Transcranial Doppler

In most cases of diagnostic use of ultrasound Dopplerography, it should be performed together with transcranial Dopplerography. Exceptions to this rule are individuals with insufficiently expressed or completely absent "temporal" windows, as well as patients for whom transcranial Dopplerography is impossible for other reasons (7-12% of the total number of patients examined). In all situations requiring verification, as well as determining the nature of the pathology that led to the formation of Dopplerographic changes, duplex scanning or other diagnostic procedures that are reference in relation to ultrasound Dopplerography are indicated.

Indications for transcranial Doppler sonography

Transcranial Doppler sonography is currently used both for diagnostics of intracranial vascular lesions and determination of flow changes in their lumens, and for the purpose of monitoring blood flow parameters in various pathological and physiological processes. Direct indications for dynamic assessment of cerebral hemodynamics are suspected microembolism in individuals with atherosclerotic, thrombotic lesions of extracranial sections of the brachiocephalic arteries, heart diseases, transient ischemic attacks of embolic genesis; pathological cerebral vasospasm. Monitoring with transcranial Doppler sonography is often used in the acute period of ischemic stroke. In addition, the method is widely used to assess cerebrovascular reactivity indices in stenotic/occlusive pathology of extra- and intracranial sections of the brachiocephalic arteries, arterial hypertension and hypotension, various forms of angiopathies and vasculitis, accompanied by damage to different sections of the cerebral circulatory bed. Using transcranial Dopplerography, intraoperative monitoring of cerebral hemodynamic indices is performed during surgical interventions on the heart and coronary arteries, the substance and vascular system of the brain, and the effectiveness of drug therapy is assessed. Transcranial Doppler sonography can be used as a diagnostic method to detect Doppler signs of stenosis of more than 50% in diameter and/or occlusion of intracranial arteries, to determine the level of arterial inflow through them in norm and with various deviations (for example, vasospasm, vasodilation, arteriovenous shunting) at rest and under load. The diagnostic significance of transcranial Doppler sonography differs slightly from that of transcranial duplex scanning, with the exception of the impossibility of Doppler angle correction. The diagnostic criteria used in this case are similar to those in ultrasound Doppler sonography.

Methodology for conducting transcranial Doppler sonography

Transcranial Doppler echolocation provides access to the middle (segments M1, less often M2), anterior (segments A1 and A2), posterior (segments P1 and P2) cerebral arteries, the intracranial part of the internal carotid artery, the basilar artery, intracranial parts of the vertebral artery (segments V4), as well as the straight sinus, veins of Rosenthal and vein of Galen. It is also possible to record the spectra of flows from other, smaller arteries and veins, but there are no methods for confirming the correctness of their location. Direct location of the connecting arteries of the circle of Willis is also fundamentally impossible.

In most areas, the cranial bones are thick and impermeable to ultrasound waves even with low frequency characteristics (1-2.5 MHz). In this regard, certain zones called ultrasound "windows" are used to locate blood flow in intracranial vessels. In these areas, the cranial bones are thinner, or they have natural openings through which the ultrasound beam can freely enter the cranial cavity. Most intracranial vessels, the fundamental possibility of locating which is not in doubt, are examined with the sensor positioned above the squama of the temporal bone. In this case, the internal carotid artery, the anterior, middle and posterior cerebral arteries are located (the so-called temporal ultrasound "window" or temporal acoustic approach). Other windows are located in the area of the craniovertebral junction (suboccipital ultrasound "window", this method is used to locate segments V4 of the vertebral and basilar arteries), above the occipital protuberance (transoccipital "window", straight sinus) and in the orbital area (transorbital "window", ophthalmic artery, internal carotid artery in the intracranial region).

