Transtemporal approach

The probe is placed on the temporal aspect of the head, above the zygomatic arch and immediately anterior and slightly superior to the tragus of the ear conch ( Fig. 10.3 , position 1 ). This is usually the most promising examination site. A more posterior window immediately cephalad and slightly dorsal to the first one (see Fig. 10.3 , position 2 ) may be more appropriate in a minority of cases, especially for insonation of the P ₂ segment of the posterior cerebral arteries (PCAs). In some patients, a more frontally located temporal window may be present (see Fig. 10.3 , position 3 ). Starting from these transtemporal windows, the ultrasound probe can be angulated anteriorly or posteriorly relative to the corresponding probe positions on the opposite side of the head. The anterior orientation of the ultrasound beam allows insonation of the M ₁ and M ₂ segments of the middle cerebral arteries (MCAs), the C ₁ segment of the carotid siphon (CS), the A ₁ segment of the anterior cerebral artery (ACA), and often the anterior communicating artery ( Fig. 10.4A ). Posterior angulation of the ultrasound beam allows examination of the P ₁ and P ₂ segments of the PCA, the top of the basilar artery (BA), and the posterior communicating arteries (see Fig. 10.4B ).

Available temporal ultrasonic windows and probe placement. The probe should first be placed in the preauricular region to identify the middle cerebral artery. Very subtle movements of the probe should be performed in each position. If position 1 is not successful, position 2 should be tried next, before position 3 is chosen. 1 , Preauricular position; 2 , posterior window; 3 , anterior window.

Position of the probe in the temporal region to insonate the anterior and posterior parts of the circle of Willis. (A) Line X – X ′ indicates a frontal plane that runs through the regular placement of the probe on either side and, simultaneously, perpendicular to the sagittal midline of the skull. Z ′ indicates the site of the intracranial internal carotid artery bifurcation. The X ′– Z ′ distance is 63 ± 5 mm. The angle µ is the angle with which the probe is aimed more anteriorly toward the middle cerebral artery and anterior cerebral artery segments. This angle was found to be 6 ± 1.1 degrees. (B) The angle ω indicates the angle with which the beam is directed more posteriorly to insonate the top ( T ) of the basilar artery (BA) and the P ₁ segments ( P′ ) on both sides. This angle was found to be 4.6 ± 1.2 degrees. The BA bifurcation could be insonated at depths of 78 ± 5 mm, corresponding to the distance X – T or X ′– T , respectively. Y indicates the fictional point at which the pathway of the beam then transits the contralateral skull (i.e., approximately 2 to 3 cm behind the external acoustic meatus). The P ₂ segments ( P ) can also be insonated if the beam is directed even more posteriorly and slightly caudally (line X′–P ). W lies approximately 5 cm behind the contralateral external acoustic meatus.

Transorbital approach

Components of the anterior cerebral circulation may be evaluated by placing the transducer against the closed eyelid. To avoid damage to the lenses of the eyes, the power of the ultrasound transmission has to be reduced. The ophthalmic artery can usually be insonated at depths of 45 to 50 mm, whereas the C ₃ segment (anterior knee of the CS) is normally met at insonation depths of 60 to 65 mm ( Fig. 10.5A ). At slightly greater insonation depths of 70 to 75 mm, the C ₂ segment shows flow away from the probe, and the C ₄ segment shows flow toward the probe. These blood flow directions apply only when the beam is nearly sagittal (slight medial obliquity) and enters the skull through the supraorbital or infraorbital fissures. Typical insonation depths and velocities are shown in Fig. 10.5B . Measurements made through the transorbital approach are less well established and validated than those made through the transtemporal and suboccipital approaches.

Insonation of the ophthalmic artery ( OA ) and carotid siphon (CS) by the transorbital approach. (A) Probe ( P ) location and relationship to the OA and CS. (B) Representative insonation depths and normal flow values within various segments of the CS ( C ₁ to C ₄ ) and OA.

