Transcranial Doppler

How to incorporate it into your practice

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The catastrophic neurologic emergency remains one of the most challenging presentations encountered by emergency physicians. Physicians routinely depend on advanced imaging modalities such as computed tomography (CT) or magnetic resonance (MR) imaging to further investigate for potential catastrophic diagnoses. Acquiring these tests introduces the risks of transport as well as delays in managing time-sensitive neurologic processes.1-4

Luckily, a more immediate, non-invasive bedside approach complementing these modalities has evolved: transcranial Doppler. While transcranial doppler should not replace or delay formal imaging, it is a valuable adjunct when the patient is not stable enough to leave the emergency department (ED) for imaging tests, or in resource-limited settings where CT and MRI access is limited. It may also be used to assess for dynamic changes after the time-sensitive imaging test has already been performed.

Transcranial doppler is a noninvasive ultrasound (US) study involving the use of a low-frequency (≤2 MHz) transducer to detect blood flow within the intracranial circulation through relatively thin bone windows.5 Originally, transcranial doppler was purely a pulsed wave technique guided by the depth, velocity, and audio characteristics of target vessels. However, due to the relatively recent introduction of B-mode, it is more feasible to identify the various intracranial vascular structures via transcranial doppler. Point-of-care transcranial doppler provides the emergency physician with a rapid, dynamic, and repeatable mechanism with which to monitor cerebral blood flow velocity (CBF-V) and intracranial vessel pulsatility through pulse wave Doppler (PWD) without requiring transport of a potentially unstable patient. 

Literature has shown that while the large majority of patients have favorable acoustic windows, imaging is inadequate in approximately 5-20% of patients due to machine limitations, poor acoustic windows, patient compliance, and operator skill.6-8 The advantages and disadvantages of transcranial doppler are provided below (Table 1):

AdvantagesDisadvantages
Safe, non‐invasiveTime intensive
Bedside techniqueHighly operator dependent
Excellent temporal resolution and repeatabilityNot gold standard imaging modality
Provides useful information on cerebral vasculatureInaccuracy due to poor acoustic window (in up to 5–20%)

Table 1. Advantages and disadvantages of transcranial doppler

Current Applications of Transcranial Doppler

While there are a variety of indications for transcranial doppler, not all are applicable in the ED. Indications for transcranial ultrasonography and Doppler in the ED are listed below (Table 2). In particular, we will cover the four major point-of-care applications for transcranial doppler in the ED (the first four listed below).9-34

Midline shift
Vasospasm (including post-subarachnoid hemorrhage)
Arterial occlusion/Ischemic stroke
Elevated intracranial pressure
Determination of brain death
Monitor cerebral perfusion pressure during resuscitation
Evaluation of thrombolysis efficacy

Table 2. Selected indications for transcranial doppler.

Principles of Spectral Doppler

The normal spectral waveform has a characteristic shape: a sharp systolic upstroke and stepwise deceleration with positive end-diastolic flow (Figure 1). The emergency physician can utilize various indices, including Gosling’s pulsatility index (PI).35,36 and the Lindegaard ratio (LR)37,38 which allow further characterization of increased cerebrovascular resistance, vasospasm, and hyperdynamic flow states. The variables of concern for transcranial doppler spectral Doppler include the following (Figure 1).5

MCA waveforem-transcranial doppler transcranial doppler - transcranial doppler waveform 1024x726 - Transcranial Doppler
Figure 1. Left MCA transcranial doppler waveform (bottom) with color Doppler (top). PSV = Peak systolic velocity, in cm/s. EDV = End diastolic velocity, in cm/s. MFV = Mean flow velocity, in cm/s. Note the depth and angle of insonation, as well as the Pulsatility Index (PI).

