How do you perform daily transcranial Doppler checks to obtain the ICA (Internal Carotid Artery) to MCA (Middle Cerebral Artery) ratio?

Technique for Daily Transcranial Doppler to Obtain ICA to MCA Ratio

To properly obtain the ICA to MCA ratio using transcranial Doppler (TCD), you must measure flow velocities in both vessels using standardized techniques, with the ratio calculation serving as a critical indicator to differentiate between vasospasm and hyperemia.

Equipment and Setup

• Use a TCD device with 2-4 MHz ultrasound probe
• Position patient in supine position with head slightly elevated
• Ensure patient is in a quiescent state (no movement or breathing activity)

Step-by-Step Technique

Step 1: Extracranial ICA Measurement

1. Position the probe submandibularly to access the extracranial ICA
2. Use color Doppler to identify the vessel
3. Sample the ICA at least 2 cm distal to the carotid bifurcation
4. Obtain flow velocity measurements:
   • Record mean flow velocity (MFV)
   • Record pulsatility index (PI)
   • Ensure angle of insonation is as close to zero as possible

Step 2: MCA Measurement

1. Use the transtemporal window approach (above the zygomatic arch)
2. Obtain an axial section at the level of the sphenoid bones 1
3. Image the circle of Willis with color Doppler
4. Select the MCA closest to the transducer
5. Visualize the entire length of the MCA
6. Zoom so the MCA occupies >50% of the image
7. Sample the MCA close to its origin from the internal carotid artery 1
8. Maintain angle between blood flow direction and ultrasound beam as close to zero as possible 1
9. Record at least 3 measurements of peak systolic velocity (PSV) and use the highest value 1

Step 3: Calculate the ICA/MCA Ratio (Lindegaard Ratio)

1. Divide the MCA mean flow velocity by the extracranial ICA mean flow velocity
2. Record this ratio daily for monitoring

Interpretation of Results

• Normal Lindegaard ratio: <3
• Mild vasospasm: 3-6
• Moderate vasospasm: 6-9
• Severe vasospasm: >9 2
• Ratio <0.5 strongly predicts MCA occlusion 3

Important Technical Considerations

• Ensure measurements are taken during periods of fetal/patient rest 1
• Repeat measurements at least 3 times and use the highest value 1
• Maintain consistent probe positioning for serial measurements
• Document the depth of insonation for reproducibility
• The procedure typically takes 5-10 minutes 1
• Avoid angle correction when possible, but if necessary due to patient positioning, apply it consistently 1

Common Pitfalls to Avoid

• Inadequate acoustic window (approximately 10-20% of patients have poor temporal windows)
• Misidentification of vessels (verify with anatomical landmarks)
• Inconsistent angle of insonation between measurements
• Failure to account for patient movement or breathing
• Not measuring at the same depth in serial examinations
• Confusing hyperemia with vasospasm (the ICA/MCA ratio helps differentiate) 2

The MCA/ICA ratio correlates well with cerebral blood flow distribution as verified by SPECT studies 2, making it a reliable indicator for distinguishing between vasospasm and hyperemia in patients with elevated TCD velocities.