ISSN:0091-2751    

DOI:10.1002/jcu.1870230202

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractVisual inspection of the spectral composition of the Doppler signal as a function of time (sonogram) has been very helpful in detecting the presence of stenoses with substantial lumen narrowing causing abnormal flow patterns. Attempts to grade a stenosis based on the spectral width at peak systole were less successful because of the obscuring effects of the ultrasound beam width with respect to lumen diameter, dimensions of the sample volume, angle of observation, and spectral broadening due to vessel branching and bends. The introduction of color flow imaging has put emphasis on the width of the velocity distribution and the consistency of flow patterns within the region of interest. This technique requires a high resolution in space, velocity, and time necessitating the development of new velocity estimation algorithms. The observed flow patterns can be related to the echogenicity and local wall thickness of peripheral vessels. In addition, the displacement behavior of arterial walls over time provides information about the elasticity of the wall. Knowing the instantaneous velocity of arterial walls, it becomes possible to suppress selectively and adaptively the arterial wall contribution, allowing for the assessment of low blood flow velocities close to the wall and, hence, of wall shear rate. The latter development enables the study of the interaction of blood velocities and the metabolism and structure of the walls, providing possible clues for atherogenesis. © 1995 John Wiley&Sons, Inc

ISSN:0091-2751    

DOI:10.1002/jcu.1870230203

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractExperimental studies in the 1960s and 1970s demonstrated the high sensitivity of Doppler ultrasound in detecting gaseous bubbles. More recent studies have shown that microscopic air bubbles, as well as glass microspheres as small as 5 μ to 20 μ, cause characteristic high‐intensity signals. Recently it has been demonstrated that less echogenic embolic materials such as thrombus, platelet aggregates, and atheroma can also be detected with a high sensitivity. Such “solid,” or formed‐element, emboli as small as 200 μ to 400 μ can be detected; the lower size limit of detection was due to an inability to make smaller embolic particles rather than to the sensitivity of the detection process itself. Analysis of the Doppler signals provides some information about embolus size and composition, but accurate characterization in clinical practice is not possible using current technology. Studies in experimental models have allowed the detailed description of embolic signals; they appear as a short‐duration, frequency‐focused increase in intensity, predominantly unidirectional in the direction of flow, and usually contained within the spectral envelope. In contrast, artifacts appear as a bidirectional, high‐intensity increase with maximum intensity at low frequencies. These differences have been exploited to develop automatic embolus detection programs, and an off‐line version has been successfully validated in an experimental model. © 1995

ISSN:0091-2751    

DOI:10.1002/jcu.1870230204

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractTranscranial color‐coded duplex sonography (TCCD), magnetic resonance angiography (MRA), and computed tomography angiography (CTA) are novel noninvasive or minimally invasive techniques for the study of the intracranial circulation. TCCD is relatively inexpensive and permits bedside examination. It improves the accuracy and reliability of conventional transcranial Doppler studies. The main limitation of TCCD are the ultrasonic windows. They restrict the area of insonation to the major cerebral arteries and the proximal part of its branches, lower the spatial resolution, and may prevent transtemporal insonation. Using MRA, both large and small intracranial arteries and veins can be imaged by selecting the appropriate imaging parameters. MRA provides morphologic information about the cerebral vessels, relying on blood flow as the physical basis for generating contrast between stationary tissues and moving spins. MRA is highly sensitive for the detection of occlusive disease in large intracranial arteries. However, with bright blood techniques the degree of stenosis tends to be exaggerated. Flow direction, eg, in collaterals, can be determined by selective or phase‐contrast MRA. Perfusion imaging techniques provide information about blood flow at the capillary level. Diffusion imaging depicts molecular motion. TCCD and MRA used in combination or alone may eliminate the need for intra‐arterial digital subtraction angiography (DSA) in most patients studied for occlusive cerebrovascular disease. DSA may be reserved for those patients where there is disagreement among the noninvasive techniques, and for the diagnosis of cerebral aneurysms and arteriovenous malformations. CTA relies on spiral CT technology and intravenous contrast injection. To date, intracranial use has been predominantly for the diagnosis of aneurysms. The role of CTA for the detection of nonaneurysmal intracranial vascular disease has yet to be established. © 1995 John Wiley&Son

