Transcranial Doppler sonography (ultrasound)

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Transcranial​ ​Doppler​ ​(TCD)​, also known as transcranial color-coded duplex sonography (TCCS) is a sonographic study of intracranial structures and blood vessels, used most commonly to identify the hemodynamic state present in the vertebrobasilar circulation and the circle of Willis.

Terminology

An extension of the non-imaging, continuous wave Doppler assessment popular among neurointensivists, the imaging of cerebral structures with grey-scale and superimposed color flow and spectral Doppler analysis is now possible using the same windows, techniques, and diagnostic goals. Some may refer to the imaging modality to be discussed below as transcranial color-coded duplex sonography (TCCS) and the non-imaging based continuous wave Doppler modality as transcranial Doppler (TCD); this article will not make this distinction, as the latter (non-imaging) modality will not be further discussed. "TCD" and "transcranial Doppler" will be used to refer to the combined 2D parenchymal imaging with or without the use of Doppler modalities.

Advantages of TCD over its "conventional" predecessor include an ability to identify structural perturbations, including the presence of masses and/or midline shift, and the presence of sonoanatomical landmarks to guide placement of a pulsed wave Doppler gate .

Neurosonography of the fetal brain, although based on similar principles, will not be discussed; this article will focus on neonatal, pediatric, and adult indications for sonographic studies of the brain and cerebral vasculature, especially as it pertains to point-of-care ultrasound.

Indications

General indications for the use of transcranial Doppler ultrasonography include:

Indications for transcranial Doppler more specific to pediatric age groups, especially pertaining to neonates, include:

Sonographic examination of the orbit will be also be discussed, as the measurement of the optic nerve sheath diameter (OSND) and spectral waveform analysis of the ophthalmic artery are essential parts of a complete transcranial Doppler examination. Sonography of the eye has a wide array of applications in point-of-care ultrasonography; indications for its use alone, distinct from the other elements of a transcranial doppler exam, include :

  • trauma to the head, eye, or orbital structures
  • acute alteration of vision
  • altered mental status
  • headache

Procedure

The performance of a transcranial Doppler study differs based on relevant patient anatomy, most notably in regards to the status of fontanelle closure; in neonates,​ these open fontanelles may ​provide​ ​acoustic​ ​windows​ ​for insonation of intracerebral structures.

Neonates

An adequate sonographic window may be often be obtained at the anterior fontanelle in infants less than 12-14 months of age. The mastoid and posterior fontanelles should also be used as sonographic windows if available, although their closure usually occurs before age 6 months. All available windows should be interrogated in both a sagittal and coronal plane, sweeping through sonographic landmarks with anteroposterior rocking and tilting of the probe. Interposed between the confluence of the frontal and parietal bones, the anterior fontanelle is a commonly used anatomic landmark, as well as the first window insonated. Relevant structures of interest in each acoustic window include :

  • anterior fontanelle
    • coronal view
    • ​sagittal view
      • a purely sagittal section through the anterior fontanelle should reveal, from near-field to far-field
      • lateral angulation to parasagittal sections will allow visualization of the caudothalamic notch, thalami, basal ganglia, lateral ventricles, and periventricular white matter
      • when indicated, this view is preferred for the measurement of the anterior cerebral artery (ACA) resistive index (RI)
        • the pericallosal branch of the ACA is often used for this purpose
  • posterior fontanelle
    • window may be located by sliding the transducer posteriorly along the midline until the characteristic depression is found, located between the occipital and parietal bones
    • visualization of the choroid plexus and occipital lobes often superior to the anterior fontanelle window
      • sagittal view
        • complementary to the sagittal anterior fontanelle view, enhancing visualization of the atrium of the lateral ventricles and periventricular white matter
        • may visualize the brainstem and subarachnoid cisterns to a superior extent
        • parasagittal sections should sweep through the thalami, choroid plexus, and the occipital horn of the lateral ventricle
      • coronal view
        • complementary to the coronal anterior fontanelle view
        • posterior sweep using the hypoechoic lateral ventricles, from trigone to occipital horn, as a sonographic landmark
        • the occipital lobes will appear in the near-field, with the tentorium and cerebellum in the far-field
  • mastoid fontanelle
    • located between the occipital, temporal, and parietal bones, with the transducer oriented roughly in the plane of the orbitomeatal line
    • the primary view to assess the cerebellum, it allows superior visualization of most posterior structures, including the cisterna magna, fourth ventricle, and cerebellar peduncles