Suboccipital (transforaminal) approach

The suboccipital (transforaminal) approach is essential for screening the distal part of the vertebral artery (VA), the so-called V4-segment and the basilar artery (BA). The probe is placed exactly between the posterior margin of the foramen magnum and the palpable spinous process of the first cervical vertebra, with the beam aimed at the bridge of the nose ( Fig. 10.6A ). The insonation depth is set at 65 mm, and the right and left VAs are tracked individually from this (deepest) point back toward the foramen magnum, using progressively smaller insonation depths (from 65 down to 35 mm). As the depth decreases, the sound beam is angled increasingly sharply toward the side of the head. The extradural part of the VA, on the posterior arch of the atlas (V3 segment), can also be screened. Flow is toward the transducer in this segment. The BA can be tracked cephalad from the point at which the VAs join. The superior end of the BA is reached at a depth of approximately 95 to 125 mm. Blood flow in the intradural VAs and the BA is normally directed away from the probe. Typical insonation depths and blood flow velocities are shown in Fig. 10.6B .

(A) Transcranial Doppler examination of the vertebral system by the suboccipital approach. (B) Representative insonation and normal flow values within the distal vertebral arteries ( V ) and the basilar trunk ( B ). The P ₁ and P ₂ velocities are measured transtemporally. P , Probe.

Submandibular approach

The submandibular approach complements the examination permitting the Doppler interrogation of the retromandibular and more distal extradural parts (C ₅ to C ₆ segments) of the internal carotid artery (ICA). This particular imaging window facilitates the detection of ICA dissection and chronic ICA occlusion with abundant collateralization through the external carotid artery. With the transducer positioned as shown in Fig. 10.7A , the beam is directed slightly medially and posteriorly. The ICA can regularly be tracked to a depth of 80 to 85 mm, at which point it bends medioanteriorly to form the CS. Typical insonation depths and blood flow velocities are shown in Fig. 10.7B .

(A) Transcranial Doppler examination of the petrous portion of the internal carotid artery ( ICA ) by the submandibular approach. The ICA can be traced from depths of 25 to 80 mm, corresponding to the C ₅ segment of the ICA. (B) Representative insonation depths and normal blood flow velocities of the distal intracranial ICA.

Basic transcranial Doppler examination

It is practical to start the examination at the transtemporal window, identify the MCA on either side at an insonation depth of 50 to 55 mm, and then track the ipsilateral arterial network, step by step, in various directions. Proof of traceability of the MCA is necessary for its unequivocal identification. This is also true for other arteries at the base of the brain. Traceability refers to the fact that the MCA (and usually other arteries) can be tracked in incremental steps from shallow insonation depths (35 mm) to deeper sites (55 mm) without showing changes in the character of the blood flow profile and direction. When tracking the MCA medially (65 to 70 mm), an abrupt change in flow direction (away from, rather than toward the probe) indicates insonation of the A ₁ segment of the ACA. Blood flow signals toward the probe at this depth usually emanate from the CS at its junction with the MCA. Typical depths and blood flow velocities are shown in Fig. 10.8 .

Typical transtemporal distances and velocities for the anterior cerebral artery and the middle cerebral artery. (A) The beam axis is in line with the C ₁ , M ₁ , and A ₁ segments of the cerebral vessels. (B) Representative insonation depths and blood flow velocities are illustrated. P , Probe.

By angling the beam more posteriorly from the transtemporal approach, the P ₁ segment of the PCA can be picked up most readily at an insonation depth of 65 to 70 mm. The PCA can then be tracked to the top of the BA (75 mm) and from there to the contralateral PCA (80 to 85 mm) (see Fig. 10.4B ). The two criteria of traceability (i.e., the display of bilateral blood flow at the junction with the BA and the change of flow direction within the contralateral PCA) are very important features for identifying the PCAs without compression tests.

