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。     

Differences in the erythrocyte aggregation level between veins and arteries of normolipidemic and hyperlipidemic individuals

The objectives of this study were to detect differences in the Doppler power backscattered by blood in vivo, and to identify factors affecting the backscattered power. The main hypothesis was that variations in the erythrocyte aggregation level between veins and arteries of normolipidemic and hyperlipidemic individuals can be detected with power Doppler ultrasound. Doppler measurements were performed at 5 MHz, with an Acuson 128 XP/10 system, over the carotid artery and jugular vein, external iliac artery and vein, common femoral artery and vein and popliteal artery and vein. Doppler signals were recorded at the center of each vessel to optimize the detection of erythrocyte aggregation, and processed off-line to obtain the backscattered power. The power of each recording was compensated for Doppler gain differences, tissue attenuation with depth and transmitted power variations occurring with pulse-repetition interval modifications. Results showed statistically stronger backscattered power in veins compared to arteries for the iliac, femoral and popliteal sites. In comparison with healthy subjects, stronger powers were observed in hyperlipidemic patients for the femoral and popliteal sites. Power differences were also found between peripheral measurements. On the other hand, no difference was observed between the power measured in the carotid artery and jugular vein for both groups of individuals. Multiple linear regression analyses were performed to identify factors affecting the backscattered power. Results showed a correlation (r) of 71.2% between the Doppler power in the femoral vein and the linear combination of two parameters: an erythrocyte aggregation index S₁₀ measured with a laser scattering method, and the diameter of the vessel measured on B-mode images. Statistically significant linear correlation levels were also found between S₁₀ and the Doppler power in various vessels. In conclusion, this study showed that power Doppler differences exist in vivo in large vessels between veins and arteries of normolipidemic and hyperlipidemic individuals. The Doppler power variations were also shown to be related to erythrocyte aggregation.     

Doppler power variation from porcine blood under steady and pulsatile flow

Although a number of recent studies have demonstrated that the echogenicity of blood varies as a function of time under pulsatile flow, the fundamental mechanisms responsible for it are still uncertain. To better understand this phenomenon, the Doppler power from porcine blood and polystyrene microsphere suspensions was measured at the center of the tube as functions of two crucial parameters, flow velocity and stroke rate (for pulsatile flow), under steady and pulsatile flow in a mock flow loop. In the present study, the experimental results were obtained with a 10-MHz pulsed Doppler system with a frequency response estimated more accurately by electronic injection, and validated by comparing to the radiofrequency (RF) signal acquired from the same Doppler instrument. The results show that the Doppler power from microspheres and porcine red blood cell (RBC) suspensions did not vary appreciably (< 2 dB), with either the speed or stroke rate (for pulsatile flow only) under steady and pulsatile flow. It was found that the Doppler power from porcine whole blood under steady flow decreased with the speed by approximately 13 dB from 3 to 33 cm/s and was only 3 dB higher than that from RBC suspension at 33 cm/s, suggesting minimal RBC aggregation in whole blood at this speed. The apparent cyclic variation from whole blood was observed at 20 and 40 beats/min (BPM). The cyclic variation became more obvious as the speed and stroke rate decreased. The mean Doppler power over a cycle increased as the peak speed decreased. The Doppler power reached a maximum near peak systole and a minimum at late diastole at the center of the tube. This pattern cannot be explained by RBC aggregation due to the shear rate alone, and may be attributed to acceleration and deceleration along with aggregation. The cyclic variation was not observed at 60 BPM, probably because of a lack of time for aggregation to occur.     

Ultrasonic observation of blood disturbance in a stenosed tube: effects of flow acceleration and turbulence downstream

Red blood cell (RBC) aggregation is known to be highly dependent on hemodynamic parameters such as shear rate, flow turbulence and flow acceleration under pulsatile flow. The effects of all three hemodynamic parameters on RBC aggregation and echogenicity of porcine whole blood were investigated downstream of an eccentric stenosis in a mock flow loop using B-mode images with Doppler spectrograms of a commercial ultrasonic system. A hyperechoic parabolic profile appeared downstream during flow acceleration, yielding another piece of evidence suggesting that the enhancement of rouleaux formation may be caused by flow acceleration. It was also found that echogenicity increased locally at a distance of three tube diameters downstream from the stenosis. The local increase of echogenicity is thought to be mainly due to flow turbulence. The hypoechoic "black hole" was also seen at the center of the tube downstream of the stenosis where blood flow was disturbed, and this may be caused by the compound effect of flow turbulence and shear rate.     

