Transcranial Doppler (TCD) is a non-invasive ultrasound technology used to assess blood flow velocity in the major basal intracranial arteries on a real time, beat-to-beat basis. Blood flow velocity is calculated and used to make determinations about intracranial hemodynamics. TCD may be used to evaluate intracranial effects of extracranial lesions, including information on collateralizing channels and tandem stenoses. In addition, TCD allows direct perioperative evaluation of middle cerebral artery blood flow velocity in carotid endarterectomy patients, and is typically used to guide shunt placement, and to monitor for re-occlusion and hyperperfusion syndrome. In recent experimental trials, TCD is being evaluated for its ability to detect and classify intracranial emboli. Researchers are investigating the appearance of high intensity transient signals in the TCD waveform as indicators of circulating microemboli. The accuracy and clinical significance of this technique has not yet been scientifically established. Future research may lead to the use of embolus detection by TCD for patient management.
Applications also include the early (sub-angiographic) bedside detection of vasospasm in subarachnoid hemorrhage patients, evaluation of stroke and transient ischemic attack, as an adjunct in the assessment of cerebral circulatory arrest, and as a monitoring tool for patients undergoing intracranial interventional procedures.
In 1979, Drs. William Stern (founder of Multigon) and Robert Barnes first used Fast Fourier Transform (or FFT) to analyze the change in frequency of the returned audio signals of ultrasound instruments. The device which carried out this analysis, the Angioscan, allowed clinicians to calculate the Doppler shift or frequency of the audio signal, resulting in reproducible and consistent quantification of blood flow velocity.
The FFT, displayed visually as a waveform, offered additional information, including direction of flow in vessel, and helped distinguish between laminar and turbulent flow. Subsequently, normal ranges of frequency values were established, and the FFT analysis of Doppler frequency became known as spectral analysis.
Ultrasound has several advantages over other diagnostic modalities, including its portability, low cost, and safety. Because ultrasound is non-invasive, studies may be performed repeatedly and without the need for contrast agents.
In 1982, Dr. Rune Aaslid discovered that it was possible to send an ultrasound beam into the brain through a thinning of the skull. He used an Angioscan to analyze a Doppler signal reflected from cerebral arteries and his discovery heralded a breakthrough in the evaluation of intracranial blood flow velocity. This technique is referred to as Transcranial Doppler (TCD).
The vessels in the circle of Willis lie at known depths in the brain. The operator selectively evaluates specific vessels by controlling the placement of the ultrasonic sample volume and the transducer's orientation. Depth of insonation, flow direction, velocity, and audio pitch assist in vessel identification. TCD signals have been extensively correlated with angiograms, and are accepted as reliable measurements of intracranial blood flow velocity.
TCD may be performed from three physical approaches, known as ultrasonic windows. The transtemporal window allows evaluation of the middle cerebral artery, internal carotid artery bifurcation, anterior cerebral artery, and posterior cerebral artery. This window lies within the area of the zygomatic arch, and three variants (anterior, middle and posterior) may be identified.
The transorbital window allows insonation of the ophthalmic artery and the internal carotid artery siphon. (When using this window, the ultrasonic power output of the instrument should be lowered to 10 or 20%.)
The sub-occipital window allows serial evaluation of major portions of the vertebral and basilar arteries. In some patients, tortuous anatomy may limit the portions of the basilar artery that are available for sampling.
To perform the examination, the operator places a 2 MHz trasnducer on the appropiate ultrasonic window. An ultrasound beam is sent out from the unit through the transducer directly into the vessel. The depth of penetration (or length of the beam) is controlled by the operator. The TCD instrument evaluates the difference between the signal sent (transmitted frequency) and the signal returned (reflected frequency). Spectral analysis then calculates this difference, also known as the Doppler frequency shift. Flow direction is also indicated, as flow coming towards the transducer is displayed above the baseline and flow going away from the transducer is displayed below the baseline.
The TCD system uses FFT (Fast Fourier Transform) to calculate and display mean flow velocity, pulsatility index, and other diagnostic parameters. By comparing established typical values with actual examination results, the interpreter can make determinations about vessel hemodynamics. For example, stenoses and occlusions may cause elevated velocity levels. Mean velocity is representative of, but does not directly measure, blood flow in the vessel. Pulsatility index describes the shape of the waveform and the relationship between peak systole and end diastole. It is believed to represent, primarily, an estimation of downstream vascular resistance. Resistance Index provides another measure of downstream vascular resistance.
Additional diagnostic criteria, such as flow direction, change or absence of signal, differences in velocity values on left and right sides, waveform shape, and others, are also used in interpretation.
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