engleski

Basic principles of ultrasound diagnostics

Doppler effect implies that reflected or scattered sound wave will be shifted to a frequency higher or lower than the incident frequency if the reflector is moving relatively toward or away from the transducer. For vascular ultrasound applications, a positive frequency shift indicates flow toward the transducer, and negative frequency shift implies the oposite. The signal received from blood is a mixture of different frequency shift components caused by reflexctors moving at a variety of different velocities within the vessel, which can be visually displayed by use of the fast Fourier transform (FFT). Different applications of neurovascular Doppler ultrasound requires different instrumentation: continuous-wave Doppler instruments are for extracranial carotid, vertebral, ophthalmic and subclavian artery evaluations. PW Doppler instruments are mainly used for transcranial Doppler (TCD) applications. Duplex instruments perform both B-mode grey scale real-time imaging and simultaneous pulse-wave Doppler sonography. Although Doppler instrumentation is used to identify, localise and quantify disease, accurate assessmment is imposible without an understanding of the hemodynamic principles. Quantification of disease relies on spectral waveform analysis, therefor hemodynamic principles must routinely be applied to the noninvasive and clinical assessment of patients with cerebrovascular disease.

neurosonology; ultrasonography; Doppler

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