Statistical variations of ultrasound signals backscattered from flowing blood

The statistical distributions of ultrasonic signals backscattered from blood have recently been used to characterize hemodynamic properties, such as red blood cell (RBC) aggregation and blood coagulation. However, a thorough understanding of the relationship between blood properties and the statistical behavior of signals backscattered from flowing blood is still lacking. This prompted us to use the statistical parameter to characterize signals backscattered from both whole blood and RBC suspensions at different flow velocities (from 10 to 60 cm/s) and hematocrits (from 20% to 50%) under a steady laminar flow condition. The Nakagami parameter, scaling parameter, backscatter amplitude profile and flow velocity profile across a flow tube were acquired using a 10 MHz focused ultrasonic transducer. The backscattered signal peaked approximately at the centerline of the flow tube due to the effects of RBC aggregation, with the peak value increasing as the flow velocity of whole blood decreased. The Nakagami parameter increased from 0.45 to 0.78 as the flow velocity increased from 10 to 60 cm/s. The probability density function (PDF) of signals backscattered from flowing whole blood conformed with a pre-Rayleigh distribution. The Nakagami parameter was close to 1 for signals backscattered from RBC suspensions at all the flow velocities and hematocrits tested, for which the PDF was Rayleigh distributed. These differences in the statistical distributions of backscattered signals between whole blood and RBC suspensions suggest that variations in the size of dynamic scatterers in the flow affect the shape of the backscattered signal envelope, which should be considered in future statistical models used to characterize blood properties. (E-mail: j648816n@ms23.hinet.net and shyhhau@cycu.edu.tw)     

High Spatial and Temporal Resolution Observations of Pulsatile Changes in Blood Echogenicity in the Common Carotid Artery of Rats

Previous studies have found that ultrasound backscatter from blood in vascular flow systems varies under pulsatile flow, with the maximum values occurring during the systolic period. This phenomenon is of particular interest in hemorheology because it is contrary to the well-known fact that red blood cell (RBC) aggregation, which determines the intensity of ultrasound backscatter from blood, decreases at a high systolic shear rate. In the present study, a rat model was used to provide basic information on the characteristics of blood echogenicity in arterial blood flow to investigate the phenomenon of RBC aggregation under pulsatile flow. Blood echogenicity in the common carotid arteries of rats was measured using a high-frequency ultrasound imaging system with a 40-MHz probe. The electrocardiography-based kilohertz visualization reconstruction technique was employed to obtain high-temporal-resolution and high-spatial-resolution time-course B-mode cross-sectional and longitudinal images of the vessel. The experimental results indicate that blood echogenicity in rat carotid arteries varies during a cardiac cycle. Blood echogenicity tends to decrease during early systole and reaches its peak during late systole, followed by a slow decline thereafter. The time delay of the echogenicity peak from peak systole in the present results is the main difference from previous in vitro and in vivo observations of backscattering peaks during early systole, which may be caused by the very rapid heart rates and low RBC aggregation tendency of rats compared with humans and other mammalian species. The present study may provide useful information elucidating the characteristics of RBC aggregation in arterial blood flow.     

High-frequency ultrasound Doppler system for biomedical applications with a 30-MHz linear array

In this paper, we report the development of the first high-frequency (HF) pulsed-wave Doppler system using a 30-MHz linear array transducer to assess the cardiovascular functions in small animals. This array-based pulsed-wave Doppler system included a 16-channel HF analog beamformer, a HF pulsed-wave Doppler module, timing circuits, HF bipolar pulsers and analog front ends. The beamformed echoes acquired by the 16-channel analog beamformer were fed directly to the HF pulsed-wave Doppler module. Then the in-phase and quadrature-phase (IQ) audio Doppler signals were digitized by either a sound card or a Gage digitizer and stored in a personal computer. The Doppler spectrogram was displayed on a personal computer in real time. The two-way beamwidths were determined to be 160 microm to 320 microm when the array was electronically focused at different focal points at depths from 5 to 10 mm. A micro-flow phantom, consisting of a polyimide tube with an inner diameter of 127 microm and the wire phantom were used to evaluate and calibrate the system. The results show that the system is capable of detecting motion velocity of the wire phantom as low as 0.1 mm/s, and detecting blood-mimicking flow velocity in the 127-microm tube lower than 7 mm/s. The system was subsequently used to measure the blood flow in vivo in two mouse abdominal superficial vessels, with diameters of approximately 200 microm, and a mouse aorta close to the heart. These results demonstrated that this system may become an indispensable part of the current HF array-based imaging systems for small animal studies.     

