Transcranial Doppler Ultrasound Imaging

Stephanie Wilson, BS, RDMS, RVT

^*Coordinator, Vascular Sonography, South Hills School of Business and Technology, State College, Pennsylvania.

Address correspondence to: Stephanie Wilson, BS, RDMS, RVT, Coordinator, Vascular Sonography, South Hills School of Business and Technology, 480 Waupelani Drive, State College, PA, 16801. E-mail: swilson@southhills.edu.

Disclosures: The author reports having no significant financial or advisory relationships with corporate organizations related to this activity.

ABSTRACT

The transcranial Doppler imaging examination is one of the least-used testing methods in ultrasound today and has yet to reach its full potential. This application of sonography is an accurate and efficient method to assist diagnosing patients with atherosclerotic disease as well as for monitoring patients for vasospasm after neurosurgery. As with learning any new technique, the first step is to begin to understand the transcranial vascular anatomy, normal physiology of intracranial blood flow, and how the body can provide numerous collateral pathways to recalculate blood where necessary. The circle of Willis is a naturally occurring collateral pathway and is the starting point in a transcranial Doppler imaging examination because the majority of the blood vessels examined are part of the circle of Willis. The circle of Willis also provides a pathway for the blood to move from the brain's posterior to anterior, or from the right to left hemispheres, if needed. This article will present the basic starting points to learn how to perform a transcranial Doppler imaging examination.

Introduction
Doppler imaging is a common application of ultrasound imaging that is used on a regular basis to evaluate blood flow in almost all areas of the body including the transcranial vessels. Transcranial Doppler imaging was first performed in 1982 in order to evaluate vasospasm of the middle cerebral artery (MCA). It was initially performed free-hand and without the assistance of an image to document the location of the signals. The nonimaging method used a 2 MHz pulsed-wave Doppler transducer and fast Fourier transform spectral analysis. The depth of the vessel, its waveform morphology, flow direction, and velocities were used to differentiate one vessel from another. This technique has a long learning curve that depends on a tremendous amount of experience and knowledge of intracranial circulation. Systems that perform spectral Doppler imaging only are not as expensive as advanced imaging systems and allow for more ability to maneuver in small places such as an operating room or an intensive care unit.

There have been significant advances in technology over the years, which allow for complex imaging systems to be smaller and more maneuverable. These advanced systems provide the compliment of the actual image of the circle of Willis as well as other intracranial landmarks that can assist the operator in correct vessel identification. The addition of the image to confirm the location of the spectral Doppler sample has shortened the examination learning curve making it far more popular than it has been in the past.¹

The first section will present the transcranial vascular anatomy that is typically evaluated as part of the examination. It will also discuss the 4 major imaging windows (ie, transtemporal, transoccipital, transorbital, and submandibular windows) along with the examination protocol and image interpretation. Finally, this article will identify the major clinical applications for the transcranial Doppler examination.

Anatomy
The vessels that are viewed in a transcranial Doppler imaging examination are the same vessels that form the circle of Willis. The Willis circle is an anastomotic ring of arteries found at the base of the skull. Its purpose is to provide a natural collateral pathway between the right and left hemispheres of the brain, or the anterior and posterior circulations in the presence of stenosis or occlusion. The vessels that comprise the circle of Willis include the MCAs, anterior cerebral arteries (ACAs), and posterior cerebral arteries (PCAs) as well as the anterior communication arteries (AcommAs) and posterior communicating arteries (PcommAs). The circle can be divided into 2 segments: the anterior circulation and the posterior circulation (Table 1).²

The anterior circulation provides the majority of blood flow to both hemispheres of the brain. The anatomy of the anterior circulation starts with the internal carotid arteries (ICAs) that travel superior, through the carotid canals, and into the brain; they terminate at the MCA and ACA. The right and left MCAs then travel laterally, and the right and left ACAs travel anteriorly. The terminal internal carotid artery (ICAt) also gives rise to the right and left ophthalmic arteries. The ACAs are connected via a short vessel the, AcommA. The posterior circulation supplies the majority of blood to the brain stem and begins with the bilateral vertebral arteries (VAs). It travels superior into the brain and then joins the basilar artery (BA). The BA then divides again into the right and left PCAs. The anterior and posterior circulation is connected by the right and left PcommAs.³ There are multiple other branches that arise from the VA and BA, however, only the vessels that are examined as part of a transcranial Doppler imaging examination are mentioned.

