Color Doppler imaging with the Philips iE33
©Walter Rasmussen, R.D.C.S.
How should color Doppler look? There are so many adjustments on current ultrasound machines that the answer is not obvious. Perhaps this is why such a wide range of variation in its appearance is seen on serial echoes when performed by different sonographers.
This article is written by a sonographer who has worked in the field for over thirty years and has seen color Doppler on many different machines from its first introduction to the clinical setting.
Color Doppler for echocardiography should demonstrate a well-defined jet area, turbulence (demonstrated by mosaicing), and fast frame rates. Adjustments that excessively smooth the map and lower frame rates and frequency should be avoided if at all possible.
LEFT: A mitral regurgitation jet (primarily blue) set at Medium Flow Optimization. There is good resolution of the jet, with color mosaic and flow convergence demonstrated.
This image demonstrates color Doppler as it has been adjusted in a standard fashion for almost 30 years.
LEFT: The same mitral regurgitation jet with color Doppler set at Low Flow Optimization, excessive smoothing and slower frame rate. The regurgitant jet appears washed-out and unfocused. Color Doppler mosaic is almost completely absent, with softening of the jet perimeter and absence of flow convergence. The low frame rate reduces temporal resolution, resulting in loss of detail.
The Philips iE33 is a versatile ultrasound machine with a large palette of adjustments for Color Doppler however, at startup your iE33 may not be set optimally for adult echocardiography and if Color Doppler is not properly adjusted, low frame rates and low resolution will result. This requires that sonographers choose which settings to use based upon their knowledge of the iE33 Color Doppler Controls, ultrasound physics and their previous experience.
Flow Optimization: The Primary adjustment for Color Doppler on the iE33
When starting Color Doppler on an adult echo exam, the first adjustment that the sonographer should check is Flow Optimzation, located on the right touch screen. Flow optimization automatically sets Color Transmit Frequency, Pulse length, Scale, Persistence and Write Priority. For adult echocardiography, Flow Optimization should initially be set at Medium and adjusted based upon the following conditions:
The iE33 has three Flow Optimization settings that relate to blood velocity and penetration and have a big impact on frame rates and resolution:
1. Low for areas of relatively slow blood flow such as is found in the venous system (pulmonary veins, IVC) and when imaging is difficult. Using Low Flow Optimization is akin to selecting a low frequency for two-dimensional echo, with better penetration than a Medium setting. Low Flow Optimization emphasizes lower velocities, slower frame rates and diminishes resolution and is therefore an undesirable initial setting for most aspects of adult echocardiography. With this setting, regurgitant jets will appear washed-out and unfocused. Laminar flow will look faint and slow.
Low Flow Optimization can produce a dramatic improvement in sensitivity when the two-dimensional ultrasound image and color Doppler at medium setting are technically difficult due to poor beam penetration, although resolution will still be poor.
2. Medium for almost all aspects of adult echocardiography when the images are of good quality, providing adequate to good frame rates with good resolution under normal physiologic conditions.
3. Fast for patients with rapid heart rates, narrow, high velocity jets (such as seen with VSD’s or Aortic regurgutation), areas very close to the transducer, or those that have very clear ultrasound images. Under good conditions, Fast Flow Optimization will provide the fastest frame rates and the highest resolution.
Alert! Changing Flow Optimization may cause other adjustments to change their settings. When changing Optimization it is therefore necessary to quickly check that Line Density, Scale and Filter are set where desired.
Located on page 2 of the right touch screen while in Color Doppler mode, Adaptive Flow may or may not improve color Doppler performance on an individual basis.
Adaptive Flow is a semi-automated approach to performing Color Doppler exams, automatically adjusting color transmit frequency and sample volume size in an attempt to optimize sensitivity for the depth of the color box.
When the sonographer wants more manual adjustments for Color Doppler to be available, Adaptive Flow should be turned-off.
Keeping the frame rate above 17 Hz is very important for maintaining accurate representation of blood flow with Color Doppler imaging however, there is no Fame Rate adjustment knob for Color Doppler on the iE33. Instead, the sonographer must configure Flow Optimization, Line Density, Scale, Color Box size and 2-D sector size.
Color Doppler frame rates of 20 to 30 Hz are possible with conscientious adjustment of the various controls described here. When frame rates fall to below 17 Hz, noticeable degradation of the Color Doppler map will become progressively more visible characterized by the flashing or choppy colors and a loss of the fluid quality to blood flow. This is the point at which Color Doppler looses it ability to adequately track the blood flow, producing images of lower diagnostic quality.
