Transcranial doppler: Technique and common findings (Part 1)

Abstract

Transcranial Doppler (TCD) can be aptly called as the doctor's stethoscope of the brain. Since its introduction in 1982, by Rune Aaslid, TCD has evolved as a diagnostic, monitoring, and therapeutic tool. During evaluation of patients with acute ischemic stroke, TCD combined with cervical duplex ultrasonography provides physiological information on the cerebral hemodynamics, which is often complementary to structural imaging. Currently, TCD is the only diagnostic tool that can provide real time information about cerebral hemodynamics and can detect embolization to the cerebral vessels. TCD is a noninvasive, cost-effective, and bedside tool for obtaining information regarding the collateral flow across various branches of the circle of Willis in patients with cerebrovascular disorders. Advanced applications of TCD help in the detection of right-to-left shunts, vasomotor reactivity, diagnosis, and monitoring of vasospasm in subarachnoid hemorrhage and as a supplementary test for confirmation of brain death. This article describes the basic ultrasound physics pertaining to TCD insonation methods, for detecting the flow in intracranial vessels in addition to the normal and abnormal spectral flow patterns.

Keywords: Ischemic stroke; intracranial stenosis; transcranial doppler.

Figure 1

(a) Ultrasound probes. Blue arrow - Transcranial Doppler, Yellow arrow - Transcranial color-coded sonography. (b) (ophthalmic), (c) (suboccipital), (d) (temporal) depict various transcranial acoustic windows and probe directions during TCD

Figure 2

PMD screen following a trans-temporal insonation. Upper panel (a) shows the M-mode appearance of various arteries. The red signal up to 65 mm means that the flow is toward the probe (MCA flow) and from 65 mm to 80 mm, away from the probe (ACA flow). Red color beyond 80 mm suggests flow in the contralateral A1 ACA and the blue signal beyond 92 mm represents contralateral MCA. Lower panel (b) shows a normal TCD spectrum at a depth of 47 mm. Note the rapid systolic acceleration and stepwise deceleration during the diastole and significant end-diastolic velocity

Figure 3

Line diagram showing the major intracranial arteries. Flow directions from various arterial segments and depths of insonation in an average human skull are shown. (Adopted with permission from 18reference 17)

Figure 4

(a) bnormal spectral waveforms. Panel A-Note delayed systolic acceleration (blunted flow). Doppler spectra obtained in a patient with moderate (50%) stenosis of right MCA (b) shows elevated flow velocities-MFV >100. Panel (c) shows the flow spectra obtained in a patient with severe (>70%) stenosis of middle cerebral artery. The white arrow shows the flow turbulence (bruit). Spectra with irregular rhythm and flow velocities (d) are diagnostic of atrial fibrillation. Panel e - shows the characteristic alternating flow signals, suggestive of cerebral circulatory arrest