After completing this on-line tutorial, the reader will be able to:

• Describe the intracranial arterial anatomy and the collateral pathways evaluated during a transcranial color Doppler imaging examination
• Explain the technical aspects of color Doppler imaging of the intracranial arteries
• Define the physiological parameters that affect the transcranial color Doppler imaging interpretation criteria

Stroke is the third leading cause of death in the United States. Approximately 160,000 of the 750,000 strokes that occur each year result in death. Stroke is also a leading cause of adult disability. Although the risk of stroke dramatically increases with age, the incidence of pediatric stroke is also on the rise. In addition, the direct (hospital, physician, rehabilitation, etc.) and indirect (lost productivity, etc.) costs associated with stroke tally more than $30 billion per year in the United States.

The prevention of stroke has been the focus for the progress made in the ultrasound evaluation of cerebrovascular disease during the past twenty years. Development of an ultrasound method to interrogate the intracranial arterial system lagged behind the evaluation of the extracranial arterial system because of the difficulty penetrating the skull with ultrasound. Recent technical advancements permit the ultrasonic evaluation of the intracranial arterial system by using transcranial Color Doppler Imaging (TCDI).

The operator must be aware that the Doppler spectral waveforms obtained during a TCDI examination are based on hemodynamics, and that the waveforms obtained do not provide anatomic information. TCDI is an advancement of intracranial ultrasound techniques since it combines the hemodynamic information with anatomic landmarks, enabling the accurate identification of the intracranial arteries. It is also important to understand that TCDI measures the velocity of the blood and does not measure cerebral blood flow. Increases in intracranial arterial velocity may be due but not limited to: increased volume flow without a lumen diameter change, a decrease in lumen diameter (stenosis) without a change in volume flow, or by a combination of an increase in volume flow and a decrease in lumen diameter.

The accurate interpretation of a patient's TCDI examination may be difficult without knowledge of the location and the extent of atherosclerotic disease present in the extracranial vasculature. Carotid duplex imaging is currently used to evaluate the cervical portion of the cerebrovascular system. Duplex imaging provides valuable information about the amount of disease present (percent diameter reduction), the length of the plaque, and the localized hemodynamic effects of the atherosclerotic plaque. Carotid and vertebral duplex imaging techniques, interpretation criteria, and diagnostic capabilities are established (link to GEMS carotid duplex imaging course).

The transcranial Doppler technique was introduced as a method to detect cerebral arterial vasospasm following subarachnoid hemorrhage. During the past 20 years, the list of clinical applications for transcranial Doppler has grown, and the addition of new areas of research will permit better understanding of intracranial cerebrovascular hemodynamics.

Currently, transcranial Doppler's role is being used or investigated for the following clinical applications:

• Diagnosis of intracranial vascular disease
• Monitoring vasospasm in subarachnoid hemorrhage
• Screening of children with sickle cell disease
• Assessment of intracranial collateral pathways
• Evaluation of the hemodynamic effects of extracranial occlusive disease on intracranial blood flow
• Intraoperative monitoring
• Detection of cerebral emboli
• Monitoring evolution of cerebral circulatory arrest
• Documentation of subclavian steal
• Evaluation of the vertebrobasilar system
• Detection of feeders of arteriovenous malformations (AVMs)
• Monitoring anticoagulation regimens or thrombolytic therapy
• Monitoring during neuroradiologic interventions
• Testing of functional reserve
• Monitoring after head trauma

An anterior (carotid) and a posterior (vertebral) circulation supply blood to the brain (Figure 1). The intracranial arteries are small and the Circle of Willis occupies the space that is approximately the size of a half dollar coin.

Figure 1. The circle of Willis and the vertebrobasilar arterial system.[From:Katz ML. Intracranial Cerebrovascular Evaluation. In: Textbook of Diagnostic Ultrasonography. Mosby, St. Louis, 2001]

The internal carotid artery is divided into four main segments: the cervical, petrous, cavernous, and the cerebral (or terminal ICA). The internal carotid artery bifurcates into its two terminal branches, the anterior cerebral artery and the middle cerebral artery, which is usually the larger terminal branch. This segment of the ICA gives origin to the ophthalmic artery, the posterior communicating artery, and the anterior choroidal artery.

