Transcranial doppler ultrasound in neurocritical care

Overview

In 1982, Aaslid et al authored a paper with the title “Noninvasive transcranial Doppler ultrasound recording of flow velocity in basal cerebral arteries,” describing the successful insonation and blood flow velocity (FV) measurement of the basal cerebral arteries with a range-gated Doppler transducer. These authors located an “ultrasonic window” above the zygomatic arch and 1 to 5 cm anterior to the ear, through which a 2-MHz ultrasonic pulse could be emitted and recorded. The velocity and direction of blood flow were recorded as a spectral display as recorded by the ultrasound transducer. Measurement of FV as well as direction of flow during unilateral common carotid artery (CCA) compression enabled Aaslid et al to describe collateral flow as well as “steal” dynamics in real time by using transcranial Doppler (TCD). They noted that compression of the CCA resulted in decreased velocity in the ipsilateral middle cerebral artery (MCA), reversal of flow or “steal” in the ipsilateral terminal internal carotid artery, as well as increase in velocity in both the ipsilateral anterior and posterior cerebral arteries, suggesting a contribution to collateral flow through the circle of Willis.

Basic principles

TCD technology is based upon the Doppler effect principle, in which the ultrasound transducer emits a frequency, fo , and this frequency is reflected back to the probe as fe . The difference between the emitted and received frequencies, or the Doppler shift, fd , can be calculated as fd = fe − fo (see Chapter 1 ). Pulsed wave Doppler refers to an ultrasound transducer that emits and receives the reflected ultrasound pulse. By using a pulsed wave Doppler, TCD can be performed at variable depths to follow the course of cerebral blood vessels. The frequency refers to the number of cycles a sound wave goes through per second. A higher frequency is used to insonate more peripheral vessels, and a lower frequency is able to insonate deep cerebral vessels. The sample volume size refers to the width of the area being insonated and is measured in millimeters (e.g., a sample volume size of 2 mm will give a more precisely localized signal compared with a sample volume size of 6 mm). The intensity or power of the ultrasound wave refers to the energy emitted through the tissue being insonated. This energy is absorbed by the tissue and converted mainly to heat. The U.S. Food and Drug Administration (FDA) regulates the amount of energy able to be transmitted by ultrasound equipment to ensure patient safety (ALARA [ A s L ow A s R easonably A chievable] principle). Ultrasound over the eye or under the chin must be used at a lower power because these locations are not covered by bone, thus exposed to greater intensity. Attenuation refers to the decrease in intensity as the ultrasound wave passes through tissue and is higher for muscle and bone and lower for fluid-filled vessels. Based on attenuation, the reflected wave will be weaker for deeper vessels.

Acoustic windows

Acoustic windows are naturally occurring areas of cerebral bone thin enough to allow transmission of ultrasound waves. There are three commonly used acoustic windows: transtemporal, transorbital, and transforaminal. Up to 10% of people may not have adequate acoustic windows. The transtemporal window allows insonation of the anterior, middle, and posterior cerebral arteries; the transorbital window is used to insonate the ophthalmic artery as well as the cavernous portion of the internal carotid artery; and the transforaminal window allows insonation of the vertebral and basilar arteries ( Figure 2-1 ).

Figure 2-1, Graphic representation of commonly used windows for insonation.

Transcranial doppler interpretation

Evaluation of cerebral hemodynamics with TCD is quick to perform, noninvasive, and relatively inexpensive compared with computed tomography, digital subtraction angiography, and magnetic resonance imaging. Parameters including waveform morphology, pulsatility index, direction of flow, and turbulence allow the interpreting physician to make inferences regarding clinically significant vascular characteristics, including stenosis, vasospasm, intracranial cerebrovascular resistance, cerebrovascular autoregulation, proximal and/or distal vessel occlusion, and presence of microemboli.

Waveform morphology

The waveform recorded by TCD reflects both systole and diastole, with systole represented by the upstroke and peak of the wave, and diastole represented by the decelerating downslope of the wave ( Figure 2-2 ). The morphology of the waveform demonstrates valuable information regarding cerebral blood flow, with a normal systolic upstroke being a quick upstroke climaxing into a peak. An upstroke that is slow and dull could be representing a proximal obstruction or focal stenosis when seen in a single vessel or may be an indication of a global low-flow state resulting from cardiac dysfunction when a widespread finding. Hassler et al described TCD waveform changes in the setting of intracranial hypertension. As diastolic pressure rises to approach the intracranial pressure (ICP), end-diastolic flow decreases and results in three stages of waveform changes: initially a decrease, followed by cessation, and lastly a reversal of flow ( Figure 2-3 ). This reversal is seen in severe intracranial hypertension near cerebral circulatory arrest (see Chapter 4 ), when the diastolic pressure rises higher than the ICP and has been coined “diastolic flow obliteration.”