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Transcranial Doppler Ultrasound
What is transcranial Doppler ultrasound?
Transcranial Doppler (TCD) ultrasound is a non-invasive method to estimate
the blood flow velocities in the large intracranial vessels of the circle of
Willis. Using established TCD techniques, sections of the internal carotid
artery (ICA), middle cerebral artery (MCA), anterior carotid artery (ACA),
posterior cerebellar artery (PCA) and the basilar and periorbital arteries
can be examined. TCD typically uses a 2 MHz pulse ultrasound which produces
a velocity spectrum throughout the cardiac cycle.
TCD was used to evaluate patients with other forms of cerebrovascular
disease in the mid-1980s and was adapted to sickle cell disease (SCD)
primarily by Medical College of Georgia researchers. This method for
detecting high-risk states is painless, relatively inexpensive, and can
usually be accomplished in 30-45 minutes.
Adults without SCD typically have time-averaged maximum mean (TAMM)
velocities in the MCA of 60 + 12 cm/sec, while children have MCA velocities
closer to 90 cm/sec. SCD, with its significant anemia, causes an increase in
cerebral blood flow velocity in the major arteries and is associated
velocities of about 130-140 cm/sec, even in the absence of vessel narrowing.
A narrowed vessel increases velocity in inverse proportion to the reduction
in the area of the vessel.
How is a TCD performed? What kind of TCD machine is
used for children with sickle cell disease?
The STOP study used a dedicated Doppler which produced the velocity spectrum
without any visual reference. The MCA is located, then, moving in 2 mm
increments, the vessel is insonated and the most representative velocity
spectrum is found by adjusting the probe's depth and angle. Depth and flow
direction assist the technician in identifying the vessels.
The same technique is used for transcranial Doppler Imaging (TCDI), which
uses imaging of the vessels and may have a shorter learning curve for
technicians. Three correlation studies indicate that TCDI provides similar
although slightly lower velocities and can readily be used for screening
children with SCD.
Why should TCD be done in children with sickle cell disease?
TCD is the only widely tested and validated method for stroke prevention in
SCD. It identifies children at elevated risk for stroke so that
consideration for prophylactic transfusion can be made. It may provide
effective surveillance after cessation of transfusion but that is currently
under study in STOP II.
TCD is recommended for children with SCD who are 2-16 years old and who have
not had a stroke. Confirmed (two separate studies) velocities over 200
cm/sec in the MCA or dICA are associated with a significant risk of stroke
unless transfusion is instituted. The risks and benefits of accepting
transfusion programs can be obtained from the STOP study data. At present
there is no role for TCD after a child has had a stroke because chronic
transfusion would already be recommended to prevent subsequent strokes.
What is a 'normal' TCD? What is an 'abnormal' one?
view movie of a normal TCD...
view movie of an abnormal TCD...
Readers of routine clinical TCD exams do not attempt to obtain precise
velocities. In the STOP study, however, an absolute cut-point, measured in
time-averaged maximum mean (TAMM) velocity, was used. Based on this velocity
number, the child's risk is categorized into one of the four exclusive
categories:
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Normal. A TCD with TAMM velocities below 170 cm/sec on both sides is associated with a low risk of stroke. However, velocities may not remain high after vessel occlusion and/or stroke has occurred, which makes interpretation of low velocities problematic.
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Conditional. Intermediate-high velocities (170-199 cm/sec) may represent increased stroke risk, depending on which arteries are involved.
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Abnormal. At least one side of the exam has evidence of high velocity (> 200 cm/sec). In STOP, a second abnormal TCD exam was required to identify patients with sustained high intracranial velocities.
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Inadequate. Inadequate TCDs cannot be interpreted, because of improper adjustments of the TCD machine, a poor ultrasound window, movement of the child, or other reasons.
How often should a TCD be repeated?
More research is needed to determine exactly when to start, how often to repeat
TCD, and for how long. The NHLBI Clinical Alert recommended that TCD be
performed every 6-12 months during early childhood when the risk for stroke is
highest.
Can other tests substitute for TCD?
Currently, only TCD has been demonstrated to predict stroke well enough to be
used for primary prevention. With further research, other tests providing
information about the brain arteries, such as angiography, may be found to
provide similar information.
Where are TCD being performed?
Many vascular ultrasound labs are capable of performing TCD. The Medical College
of Georgia and others have conducted courses in performing and interpreting TCD
in SCD.
In addition, the Intersocietal
Commission for the Accreditation of Vascular Labs (ICAVL) is an independent certifying agency that
provides and ensures quality standards for vascular laboratories. Certified labs
which perform TCD are identified in the ICAVL Website as having the capability
of doing intracranial exams.
Click here to view a list of TCD Exam Locations in
North America.
Where can TCD training be obtained?
The Medical College of Georgia's Division of Continuing Education sponsors
courses on the use and interpretation of TCD in sickle cell disease. For more
information, call 706-721-3967 or 800-221-6437 or email
lhord@mcg.edu.
Special arrangements for TCD training can be pursued through
Dr.
Fenwick Nichols, Director of Neurosonology at the Medical College of
Georgia. He is a collaborating investigator in the STOP and STOP II studies in
charge of TCD training and examiner certification.
Save these Dates! July 13-15, 2006
Presented by
The Medical College of Georgia:
Using TCD and MR
to STOP Stroke
in Sickle Cell Disease
Presented at Kiawah Island, South Carolina
This activity has been approved for AMA PRA credit.
For more information,
contact:
Division of Continuing Education, Medical College of Georgia
1120 15th Street
Augusta, GA 30912
Phone: 800-221-6437 or 706-721-3967
Beginning January 9, 2006, access the brochure on line at
http://www.mcg.edu/ce/med.html