Pulsed Doppler ultrasound detection of flow disturbances in arteriosclerosis.

Pulsed Doppler ultrasound blood flow detection has been used in a noninvasive manner to detect arterial abnormalities associated with arteriosclerosis. Sound spectrograms of ultrasound signals obtained from in vitro and animal studies in which flow was disturbed by obstacles placed in the flow stream showed a different distribution of energy over frequency than signals obtained from studies with no flow disturbances. Similar findings were seen clinically. A technique has been developed which can detect disturbed flow patterns resulting from partial occlusion in important superficial arteries (e.g. femoral and carotid) up to 15 cm distal to localized arterial wall abnormalities. Thirty-five arterial examinations of normal and arteriographically abnormal arteries in 12 patients revealed a sensitivity of 83 percent and a specificity of 61 percent. This study suggests that pulsed Doppler ultrasound may be useful as a screening technique for detection of arteriosclerotic lesions in major superficial arteries.     

Cyclic changes in blood echogenicity under pulsatile flow are frequency dependent

Previous in vivo and in vitro studies have demonstrated that blood echogenicity varies under pulsatile flow, but such changes could not always be measured at physiological stroke rates. The apparent contradiction between these studies could be a result of the use of different ultrasound frequencies. Backscattered signals from porcine blood were measured in a pulsatile Couette flow apparatus. Cyclic changes in shear rate for stroke rates of 20 to 70 beats per minute (BPM) were applied to the Couette system, and different blood samples were analyzed (normal blood and blood with hyperaggregating erythrocytes promoted with dextran). To confirm that cyclic echogenicity variations were observable, spectral analysis was performed to verify if changes in echo-amplitude corresponded to the stroke rate applied to the flow. Echogenicity was measured with two single-element transducers at 10 and 35 MHz. At 35 MHz, cyclic variations in backscatter were observed from 20 to 70 BPM. However at 10 MHz, they were detected only at 20 BPM. For all cases except for hyperaggregating red blood cells (RBCs) at 20 BPM, the magnitude of the cyclic variations were higher at 35 MHz. We conclude that cyclic variations in RBC aggregation exist at physiological stroke rates, unlike what has been demonstrated in previous in-vitro studies at frequencies of 10 MHz. The increased sensitivity at 35 MHz to small changes in aggregate size might be the explanation for the better characterization of RBC aggregation at high stroke rates. Our results corroborate in-vivo observations of cyclic blood echogenicity variations in patients using a 30-MHz intravascular ultrasound catheter.     

Rapid volume flow rate estimation using transverse colour Doppler imaging

A system is described in which the volume flow rate of blood in a vessel is determined using transverse colour Doppler ultrasound imaging. The system measures rapidly the two-dimensional velocity profile of blood flowing through a vessel. By integration of the measured velocity profiles the volume flow rate of blood in the vessel is obtained. The Doppler angle is obtained from the included angle between two imaging planes, and their respective average measured flows. This technique yields instantaneous and average flow rate in real time, and permits long flow recordings to be made and stored digitally. The error is less than 5% over a 8:1 flow rate range.     

彩色多普勒超声对皮肤血管瘤的诊断价值

本文报道应用彩色多普勒超声对２１个皮肤血管瘤的检测结果。小的单纯血管瘤瘤体近圆形、大的海绵状血管瘤多不规则，均无明显包膜回声，内部回声低而不均，８个瘤体侵犯了深部组织。小的瘤体比大的瘤体易于获取血流信号，彩色多普勒能量图（ＣＤＰＩ）敏感性优于彩色多普勒血流显像（ＣＤＦＩ）。在１７个瘤内可记录到的动脉频谱中，其平均最大流速为１１．３ｃｍ／ｓｅｃ（４．１～１５．５ｃｍ／ｓｅｃ），平均阻力指数为０．６５（０．５９～１．０）。     

High-frequency subharmonic pulsed-wave Doppler and color flow imaging of microbubble contrast agents

