(A) Basic principles of Doppler ultrasound for blood flow velocity detection
Transcranial Doppler is a noninvasive method that uses the ultrasound Doppler effect to detect the hemodynamic and physiological parameters of the major intracranial cerebral base arteries. 1982 Aaslid, a Norwegian scholar, was the first in the world to report transcranial Doppler ultrasound technology, which combines low emission frequency (2 M Hz) with pulsed Doppler technology, allowing the ultrasound beam to pass through the thin part of the skull (specific acoustic window This technique combines a low emission frequency (2 M Hz) with pulsed Doppler, allowing the ultrasound beam to pass through a thin part of the skull (a specific acoustic window) and directly obtain a Doppler shift signal of the vasculature at the base of the brain at a defined distance and within a defined sampling volume.
In recent years, transcranial Doppler has been used to perform fast Fourier transform analysis of Doppler spectrum via microcomputer, which shows and calculates a series of physiological parameters such as peak systolic velocity, end-diastolic velocity, mean velocity, peak systolic to end-diastolic velocity ratio (S/D), pulsatility index (PI), resistance index (RI), etc., which can help clinical analysis of various cerebrovascular diseases. It can help clinical analysis of various cerebrovascular diseases.
In 1842, the Austrian scholar Kjauschen Johann Doppler described a physics effect. When observing the change of light color from the planet, he found that when the planet was moving opposite to the Earth, the light color shifted toward the purple end of the spectrum, indicating an increase in the frequency of light waves; while when the planet was moving backward to the Earth, the light color shifted toward the red end of the spectrum, indicating a decrease in the frequency of light waves. This physics phenomenon is named as Doppler effect.
Doppler ultrasonography is an instrument that uses the Doppler effect for the detection of blood flow. The probe acts as a transmitter and receiver of ultrasound waves, and the change in frequency detected by such a structure is caused by the displacement of the reflector (blood cells). When measuring blood flow velocity, the propagation speed and emission frequency of ultrasonic waves in the tissue are fixed. Thus, there is a very simple relationship between the detected blood flow velocity V and the true blood flow velocity|V|: V =|V|cosθ
where θ is the angle between the ultrasound beam and the direction of blood flow. It is easy to see from the equation that the smaller the angle between the ultrasound beam and the direction of blood flow, the closer the result is to the true blood flow velocity. When performing cerebral vascular testing, we cannot estimate the angle between the ultrasound beam and the direction of the vessel. However, because the anatomical position of the vessels at the base of the brain is relatively constant with respect to the ultrasound window, this provides an anatomical basis that facilitates the measurement of true blood flow velocity, i.e., the limitation of the ultrasound window to the site of incidence of the ultrasound beam determines that the blood flow velocity of intracranial vessels can only be detected at a small angle.
Therefore, the error formed by this angle can be omitted, i.e., the angle between the ultrasound beam and the course of the vessel is considered to be zero.
(II) Detection methods
1.Extracranial vascular detection
The extracranial segment vascular detection includes common carotid artery, external carotid artery and extracranial segment of internal carotid artery. A comprehensive examination of the extracranial arteries is important for the correct identification of intracranial hemodynamic changes. Stenosis or occlusion is most likely to occur at the bifurcation of the common carotid artery and the internal carotid artery, and if the lesion progresses slowly, collateral circulation can be established intracranially.
The patient is placed in a supine position with the head tilted to the opposite side, and the 4 M Hz probe is placed lateral to the sternocleidomastoid muscle, moving the probe from proximal to distal to provide a complete view of the common carotid artery, taking care to keep the ultrasound at a 45° angle to the direction of vascular travel. An angle that is too large or too small can affect the calculated blood flow velocity.
The external carotid artery is usually divided at the level of the thyroid cartilage, and it is traced and recorded anteriorly and superiorly. The internal carotid artery is traced posteriorly and laterally from the common carotid artery bifurcation until it cannot be detected. Under normal circumstances, there is no difficulty in identifying the common and external carotid arteries. The spectral patterns of the common carotid artery, external carotid artery, and internal carotid artery are distinctly different, and the first two are highly pulsatile.
