It is certainly easy to set up the TCD machine and calibrate it. It is also easy to apply it to the patient's head. However, the success rate of insonating a specific vessel is quite variable. It may be difficult, especially in elderly females who have thicker calvariums, to obtain a good velocity wave form. In approximately 30% of this population, even the experienced sonographer will not be able to get an acceptable velocity tracing.
The first step in TCD examination is to localize a cranial 'window' where the ultrasonic beam can penetrate without being excessively damped. The next step is that of identifying the signals from the different segments of the arterial network at the base of the skull. Three main pathways (Fig. 9) to access the intracranial arteries have been described: (a) the transtemporal, (b) the transorbital and (c) the transforaminal route.
The insonation from the temporal region has to be performed through solid bone tissue. This is only possible if the bones are thin. In young adults it is usually possible to obtain good signals from a relatively large area, but in some elderly patients it may be barely possible to obtain signals through a very small window. The temporal windows are found above the zygomatic arch. It is usefull to discriminate between four different locations of the temporal window (Fig. 10A/B): (a) the frontal, (b) the anterior, (c) the middle and (d) the posterior window. The middle temporal window facilitates an approximately direct medial insonation, whereas the probe has to be aimed obliquely in a slightly posterior direction from the anterior temporal window and it has to be aimed anteriorly to reach the arteries in the circle of Willis.
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Fig. 9 The three natural accoustic windows that allow penetration of the ultrasound beam into the cranium and insonation of the major intracranial arteries. The most important i.e. the most frequently used approach is the transtemporal window; it is used for investgation of the anterior, middle, and posterior cerebral arteries (adapted from KA Fujioka, Douville CM. Anatomy and freehand examination. In Newell DW, Aaslid R: Transcranial Doppler, Raven Press, New York, 1992).
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Fig. 10 (A) The transtemporal window is divided into four distinct areas: the posterior (P), middle (M), anterior (A), and frontal (F) window. (B) Transducer angulations vary according to which transtemporal window is being utilized (adapted from KA Fujioka, Douville CM. Anatomy and freehand examination. In Newell DW, Aaslid R: Transcranial Doppler, Raven Press, New York, 1992).
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There are three main sources of information for artery identification: (a) the spatial resolution of the signal to other intracranial signals (including information of depth and angle of the probe), (b) the direction of flow (towards or away from the transducer) and the spatial distribution and, (c) the response of the signal to compression or vibration maneuvers (Tab. 1). By using proper examination techniques, it is generally possible to achieve a relatively high degree of accuracy in artery identification even without compression maneuvers. The main 'landmark' for orientation is the branching of the supraclinoid ICA into the ACA and the MCA.
To monitor physiologically (eg CO2 reactivity, cerebral autoregulation) or pharmacologically (eg anesthetics) induced changes in CBFV only the MCA is of interest. The MCA runs laterally and slightly anteriorly as a continuation of the intracranial ICA. It has the highest volume flow of the branches from the circle of Willis, carrying about 80% of the flow to the hemisphere. An anterior-temporal window allows almost zero degree insonation, whereas from a posterior window a somewhat blunter angle may be expected. The MCA (together with its branches) is normally the only artery seen between depth of 50 and 25 mm from the temporal window. The criteria for MCA identification are: (a) the Doppler signal can be followed laterally with only slight probe movements from the termination of the ICA up to about 30 mm, (b) the flow is toward the probe, and (c) the signal responds to ipsilateral vibration or compression of the lower CCA.
Traditionally, systolic, diastolic and mean values are used to describe pressure, flow and flow velocity in the arterial system. Of these values, the mean carries the highest physiological significance because it depends less on central cardiovascular factors such as heart rate, contractility, total peripheral resistance and aortic compliance than do systolic or diastolic values. Moreover, the mean velocity correlates better with perfusion than the peak and trough values.
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Tab. 1 Summary of vessel identification criteria using transcranial Doppler techniques (adapted from KA Fujioka, Douville CM. Anatomy and freehand examination. In Newell DW, Aaslid R: Transcranial Doppler, Raven Press, New York, 1992)
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Transcranial Doppler shifts i.e. mean velocity values from the various basal cerebral arteries in normal subjects were reported (Tab.1). The values reported are in close agreement, but in individual cases there is a considerable range of values of these Doppler shifts, with a relative standard deviation of about 90 percent higher than the reported for CBF measured by indicator methods. Variations in insonation angle cannot account for this increased deviation, and the variability of flow velocity must therefore be attributed to differences of the cross-sectional lumen of the arteries.
Transcranial Doppler sonography measures the flow velocity in the cerebral arteries that change according to the cardiac phases. The velocity is obtained by recording frequency shifts in ultrasound reflected from flowing blood. The frequency shifts then are coded by spectral analysis. Under normal conditions in a relatively straight artery, a number of different frequency shifts occur during laminar flow, due to the parabolic flow profile. These different frequency shifts are displayed on the spectrum analyzer as different velocities.
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Fig. 11 Parameters of the Doppler signals. a1 = a2, A1 = A2 (Abbreviations are explained in the text).
