Transcranial Doppler (TCD) is a portable device that uses a handheld 2-MHz transducer. It is most commonly used in subarachnoid hemorrhage where cerebral blood flow velocities in major intracranial blood vessels are measured to detect vasospasm in the first 2 to 3 weeks. TCD is used to detect vasospasm in traumatic brain injury and post-tumor resection, measurement of cerebral autoregulation and cerebrovascular reactivity, diagnosis of acute arterial occlusions in stroke, screening for patent foramen ovale and monitoring of emboli. It can be used to detect abnormally high intracranial pressure and for confirmation of total cerebral circulatory arrest in brain death.
Key points
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Transcranial Doppler (TCD) is a bedside procedure that measures linear cerebral blood flow velocity (CBFV) through the intracranial circulation and pulsatility index (PI).
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Several different disease processes can lead to intracerebral vasospasm, for example after subarachnoid hemorrhage, and traumatic brain injury. Intracerebral vasospasm will be represented by abnormally high CBFV.
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The PI’s changes can be used for evaluation of high intracranial pressure (ICP). The PI is the reflection of downstream resistance and will be affected by abnormally high ICP.
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TCD can be used for detection of cerebral vessel occlusion and estimation of cerebrovascular reactivity. Contrast TCD is used for the diagnosis of right to left cardiac shunts, for patients with cryptogenic stroke.
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TCD is the unique standard for the detection of microembolic signals in real-time.
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TCD has high accuracy to confirm total cerebral circulatory arrest and has been used as an ancillary test to support clinical diagnosis of brain death.
Introduction
Transcranial Doppler ultrasonography (TCD) was introduced into the practice of medicine in 1986 and has been used extensively in a variety of inpatient and outpatient settings. TCD ultrasonography uses a handheld 2-MHz transducer that is placed on the surface of the scalp to measure the cerebral blood flow velocity (CBFV) and pulsatility index (PI) within the intracranial arteries. Because of its noninvasiveness and easy applications, TCD examinations have gained an important role in the very early phase, as well during the repetitive assessment of patients with cerebrovascular diseases (CVDs). This has led to a broad application of TCD in outpatients and inpatients, and emergency and intraoperative settings. This article describes specific clinical applications of TCD to diagnose and monitor vasospasm (VSP) for patients after subarachnoid hemorrhage (SAH) of different etiologies (aneurysm rupture, traumatic brain injury [TBI]) and cerebral hemodynamic changes in patients after stroke (including cryptogenic stroke). Other important clinical applications of TCD discussed are emboli monitoring, management of patients with sickle-cell disease, and so-called functional TCD. Advanced TCD application for diagnosis and monitoring of patients with intracranial hypertension and confirmation of clinical diagnosis of brain death are also presented.
It should be noted that TCD has also been frequently used for the clinical evaluation of cerebral autoregulatory reserve, and to monitor cerebral circulation and emboli during cardiopulmonary bypass, carotid endarterectomies, and carotid artery stenting. Over the past decade, Power M mode, color Doppler imaging, and use of ultrasound contrast agents have extended the scope of TCD clinical applications. In addition, TCD is being increasingly used as a research tool.
Basic Concepts
TCD examination involves placement of the probe of a range-gated ultrasound Doppler instrument, allowing the velocities in the arteries to be determined from the Doppler signals. At 2-MHz frequency, the attenuation in bone and soft tissues is considerably less as compared with higher frequencies and provides satisfactory recordings of intracranial CBFVs. An ultrasonic beam transmitted by the probe crosses the skull at prespecified locations and is reflected back from the flowing erythrocytes in the blood vessels. These erythrocytes move at different speeds and the resultant Doppler signal obtained is a mixture of different frequency components. The Doppler shift is the difference between the transmitted signal and the received signal and the time interval from pulse emission to reception determines the depth from which any Doppler frequency shift is detected. Spectral analysis then presents 3-dimensional Doppler data in a 2-dimensional format. The time vector is represented on the horizontal scale while velocity (frequency shift) is displayed on the vertical scale. The brightness of color represents the signal intensity. Mean CBFV is calculated using a spectral envelope (also known as Fast Fourier transformation [FFT]), which corresponds to the time averaged flow velocity throughout a cardiac cycle : Mean CBFV = [PSV + (EDV × 2)]/3, where PSV is peak systolic CBFV and EDV is end-diastolic CBFV.
The relationship between the velocity and pressure exerted by blood flowing through the cerebral arteries is described by the Bernoulli principle, which states that as the velocity of flow increases, the pressure exerted by that fluid decreases. TCD ultrasonography is based on the principle that the CBFV in a given artery is inversely related to the cross-sectional area of that artery. Thus, TCD ultrasonography provides an indirect evaluation of the vessel diameter by calculating the Doppler shift. TCD also allows measurement of PSV and EDV. Using these values, the mean CBFV, PI, and resistance index (RI) can be calculated.
There are several physiologic factors affecting CBFV, among them age, hematocrit, vessel diameter, gender, fever, metabolic factors, exercise, and brain activity. Table 1 outlines mean CBFV based on different age groups in anterior and posterior circulation. Other variables measured with TCD examination are a PI (Gosling Index) and/or RI (Pourcelot Index): PI = (PSV − EDV)/mean CBFV and RI = (PSV – EDV)/PSV. The physiologic meaning of these indices is the reflection of downstream resistance. Table 2 outlines Mean CBFVs and associated conditions.
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