Color Doppler Flow and Displacement Imaging

Ultrasonic B-mode real-time imaging can be combined with Doppler in a scanner so that the scanner is capable of providing not only anatomical information but also blood flow data. Both sets of information are displayed simultaneously. A cursor line is typically superimposed on the B-mode image to indicate the direction of the Doppler beam. An FFT algorithm is used to compute the Doppler spectrum that is displayed in real time. This type of scanner is called a duplex scanner; a duplex image is shown in Figure below. Alternatively, blood flow data can be displayed in real time and superimposed with the B-mode image if the data acquisition rate and image processing algorithms are sufficiently fast.

where the magnitude and phase of H(τ) are an even function and an odd function, respectively. The symbol A is used to represent the magnitude of H(τ) here. where the dot operation represents the first derivative=∂H(τ)/∂τ. Let< ω > denote the mean of ω and, from the definition of mean angular frequency and Equation However, limited by the frame rate and field of view, the pulse repetition frequency in most color Doppler systems is between 8 and 16 kHz. As a result, aliasing frequently occurs with color Doppler in cardiac imaging. To overcome these
problems, one can reduce the image size or use M-mode color Doppler, in which the beam is fixed in one direction. In the heart, the myocardium is in motion during a cardiac cycle. Tissue color
Doppler images of this motion can be acquired with color Doppler methods previously described as well. The difference lies in that myocardial motion is slower than blood flow and myocardial echoes are stronger than blood. The spurious Doppler signals from blood in this case can be eliminated by thresh holding the echoes. A tissue Doppler image of the heart in which the color indicates the velocity of myocardial motion is shown in Figure 6.5. Many clinical applications have been found for color Doppler flow imaging, including diagnosing tiny shunts in the heart wall and valvular regurgitation and stenosis. It considerably reduces the examination time in many diseases associated with flow disturbance. Problematic regions can be quickly identified first from the flow mapping. More quantitative conventional Doppler measurements are then made on these areas. Color Doppler has been now widely used in a variety of medical disciplines; however, it has several shortcomings:

• Flow perpendicular to the beam cannot be reliably detected.
• Higher blood flow velocity results in aliasing.
• Its spatial resolution is poorer than B-mode gray-scale imaging.
• The mean velocity estimated is the average velocity within a pixel or voxel.
• Because the color Doppler image is overlaid over the gray-scale B-mode, the overlay process is determined arbitrarily by thresh holding, which may result in vessel wall overwrite obscuring the slow blood flow signal near the wall.
• Large echoes due to slow moving tissues can cause the “color flash artifact” because they overlap echoes from flowing blood.