After the completion of the examination from both temporal windows, additional information may be obtained through the orbital, suboccipital, or submandibular windows. The arterial segments that are accessible from these windows and insonation depths, as well as the techniques for identifying these vessels, have previously been described. A protocol for TCD examination is described in Table 10.1 .

Transcranial color-coded duplex sonography examination

TCCS is a well-established diagnostic method, allowing direct noninvasive imaging of intracranial vascular structures. This visual approach provides for more rapid and reliable vessel identification, permitting exact localization of the Doppler sample volume, and shortening the examination time. This technique has evolved rapidly and now includes not only vascular (i.e., arterial and venous) imaging but also imaging of the brain parenchyma. Usually the transtemporal and suboccipital approaches are used during TCCS examinations. No systematic data exist for the submandibular approach. The transorbital TCCS investigation now offers a means of monitoring intracranial pressures by measurement of the width of the sheath of the optical nerve (i.e., in intensive care units [intracranial pressure]), or in cases of suspected benign intracranial hypertension.

For transtemporal insonation, the probe is positioned axially along the orbitomeatal line and the hypoechoic, butterfly-shaped midbrain is visualized as an anatomic landmark at a depth of 6 to 8 cm. From this perspective, the circle of Willis can then easily be depicted ( Fig. 10.9 ). Other landmarks that can be seen as the beam is tilted more cranially are the third ventricle in the midline, the pineal gland and the choroidal plexus (both hyperechoic), and the cella media. For the suboccipital approach, the hypoechoic foramen magnum and the hyperechoic clivus serve as the anatomic landmarks, with both VAs located at their lateral edges ( Fig. 10.10 ). The origin of the BA can also be visually identified in most cases at a depth of 75 to 95 mm. Generally, the reference depths of the target vessels are similar to the values given previously for the TCD examination. TCCS also allows for the examination of cerebral venous sinuses and large basal cerebral veins, although this has not become part of the clinical routine. Transcranial B-mode ultrasound can be used to follow intracranial hemorrhages and to assess the brain parenchyma in movement disorders. The method is especially helpful in differentiating Parkinson syndromes.

Illustration of a typical transtemporal transcranial color-coded Duplex sonography examination. (A) For initial spatial orientation, the examination is started with a large-scale, B-mode cranial view, which is usually achieved at a depth of 14 to 17 cm. Visualization of the hyperechoic contralateral skull ( arrowheads ) proves the presence of adequate transcranial ultrasound penetration. If the hypoechoic, butterfly-shaped midbrain ( arrows ) and the hyperechoic sphenoid bone ( asterisks ) can be visualized, then the correct insonation plane has been achieved. (B) For the color-mode examination, the insonation depth is reduced to 8 to 10 cm; the precommunicating (P ₁ ) and postcommunicating (P ₂ ) segments of the posterior cerebral artery ( PCA ) can be visualized as they follow the edge of the midbrain. More anteriorly, the sphenoidal (M ₁ ) and the insular (M ₂ ) parts of the middle cerebral artery ( MCA ), and the precommunicating (A ₁ ) part of the anterior cerebral artery ( ACA ) can be depicted. In rare cases and with excellent bone insonation conditions (as illustrated), the entire circle of Willis can be displayed. The distal part of the internal carotid artery is also assessable with the probe tilted downward.

Illustration of a typical suboccipital (or transforaminal) transcranial color-coded duplex sonography TCCS examination. (A) For initial spatial orientation, the examination is started with a large-scale, B-mode cranial view, which is usually achieved at a depth of 11 to 13 cm. Visualization of the hypoechoic foramen magnum ( asterisks ) and the hyperechoic clivus ( arrow ) proves the adequacy of transcranial ultrasound penetration. (B) For the color-mode examination, the insonation depth is usually reduced to 8 to 11 cm, visualizing segments (V4) of both vertebral arteries ( VAs ) as they follow the edges of the foramen magnum. The Y-shaped conjunction of the VAs with the basilar artery ( BA ) is usually located close to the clivus. Note, however, that the origin of the BA is highly variable and all three arteries are not always visible within the same insonation plane.

