Color Doppler Echocardiography

Point-by-point determination of blood flow velocities by PW Doppler is too time consuming and does not reveal the instantaneous distribution of flow velocities within the cross-sectional image that is required for many diagnostic decisions. Color Doppler imaging was developed for this purpose. In color Doppler, which is a form of PW Doppler, a color code is used to depict flow toward (red) and away (blue) from the transducer; lighter and darker shades of red and blue respectively denote relatively faster and slower velocities. Continuous color maps of flow are superimposed on gray-scale cross-sectional echocardiograms. However, color Doppler is generally a semiquantitative technique and, like PW Doppler, will alias (color reversal) when the Nyquist limit is exceeded. Two aliasing patterns are easily recognized. The first is "normal" aliasing in which the area of apparent flow reversal forms one or more broad, relatively homogeneous color surfaces ( Plate 33-1 ). Blood flow velocities within a normal heart often produce this type of aliasing because they exceed the Nyquist limit for color Doppler (0.6 to 0.8 m/sec). The second type of aliasing results from disturbed or turbulent flow within the heart (e.g., mitral regurgitation) and is never normal ( Plate 33-2 ). When the ultrasonograph detects two different velocities

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Figure 33-5 Continuous-wave Doppler measures high-velocity flow without aliasing. Continuous-wave Doppler measurement of blood flow velocities in a mitral valve orifice during four cardiac cycles is shown. At the top of the figure is a still-frame image of the two-dimensional cross section used to position the Doppler sample cursor (the diagonal white line). On the bottom two thirds of the figure is the display in white of all the instantaneous blood flow velocities (vertical axis) versus time (horizontal axis) occurring anywhere along that cursor. The electrocardiogram provides timing, and the bold horizontal line is the baseline (zero flow) for the flow velocities. Flow velocities above this line are positive (i.e., toward the transducer) to a maximum of 753 cm/sec. Flow velocities below this line are negative (i.e., away from the transducer) to a maximum of -316 cm/sec. This tracing documents significant mitral regurgitation (the positive systolic velocities) with a peak blood flow velocity of approximately 5 m/sec (each white dot on the vertical axis equals 100 cm/sec or 1 m/sec). LA, left atrium; LV, left ventricle. (From Cahalan MK: Intraoperative Transesophageal Echocardiography. An Interactive Text and Atlas. New York, Churchill Livingstone, 1997.)