The term echocardiography refers to the evaluation of cardiac structure and function with images and recordings produced by ultrasound. 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.1 A one-dimensional (1D) method performed from the precordial area to assess cardiac anatomy has evolved into a two-dimensional (2D) modality performed from either the thorax (TTE) or from within the esophagus (TEE), capable of also delineating flow and deriving hemodynamic data.2 Newly evolving technical developments have extended the capacity of ultrasound to routine three-dimensional (3D) visualization3 and the assessment, in conjunction with contrast agents,4 of myocardial perfusion.
The development of echocardiography is usually credited to Elder and Hertz in 1954.5 For nearly two additional decades, clinical echocardiography consisted primarily of 1D time-motion (M-mode) recordings, as popularized by Feigenbaum.6 In the mid-1970s, Bom and colleagues7 developed a multielement linear-array scanner that could produce anatomically correct images of the beating heart. 2D images of superior quality were soon achieved by mechanical sector scanners8 and ultimately by phased-array instruments developed by VonRamm and Thurstone9 as the present-day standard. Recently, 3D instruments capable of real-time volumetric imaging have been developed.10 Miniaturization of ultrasound transducers has also led to handheld echographs that can be carried in a lab coat and incorporation into gastroscopes and cardiac catheters to achieve transesophageal and intravascular images.11,12
Although efforts to use the Doppler principle to measure flow velocity by ultrasound were begun in the early 1970s by Baker,13 clinical application of this technique did not thrive until the work of Hatle in the early 1980s.14 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.
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. 18–1). 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 for diagnostic imaging, because, like light, it can be directed as a beam that obeys the laws of reflection and refraction.15,16 Thus an ultrasound ...