Vessel identification

The primary TCD parameters for identifying the cerebral arteries are the following:

• 1.
  

  Insonation depth

  
  
• 2.
  

  Direction of blood flow at insonation depth

  
  
• 3.
  

  Flow velocity (mean flow velocity and systolic or diastolic peak flow velocity)

  
  
• 4.
  

  Probe position (e.g., temporal, orbital, suboccipital, submandibular)

  
  
• 5.
  

  Direction of the ultrasonic beam (e.g., posterior, anterior, caudad, cephalad)

  
  
• 6.
  

  Traceability of vessels

Blood flow velocity measurements

The mean blood flow velocities of various arterial segments, and their age dependency, are shown in Tables 10.2 and 10.3 . Normal blood flow velocity values in adults show little variation among different investigators. The highest velocities are almost always found in the MCA or the ACA. The PCAs and BAs have lower Doppler frequency shifts than the MCA in normal patients. This pattern has not been observed in cerebral blood flow studies where volume blood flow is measured in cubic centimeters per second. Two explanations have been offered for this discrepancy between velocity and volume flow: (1) the measurement sites may be different or (2) more likely, different velocities occur as a compensatory mechanism to keep volume flow constant in arteries of different sizes. Thus velocities are slower in large vessels and faster in small ones. Normal angle-corrected blood flow velocity values using TCCS have likewise been established and are only slightly higher than those obtained with TCD. The TCD documentation of decreasing flow velocities with increasing age correlates well with age-related changes in cerebral blood flow and underlines the validity and accuracy of TCD and TCCS for estimating cerebral blood flow.

Functional reserve testing

TCD is an ideal functional test for detecting rapid changes in cerebral perfusion because the technique provides excellent resolution of blood flow velocity changes occurring over time. Functional tests are predominantly aimed at the evaluation of the reserve mechanism of the cerebral vasculature, using various stimuli such as hypocapnia or hypercapnia, shift in the pH by acetazolamide, increased or reduced systemic arterial pressure, and hypoxia. The CO ₂ (carbon dioxide) dilatory effect is mainly restricted to the peripheral arterial vascular bed, particularly the small cortical vessels. With changing CO ₂ concentrations, the relationship between flow velocity and volume flow within a large cerebral artery is linear, provided that the CO ₂ level does not directly affect the diameter of the large proximal arterial segment. Velocities measured from the MCA with changing CO ₂ concentrations show a biasymptotic, S-shaped curve ( Fig. 10.11 ).

Vasomotor reactivity in 40 normal individuals (ages 20 to 75 tears). Blood flow velocity changes are shown during CO ₂ -induced hypercapnia ( upper curve ) and hypocapnia ( lower curve ). The average change was 87.8% (52.5% and 35.3% hypercapnia and hypocapnia, respectively).

(From Ringelstein EB, Sievers C, Ecker S, et al. Noninvasive assessment of CO ₂ -induced cerebral vasomotor response in normal individuals and patients with internal carotid artery occlusions. Stroke . 1988;19:964. Copyright © American Heart Association.)

A “preserved” vasomotor reserve implies that a drop in perfusion pressure can be counterbalanced by vasodilatation of cortical arterioles to maintain sufficient cortical blood supply. The vasomotor reserve may become exhausted if the resistance vessels in brain areas with low perfusion pressure distal to a high-grade stenosis are already maximally dilated. In this state, the resistance vessels are refractory to any further vasodilatory stimuli, and hypercapnia cannot increase blood flow. This condition may be critical because ischemic brain injury can occur if the perfusion pressure is further reduced for any reason. Measurements of the vasomotor reserve capacity are useful in evaluating the hemodynamic impact of extracranial occlusive carotid disease or of a high-grade proximal middle cerebral artery stenosis.

