Dopplerography of cerebral vessels

Ultrasound of the brain is considered one of the safest and at the same time effective methods. This procedure is carried out for both children and adults. With the help of ultrasound diagnostics, it is possible to identify quite serious diseases at the initial stage of development and prescribe their treatment.

The primary objective of cerebral vascular examination using color duplex sonography is to determine and quantify the degree of stenosis caused by atherosclerotic changes in patients with complaints and a history of transient ischemic attack or stroke. The examination should establish the degree of stenosis and the extent of the affected vessel segment. The collateral system should be assessed for preoperative or preinterventional determination of the risk of complications. The examination requires knowledge of cerebral vascular anatomy and normal ultrasound imaging, which will be discussed in this chapter before presenting the semiotics of cerebrovascular disease in the carotid and vertebral artery basins.

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Ultrasound anatomy of the carotid artery system, research methods

Many physicians prefer to sit behind the patient's head while the patient is lying down. Scanning can also be started anteriorly, with the transducer positioned near the midline and displaying a cross-section of the common carotid artery. This vessel lies posterior and medial to the internal jugular vein. The diameter of the jugular vein can be increased by performing a Valsalva maneuver, which usually results in immediate visualization of the vessel in B-mode. The cross-section is displayed as shown below, with the right and left sides reversed.

When the transducer is rotated 90° along the longitudinal axis, the right side of the image is at the bottom and the left side is at the top, just as in abdominal ultrasound. Watch for the physiological separation of the eye folds that occurs at the level of the bifurcation of the common carotid artery and the transition to the carotid bulb of the internal carotid artery. This abrupt widening creates a rounded swirl that should not be mistaken for pathological poststenotic flow back, turbulence, or blurring.

The Doppler spectrum from the common carotid artery typically shows a slight increase in peak systolic velocity compared to the internal carotid artery due to relatively low intracranial peripheral resistance. This pattern differs from the external carotid artery, which may show a "whistling" audio signal with relatively high systolic and low diastolic velocities. A triphasic spectrum can be obtained from the external carotid artery that includes a component of reverse flow. The superior thyroid artery is visible here in color mode.

Anatomical orientation

When visualized in the longitudinal axis, the internal carotid artery is normally located posterior and lateral to the transducer, whereas the external carotid artery remains close to it for a long distance. If there is doubt about the vessel, repeated compression of the superficial temporal artery leads to oscillations in the external carotid artery spectrum. The internal jugular vein is easily distinguished from the internal carotid artery by the direction of blood flow and a flat spectral trace.

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Stenotic lesion of the internal carotid artery

Atherosclerotic deposits do not always contain calcifications with shadowing. “Soft plaques” appear as hypoechoic, crescent- or circular-shaped voids in the color lumen along the vessel wall. With color duplex sonography, the craniocaudal extent of the plaque can be accurately determined. Eccentric increased blood flow can often be seen.

Vessel wall stratification

Dissecting vessel wall with blood between the layers is a special condition that usually occurs spontaneously, but can also be associated with neck trauma or physical overload at any age. It is characterized by the presence of a hypoechoic intramural hematoma, causing significant blood flow disturbances.

A wall aneurysm usually develops as a complication. The intimal flap may occlude the original vessel lumen, which on ultrasound appears to end in an acute angle. Recanalization may occur after several weeks and can be accurately documented using color duplex sonography.

Ultrasound anatomy of the vertebral artery system, research methodology

The vertebral artery is scanned in longitudinal section from the anterolateral approach with the patient in the supine position, starting from its origin (V _{0 ), and the examination is continued to a point in the region of the C1} vertebral loop (including the V ₂ segment ). It is best to use a linear transducer with variable frequency (5.0-7.5 MHz). The intraforaminal segment V ₂ of the vertebral foramina is best accessible to duplex scanning. It can be clearly visualized together with the accompanying vein between the acoustic shadows of the cervical vertebral bodies.

In hypoplastic vertebral artery, most often one of the arteries (usually the right one) has a diameter of less than 2.5 mm, while the opposite one is enlarged to more than 4 mm in diameter (the discrepancy is more than 1: 1.7). The normal diameter of the vertebral artery is approximately 3.8 ± 0.5 mm. In a hypoplastic vertebral artery, a decrease in the end-diastolic component of blood flow (Vdiast) is noted. Sometimes it is difficult to distinguish hypoplastic vertebral artery from distal stenosis or occlusion, since in all cases there is a decrease in Vdiast. Favorite locations for stenosis are the origin of the vertebral artery from the subclavian artery, as well as the area at the level of the C1 vertebra, which is scanned from a posterior approach behind the mastoid process. It is best to use a 5.0 MHz transducer, placing it immediately below the mastoid process and posteriorly, tilting it toward the opposite orbit with a slight turn of the head to the other side.