2.Intracranial segmental vessel detection
Determining the acoustic window is the first step to successful transcranial multispectral testing. The acoustic window is the channel through which ultrasound can penetrate the skull without severe attenuation. There are three main “windows” that have been identified and can be used in clinical practice, namely the temporal, orbital and occipital windows. The acquisition of a good intracranial vascular Doppler signal depends on the operator’s skillful technique to position the probe at the best position and angle in order to obtain sufficient Doppler ultrasound signal after the acoustic window is determined.
(1) Temporal window: The temporal window is within the area above the zygomatic arch, between the outer edge of the orbit and the ear. This area can be further divided into anterior, middle and posterior windows. Because ultrasound penetration of the temporal window depends on the thickness of the skull in this area, there are varying degrees of variation between people of different ages and genders. In young adults, there is usually a large area where the ideal signal can be obtained, while in older adults the temporal window is often reduced or even absent due to bone thickening, especially in older female subjects.
During temporal window detection, the patient is placed in a supine position with the head in an upright position, and sufficient acoustic coupling agent is applied to the detection area to maintain good contact between the probe and the skin with moderate pressure without squeezing out the coupling agent and causing discomfort to the patient. The frequency of 2 M Hz focused emission probe is used, and the depth is generally set between 55 and 60 cm, at which the Doppler signal is most easily obtained.
When the Doppler signal is found, the probe is then slightly moved or tilted to select the best probe position to obtain the strongest and clearest Doppler frequency shift signal. The anterior cerebral artery, anterior communicating artery, middle cerebral artery, terminal segment of internal carotid artery, posterior communicating artery, posterior cerebral artery and basilar artery bifurcation can be detected through the temporal window.
(2) Orbital window: The subject is placed supine with the head in an upright position and the eyes closed. A 2 M Hz probe is placed on the eyelid without forceful pressure, as long as the probe is kept in contact with the skin. The Doppler energy is reduced to 5% and the detection time in the eye is kept as short as possible. The transorbital window is focused on the siphon segment of the internal carotid artery and the ophthalmic artery.
(3) Occipital window: The subject lowers the head and flexes the neck so that the space between the skull and the cricoid spine is open. The probe is placed 1.5-2 cm below the occipital ridge in the posterior midline of the neck, and the acoustic beam is directed toward the brow arch so that it enters the skull through the foramen magnum. The intracranial segment of the vertebral artery, the posterior inferior cerebellar artery, and the basilar artery can be detected in this window.
(C) Normal cerebrovascular Doppler spectrum and various flow parameters
A typical normal transcranial Doppler spectrum pattern consists of a series of continuous and regular pulsed fluctuation patterns that are consistent with the cardiac cycle. It is formed approximately as a right triangle, with each frequency occupying one cardiac cycle. The outer frequency curve consists of an ascending branch and a descending branch, and the angle between the ascending branch and the zero baseline is referred to as the alpha angle, with two peaks in systole, S1 and S2, and a third peak in early diastole, D. The time from the beginning of systole to the highest blood flow velocity is called the peak time (Figure 44).
The range of velocity distribution between zero baseline and maximum blood flow velocity at a given instant in the spectrum is called the bandwidth. High-energy signals are concentrated in the periphery, with darker colors, while low-energy signals are distributed in the lower part of the spectrum, with lighter colors. This results in a window, called the “frequency window”. The formation of the frequency window is mainly due to the “laminar flow” of blood in the blood vessels. It is worth noting that sometimes there is an artifact of the disappearance of the frequency window, such as improper angle between the sound beam and the vessel, and excessive ultrasound reflection energy. Therefore, when testing, we should strive to find the best transmission angle and choose the appropriate ultrasound emission power.
