Transcranial Doppler (TCD) is a non invasive ultrasound technology used to evaluate blood flow velocity in the major basal intracranial arteries. TCD may be used on its own or in conjunction with other imaging modalities. A thorough neurovascular exam should include TCD, as carotid obstructions seen with duplex imaging may have significant intracranial effects that should be considered in patient management decisions.
Basics of the TCD Examination
Basic Concepts
The basic technique for transcranial Doppler examination consists
of insonating the basal portions of the major cerebral arteries (around the circle of Willis) through different “windows” found at various locations in the skull. The smallness and specific localization of these “windows” has resulted in a restricted variability of the angle of insonation found upon sampling the cerebral blood vessels. This close spatial relationship between the position of the artery and the “window” which allows its insonation has also been of considerable importance for the proper identification of the different vessels based on relatively specific criteria. These standard criteria of identification result in highly reproducible findings and comparable inter-laboratory results. They include the following parameters:
- The window through which the vessel is being insonated.
- The angle of the transducer during insonation.
- The depth of the sample volume position.
- The direction of the flow with respect to the transducer and the spectral distribution.
- The response of the signal to vibration or compression maneuvers of proximal feeding vessels.
The windows that are commonly used for transcranial Doppler examination include areas where the skull bones are relatively thin or where the naturally occurring forage allow proper penetration of the ultrasound beam. The need for adequate ultrasonic window utilization cannot be overemphasized since recent intraoperative recordings have documented that, in the best of cases, only approximately 6% of the intensity of the ultrasound used reaches the brain substance. The windows commonly utilized are:
- TRANSCRANIAL (TRANSTEMPORAL) WINDOW: The vessels of the circle of Willis are insonated through the thinnest portion of the squamous component of the temporal bone.
- THE TRANSORBITAL WINDOW: The carotid siphon and the ophthalmic artery can be directly insonated by having the ultrasound beam cross the contents of the orbit. In some cases, cross-insonation of the contralateral half of the circle of Willis is also possible.
- THE SUB-OCCIPITAL OR TRANSFORAMINAL WINDOW: Insonation of the vertebral and the basilar arteries can be accomplished by taking advantage of the space between the atlas and the base of the skull, as well as passing the ultrasound beam through the foramen magnum.
“Static” Transcranial Doppler Examination
A comprehensive approach to the concepts which were outlined above results in the incorporation of different technical strategies and multiple sampling to obtain a large body of information. The spatial and temporal correlation of the findings from all the vessels studied will then provide an overall picture of the hemodynamic status of the major intracranial blood vessels. The vessels that can be examined through the various windows and the most common finds obtained after their evaluation will serve as the basis for our comparison when we later describe the dynamic variations of transcranial Doppler examination techniques.
The Transtemporal Window
The insonation of the basal cerebral arteries and the circle of Willis itself was made possible by having the ultrasonic beam cross the thinnest portion of the squamous part of the temporal bone. Since the passage of ultrasound through the bone will depend upon the thickness of this, a certain degree of variability exists from patient to patient and is relative to age. In children and young adults it is particularly easy to obtain good signals from almost any of the vessels desired. On the other hand, it is not unusual in certain elderly individuals to barely be able to obtain adequate signals. Harvey (personal communication) has seen this difficulty in up to 20% of the patients studied. Our experience, however, is approximately 5-10%. Racial and gender differences have also been noted, with black women apparently having the thickest temporal bones and hence the smallest windows. As it has been previously described, the temporal window can be localized quite anteriorly (close to the vertical portion of the zygomatic bone) or, more frequently, posteriorly (close to the pinna of the ear). The vessels that can be examined through this window include the terminal portion of the internal carotid artery, its bifurcation into the middle and the anterior cerebral arteries, and the posterior cerebral artery. The communicating arteries (either anterior or posterior) can be at times sampled, but they are more dependent upon the hemodynamic variability of the circle of Willis.
The Transorbital Window
The use of this approach requires that the power output of the Doppler instrument is decreased (to 10-20%) to reduce the ultrasonic exposure of the eye. This “window” allows the direct insonation of the carotid siphon ipsilateral to the orbit selected and of one of its collateral, the Ophthalmic artery. The importance of the utilization of this approach for the identification of extracranial and intracranial collateral circulation through the latter vessel cannot be underscored. Additionally, in cases where it has been impossible to insonate the middle and anterior cerebral arteries through the transtemporal window, it is also possible to use the transorbital approach to sample these vessels contralaterally. The technique for the performance of this cross examination has been described previously and it is based upon repositioning the transducer in a rather lateral fashion, directing the beam of ultrasound medially and changing from sampling volume depth to 80mm.
