Merrill P. Spencer

Ultrasonic Doppler Technique for Imaging Blood Vessels

Present ultrasonic Doppler flow detectors that use the Doppler effect on waves scattered from moving blood have provided useful information when directed by hand to trace the circulation of animals and man. By scanning with a highly directive flow detector, the areas of flow can be localized. Images can be formed of the interior of blood vessels. These images have the appearance of arteriograms and venograms made by dye contrast radiography, but have none of its hazards. The resolution appears adequate for useful images.

LOCAL DOPPLER AUDIO SPECTRA IN NORMAL AND STENOSED CAROTID ARTERIES IN MAN

Abstract In this study spectral analysis was performed of signals obtained with a focussing 5 MHz CW Doppler flowmeter along normal and stenosed carotid arteries. The position of the sound beam in the artery was determined using a Doppler imaging system which converts velocity signals into a map of the cervical carotid arteries. In normal carotid arteries maximum frequencies usually ranged between 2 and 4 kHz. The regular outline of the sonagram and the concentration of frequencies near the maximum frequency is indicative for plug flow and explains the smooth sound heard. In stenosed internal carotid arteries (degree: 65–74%) five identifiable zones could be detected. Upstream to the stenosis, generally normal maximum frequencies and sonagrams were found. Within the stenosis, increased frequencies with even spectral distribution and usually a regular outline of the sonagram were found, which is compatible with parabolic flow. Just distal to the stenosis, the outline of the sonagram became irregular and high maximum frequencies persisted but were associated with high amplitude low frequencies, producing a superimposed gruff quality sound. This quality is considered to represent artery wall vibration caused by high velocity turbulence. Approximately 2 cm distal to the stenosis lower maximum frequencies were found, but still higher than normal. The outline of the sonagram remained irregular. A fluttering quality sound was heard likely to be produced by low velocity turbulence. About 3 cm distal to the stenosis a normal distribution of blood velocities was found. It is suggested that the detection of flow disturbances along stenosed internal carotid arteries, which occur at relatively slight degrees of stenosis, may contribute to the early diagnosis of internal carotid artery disease.

Cervical Carotid Imaging With a Continuous-Wave Doppler Flowmeter

A noninvasive technique for carotid arteriography using an ultrasonic directional Doppler flowmeter to image the carotid bifurcations is described. The technique uses a position sensing arm to hold the sharply focusing probe and translates the position of arterial flow onto an image storage screen. By multiple manual sweeps across the cervical carotids, a two-dimensional projection of the locus of arterial flow is developed. The probe beam is then applied through the eyelids to assess the posterior orbital ophthalmic flow. The adequacy of the internal carotid circulation and the presence of stenosis and calcified plaques are determined. Experience with the first 60 patients surveyed using the Doppler technique demonstrated a high degree of accuracy and reproducibility. The ultrasonic angiography provided local flow and velocity information that x-ray angiography did not. X-ray angiography is frequently indicated by the ultrasonic findings when risks of x-ray angiography might not otherwise be taken. The technique was found especially sensitive in detecting calcified atherosclerotic plaques and may be used in screening for the stroke-prone patient and following arterial lesions over extended periods of time.

Cerebrovascular evaluation with Doppler ultrasound

1. An Overview of Non-invasive Cerebrovascular Evaluation Using Doppler Ultrasound.- 2. Sound and Ultrasound.- 3. Doppler Flowmeter Systems.- 4. Doppler Imaging Systems.- 5. Other Ultrasonic Techniques and Current Research.- 6. Technique of Doppler Examination.- 7. Vascular Murmurs.- 8. Blood Flow in the Arteries.- 9. Hemodynamics of Carotid Artery Stenosis.- 10. Audio Spectral Analysis.- 11. The Periorbital Collateral Arteries.- 12. Ultrasonic Detection of the Non-Stenotic Plaque.- 13. Obstructive Lesions Diagnosed by Doppler Ultrasound.- 14. Clinical Management Decisions Based on Doppler Cerebrovascular Evaluation.- 15. Full Capability Doppler Diagnosis.- Doppler Cerebrovascular References.- Credit and Recognition List.- Index of Subjects.

The Use of Diastolic Reactive Hyperemia to Evaluate the Coronary Vascular System

Abstract Instantaneous and mean coronary blood flow (MCBF) and the diastolic reactive hyperemia (DRH) response were measured at the conclusion of aorta-to-coronary vein bypass operations and in dogs under various experimental circumstances. Extended hyperemia following surgical occlusion can abolish the DRH response for several hours in the presence of a high MCBF. Critical stenosis can be overlooked by recording MCBF alone without evaluation of the DRH response. The presence of a DRH response indicates that the grafts can participate in increased myocardial oxygen supply. The wide range in clinical MCBF values found by us and other investigators can be explained by various degrees of vasodilatation or local variations, or both, in intramyocardial pressure.