The MCAs, ACAs, and PCAs can be further divided into the following segments: M¹, M², M³, A¹, A², P¹, and P². Essentially, the arteries are divided based on the branches that arise from them. For example, the A¹ segment of the ACA is located proximal to the branch of the AcommA. Thus, the A² segment is the section distal to the communicator. The same occurs with the PCA in that the P¹ segment is proximal to the PcommA branch, and the P² segment is distal to the branch. Anatomic variants are common, however, and it has been reported that only 20 percent of the population actually has a complete circle of Willis.⁴

Transtemporal Window
There are 4 major imaging windows for a transcranial Doppler imaging examination; they include the transtemporal, transoccipital, transorbital, and submandibular windows. The transtemporal window is the most common window and shows the most anatomy. It is located through the thinnest part of the transtemporal bone which can be found just anterior to the superior portion of the ear. Thus, the sound beam travels into the brain from lateral to medial. The indicator notch on the transducer should be pointed anteriorly so that the orientation of the image appears as demonstrated in Figure 1. The B-mode landmarks include the petrous ridge (Figure 1, double arrows), sphenoid bone (Figure 1, arrow), and the cerebral peduncles (Figure 2).⁵

Anterior Circulation
As the sound beam travels into the skull from lateral to medial, the first visualized vessel should be the ipsilateral MCA. From this position, it only requires very slight angulations of the transducer to identify the ACA, PCA, PcommA, and ICAt. The approximate depth of this vessel is between 30 and 67 mm.⁵ The examination requires the Doppler map for both the color and spectral applications to be left in the standard format. This means that positive shifts flow will appear color-coded red and displayed above the baseline on the spectral analyzer. The negative shifts flow will be color-coded blue and displayed below the baseline on the spectral analyzer. Using this format, MCA should be flowing laterally toward the ear, which is also toward the transducer. This is demonstrated in Figure 3 where the vessel color-coded in red represents the MCA. The ipsilateral ACA can be identified at the approximate depths of 60 mm to 80 mm using the landmarks of the MCA and also the sphenoid bone.⁵ Once the ACA is visualized, it curves medial and anteriorly; therefore, the flow direction will be away from the transducer, or color-coded blue, as demonstrated in Figure 3.⁶

To perform the examination, the MCA should be interrogated with spectral Doppler by sweeping the sample volume throughout the length of the vessel. Initially, the sample volume can be increased to aid in searching for the initial signal; however, once found, the sample volume size should be decreased slightly to avoid interference from other vasculature. Three to 4 representative spectral Doppler recordings with measurements should be obtained starting at a shallow depth and working deeper. For example, signals could be obtained at 3.5 mm, 4.0 mm, 4.5 mm, and 5.0 mm. Widening the sample volume will help in the initial identification of the vessel. During a transcranial Doppler imaging examination, all spectral Doppler signals should be obtained with the cursor angle at 0^°.⁷ The theory is that most vessels will be located between 0^° and 30^° compared to the spectral beam, and there is very little difference in velocity calculation using angles that are this acute. The amount of error in the calculation is within acceptable limits to leave the curser angle at 0^°.⁸

