Ultrasonic imaging of the carotid arterial system

Blood flow velocity waveform in the common carotid artery and its analysis in elderly subjects

We investigated common carotid artery blood flow in 99 elderly nursing home residents using an ultrasonic quantitative blood flow measurement system. Systolic velocity and end-diastolic velocity were obtained from the waveform and classified, using the end-diastolic/systolic volume as Type A which is normal flow (greater than 20% bilaterally), Type B which is unilateral decreased flow (less than 20% unilaterally), Type C which is bilateral decreased flow (less than 20% bilaterally), Type D which is Type C plus a saw tooth pattern and Type E which is no diastolic pattern, i.e. a value of 0 unilaterally or bilaterally. Five per cent of subjects showed Type A of blood flow velocity waveform (normal flow pattern) and about 40% showed Type E (no diastolic pattern). The subjects were divided into three groups according to blood flow velocity waveform. Groups 1, 2 and 3 showed Types A or B, C or D, and E, respectively. The total blood flow volume and mean blood flow velocity of Group 1 were significantly higher than those of Groups 2 and 3 in both the supine and sitting positions. Although both total blood flow volume and mean blood flow velocity of Group 1 decreased significantly in postural change, those of Groups 2 and 3 did not change. These results suggest that total blood flow volume and mean blood flow velocity decrease in proportion to changes in the blood flow velocity waveform.

Detection of moving flow separation in pulsatile flow and the degree of stenosis by power of Doppler shift signals

Power ratios were derived from the principle of ultrasonic Doppler velocimetry to determine a ratio of the volume of vortices to the total vessel volume under the ultrasonic beam. This ratio also equals the ratio of the cross-sectional area of vortices to the vessel lumen. In vitro pulsatile flow experiments were performed in a Reynolds number range of 1,230-4,320 with axisymmetric constrictions with area reductions of 55%, 65%, 70%, and 85% to model carotid stenosis. Flow separation downstream from the model stenoses was detected, and the power ratio fluctuated when vortices with the forward- and reverse-flow velocity components passed by the measurement position. The power ratio estimated the degree of stenosis within 10% of error. Ensemble average of the power ratio was computed to obtain the statistically averaged separated flow region. The moving flow reattachment point was revealed downstream from the 85% stenosis at a Reynolds number of 900.

Determination of volume of vortices in poststenotic pulsatile flow by ultrasonic Doppler power ratio and spectrum analysis

Based on the principle of ultrasonic Doppler flowmetry, a power ratio was derived from independent forward and reverse flow Doppler shift signals to measure a ratio of the volume of vortices to the total vessel volume in poststenotic separated flow. The ratio was also proportional to the ratio of the cross-sectional areas of vortices to the vessel lumen. In vitro pulsatile flow experiments were performed to test the methodology and to study flow separation and vortex shedding downstream from model stenoses. The averaged flow cross-sectional area ratio linearly correlated (r = 0.91) with the actual area reduction of the stenosis.

Hemodynamics of the normal human carotid bifurcation: in vitro and in vivo studies

The spatial and temporal characteristics of blood flow in the normal adult human carotid bifurcation are investigated by two different methods: in vitro pulsatile flow model experiments using laser Doppler anemometry and in vivo studies employing pulsed Doppler velocity measurements obtained with an ultrasound duplex scanner. Glass and Plexiglas models based upon arteriographic measurements were evaluated with laser Doppler anemometer methods for pulsatile flow. A similarity approach permits the model study to be geometrically and hydrodynamically accurate with respect to the human carotid bifurcation. These parallel but separate approaches were originally performed by the principal authors without knowledge of each others' work. Normal flow patterns in the proximal internal carotid artery are demonstrated to include: unidirectional, helical, transient reversal, and low velocity regions of flows. The characterization of these complex temporal and spatially variant flow fields required the high sample volume resolution afforded by the model study. Pulsed Doppler ultrasound and a novel method of positioning the sample volume permitted a qualitative description of the complex flow velocity fields in the normal human bifurcation. Results of the two methods are compared and a striking similarity between the two methods is observed for the primary and secondary flow features. The problem of associating blood flow velocity disturbances with the presence of intralumenal disease is addressed in the discussion. It is suggested that the flow disturbances associated with the normal carotid bifurcation are different from those associated with intraluminal disease and further, that the secondary flow structures can be usefully employed to establish normalcy.

