History:
Christian Andreas Doppler was an Austrian mathematician who described the “Doppler effect” in 1842 to explain the changing color of the moving stars. Doppler's first descriptions concerned changes in the wavelength of light and therefore colors of stars as they move relative to earth. Interestingly, Doppler never extrapolated his postulates to sound waves.
In 1845, his theory was confirmed for acoustic frequencies by an experience while musicians on a train were playing notes and others were recording which notes they were hearing. The 1st practical application of Doppler effect was after the Titanic Disaster. There was a necessity to develop an underwater detection system (Sonar) that was refined during the following years and was of great help for the submarines in World War II.
In 1961, Franklin & colleagues adapted the Doppler technology to blood flow measurement. The first arteries to be studied were the carotids & lower limbs arteries. Doppler examination of the extracranial carotid arteries became very popular in the 1970’s. In the 1980’s, it stated to be coupled with vessel wall imaging (Echography) and the term of carotid Duplex was used to designate this combined examination. Finally, in the 1990’s, the problem of the skull being an obstacle for ultrasounds was resolved and transcranial Doppler became a routine technique with multiple applications in cerebrovascular disease & anesthesia.
The advent of technology has transformed the practice of modern medicine to become science-based, where every minute pathological and physiological change can be measured accurately. Doppler's principle is the scientific tool available for measuring blood flow characteristics anywhere in the circulatory system in a simple logical sequence. Cardiologists have been using this tool since the early sixties to evaluate cardiovascular flow. This valuable tool is now used by neuro-sonologlists in the investigation of cerebrovascular flow to assess pathological changes and various disease process affecting intracranial structures.
General principle of Doppler:
Wherever there is movement of particles, of any nature and which can not be directly seen, Doppler's principles comes into play to detect the velocity of that movement . This applies to gases moving in tubes at high speed in a chemical factory, fighter jets in the sky, or overspeeding motorists on the road. The same Doppler's principle is utilized.
Blood flow dynamics:
A moving particle inside a lumen requires a force to drive it from one point to another. This force is dependent on the pressure gradient between two points. The blood flow inside a vessel is dependent on this phenomenon. This is called the flow rate and guided by two major factors, which are the pressure difference between two points, mainly created by the force of cardiac contractility (output) and the blood vessel wall resistance (Figure 1).
Figure 1
The flow rate is directly proportional to the pressure difference and inversely proportional to the vascular resistance (Figure 2). The Other minor factors that have a role in the blood flow rate include the hydrostatic component of the blood flow, which is largely related to gravity, and the static component, which is related to the volume of blood.
Figure 2
Other important factors that influence the blood flow rate are related to the blood vessel wall, which is the dimensions of the blood vessel, reflected by its cross sectional area. Thus, and safely, we can predict that the velocity of blood flow inside a vessel is mainly dependent on the flow rate and the cross sectional area of the blood vessel (Figure 3).
Figure 3
Figure 4 demonstrates the blood flow velocity differences in the carotid arteries, reflected by different colors, in different segments of the arteries due to differences in the vessel cross sectional areas at different segments.
Figure 4
Dynamics of intracranial vessels:
Various normal anatomical and physiological factors have influence on the blood flow velocity of the intracranial circulation. These factors are as follow:
A) Vessel geometry:
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Blood vessels divide and make smaller vessels. Those divisions can take different angles and accordingly blood flow velocity changes at division points, creating a mild turbulence at division zones (Figure 5). This turbulence is usually not significant.
Figure 5
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The course of any blood vessel is not straight but usually is curved and the flow velocity is naturally higher at the inner aspect of those curves, with mild and insignificant turbulence. However, the change is minimal and would not affect the total measurements of velocity, unless vessel is markedly tortuous (Figure 6).
Figure 6
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Intracranial blood vessel wall is different in its compliance and tension, when compared to the rest of blood vessels in the body. This compliance allows blood to flow even in diastole (Figure 7)
Figure 7
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Blood vessels progressively narrows normally as the go forward, however, if there is a significant and abnormal narrowing, there will be an acute elevation of velocity at the narrow segment (Figure 8).
Figure 8
B) Autonomic and Posture effects:
Changes in respiration, posture, blood pressure and heart rate have their influence on blood flow velocity, and should always be considered with insonation of intracranial vessels.
C) Age effect:
As people advances in age, blood vessels wall becomes more rigid. This change in vessel wall would influence the blood flow velocities.
D) Intracranial blood flow autoregulation;
Cerebral blood flow (CBF) is around 50 ml/100 Gm of brain tissue/min and this flow is maintained throughout different physiological and even pathological processes. This involves a delicate balance between the systemic and intracranial pressures. If there is any fall in the CBF, protective mechanisms operate to enhance CBF. Those mechanisms are believed to be through the vasomotor reserve, involving myogenic and metabolic mechanisms (Figure 9).
Figure 9
The changes of intracranial PH (through changes of arterial carbon dioxide tension) have the most powerful effect on cerebral blood flow. This well known fact made it possible to measure the vasomotor reserve of cerebral hemispheres in normal and various pathological diseases by inducing hypo or hypercapnia (Figure 10).
Figure 10
E) In case of vessel stenosis, certain blood flow dynamics take place. They are classified into direct effects, which include acute elevation of blood flow velocities, along with flow turbulence, based on degree of stenosis, or indirect effects, which are related to flow disturbances proximal or distal to the stenotic segment (Summarized in table 1).
Table 1
Figure 11 explains the direct and indirect effects in a case of internal carotid artery (ICA) stenosis.
Figure 11
Conclusion:
Transcranial Doppler (TCD) as a method for the investigation of lumen narrowing (or widening) within the insonated vascular segment is well known. By careful analysis of flow pattern TCD, as well as by utilizing stimulation techniques TCD can also illuminate general or local physiology and pathophysiology of cerebral hemodynamics. Since the cerebral blood flow velocity in the large brain supplying artery, which is measured by TCD, perfectly reflects cerebral blood flow (CBF) TCD can be used 1.) to monitor qualitatively cerebrovascular resistance (i.e. increased in the case of elevated intracranial pressure or decreased in encephalitis); 2.) to illuminate cerebrovascular reserve capacity by analyzing flow changes to either hypo- as well as hypercapnia or by administration of acetazolamide (i.e. to uncover exhaustion of collateral blood supply distal to high grade arterial obstruction); and 3.) to measure cerebral autoregulation due to blood pressure alterations (i.e. in brain trauma or patients suffering from syncope).