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 beam travels in a straight line through a homogeneous medium. If the beam meets an interface of different acoustic impedance, however, part of the energy reflects, and the remaining attenuated signal is transmitted. The reflected energy, or echo, is used to construct an image (Fig. 18–2).
Sound energy results in alternating compression and rarefaction of particles in a conducting medium. This alternation, which can be plotted against time (or distance), conforms to a sine-wave pattern (bottom panel). Reproduced with permission from Hagan AD, DeMaria AN. Clinical Applications of Two-Dimensional Echocardiography and Cardiac Doppler. Boston, MA: Little, Brown; 1989.
Upper panel: Attenuation of an ultrasound beam emitted from a transducer. There is reflection and progressive loss of energy at each interface encountered. Lower panel: The reflected wavefronts are recorded as signals of varying amplitudes (A mode) via the piezoelectric crystal. Upper panel modified with permission from Hagan AD, DeMaria AN. Clinical Applications of Two-Dimensional Echocardiography and Cardiac Doppler. Boston, MA: Little, Brown; 1989.
The transducer is responsible for both transmitting and receiving the ultrasound signal. The transducer consists of electrodes and a piezoelectric crystal, whose ionic structure results in deformation of shape when exposed to an electric current. Thus piezoelectric crystals are composed of synthetic materials, such as barium titanate, which, when exposed to electric current from the electrodes, alternately expand and contract to create sound waves. When subjected to the mechanical energy of sound returning from a reflecting surface, the same piezoelectric element changes shape, thereby generating an electrical signal detected by the electrodes (Fig. 18–3). Thus the transducer both produces and receives ultrasonic signals.
The basic principle of ultrasonic imaging. The piezoelectric crystal is activated, producing a transmitted pulse (T), which reflects off the interface. The reflected pulse (R) excites the crystal, producing an electric current. Because the velocity of the pulse is constant, distance can be calculated based on the transit time. (Because the pulse must travel back and forth from the interface, the time is divided by 2.) Modified with permission from Weyman.20
In the past, echographs have both transmitted and received signals of the same frequency. Recently, harmonic imaging, in which ultrasound energy is transmitted at a baseline (fundamental) frequency but then received at a higher multiple (harmonic) of that frequency (usually the first harmonic) has been implemented to enhance the signal-to-noise ratio. Harmonic imaging is based on the change in the ultrasound frequency of a transmitted wave induced by the interaction with a reflecting target. The sinusoidal waveform becomes peaked as it travels through tissue, thereby undergoing a change in frequency.
A simplistic analogy of this phenomenon would ...