The ophthalmic artery (OA) is the first branch of the internal carotid artery in the cerebral segment. The branches of the ophthalmic artery often play an important role in collateral pathways that form as a result of disease of either the internal or external carotid arteries.

The posterior communicating artery (PCoA) courses posteriorly and medially from the internal carotid artery to join the posterior cerebral artery (PCA). It can be large when the P1 segment of the posterior cerebral artery is hypoplastic, which occurs in 15-22% of the cases. When the PCA is supplied from the ICA, this anatomic variant is termed a "fetal" origin of the posterior cerebral artery. The PCoA generally does not function as an important collateral pathway unless the patient has extensive extracranial occlusive disease bilaterally or incomplete anterior cross-filling via the anterior communicating artery.

The larger (diameter and length) terminal branch of the internal carotid artery is the middle cerebral artery (MCA), which is the blood supply to most of the lateral surface of the cerebral hemisphere. From its origin, the MCA extends laterally and horizontally in the lateral cerebral fissure. The MCA either bifurcates or trifurcates and the terminal branches of the MCA anastomose with terminal branches of the anterior cerebral and the posterior cerebral arteries

The anterior cerebral artery (ACA), is the smaller of the two terminal branches of the internal carotid artery. From its origin, the ACA courses antero-medially to the interhemispheric fissure. The proximal, horizontal segment of the anterior cerebral artery is known as the A1 segment and is connected to the contralateral A1 segment via the anterior communicating artery. Distal to the anterior communicating artery, the anterior cerebral arteries turn superiorly and run side by side in the interhemispheric fissure. The segment of the ACA from the anterior communicating artery to the distal anterior cerebral artery bifurcation is termed the A2 segment.

The anterior communicating artery (ACoA) is a short vessel that connects the anterior cerebral arteries (A1) at the interhemispheric fissure. The ACoA may be absent, be a single, duplicated, or multi-channeled system.

The vertebral arteries (VA) are large branches of the subclavian arteries. In the majority of cases, the left vertebral artery is the dominant artery. The vertebral artery can be divided into four segments: the extravertebral, intervertebral, horizontal, and the intracranial. Several major branches arise from the intracranial segment of the vertebral artery, the posterior and anterior spinal arteries, and the posterior inferior cerebellar artery (PICA), which is the largest branch of the vertebral artery.

The basilar artery (BA) is formed by the union of the two vertebral arteries, and is variable in its course, size, and length (2 - 3.5 cm). The basilar artery is often tortuous. During its course the basilar artery gives off several branches including; the anterior inferior cerebellar artery (AICA), the internal auditory (labyrinthine) artery, the pontine branches, and the superior cerebellar arteries (SCA) just proximal to the posterior cerebral arteries.

The posterior cerebral arteries (PCA) originate from the terminal basilar artery and course anteriorly and laterally. The segment of PCA from its origin to its junction with the posterior communicating artery is termed P1 and the portion of the vessel extending posteriorly from the PCoA to the posterior aspect of the midbrain is the P2 segment.

Sir Thomas Willis first described the Circle of Willis in 1664. The circle is composed of the A1 segments of the two ACAs, the ACoA, the two PCoAs, the two ICAs, and the P1 segment of the two PCAs. It is a polygonal-shaped anastomotic ring at the base of the brain that permits shunting of blood between the right and left cerebral hemispheres (by the ACoA), and between the anterior and posterior systems (by the PCoAs). These communications are important when there is significant disease or occlusion of one of the major cervical arteries. Variations of the circle of Willis are common. A classic "normal" circle of Willis is found in only approximately 20% of the cases.

General Information

The quality of the intracranial image is dependent upon proper adjustment of many instrument controls. Increasing the power setting and the color gain to the appropriate level during a TCDI study are probably the most important instrument control adjustments. Adjusting the focal zone in the range of 6-8 cm will improve the image and color resolution. Maintaining a small image sector width and color box width will keep the highest possible frame rates. Checking for the appropriate color PRF, sensitivity, and persistence settings are also very important to obtain good quality color Doppler intracranial images.

