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2.6 Color Doppler
Color Doppler is an incredibly powerful tool. You can easily detect jets and study how blood flows within the heart. In Chapter 1 (Physical Principles of Echocardiography) we discussed how color Doppler works and the importance of scanner settings. In this section we will describe how color Doppler is integrated into a standard exam.
2.6.1 Practical Issues
Trainees tend to "overuse" color Doppler. Don't forget the 2D image. Look at the 2D morphology and function first. Color Doppler will yield insufficient results when the 2D image is not optimized. Before you use color Doppler, also reduce the 2D gain. This enhances the color-to-background contrast and avoids color artifacts on "tissue" structures. Finally, optimize the scanner settings (box size, color gain, PRF) and "readjust" your transducer position if necessary. The interpretation of color Doppler findings can be quite difficult in the beginning. To gain a better understanding, use the ciné function and combine spectral Doppler with color Doppler. This will make it easier to understand the sequence and timing of flow events.
2.6.2 Parasternal Long Axis - Color Doppler
The parasternal long axis displays the aortic and the mitral valve. Thus, color Doppler can be used here to display jets associated with these valves (stenosis and regurgitation). You will also see turbulent flow in the left ventricular outflow tract (hypertrophic cardiomyopathy). In theory the jet orientation in these pathologies is perpendicular to the Doppler orientation. Therefore, one would not anticipate a good display of jets. In clinical practice the opposite is the case. As jets are more or less eccentric, you will see jets quite clearly.
You can also use slightly atypical views that help to align the jets in a more parallel fashion. Remember that we are very close to the valves at this site; this fact is more important than perfect jet alignment. The color box can be moved back and forth between the mitral valve and the aortic valve, depending on what you want to focus on. Moving a sector box back and forth is much better that using a large color box that covers both valves and where the color resolution is poor.
|2D before color!|
|Look for aliasing to detect jets|
|Use higher frame rates|
|Color as guide for CW/PW|
|Reduce PRF to detect low velocity flow|
|Use multiple views|
2.6.3 Parasternal Short Axis Base - Color Doppler
Color Doppler in the short-axis view at the base can be used to assess aortic regurgitation. In this view you are orthogonal to the jet. Therefore you will not be able to see the extension of the jet into the left ventricle. However, this view has a major advantage: you can see the origin of the aortic regurgitation jet(s). Thus, you will be able to determine the geometry, size and number of the regurgitant orifice(s). This is helpful to quantify aortic regurgitation and establish its "mechanism". Use this view especially in patients with an aortic valve prosthesis.
On this view you will be able to see ventricular septal defects and determine the type of VSD (see Chapter 20, Congenital Heart Disease).
Color Doppler should also be used to assess flow in the right ventricular outflow tract, across the pulmonary valve, and in the main pulmonary artery (including its branches). To do this you need to optimize your short axis view for the pulmonary valve first (see above). As the orientation of flow is parallel here, you will obtain a clear view of pulmonary regurgitation. Turbulent (aliased) flow across the pulmonary valve is usually indicative of pulmonary stenosis. However, increased flow is also present in large atrial septal defects and RVOT obstruction. Don't forget to assess flow in the distal pulmonary artery. At this site you will be able to identify a patent ductus arteriosus. Besides, it is also important for quantification of pulmonary regurgitation.
2.6.4 Short-Axis View - Mitral Valve - Color Doppler
This view allows you to study mitral regurgitation. First adjust the 2D image so that you are at the tip of the leaflets. Color Doppler will then show you the origin of mitral regurgitation. Again, you are displaying the jet in orthogonal orientation and will therefore not be able to see the "longitudinal" component of the jet. Similar to the assessment of aortic regurgitation in short-axis orientation, this view should be used to determine the "origin" and geometry of the jet.
You will read more about this subject in Chapter 12, Mitral Regurgitation. As this information is only relevant if significant regurgitation is present, you need not use color Doppler in this view in all patients.
2.6.5 Four-Chamber View - Color Doppler
The four-chamber view shows the mitral valve and the tricuspid valve. The primary use of color Doppler in this setting is to assess these valves. Optimize the 2D image for each valve individually before using color Doppler. Further adjustments of transducer orientation and position should be made in accordance with the origin and direction of the jet.
For the tricuspid valve use an optimized right ventricular four-chamber view (see above). Sometimes it is also necessary to move up one intercostal space. Color Doppler should always be used to guide CW Doppler for quantification of TR velocity. Here one should primarily focus on the origin of the jet (vena contracta, PISA). Color Doppler in the four-chamber view is also used to visualize blood flow in the right upper pulmonary vein and the inferior vena cava. Modifications of the four-chamber view are used to exclude atrial or ventricular septal defects.
2.6.6 Five-Chamber View - Color Doppler
The main focus of color Doppler here is the aortic valve and the left ventricular outflow tract. The orientation is ideal to visualize aortic regurgitation and turbulences in the left ventricular outflow tract as they occur, for instance, in hypertrophic obstructive cardiomyopathy. Color Doppler in the five-chamber view also enables you to display a membranous ventricular septal defect.
2.6.7 Two-Chamber View - Color Doppler
As you will see in Chapter 12 (Mitral regurgitation), this view is very useful to quantify mitral regurgitation and determine the origin of a jet. As you are traversing the mitral valve along the "line of closure", this view provides an accurate impression of the extent of coaptation defects at this site. As the line of closure is not a straight line, you should tilt and rotate the transducer back and forth with the color on until you see the entire vena contracta and the convergence zone of proximal flow. How you determine the origin of MR jets will be discussed in Chapter 12 (Mitral Regurgitation).
2.6.8 Three-Chamber View - Color Doppler
The three-chamber view has significant advantages as compared to the parasternal long-axis view when using color. While both views display the same "chambers" and valves (mitral and aortic valve), the three-chamber view has a more parallel orientation to the Doppler signal. Therefore, jets such as those of mitral regurgitation, aortic regurgitation and also the flow in the LVOT are easier to interpret. Optimize color Doppler to the individual valves and keep the sector as small as possible. Remember you are visualizing a different cross-section of the mitral valve than in the other apical views. This is the reason why color Doppler provides additional information.
2.6.9 Subcostal Views - Color Doppler
In general the quality of color Doppler from the subcostal window is inferior to that from other windows. Nevertheless, color Doppler has several uses. The most important of these is the visualization of atrial septal defects. As the interatrial septum is perpendicular to the angle of insonation, you will be able to visualize such defects here. Color Doppler is also useful to display regurgitant jets, especially those of the tricuspid valve. Other applications include visualization of right atrial inflow, hepatic veins, and pulmonary flow.
2.6.10 Suprasternal Window
The suprasternal window provides information about aortic flow velocity. Flow in the ascending aorta is shown in red (towards the transducer) while flow in the descending aorta in shown in blue (flow away from the transducer). In normal individuals the flow velocity is below 1.2 m/sec. Therefore, you will not see much aliasing. The presence of significant aliasing usually denotes that a disorder is present (i.e. aortic stenosis, coarctation). One of the most important applications of color Doppler in this setting is the assessment of aortic regurgitation. Although this is usually performed with PW Doppler, color Doppler provides you with a rough estimate of retrograde flow and helps you to guide the position of the PW sample volume.
From the suprasternal window it is also possible to visualize a patent ductus arteriosus (PDA). More on this subject can be found in Chapter 20.