Ultrasound Doppler vascular ultrasonography

It is well known that stenotic and occlusive lesions of the main arteries of the head are of great importance in the pathogenesis of cerebrovascular diseases. At the same time, not only initial, but also severe stenosis of the carotid and vertebral arteries can proceed with few symptoms. In the development of angioneurological pathology, the contribution of venous discirculation is also important, sometimes also proceeding subclinically. Timely diagnostics of these diseases is largely associated with such modern ultrasound methods as TCDG, duplex and triplex examination with three-dimensional reconstruction of the image, etc. Nevertheless, the simplest and most common method of ultrasound location of human vessels to this day remains ultrasound Dopplerography (USDG). The main task of ultrasound Dopplerography in angioneurology is to identify blood flow disorders in the main arteries and veins of the head. Confirmation of subclinical narrowing of the carotid or vertebral arteries detected by ultrasound Dopplerography using duplex imaging, MRI or cerebral angiography allows for active conservative or surgical treatment to prevent stroke. Thus, the goal of ultrasound Dopplerography is primarily to identify asymmetry and/or direction of blood flow in the precerebral segments of the carotid and vertebral arteries and ophthalmic arteries and veins. In most cases, it is possible to determine the presence, side, localization, length, and severity of the indicated blood flow disorders.

A big advantage of ultrasound Dopplerography is the absence of contraindications to its implementation. Ultrasound location can be carried out in almost any conditions - in a hospital, intensive care unit, operating room, outpatient clinic, ambulance and even at the site of an accident or natural disaster, provided that an autonomous power supply unit is available.

The method of ultrasound Dopplerography is based on the effect of H.A. Doppler (1842), who applied mathematical analysis of the frequency shift of a signal reflected from a moving object. The formula for the Doppler frequency shift is:

F _d = (2F ₀ xVxCosa)/c,

Where F ₀ is the frequency of the transmitted ultrasound signal, V is the linear flow velocity, a is the angle between the vessel axis and the ultrasound beam, c is the speed of ultrasound in tissues (1540 m/s).

One half of the sensor emits ultrasonic vibrations with a frequency of 4 MHz in the "continued wave" mode. The other half of the sensor, located at an angle to the surface of the transmitting part, records ultrasonic energy reflected from the blood flow. The second piezoelectric crystal of the sensor is installed in such a way that the area of maximum sensitivity is a cylinder measuring 4.543.5 mm, located 3 mm from the acoustic lens of the sensor.

Thus, the transmitted frequency will differ from the reflected one. The specified difference in frequencies is isolated and reproduced by an audio signal or graphic recording in the form of an "envelope" curve or by means of a special Fourier frequency analyzer in the form of a spectrogram. Moreover, it is possible to determine the direction of blood flow, since the circulation going to the ultrasound sensor increases the received frequency, while the flow directed in the opposite direction decreases it.

There is a peculiarity of circulation in the main arteries of the head: normally, the blood flow does not fall to zero in any phase of the cardiac cycle, i.e. blood flows to the brain continuously. In the brachial and subclavian arteries, the linear velocity of blood flow between two adjacent cycles of cardiac contraction reaches zero without changing direction, and in the femoral and popliteal arteries, at the end of systole, there is even a short period of reverse circulation. According to the laws of hydrodynamics (blood can be considered as one of the variants of the so-called Newtonian fluid), there are three main types of flows.

• Parallel, where the flow rate of all layers of blood, both central and parietal, is essentially equal. This flow pattern is typical for the ascending aorta.
• Parabolic, or laminar, in which there is a gradient of central (maximum velocity) and parietal (minimum velocity) layers. The difference between the velocities is maximum in systole and minimum in diastole, and these layers do not mix with each other. A similar variant of blood flow is noted in unaffected main arteries of the head.
• Turbulent or vortex flow occurs due to unevenness of the vascular wall, primarily in stenosis. Then the laminar flow changes its properties depending on the approach of the direct passage and exit from the stenosis site. Ordered layers of blood are mixed due to chaotic movements of erythrocytes.