The internal carotid artery and the external carotid artery have significantly different spectral patterns. The internal carotid artery has the same flow characteristics as the intracranial artery, with relatively low resistance and gentle descending branches, while the external carotid artery is a high resistance type with the characteristics of a peripheral vasculature, with a high pointed systolic peak, steep descending branches and obvious diastolic cut.
Transcranial Doppler spectral parameters include flow velocity, beat index, peak systolic to end diastolic flow velocity ratio, and resistance index. Blood flow velocity measurement is the main parameter of transcranial Doppler spectral analysis, which includes peak systolic blood flow velocity (V S), mean blood flow velocity (V m ), and end-diastolic blood flow velocity (Vd). The pulsatility index (PI) and the ratio of peak systolic to end-diastolic flow velocity (S/D), which are indicators of vascular compliance and vascular elasticity, and the resistance index (RI) are indicators of the diastolic state of the cerebral vasculature, i.e., the resistance condition. The formula is as follows.
PI = peak systolic flow velocity – end-diastolic flow velocity / mean flow velocity
S/D = peak systolic flow velocity/end diastolic flow velocity
RI = peak systolic flow velocity – end-diastolic flow velocity / peak systolic flow velocity
II. TCD diagnosis of cerebrovascular diseases
(I) Cerebral arteriovenous malformation (A V M)
Cerebral arteriovenous malformation A V M is a congenital abnormality of cerebral vascular development, the main pathophysiology of which is blood theft from malformed vessels, and a large amount of cerebral arterial blood is lost through arteriovenous short circuit, causing cerebral hemodynamic changes. Using transcranial Doppler technique, we can detect not only the abnormal blood flow at the site of malformed vessels, but also all the arteries involved in blood supply and blood theft in the contralateral or ipsilateral hemisphere.
1. TCD manifestations of cerebral AV M
(1) Alteration of blood flow velocity
In normal arterioles, a capillary network exists between the arteries and veins, which can create a normal vascular resistance. In arteriovenous malformation, the presence of fistula between artery and vein decreases vascular resistance, resulting in increased blood flow and significantly accelerated blood circulation time, showing high flow velocity and low resistance Doppler flow characteristics, blood flow velocity can be two to three times higher than normal, and the relative increase in diastolic phase is significant, so the systolic to diastolic flow velocity ratio (S/D) is significantly reduced. The low resistance characteristics of the blood supply artery are reflected by a decrease in the pulsatility index (PI), which decreases significantly with increasing flow velocity.
(2) Spectral characteristics of blood flow
As the flow velocity of the blood supply artery increases, the ratio of systolic to diastolic flow velocity decreases significantly, resulting in a decrease in the difference between the systolic and diastolic frequency shifts in the spectrum, i.e., the spectrum widens significantly, and the closer to the malformed vascular mass, the higher the flow velocity, the more disorganized the blood flow is, the more dilated and tortuous the vessels are, the more the spectrum loses its normal “frequency window” characteristics, showing a burr-like border and a filled frequency window. Disturbed blood flow signals of different frequencies, unclear layers, eddy currents, enhanced low-frequency signals, weakened high-frequency signals, or bidirectional blood flow spectrum.
(3) Blood flow audio characteristics
Normal Doppler blood flow audio is soft and clear, when the arteriovenous malformation, blood flow velocity and flow rate increase, the direction of blood flow in the malformed vascular group is different, and the audio signal varies greatly. When detected, the blood flow audio signal can be heard as loud, rough and garbled, like the “rumble-like” vascular murmur of a machine, and can also be accompanied by sharp musical murmur audio frequency.
High flow velocity and low resistance characteristics of cerebral AV M supply artery blood flow spectrum
(4) Intracranial blood steal
Due to the decrease in resistance and increase in flow velocity in the malformed vascular mass, the pressure in the blood supply artery decreases, and the blood flow to the normal brain tissue is stolen by the cerebral AVM, resulting in “blood theft syndrome”. TC D test can detect the opening of abnormal traffic arteries such as anterior cerebral artery (A C A ), posterior cerebral artery (PC A ), etc.
