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INTRODUCTION

Echocardiography utilizes ultrasound to produce images of the heart and vascular structures (using M-mode, two-dimensional [2D], and and three-dimensionall [3D] echocardiography) and to provide information about the direction and velocity of blood flow within these structures (using spectral Doppler and color flow Doppler mapping). This chapter presents a basic review of the physics of ultrasound and the principles of cardiovascular ultrasonic imaging for all clinicians and medical personnel who perform echocardiography.

WHAT IS ULTRASOUND?

Ultrasound refers to sound waves with a frequency well beyond the range of human hearing (the normal human range is 20-20,000 Hz). Because the ability of ultrasound waves to penetrate human tissue carries an inverse relationship to the frequency of the transmitted ultrasound (as shown in Table 1-1), the range of frequencies that are suitable for transthoracic echocardiography generally falls between 2 to 10 MHz,1 which corresponds to an image depth of 6 cm in premature neonates (using a 10-MHz probe) down to 30 cm in adults (using a 2-MHz transducer). High ultrasonic frequencies are required to produce the necessary resolution for diagnostic imaging of small cardiovascular structures in newborns (Table 1-1). Because image resolution is directly related to the ultrasound frequency, the highest frequency that can penetrate to the structure of interest is generally preferred for M-mode, 2D, and 3D echocardiography. The opposite is true for Doppler and color flow mapping, in which lower ultrasonic frequencies generally provide better information.

TABLE 1-1.Penetration (imaging Depth) and Axial Resolution (using Two-Cycle Pulse) of Transducer Used in Echocardiography

HOW IS AN ECHOCARDIOGRAPHIC IMAGE (M-MODE, 2D, AND 3D) CREATED?

Echocardiography utilizes properties of sound wave interaction with human tissue called reflection and scattering (Figure 1-1). The human body consists of various tissues, each with a different composition and density, that result in a sound-related property called acoustic impedance that is specific to the tissue. When sound propagates through a smooth and long interface between homogeneous tissues with two different acoustic impedances (such as between blood and endocardium), both transmission and reflection of the sound wave occur; the degree of transmission versus reflection depends upon the degree of matching between the two acoustic impedances (the greater the degree of mismatch, the greater the amount of reflection). Reflection of sounds waves is similar to reflection of light upon a mirror or a flat and smooth surface. Most human tissues, however, are either irregular or inhomogeneous. Reflection of sound ...

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