Noninvasive assessment of atherosclerosis in nonhuman primates

Atherosclerosis is a progressive and degenerative disease of the artery wall which begins relatively early in life, years if not decades prior to the onset of clinical signs and symptoms. One of the major challenges which face investigators in this field of research is to establish the validity and reliability of noninvasive methods which can detect atherosclerotic plaques before they become severe enough to result in tissue ischemia, and to determine whether or not atherosclerotic lesions can be monitored both in terms of rate and direction over time. Although several methods of direct arterial visualization are available currently, high-resolution B-mode ultrasound imaging appears to have the most advantages. This technique is noninvasive, relatively inexpensive, and can visualize not only lumen contour and configuration, but also the atherosclerotic plaque in the underlying wall. Preliminary experiments in animal models suggest that lesions as small as 0.5 mm in the carotid arteries can be detected using this method. Whether or not atherosclerotic plaques can be monitored over time, however, has not been demonstrated.

Algorithms in the management of cerebrovascular disease

We present a diagnostic approach to the various clinical presentations of patients with cerebrovascular disease. It involves grouping the patients into five categories: (1) asymptomatic, (2) transient neurologic episodes, (3) unstable neurologic deficits, (4) prolonged neurologic deficits and (5) completed stroke. An algorithm is given for each category and an approach to management is outlined. The algorithms are clinically validated by reviewing the results of 185 reconstructive operations performed on patients with manifestations of cerebrovascular disease who were managed following the algorithms.

Quantitative analysis of carotid artery Doppler spectral waveforms: diagnostic value of parameters

Continuous-wave ultrasonic Doppler recordings made at the carotid bifurcation have been processed by spectrum analysis and off-line microprocessor software. Data reduction algorithms were defined and tested prior to incorporation in an integrated, hard-wired device. Subsequent analysis of waveforms in real-time provided measures of peak and mode frequencies and degree of spectral broadening. Correlation of results with 123 angiographically visualized internal carotid arteries showed that peak and mode frequencies generally increased with degree of stenosis while the systolic window decreased. Ratios of peak and mode frequencies in the proximal to the distal internal carotid artery and the systolic window at the distal internal carotid artery were retrospectively found to have diagnostic value with a combination of these three parameters providing the best overall accuracy.

The limitations of B-mode imaging of the carotid bifurcation: a comparison with three flow-dependent non-invasive tests

Three hundred and four carotid arteries in 152 patients presenting with cerebral symptoms or asymptomatic carotid bruits were scanned with a real time B-mode scanner. These patients were also submitted to three other non-invasive screening modalities of Oculoplethysmography (OPG), Periorbital Doppler (POD) and Carotid Phonoangiography (CPA). Contrast angiography was performed on 57 patients (114 arteries). Lesions greater than 50% luminal reduction were considered haemodynamically significant. B-mode identified these lesions with a sensitivity of 75.5% and a specificity of 80.3%. Combined with OPG the sensitivity reached 96.2% but with a reduction of the specificity to 73.8%. In defining the need for angiography, no individual with a potentially surgically correctable lesion was incorrectly refused angiography. The limitations of real time B-mode scanning of the carotid bifurcation are enumerated.

The current status of carotid artery surgery

The natural history of extracranial cerebrovascular disease and available alternatives in its treatment are reviewed. An evaluation of the evidence suggests that carotid endarterectomy is the treatment of choice in patients with transient ischemic deficits, provided that an anatomically appropriate lesion can be identified. These patients have a 25-38% chance of stroke if untreated, which can be reduced to 5-10% by carotid endarterectomy. Patients with asymptomatic carotid stenosis who are good operative risks are also candidates for surgery, although this issue remains controversial. Patients with small asymptomatic ulcerated carotid plaques have a relatively benign prognosis and should not undergo preventive carotid surgery. Carotid surgery is occasionally indicated in patients with vertebral basilar insufficiency and carotid stenoses, fibromuscular dysplasia, or carotid kinks associated with symptoms of ischemia. Carotid endarterectomy may be performed with an overall mortality of 1-2% and morbidity of 2-5% if the patients are carefully selected and the surgical team is expert.

General principles and applications of ultrasound

The scope of diagnostic applications of ultrasound is great but differs from that of computed tomography and other noninvasive imaging techniques. Knowledge of general principles of ultrasound can aid in understanding its applications, scanning techniques, and limitations.