Transcranial Doppler (TCD) ultrasound is a non-invasive tool used to assess and evaluate intracranial blood flow. While most forms of ultrasonography deliver images of the tissue studied, TCD delivers audible sounds that can be heard, recorded, and examined. It has many uses including the evaluation of extra- and intracranial stenosis, vasospasm, cardiac shunts (PFO), cerebral circulatory arrest, autoregulation, vasomotor reactivity, sickle cell disease, surgical monitoring, emboli monitoring, and more.1
Similar to other ultrasound-based tests, TCD uses sound waves to evaluate structures inside the body, in this case the blood flow in the brain. However, the brain is protected by the skull and ultrasound waves cannot easily pass-through bone. Age, gender, and race are all contributing factors to varying skull thickness. TCD testing requires the operator to be able to find the transtemporal window (thinnest part of the bone in the skull) that will allow the ultrasound waves to penetrate the skull and find the blood vessels that need to be evaluated. Unique to this form of ultrasound, finding the window is one of the most difficult parts of the exam and results in a high learning curve compared to other ultrasound tests. Newer robotic TCD systems are designed to autonomously find the optimal temporal window.
The two main features of a TCD ultrasound system that provide important flow characteristic information are the M-mode and the spectral waveform.
Motion Mode (M-mode)
M-mode uses Doppler, or power Doppler in the case of power M-Mode, to visually display the detection of motion, in this case blood flow. M-mode displays a ‘map’ of the blood flow using red and blue colors to denote flow moving toward or away from the ultrasound transducer across 64mm of depth. The brightness of the M-mode signal corresponds to the amplitude of the signal. A brighter signal means more red blood cells are being reflected. Evaluation of any segment within the ‘map’ is possible by moving the depth marker, or sample volume. The system will provide flow velocity, pulsatility index, and display the spectral waveform for the specific point where the marker is placed.
The basic TCD waveform is shown with time along the horizontal axis, velocity (in cm/s) along the vertical axis, and amplitude as the signal brightness. The white line tracing the waveform, called the envelope, allows for auto-calculation of diagnostic TCD parameters.
The TCD waveform illustrates detailed information about blood flow dynamics, providing important insight into the entire cerebrovascular system.
Peak Systolic Velocity (PSV)
Corresponds to each tall peak in the spectrogram.
End Diastolic Velocity (EDV)
Corresponds to the point at the end of the cardiac cycle, just prior to the systolic peak.
Mean Flow Velocity (MFV)
Calculated as EDV plus one-third of the difference between PSV and EDV.
The initial slope of the velocity spectrum during systolic acceleration.
Pulsatility Index (PI)
Calculated by subtracting EDV from PSV and dividing the value by MFV. This is the most frequently used TCD parameter to determine flow resistance.
Pulsatility Index (PI) is a key TCD parameter that describes the relative difference between the peak systolic velocity (PSV) and the end diastolic velocity (EDV). This difference provides insight into how cerebral blood flow is being affected by the dynamic upstream and downstream processes.In the brain, the EDV is considered normal when it is approximately 50-60% of the PSV. An elevated PI occurs when the EDV is lower than normal, resulting in a larger difference between the PSV and EDV. Causes of elevated PI include bradycardia, hyperventilation, aortic valve incompetence, increased intracranial pressure, and increased peripheral resistance. A decreased PI occurs when the EDV is higher than normal, resulting in a smaller difference between the PSV and EDV. Causes of decreased PI include proximal stenosis, hypercapnia, arteriovenous malformation, and decreased peripheral resistance.
Systolic upstroke is the initial slope of a waveform during systolic acceleration. A sharp upstroke (70-90 degrees) is considered normal and indicates an absence of significant proximal obstruction. A delayed upstroke, known as a rounded or blunted peak, is considered abnormal and indicates proximal obstruction.
Through the temporal window, TCD evaluates arteries in the Circle of Willis which supply blood flow to the frontal, temporal, and parietal lobes of the brain.