Once the MCA evaluation is complete, the sample volume can be swept deeper in the bifurcation of the MCA and ACA and ultimately into the ACA itself. Many times, a signal is obtained right at the level of the bifurcation, which will show flow from both vessels. This is commonly referred to as a butterfly signal where the positive shifted flow is from the MCA, and the negative shifted flow is from the ACA (Figure 4). Placing the sample volume right at the MCA/ACA bifurcation helps provide confirmation of the correct location. Then the examination continues into the ACA, and again, the sample volume is swept throughout the course of the ACA. Two to 3 representative spectral Doppler recordings, with measurements, should be obtained.⁹

Finally, the last major vessel of the anterior circulation to examine through the transtemporal window should be the terminal portion of the ICA. It is located inferior to the level where the MCA and ACA are identified; therefore, the transducer is tilted inferiorly to locate the correct landmark. The landmark is the foramen lacerum, which is a triangular-shaped opening that the ICA passes through (Figure 4). The approximate depth where the ICAt can be identified is 55 mm to 67 mm, and the direction of flow can be either positive or negative.⁵ This is because the ICAt is tortuous, and the flow direction will depend on the patient's specific anatomic configuration. At least 1 representative spectral Doppler signal recording should be obtained with measurements in the ICAt. The AcommA is also part of the anterior circulation; however, it is not routinely visualized due to its small size.¹⁰

Posterior Circulation
The vessels seen through the transtemporal window from the posterior circulation include the PCAs and the PcommAs. The 2-dimensional (2D) landmarks that assist in PCA identification are the cerebral peduncles (Figure 2). The vessels arc around the peduncles and can be found at the approximate depths of 55 mm to 75 mm.¹¹ The flow direction may be either a positive or negative shift and will depend on the exact location of the sample site. Spectral Doppler tracings should be obtained throughout the visualized segments of the PCA to look for any focal increases in velocity. The PcommA connects the ICAt to the PCA and may have flow in either a positive or a negative direction on Doppler. Nonvisualization of the PCA is common because it is a very short segment; however, it hypertrophies depending on the collateral need of the patient.¹²

Transoccipital Window
This imaging window requires access to the back of the patient's neck. The patient can lie on their left side and tuck their chin to their chest or be seated in a chair with the chin tucked to the chest. The transducer is placed at the back of the head with the indicator light facing the patient's right side. This means that the anatomy to the left of the screen corresponds to the patient's right side, which is a standard presentation for imaging. The vessels to be identified include the intracranial segments of the bilateral VAs and the BA. The views are obtained through the foramen magnum (Latin for "great hole") and is a large opening in the occipital bone of the cranium.¹³

The vertebral vessels should appear at the approximate depths of 40 mm to 85 mm, and the BA should appear deeper than 80 mm using the landmark of the foramen magnum.¹⁴ Flow in the VA should be away from the transducer or a negative shift. If more shallow segments of the VAs are visualized (less than 40 mm), flow may appear as a positive shift due to vessel tortuosity. The landmark and goal to identify the BA correctly is to the confluence of the VAs. In the adult population, this normally occurs at depths greater than 80 mm.¹⁵ A color Doppler image of the bilateral VAs and spectral Doppler of the BA can be seen in Figure 5. Notice how the confluence of the VAs forms the shape of a "Y." As with all prior vessels, the sample volume for spectral Doppler should be swept throughout the length of both VAs and the BA. Two to 4 representative spectral Doppler recordings with measurements should be obtained in both VAs.