The color display is important because it assists in the proper placement of the Doppler sample volume. The interpretation of the TCDI examination is made from the Doppler spectral waveform information. Therefore, Doppler signals are obtained from various depths along the artery's path. The color Doppler display helps guide the operator, as the Doppler sample volume is "swept" through the intracranial arteries to obtain the Doppler spectral waveforms. At each depth setting it is important to adjust the position of the sample volume on the color display and angle the transducer to optimize the Doppler signal.

Conventional color orientation for TCDI examinations is set for shades of red indicating blood flow toward the transducer and shades of blue indicating blood flow away from the transducer. By keeping this color assignment constant, intracranial blood flow direction in the arteries can be readily recognized. The appearance of intracranial arterial blood flow is dependent upon many instrument controls that can affect its presentation. Therefore, estimations of arterial size are not accurate from the color Doppler display.

The Doppler evaluation of the intracranial arteries is performed with a low frequency (2-3 MHz) phased array imaging transducer. A large sample volume is used to obtain a good signal-to-noise ratio. With TCDI, a smaller gate (i.e. 5-10 mm) can be placed on a specific arterial segment that is readily identified from a color flow image. Intracranial arterial velocities acquired with TCDI are acquired assuming a zero degree angle.

Additionally, with the use of TCDI, many investigators are reporting results using peak systolic and end diastolic velocities instead of the traditionally accepted mean velocities (time average peak velocities). Each institution will have to decide which velocity value to report, and adjust the diagnostic criteria accordingly.

Patient Position

TCDI examinations are performed with the patient in the supine position. The supine position allows access to the transtemporal, transorbital, and submandibular windows, whereas other positions are used for the suboccipital approach. The suboccipital examination can be performed with the patient lying supine and the head turned to one side, with the patient sitting and the head lowered slightly towards their chest, or with the patient lying on the side with the head bowed slightly so the chin touches their chest. We have found the suboccipital examination easier if the patient is able to tolerate lying on their side.

Operator Position

The operator sits near the patient's head and stabilizes the examining arm by resting the elbow on the examination table. This placement of the arm eliminates minor spontaneous hand movements that may cause intracranial arterial Doppler signals to be lost, and prevents motion artifact from being introduced into the signal. From this position, the operator also has equal access to both sides of the patient's head, permitting the best orientation of the transducer to the body. It is critical that the operator maintains a comfortable and ergonomically sound position when performing TCDI examinations. Proper body mechanics and postural alignment will assist in avoiding pain and injury.

TCDI Examination Sites

A complete TCDI examination incorporates the following four ultrasound approaches (Figure 2):

• Transtemporal
• Transorbital
• Suboccipital
• Submandibular

Figure 2. Transducer position is displayed for the four TCD windows. (A) transtemporal, (B) transorbital, (C) suboccipital, and (D) submandibular. [From: Katz ML. Intracranial Cerebrovascular Evaluation. In: Textbook of Diagnostic Ultrasonography. Mosby, St. Louis, 2001]

The depths (mm) and the mean velocities (cm/sec) of the intracranial arteries evaluated by each transcranial Doppler approach are listed below.

Transtemporal Window

The transtemporal approach is performed with the patient in the supine position and their head straight. The transducer is placed on the temporal bone cephalad to the zygomatic arch, and anterior to the ear. A generous amount of acoustic gel is necessary to ensure good transducer-to-skin contact, especially in patients where angling the transducer to optimize the Doppler signal requires the transducer's footprint to be elevated from the skin's surface.

Finding this window can be difficult because ultrasound penetration of the temporal bone is required. The transtemporal window varies in size and location with each patient, and may vary in an individual from one side to the other. A transtemporal window is not located in up to 30% of the population when performing a TCDI examination, and is more difficult to find in older individuals, females, and in African Americans.