Peak systolic velocity (PSV in cm/s)

PSV is the initial peak on the transcranial doppler waveform during each cardiac cycle. Most commonly, a rapid upstroke indicates an absence of severe stenotic lesions between the visualized intracranial arterial segment and heart.39

End-diastolic velocity (EDV in cm/s)

The EDV should be between 20 and 50% of the peak systolic velocity values, indicating low resistance intracranial arterial flow which is seen in all structurally normal intracranial arteries.40

Mean flow velocity (MFV in cm/s)

The mean flow velocity is calculated as EDV plus one-third of the difference between PSV and EDV.41 Among the examined intracranial arteries in transcranial doppler, the MCA should have the highest MFV.5

Pulsatility index (PI)

Most commonly in transcranial doppler, resistance to intra-arterial flow is assessed by using the PI, which is calculated by subtracting end diastolic velocity from peak systolic velocity and dividing the resulting value by the mean flow velocity.42 The PI is independent of the angle of insonation, and a value greater than 1.2 represents high resistance blood flow.5,43

Pulsatility index = (Peak systolic velocity-End diastolic velocity)/(Mean flow velocity) or  PI = (PSV-EDV)/MFV

Resistance index (RI)

Likewise, another transcranial doppler parameter used to assess flow resistance is the resistance index, which represents flow resistance distal to the area insonated.44 RI is calculated by subtracting EDV from PSV and dividing the value by PSV, with a normal value being less than 0.75.45

Resistance index = (Peak systolic velocity-End diastolic velocity)/(Peak systolic velocity) or RI = (PSV-EDV)/PSV

Heart rate

Heart rate may influence the aforementioned flow parameters, and is an important clinical component of transcranial doppler. Below is a table with the insonation characteristics of selected cerebral vasculature (Table 3).

WindowArteryProbe AngleDepth (mm)Flow Direction Adult MFV (cm/sec)
TranstemporalMCAStraight/Anterior-superior30–65Away55 ± 12
TranstemporalACAStraight/Anterior-superior60-75Away50 ± 11
TranstemporalPCA- Segment 1Straight/Posterior60-70Toward39 ± 10
TranstemporalPCA- Segment 2Straight/Posterior-superior60-70Away40 ± 10
SuboccipitalBASuperior80-120Away41 ± 10
SuboccipitalVASuperior lateral60-75Away38 ± 10
TransorbitalOAStraight45-55Toward21 ± 5

Table 3. Characteristics of selected cerebral vasculature. MFV: mean flow velocity, MCA: middle cerebral artery, ACA: anterior cerebral artery, PCA: posterior cerebral artery, BA: basilar artery, OA: ophthalmic artery.

Transcranial Doppler Technique

The transcranial doppler examination is best performed using a 2-MHz frequency US transducer (Figure 2). The higher frequency transducers routinely used to perform extracranial Doppler studies are not ideal for intracranial imaging, as the higher frequency waves are not able to adequately penetrate through the skull.46 While there are several manuals and protocols published, all comprehensive examinations center around four ‘windows’ used to insonate the relevant cerebral arteries (Figure 3).47

Transcranial doppler technique transcranial doppler - Transcranial doppler technique - Transcranial Doppler
Figure 2. Four acoustic windows commonly used in transcranial Doppler examination: (A) transtemporal window (B) submandibular window (C) suboccipital window (D) transorbital window. Note the 2 MHz ultrasound transducer selection.
Transcranial doppler arteries and depth transcranial doppler - Transcranial Doppler Line Diagram 993x1024 - Transcranial Doppler
Figure 3. Line diagram showing the major intracranial arteries and transcranial doppler windows. Flow directions from various arterial segments and depths of insonation (in mm) for an average human skull are shown.

Despite each view having unique advantages for different arteries and clinical indications, a comprehensive transcranial doppler examination includes measurements from all four windows and assessment of blood flow at various depths within each major branch of the circle of Willis. These arteries’ location is determined through a combination of:

  1. Relative direction of the transducer within the acoustic window.
  2. Direction of blood flow relative to the transducer’s orientation.
  3. Depth of insonation
  4. In difficult cases when one is able to differentiate anterior from posterior circulation, blood flow response to carotid compression or vibration.7

However, for applications in the ED and intensive care unit (ICU) settings, a limited point-of-care protocol emphasizes measurements of the MCA due to the ease of access through the temporal window and the quality of the signal.48 Additionally, the MCA carries approximately 50–60% of the ipsilateral carotid artery blood flow, and thus can be taken to represent blood flow to the hemisphere 42. While reviewing a comprehensive transcranial doppler protocol is outside of the scope of this post, readers may pursue further exposure through published texts.47,49,50

Transtemporal window in transcranial doppler

The transtemporal window view in a limited point-of-care protocol emphasizes measurements of the MCA, although a skilled ultrasonographer may be able to evaluate the ACA, PCA, and PCOM as well.

