Transcranial Doppler

In most cases of diagnostic use of ultrasound dopplerography, it should be carried out together with transcranial dopplerography. The exception to this rule is made up of persons with insufficiently pronounced or completely absent "temporal" windows, as well as patients who are unable to perform transcranial dopplerography for other reasons (7-12% of the total number of subjects). In all situations that require verification, as well as determining the nature of the pathology that led to the formation of Doppler changes, duplex scanning or other diagnostic procedures referring to ultrasound Dopplerography is indicated.

Indications for transcranial dopplerography

Transcranial dopplerography is currently used both for diagnosis of intracranial vascular lesions and for determining changes in flow in their lumens, and for monitoring blood flow rates in various pathological and physiological processes. Direct indications for the dynamic evaluation of cerebral hemodynamics - suspicion of microemboli in persons with atherosclerotic, thrombotic lesion of extracranial divisions of brachiocephalic arteries, heart diseases, transient ischemic attacks of embolic origin; pathological cerebral vasospasm. Monitoring using transcranial dopplerography is often used in the acute period of ischemic stroke. In addition, the method is widely used to assess the rates of cerebrovascular reactivity in the stenotic / occlusive pathology of extra- and intracranial divisions of brachiocephalic arteries, arterial hypertension and hypotension, various forms of angiopathy and vasculitis, accompanied by the defeat of various parts of the circulatory bed of the brain. With the use of transcranial dopplerography, intraoperative monitoring of cerebral hemodynamics indices in surgical interventions on the heart and coronary arteries, the substance and the cerebral vascular system is performed, and also the evaluation of the effectiveness of drug therapy is performed. Transcranial dopplerography can be used as a diagnostic method for detecting Doppler signs of stenosis more than 50% in diameter and / or occlusion of the intracranial arteries, determining the level of arterial inflow over them in norm and with various deviations (for example, vasospasm, vasodilation, arteriovenous shunting) at rest and at loads. The diagnostic significance of transcranial dopplerography is slightly different from that of transcranial duplex scanning, except for the impossibility of correcting the Doppler angle. Diagnostic criteria used in this case are similar to those for ultrasound dopplerography.

The technique of transcranial dopplerography

In the transcranial dopplerography of the echolocation, the middle (segments M1, less often M2), the anterior segments (segments A1 and A2), the posterior segments P1 and P2, the cerebral arteries, the intracranial section of the internal carotid artery, the main artery, the intracranial parts of the vertebral artery (V4 segments), as well as a straight sine, the veins of Rosenthal and the Vienna of Galen. It is also possible to record the spectra of flows from other, smaller arteries and veins, but there are no methods to confirm the correctness of their location. Direct location of the connecting arteries of the Willis circle is also fundamentally impossible.

In most departments, the skull bones are of considerable thickness and impermeable to ultrasonic waves, even with low frequency characteristics (1-2.5 MHz). In this regard, for the location of blood flow in the intracranial vessels use certain zones, called ultrasound "windows". In these areas, the skull bones are thinner, or they have natural holes through which the ultrasonic beam can freely enter the cavity of the skull. Most of the intracranial vessels, the principal possibility of locating which is beyond doubt, is examined when the sensor is positioned above the scales of the temporal bone. In this case, the internal carotid artery, anterior, middle and posterior cerebral arteries (the so-called temporal ultrasound "window", or temporal acoustic access) are losers. Other windows are localized in the area of the craniovertebral articulation (suboccipital ultrasound "window", thus securing V4 segments of the vertebral and main artery), over the occipital mound (transcicillary "window", straight sinus) and in the orbit (transorbital "window", eye artery, internal carotid artery in the intracranial part).

