinear general-purpose transducer (Aspen) and the PVM375AT curvilinear general-purpose transducer (Nemio). The colour Doppler sensitivity performance was found to be significantly dependent on the clutter filter setting and the output power setting for both transducers tested. Users need to be aware of the effect of these settings on the colour Doppler sensitivity performance of their US scanner when interpreting the clinical significance of the colour Doppler information.     

The use of a Diasonics DRF400 duplex ultrasound scanner to measure volume flow in arterio-venous fistulae in patients undergoing haemodialysis: an analysis of measurement uncertainties

A pilot study to measure volume flow in arterio-venous fistulae in patients undergoing haemodialysis was undertaken to determine the accuracy and precision of values obtained using a modern duplex ultrasound scanner. A Diasonics DRF400 duplex ultrasound scanner was used with a small parts 10 MHz mechanical sector probe incorporating a 4.5 MHz Doppler transducer. Volume flow was measured in the brachial artery of 13 patients with surgically created arterio-venous fistulae in the distal circulation. Volume flow was also measured using a flow phantom in which the true volume flow was known. An analysis of the measurements made by the duplex scanner was performed to determine the sources of random and systematic uncertainty. Repeatability was improved with the probe clamped in position over the brachial artery giving a coefficient of variation in repeated measurements of volume flow in a patient of +/- 12%. Measured flows ranged from 884-3088 mL min-1. Comparison with measurements on the flow phantom showed these values from patients to be overestimated by 200-600 mL min-1. The values from the phantom should therefore be used to calibrate patient measurements. Analysis of uncertainties showed that precision was limited by random uncertainties in the measurement of vessel diameter and Doppler angle.     

Assessment of the effect of vessel curvature on Doppler measurements in steady flow

Blood vessel curvature is responsible for the appearance of nonaxial velocity components and for minor changes in the pattern of the axial flow. All the velocity components are expected to contribute to the Doppler signal produced by the ultrasound (US) backscattered by the insonated blood cells, the axial velocity, contributing to the actual volumetric blood flow, and the transverse velocity, causing the recirculating vortices. A detailed, separate analysis of the velocity components is, therefore, mandatory to quantify how vessel curvature can affect results and clinical diagnosis. Both experimental in vitro measures and numerical simulations were performed on a curved tube and the Doppler power spectra so obtained were compared. The satisfactorily agreement of the above spectra shows that the nonaxial velocity components are easily detectable with clinical equipment and that their amplitude, as expected, is not negligible and can bias Doppler measurements and resulting clinical diagnosis.     

Doppler spectral characterization of flow disturbances in the carotid with the Doppler probe at right angles to the vessel axis

This paper reports on a new method intended to detect early flow disturbances generated by small lesions, using conventional clinical instrumentation. In vitro experiments on models of stenotic vessels are presented which prove that ultrasound Doppler, with the beam directed at right angles to the vessel axis can detect vortices and other flow disturbances caused by wall irregularities. These disturbances characterized by small velocity components first toward and then away from the transducer correlate with the spectrum of vortices caused by small artificial lesions. We found these disturbances in flow to be too small to cause detectable broadening in the Doppler spectrum acquired in the traditional way (i.e. with the beam at an angle less than 90 degrees). The detected flow disturbances were found to depend on the surface roughness, the profile of the obstructive lesion and the narrowing of the vessel. Similar flow disturbances to those detected in vitro were demonstrated in vivo for this new beam orientation in regions of the carotid, such as the bulb and the beginning of the common carotid, where vortex-like flows are expected.     