The Suboccipital (Transforaminal) Window
The ability to introduce a beam of ultrasound in between the atlas and the base of the skull, and through the foramen magnum allows the sampling of the vertebral arteries (segments V3 and V4) as well as the basilar artery, from its origin often up to the level of its bifurcation into the posterior cerebral arteries. In addition to this, it is also possible to sample some of the other branches of the posterior circulation (e.g. posterior inferior cerebellar artery).
Examination Guide
Transcranial spectral analysis is a non-invasive technique by which the basal cerebral arteries may be investigated to detect abnormal flow velocity (stenosis, vasospasm, etc.).
The following items will be used in the examination:
- Examining Table/Bed
- Acoustic Coupling Gel
- Neurovision™ 500M Transcranial Doppler System with 2 MHz Pulsed Doppler
- Hard Copy Device (Printer, Video Recorder, etc.)
Temporal Approach
The patient is placed on the examining table/bed in a supine position and acoustic coupling gel is applied to the temporal area. The pulsed Doppler is positioned at approximately 55mm depth, placed on the temporal area and moved over the area until the middle cerebral artery is located (anterior, posterior, or medial windows). Once an adequate signal is obtained at that depth, the pulser and receiver power is adjusted to optimize visual display. The pulsed Doppler sample is then moved shallower in 1 mm depth increments until the middle cerebral artery can no longer be followed. This permits confirmation of actually being in the middle cerebral artery. Common carotid compression by a physician on the ipsilateral side is sometimes performed to ascertain correct anatomical positioning and/or autoregulatory response. Mean flow velocity is determined at each site and a hard copy produced. The Doppler sample volume is then returned to the 55mm depth and gradually increased until a bidirectional display (mirror-like) is located. This is the bifurcation of the middle cerebral and anterior cerebral arteries and is usually found between 60 and 70mm depth. A hard copy picture is also taken of this area. The depth is then slowly increased insonating either the anterior or posterior cerebral arteries. An optimal signal is obtained in both of these vessels, mean flow velocity is determined, and a hard copy produced. Flow in the anterior cerebral artery should be away from the probe and flow in the posterior cerebral artery should be toward the probe. Usually, the anterior cerebral artery anatomical position is slightly superior and anterior, and the posterior cerebral artery is inferior and slightly posterior. These vessels should also be followed to the anterior and posterior communicating arteries to determine their presence. The sample volume is then returned to the bifurcation and the probe shifted interiorly until the terminal internal carotid artery signal is detected. This signal normally has much lower velocity due to the angle of insonation. The Doppler sample volume may then be moved deeper and the probe adjusted for maximum signal strength. Mean flow velocity and hard copy results are also obtained over this vessel.
Orbital Approach
The patient is asked to remove any contact lenses prior to examination and is instructed to close their eyes. Acoustic coupling gel is applied over the eyelid. The probe is then placed over the eye, sample depth set in the 40-50mm range and the ophthalmic artery (low diastolic flow, high resistance vessel) signal obtained and optimized. Power output level should be adjusted to the minimum level for adequate visualization. Mean flow velocity and hard copy of the ophthalmic artery are then taken. The sample volume depth is then moved deeper to 55-70mm where the genu, paracellar, and the superclinoid can be insonated. These vessels are then followed with hard copy results taken and mean flow velocities determined as in the temporal approach.
Occipital Approach
The patient is placed in the prone position (or sitting) and acoustic coupling gel applied to the suboccipital area (sometimes it is necessary to have the patient place their chin upon their chest to open the window slightly). The vertebral arteries are first insonated by moving the probe laterally or medially to a depth of approximately 60-70mm. Flow of these vessels (depending on probe angle) is generally away from the probe. Once these vessels are insonated the bifurcational signal is obtained. The depth is then increased (normally between 80 and 100mm depth) until the basilar artery is insonated. Mean flow velocities and hard copy results are done in 5mm increments as in other vessels. In any areas of significant mean flow velocity changes or abnormalities, hard copy results and mean flow velocities are obtained more frequently.
This completes the examination.
MULTIGON would like to thank Mr. Robert McGrath and the Vascular Department of the University of Tennessee for their contribution to this guide. MULTIGON recommends accredited clinical training courses for all clinicians performing TCD exams.