While sweeping through the area being investigated, the transducer repeatedly emits pulses that result in radial lines of information. Adjusting Line Density higher or lower changes the number of lines of Color Doppler information, which affects resolution. Lowering the Line Density allows the machine to dedicate more resources to frame rate. On the iE33, the Medium setting is the general starting point for the adult echo exam however, if frame rates start to fall, the Low setting can be used to help to improve temporal resolution without significant loss of Doppler information.
Color Doppler Filter:
While not affecting frame rate, the Color Doppler filter is used to eliminate an excessive amount of low velocity signals that can interfere with viewing the useful information. In adult echocardiography, the Filter adjustment should initially be set to medium but may be adjusted lower while looking at venous blood flow or shunts. A high Color Doppler filter setting is not recommended for adult echocardiography, since it has the potential to diminish sensitivity and jet area. The sonographer should check that this adjustment is correctly set at the beginning of every exam.
Color Doppler Scale:
In echocardiography, the standard Color Doppler Scale is 50-60 cm/sec for patients with a normal heart rate however, this adjustment also influences frame rate. Since the higher the scale is, the faster the frame rate will be, it is logical to set the scale as close as possible to 60 cm/sec. It makes no noticeable difference to the jet area if the Color Doppler Scale is set slightly above 60 cm/sec, and it is reasonable to do this if it helps to maintain the frame rate above 17 Hz.
Color Doppler scale can be temporarily increased beyond 60 cm/sec in order to see individual regurgitant jets and shunts more clearly in areas of high turbulence and hyper-dynamic blood flow. Scale can also be temporarily decreased to 50 cm/sec and lower in order to record low velocities in the pulmonary veins, the inferior vena cava and atrial septal shunts without adjusting Flow Optimization.
A technique that combines Fast Flow Optimization (which raises frame rates) and a lower Scale (50 cm/sec) will improve color Doppler resolution in patients that are average to easy to image (good beam penetration).
Color Doppler Box size and Distance:
After all of the other adjustments have been set, the color Doppler box size and two-dimensional image width become the primary tools for maintaining the frame rate and resolution of the Color Doppler exam. Many sonographers don’t realize the importance of frame rate and routinely produce echocardiograms with color boxes that take-up more than half of the 90 degree sector.
The Color Doppler box size should therefore be tailored to each area being explored, widening or lengthening it whenever necessary according to the following principles:
1. Always adjust the size of the Color Box size so that it covers only the area of immediate interest. For example: For mitral regurgitation, the width should be only slightly wider than the mitral valve annulus. Once a regurgitant jet is located and recorded, the Color Box can be narrowed even more, in order to further increase Color Doppler frame rate and resolution.
2. The farther from the transducer the Color Box is placed, the slower the frame rate will be due to increased transit time. Reducing the size of the color box and two-dimensional sector width counteracts this effect.
3. In general, the color box should always be slightly taller than its width. Unnecessary widening of the color box dramatically lowers frame rate. The only situation where widening the box becomes necessary is when aortic regurgitation is encountered from the parasternal long axis. In this instance, if the frame rate starts to affect visualization, one suggestion is to provide two clips, one wide for demonstration of the entire jet, and one narrow to more accurately document the vena contracta.
4. Avoid making the color box longer than necessary. The deeper the sampling, the slower the frame rate will be, due to increased sampling area and transit time.
5. Use B&W Suppress to automatically narrow the background two-dimensional image while using Color Doppler if an increase in frame rate is desired.
6. Avoid covering more than one valve with the color box at the same time. This only reduces resolution for both valves.
When activated, B&W Suppress automatically narrows the background sector image every time Color Doppler is activated, thus helping to raise the frame rate. Since the width of the background two-dimensional image greatly affects frame rates and because it is only used to help establish the location of the blood flow being investigated, it should be no more than a few centimeters wider than the color box. Image width can also be manually adjusted before starting Color Doppler if a larger two-dimensional reference image is desired.
Another way of narrowing the sector image while in Color Doppler is by using the 2-D Write Zoom feature however, this needs to be distinguished from Magnify, which only enlarges pixels and does not help maintain frame rate.