While the aforementioned windows provide an adequate window in the majority of studies, additional views may be opportunistically sought in the presence of the following;

  • a burr hole
  • craniotomy defect
  • abnormally wide sutures
Adults

Four acoustic windows persist after fontanelle closure; the transtemporal, orbital, suboccipital and retromandibular windows . The transtemporal window is the most easily accessible window, and the most widely used at the point of care; some authors define various "sections" of the transtemporal window (i.e. probe positions); the cardinal probe position is superior to the zygomatic arch and nasal to the pinna of the ear , with subsequent adjustments in probe position from anterosuperior ("frontal" section) to slightly caudad ("anterior" section) and sequentially posterior ("medial" and "posterior" sections) .

  • transtemporal window
    • the hypoechoic cerebral peduncles encircled by the hyperechoic suprasellar cistern are often used as sonographic landmarks
      • roughly assume the shape of a butterfly
    • ipsilateral and contralateral temporal bones delineate the extent of the sonographic near and far field, respectively
      • angulation to include the presence of the third ventricle, and measurement of the distances between the ipsilateral temporal bone and the:
        • third ventricle
        • contralateral temporal bone
      • in the absence of midline shift, these measurements are equal
    • color flow Doppler is then implemented, with the near-field middle cerebral artery (M2 or insular segment, flowing toward the transducer, thus depicted in red) often the most conspicuous vessel, mirrored by its counterpart in the far field (flowing away from the transducer, color scale blue) 
      • the Nyquist limit may have to be adjusted for smaller vessels such as the posterior cerebral artery (PCA); the ipsilateral PCA will appear as blue and red flow signals just posterior and anterior to the ipsilateral cerebral peduncle, respectively 
    • the anterior (ACA) and middle cerebral (MCA) arteries may be differentiated by their course and the color scale
      • the ipsilateral ACA (A1 segment) will be depicted in blue coursing anteriorly, perpendicular to the plane of insonation
      • the ipsilateral MCA courses parallel, toward the footprint of the probe, and is depicted in the aforementioned red
    • the internal carotid (ICA) may be insonated from its cavernous segment caudad (depicted in blue) cephalad, with a color scale transition (to red) immediately prior to its bifurcation
  • suboccipital window
    • transducer at the junction between the posterior midline of the neck and the occiput, aiming cephalad toward the imagined location of the foramen magnum
    • the paired vertebral arteries (VA) course away from the transducer (appearing blue with color flow Doppler) and converge, forming the basilar artery (also flowing away from the transducer)
      • conformation said to resemble a "Y"
  • orbital window
    • identical to the performance of other ocular ultrasonography
    • behind the elliptical globe the homogenous, linear hypoechoic optic nerve should be identified, invested in a hyperechoic sheath
    • a surrogate of intracranial pressure measurement, the optic nerve sheath diameter (OSND) should be noted
      • measured 3 mm posterior to the retina, normal diameter in adults should be under 5 mm
    • color flow Doppler should be utilized to identify the ophthalmic artery, external to, and running in a course parallel to the optic nerve in the posterior orbit
      • the central retinal artery (and vein) may appear "within" the optic nerve body, just proximal to its confluence with the posterior globe
Spectral Doppler

Once sonographic landmarks and vessels are identified using the abovementioned combination of grey-scale (B-mode) and color flow Doppler, spectral Doppler modalities may be implemented. A suggested approach to a pulsed wave Doppler (PWD) (with a gate width of 5 mm) examination of the vasculature may proceed as follows, starting at the transtemporal window and proceeding to the suboccipital window :

  • the pulsed wave Doppler gate is advanced from the bifurcation of the internal carotid artery along the middle cerebral artery to ascertain the maximum velocity (Vmax) or peak systolic velocity (PSV)
    • the PSV and end-diastolic velocity (EDV) in each vessel insonated should be recorded, and (when bilateral) both ipsilateral and contralateral structures should be examined
  • along the A1 segment of the ACA
  • from the ICA bifurcation to the cavernous portion
  • near-field extent of the vertebral artery to the basilar artery

