Knowledge – Transfers

Doppler echocardiography is a method for detecting the direction and velocity of moving blood within the heart. As will be seen in this program, the technique may be used for detection of cardiac valvular insufficiency and stenosis as well as a large number of other abnormal flows. The current interest in Doppler echocardiography has reached a remarkable level in just the past few years. Doppler methods extend the use of cardiac ultrasound into the evaluation of normal and abnormal flow states and provide quantitative data that are essential in the clinical decision making process concerning patients with heart disease.

Understanding Doppler echocardiography begins with an understanding of the Doppler principle. All readers are familiar with the Doppler effect in every day life. For example, an observer stationed on a highway overpass readily notices that the pitch of the sound made from the engine of a passing automobile changes from high to low as the car approaches and then travels into the distance. The engine is emitting the same sound as it passes beneath, but the observer notices a change in pitch dependent upon the speed of the auto and its direction. Figure 1.1 demonstrates the changes in the frequency from an approaching and departing sound source (the moving automobile) relative to a stationary sound source.

Fig.1.1

The first description of the physical principles used in Doppler echocardiography is attributed to Johann Christian Doppler, an Austrian mathematician and scientist who lived in the first half of the 19th century. Doppler’s initial descriptions referred to changes in the wavelength of light as applied to astronomical events. In 1842, he presented a paper entitled “On the Coloured Light of Double Stars and Some Other Heavenly Bodies” where he postulated that certain properties of light emitted from stars depend upon the relative motion of the observer and the wave source. He suggested that the colored appearance of some stars was caused by their motion relative to the earth, the blue ones moving toward earth and the red ones moving away.

He drew an analogy of a ship moving to meet, or retreat from, incoming waves. The ship moving out to sea would meet the waves with more frequency than a ship moving toward the shoreline. Interestingly, Doppler never extrapolated his postulations to sound waves.

The Doppler principle is now used in many complex technologies. It is the fundamental principle upon which complex radar weather systems detect the severity of approaching storms and tracks its speed. It is also used by police to determine the speed of fast moving automobiles.

The Doppler Principle and the Study of Cardiac Flows
Blood Flow Patterns

Fig.1.2

Blood flow through the heart and great vessels has certain characteristics that can be measured using Doppler instruments designed for medical use. For the purpose of understanding flow patterns in the heart, it is important to recognize the difference between laminar flow and turbulent (or disturbed) flow. Laminar flow is flow that occurs along smooth parallel lines in a vessel so that all the red cells in an area are moving at approximately the same speed and in the same direction (Fig. 1.2). Due to friction, flow is always slightly slower near the walls of a vessel. With the pulsations of the heart, the red cells generally accelerate and decelerate at approximately the same speed. Flow in most of the cardiovascular system, including the heart and great vessels, is normally laminar and rarely exceeds the maximum velocity of 1.5 m/sec.

In contrast, turbulent or disturbed flow is present when there is some obstruction that results in a disruption of the normal laminar pattern. This causes the orderly movement of red blood cells to become disorganized and produces various whirls and eddies of differing velocities and directions. Obstruction to flow usually also results in some increase in velocity. Thus, turbulent flow is characterized by disordered directions of flow in combination with many different red cell velocities. If the obstruction is significant, some of the red blood cells may be moving at higher velocities than normal and may reach speeds of 7 m/sec. Turbulent flow is usually an abnormal finding and is considered indicative of some underlying cardiovascular pathology.

Fig.1.3

Abnormal flows are therefore generally characterized by turbulence and any increase in velocity. As an example, consider blood flow in the ascending aorta during systole. If the aorta and aortic valve are normal, then this flow is laminar. However, the presence of a valvular stenosis will induce a turbulent flow pattern.Figure 1.3 shows that a narrowed aortic valve orifice interrupts the parallel lines of normal laminar flow and produces turbulent flow. The resulting jet of blood creates a short segment within the proximal aorta with complex flow and velocity characteristics.

The Doppler Principle and the Study of Cardiac Flow
The Frequency of Sound Waves

Conventional two-dimensional echocardiographic systems emit high frequency bursts of sound (ultrasound) into the tissues. In standard echocardiographic imaging a given pulse of ultrasound is transmitted into the body and then reflected back from the various tissues. Since the speed of sound in tissue is known (approximately 1540 m/sec), a standard ultrasound imaging system can wait for a given time for the transmitted pulse to travel to a target (time X) and then back (time 2X) and the given target will be received and recorded. In complex two-dimensional imaging systems this alternating process is repeated in a variety of directions thousands of times each second. The best ultrasound images are made when the target is perpendicular (or specular) to the sound waves.

_

Fig.1.4

Frequency is a fundamental characteristic of any wave phenomenon, including sound, and refers to the number of waves that pass a given point in one second (Fig. 1.4). It is usually described in units of cycles per second or Hertz (Hz). Thus, the top of the illustration in Figure 1.4 shows an example of a waveform of 10 Hz while the one below is 5 Hz. Ultrasound is emitted in waveforms of a known frequency.

Doppler echocardiography, on the other hand, depends entirely on measurement of the relative change in the returned ultrasound frequency when compared to the transmitted frequency. Depending on the relative changes of the returning frequencies, Doppler echocardiographic systems measure these characteristics of disturbed flow: direction, velocity and turbulence. This enables examiners to differentiate between normal and abnormal flow patterns and, in some cases, to quantitate those characteristics that are helpful in determining the severity of abnormal flow states.

Most readers understand frequencies in relationship to the pitch of audible sound. The relationship between pitch and frequency is simple: the pitch of any given sound is proportional to its frequency. As sound wave frequency increases, pitch gets higher; and as frequency decreases, pitch declines.

_

Fig.1.5

Doppler systems are totally dependent on the changes in the frequency of the transmitted ultrasound that result from the encounter of the wavefront with moving red blood cells. Figure 1.5shows a transducer on the left that is emitting a given frequency of ultrasound toward the right and into the tissues. The transmitted sound waves encounter a group of red cells moving toward the transducer and are reflected back at a frequency higher than that at which they were sent producing a positive Doppler shift. The opposite effect occurs when a given frequency sent into the tissues encounters red cells moving away. The result is the return of a frequency lower than that transmitted, and the Doppler shift is negative.

The Doppler Principle and the Study of Cardiac Flows
The Doppler Equation

_

Fig.1.6

This Doppler effect in tissues maybe expressed as an equation as shown in Figure 1.6. Simply stated, the Doppler shift (Fd) of ultrasound will depend on both the transmitted frequency (fo) and the velocity (V) of the moving blood. This returned frequency is also called the “frequency shift” or “Doppler shift” and is highly dependent upon the angle (?) between the beam of ultrasound transmitted from the transducer and the moving red blood cells. The velocity of sound in blood is constant (c) and is an important part of the Doppler equation.