A recent study has shown the feasibility of subharmonic (SH) flow imaging at a transmit frequency of 20 MHz. This paper builds on these results by examining the performance of SH flow imaging as a function of transmit pressure. Further, we also investigate the feasibility of SH pulsed-wave Doppler (PWD) imaging. In vitro flow experiments were performed with a 1-mm-diameter wall-less vessel cryogel phantom using the ultrasound contrast agent Definity and an imaging frequency of 20 MHz. The phantom results show that there is an identifiable pressure range where accurate flow velocity and power estimates can be made with SH imaging at 10 MHz (SH10), above which velocity estimates are biased by radiation force effects and unstable bubble behavior, and below which velocity and power estimates are degraded by poor SNR. In vivo validation of SH PWD was performed in an arteriole of a rabbit ear, and blood velocity estimates compared well with fundamental (F20) mode PWD. The ability to suppress tissue signals using SH signals may enable the use of higher frame rates and improve sensitivity to microvascular flow or slow velocities near large vessel walls by reducing or eliminating the need for clutter filters.     

彩色多普勒超声在下肢深静脉栓塞诊断中的应用价值

目的探讨彩色多普勒超声诊断下肢深静脉血栓(DVT)的应用价值。方法彩色多普勒超声对212例临床怀疑DVT的患者进行血管超声检查,观察双下肢深静脉的走行、管径、血管内壁、管腔内透声及血栓回声特征。结果彩色多普勒超声明确诊断201条深静脉血栓形成。彩超表现为患肢深静脉管径增宽,管腔内可见絮状弱回声、实质低回声或不均匀低弱回声,管腔结构不清,探头轻压不能闭合,病变部位无彩色血流信号或可见狭窄细条状的彩色血流信号。结论彩色多普勒超声能及时诊断急性DVT,为临床提供及时、准确、安全、无创的检查手段,对早期诊断、指导治疗、减少并发症具有重要的临床应用价值。     

肾静脉血栓的彩色多普勒超声诊断

目的探讨彩色多普勒诊断肾静脉血栓的超声显像及诊断。方法回顾性分析6例肾静脉血栓患者肾脏大小结构,肾静脉主干回声和血流信号,肾内血流等彩色多普勒超声结果。结果肾静脉主干血栓5例,超声显示肾静脉主干充满实性条索状回声,管腔内完全或部分无血流信号显示;肾内小静脉血栓1例,局部区域肾结构模糊,无静脉血流信号。肾内动脉RI均升高(范围0.70～0.92,平均0.86),部分患者出现肾内动脉舒张期反向波。结论彩色多普勒超声诊断本病准确、实用、有助于指导临床治疗和随访观察。     

Doppler evaluation of fetal hemodynamics

Human umbilical hemodynamics was investigated in 27 pregnant patients with gestational ages from 29 to 43 weeks using a pulsed Doppler flowmeter. The signals were analyzed by Fast Fourier Transform using a minicomputer. Coherent averaging of the signals, along with the computation of maximum velocity, mean velocity and the first moment of Fourier spectra about the zero frequency axis were performed. Hemodynamic indices were measured utilizing a comprehensive feature characterization technique of the blood flow velocity waveform. It was observed that coherent averaging reduced spectral variance thus relatively simplifying the analyses. The Pulsatility Index based on the first moment of the Fourier spectra indicated a higher umbilical flow change during the fetal cardiac cycle, than that predicted by the peak velocity Pulsatility Index. We also observed a high Pulsatility Index in association with three cases of oligohydramnios indicating the possibility of an elevated placental circulatory impedance. Variation in the peak velocity up to 20% was also noted with fetal breathing movements. Further studies are necessary to determine the clinical relevance of these findings.     