  1. Using the 1–5 MHz phased array transducer with a transcranial doppler preset (or if unavailable, a cardiac preset), begin by placing your patient in the supine position with the head of bed >30°.
  2. Place the ultrasound transducer (with the transducer marker oriented to screen left) over the temporal area, slightly above the zygomatic arch and immediately in front of the tragus of the ear. Orient the transducer slightly upward and anterior (Figure 2). Through the transtemporal window, the flow velocities in MCA, ACA, PCA, and PCOM can be obtained by pulsed wave Doppler.
  3. Identify the ipsilateral and contralateral temporal bones, and the third ventricle (midline).
  4. Decrease the depth to the distance of the third ventricle in the far-field and identify the cerebral peduncles and echogenic basal cisterns, approximately 3-8 cm in depth in the average adult.
  5. Place a color Doppler box over the top half of the screen (the near field) just lateral to the cerebral peduncles, in order to locate the MCA. A red color signal (flow towards the transducer) between 40 and 65 mm represents flow in the ipsilateral MCA, while the blue color signal between 65 and 80 mm represents flow from the ipsilateral ACA.5 In patients with good windows and favorable insonation, a red signal beyond 80 mm may be seen, representing flow in the contralateral A1.
  6. Place the pulse wave gate on top of the MCA in order to obtain spectral Doppler waveforms. Angle correction should be utilized in order to adjust the pulse wave Doppler for the angle of insonation. MCA interrogation can be further refined, obtaining signals at 50 mm for the M1 segment, and 40 mm for the M2 segment.51
  7. Angle the transducer caudally, visualizing the terminal ICAs between 60 and 70 mm in order to obtain spectral Doppler waveforms.7
  8. After obtaining flow signals from the MCA and ACA, orient the ultrasound transducer posteriorly by roughly 10 to 30 degrees, in order to evaluate the PCOM and PCA. In most patients, there is a loss of flow while angling the transducer posteriorly. This is followed by flow signals from the ipsilateral PCA, visualized between 55 and 70 mm, as the transducer continues to be angled posteriorly.5 An absence of the flow gap while angling the transducer posteriorly after evaluating the MCA/ACA most often represents flow signals from the PCOM.7
  9. Repeat the above protocol for the contralateral hemisphere.

Submandibular window in transcranial doppler

Place the transducer laterally under the jaw anterior and medial to the sternocleidomastoid muscle, aiming the transducer upwards and slightly medially with a depth of 50 mm. The distal ICA should be visualized as a low-resistance flow signal directed away from transducer. Occasionally, the external carotid artery may be confused for the ICA, and it is important to perform a temporal artery tap to differentiate the two. The temporal artery tap consists of tapping over the ipsilateral superficial temporal artery while simultaneously assessing the carotid bifurcation for evidence of a reflected flow in the external carotid artery.52 Note that this method alone may not reliably distinguish between the external and internal carotid arteries.53

Suboccipital window in transcranial doppler

Turn the patient to one side and place the transducer just below and medial to the mastoid process, directing the transducer slightly medially toward the bridge of the nose or contralateral eye. The suboccipital window is key to obtaining flow signals from the ipsilateral vertebral artery, commonly found between 50 and 75 mm, with the signals presenting away from the transducer (appearing as blue). In patients with favorable anatomy, the basilar artery may be visualized by aiming the transducer slightly upward and medial, increasing the depth to between 75 and 110 mm. If the basilar artery cannot be visualized, one may place the transducer just below the occipital protuberance and oriented toward the nasal bridge, commonly called the transforaminal window.54 The flow from the basilar artery is also away from the transducer.