Velocities obtained are analyzed using principles derived from the Doppler equation and the modified Bernoulli principle; erroneous results (e.g. through angle dependence) may be mitigated through the use of derived parameters as follows:

  • peak systolic and end-diastolic velocities should be used to calculate a mean flow velocity
    • mean flow velocity (MFV) = (PSV + [2 x EDV]) / 3
  • the pulsatility index (PI) may also be useful to calculate (the difference between PSV and EDV, divided by the PSV)​

Normal and abnormal spectral doppler envelopes and measured velocities differ based on the manner of pathology present and the vessel in question:

  • middle cerebral artery (MCA)
    • high velocity, sudden systolic inflection (ipsilateral artery above baseline, contralateral below) with step-wise velocity decrements in diastole
      • in adults, a pulsatility index (PI) may be converted into a rough estimate of intracranial pressure with the equation ICP = (10.93 x PI) - 1.28
    • mean flow velocity (adults) usually between 46 to 86 cm/s
      • may be altered by an increase or decrease in regional blood flow, changes in blood viscosity, or a change in luminal caliber
  • ophthalmic artery (OA)
    • spectral envelope above baseline (toward transducer)
    • high resistance pattern (supplied by the external carotid artery) with a mean velocity between 16-26 cm/s
    • flow reversal (envelope below baseline) may indicate occlusion of the ipsilateral internal carotid artery 
      • may also demonstrate "internalization," or change in the flow pattern to low-resistance
  • anterior cerebral artery (ACA)
    • mean flow velocity (MFV) between 41 and 76 cm/s

Treatment and prognosis

Stroke and intracranial hemorrhage

The degree of midline shift (MLS) measured by transcranial sonography directly correlates with prognosis and likelihood of cerebral herniation in space-occupying hemispheric stroke. Quantitation of MLS with sonography is useful for predicting the need for, and the neurological outcome after, decompressive surgery . Furthermore, when performed three days after an ischemic stroke affecting the middle cerebral artery territory, measurement of midline shift may assist in risk stratification.

Cerebral vasospasm is a common complication occurring after subarachnoid hemorrhage (SAH) resulting in increased morbidity and mortality. TCD allows the clinician to both recognize the presence of vasospasm, a harbinger of cerebral ischemia, and grade the severity ;

  • within three days status post-SAH, an increase in the middle cerebral artery baseline velocity by more than 21 cm/seconds/day is considered highly suggestive of vasospasm
  • velocities exceeding 160 cm/s are usually required to produce symptoms
  • a lindegaard ratio above 3.0 is considered diagnostic
    • ratios above 6.0 are indicative of severe vasospasm
    • the peak systolic flow velocities in the middle cerebral artery and the ipsilateral internal carotid artery (extracranial portion) may be compared
      • a ratio (MCA PSV/ICA PSV) >3.9 is consistent with either moderate or severe vasospasm, with higher specificity in the presence of an MCA mean flow velocity (MFV) between 94 and 108 cm/s
Cerebral circulatory arrest

A proposed future ancillary test for the diagnosis of cerebral circulatory arrest, some authors have suggested the following criteria for this diagnosis :

  • biphasic flow pattern, with equalization of systolic and diastolic flow amplitudes
  • measured peak systolic velocities (PSVs) <50 cm/s with duration of systolic "spikes" <200 ms
  • obliteration of all intracranial flow, in the presence of extracranial flow

To fulfill diagnostic criteria, an examination should be recorded before the onset of cerebral circulatory arrest, one of the aforementioned three criteria must be present in bilateral vessels and be recorded twice (sequential examinations 30 minutes apart).

Differential diagnosis

  • headache and decreased level of consciousness (LOC)
    • elevated intracranial pressure
      • an MCA pulsatility index (PI) above 2.13 correlates to intracranial pressures above 22 mmHg (normal ICP between 5 and 15 mmHg)
      • optic nerve sheath diameter >5 mm (adults) correlates with elevated ICP
    • mass effect leading to midline shift
    • presence of brain death
  • status post subarachnoid hemorrhage with decreased LOC
    • vasospasm
      • high-velocity MCA flow with Lindegaard ratio >3
    • hemorrhage