On the relation between aggregation, packing and the backscattered ultrasound signal for whole blood

Previous studies have shown that the backscattered ultrasound (US) power from blood depends on the manner in which red blood cells (RBCs) are packed and, in particular, on spatial variations in the red blood cell number density (i.e., the RBC concentration variance). Experimental measurements have also shown that the backscattered US power depends on the degree of RBC aggregation, and it has been hypothesized that this is primarily due to the effect of RBC aggregation on the concentration variance. An initial simulation study of the relationship between RBC aggregation and packing statistics is presented, in which the effects of hematocrit, aggregate size, shape and size distribution on concentration variance are investigated. Both two-dimensional (2-D) and 3-D samples of aggregated and disaggregated RBCs were simulated; these enabled the concentration variance to be calculated. In agreement with theoretical predictions and experimental US results, the concentration variance for disaggregated RBCs is shown to be lowest at low and high hematocrits, and to peak at intermediate hematocrits. The concentration variance is shown to be particularly sensitive to changes in aggregate size and size distribution, and less sensitive to the shape of small aggregates. The results of this study provide a foundation for relating the state of aggregation in a blood sample to the manner in which RBCs are packed and, therefore, to the backscattered US power.     

Denoising of arterial and venous Doppler signals using discrete wavelet transform: Effect on clinical parameters

In this paper, the effects of a wavelet transform based denoising strategy on clinical Doppler parameters are analyzed. The study scheme included: (a) Acquisition of arterial and venous Doppler signals by sampling the audio output of an ultrasound scanner from 20 healthy volunteers, (b) Noise reduction via decomposition of the signals through discrete wavelet transform, (c) Spectral analysis of noisy and noise-free signals with short time Fourier transform, (d) Curve fitting to spectrograms, (e) Calculation of clinical Doppler parameters, (f) Statistical comparison of parameters obtained from noisy and noise-free signals. The decomposition level was selected as the highest level at which the maximum power spectral density and its corresponding frequency were preserved. In all subjects, noise-free spectrograms had smoother trace with less ripples. In both arterial and venous spectrograms, denoising resulted in a significant decrease in the maximum (systolic) and mean frequency, with no statistical difference in the minimum (diastolic) frequency. In arterial signals, this leads to a significant decrease in the calculated parameters such as Systolic/Diastolic Velocity Ratio, Resistivity Index, Pulsatility Index and Acceleration Time. Acceleration Index did not change significantly. Despite a successful denoising, the effects of wavelet decomposition on high frequency components in the Doppler signal should be challenged by comparison with reference data, or, through clinical investigations.     

Simultaneous measurement of red blood cell aggregation and whole blood coagulation using high-frequency ultrasound

This study aims to investigate the feasibility of using high-frequency ultrasound (HFUS) for simultaneous monitoring of blood coagulation and red blood cell (RBC) aggregation. Using a 35-MHz ultrasound scanner, ultrasound speckle data were acquired from whole blood samples of three experimental groups of rats, including 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS)-treated, noncoagulation and normal control groups. The variations of blood echogenicity, the shape parameters of probability distribution of speckle intensity (skewness and kurtosis) and the correlation coefficient between two consecutive speckle data were calculated as a function of time starting from immediately after taking blood. The blood echogenicity increases rapidly to plateaus at the early stage of measurement for all the experimental groups caused by the formation of RBC aggregates. The DIDS-treated group exhibits the lowest echogenicity level due to the inhibitory effect of DIDS on RBC aggregation. The correlation analysis between consecutive speckle patterns seems to be useful to examine the variation of blood fluidity and the progress of clot formation. Whole blood coagulation is observed to be accelerated by DIDS treatment. In addition, the results of skewness and kurtosis analysis indicated that RBC aggregates may be disrupted during blood coagulation. The present study suggests that HFUS has good potential for simultaneous monitoring of RBC aggregation and blood coagulation to examine the relationship between them.     

彩色多普勒和三维血流能量成像在肝脏肿瘤的应用

目的：探讨新型彩色超声多普勒在肝脏肿瘤的应用。方法：对４１个肝脏肿瘤进行了彩色超声多普勒和三维血流成像检查。结果：原发性肝细胞癌（ＨＣＣ）（１９例２２个肿块）和肝脏转移瘤（１１例１８个肿块）的瘤内血流信号检出率有明显差别（１００％,４４．４％）;ＨＣＣ血流抵达瘤体中心区域者占３６．４％（８／２２）。而转移瘤的血流多位于肿块内的外侧。一例巨大肝细胞腺瘤周围和瘤体内也显示了丰富血流。结论：数字化程度高的超声仪器可望获得更敏感的低速度血流。在部分较大瘤体内的不同部位能测到不同速度的血流。三维血流图像（３Ｄ）对鉴别肿瘤边缘部位的血流归属有重要的帮助作用,使肝内肿瘤的定位更加准确;为鉴别肝内和肝外肿瘤能提供重要依据