Transcranial Doppler (TCD)

Transcranial Doppler — windows, normal velocities, criteria for stenosis and vasospasm, the Lindegaard ratio, collateral patterns, and sickle-cell screening.

TCD Diagnostic Values

Degree of MCA vasospasm

Degree of basilar vasospasm

Transcranial Doppler (TCD)

Equation

• Reflector speed = (Doppler shift × propagation speed) / (2 × incident frequency × cosθ) = (770 × new frequency) / (source frequency × cosθ).
• Propagation speed = 1540 m/s for soft tissue.
• θ = angle of insonation; the larger the angle, the greater the error in measurement. As the angle of insonation increases, waveforms become blunted and ultimately disappear at 90°.

Factors That Affect TCD Velocities

• Age & sex: TCD velocities tend to get slower with age; small difference between premenopausal women and men.
• Viscosity & hematocrit: inversely related to velocity. A drop of 10% of hematocrit will increase velocity by 20%.
• Blood pressure: despite autoregulation, increased BP increases TCD velocities.
• CO2 pressure.

TCD Machines

• Non-duplex and duplex (transcranial color-coded duplex, TCCD).
• TCD shows the anatomy in B-mode besides the spectral Doppler. It also allows measurement of the angle of insonation to correct flow velocities.
• The probe is 2 MHz; higher-frequency usual US probes can’t penetrate the skull.

TCD Windows

• Transtemporal.
• Transorbital.
• Suboccipital.
• Submandibular.

Clinical Use

• SAH for vasospasm (see table below).
• Collateral flow.
• Vasodilatory reserve.
• Micro-emboli detection: depends on backscatter seen with emboli (gaseous > solid). Used in patients with carotid stenosis, MI, atrial fibrillation & mechanical valves. CARESS trial (Clopidogrel and Aspirin for Reduction of Emboli in Symptomatic Carotid Stenosis): repeating TCD at day 7 after treatment showed reduction of microemboli with dual antiplatelets.
• Cerebral circulatory arrest: make sure SBP > 70 during the procedure. With increased ICP, initially waves are more spiked, then the diastolic component is lost, then flow reversal in diastole. The retrograde oscillatory diastolic flow along with systolic spikes are characteristic for circulatory arrest. Sensitivity 96%, specificity 100%.
• Research for: cerebral autoregulation, neurovascular coupling (functional hyperemia), intraoperative TCD.

Sickle Cell Disease

• Hemolysis causes low hemoglobin and increased velocities. Sickle cells also irritate arteries and cause vaso-occlusion. Increased Vmean > 200 is associated with increased risk for ischemic stroke.
• Vmean > 200 will benefit from transfusion to lower HbS < 30% (STOP trial).
• STOP trial: children with increased Vmean > 200 allocated to periodic transfusion to lower HbS < 30% had less risk of stroke (92% risk reduction). Over an 8-year period, a 5-fold decrease in the rate of stroke.
• Technique: use TAMV (time-averaged mean velocity), not PSV. To be adequate, both MCA and BIF (bifurcation) have to be captured.
• Screening categories:
  • Normal: TAMV < 170 in all segments — repeat annually until age 16.
  • Conditional: TAMV 170–200 — repeat in 2 weeks to 2 months.
  • Abnormal: TAMV > 200 — either transfuse or repeat in 2 weeks.
  • If inadequate: repeat the study; if still inadequate, you may get MRI/MRA.
• STOP II trial: tried to see if TCDs could be stopped after 30 months of transfusions; stopped early due to increased incidence of stroke in the group that stopped TCD.
• NHLBI TCD recommendations 2014: screening typically starts at age 2 and continues until age 16. Refer to a transfusion expert if TCD is abnormal (> 200) or conditional (170–200).

Parameters

• Peak systolic velocity and EDV: EDV is typically 25–50% of PSV.
• Mean velocity: MV = (PV + 3·EDV) / 3 = EDV + (PV − EDV)/3. MV is the average of the edge frequency over the cardiac cycle and varies between laboratories.
  • Wake Forest normal ranges: MCA 40–80, ACA 35–60, ICA 40–70, PCA 30–55, BA 25–60, VA 25–50.
  • MCA vasospasm: mild 80–120, moderate 120–200, severe > 200.
• Lindegaard ratio (LR): ratio between Vmean of MCA and ICA. Used to differentiate hyperemia from vasospasm; hyperemia will increase Vmean in both MCA and ICA. LR < 3 is normal or mild spasm, 3–6 is moderate spasm, > 6 is severe spasm. LR > 6 indicates preferential increased MCA > ICA.
• Pulsatility Index (PI): PI = (PV − EDV) / mean. Normal values 0.6–1.2 (except in OA it is > 1.2). TCD can only measure proximal vessel velocities, not distal vessels. Increased PI indicates increased distal resistance. A rounded waveform has low PI; a peaked waveform has high PI. An isolated artery with increased PI can be focal stenosis — compare with baseline PI to determine if increased from baseline. A diffuse increase in PI may indicate diffuse spasm or increased ICP.
• Resistive Index (RI): RI = (PV − EDV) / peak. A measure for downstream resistance. Normal value < 0.75.
• Flow Acceleration (FA): FA = (PV − EDV) / Δt. A measure for upstream systolic flow. Low FA indicates increased upstream resistance, as in severe proximal ICA stenosis or aortic stenosis.

Conditions — Cerebral Artery Stenosis

• Mild stenosis: ↑ PSV — no change in Doppler pattern.
• Moderate stenosis: ↑ PSV; spectral broadening; ↑ EDV; post-stenotic depression of PSV; turbulent flow.

Abnormal Waveforms

• Absent: no flow detectable.
• Minimal signal: flow signal present but no end-diastolic flow; PI > 1.2.
• Blunted signal: delayed acceleration; step-wise maximum velocity arrival in mid-late systole; focal ↓ MFV; PI < 1.2.
• Dampened signal: turbulent flow; normal flow acceleration; ↓ MFV; any PI.

Collateral Patterns

• ACA reversal: reversed flow in the ipsilateral A1; ↑ velocity in the contralateral ACA; contralateral ACA velocity > MCA by at least 25%.
• PCOM reversal (blood from PCA through PCOM to MCA and ACA): ↑ PCA velocity; PCA > MCA.
• ECA to ICA collaterals: reversed ophthalmic; ↓ PI in ophthalmic (internalization of OA).
• Signs of ICA stenosis — any one of the following has 95% sensitivity for 70% ICA stenosis:
  • Reversed flow in the ipsilateral OA.
  • Reversed flow in the ipsilateral ACA.
  • ↑ velocity > 80 in the contralateral ACA.
  • No Doppler signal in the ipsilateral OA or bifurcation.

Predictors of Vasospasm in SAH

• Rise in velocity > 20 cm/s/day between days 3–7.
• > 25% velocity rise per day.

Criteria for Normal TCD

Ahmed Koriesh, MD