PART 3: Basic Spectral Doppler and Color Flow Protocols
DOPPLER APPLICATIONS AND TECHNIQUE
Doppler echocardiography is established as a valuable noninvasive tool in clinical cardiology to provide hemodynamic information about the function of the cardiac valves and chambers of the heart. When combined with conventional 2D echo, Doppler techniques may be focused to provide specific information on the velocity flow patterns of a particular area within the heart. The ability to provide qualitative and quantitative information in evaluating valvular function, intracardiac shunts, dysfunction of a native or prosthetic valve, or the obstruction of a surgically inserted shunt has contributed to the clinical care of the cardiac patient. Understanding of cardiac physiology and hemodynamics is critical to the interpretation of the Doppler information. In addition, the sonographer must clearly understand Doppler principles, artifacts, and pitfalls in order to produce a quality study.
Doppler Effect. The Doppler effect is the apparent change in frequency of sound or light waves emitted by a source as it moves away from or toward an observer, Figure 1.
Sound that reflects off a moving object undergoes a change in frequency. Objects moving toward the transducer reflect sound at a higher frequency than that of the incident pulse, and objects moving away reflect sound at a lower frequency. The difference between the transmitted and the received frequency is called the Doppler frequency shift. This Doppler effect is applied when the motion of laminar or turbulent flow is detected within a vascular structure.
In the medical application of the Doppler principle, the frequency of the reflected sound wave is the same as the frequency transmitted only if the reflector is stationary. If the red blood cell (RBC) moves along the line of the ultrasound beam (parallel to flow), the Doppler shift is directly proportional to the velocity of the RBC. If the RBC moves away from the transducer in the plane of the beam, the fall in frequency is directly proportional to the velocity and direction of RBC movement. The frequency of the echo will be higher than the transmitted frequency if the reflector is moving toward the transducer, and lower if the reflector is moving away,
Doppler Shift. The difference between the receiving echo frequency and the frequency of the transmitted beam is called the Doppler shift. This change in the frequency of a reflected wave is caused by relative motion between the reflector and the transducer’s beam. Generally the Doppler shift is only a small fraction of the transmitted ultrasound frequency.
The Doppler shift frequency is proportional to the velocity of the moving reflector or blood cell. The frequency at which a transducer transmits ultrasound influences the frequency of the Doppler shift. The higher the original, or transmitted, frequency, the greater is the shift in frequency for a given reflector velocity. The returning frequency increases if the RBC is moving toward the transducer and decreases if the blood cell is moving away from the transducer. The Doppler effect produces a shift that is the reflected frequency minus the transmitted frequency. When interrogating the same blood vessel with transducers of different frequencies, the higher-frequency transducer will generate a larger Doppler shift frequency.
Doppler Angle. The angle that the reflector path makes with the ultrasound beam is called the Doppler angle. As the Doppler angle increases from 0 to 90 degrees, the detected Doppler frequency shift decreases, Figure 2.
At 90 degrees, the Doppler shift is zero, regardless of flow velocity. The frequency of the Doppler shift is proportional to the cosine of the Doppler angle. The beam should be parallel to flow to obtain the maximum velocity. The closer the Doppler angle is to zero, the more accurate is the flow velocity. If the angle of the beam to the reflector exceeds 60 degrees, velocities will no longer be accurate.
Spectral Analysis. Blood flow through a vessel may be laminar or turbulent, Figure 3.
Laminar flow is the normal pattern of vessel flow, which occurs at different velocities, as flow in the center of the vessel is faster than it is at the edges. When the range of velocities increases significantly, the flow pattern becomes turbulent. The audio of the Doppler signal enables the sonographer to distinguish laminar flow from turbulent flow patterns. The process of spectral analysis allows the instrumentation to break down the complex multifrequency Doppler signal into individual frequency components.
The spectral display shows the distribution of Doppler frequencies versus time. This is displayed as velocity on the vertical axis and time on the horizontal axis, Figure 4.
Flow toward the transducer is displayed above the baseline, and flow away from the transducer is displayed below the baseline. In normal arterial flow, the spectral window is clear, the spectral width is not increased, and the spectral intensity is not bright.
When the area of the vessel that is examined contains RBCs moving at similar velocities, the cells will be represented on the spectral display by a narrow band. This area under the band is called the “window.” As flow becomes more turbulent or disturbed, the velocity increases, producing spectral broadening on the display. A very stenotic (high-flow velocity) lesion would cause the window to become completely filled in, with increased spectral width and intensity.
The Doppler and Color flow Doppler Examination. The Doppler color flow mapping (CFM) examination is generally performed along with the conventional 2D examination. The advantage of CFM is its ability to rapidly investigate flow direction and movement within the cardiac chambers. The sonographer should acquire the respective cine loop(s) for 2D and color flow Doppler and acquire the representative still frames for M-modes and PW/CW Doppler.
|DOPPLER AND COLOR DOPPLER WINDOWS
Mitral valve, tricuspid valve, left ventricular outflow tract, aortic valve, pulmonary vein inflow, superior vena cava inflow, interventricular septum, interatrial septum
|Parasternal Short-Axis Window
Pulmonary valve, main pulmonary artery, right and left branches pulmonary artery (patent ductus arteriosus flow), tricuspid valve
|Suprasternal Notch Window
Ascending aorta, descending aorta, patent ductus arteriosus flow, right pulmonary artery
Interatrial septum, interventricular septum, inferior vena cava flow, superior vena cava flow
|Parasternal Long-Axis Window
Mitral regurgitation, tricuspid regurgitation, aortic regurgitation
|Right Parasternal Window
Color Flow Doppler
Color flow Doppler is sensitive to Doppler signals throughout an adjustable portion of the area of interest. A real-time image is displayed with both gray scale and color flow in the vascular structures. Color flow Doppler is able to analyze the phase information, frequency, and amplitude of returning echoes.
Velocities are quantified by allocating a pixel to flow toward the transducer and flow away from the transducer. Each velocity frequency change is allocated a color. Color maps may be adjusted to obtain different color assignments for the velocity levels; signals from moving red blood cells are assigned a color (red or blue) based on the direction of the phase shift (i.e., the direction of blood flow toward or away from the transducer). Flow toward the transducer is recorded in red, and flow away from the transducer is blue, (see Figure 5).
This is denoted on the color bar on the right upper side of the image. As the velocities increase, the flow pattern in the variance mode turns from red to various shades of red, orange, and yellow before it aliases. Likewise flow away from the transducer is recorded in blue; this color turns to various shades of blue, turquoise, and green before it aliases. Depending on the location of the transducer, the flow signals from various structures within the heart appear as different colors. An understanding of cardiac hemodynamics helps the examiner understand the flow patterns.
Aliasing occurs in color flow imaging when Doppler frequencies exceed the Nyquist limit, just as in spectral Doppler. This appears as a wrap-around of the displayed color. The velocity scale, (pulse repetition frequency), may be adjusted to avoid aliasing. Color arising from sources other than moving blood is referred to as flash artifact or ghosting.