Transorbital Window
This imaging window demonstrates ophthalmic artery flow and flow in the ICA siphon. Access to these vessels is obtained through the optic canal, which is a round opening at the back of the orbit. This is the most uncomfortable part of the examination for the patient. The patient lies supine on the examination table, and a small amount of gel is placed on the eyelid. The transducer is gently placed on the eyelid with very light probe pressure. This window also requires an adjustment to the machine settings. The manufacturers should be contacted to determine if the correct mechanical index and power levels are at the correct settings. The literature recommends that the mechanical index be set at 0.28 mW/cm² and the spatial peak temporal average be set at 17 mW/cm².¹⁶ This is extremely important to ensure that there are no bioeffects to the eye during the examination, which can be accomplished using the "As Low As Reasonably Achievable" (ALARA) principle.¹⁷ The indicator light should be facing to the right of the patient to maintain standard imaging presentation on the screen. The orbit will appear as a large anechoic circle on the B-mode image. At 40 mm to 60 mm deep, the ophthalmic artery can be visualized with normal flow direction as a positive shift or towards the transducer.¹⁸ Constantly be aware of the sample volume depth, because the retinal artery is located at depths less than 40 mm. It is a smaller vessel compared to the ophthalmic artery; however, the flow direction will also be a positive shift. Also, keep in mind that the optic nerve is centered and immediately below the orbit. The retinal artery travels within the optic nerve; however, the ophthalmic artery is adjacent to it (Figure 6).

The landmark for the ICA is the ophthalmic artery. This vessel is very tortuous and can be located at depths of 60 mm to 80 mm.¹⁹ Because of the tortuosity, flow may be either a positive shift or a negative shift and is dependent upon the patient's anatomic configuration. Again, the sample volume is swept through all visualized segments, and representative spectral Doppler recordings with measurements should be obtained. Figure 7 demonstrates color and spectral Doppler of the ICA siphon.

Submandibular Window
The submandibular window is obtained through the carotid canal. The patient should be supine with his or her chin slightly elevated and turned away from the side being examined. The ICA is identified in its retromandibular segment. The depth should be more than 35 mm, and the flow direction will be a negative shift. A representative spectral Doppler waveform with measurements is obtained as shown in Figure 8.

Technique
There are correct scanning techniques for various applications of ultrasound during a transcranial Doppler imaging examination, which include the following:

B-Mode: Transcranial Doppler imaging is performed with the image fairly dark. Although there are some b-mode landmarks, the goal is to obtain the spectral Doppler information rather than evaluate the parenchyma of an organ. This will also help enhance the acoustic interface between the bony structures that are used as landmarks, such as the sphenoid bone. As with most imaging examinations, be sure that the focal zone or zones are set at approximately the same level or just below the structure of interest. The frame rate should be maximized, and this can be done by making the 2D sector width fairly narrow. The smaller the sector window, the less scan lines per frame, and thus, a faster frame rate. The dynamic range setting should be fairly low which gives the image more contrast with less shades of gray. This will help enhance the interface between bony structures.

Color Doppler: The color box, or region of interest, should be kept as narrow as possible to increase the frame rate. The same principle for the b-mode image exists ie, the less lines per frame, the higher the frame rate. The color map should be in the standard format of a positive shift color-coded in red and a negative shift color-coded in blue. This format is also known as BART ("Blue Away, Red Towards").²⁰ Wall filter for the color Doppler application rarely changes; however, the filter should be set low to avoid slow flow echoes. Turn the color gain up higher than normal to initially search for the vessels. This will fill in the color box with a mosaic of colors. Then, slowly back off the gain by turning the color box down in small increments. This will ensure a gain of maximum color filling of the vessels as possible. Once the vessels are found, the gain can be decreased to the proper setting.

The next 2 color tips are also lesser-used features that the manufacturers provide; however, they can be irreplaceable tools for difficult imaging situations. Increasing the color sensitivity or persistence improves the machine's ability to pick up slow or flow that's difficult to identify. By turning this feature up, differences that can be observed throughout the cardiac cycle and the rapid changes between systole and diastole may be masked. Color priority is a feature that tells the machine to put less emphasis on b-mode and more emphasis on color. By nature, color Doppler imaging overwrites the b-mode image, and this feature tells the machine to enhance it even more than normal. This will again increase the likelihood of finding the flow if it's there.