Segment V4 is scanned with a 2.5 or 2.0 MHz sector transducer, which is positioned below the occipital protuberance and angled toward the orbit.

It should be noted that there are no significant criteria for determining the degree of vertebral artery stenosis, unlike the carotid artery.

With normal patency of the vertebral artery, there is a biphasic spectrum with a clear spectral window, whereas stenosis is characterized by a significant increase in blood flow and filling of the spectral window.

Dissection of the vertebral artery after trauma can lead to embolic cerebral ischemia, ending in stroke. The results of color duplex sonography can be very diverse - from the presence of an intramural hematoma to occlusion of the affected segment of the artery. Sometimes the detached intimal flap itself can be seen.

The thin squamous portion of the temporal bone provides the best acoustic window for scanning the circle of Willis with a 2.0 MHz transducer.

Transcervical examination of the basilar artery

Transcervical scanning can be performed with the patient sitting with the head tilted forward or with the patient lying supine with the head turned to the side. This allows both V4 segments to be seen where they merge into the basilar artery.

Anatomy of the cerebral vessels

The circle of Willis is normally formed by the carotid (anterior basin) and vertebral (posterior basin) arteries. Atherosclerotic plaques rarely form at the site of the common carotid artery's origin from the aortic arch on the right and from the brachiocephalic trunk on the left. Stenosis usually develops at the bifurcation of the common carotid artery into the internal carotid artery and the external carotid artery. The first intracranial branch of the internal carotid artery is the ophthalmic artery. Immediately after it, the internal carotid artery divides into the middle cerebral artery and the anterior cerebral artery.

The vertebral arteries arise from the aortic arch in 4% of cases, but their source is usually the subclavian artery. The left vertebral artery often begins more proximally than the right. Each vertebral artery is divided into 5 segments. The proximal segment from the origin is called Vo. Segment Vi continues to the transverse process of the C6 vertebra, but sometimes the artery enters the foramen at the level of Cs. Segment V2 is most accessible for examination in the middle of the neck. The loop of the vertebral artery at the level of the first cervical vertebra corresponds to segment V3. Segment V4 is located within the skull, and from its distal segment the posterior inferior cerebellar artery arises. In certain segments or along its entire course, the vertebral artery may be hypoplastic. The right and left vertebral arteries merge, forming the basilar artery, which divides into the right and left posterior cerebral arteries.

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Collateral pathways

1. Severe stenosis or occlusion of the internal carotid artery. With the main collateral pathway from the external carotid artery to the internal carotid artery basin, blood enters the brain retrogradely via the supratrochlear and ophthalmic arteries. Another way to compensate for high-grade stenosis of the internal carotid artery is cross-flow via the anterior communicating artery. To avoid risk during surgery, the surgeon should be aware of the possibility of hypoplasia or aplasia of the proximal A1 segment of the anterior cerebral artery. The vertebral artery system may receive collateral blood flow via the posterior communicating artery if the P1 segment of the posterior cerebral artery on the corresponding side is not underdeveloped.
2. Severe stenosis or occlusion of the vertebral artery. Collaterals in proximal stenosis of the vertebral artery may be the deep artery of the neck, coming from the thyrocervical trunk or the branch of the occipital artery from the basin of the external carotid artery. In stenosis of the basilar artery, the only collateral pathways are the posterior communicating arteries or leptomeningeal anastomoses from the basin of the middle cerebral artery. In such cases, aplasia of the P segment, the posterior cerebral artery with direct origin of the posterior cerebral artery from the internal carotid artery, have a positive side.

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Quantitative assessment of internal carotid artery stenosis

The local degree of stenosis can be calculated cross-sectionally by measuring the intrastenotic color residual lumen (Ag) and relating it to the original transverse diameter of the vessel in the affected area (AN) using the cross-sectional area reduction formula. The more sensitive power Doppler mode is used to accurately determine the cross-sectional area of the residual perfused lumen.

In both images, the hypoechoic plaque within the lumen is clearly differentiated from the hyperechoic calcifications.