To confirm the correctness of echolocation, a set of features is used: the depth of the vessel, the direction of blood flow in the lumen of the vessel in relation to the scanning plane of the sensor, as well as the response of the blood flow in the lumen to compression tests. The latter involve short-term (for 3-5 s) compression of the lumen of the common carotid artery above the orifice (or distal) on the location side. A drop in pressure in the lumen of the common carotid artery distal to the compression site and a slowdown or complete cessation of blood flow in it lead to a simultaneous decrease (cessation) of the flow in the located section of the middle cerebral artery (segment M1 or M2). Blood flow in the anterior cerebral artery (A1) and posterior cerebral artery (P1) during compression of the common carotid artery depends on the structure of the circle of Willis and the functional capacity of the anterior and posterior communicating arteries, respectively. In the absence of pathology, blood flow in the connecting arteries (if any) at rest may be absent, bidirectional, or oriented toward one of the connecting arteries, which depends on the pressure level in their lumens. In addition, the length of the connecting arteries and the extreme variability of their location do not allow the use of the indirect signs given above to confirm the correctness of echolocation. Therefore, compression tests are also used to determine the functional capacity (and not the anatomical presence or absence) of the connecting arteries of the Willis circle. The main diagnostic limitations of transcranial Dopplerography are related to the fundamental impossibility of visualizing the vascular wall and the associated hypothetical nature of qualitative interpretations of the data obtained, difficulties in correcting the Doppler angle during “blind” location of flows in intracranial vessels, as well as the existence of multiple variants of the structure, origin, location of intracranial arteries and veins (the frequency in the population reaches 30-50%), in which the value of signs that allow verification of the correctness of echolocation is reduced.

Interpretation of transcranial Doppler ultrasound results

Objective information on the state of cerebral blood flow according to transcranial Doppler sonography is based on the results of determining linear velocity indices and indices of peripheral resistance. In practically healthy people, when examined at rest, Doppler characteristics of flows in intracranial arteries can vary quite significantly, which is due to many factors (functional activity of the brain, age, level of systemic arterial pressure, etc.). The symmetry of blood flow and its indices in paired arteries of the base of the brain are much more constant over time (usually the asymmetry in the values of absolute indices of linear velocity characteristics of flows in the anterior, middle and posterior cerebral arteries does not exceed 30%). The degree of asymmetry of linear velocities and peripheral resistance in the intracranial sections of the vertebral artery is expressed to a greater extent than in the carotid basin, due to the variability of the structure of the vertebral artery (permissible asymmetry is 30-40%). Determination of blood flow indicators in intracranial vessels at rest provides important information about the state of blood circulation in brain tissue, but its value is significantly reduced due to the presence of the autoregulation system of cerebral blood flow, due to its functioning the level of perfusion remains constant and sufficient in a wide range of systemic (local intraluminal) arterial pressure and partial pressure of blood gases (pO ₂ and pCO ₂). This constancy is possible due to the functioning of local mechanisms of vascular tone regulation, which form the basis of autoregulation of cerebral circulation. Among the above mechanisms, myogenic, endothelial and metabolic are distinguished. To determine the degree of their functional stress, transcranial Dopplerography tests the indices of cerebrovascular reactivity, which indirectly characterize the potential ability of cerebral arteries and arterioles to additionally change their diameter in response to the action of stimuli that selectively (or relatively selectively) activate various mechanisms of vascular tone regulation. Stimuli close in action to physiological ones are used as a functional load. Currently, there are methods for determining the functional state of the myogenic and metabolic mechanisms of cerebral blood flow autoregulation for the cerebral vascular pool. To activate the myogenic mechanism (the degree of its dysfunction approximately corresponds to that of the endothelial mechanism), orthostatic (rapid lifting of the upper half of the body by 75° from the initial horizontal lying position), antiorthostatic (rapid lowering of the upper half of the body by 45° from the initial horizontal lying position) and compression (short-term, 10-15 s compression of the lumen of the common carotid artery above the mouth) tests are used, with the introduction (usually sublingual) of nitroglycerin. The latter leads to the simultaneous activation of the endothelial and myogenic mechanisms of vascular tone regulation, since the action of this drug is realized directly through the smooth muscle elements of the arterial wall and indirectly - through the synthesis of vasoactive factors secreted by the endothelium. To study the state of the metabolic mechanism of autoregulation of cerebral blood flow, a hypercapnic test (inhalation for 1-2 minutes of a 5-7% mixture of CO ₂ with air), a breath-hold test (short-term breath-hold for 30-60 sec), a hyperventilation test (forced breathing for 45-60 sec), and intravenous administration of the carbonic anhydrase inhibitor acetazolamide are used. In the absence of signs of functional stress of the regulatory mechanisms at rest, the reaction to the tests is positive. In this case, a change in the velocity indicators of blood flow and peripheral resistance corresponding to the applied load is noted, assessed by the values of the reactivity indices reflecting the degree of change in the Doppler parameters of blood flow in response to load stimulation in comparison with the initial ones. With stress of the autoregulation mechanisms due to an increase or decrease in intraluminal pressure in the cerebral arteries or pCO ₂in the brain tissue, relative to their optimal values, negative, paradoxical or enhanced positive reactions are recorded (depending on the initial direction of changes in tone, the diameter of the cerebral vessels and the type of load stimulation used). In case of failure of autoregulation of cerebral circulation, usually characterized by uneven distribution in the brain tissue, reactions to both myogenic and metabolic tests change. With pronounced tension of autoregulation, a pathological direction of myogenic reactions is possible with a positive nature of responses to metabolic tests. In individuals with stenotic/occlusive pathology, tension of autoregulatory mechanisms occurs due to failure or insufficient development of collateral compensation. In arterial hypertension and hypotension, deviations of systemic arterial pressure from its optimal value lead to the inclusion of the autoregulation system. In vasculitis and angiopathies, limitations of tonic reactions are associated with structural transformation of the vascular wall (fibrosclerotic, necrotic changes and other generalized processes leading to structural and functional disorders).