The transducer's orientation marker/light should be pointing in the anterior direction, with the transducer angled slightly superiorly. This orientation of the transducer produces an imaging plane that is a transverse oblique view that has the advantage of simultaneous visualization of the anterior and posterior intracranial circulation in many patients. The ipsilateral hemisphere is at the top and the contralateral hemisphere at the bottom of the monitor, with anterior being to the left and posterior to the right side. With a good transtemporal window and a deep depth setting (14-16 cm), the contralateral skull produces echoes near the bottom of the image.

The transducer should be angled and moved slowly along the skin surface to find the best transtemporal window. Once located, the image depth setting is adjusted to a depth range of 8-10 cm. Visualization of the contralateral arterial system is possible in many patients, but each hemisphere should be separately studied through ipsilateral windows to obtain the best artery-to-transducer angle. In patients with only a unilateral transtemporal window however, evaluating the contralateral hemisphere is possible and may provide valuable information.

After locating the transtemporal window, the transducer is angled slightly inferior to identify the bony landmarks. These anatomic structures ensure the operator that s/he is at the correct level within the skull to locate the circle of Willis. The reflective echo extending anteriorly is the lesser wing of the sphenoid bone, and the petrous ridge of the temporal bone extends posteriorly. The ipsilateral temporal lobe is at the top of the image. After obtaining visualization of these intracranial landmarks, the gray scale is optimized using the overall gain and times gain compensation (TGC) controls.

The color control is turned on and examination is begun by visualizing the terminal internal carotid artery (t-ICA) as it courses near the foramen lacerum. The t-ICA is located at a depth of 60-67 mm , the mean velocity is normally 39 +/- 9 cm/sec, and the direction of blood flow depends upon the artery's anatomic configuration.

Although the anterior and posterior circulations can be simultaneously visualized in many patients (Figure 3 and Figure 4), often the patient's anatomy requires separate evaluation. Minor changes in the transducer's position on the skin or its angle permits individual evaluation of either the anterior or posterior intracranial circulations.

Figure 3. A schematic of the Circle of Willis visualized from the transtemporal approach. Middle cerebral artery (M1 and M2); anterior cerebral artery (A1 and A2), and posterior cerebral artery (P1 and P2)[From: Katz ML. Intracranial Cerebrovascular Evaluation. In: Textbook of Diagnostic Ultrasonography. Mosby, St. Louis, 2001]

Figure 4. The Circle of Willis visualized from the transtemporal approach. Middle cerebral artery (MCA); anterior cerebral artery (ACA), and posterior cerebral artery (PCA). Click on the image for a larger view.

Figure 5. Display of the entire Circle of Willis from the transtemporal window. Click on the image for larger view in motion.

The transducer is then angled anteriorly and superiorly so that the remainder of the anterior circulation (middle and anterior cerebral arteries) can be examined. The sphenoid wing can be used as a bony landmark since the middle cerebral artery (MCA) courses adjacent to it. The main trunk of the MCA is displayed in red since blood flow is normally toward the transducer. The depth is usually from 30-67 mm and the mean velocity is normally 62 +/- 12 cm/sec (Figure 6). The M2 branches are usually displayed in red but may appear blue as they curve and blood flows away from the transducer.

Figure 6. The Doppler signal from the Middle Cerebral Artery (MCA). The mean velocity is 56 cm/sec. Click on the image for a larger view.

The anterior cerebral artery (ACA) is displayed in shades of blue as it courses away from the transducer toward the midline. The transducer may need to be angled slightly anterior and superiorly to visualize the ACA. The depth is usually from 60-80 mm and the mean velocity is normally 50 +/- 11 cm/sec, and the color PRF may need to be decreased to visualize this artery due to its lower velocity. Although not part of the routine TCDI examination, the initial portion of the A2 segment often can be visualized and is displayed in blue extending in an anterior direction at midline into the interhemispheric fissure.