Spectral Doppler: The initial search for signals should start by increasing the sample volume to 5 mm to 10 mm. This will boost the chance of landing in the small transcranial vessel. Be aware that doing so often results in obtaining signals from multiple vessels. Once the right vessel is found, narrow the sample volume back down to discriminate between vessels. As mentioned previously, the cursor angle should be kept at 0^°. Do not invert the spectral analysis display so that positive shifts flow is presented above the baseline and the negative shifts flow is presented below. Flow direction for this examination is crucial and can be very confusing, so keeping the spectral display in its standard format is very important. The gain for control of any application is to amplify the signal. During a transcranial imaging examination, the gain can be increased more than normal, and then it can be slowly backed off until a clear envelope is visualized. ²¹

Adjusting the dynamic range of the spectral Doppler will also improve the clarity of the signal, which may mean that background echoes on the spectral Doppler tracing will be present. This can be a significant downfall when attempting to use an automatic tracing feature for measurements. In those cases, the signal can be manually traced. As with any spectral display, adjust the pulse repetition frequency, or scale, such that there is no aliasing demonstrated, and the output power will be maximized on the transcranial preset. When performing the transorbital component, the output power may need to be adjusted each time a new application is entered. The power will need to be decreased with the B-mode image. Once color is turned on, it should be checked again; when spectral Doppler is turned on, it should also be checked again.

Power Doppler: Power Doppler is another tool that can be used to image difficult cases. It is very sensitive to flow, less dependent on angle, and free of aliasing. The spectral Doppler information can be obtained using power Doppler as the guide rather than color Doppler. Disadvantages include not being able to differentiate flow direction on some machines. The signal is free of aliasing, and it does not assist in the determination of the flow's speed and character. Again, that information can be better assessed with spectral Doppler, so it is not of critical importance.²²

Protocol
As indicated by the Intersocietal Commission for the Accreditation of Vascular Laboratories (ICAVL), first and foremost, a written protocol should be in place that defines the components, documentation, and number of samples that need to be recorded. It should also describe how the entire course of accessible portions of the anterior and posterior circulations is obtained. ICAVL defines that a complete examination include: spectral Doppler sampling of the intracranial ICAs, an A¹ segment of the ACAs, M¹ and proximal M² segments of the MCAs, P¹ or P² segments of the PCA, the ophthalmic artery, the ICA siphon, the retromandibular ICA on the neck if indicated, the distal vertebral arteries, and the proximal and distal BA. If the examination does not include any of these vessels, or is only a unilateral examination, it would be considered a limited examination. Pathology should be documented by recording waveforms proximal to, within, and distal to the stenosis. The inability to visualize vessels must be documented, and velocity data needs to be recorded from communicating arteries if they can be identified. Additional written protocols must be in place and must define specific requirements for any other types of transcranial Doppler examinations performed in the laboratory. For example, if the department participates in the detection of emboli, CO₂ reactivity and testing for shunts and vasospasm monitoring should all have individual protocols.²³

Interpretation
Vessel Identification
The first step in interpretation is to correctly identify the vessels. This is based in part on what can be visualized on the B-mode and color Doppler image. It is important to be familiar with the appropriate sample volume depths for the specific vessel, the expected direction of blood flow relative to the transducer, the relationships of one Doppler signal to another, and the ability to trace the artery. The best example of this scenario is the ability to sweep the sample volume through the MCA, into the MCA/ACA bifurcation, and then into the ACA. The MCA should be a long segment of vessel between 30 mm and 55 mm deep, with flow toward the transducer. As the sample volume is swept from superficial to deep, the MCA is identified first, then the MCA/ACA bifurcation, which gives the butterfly signal. Even deeper is the ACA where the flow direction will change to a negative shift and should occur at depths greater than 65 mm.²⁴

Waveform Morphology
Evaluating the overall waveform characteristics is also subjective in that each signal should demonstrate a low resistance and high diastolic flow pattern throughout the cardiac cycle. The quality of the waveform envelope will have a great effect on the accuracy of the examination. There are normal velocity ranges for these vessels in the adult population; however, it is important to realize that there will be differences depending on the age of the patient.

