Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

information is displayed on the video monitor, typically below the 20 B-modeimage, as a moving trace, with the blood velocity (proportional to Doppler frequency)as the vertical axis (from - Vmax to + Vmax) and time as the horizontal axis.The intensity of the Doppler signal at a particular frequency and moment in timeis displayed as the brightness at that point on the display. Velocities in one directionare displayed as positive values along the vertical axis, and velocities in theother direction are displayed as negative values. As new data arrive, the informationis updated and scrolled from left to right. Pulsatile blood takes on theappearance of a choppy sinusoidal wave through the periodic cycle of the heartbeat.Interpretation of the spectral display provides the ability to determine the presenceof flow, the direction of flow, and characteristics of the flow. It is more difficultto determine a lack of flow, since it is also necessary to ensure that the lack ofsignal is not due to other acoustic or electric system parameters or problems. Thedirection of flow (positive or negative Doppler shift) is best determined with a smallDoppler angle (about 30 degrees). Normal flow is typically characterized by a specificspectral Doppler display waveform, which is a consequence of the hemodynamicfeatures of particular vessels. Disturbed and turbulent flow produce Dopplerspectra that are correlated with disease processes. In these latter situations, the spectralcurve is "filled in" with a wide distribution of frequencies representing a widerange of velocities, as might occur with a vascular stenosis. Vascular impedance andpulsatile velocity changes concurrent with the circulation can be tracked byDoppler spectrum techniques. Pertinent quantitative measures such as the pulsatilityindex [PI = (max - min)/average] and the resistive index [RI = (max -min)/max] are dependent on the characteristics of the Doppler spectral display (Fig.16-51 describes max, min, and average).FIGURE 16-51. Left: Aliasing of the spectral Doppler display is characterized by "wraparound"of the highest velocities to the opposite direction when the sampling (PRF) isinadequate. Right: Without changing the overall velocity range, the spectral baseline isshifted to incorporate higher forward velocity and less reverse velocity to avoid aliasing.The maximum, average, and minimum spectral Doppler display values allow quantitativedetermination of clinically relevant information such as pulsatility index and resistiveindex.