The posterior circulation is visualized by angling the transducer slightly posterior and inferiorly using the cerebral peduncles as an anatomic landmark. Normally, the two cerebral peduncles are identical in size and shape, and are of intermediate echogenicity. The posterior cerebral artery (PCA) wraps around the cerebral peduncle. The P1 segment of the posterior cerebral artery is displayed in red because blood flow is normally toward the transducer. The color PRF may need to be decreased to visualize the PCAs due to the lower blood flow velocities. The depth is usually from 55-80 mm and the mean velocity is 39 +/- 10 cm/sec. The P2 segment of the PCA may be displayed in red (toward the transducer) just distal to the origin of the posterior communicating artery, but will be displayed in blue distally as it wraps around the cerebral peduncle. This is variable due to the vessel tortuousity and orientation to the transducer. Often the ipsilateral and contralateral P1 segments can be visualized near their origin as the basilar artery terminates. The ipsilateral P1 segment is in shades of red and the contralateral P1 segment is displayed in shades of blue.

The anterior communicating artery (ACoA) is not visualized because it is short in length. However, the posterior communicating (PCoA) is longer in length and often is visualized connecting the anterior and posterior circulations in patients. The mean peak velocity in the PCoA is 36 +/- 15 cm/sec and the direction of blood flow may be toward or away from the transducer. The color PRF may need to be decreased to visualize the PCoA. Additionally, using power Doppler imaging may be helpful in locating the PCoA because this artery often courses parallel to the skin line.

It is important to remember that the transcranial color Doppler images are only in two dimensions. Tortuous intracranial arteries frequently cannot be displayed along their length as a continuous color pathway. In these instances, the gray scale image and color pixel information guides movement of the Doppler sample volume through the anatomically correct location, and the operator uses the imaging transducer to obtain the hemodynamic information similar to performing a nonimaging TCD examination.

Subocciptal Window

When evaluating the vertebrobasilar system, the best results are obtained with the patient lying on her/his side with her/his head bowed slightly toward the chest. This position increases the gap between the cranium and the atlas. The orientation marker/light on the transducer should be pointing to the patient's right side and the transducer placed on the posterior aspect of the neck inferiorly to the nuchal crest. The best images from the suboccipital approach are found with the transducer slightly off midline, with the ultrasound beam directed toward the bridge of the patient's nose. When compared to the nonimaging TCD approach for this window, the imaging transducer (wide ultrasound beam) is placed more inferiorly on the neck and angled more superiorly to obtain images of the vertebrobasilar arterial system.

The large circular anaechoic area visualized from this approach is the foramen magnum, and the bright echogenic reflection is from the occipital bone. The depth of these anatomic structures will vary with each patient depending upon the thickness of the suboccipital soft tissue. When these anatomic landmarks have been located, the gray scale image is optimized using image gain and TGC controls. The color control is turned on, and the right vertebral artery will be displayed on the left of the image (monitor) and the left vertebral artery is on the right side. The basilar artery is deep (bottom of the image) to the vertebral arteries. The depth range of the image should be set for 10-12 cms. Blood flow in the vertebrobasilar system is normally away from the transducer, so the arteries should be displayed in shades of blue. The vertebrobasilar system appears as a blue "Y" on the image. The operator may not always be able to visualize the vertebrobasilar system as a continuous "Y" due to the tortuousity associated with the vertebral and basilar arteries. Even if a "Y" -sign is visualized with color flow imaging, the most distal part of the basilar artery may not be properly displayed from this imaging plane. The basilar artery is often tortuous and visualization of its distal position requires an additional "dig-in" maneuver with the transducer. Relative to the "Y"-sign plane, an operator needs to move the transducer slightly inferiorly but aim more rostrally to visualize the course of the distal basilar artery. An important rule to remember is that the distal basilar artery is usually found at 10 cm depths whereas the "Y"-junction of the vertebral arteries is usually found from 7 - 8.5 cm.