Doppler Measurement of the Pressure Drop Caused by Arterial Stenosis: An Experimental Study: A Case Report

A stenotic arterial lesion which reduces the cross-sectional area of the artery causes an increased velocity and, as a consequence, a loss in kinetic energy and a pressure drop. A simplified formula, derived from the Bernoulli principle, relates the pressure drop to the maximum velocity of the blood flow in the stenotic segment: ΔP (mmHg) =4 Vmax2 (m/sec). This formula has been validated for stenosis of cardiac valves. Aim of our study was to test the hypothesis that this formula applies in the major arteries using Doppler ultrasound with spectrum analysis. In our experiments we created artificial graded stenoses of varied geometry in the thoracic aorta of dogs. Invasive pressure measurements were obtained using intra-arterial needles on both sides of the stenosis. A Doppler signal was obtained with a 2.5 MHz CW probe, insonating the stenotic area from a distance, with an almost parallel approach. In these conditions the maximum Doppler frequency shift is an accurate estimate of the maximum flow velocity, according to the Doppler equation. We compared the Doppler derived (ΔP=0.36 Fmax2) and the invasive measurements of pressure drops. Our results show a highly significant correlation between the intra-arterial and the Doppler measurements of the pressure drops caused by arterial stenoses and encourage efforts in applying similar techniques in the noninvasive evaluation of vascular patients.

Doppler ultrasound in the evaluation of the peripheral arterial circulation.

This review summarizes the principle, advantages, and limitations of Doppler ultrasound flowmeters, and discusses the possibilities of applying these systems in evaluating the functional state of the peripheral arterial circulation. This discussion is not limited to the assessment of blood flow velocity and poststenotic arterial pressure with Doppler flowmeters, but includes among other things the recordings of flow patterns, the determination of the transit time of velocity wave forms, and the imaging of the arterial circulation.

Doppler Imaging Systems

The search for a practical, clinically applicable system for non-invasive detection and diagnosis of atherosclerosis followed early demonstrations that ultrasound could be used. Pulse-echo systems were the first to be investigated and showed that, although plaque could be visualized or its absorption properties made apparent in vivo, some types of plaque would not be visualizable[l]. Apparently, fatty plaque or plaque whose blood tissue interface was not at right angles to the sound beam could not be differentiated from blood. Interest was then focused on imaging of the blood itself through various types of imaging systems based on the ultrasonic Doppler effect. The first of these was a pulse-Doppler system which produced cross-section flow images [2]. The present authors developed a continuous wave (c-w) Doppler system which, by ignoring the depth coordinate, produced a plan view geometrically similar to an arteriogram [3]. The c-w system to be described here was devised as a clinically-applicable screening tool for diagnosis of surgically correctable flow disturbance in large arteries. It has been applied to the carotid and vertebral arteries of the neck and evaluated on more than 3,000 patients in our clinics alone. Other centers have reported active use of this equipment [4, 5].

Physics for ultrasonic diagnosis

Ultrasound is a mechanical vibration which is basically no different than audible sound waves. The limits of human hearing ability are between 20 and 20,000 Hertz (cycles per second) but the upper limit decreases with age. Middle-C on the piano is a note caused by vibrations of the piano string 262 times per second (262 Hz). This frequency of vibration is usually called the fundamental carrier frequency on which some higher frequencies (harmonies) may be superimposed. Each octave, on the piano, represents a doubling of the fundamental frequency as we go up the scale and higher frequencies have a higher pitch. Sound waves having frequencies higher than the human hearing are called ultrasound. Frequencies which are 100 times higher than those of the human hearing range 2-10 million Hertz or megahertz (MHz) are the most commonly used frequencies in medical diagnosis. The concepts summarized in this chapter are discussed in more detail in many available textbooks [1–11].

Noninvasive Detection of the Atherosclerotic Plaque: The Carotid Bifurcation

A new Doppler ultrasonic imaging technique for mapping the course of arterial channels has been applied to the carotid bifurcation in over 800 patients. The technique has proven its ability to detect stroke producing wall disease including calcified plaques and flow disturbances. (Stroke 5:145, 1974) Comparisons with x-ray angiography demonstrates agreement in 89% of hemodynamically significant lesions. When including wall plaques without obstruction, agreement dropped to 79%. The ultrasonic method is more sensitive to elements of the atherosclerotic plaque, including calcium deposits, than is x-ray. Abnormal doppler flow signals associated with the presence of an atherosclerotic plaque beneath the sound beam include low amplitude, coarse quality, and low frequency sounds, as well as weak, biphasic, and inverted velocity flow tracings. Calcium deposits may be responsible for most of these abberations and when present in large quantities, produces silent nonvisualizing segments in the arterial image. The association of these blank segments with symptoms and signs suggestive of embolization leads to the possibility of ulcerated plaque and treatment with antithrombotic agents or endarterectomy.