To confirm the correctness of the echolocation, a complex of features is used: the depth of the vessel, the direction of blood flow in the lumen of the vessel relative to the scanning plane of the sensor, and the reaction of the blood flow in the lumen to the compression samples. The latter suggest a short-term (within 3-5 s) compression of the lumen of the common carotid artery above the mouth (or distal) on the side of the location. The drop in pressure in the lumen of the common carotid artery distal to the place of compression and the slowing or complete cessation of blood flow in it lead to a simultaneous decrease (stop) of the flow in the region of the middle cerebral artery (segment M1 or M2). The blood flow in the anterior cerebral artery (A1) and the posterior cerebral artery (P1) when the common carotid artery is compressed depends on the structure of the Willis circle and the functional consistency of the anterior and posterior connective arteries, respectively. In the absence of pathology, the blood flow in the connective arteries (if any) at rest may be absent, bi-directional or oriented toward one of their connected arteries, depending on the level of pressure in their lumens. In addition, the length of the connective arteries and the extreme variability of location do not allow us to use the indirect features given above to confirm the correctness of the echolocation. Therefore, to determine the functional consistency (rather than anatomical presence or absence) of the connecting arteries of the Willis circle, compression tests are also used. The main diagnostic limitations of transcranial dopplerography are associated with the fundamental impossibility of visualization of the vascular wall and the associated alleged character of qualitative interpretations of the data obtained, difficulties in correcting the Doppler angle in the "blind" location of flows in the intracranial vessels, as well as the existence of multiple variants of the structure, arteries and veins (frequency in the population reaches 30-50%), in which the value of the signs allowing Verify the accuracy of echolocation, is reduced.

Interpretation of results of transcranial dopplerography

Objective information on the state of cerebral blood flow from the data of transcranial Dopplerography is based on the results of determining linear velocity indices and indexes of peripheral resistance. In practically healthy people, when examining at rest, the Doppler flow characteristics in the intracranial arteries can vary quite significantly, which is due to many factors (functional brain activity, age, systemic blood pressure level, etc.). The symmetry of the blood flow and its indices in the paired arteries of the base of the brain are much more constant in time (usually asymmetry does not exceed 30% according to the absolute values of the linear velocity characteristics of the flow in the anterior, middle and posterior cerebral arteries). The degree of asymmetry of linear velocities and peripheral resistance in the intracranial segments of the vertebral artery is more pronounced than in the carotid basin due to variability in the structure of the vertebral artery (acceptable asymmetry is 30-40%). Determination of blood flow in the intracranial vessels at rest gives important information about the state of the blood circulation in the brain tissue, but its value is significantly reduced due to the system of autoregulation of the cerebral blood flow, thanks to its functioning, the level of perfusion remains constant and sufficient in a wide range of systemic (local intraluminal) blood pressure and partial pressure of blood gases (pO ₂ and pCO ₂ ). Ensuring this consistency is possible due to the functioning of local mechanisms of regulation of vascular tone, which form the basis of autoregulation of cerebral circulation. Among the mechanisms mentioned are the myogenic, endothelial and metabolic ones. To determine the degree of their functional stress in transcranial dopplerography, tests of cerebrovascular reactivity, which indirectly characterize the potential capacity of cerebral arteries and arterioles to further change their diameter in response to the action of stimuli, selectively (or relatively selectively) activate various mechanisms of regulation of vascular tone. As a functional load, stimuli that are close in action to physiological stimuli are used. Currently, for the cerebral vascular pool, there are methods for determining the functional state of the myogenic and metabolic mechanisms of autoregulation of cerebral blood flow. To activate the myogenic mechanism (the degree of disruption of its functions approximately corresponds to that of the endothelial mechanism), use orthostatic (rapid lifting of the upper half of the trunk by 75 ° from the initial lying position horizontally), antiorthostatic (rapid lowering of the upper half of the trunk by 45 ° from the initial lying position horizontally) and compression (a short-term, within 10-15 s compression of the lumen of the common carotid artery above the mouth) of the sample, the introduction (usually sublingual) of nitroglycerin. The latter leads to the simultaneous activation of endothelial and myogenic mechanisms of vascular tone regulation, since the effect of this drug is realized directly through the smooth muscle elements of the artery wall and indirectly through the synthesis of vasoactive factors released by the endothelium. To study the state of the metabolic mechanism of autoregulation of cerebral blood flow, a hypercapnic sample (inhalation of 5-7% of a mixture of CO ₂ with air for 1-2 minutes ), a breath-holding test (short-term delay of 30-60 s), a hyperventilation test (forced breathing in for 45-60 s), intravenous administration of an inhibitor of carbonic anhydrase acetazolamide. In the absence of signs of functional tension of regulatory mechanisms at rest, the response to tests is positive. In this case, the change in the velocity indices of the blood flow and peripheral resistance corresponding to the applied load is estimated, which is estimated from the values of the reactivity indices reflecting the degree of change in the Doppler blood flow parameters in response to loading stimulation in comparison with the initial ones. When voltage of autoregulation mechanisms is increased due to increase or decrease of intraluminal pressure in cerebral arteries or pCO ₂ , negative, paradoxical or enhanced positive reactions (depending on the initial direction of changes in tone, diameter of cerebral vessels and type of load stimulation used) are recorded in brain tissue relative to their optimal values. When the autoregulation of the cerebral circulation, which is usually characterized by an uneven distribution in the brain tissue, changes, both the myogenic and metabolic tests are changed. When the autoregulation voltage is expressed, the pathological orientation of myogenic reactions is possible with a positive response to metabolic tests. In persons with stenotic / occlusive pathology, the stress of autoregulatory mechanisms occurs when the collateral compensation is inadequate or insufficiently developed. With arterial hypertension and hypotension, the systemic arterial pressure deviations from its optimum value lead to inclusion of the autoregulation system. In vasculitis and angiopathy, the limitations of tonic reactions are associated with the structural transformation of the vascular wall (fibro-sclerotic, necrotic changes and other generalized processes, leading to structural and functional disturbances).