A basic understanding of flow through a stenosis is necessary when performing any Doppler examination. As blood moves through a channel that is narrower than the original diameter of the vessel, it increases. If the diameter is more than 50% narrowed, it is considered a hemodynamically significant stenosis. This means that the velocity of the blood will flow increase approximately 2 fold throughout a hemodynamically significant stenosis. As the flow exits the stenosis, it becomes chaotic and turbulent. This is called poststenotic turbulence. There are 2 crucial elements to document the presence of a hemodynamically significant stenosis: an increased velocity at the site of the lesion and poststenotic turbulence. The expected amount of flow through each of these vessels is subtly different. The highest flow should be within the MCA. The flow in the ACA should be slightly less, and then the PCA, the basilar, and finally the VA, respectively; the flow in the VA should be the lowest. Additionally, side-to-side asymmetry and any significant differences between the hemispheres should be documented.²⁵

Measurements
Each spectral Doppler waveform for this examination should include a measurement of peak systole and end diastole. Beyond this, the mean peak velocity is also measured, which is a different measurement than the overall mean velocity. As a result, it is important to make sure that the correct measurement is selected. Mean peak velocities are calculated by tracing the envelope of the spectral waveform. The theory behind this is that the mean peak velocity will be less affected by other central cardiovascular factors. This can be performed by both auto and manual trace techniques. Figure 9 demonstrates typical measurements. Notice the differences between the mean velocity tracing and the time averaged velocity mean. The tracing that measures the envelope of the spectral waveform from this manufacturer is labeled "time-averaged mean velocity." Table 2 summarizes the normal mean velocities.⁴

Calculations
There are several indices and ratios that can be calculated as part of a transcranial Doppler examination. The first index that is often calculated for a transcranial Doppler examination is described by Gosling et al for peripheral artery disease and is known as the pulsatility index (PI).²⁶ The PI is calculated by subtracting the diastolic velocity from the systolic velocity, and then dividing it by the mean velocity. This is the most widely used index parameter; normal values range from 0.5 to 1.1. The goal is to measure resistance with each cardiac cycle. Relatively high pulsatility indices are found proximal to an obstruction demonstrating the downstream resistance to flow. A relatively low PI can be found distal to the obstruction representing the capillary bed vasodilatation.

The resistive index is used to evaluate distal vascular resistance.²⁷ The resistive index is calculated by subtracting the diastolic velocity from the systolic velocity and dividing the resulting number by the systolic velocity. The values for the index should also be higher proximal to a stenosis and lower distal to a stenosis. An MCA/ACA ratio can be calculated, and if it is above normal, it could indicate pathology such as hypoplasia, stenosis, collateralization, branch occlusion, or distribution infarction. An abnormal value for this ratio is greater than 1.2.

An MCA/ICA ratio can also be calculated, which is also known as the cervical or Lindegaard ratio. The theory behind this ratio is that the MCA velocities change due to stenosis or volume flow. The ICA velocity used should be obtained from the submandibular window. This ratio has been shown to help improve the diagnosis of vasospasm. Normal values for the MCA/ICA ratio are 1.1 to 3.0 with a mean value of 1.8.

Finally, a BA to extracranial VA ratio can be calculated to help determine BA vasospasm. This ratio provides the interpreter the ability to differentiate between basilar artery hyperemia and true vasospasm. Vasospasm is often a complication in patients following subarachnoid hemorrhage and subsequent aneurysm clipping. Values for the BA/VA ratio greater than 2.0 indicate BA vasospasm.²⁸

Clinical Applications
The most common indication for this examination is suspected intracranial stenosis, which is suspected in patients that have typical stroke-like symptoms or transient ischemic attacks. Another indication would be an inconclusive extracranial carotid examination. In most cases, the transcranial Doppler imaging examination is performed after a regular carotid examination. If the carotid examination shows extracranial stenosis or occlusion, more information may be needed about the intracranial vessels to determine the effect of the extracranial stenosis and the collateral pathways that may be in play.