The degree of stenosis can also be assessed using longitudinal scanning by measuring peak flow velocities with their angular correction. Digital subtraction angiography, for example, cannot assess flow velocity. The method used in the largest multicenter study to date (North American Symptomatic Carotid Endarterectomy Trial: NASCET) measured carotid stenosis by determining the ratio of the lumen diameter at the narrowest part of the stenosis (ds) to the normal carotid diameter distal to the stenosis.

When considering the use of color duplex sonography for stenosis assessment, it was shown that the degree of stenosis can be determined with high accuracy using this technique. It is important to differentiate pre-occlusive "pseudo-occlusion" from true occlusion for planning appropriate treatment. A thread-like residual lumen, invisible on native images, can sometimes be detected with intravenous contrast. It should be remembered that sometimes higher peak blood flow velocity can be determined after contrast administration. Color duplex sonography also allows non-invasive monitoring after carotid thromboendarterectomy or stent implantation to exclude recurrent stenosis. Several multicenter studies have shown that thromboendarterectomy reduces the individual risk of stroke in patients with clinically evident high-grade (>70%) internal carotid artery stenosis.

Intima-media thickness in the carotid artery system

Long-term epidemiological studies have shown that carotid intima-media thickness is a prognostic factor for stroke or myocardial infarction after all other risk factors (hypercholesterolemia, hypertension, smoking, etc.) are taken into account. How is it determined?

The examination is performed with a linear transducer with a frequency of more than 7.5 MHz, recording images with 60 dB compression, and measuring vessels in systole. Harmonic components and artifactual contrast agents are not used. If the examination is started from the lumen of the carotid artery, the first sonographically determined layer is the echogenic junction of blood and intima, followed by the hypoechoic image of the intima-media, and finally the media and adventitia. For physical reasons, the intima-media thickness can be measured more accurately at the far wall (4=) than at the near wall, where the transition is less clearly defined. The intima-media thickness at the far wall is measured as the total thickness of this entire complex, since accurate separate measurement of both layers is impossible.

In research studies, it is common to make 5-10 measurements in three segments of the carotid artery - the common carotid artery, the bifurcation region, and the internal carotid bulb - and calculate the average value for all three segments. These studies often use semi-automated processing modules that sequentially record multiple IMT values using a gray scale, which improves the reproducibility of the measurements.

For practical application of this technique, it is necessary to limit the examination to a segment of the common carotid artery. One protocol consists of measuring a well-visualized segment of 10 mm in length, 5 to 10 individual measurements, and calculating the average value. The resulting data depend on age and correlate with established risk factors. It has been found that effective intervention of cardiovascular risk factors for 1 to 2 years reduces intima-media thickness.

Ultrasound semiotics of intracranial vascular lesions

In patients with high-grade internal carotid artery stenosis or unilateral occlusion, it is important to determine the presence of retrograde collateral blood flow through the ophthalmic artery from the external carotid artery basin, opposite to zero or normal. The picture of intracranial collateralization can be assessed by comparing the Doppler spectra from the arteries.

In bilateral occlusion of the internal carotid arteries, collateral blood flow comes from the vertebral arterial system via the intact circle of Willis or via the orbital collaterals. To avoid erroneous interpretation, it is always necessary to examine all major arteries of the circle of Willis that are accessible to Doppler ultrasound.

Increased blood flow can occur for reasons other than stenosis. For example, anemia can cause functional increased blood flow in the internal carotid artery, as shown in this patient with a hemoglobin level of only 6.2 g/L. Increased blood flow can also occur with aneurysms, which can be detected by color duplex sonography when they are larger than 5-10 mm and located in areas accessible to scanning.

Critical assessment

The carotid arteries, due to their superficial location and the possibility of scanning with good resolution at high frequencies, are ideal for examination using non-invasive color duplex sonography. To a certain extent, the same applies to the vertebral arteries. It is quite difficult to visualize using color duplex sonography the origin of the left vertebral artery, which is often located at a fairly low level. A similar problem also exists in 4% of cases of the origin of the vertebral artery from the aortic arch. An alternative non-invasive examination technique when excluding dissection of the vertebral or carotid artery is MR angiography (MRA), which can be performed in the time-of-flight mode or with the introduction of a contrast agent.

Another, more invasive method is digital subtraction angiography. Its main advantages are the ability to detect slow blood flow in stenoses with a very narrow lumen and to identify the lumens of small intracranial vessels. In this case, a small aneurysm was detected. Digital subtraction angiography can also determine collaterals and venous drainage when venous sinus thrombosis is excluded.

In 15% of cases, ultrasound penetration during Doppler examination is so difficult (for example, with thick bones of the vault) that contrast agents must be used.

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