The basis of ultrasound detection of cerebral microembolism is the ability to determine atypical signals in the Doppler spectrum of distal blood flow (in the arteries of the base of the brain) that have characteristic features that allow them to be differentiated from artifacts. When monitoring blood flow in intracranial vessels using transcranial Dopplerography, it is possible not only to record microembolic signals, but also to determine their number per unit of time, and in some situations - the nature of the microembolic signal (to distinguish air embolism from material), which can significantly affect the further tactics of patient management.

Diagnostics and monitoring of cerebral vasospasm is one of the most important methodological tasks of transcranial Dopplerography, given the significance of angiospasm in the genesis of ischemic damage to the brain tissue caused by a breakdown in the metabolic mechanism of autoregulation with subsequent formation of a hemodynamic phenomenon similar to arteriolar-venular shunting. Pathological cerebral vasospasm develops in hemorrhagic disorders of cerebral circulation, severe craniocerebral trauma, inflammatory lesions of the brain tissue and its membranes (meningitis, meningoencephalitis). Less common causes of this condition are the use of medications (for example, some cytostatics), as well as head irradiation for the purpose of ablation in cancer patients. Diagnostic signs of cerebral vasospasm in transcranial Dopplerography are a significant increase in linear blood flow velocity indices, a decrease in peripheral resistance, Doppler signs of generalized turbulence in the flows of spasmodic arteries, paradoxical or negative reactions during stress testing of the metabolic mechanism of cerebral blood flow autoregulation. As vasospasm progresses, spastic reaction of large extra- and intracranial arteries of varying severity is noted, with its prevalence in the latter. The more severe the spasm, the higher the linear flow velocities and the lower the indices of peripheral resistance. Since the extra- and intracranial spastic reaction is expressed differently, but with a very specific ratio, increasing with increasing severity of spasm (due to ever greater severity in the intracranial sections), special calculated 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 velocity in the middle cerebral artery to that in the extracranial section of the corresponding internal carotid artery. An increase in this index indicates a worsening of the vasospasm.

Studies of the cerebral venous system using transcranial Doppler are determined, on the one hand, by the variability of the cerebral vein structure, and on the other hand, by the limitations of acoustic approaches and methods for verifying the correctness of echolocation (which is especially important for deep veins and sinuses). Of greatest practical importance is the determination of Doppler characteristics of blood flow in the straight sinus at rest and during functional load tests aimed at changing (increasing) intracranial pressure. The importance of such procedures is determined by the possibility of non-invasive verification and assessment of the severity of intracranial hypertension, as well as a number of other pathological conditions (for example, thrombosis of the sinuses of the dura mater). In such situations, diagnostically significant Dopplerographic criteria are an increase in linear blood flow indicators in the deep veins and straight sinus, as well as atypical reactions during antiorthostatic loads with a shift in the “inflection point” due to a limitation of the reserve of volumetric and elastic compensation.

In cases with a significant increase in intracranial pressure (to a level comparable to or exceeding arterial pressure), a hemodynamic situation develops characterized by a significant decrease or complete cessation of arterial flow to the brain ("cerebral circulatory arrest"), leading to brain death. In this case, the Doppler spectrum of blood flow from the intracranial arteries cannot be obtained (or a bidirectional flow with a sharply reduced velocity is located), in the extracranial sections of the brachiocephalic arteries, the time-averaged linear velocity of blood flow is reduced or equal to zero. The advisability of research using ultrasound Dopplerography of blood flow in the extracranial (internal jugular) veins has not yet been determined.