The pulsatility index, as defined by Gosling (see Chapter 3 ), reflects the resistance in the peripheral vascular bed and has been suggested as a sensitive index of diastolic runoff, that is, with increased peripheral vasodilatation, diastolic runoff is expected to increase and the pulsatility index to decrease. However, in a large series of patients with carotid artery occlusion, the pulsatility index appeared to be much poorer for predicting the intracranial hemodynamic situation than the vasomotor reserve capacity.

Definition of stenosis with transcranial Doppler

The following are typical TCD features of circumscribed stenosis of a large basal cerebral artery ( Fig. 10.12 ): (1) increased flow velocity; (2) disturbed flow (spectral broadening and enhanced systolic and low-frequency components); and (3) covibration phenomena (vibration of vessel wall and surrounding soft tissue). It is unclear whether the PSV (>120 to 160 cm/s) or the mean systolic velocity (>80 to 120 cm/s) should be used as a threshold value. When detecting intracranial stenoses with a diameter of 50% or more with a mean velocity value of 100 cm/s, a sensitivity of 100%, a specificity of 97.9%, as well as positive and negative predictive values of 88.8% and 94.9% were reported. For the vertebrobasilar system, a threshold of more than 2 kHz peak-systolic Doppler shift showed a sensitivity of 80% and a specificity of 97% in detecting stenoses of 50% or more. Most authors agree that, in comparison with the contralateral vessel segment, a relative increase in PSV of more than 30% is suspicious for hemodynamically significant stenosis and a relative increase of more than 50% indicates a definite intracranial artery stenosis.

Middle cerebral artery stenosis and associated transcranial Doppler changes: ( 1 ) normal proximal flow; ( 2 ) increased systolic and diastolic peak velocity and spectral broadening (turbulent flow) at the center of the stenosis; ( 3 ) distal turbulent flow.

Definition of occlusion with transcranial Doppler

Basal cerebral artery occlusion can be detected by three observations: (1) the absence of arterial signals at an expected depth; (2) the presence of signals in vessels that communicate with the occluded artery; and (3) altered flow in communicating vessels, indicating collateralization. For example, occlusion of the MCA is diagnosed from the lack of an MCA signal in the presence of Doppler signals from other vessels (i.e., the PCA, the ACA, or the distal CS). This combination of findings also confirms that the temporal window is satisfactory. TCD has shown a sensitivity of 83% and a specificity of 94.4%, with an overall accuracy of 91.6%, in the detection of intracranial vessel occlusion. Using coronary angiography-based Thrombolysis in Myocardial Infarction (TIMI) criteria as a model, Demchuk and colleagues have proposed Thrombolysis in Brain Ischemia (TIBI) criteria for the TCD-based classification of the MCA status during and after thrombolysis. The TIBI scale, ranging from 0 (MCA occlusion) to 5 (normal MCA), is given in Table 10.4 . The TIBI criteria were found to be accurate in the prediction of the clinical outcome in acute stroke patients undergoing thrombolytic therapy.

TABLE 10.4

Thrombolysis in Brain Ischemia Criteria for Transcranial Doppler Monitoring of the Middle Cerebral Artery Recanalization During and After Thrombolytic Therapy.

TIBI Score	Status of the MCA Flow	TCD Criteria
0	Occlusion	• No flow signal
1	Near occlusion or minimal residual flow	• Early systolic low-flow signal • No diastolic flow signal
2	Strongly reduced	• Reduced systolic and diastolic velocity • Flattened early systolic increment • Pulsatility index <1.2
3	Moderately reduced	• Normal systolic increment • Pulsatility index >1.2 • Relative reduction of blood flow velocity of >30% as compared with the contralateral side
4	Stenotic signal	• Mean blood flow velocity >80 cm/s or relative increase of velocity >30% as compared with the contralateral side • Detection of turbulent flow
5	Normal signal	• Side-to-side difference of blood flow velocity <30% • Comparable values of pulsatility index

 

MCA , Middle cerebral artery; TCD , transcranial Doppler; TIBI , Thrombolysis in Brain Ischemia.

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