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Transcranial Doppler
Introduction
Transcranial Doppler (TCD) is the use of Doppler technology to evaluate cerebrovascular blood flow within the basal arteries of the brain. The information from such an examination supplements that obtained from other modes of vascular imaging. The advantage however relies on its real time, noninvasive bedside use as it can be used to evaluate vascular changes in response to interventions performed during acute cerebrovascular events. Here are some examples of the conditions that can evaluated with TCD:
In blue, conditions that can be evaluated with point of care ultrasound TCD and that we will be exploring in this chapter. Please be advised that POCUS TCD is not intended to supplant an full TCD exam but be able to answer if the conditions are present or absent.
Principle: the Doppler effect
TCD uses the Doppler effect so its understanding is paramount in its clinical application. The Doppler effect is the measured change in frequency that corresponds to a change in velocity which in turn is related to a change in flow. Lets dig a bit deeper for better understanding.
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The ultrasound probe emits a frequency that travels to a vessel. This vessel contains moving red blood cells that interacts and reflect this sound wave. The reflected sound wave then returns towards the probe with a different frequency to that which was emitted. If the red blood cell is moving away from the probe, its wavelength is higher and has a lower frequency.
Let imagine we have an ambulance with a siren emitting the same frequency. If the vehicle remains stationary we perceive one unique frequency but if we hear and ambulance coming towards us and then moving away from us we hear a difference in frequency due to the movement of the vehicle. If its coming towards us the frequency appears higher since the wavelengths are compressed together. Once the ambulance moves away from us the wavelengths now are further and further away from each other and we perceive a lower frequency. The faster the moving object, the greater the perceived change in frequency. We can also display this information visually which is what the US machine would do for us. If this Doppler effect of an object moving away from us we would be displaying it blue and red if the contrary were true which is what happens when you turn on the Color Flow Doppler (CFD) feature of the machine.
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1
2
Schematic representation of the Doppler effect with the use of a an ambulance and a active siren. On the left, a human ear representing the ultrasound sensor. 1, stationary ambulance. 2, moving ambulance
This Doppler shift is thus directly proportional to the speed of moving red blood cells and ultrasound machine creates a spectral display of the different changes in velocity observed. It is important to note that to be able to measure this shift in frequency we need to be as parallel as we can to the flow and that at 90 degrees from flow we are unable to detect any doppler shift. An important feature of Pulsed Wave Doppler which is a technology that uses Doppler is that we can measure this shift in a particular point in space with the use of a gate.
This doppler change represents flow velocity with the following mathematical formula:
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This math formula is important since it describes the velocity as being directly proportional to the Doppler shift. Of the parameters, C is the speed of sound in the medium and corresponds to 1541m/s in soft tissue. Ft is the frequency of the probe and in order to penetrate to deeper tissues we need a lower frequency and thus this parameter does not change much. Theta (θ) is the angle between the emitted wave and the direction of flow. The larger this angle, the worse the error in measurement.
Doppler Shift x C
Flow velocity =
2 x Ft x Cos θ
The spectral information displayed from this doppler analysis (see image below) includes measures of blood flow velocities (y axis) across time (x axis). Parameters that can be measured from this include peak systolic velocity (Vs), end systolic velocity (Vd), acceleration time or systolic upstroke, time-averaged mean velocity (V mean) and pulsatility index (PI). Vmean is calculated by using the following Vmean = [Vs + (Vd x 2)]/3. The PI is a non invasive measure assessing vascular resistance and is thus used to assess downstream vascular resistance and is calculated as PI= (Vs-Vd)/Vmean.
Vs
Vd
At
Example of spectral Doppler frequency display of the middle cerebral artery. B-mode US with Color Flow Doppler sector overlaid. PWD sample box over the middle cerebral artery. Peak systolic velocity (Vs), end systolic velocity (Vd), acceleration time or systolic upstroke (At), pulsatility index (PI), time-averaged mean maximum velocity (Vmean) (traced in purple).
Blood flow velocity indices and physiologic factors
There are physiologic variables that influence cerebral blood flow as measured by TCD so it is important to have them in mind. These include age, gender, blood pressure, temperature, carbon dioxide, hematocrit and mental or motor activity. Any measured differences in blood flow velocity should have these parameters in mind.
Age
Gender
BP
CO2.
Hematocrit
Blood flow velocities decline up to 0.5% per year from ages 20-70.
