TCD SCANNING FOR SICKLE CELL DISEASE

Suzanne VERLHAC - 9 février 2010

Objectives
1. To understand principles of imaging TCD
2. To be able to perform TCD assessment
3. To be able to interpret the results of TCD studies for sickle cell disease patients

Background Notes

Sickle cell disease
• Stroke occurs in 11% of young children with sickle cell disease due to an occlusive vasculopathy (intima-media hyperplasia)
• Intima-media hyperplasia favours the distal intracranial ICA and the proximal parts of the MCA and ACA but spares the PCA and basilar arteries.
• The stroke Prevention Trial in Sickle Cell (STOP 1998) established transcranial Doppler velocity criteria for identifying children at risk for stroke that would benefit from blood transfusion.
• Treatment is by blood transfusion which reduces the risk of stroke.

Protocol

1. Equipment : Duplex ultrasound colour flow mapper with TCD imaging transducer. The probe is either a sector or phased array with a small imaging foot print and an imaging frequency range of 1-5 MHz and Doppler frequency of 1.6 or 2 MHz.

2. Imaging landmarks
• Bony: Bright echogenic signal from the lesser sphenoid wing
• Parenchymal : echolucent signal from the heart shaped brainstem
• Both structures should be imaged clearly and indicate the quality of the acoustic window
1. Middle cerebral artery
• Projects anterior to the brainstem
• Colour flow red indicating flow towards the transducer
• Record serial velocity recordings at 2 mm intervals from the ICA bifurcation to the periphery
2. Anterior cerebral artery
• Increase sample volume depth for flow in A1 segment –colour flow blue

3. Posterior cerebral artery
• Visualised as it courses around the brainstem
• P1 segment colour flow red, towards transducer
• P2 segment colour flow blue, away from transducer as it encircles cerebral peduncle

4. Distal internal carotid artery
• Visualise bifurcation (bi-directional flow), angle transducer inferiorly and increase sample volume depth by 5 mm, colour flow red, towards transducer then blue away from transducer.

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Diagnostic criteria From the STOP trials

• Normal – All velocities less than 170 cm/s
• Conditional – A velocity of at least 170 cm/s but less than 200 cm/s
• Abnormal – Velocity at least 200 cm/s in either the internal carotid artery, the middle cerebral artery or the anterior cerebral artery
• Low velocities- MCA velocity less than 50 cm/s
• Inadequate – no imaging window

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Surveillance intervals

TCD since the second year of age
• Normal findings: Re-scan annually
• Conditional findings: Re-scan in 3 months
• Abnormal findings: Transfusion treatment immediately. A monthly transfusion for 3 months, then re-scan.
o If values drop to normal values, transfusions can be stopped, but TCD has to be repeated in 3 months and transfusions reinitiated if velocities return to abnormal or conditional values. Hydroxyurea?
o If values remain abnormal, consider chronic transfusions
• Low velocities: Perform additional imaging (MRA) for confirmation of pathology
• Consider alternative imaging (MRA) for non-diagnostic scans

Example

Example of normal velocities in the left middle cerebral artery

Pitfalls

o A small percentage of children will have limited scans due to attenuation of ultrasound. They are identified by poor or absent visualisation of parenchymal , 2005
o Risk of underestimation of velocity. Operator must optimise transducer/vessel orientation to achieve smallest Doppler angle and thus highest Doppler velocity.

TCD report

The highest velocity of each artery recorded to data sheet.
Classification of the patient into one of 5 groups:
• Normal- time average maximum velocity <170 cm/sec
• Conditional- time average maximum velocity =170cm/s-199cm/s
• Abnormal -time average maximum velocity =200 cm/s
• Low velocities - MCA velocity =50cm/s
• Inadequate - non-diagnostic scan

Bibliography

1. TCD in sickle cell disease: an important and useful test. Adams RJ. Pediatr Radiol. 35: 229-34, 2005.
2. Screening children for sickle cell vasculopathy: guidelines for transcranial Doppler evaluation. Bulas D. Pediatr Radiol 35 : 235-241