Transcranial Doppler and Transcranial Color-Coded Duplex Sonography
Transcranial Doppler (TCD) is a safe, noninvasive, bed-side technique for the measurement of cerebral arteries blood flow velocity, and it is commonly used in standard care of neurocritical care patients. TCD-derived indices, including flow velocities and
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Chiara Robba and Andrea Rigamonti
24.1 Basic Principles of Transcranial Doppler Transcranial Doppler (TCD) ultrasonography was first described by Aaslid and collaborators in 1982 [1]. It is a noninvasive technique able to monitor dynamics of cerebral blood flow (CBF) and vessel pulsatility in the basal cerebral arteries, such as middle cerebral artery (MCA), anterior cerebral artery (ACA), and posterior cerebral artery (PCA) (Fig. 24.1). TCD technique is based on the phenomenon described by the physicist Christian Andreas Doppler in the nineteenth century, called Doppler Effect. According to this principle, when a sound wave with a certain frequency strikes a moving object (such as red blood cell inside an insonated artery), it is reflected with a different frequency, the Doppler shift, 𝑓d, which is directly proportional to the velocity of the
C. Robba (*) Neurocritical Care Unit, Addenbrookes Hospital, Cambridge, UK e-mail: [email protected] A. Rigamonti Departments of Anaesthesia and Critical Care, St. Michael’s Hospital, University of Toronto, Toronto, ON, Canada
object (V). Echoes received by the ultrasound (US) probe are processed to produce a spectral waveform with peak systolic velocity and diastolic velocity values (Fig. 24.2). V=
( c ´ fd )
2 ´ f 0 ´ cos q (𝑐 is the speed of the incident wave, 𝑓0 is the incident pulse frequency, and 𝜃 is the angle of the reflector relative to the US probe). The spectral waveform is then processed and combined with indices derived from cerebral blood flow velocity (FV) (such as Gosling’s and spectral pulsatility index) allowing the calculation of secondary indices (including autoregulation, critical closing pressure (CrCP), noninvasive intracranial pressure [ICP]) useful for the analysis of cerebral hemodynamics. The main obstacle to vessel insonation and ultrasound penetration of the skull is the bone. Therefore, TCD is performed through acoustic windows representing specific points of the skull where the bone is thin enough to allow ultrasound waves to penetrate. The equipment used is a duplex US color flow mapper. The probe is either a sector or phased array cardiac or dedicated probe with a small imaging footprint and a Doppler frequency of 1.8 or 2 MHz.
© Springer International Publishing Switzerland 2017 Z.H. Khan (ed.), Challenging Topics in Neuroanesthesia and Neurocritical Care, DOI 10.1007/978-3-319-41445-4_24
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C. Robba and A. Rigamonti
276 PCA
MCA
ACA
ICA
Basilar
Fig. 24.1 Typical transcranial color-coded duplex sonography (TCCS) view of the circle of Willis
Fig. 24.2 Spectral waveform highlighting the peak systolic and the diastolic velocity values
24.2 Acoustic Windows The acoustic windows used to insonate the vessels are transtemporal, suboccipital, transorbital, and retromandibular (Fig. 24.3). The temporal window is the most commonly used for the insonation of the MCA (responsible for the 75% of the brain blood flow and it is the vessel of choice for most of the TCD derived signals), ACA, and PCA (Fig. 24.4). The posterior circula
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