The term echocardiography refers to the evaluation of cardiac structure and function with images and recordings produced by ultrasound. The development of echocardiography is usually credited to Edler and Hertz in 1954.1 In the past 30 years, it has become a fundamental component of the cardiac evaluation. Currently, echocardiography (echo) provides essential (and sometimes unexpected) clinical information and is the second most frequently performed diagnostic procedure.2 The history of this technique has been closely linked with advances in computer processing, storage and miniaturization.
The first two decades of clinical echocardiography involved one-dimensional (1D) time-motion (M-mode) recordings, performed from the precordial area to assess cardiac anatomy.3 In the mid-1970s, a multielement linear-array scanner was created to produce anatomically correct, two-dimensional (2D) images of the beating heart.4 This was followed by mechanical sector scanners5 and, ultimately, by the present-day phased-array instruments.6 After initial forays into three-dimensional (3D) ultrasound using reconstruction, 3D instruments capable of real-time volumetric imaging were developed, and 3D visualization is now routine.7 This process of miniaturization permitted evolution from transthoracic echocardiography (TTE) to transesophageal echocardiography (TEE),8 as well as handheld echographs that can be carried in a lab coat and devices incorporated into cardiac catheters to achieve intravascular images. Advances in image processing have led to the assessment of myocardial mechanics, including myocardial strain.9 However, other technical developments, including the assessment of myocardial perfusion with contrast agents, have had a longer pathway to acceptance.10
Although efforts to use the Doppler principle to measure flow velocity by ultrasound were begun in the early 1970s,11 clinical application of this technique as a partner to clinical echocardiography did not thrive until the work of Hatle in the early 1980s.12 The combination with Doppler led to machines capable of also delineating flow and deriving hemodynamic data.13 Pulsed and continuous-wave Doppler recordings soon were expanded to full 2D color-flow imaging. Most recently, Doppler velocity recordings have been obtained from myocardium itself, enabling measurement of tissue velocities and the derivation of values for regional strain.
PRINCIPLES OF ECHOCARDIOGRAPHY
Physics and Instrumentation
Sound is an energy form that travels through a medium as a series of alternating compressions and rarefactions of the molecules (Fig. 15–1). It is typically characterized by its wavelength, which is the distance between any two consecutive phases of the cycle (eg, peak compression to peak compression), and by its frequency, which is the number of wavelengths per unit time (customarily expressed as cycles per second, or Hertz [Hz]). The velocity of sound is the product of wavelength and frequency; thus, there is an inverse relationship between these two characteristics: the greater the frequency, the shorter the wavelength. Ultrasound is sonic energy with a frequency more than the audible range of the human ear (> 20,000 Hz) and is useful ...