2.TCD evaluation of the efficacy of endovascular embolization of cerebral AV M
The main pathological change of cerebral A V M is the alteration of intracranial hemodynamics. High flow velocity and low resistance of the blood supply artery and opening of the traffic artery occur due to malformation and blood theft. When a malformed vascular mass is embolized, the increase in its internal resistance inevitably causes a rise in pressure and varying degrees of decrease in flow velocity within the blood supply artery, and the blood flow supplying the malformed lesion is redistributed intracranially. Therefore, the change of blood flow velocity in the original blood supply artery after embolization therapy is an important index for evaluating the efficacy.
We observed the TC D of 170 cerebral A V M cases and found that the end-diastolic flow velocity (Vd) reduction rate was greater than the peak systolic flow velocity (Vd) reduction rate in fully embolized supply arteries, indicating that the change in Vd is more sensitive to the change in the distal resistance of the supply artery; the efficacy of embolization of single-branch supply A V M is much more satisfactory than that of multi-branch supply A V M. The situation of multi-branch supply A V M is more complicated. The mean flow velocity and PI value of the embolized donor artery recovered or approached normal, while the flow velocity of the unembolized donor artery increased or decreased unequally, and the PI value of the more significantly increased donor artery decreased significantly.
This indicates that the blood supply to the malformation from the embolized blood supply artery has stopped, while the blood supply to the malformation from the unembolized blood supply artery has increased, which is thought to be related to the redistribution of intracranial blood flow after embolization.
3.Diagnosis and differential diagnosis
Diagnosis of TC D in cerebral A V M.
①The blood supply artery exhibits high flow velocity, low resistance, reduced or significantly reduced PI, and decreased systolic/diastolic ratio;
(2) The blood flow spectrum shows a decrease in the difference between systolic and diastolic frequency shift, i.e., a significant broadening of the spectrum, diffusion in diastole, indistinct systolic peaks, or a disorganized spectrum with irregularities and hairy borders;
③The blood flow sound frequency is loud and rough, such as machine “rumble-like” vascular murmur or sharp musical murmur sound frequency; ④Intracranial blood theft signs.
According to the characteristics of cerebral A V M hemodynamic changes, combined with the clinical history, it is helpful to distinguish cerebral vasospasm from vascular stenosis.
Cerebral vasospasm is closely related to subarachnoid hemorrhage, and Doppler flow velocity is uniformly elevated, with sharp systolic peaks and symmetrically elevated systolic and diastolic flow velocities, and PI is within the normal range.
Arterial stenosis is usually associated with various causes of atherosclerosis, nonspecific intracranial arteritis, and arterial thrombosis. The elevated Doppler flow velocities are characterized by increased segmental flow velocities, reduced or normal flow velocities distal to the stenosis, a turbulent spectrum, a filled window, and weak energy, and a “narrow band” systolic spectrum.
Based on the characteristic changes of cerebral A V M flow, TC D can make a definite conclusion, but for small distal artery (straight <2 cm) malformations, the flow velocity and pulsatility index of the supply artery are not significantly changed, so TC D diagnosis is more difficult. Therefore, the presence of cerebral A V M cannot be ruled out even if there is no obvious abnormal change in TC D examination, and cerebral angiography should be performed to confirm the diagnosis if necessary.
4.Clinical evaluation
The diagnosis of cerebral A V M generally depends on cerebral angiography and CT scan, but neither of them can obtain intracranial hemodynamic information. TC D can provide real-time and dynamic observation of the high-flow and low-flow characteristics of malformed blood supply arteries, understand the situation of draining veins and intracranial blood steal, provide hemodynamic parameters, and at the same time, objectively evaluate the effect of endovascular embolization treatment according to the changes of hemodynamic parameters. Thus, TC D can be used as a non-invasive test for diagnosis, efficacy assessment and follow-up of cerebral A V M.