Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

eam is aligned at a particular orientation and with appropriate range delay toobtain Doppler information.Instrumentation for duplex scanning is available in several configurations. Mostoften, electronic array transducers switch between a group of transducers used to createa B-mode image and one or more transducers used for the Doppler information.The duplex system allows estimation of the flow velocity directly from theDoppler shift frequency, since the velocity of sound and the transducer frequencyare known, while the Doppler angle can be estimated from the B-mode image bythe user and input into the scanner computer for calculation. Once the velocity isknown, flow (in units of cm 3 /sec) is estimated as the product of the vessel's crosssectionalarea (cm 2 ) times the velocity (cm/sec).Errors in the flow volume may occur. The vessel axis might not lie totallywithin the scanned plane, the vessel might be curved, or flow might be altered fromthe perceived direction. The beam-vessel angle (Doppler angle) could be in error,which is much more problematic for very large angles, particularly those greaterthan 60 degrees, as explained previously. The Doppler gate (sample area) could bemispositioned or of inappropriate size, such that the velocities are overestimates(usually too small) or underestimates (usually roo large) of the average velocity.Noncircular cross sections will cause errors in the area estimate, and therefore errorsin the flow volume.Multigate pulsed Doppler systems operate with several parallel channels closelyspaced across the lumen of a single large vessel. The outputs from all of the gatescan be combined to estimate the velocity profile across the vessel, which representsthe variation of flow velocity within the vessel lumen. Velocities mapped with acolor scale visually separate the flow information from the gray-scale image, and areal-time color flow Doppler ultrasound image indicates the direction of flowthrough color coding. However, time is insufficient to complete the computationsnecessary for determining the Doppler shifts from a large number of gates to getreal-time image update rates, particularly for those located at depth.Doppler Spectral InterpretationThe Doppler signal is typically represented by a spectrum of frequencies resultingfrom a range of velocities contained within the sampling gate at a specific point intime. Blood flow can exhibit laminar, blunt, or turbulent flow patterns, dependingon the vessel wall characteristics, the size and shape of the vessel, and the flow rate.Fast, laminar flow exists in the center of large, smooth wall vessels, while slowerblood flow occurs near the vessel walls, due to frictional forces. Turbulent flowoccurs at disruptions in the vessel wall caused by plaque buildup and stenosis. Alarge Doppler gate that is positioned to encompass the entire lumen of the vesselwill contain a large range of blood velocities, while a smaller gate positioned in thecenter of the vessel will have a smaller, faster range of velocities. A Doppler gatepositioned near a stenosis in the turbulent flow pattern will measure the largestrange of velocities.With the pulsatile nature of blood flow, the spectral characteristics vary withtime. Interpretation of the frequency shifts and direction of blood flow is accomplishedwith the fast Fourier transform, which mathematically analyzes thedetected signals and generates amplitude versus frequency distribution profileknown as the Doppler spectrum. In a clinical instrument, the Doppler spectrumis continuously updated in a real-time spectral Doppler display (Fig. 16-50). This