The Doppler sample volume is placed in a vertebral artery and the spectral Doppler waveform information is obtained. The sample volume can be moved from side to side or can be swept through each vertebral artery individually. The mean velocity in the vertebral arteries is normally 38 +/- 10 cm/sec, and the mean velocity is 41 +/- 10 cm/sec in the basilar artery. Since the posterior circulation has lower velocities than the anterior circulation, the operator may need to decrease the color PRF to visualize the vertebrobasilar system. Additionally, the operator may need to adjust the transducer's position on the neck or its angle when trying to locate the deeper distal basilar artery. Moving the transducer slightly inferiorly on the neck and angling superiorly often allows better visualization of the distal basilar artery. Following the basilar artery distally (deep), the operator may not visualize color Doppler information. In this case, the operator may use the imaging transducer to obtain the Doppler hemodynamic information similar to performing a nonimaging TCD examination. The sample volume is moved along the expected course of the basilar artery. The terminal portion of the basilar artery as it bifurcates into the PCAs cannot be visualized from this approach.

Transorbital Window

The transorbital evaluation provides information about the ophthalmic artery and the carotid siphon. The United States Food and Drug Administration (FDA) has approved certain imaging transducers on various manufacturers' equipment for the evaluation of the orbit. It is important for each operator to contact the appropriate ultrasound manufacturing company and determine which transducer is approved for orbital imaging for their imaging system.

It is important when imaging the orbit to decrease the power setting significantly prior to evaluating the orbit. Using a low power output, successful color Doppler imaging of the orbit has been reported. Additionally, the examination time should be minimized. Although there are not any known observed bioeffects from the ultrasound evaluation of the eye, the power settings of color Doppler imaging systems raise concern for ocular damage. The current FDA maximum acoustic output allowable levels (derated) for ophthalmic imaging are a spatial peak temporal average (SPTA) intensity of 17 mW/cm2 and a mechanical index (MI) of 0.28. The guiding principle for the use of diagnostic ultrasound is the ALARA ("as low as reasonably achievable") principle.

The examination is performed with the patient in the supine position and the transducer gently placed on the closed eyelid. Removal of contact lenses is only necessary for patient comfort. A liberal amount of acoustic gel is important and firm pressure on the eyelid is not necessary or recommended. Careful attention to instrument controls is important to minimize artifacts caused by involuntary eye movement. Sometimes it is helpful to instruct the patient to focus their eyes to the opposite side from that being examined.

The orientation marker on the imaging transducer should be pointed medially, toward the nose, when performing the right or left examination. Examination of either eye produces an image with medial (nasal) on the left side and temporal on the right side of the monitor. A variation of this technique is to have the transducer's orientation marker directed to the patient's right side when evaluating either eye. This transducer orientation will produce an image with medial (nasal) on the monitor's left when examining the left eye and medial on the monitor's right side when evaluating the right eye. The globe is at the top of the image (monitor).

The carotid siphon is located at a depth of 60-80 mm and the mean velocity is 47 +/- 14 cm/sec. Direction of blood flow depends on which segment (parasellar, genu, supraclinoid) of the carotid siphon is insonated. Blood flow is bi-directional at the genu, toward the transducer in the parasellar portion, and away from the transducer in the supraclinoid segment.

To evaluate the ophthalmic artery (OA), the depth setting of the Doppler sample volume should be in the range of 40-60mm. A better image of this superficial segment may be obtained using a higher frequency (7.5-10MHz) linear array transducer. The ultrasound beam should be directed slightly medially along the anterior-posterior plane. The OA is generally identified adjacent to the optic nerve. The color PRF needs to be decreased to visualize the ophthalmic artery and blood flow is normally toward the transducer with a mean velocity of 21 +/- 5 cm/sec. The Doppler signal has a high pulsatility because the OA supplies blood to the globe and its structures.

Submandibular Window

The submandibular approach is a continuation of the duplex imaging evaluation of the extracranial distal internal carotid artery. The transducer is placed at the angle of the mandible and angled slightly medially and cephalad toward the carotid canal. The transducer's orientation marker/light is pointed in a superior direction. Distal ICA blood flow is away from the transducer and is displayed in shades of blue. Careful Doppler evaluation will distinguish the ICA's low resistance signal from the higher resistance signal from the ECA. Venous blood flow is less pulsatile and is directed toward the transducer (shades of red) at this location. The transducer's angle should be adjusted to follow the retromandibular portion of the ICA. The mean velocity in this segment of the ICA is normally 37 +/- 9 cm/sec.

