Color Duplex Imaging Limitation

As a result of the vast amount of information to be processed, color duplex scanning has a much poorer spatial and temporal resolution than pure B-mode imaging. Axial resolution is proportional to the wavelength in B-mode imaging while it is dependent on the number of sample volumes placed along the color Doppler scan line in the color duplex mode. The use of smaller sample volumes improves axial resolution but at the expense of sensitivity and accuracy in Doppler shift evaluation as the signal-to-noise ratio deteriorates. Lateral resolution in color-coded duplex scanning is determined by the number of color Doppler lines processed per centimeter. The frame rate decreases as the number of Doppler lines increases, resulting in a lower temporal resolution, in particular at greater scan depths. As a result of these limitations, the axial resolution of color duplex ultrasound on the order of 0.4–1.0 mm with a lateral resolution of only 1.0–2.0 mm, which is 4–10 times lower than B-scan resolution (Widder 1995). The frame rate in the color duplex mode ranges from 50–200 ms (corresponding to an image repetition rate of 5 Hz), depending on the scanning depth and size of the color box. When a low pulse repetition frequency is selected, the speed at which the color Doppler scan lines sweep the sector is similar to or slightly below the mean flow velocity in arteries. Therefore, a single ultrasound scan may simultaneously depict systolic flow (e.g. displayed in red) and early diastolic flow(e.g. displayed in blue) . Due to the low temporal resolution, however, the color coding does not fully reflect the pulsatile character of flow. Slow flow produces smaller Doppler frequency shifts, which have to be extracted from short echo pulse packets for each scan line consisting of a number of individual pulses. The scan lines must be processed successively. Though the insonation angle should ideally be as small as possible for optimal Doppler scanning, this is not always practical because there will be a longer delay when the color box is tilted (beam steering). This is why one must find a compromise, in particular when examining deeper vessels. Tilting the color box 20° and 30° prolongs the echo arrival time by 13% and 31%, respectively.

Color duplex imaging, like all diagnostic ultrasound techniques, is impaired by scattering and acoustic shadowing caused by bowel gas or calcified structures (bone or calcified plaques on vessel walls). The examiner can circumvent such interfering structures by moving the transducer, but this is frequently achieved only at the cost of a longer echo arrival time due to a greater distance from the structure of interest. Strong reflectors that are oblique to the beam axis act like mirrors and generate phantom images in another area of the scan. Such mirror images can be identified by angling the transducer, which will make the mirror artifacts disappear or appear in a different location. When the ultrasound beam strikes interfaces of high acoustic impedance at a right angle, reverberations (repeat echoes) may occur with the ultrasound pulses being reflected to and fro, resulting in a kind of ping-pong effect. Slight angulation of the transducer prevents reverberations but will also reduce reflection from the interface and thus degrade image quality.