Color Doppler Imaging Adjustment Guide
©Walter Rasmussen, R.D.C.S.
Starting Color Doppler is as simple as pressing a single button, but the sonongrapher needs to know how to compensate for how well the ultrasound beam is penetrating and blood flow properties at the area of interest. Because of the rapid velocities encountered in Echocardiography, the adjustment of color Doppler Gain, Scale, Frame Rate and frequency resolution are of primary importance.
How to keep Frame Rates and Resolution high:
Accurate tracking of the typical velocity of blood in cardiac chambers and vessels requires that the image is updated at or above 17 Hz and any adjustment that obtains even higher frame rates will provide even better resolution of blood flow characteristics, particularly at the core and borders of the jet.
The most important background setting for the whole color Doppler system is called Flow Optimization or Frequency Optimization on Philips systems and Frame Rate on GE systems. All current ultrasound machines have low, medium and high optimization adjustments.
The Medium Flow or Frequency Optimization setting is a good starting point for all adult echcocardiograms. This setting helps to maintain adequate frame rates, good spatial resolution and penetration. Once the sonographer has observed the color Doppler map at a medium setting, the machine can then be adjusted higher or lower as needed in order to optimize frame rate, penetration and resolution. Knowing and using this adjustment gives the sonographer a distinct advantage for creating color Doppler images with the best possible resolution.
The Low Flow Optimization setting is useful when encountering difficult to image patients the same way that lower frequencies are used for difficult two-dimensional imaging. Low flow optimization can also be helpful where very low blood velocities are typically found such as in the pulmonary veins, large ASD’s, and the inferior vena cava, but this is seldom necessary because it is easier to stay in Medium Flow Optimization and temporarily lower the scale than to use another optimization setting.
Some sonographers always set their machines at Low Flow Optimization either because they don’t know what it does or because it makes performing color Doppler easier however the down-side to routinely using the Low optimization setting is that it decreases frame rates, resolution and exaggerates jet area.
Above: Well-defined jet border (blue)with mosaic recorded with Medium Flow Optimization.
Above: The same MR jet as above at Low Flow Optimization showing excessive smoothing and ill-defined borders.
A good hint that color Doppler optimization is set too low is when the colors look faint or washed-out, move slowly and the edges are blurred. In this situation, turning the optimization to a medium or high setting should be attempted for increasing the resolution. For advanced sonographers, using medium to high optimization at a higher gain setting and lowering the scale closer to 50 cm/sec usually compensates for any potential loss of jet area while improving the temporal and spatial resolution.
The High Flow/Frequency/Frame Rate Optimization can be used for patients with normal to high blood flow velocities and who have average to crystal clear images. With High Flow Optimization, lowering the Scale toward 50 cm/sec further enhances the sensitivity of the color Doppler map without significantly sacrificing Frame Rate and the benefit is higher resolution
Setting flow optimization is not the total solution for improving color Doppler resolution. Once the background color Doppler optimization is set, the primary method of adjusting frame rate is by limiting the width of the two-dimensional reference image and the size and depth of the color box. Color Doppler scale is another critical adjustment for achieving higher frame rates. Sonographers who never narrow the two-dimensional sector (despite being taught to do so in textbooks and ultrasound school), are not utilizing the full capabilities of the ultrasound machine and creating images that are of considerably lower diagnostic value.
The resolution of the color Doppler image is also influenced by image processing. Excessive use of the Smoothing function, Data Dependent Processing (DDP), Radial Averaging, Lateral Averaging on the GE and Color Smooth on the Philips results in progressive blurring of the color Doppler map. Color Doppler adjustments should always have the goal of demonstrating sharp jet borders, layers of velocity within the jet and “mosaicing” of high velocity or turbulent areas.
Checklist for the Adjustment of Color Doppler:
- Adjust Flow Optimization or Frame Rate to Medium, adusting it up or down only when necessary to either increase penetration (Low) or increase resolution (High).
- Adjust the size of the color box so that it covers only the area of immediate interest.
- Avoid making the color box wider than necessary. Widening the sampling area dramatically slows the frame rate.
- Avoid making the color box longer than necessary. The deeper the sampling, the lower the scale due to increased transit time and sampling area.
- The two-dimensional sector image should be narrowed to no more than a few centimeters wider than the color box.
