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Ultrasonic dopplerography of vessels
Last reviewed: 18.10.2021
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The great importance of stenosing and occlusive lesions of the main arteries of the head in the pathogenesis of cerebrovascular diseases is well known. In this case, not only the initial, but also severe stenosis of the carotid and vertebral arteries can proceed little. In the development of angioedema pathology is important and the contribution of venous dyskirkulyatsii, also sometimes taking place subclinically. Timely diagnosis of these diseases is largely associated with such modern ultrasonic methods as TCD, duplex and triplex studies with 3D image reconstruction, etc. Nevertheless, ultrasonic dopplerography (UZDG) remains the simplest and most widely used method of ultrasound location of human vessels. The main task of ultrasonic dopplerography in angioneurology is to detect a violation of blood flow in the arteries and veins of the head. Confirmation of subclinical narrowing of carotid or vertebral arteries revealed by ultrasound dopplerography with duplex, MRI or cerebral angiography allows the use of active conservative or surgical treatment that prevents stroke. Thus, the goal of ultrasound dopplerography is primarily to detect asymmetry and / or direction of blood flow along the precerebral segments of carotid and vertebral arteries and orbital arteries and veins. In most cases, it is possible to determine the presence, side, location, extent, severity of these disorders of blood flow.
A great advantage of ultrasonic dopplerography is the absence of contraindications to its conduct. Ultrasound location can be carried out practically in any conditions - in a hospital, resuscitation block, operating room, outpatient clinic, ambulance car and even at the scene of an accident or natural disaster, provided there is an autonomous power supply unit.
The method of ultrasonic dopplerography is based on the effect of H.A. Doppler (1842), who applied a mathematical analysis of the frequency shift of a signal reflected from a moving object. The formula of the Doppler shift of frequencies:
F d = (2F 0 x V x Cosa) / c,
Where F 0 is the frequency of the ultrasound signal being sent, V is the linear flow rate, a is the angle between the axis of the vessel and the ultrasonic beam, and c is the ultrasound velocity in the 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 some angle to the surface of the transmitting part, registers ultrasonic energy reflected from the blood stream. The second piezoelectric crystal of the sensor is installed in such a way that the area of maximum sensitivity is a cylinder of dimensions 4.543.5 mm, located 3 mm from the acoustic sensor lens.
Thus, the frequency sent will differ from the reflected frequency. The indicated difference in frequencies is allocated and reproduced by a sound signal or graphic registration 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 the blood flow, t. The circulation going to the ultrasonic sensor increases the received frequency, while the flow directed to the opposite side reduces it.
There is a peculiarity of circulation in the main arteries of the head: normally, the blood flow does not drop to zero in any phase of the cardiac cycle, that is, the blood enters 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 at the end of the systole there is even a short period of reverse circulation. According to the laws of hydrodynamics (blood can be regarded as one of the variants of the so-called Newtonian fluid), there are three main types of flows.
- Parallel, where the velocity of flows of all blood layers and central and parietal is essentially equal. Such a flow model is characteristic of the ascending part of the aorta.
- Parabolic, or laminar, in which there is a gradient of the central (maximum speed) and near-wall (minimum speed) layers. The difference between the speeds is maximum in systole and minimal diastole, and these layers do not mix with each other. A similar variant of the blood flow is noted in the unbroken arteries of the head.
- Turbulent, or vortex, flow arises from the unevenness of the vascular wall, primarily in stenoses. Then the laminar flow changes its properties depending on the approach of direct passage and exit from the site of stenosis. Ordered blood layers are mixed due to chaotic red blood cell movements.