Over-zooming Color Doppler can make the color Doppler map pixels larger, thus making it difficult to perceive the jet as a whole. The assessment of jet area is also more difficult if most of the cardiac chamber is not included within the imaging area because it is then not possible to compare the chamber area with the jet area.
Color Doppler Gain:
Proper Color Doppler Gain level should be customized for each patient by slightly increasing it to the point that random colored pixels, representing noise are observed, and then backing off to eliminate most of the noise, but not important blood flow information. If the gain level is set too low, the jet area will be reduced, and can lead to the under-estimation of the severity of regurgitant lesions and shunts. Color Doppler gain should be re-adjusted as necessary during the exam with each imaging window used.
Color Doppler Frequency:
Because lower frequencies allow the ultrasound beam to penetrate farther into the tissue and produce stronger returning signals, the sonographer should assure that the frequency at which the machine is set for Color Doppler is right for the patient being examined. For adult echocardiography, Color Doppler Frequency should be set at about 2.2 mMhz and adjusted lower for more difficult to image patients. This will result in substantially brighter colors and higher sensitivity to smaller jets. If Adaptive Flow is selected, manual adjustment of color Doppler frequency is not possible.
On page 2 of the iE33 touch screen the Color Smooth adjustment becomes available. Color Smooth provides incremental averaging of the Color Doppler frames from zero to the point that detailed velocity information becomes washed-out and the edges blurred due to reduced lateral and axial resolution. Excessive Color Smooth results in artifactual widening of the jet area. Color Smooth also makes it progressively difficult to clearly distinguish layers of velocities within jets and will blend small jets that are adjacent to each other. As Color Smooth is increased, it can also be observed that color mosaic, indicating high velocity and turbulence is progressively eliminated.
In order to preserve the detailed qualities of jets such as an accurate PISA radius, vena contracta width, jet velocity layers and color mosaic, Color Smooth should be set no higher level 2, in most cases. If the Color Doppler signal is weak, increasing Color Smooth can fill-in the resultant small black holes in the color map, but the actual Doppler information is simply being interpolated and does not represent the actual signal.
Write Priority gives more priority to either the Color Doppler map or the background two-dimensional sector image. The default setting gives equal emphasis to both the background image and Color Doppler. To increase the visibility of the Color Doppler Map, turn the dial to the left and a small green bar will move upward on the Color Scale.
Moving the dial clockwise will gradually remove the Color Doppler map overlay and show only the outline of the Color Box within the two-dimensional image. Reducing Color Doppler Priority beyond the mid-point is not recommended because it reduces Color Doppler sensitivity, jet area and saturation.
The two-dimensional image plays only the role of orienting the observer to where the Color Doppler Box was placed and should never be so bright that it competes with the Color Doppler map. Considerable improvement in color saturation is possible when the sonographer lowers the intensity of 2-D Gain.
In ultrasound imaging, the highest resolution will occur within the focal zone therefore; the Color Doppler map will have higher accuracy if the focal zone is set at the area of greatest interest. In Color Doppler systems, the focus also helps to tell the machine which frequency to use; higher for structures closer to the transducer and lower for those that are farther away, for better penetration and stronger reflections.
Persistence is an adjustment that causes individual frames of the scan to linger, thus blending them with the images in the successive frames. This causes incremental degrees of smoothing to the ultrasound image. For echocardiography, the iE33 should be set at zero Persistence for both two-dimensional and Color Doppler exams due to the necessity for tracking rapidly moving blood flow and mechanical parts.
It is not unusual to find an ultrasound machine that is adding too much persistence at start-up and the sonographer should know its effect and how to set it.
In Review: Recommended Initial Color Doppler Settings for Adult Echocardiography:
Flow Optimization: Medium
Line Density: Medium or Low
Wall Filter: Medium
Color Smooth: 1
B&W Suppress: On or manually narrow the 2-D Sector.
Color Box: Small as possible to cover the area of interest.
Scale: 60 cm/sec
Write Priority: Favoring Color Doppler
Saving a Preset:
Once the desired startup settings for Color Doppler are adjusted, the sonographer can save a preset so that everything is correct at the beginning of an exam. On the top row of the pull-out keyboard, is a key marked, “Save”. When the machine is customized for two-dimensional imaging and Color Doppler, press Save and follow the prompts in order to give it a name. The preset is linked to the transducer selected and will be available on the main menu on the left touchscreen, making it possible to instantly configure the preferred 2-D, Color Flow, Spectral PW and CW Doppler.