Measurement of the Doppler Power of Flowing Blood Using Ultrasound Doppler Devices

Measurement of the Doppler power of signals backscattered from flowing blood (henceforth referred to as the Doppler power of flowing blood) and the echogenicity of flowing blood have been used widely to assess the degree of red blood cell (RBC) aggregation for more than 20 y. Many studies have used Doppler flowmeters based on an analogue circuit design to obtain the Doppler shifts in the signals backscattered from flowing blood; however, some recent studies have mentioned that the analogue Doppler flowmeter exhibits a frequency-response problem whereby the backscattered energy is lost at higher Doppler shift frequencies. Therefore, the measured Doppler power of flowing blood and evaluations of RBC aggregation obtained using an analogue Doppler device may be inaccurate. To overcome this problem, the present study implemented a field-programmable gate array-based digital pulsed-wave Doppler flowmeter to measure the Doppler power of flowing blood, in the aim of providing more accurate assessments of RBC aggregation. A clinical duplex ultrasound imaging system that can acquire pulsed-wave Doppler spectrograms is now available, but its usefulness for estimating the ultrasound scattering properties of blood is still in doubt. Therefore, the echogenicity and Doppler power of flowing blood under the same flow conditions were measured using a laboratory pulser–receiver system and a clinical ultrasound system, respectively, for comparisons. The experiments were carried out using porcine blood under steady laminar flow with both RBC suspensions and whole blood. The experimental results indicated that a clinical ultrasound system used to measure the Doppler spectrograms is not suitable for quantifying Doppler power. However, the Doppler power measured using a digital Doppler flowmeter can reveal the relationship between backscattering signals and the properties of blood cells because the effects of frequency response are eliminated. The measurements of the Doppler power and echogenicity of flowing blood were compared with those obtained in several previous studies.     

A real vessel phantom for flow imaging: 3-D Doppler ultrasound of steady flow

Vascular phantoms are used to assess the capabilities of various imaging techniques, such as x-ray CT and angiography, and B-mode, power Doppler, and colour Doppler ultrasound (US). They should, therefore, accurately mimic the vasculature, blood, and surrounding tissue, in regard to both imaging properties and vessel geometry. In the past, a variety of walled and wall-less vessel models have been used. However, these models only approximate the true vessel geometry, and generally lack pathologic features such as plaques or calcifications. To amend these deficiencies, we have developed a real vessel phantom for US and x-ray studies, which comprises a fixed human vessel specimen, cannulated onto two acrylic tubes, and embedded in agar in an acrylic box. Earlier, we demonstrated a good overall correlation between x-ray angiography, CT, and 3-D B-mode US images of this phantom. Here, we extend its use to flow imaging with 3-D power and 3-D colour Doppler US.     

向量血流成像技术评估兔腹主动脉早期动脉粥样硬化的价值

目的:探讨全新的向量血流成像技术(vector flow imaging technique,V Flow)测量的血管壁应力(wall shear stress,WSS)在评估兔腹主动脉早期动脉粥样硬化(atherosclerosis,AS)中的价值。方法:选用健康雄性新西兰兔7只,高脂饮食喂养建立兔腹主动脉AS模型。喂养14周后,每周采用常规灰阶超声观察兔腹主动脉管壁是否有硬化斑块形成,采用彩色多普勒血流图(color Doppler flow imaging,CDFI)测量腹主动脉血流量(blood flow volume,BFV)及收缩期峰值流速(peak systolic velocity,PSV),采用V Flow观察并测量兔腹主动脉前、后壁WSS值。34周后,以病理检查结果为金标准,对比并分析WSS在兔腹主动脉AS形成及发展过程中的动态变化。结果:病理结果显示,高脂饮食喂养34周后兔腹主动脉出现AS脂纹期的典型改变,37周后兔腹主动脉有AS纤维斑块的表现。腹主动脉前、后壁WSS随着AS病程的进展动态变化:从14周开始呈降低趋势,20/21周至27周表现为增高趋势,27周之后又逐渐降低。腹主动脉前壁和后壁WSS最大值(WSS_(max))、WSS平均值(WSS_(mean))的拐点值与第0周基线值差异具有统计学意义(P0.05),WSS_(max)、WSS_(mean)变化曲线出现拐点的时间早于常规CDFI测得的PSV及BFV值。结论:V Flow较传统的CDFI可更早预测腹主动脉AS斑块及评估相应血流动力学改变,具有潜在的临床应用价值。