Women tend to have higher flow velocities than men between 20-60 years of age with a magnitude of difference of up to 15%. This difference no longer exists for those >70.
Measured velocities may be higher in patients with high blood pressure despite an intact autoregulatory system.
Partial pressure of carbon dioxide is directly proportional to cerebral blood flow.
Viscosity is inversely related to cerebral blood flow velocity. Approximately 20% increase in CBF occurs from decreasing hematocrit from 40 to 30%
Equipment and Technique:
We will be using a lower frequency probe that allows penetration through the thinnest portions of the skull. This corresponds to a 2Mhz probe.
TCD Windows
The acoustic windows that allow US transmission to evaluate the basal cerebral circulation are 4 (see diagram below). These are the transtemporal, suboccipital (transforaminal), transorbital, and submandibular (retromandibular). The transtemporal window is located above the zygomatic ridge between the lateral canthus of the eye and auricular pinna and has 4 interrogation zones (with a frontal [F], anterior [A], middle [M] and posterior [P] window) and is the most frequently used to interrogate the middle (MCA), anterior (ACA), posterior cerebral arteries (PCA), and terminal internal carotid artery (ICA). Bear in mind that in up to 20% of patients have inadequate transtemporal windows.
TCD Windows
F
A
M
P
Diagram showing the 4 TCD windows. In the trans-temporal window (F, A, M, P) notice the location of the red indicator which corresponds to the direction of the marker in the actual probe. See text for details.
The target artery is interrogated by selecting an appropriate acoustic window, probe angle and selecting a Color Flow Doppler window. The vessel is then recognized through flow direction, pulsatility, and velocity. Velocity is measured after turning continuous wave Doppler (CWD) and selecting an appropriate depth gate. We can also evaluate waveform changes induced by dynamic maneuvers such as proximal carotid artery compression.
During TCD examinations the sonographer should follow the blood flow in each major branch of the circle of Willis and attempt to identify 2 key points per artery. The MCA should be interrogated as proximal , mid and distal. The vertebral artery measured two distances apart. Basilar artery measurements taken as proximal mid and distal. There are age specific depth ranges and flow ranges for each major artery. In cases were it may not be possible to differentiate the anterior from a posterior circulation the blood response to carotid compression or vibration may be used. Bear in mind that our focus will be on the trans-temporal window since it will provide us with the highest yield when interrogating it as part of POCUS.
The following measurements for adults represent the location of the arteries in their corresponding window. We will be focusing on the trans-temporal window for the rest of this chapter :
Transtemporal window
Intracranial carotid artery (ICA) bifurcation
Simultaneous flow towards and away from probe
Middle cerebral artery (MCA)
Flow towards the probe
Anterior cerebral artery (ACA)
Flow away from the probe
Posterior cerebral artery (PCA)
2cm posterior to the ICA bifurcation. P1 segment has flow towards while the P2 has flow away from the probe.
Transorbital window
Opthalmic artery
Use under 10% max power to minimize subluxation of the lens. Probe directed towards the optic canal. Flow is in the direction of the probe.
Suboccipital window
Basilar artery
Flow away from the probe
Submandibular window
Distal ICA
Depth
55-65mm
35-55mm
60-70mm
60-70mm
55-70mm
60-70mm
40-60mm
Schematic diagram of the circle of Willis. Arrows depict direction of flow. The color in the arrows represent the resultant CFD in their respective window. Image by Rhcastihos.
Trans-temporal window and views.
Now that we have some idea of the structures and principles for TCD we will be exploring the trans-temporal window moving forward. We start with the probe orientation and the structures that should be seen and then start using color coded duplex sonography.
Probe position and 2D image for the trans-temporal window. The probe should be just above the zygomatic arch with the probe indicator pointing forward or towards the patient's forehead. Small adjustments with rotational movements of the probe will be of paramount importance for us to be able to identify structures within the cranium.
The following images have a side by side comparison of 2D ultrasound and anatomic landmarks that are necessary for TCD. The preset for these images is the cardiac preset (flipped images from a TCD preset). We first focus our attention to the middle of the field and look for 2 hypoechoic structures that look like a butterfly on its side (see image below). These structure represent either the thalami or the peduncles. The third ventricle lies anterior to the 'butterfly'. The third ventricle is an anechoic space with hyperechoic walls.