Cerebrovascular Reactivity - Transcranial Doppler (CVR)

What is Cerebrovascular Reactivity (CVR) test?

Cerebrovascular Reactivity (CVR) test is a specialized Transcranial Doppler (TCD) ultrasound test. It helps to assess how effectively your brain regulates blood flow when it requires more oxygen.

Assess conditions affecting blood flow in the brain, such as Moyamoya disease.

How should I prepare for CVR?

• Please let your doctor know prior to the test date if you have any breathing conditions, such as asthma or chronic obstructive pulmonary disease (COPD).
• Not required to fast
• Continue with your regular diet and medications
• If you feel unwell, please inform the lab prior to the test

How is CVR performed?

The test is done by a sonographer and takes about 45 minutes to 1 hour.

• The test is performed while you are lying on a bed.
• A clear, water-soluble gel is applied to two small ultrasound probes to help transmit sound waves.
• The probes are placed at your temples and secured with a soft, padded headframe.
• You may hear a continuous “whooshing” sound, which represents blood flow in your brain arteries.
• You will be asked to:
  • Hold your breath for about 30 seconds
  • Breathe in a small amount of carbon dioxide (CO₂) gas through a mask for a short period. This is carefully monitored throughout the test.
  • Breathe in and out more rapidly (in ambient air) for a short period
• Once the test is complete, the headframe is removed, and the gel is cleaned off.
• A report will be sent to your doctor, and he/she will discuss the results with you at your next appointment.

Frequently Asked Questions about CVR

Is the test safe?

• CO₂ inhalation is widely used in CVR reactivity studies^1,2. It is safe with mild temporary side effects, sometimes experienced; this includes shortness of breath or dizziness.   
• The test is radiation-free
• A sonographer will be present throughout the process. If you feel unwell, please inform the sonographer.

¹Willie C. K., Tzeng Y., Fisher J. A., Ainslie P. N. (2014). Integrative regulation of human brain blood flow. J. Physiol. 592 841–859. 10.1113/jphysiol.2013.268953

²Willie C., Colino F., Bailey D., Tzeng Y., Binsted G., Jones L., et al. (2011). Utility of transcranial doppler ultrasound for the integrative assessment of cerebrovascular function. J. Neurosci. Methods 196 221–237. 10.1016/j.jneumeth.2011.01.011

Have questions or concerns? Please contact the neurodiagnostic laboratory managing your test.

Are there any limitations to the test?

In some patients, the ultrasound signal cannot pass effectively through the skull at the temple area due to thicker bone (this is known as a limited or absent temporal window). When this occurs, it may not be possible to fully assess the brain arteries, and in some cases the test cannot be done. 

This is more common in older adults and may occur more frequently in Asian women due to natural variations in skull thickness. If this occurs, your doctor will advise on the next appropriate steps. In such cases, there will be no charge for the CVR test.

Can I use Medisave to pay for this test?

This test may be claimable under:

• Flexi-MediSave for Seniors; or
• Outpatient scans for diagnosis or treatment of a medical condition. 

Please refer to the Ministry of Health's Outpatient Care page for the latest claim limits and eligibility criteria.

The information is correct as of July 2026 and subject to revisions without notice.