![]() When Pellett, Ph.D., Associate Professor of Cardiopulmonary Sci- ence, Louisiana State University Health Sciences Center, a traveling wave of ultrasound strikes red blood cells that are Dept. Representation of a positive Doppler shift. The difference between the emitted (f0) and the reflected (fr) ultrasound frequencies, termed theĪddress for correspondence and reprint requests: Andrew Figure 1. moving away from the transducer, the reflected frequency is lower (a negative Doppler shift). If the blood or tissue is ferent ultrasound reflector velocities present. The number of reflected frequen- The increase in frequency is termed a positive cies is also influenced by the number of dif- Doppler shift (Fig. ![]() As transducer, the frequency of the reflected ul- a result, there are multiple reflected frequen- trasound is greater than the emitted frequency. If the target is traveling toward the cies are present in the emitted beam or pulse. Although there is a primary sulting emitted ultrasound signal strikes the transmitted frequency termed the center fre- red blood cells or tissue, a small proportion of quency, which is typically ∼2 MHz in commer- its energy is reflected back to the ultrasound cial ultrasound machines, additional frequen- probe. To be- ously (continuous wave) or in bursts (pulsed gin, the presence of f0 in the Doppler equa- wave) is aimed directly at a stream of flowing tion suggests that there is one emitted ultra- blood or moving cardiac tissue. As applied to Doppler v cfD 2f0(cos ) (2) echocardiography, an ultrasound transducer With regard to the Doppler equation, there are that is electrically stimulated either continu- several points that require explanation. By rearranging the that is different from the emitted frequency.1 equation, one may solve for velocity: This change in perceived frequency is known = / θ as the Doppler effect. When either the source or first acts as the sound recipient, and then as the recipient of a traveling wave is moving, the source as ultrasound strikes it and reflects then the wave recipient perceives a frequency back toward the transducer. There are actually blood flow (continuous wave and pulsed wave two Doppler shifts that occur (hence the num- Doppler) and cardiac tissue (tissue Doppler ber 2 in the numerator), for the moving target imaging) velocities. tissue, and c is the average speed of ultrasound Doppler echocardiography is used to measure in soft tissue (1540 m/sec). We will begin of the ultrasound beam and that of blood flow or with a discussion of the Doppler equation. ![]() The purpose of this and subsequent fD (fr f0) 2f0v(cos ) c (1) reviews is to discuss the areas of ultrasound where v is the red blood cell or tissue velocity, physics and instrumentation that are particu- cos θ is the cosine of the angle between the path larly significant to the clinician. Knowledge of ultrasound physics and in- Doppler shift frequency (fD), is determined ac- strumentation enables the ultrasound user to cording to the following equation: optimally employ available echocardiographic = − = θ / modalities. Kerut, M.D., F.A.C.C., F.A.S.E.† ∗Department of Cardiopulmonary Science, Louisiana State University Health Sciences Center, New Orleans, Louisiana †Heart Clinic of Louisiana, Marrero, Louisiana Cardiovascular Research Laboratory, LSU Health Sciences Center, New Orleans, Louisiana Division of Cardiology, Tulane University School of Medicine, New Orleans, Louisiana ![]()
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