Another common application would be for diagnosing vasospasm. Vasospasm occurs when there is vasoconstriction of the intracranial vessels and is a complication after a subarachnoid hemorrhage. A subarachnoid hemorrhage could be the result of a ruptured intracranial aneurysm and, therefore has a high morbidity and mortality rate. These patients are treated with neurosurgery and are prone to vasospasm in the postoperative period. Transcranial Doppler imaging examinations are performed several times per week on the patient to monitor their velocities and determine the presence or absence of vasospasm. Treatment for these patients can be directly impacted by the velocity information provided from the examination including hydration, blood volume control, and blood pressure control to help fend off the vasospasm attacks, and ultimately, prevent a secondary stroke. The examination itself is usually limited because vasospasm is most likely going to occur in the MCAs, ACAs, and PCAs unless it is indicated elsewhere. The interpretation criteria are mainly based on the mean peak velocities as described earlier as well as the MCA/ICA ratio. The 4 major categories of vasospasm in adults are listed in Table 3.⁴

Vasospasm can also occur in the pediatric population particularly in patients who suffer from sickle cell anemia. The pediatric interpretation criteria are slightly different than the adult population. Sickle cell disease is an inherited blood disorder more common in people of African or Mediterranean decent. The red blood cells become "sickle," or crescent shaped, and are unable to normally pass through the microcirculation. The result of this is tissue ischemia or infarction resulting in a stroke. There is no cure for this disease.

Another childhood disease that transcranial Doppler is used for is called Moyamoya disease. It is rare, except in Japan, and the etiology is unknown. It affects mostly children and young adults and the disease is more common in women than men. The term "moyamoya" is actually Japanese for "cloud of smoke," because this is how the circulation presents itself on imaging tests.²⁹ There is stenosis/occlusion of the distal ICA/MCA and/or ACA, so the blood flows up the ICA, then the major pathways for blood disappear into a "cloud of smoke," which is the collateral network formed as a result of the stenosis/occlusions. Unfortunately, there is also no cure for this disease. The main focus for monitoring these patients is to determine the presence of vasospasm in the MCA and terminal ICA; as a result, the majority will be limited examinations rather than the full complete protocol described earlier.

Conclusions
The purpose of the article is to introduce the readers to an application of sonography that is not well known or understood. Transcranial Doppler imaging examinations are one of the least understood applications of sonography. There are few facilities that perform numerous transcranial Doppler imaging examinations, which makes this examination relatively uncommon and misunderstood. As with any new sonography examination, if it is broken down to the necessary individual components, it can be learned and mastered just as any other sonogram. Those crucial components begin with a thorough knowledge of the anatomy being examined. Once the anatomy is mastered, it needs to be understood, in addition to which imaging windows can demonstrate which component, since it is very unusual to be able to visualize the anatomy all from one window.

Throughout this article, the normal transcranial vascular anatomy was presented and the 4 major transcranial Doppler imaging windows were described in detail. A broad knowledge of the foundation of sonography is necessary so that the equipment is optimized accordingly. This can make or break the operator's ability to find the anatomy or identify any pathology. An explanation of the correct equipment settings was discussed including, B-mode, color Doppler, spectral Doppler and power Doppler applications.

It is very important to be able to think "outside the box" in terms of sonography and know where further investigation is needed next. The ability to do this begins with being able to interpret the information that is being gathered in real-time. The waveform morphology, flow direction, and depth of the vessels are all crucial elements in vessel identification for the free-hand transcranial Doppler examination. Although imaging provides an overall map, these elements are still of critical importance to help the operator determine which vessel is being insonated. Observation and demonstration of the techniques described is required to become competent in performing transcranial Doppler imaging examinations.

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