ISSN:0091-2751    

DOI:10.1002/jcu.1870230205

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractVarious methods were developed for the quantification of the degree of stenosis: B‐mode imaging, CW Doppler with spectral analysis, PW‐duplex, and colorflow imaging. The degree of stenosis can be evaluated using transverse views of the carotid in conventional B‐mode imaging. The diameters of the residual lumen (Ds) and the external diameter (De) of the artery at the same level are measured and the degree of stenosis (in area) is calculated. Two different Doppler methods have been designed and validated for the quantification of the stenosis degree. The first one is based on the determination of the maximal velocity inside the stenosis and requires the use of a duplex device. The second one, based on the quantification of the hemodynamic disturbances at the outlet of the stenosis, can be used either with a duplex system or with a continuous‐wave Doppler system (pencil probe). The color display mode facilitates the determination of the residual lumen and therefore contributes to making the B‐mode method more accurate. Moreover, it provides a visualization of the blood flow which avoids most of the misinterpretations of B‐mode or Doppler data. Several classification systems based on subjective or semi‐quantitative criteria (morphological or hemodynamic) were used to identify different grades of stenosis (<40%, 40% to 60%, 60% to 90%, etc.) These classifications provide an evaluation of the degree of stenosis accurate enough to discuss the possibility of performing an endarterectomy on a symptomatic patient. When the surgical treatment seems to be inappropriate, the patient is treated medically; the follow‐up of the lesion (ie, every 6 months) requires a more precise evaluation of the plaque changes than does stenosis classification. Several parameters have been designed and validated for the quantification of the degree of stenosis by ultrasound. These parameters, whether measured with the B‐mode and color image or with the Doppler spectrum, allow quantification of the stenosis degree in percentage of lumen reduction with a precision of approximately ± 10%. Such a quantitative assessment of carotid lesions for a long period of time may be very helpful in evaluating the beneficial effects of medical treatment or in detecting any significant increase of the stenosis that could lead to surgical treatment. A new method for the plaque volume assessment has been recently validated. The plaque volume index expressed in mm3is calculated from logitudinal and transversal B mode views of the bifurcation. © 1995 J

ISSN:0091-2751    

DOI:10.1002/jcu.1870230206

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractMaintenance of cerebral perfusion pressure is a prerequisite for the prevention of cerebral ischemia. Physiological fluctuations in systemic perfusion pressure are compensated by cerebrovascular autoregulation. Cerebral hypoperfusion could result from (1) systemic hemodynamic failure (eg, distal to severe arterial stenosis), overcharging the vasoregulatory capacity; (2) dysfunction and exhaustion of cerebrovascular autoregulation; or (3) both.Ultrasound offers an excellent temporal resolution, is noninvasive, and is easily applicable for follow‐up investigations. Despite its poor spatial resolution, transcranial Doppler sonography has been used for determination of cerebral perfusion reserve studies measuring cerebral blood flow velocity (CBFV) during hypercapnia or application of vasoactive agents (eg, acetazolamide). This approach evaluates vasomotor regulation in patients with hemodynamic compromise distal to severe stenosis or occlusion of the brain supplying arteries.Monitoring CBFV during tilt table examinations directly measures cerebral autoregulation. In patients with systemic orthostatic hypotension, maintainance or failure of cerebrovascular compensation and, even more importantly, cerebrovascular dysautoregulation, despite normal systemic blood pressure regulation, may be demonstrated.Vasoneuronal coupling is reflected by CBFV variations during appropriate neuronal stimulation. Neuronal dysfunction is associated with CBFV abnormalities as exemplified by preconditions of focal cerebral dysfunction in the posterior cerebral artery (PCA) in migraineurs with aura, where massive alteration of vasoneuronal coupling and ischemia is threatening during spreading depression. A highly significant asymmetric gain of vasoneuronal coupling in the interictal state may act as a trigger mechanism in these patients.Testing for vasoneuronal coupling within the middle cerebral artery (MCA) territory is more difficult due to the poor spatial resolution with various neuronal stimuli (eg, motorsensory or cognitive paradigms), only eliciting local neuronal areas underrepresented in the MCA CBFV global changes. However, motor stimulation evoked CBFV may be used to indicate dysintegation of vasoneuronal coupling in the course of acute cerebral ischemia with sensorimotor hemiparesis and, moreover, seems to be of prognostic value regarding the motor deficit. © 1995 John Wiley&Sons, I

ISSN:0091-2751    

DOI:10.1002/jcu.1870230207

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

摘要:

AbstractFour‐dimensional (4D) analysis of atherosclerotic plaque and wall motion, the application of 4D ultrasound to the study of atherogenesis, and the incorporation of ultrasound data into flow models for simulation of cerebrovascular hemodynamics are new frontiers in diagnostic ultrasound that use computer vision and optical flow techniques to exploit the full potential of real‐time imaging and Doppler studies. New approaches to improve blood vessel delineation with ultrasound include application of contrast agents, harmonic imaging, and red blood cell density imaging. An assessment of the potential clinical utility of these new developments in cerebrovascular ultrasound requires an analysis of comparable trends in magnetic resonance (MR) technology, eg, rapid advances in the fields of MR angiography, dynamic contrast‐enhanced MR, and MR diffusion imaging. Likewise, the value of ultrasound techniques for the measurement of blood flow to evaluate cerebrovascular hemodynamics must be compared to related methods in magnetic resonance, such as dynamic MR inflow tracking. This article addresses several new and future developments in cerebrovascular ultrasound and discusses their relative merits in terms of ongoing research in the field of magnetic resonance angiography, imaging, and related techniques. © 1995 John Wiley&Son

ISSN:0091-2751    

DOI:10.1002/jcu.1870230208

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:0091-2751    

DOI:10.1002/jcu.1870230209

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:0091-2751    

DOI:10.1002/jcu.1870230210

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY

ISSN:0091-2751    

DOI:10.1002/jcu.1870230201

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1995

数据来源: WILEY