Transorbital window in transcranial doppler

Transorbital insonation focuses on evaluating the ipsilateral ophthalmic artery and the ICA siphon. The transducer is placed gently over the eyelid and angled slightly medial and upward. Flow signals at a depth of less than 60 mm toward the transducer represent the ophthalmic artery. In order to visualize the siphon, one must move the depth beyond 60 mm. Due the ICA siphon being a curved artery, the flow signals may vary in direction either toward or away from the transducer.55 Additional care must be taken if the genu of the siphon is insonated, as this may present as bidirectional signals.56

Evaluation of midline shift (MLS)

Diagnosis of midline shift using transcranial doppler is important for both recognizing further secondary neurological injury and for neuro-prognostication in the acute setting.51 While any amount of midline shift is abnormal, a clinically significant midline shift as little as 0.5 cm is associated with a poor prognosis.57 Given its high temporal resolution and repeatability,11 transcranial doppler allows closer monitoring of critically injured patients. MLS calculated by use of transcranial doppler has demonstrated a high degree of correlation with CT and has been associated with predicting poor outcome secondary to midline shift in a variety of conditions, including acute stroke, hemorrhage, and traumatic brain injury.11,58-60

There have been several methods published regarding MLS measurement by transcranial doppler, but the most reliable method includes measuring the distance from the bilateral temporal bones to the midline third ventricle.55 Of note, distance A is measured from the ipsilateral side, whereas distance B is from the contralateral side (Figure 4). The full length from the ipsilateral to the contralateral temporal bone should be measured. Then, MLS is calculated by using the following equation, with an example provided below: Midline shift (MLS) = (distance A – distance B)/2

Transcranial doppler measurement of midline shift transcranial doppler - transcranial doppler midline shift - Transcranial Doppler
Figure 4. Transcranial imaging for midline shift. A. Insonation from right temporal bone to third ventricle, representing distance A (6.36 cm). B. Follow-up CT scan post transcranial doppler which confirms distance from right temporal bone to third ventricle found on transcranial doppler.

If the MLS is positive, then the MLS is expanding away from the ipsilateral side. Conversely, if the MLS is negative, the MLS is toward the ipsilateral side. In order to reduce user error and improve repeatability, the physician should make two independent measurements from each side. There should also be an internal check for correct measurements (i.e. the sum of distance A and B should equal the full distance from the ipsilateral to contralateral temporal bones).55 Care must be taken, however, as measuring MLS is reliant on proper trans-temporal windows which may be challenging to obtain.61 Likewise, there is no data correlating angle of insonation and accuracy of transcranial US midline shift measurements, although official guidelines recommend an upward angle of insonation no greater than 10–15°.55

Evaluation of Vasospasm

Transcranial doppler for vasospasm has been studied extensively and emerged as a validated screening tool, particularly in patients with subarachnoid hemorrhage (SAH).62-64 Post-SAH vasospasm greatly influences prognosis, leading to significant morbidity and mortality due to strokes from cerebral ischemia.65,66 By utilizing the relationship between MCA diameter and mean blood flow velocities, the emergency physician can quantify and subcategorize vasospasm.12 Compared to the gold standard of CT angiogram, the sensitivity and specificity of transcranial doppler for diagnosis for SAH-associated vasospasm is between 89 and 98%.67 However, the transorbital and transforaminal windows are less reliable compared to the transtemporal window. The MCA’s normal spectral Doppler profile has a sharp systolic upstroke followed by a step-wise diastolic deceleration and a normal MFV < 80 cm/s. Further categorization of vasospasm based on differing MFV is provided below (Table 4). Of note, these cutoffs were originally derived using traditional transcranial doppler transducer, not the phased array transducer common in point-of-care use. Furthermore, progressive increases in MFV early on in SAH have been shown to be predictive of vasospasm.68 Another method of quantifying vasospasm severity is the formula known as the Lindegaard ratio, although this requires more measurements.49

Lindegaard ratio = Ipsilateral MCA mean flow velocity/ipsilateral extracranial ICA mean flow velocity

As vasospasm is not the only cause of increased mean flow velocities, the Lindegaard ratio does not necessarily grade vasospasm, but rather aids in the differentiation of hyperemia and the onset of vasospasm.55 Grading of vasospasm by Lindegaard ratio is provided below (Table 4).