At the heart of ultrasound detection of cerebral microembolism lies the possibility of determining in the Doppler spectrum of the distal blood flow (in the arteries of the base of the brain) atypical signals that have characteristic features that allow them to differentiate from artifacts. When monitoring blood flow in intracranial vessels using transcranial dopplerography, it is possible not only to fix microembolic signals, but also to determine their number per unit of time, and also in the part of situations - the nature of the microembolic signal (distinguish air embolism from material), which can significantly influence further tactics management of the patient.

Diagnosis and monitoring of cerebral vasospasm is one of the most important methodological tasks of transcranial dopplerography, taking into account the importance of angiospasm in the genesis of ischemic damage to the brain substance caused by disruption of the metabolic mechanism of autoregulation followed by the formation of a hemodynamic phenomenon similar to arteriolar-venular shunting. Pathological cerebral vasospasm develops with hemorrhagic disorders of cerebral circulation, severe craniocerebral trauma, inflammatory lesions of the brain substance and its membranes (meningitis, meningoencephalitis). More rarely, the cause of this condition is the use of medication (for example, some cytotoxic drugs), as well as head irradiation for ablation in cancer patients. Diagnostic signs of cerebral vasospasm in transcranial dopplerography - a significant increase in linear velocity indices of blood flow, a decrease in peripheral resistance, Doppler symptoms of generalized turbulence in spasmodic artery streams, paradoxical or negative reactions in stress testing of the metabolic mechanism of autoregulation of cerebral blood flow. As vasospasm progresses, spastic responses of large extra- and intracranial arteries with a predominance in the latter are noted with varying degrees of severity. The heavier the spasm, the higher the linear flow rates and the lower the peripheral resistance indexes. Because the extra- and intracranial spastic reaction is expressed in different ways, but with a certain ratio increasing with the increase in the severity of the spasm (due to the increasing severity in the intracranial divisions), special calculation indices are used for its verification and gradation. In particular, to characterize the degree of vasospasm in the carotid system, the Lindegard index is used, reflecting the ratio of the peak systolic flow rate in the middle cerebral artery to that in the extracranial part of the corresponding internal carotid artery. An increase in this index indicates an aggravation of angiospasm.

Studies using transcranial dopplerography of the venous system of the brain are determined, on the one hand, by the variability in the structure of the cerebral veins, on the other hand by the limitations of acoustic access and methods of verifying the accuracy of echolocation (which is especially important for deep veins and sinuses). The most practical value is the determination of Doppler blood flow characteristics in the forward sinus at rest and during functional stress tests aimed at changing (increasing) intracranial pressure. The importance of such procedures is determined by the possibility of non-invasive verification and evaluation of the severity of intracranial hypertension, as well as of a number of other pathological conditions (eg, thrombosis of the dura mater of the dura mater). In such situations, diagnostically significant Doppler ultrasound criteria are an increase in linear blood flow in deep veins and a direct sinus, as well as atypical reactions in the case of antiorthostatic loads with a displacement of the "inflection point" due to the limitation of the reserve of volumetric and elastic compensation.

In cases with a significant increase in intracranial pressure (to a level comparable to or greater than the arterial pressure), a hemodynamic situation develops, characterized by a significant reduction or complete cessation of the arterial influx to the brain ("cerebral circulatory arrest"), leading to brain death. In this case, the Doppler spectrum of blood flow from the intracranial arteries can not be obtained (or a bidirectional flow with a sharply reduced rate is established), in the extracranial regions of the brachiocephalic arteries, the averaged linear velocity of blood flow is reduced or equal to zero. Expediency of research using ultrasonic dopplerography of blood flow in extracranial (internal jugular) veins has not been determined to date.