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Most Doppler instruments display the spectral outline velocity, which is obtained by assigning an outline on the spectral tracing, thereby reflecting only the maximum velocities occuring at the central portion of the artery (Fig. 11). From this envelope curve of the Doppler frequency spectra systolic peak flow velocity (vs), mean peak flow velocity (vm) and enddiastolic peak flow velocity (vd) can be derived. In contrast, the 'true mean' velocity is derived from an average weighting of all the spectral signals in a cross section of the artery being recorded from. The true mean includes the high velocity signals at the center of the artery and the low velocity signals near the walls. From all these registered Doppler frequencies, averaged systolic flow velocity (vS), averaged mean flow velocity (vM) and averaged enddiastolic flow velocity (vD) can be derived. In the absence of any disturbance in the velocity profile a change in blood flow will produce proportional changes in maximal flow velocity and true mean velocity. For practical use the maximal flow velocity and its derived parameters are used and reported. From the different defined parameters the resistance index according to Pourcelot (RI = ( vs - vd ) / vs ) and the pulsatility index according to Gosling (PI = ( vs - vd ) / vm ) can be computed.
In relating CBFV to CBF some expressions need clarification. When considering a relationship between CBFV and CBF, two questions arise that must be dealt with separately: (a) how do absolute CBFV and absolute CBF relate, and (b) do relative changes in CBFV reflect corresponding relative changes in CBF.
In a cerebral artery with lumen A (cm2), the blood flow Q (ml.s-1) and the cross-sectional average blood velocity Vav (cm.s-1) are related by the equation:
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As blood is neither added nor substracted in the cerebral circulation, the same amount of blood will also pass through a certain part of the cerebral microcirculation. The brain area is the perfusion territory of this artery. When investigating with indicator methods (vide 3.), brain tissue perfusion is usually expressed in (ml/100gtissue/min). However, it can also be expresed as (cm3/100g tissue/60s). Thus, the relationship between the arterial blood flow Q' in (cm3/60s) and the rCBF in (cm3/100gtissue/60s) in the perfusion territory T (expressed in 100g of brain tissue) of a given cerebral artery is:
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By combining the 2 equations, remembering that Q' = 60 Q, and solving for Vav, we obtain:
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The Vav is very difficult to measure accurately since the anatomical configuration of the basal cerebral arteries is very complex. Further, when recording flow velocities in basal cerebral arteries through the intact scull, the ultrasonic field is inhomogeneous and the signal-to-noise ratio is low. Also, Doppler instruments usually remove the lowermost frequencies of the Doppler-shifted spectrum through high-pass filtration. With the current TCD instrumentation we therefore prefer considering the maximal velocity, here denoted as V, as measuring the maximal velocity (the outline of the Doppler-shifted velocity spectrum) that is less encumbered by these limitations (vide 7.4.). When pulsatility is low, such as in normal cerebral arteries, maximal blood velocity may be substituted for cross-sectional average blood velocity by introducing a constant K. Substituting V for Vav we obtain:
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Given a situation permitting no collateral blood flow into the perfusion territory of a cerebral artery, tissue perfusion will cease upon clamping of this artery. V and rCBF therefore both become zero. By rearranging the above equation into V/rCBF = K 1/60 T A-1, we see that the slope of this relationship V/rCBF is influenced by the size of the perfusion territory T of the artery and the arterial lumen A. Moreover, the slope increases with an increase in T and with a decrease in A.
In practice, when relating absolute V to absolute rCBF, the angle of incidence between the blood flow and the ultrasound beam must be considered. The observed blood velocity V0 relates to V as follows: V0 = V, where is the cosine of the angle of incidence between the blood flow and the axis of the ultrasound beam.
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Thus, to calculate the blood flow in a region perfused by a cerebral artery from the blood velocity of this artery, one must know the luminal area A and the perfusion territory T of the artery, the constant K and the angle of incidence between the blood flow and the ultrasound beam.
Investigating the cerebral circulation of a subject in two different situations, the relationship between V0 and rCBF becomes:
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and
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Expressing the relative change in V0 and rCBF from situation 1 into situation 2 as indices, we obtain:
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which can be rearranged into:
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During variation in blood velocity within ± 50% [96] or variation in blood flow within ± 35% [108], the velocity profile in the basal arteries does not seem to vary unduly. Therefore, within these limits, any change in K are probably minor, and hence K2 / K1 is approximately 1. Further, if there is no change in the angle of incidence between the blood flow and the ultrasound beam, 2 / 1 equals 1. Given these assumptions, relative changes in V0 and rCBF of a defined cerebral artery system relate as follows:
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Relative changes in V0 thus correspond to relative changes in rCBF only when the luminal area A and the perfusion territory T of the examined artery remain constant. Given an increase in T or a decrease in A, relative changes in V will exceed relative changes in rCBF, with increases in A or decreases in T having the opposite effect.
It is important to understand that TCD is not a CBF monitor. It only measures flow velocity, although research is ongoing into ways to utilize it as a flow monitor. How it will correlate with CBF in the spectrum of research work presented in the following chapters has to be described individually for every study. However, it is clear that zero velocity should correlate nicely with zero flow suggesting that at low flows - of most interest to research work and/or intraoperative monitoring - it might be a useful quantitative indicator of cerebral perfusion, although not producing a quantitative indication (in ml/100g/min) of flow change.