Mean Flow Velocity (MFV)Lindegaard ratio
Normal≤ 80 cm/s<3
Mild81–119 cm/s<3
Moderate120–159 cm/s3–5
Severe160–199 cm/s5-6
Critical≥ 200 cm/s>6

Table 4. Doppler Characteristics of MCA Vasospasm

It must be noted, however, that several factors make the diagnosis of vasospasm by transcranial doppler challenging. For instance, cerebral blood flow is influenced by many factors, including PaO2, PaCO2, blood viscosity, and collateral flow, all of which may affect MFV. Additionally, the operator should be experienced, as improper vessel identification, various lesions proximal to the insonated area, and tortuous vessel course can lead to improper measurements.

Evaluation of acute ischemic stroke

Transcranial doppler is particularly useful in acute ischemic stroke as repeated transcranial doppler studies can be used to follow the course of an arterial occlusion after thrombolysis.34 From recent studies, transcranial doppler is able detect acute MCA occlusions with a sensitivity and specificity of greater than 90%.69-72 transcranial doppler detection of occlusions in the ICA siphon, vertebral, and basilar arteries has a sensitivity between 70 and 90%, and a specificity greater than 90%.73

Additionally, transcranial doppler plays an important role in augmenting thrombolytic-induced arterial recanalization, while simultaneously monitoring its efficacy.74 Transcranial doppler is thought to act synergistically with thrombolytic agents by delivering mechanical pressure waves to increase the amount of thrombus surface area exposed to circulating tissue plasminogen activator (tPA).75,76  A meta-analysis found that complete recanalization rates have been higher in patients receiving combination of transcranial doppler with tPA, 37.2% (95% CI, 26.5%- 47.9%) compared with patients treated with tPA alone 17.2% (95% CI, 9.5%-24.9%).77 Of note, transcranial doppler-enhanced thrombolysis has not been associated with an increased risk of symptomatic intracerebral hemorrhage.77

The presence of detectable residual flow signals on transcranial doppler prior to intravenous thrombolysis can be used to predict recanalization. Patients with detectable residual flow signals before intravenous thrombolysis are twice as likely to have early complete recanalization.78,79  In contrast, those patients with no detectable flow signals on transcranial doppler have a less than 20% chance for complete recanalization within two hour transcranial doppler can also be used to accurately identify these patients with incomplete recanalization. Compared to angiography (digital subtraction or magnetic resonance), transcranial doppler has a sensitivity of 91% and specificity of 93% for detecting successful recanalization after thrombolysis.34 In the acute care setting, this finding may prompt consideration of alternative therapies, including mechanical thrombectomy or intra-arterial thrombolysis.79 The evaluation for stenosis or acute ischemic stroke centers on the presence or absence of the following signs (Table 5).36

Signs of OcclusionPrimary signs of stenosisSecondary signs of stenosis
Absence of signal from arteryIncrease in mean flow velocity at the site of luminal narrowingDecreased velocity upstream from the lesion
Sonographic evidence of collateral flowIncreased pulsatility upstream from the lesion
Abnormal flow immediately downstream from the lesion

Table 5. Transcranial Doppler evaluation of stenosis and occlusion

Measurement of Intracranial pressure (ICP)

Although ICP can guide patient management in emergency care, it is not commonly monitored in many clinical conditions. This is due to the invasive nature of the standard methods for ICP monitoring (epidural, subdural, intraparenchymal, and intraventricular monitors) and their associated risks to the patient (infections, brain tissue lesions, and hemorrhage). transcranial doppler, on the other hand, provides rapid, reliable clinical information at the bedside with high temporal resolution.21 There is extensive literature proposing transcranial doppler as a tool for noninvasive measurement of ICP, showing a good concordance between invasive ICP measurements and transcranial doppler-based ICP measurements.43,80-82

Furthermore, in comparison to ultrasound measurement of the optic nerve sheath diameter, transcranial doppler has a higher specificity and sensitivity for detecting elevated ICP 83. Similarly, transcranial doppler has been demonstrated to accurately screen patients with mild or moderate TBI at risk of secondary neurological deterioration due to elevated ICP, which may play an important role in determining patient disposition and level of care.84 For instance, multiple studies, including a recent multicenter study in 356 traumatic brain injury patients, have shown that transcranial doppler had a negative predictive value of 98% for predicting neurologic worsening over the first week in patients admitted to the hospital with mild to moderate TBI.85,86

Transcranial doppler-based ICP measurements are based on approximate semi-quantitative relationships between cerebrovascular dynamics and ICP.  As ICP increases, the flow in intracranial vessels is affected. Initially, there is an increase in systolic velocity (as measured by PSV) as the increased ICP externally compresses cerebral vessels, causing the intraluminal diameter to narrow. Furthermore, diastolic flow becomes blunted, as raised ICP becomes the predominant external pressure opposing forward arterial flow during diastole.55 In extreme cases, ICP can exceed normal forward flow during diastole, leading to diastolic flow reversal and catastrophic ischemia, as evidenced below (Figure 5).