At the end of the TCDI examination, the ultrasound gel should be removed from the patient. Excess gel should be removed from the ultrasound transducer and it should be cleaned using a disinfectant.

Most published TCD data are in velocities (centimeters/second; cm/sec), and that is the currently accepted format. It is customary to assume a zero degree angle when performing TCD examinations because the exact angle between the ultrasound beam and the intracranial arteries is unknown. Mean velocity (time-averaged peak) are usually reported for TCD examinations since this parameter is less affected by changes of central cardiovascular factors (heart rate, peripheral resistance, etc.) than systolic or diastolic values. In addition, intracranial arterial velocities decrease with increasing age of the patient.

Differences between intracranial arterial velocities are more important than the absolute values recorded from an individual. General observations of the mean velocities in the intracranial arteries are: 1) the velocities are highest in the middle cerebral artery (MCA), 2) if an anterior cerebral artery (ACA) velocity is more than 25% greater than the MCA velocity, the ACA may be hypoplastic, stenotic, serving as a collateral vessel, or there may be a MCA distribution infarction, and 3) the velocities in the anterior circulation are higher than the posterior circulation.

In the asymptomatic adult, side-to-side asymmetry should be minimal. The difference between sides has been reported to be less than 30% for normal vessels and undisturbed anatomy of the circle of Willis. Small differences generally do not indicate an underlying pathological condition. If a side-to-side difference is noted during the TCDI examination, try repeating the study on the side with the lower velocity. If side-to-side differences occur, they should not be considered abnormal unless they exceed 30%. Additionally, it may be difficult to interpret marked asymmetries in the posterior cerebral and intracranial vertebral arteries due to variable anatomy and/or hypoplasia that often affect these vessels.

The pulsatility index (P.I.) was first described by Gosling and colleagues in an attempt to quantify Doppler waveforms during the evaluation of lower extremity arterial disease. When using the P.I., the resistance that is encountered with each cardiac cycle is considered. For example, damped blood flow distal to an obstruction will have a decreased P.I. (diastolic velocity greater than 50-60% of peak systolic). Doppler signals obtained proximal to a high resistance (i.e. increased intracranial pressure), however, will have an increased P.I. (pulsatile spectral waveform). The P.I. in the MCA is normally in the range of 0.5 - 1.1.

Spectral broadening is observed in most TCDI examinations. The Doppler sample volume size is usually larger than the intracranial arteries. In fact, the sample volume of 10 mm exceeds the entire cross-sectional lumen of an intracranial artery and includes bifurcations and small arterial branches. Therefore, even though historically spectral broadening was used as a diagnostic criterion for moderate degrees of stenosis in the extracranial carotid arteries, it is not helpful in refining the TCDI interpretation.

TCDI offers new and important advantages to the TCD technique. Using the anatomic landmarks and proper instrument controls, a more accurate and reproducible evaluation of the complex intracranial arterial hemodynamics can be performed.

It is important to remember to: 1) take a patient history (focus on history, risk factors, and symptoms), 2) be aware of the status of the extracranial vessels, 3) be familiar with intracranial arterial anatomy, 4) understand how each color control affects the image and how the controls affect each other, 5) use the color/power Doppler display as a guide to obtain the Doppler spectral waveform information, 6) use a large Doppler sample volume (10-15mm) and assume a zero degree angle, 7) be aware of the Doppler spectral waveform configuration, 8) compare the Doppler spectral waveforms from the anterior and posterior circulations, and from left and right sides, and 9) establish institutional diagnostic criteria for the various clinical applications.

National Stroke Association, Retrieved April 2003 from www.stroke.org

Katz ML and Alexandrov AV. A Practical Guide to Transcranial Doppler Examinations. Summer Publishing Company, Littleton, CO, 2003.

• www.stroke.org
• Summer Publishing Company

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