- The proper color Doppler gain should customized for each patient by slightly increasing it to the point that random colored pixels, representing noise –are seen, and then backing off to eliminate most of the noise, but not the important blood flow information.
- The focal zone should be directly adjacent to the area of interest and may require frequent adjustment during and after using color Doppler.
- The color Doppler filter or low velocity reject, is used to eliminate low velocity signals and wall motion that clutter the color map. In echocardiography this adjustment should initially be set to medium but may be adjusted lower while looking at venous blood flow or shunts and higher as long as important flow is not diminished. The sonographer should make certain that this adjustment is correct at the beginning of each exam.
- The standard color Doppler scale is 50-60 cm/sec for patients with a normal heart rate however, it is better to keep the adjustment at or nearer 60 cm/sec for typical heart rates between 45 and 90 bpm because smaller jets will be more visible in turbulent areas and frame rates will be higher. Color Doppler scale can be increased beyond 60 cm/sec in order to help see jets more clearly in areas of high turbulence and hyper-dynamic blood flow. Scale can be decreased to 50 cm/sec and lower than 50 cm/sec in order to record low velocities such as those typically found in the pulmonary veins, the inferior vena cava and atrial septal shunts.
- Color Doppler frequency can seriously affect the sensitivity of the displayed information because at lower frequencies, the ultrasound beam will penetrate farther and thus produce a stronger signal. Periodically check to make certain that the ultrasound machine is set at 1.9 to 2.2 MHz for adult echocardiography. If the ultrasound machine has a frequency adjustment (this option is being taken away by some ultrasound companies), there are several ways to determine whether is set appropriately. First, if the color Doppler image is cluttered with low velocity and wall artifact that does not go away even with lowering the frequency, increase the frequency to ascertain whether the image cleanes-up. On the other hand, if the color Doppler image is faint and adjustment to higher gain setting does not help, decrease the frequency. It is not uncommon to find machines adjusted to a higher than necessary frequency with a dramatic decrease in sensitivity. Some ultrasound machines prohibit the adjustment of color Doppler frequency and demonstrate lower sensitivity to blood flow.
- Limit the use of Color Smooth, Radial Averaging, Lateral Averaging and DDP because they blur or blend velocities on the color Doppler map. A properly adjusted color Doppler system enables the viewer to see distinct layers of velocities within blood flow jets as well as color mosaic where there is high velocity or turbulence. Lowering Line density can help increase frame rates but should not be lowered to the point that excessive dropout of color Doppler information occurs.
- Limit the use of Tissue Priority or Write Priority because it will incrementally remove the color Doppler map from the image, diminishing the sensitivity of the Color Doppler display itself. It is not uncommon to find machines that are preset with this Write Priority or Tissue Priority turned to a high level.
- Monitor the color Doppler velocity scale at all times because if the machine automatically decreases the scale, or if it is adjusted manually, the machine may not automatically adjust it back upward as the box is moved closer to the transducer, where a higher scale is available.
- Recordings of long jets that require that the color box be excessively widened may lower the frame rate to the point that jet area is overly reduced. An example of this is aortic insufficiency from the parasternal long axis view. If the Sonographer suspects that there is too much loss of detail, the jet can be broken-up into two clips using a narrower color box in order to preserve the adequate assessment of jet area.
- Two-dimensional image colorization, other wise known as B-Color, should not be used in combination with Color Doppler because a strong red, yellow, orange or blue background will obscure the same colors on the color Doppler map and thus affect the sensitivity of the system, resulting in missed pathology.
- Excessive brightness of the 2-D image can reduce the visibility of the color Doppler map because both modalities compete for pixel dominance. Manually adjust the 2-D image so that the color Doppler has the maximum possible saturation.
Color Doppler in Review:
When initializing Color Doppler, think of ways to maximize the frame rate by adjusting the 2-D sector width, color box size and the flow setting to see fluid movement of blood rather than just flashing or shredded colors. Increase or decrease the scale appropriate to the blood flow that is being investigated. Adjust the color Doppler gain for good color map saturation and to avoid signs of excessive gain. Reduce the 2-D image gain so that the Color Doppler information is not washed-out. Adjust the focus to the level where the Color Doppler box is positioned. Don’t use two-dimensional image colorization with Color Doppler. Avoid using image processing adjustments that smooth, average or diminish the color Doppler information.
More on this subject can be seen at the Philips iE33 Color Doppler Page.