Transcranial doppler waveform in increased intracranial pressure transcranial doppler - transcranial doppler intracranial pressure elevation - Transcranial Doppler
Fig 5. Progression of transcranial Doppler waveforms with decreasing cerebral perfusion pressure (CPP) and increasing intracranial pressure (ICP) after head injury. A. Normal systolic upstroke with normal step-down of diastolic flow. B-C. Increased peak systolic flow with decreasing diastolic flow and eventual blunting of diastolic flow. D. Diastolic flow reversal (AKA Biphasic or oscillating flow)—where diastolic flow reversal begins to approach equal size to systolic flow. This denotes one state in which cerebral circulatory arrest can be diagnosed.

An increased ICP can be estimated by the Gosling’s pulsatility index (PI), which reflects peripheral resistance, equal to the difference between the peak systolic velocity (PSV) and end-diastolic velocity (EDV), divided by the mean flow velocity (MFV).47

Pulsatility index (PI) = (PSV – EDV)/MFV

A formula has been derived to convert pulsatility index into ICP (from all causes), with an associated sensitivity of 89%, and specificity of 92%.43

ICP = (10.93 × PI) – 1.28

Therefore, based on this formula, a PI of greater than 2.13 would correlate to an ICP greater than 22 mmHg, which is the clinically significant cutoff for raised ICP,  whereas normal PI is less than 1.2, corresponding to an ICP of 12 mmHg. There are drawbacks to using this technique, however. The main limitation of utilizing transcranial doppler to measure ICP is that wide confidence intervals have been reported when compared directly to ICP monitors.36,87 Furthermore, cerebral blood flow and the PI are influenced by many factors, including arterial blood pressure, PaCO2, and cerebral perfusion pressure. Likewise, lack of pulsatile flow, as is the case in patients with left ventricular assist devices, renders the systolic and diastolic ratios for pulsatility index uninterpretable. Nevertheless, transcranial doppler-based methods for ICP measurement generally present a positive degree of agreement and acceptable correlations with measured ICP while retaining a high degree of temporal resolution.

Conclusion

With the advent and wide dissemination of bedside ultrasound within the emergency department, there is a unique opportunity for the emergency physician to utilize transcranial doppler for a variety of conditions. The core applications include the evaluation of midline shift, vasospasm after SAH, acute ischemic stroke, and ICP.  Although there are numerous formalized transcranial doppler protocols utilizing four views (transtemporal, submandibular, suboccipital, and transorbital), point-of-care transcranial doppler is best accomplished through the transtemporal window. While barriers to training exist, emergency physician performance of limited point-of-care transcranial doppler is feasible and may provide rapid and reliable clinical information with high temporal resolution.

Take Home Points:

  • Point-of-care transcranial doppler can be performed rapidly by emergency physicians to evaluate for midline shift, vasospasm, acute ischemic stroke, and increased intracranial pressure.
  • A limited point-of-care protocol focuses on the following windows- Transtemporal, Submandibular, Suboccipital, and Transorbital.
  • When measuring midline shift, the most reliable method includes measuring the distance from the bilateral temporal bones to the midline third ventricle.
  • Compared to the gold standard of CT angiogram, the sensitivity and specificity of transcranial doppler for diagnosis for SAH-associated vasospasm is between 89 and 98%.
  • Evaluating tPA administration with transcranial doppler may prompt consideration of alternative therapies, including mechanical thrombectomy or intra-arterial thrombolysis.
  • In comparison to ultrasound measurement of the optic nerve sheath diameter, transcranial doppler has a higher specificity and sensitivity for detecting elevated ICP.

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References

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