Principles and Technology of Two-Dimensional Echocardiography
Principles and Technology of Two-Dimensional Echocardiography
Albert C. Perrino Jr.
Two-dimensional (2D) echocardiography generates dynamic images of the heart from reflections of transmitted ultrasound. The echocardiography system transmits a brief pulse of ultrasound that propagates through, and is subsequently reflected from, the cardiac structures encountered. The sound reflections travel back to the ultrasound transducer which records the time delay for each returning reflection. As the speed of sound in tissue is constant, the time delay allows for a precise calculation of the location of the cardiac structures from which the echocardiography system can then create an image map of the heart. Not surprisingly, successful cardiac imaging requires a firm understanding of the interactions of sound and tissue. This chapter reviews the basic principles of ultrasound, its propagation through tissues, and the technologies which create moving images of the heart.
PHYSICAL PROPERTIES OF SOUND WAVES
Sound is the vibration of a physical medium. In clinical echocardiography, a mechanical vibrator, known as the transducer,
is placed in contact with the esophagus (transesophageal echocardiography [TEE]), skin (transthoracic echocardiography), or the heart (epicardial echocardiography) to create tissue vibrations. The resulting tissue vibrations or sound waves consist of areas of compression
(areas where molecules are tightly packed) and rarefaction
(areas where molecules are dispersed) resembling a sine wave (Fig. 1.1
The amplitude of a sound wave represents its peak pressure and is appreciated as loudness. The level of sound energy in an area of tissue is referred to as intensity. The intensity of the sound signal is proportional to the square of the amplitude and is an important factor regarding the potential for tissue damage with ultrasound. For example, lithotripsy uses high-intensity sound signals to fragment renal stones. In contrast, cardiac ultrasound uses low-intensity signals to image tissue, which produces only limited bioeffects. Since levels of sound pressure vary over a large range, it is convenient to use the logarithmic decibel (dB) scale:
where A is the measured sound amplitude of interest and Ar is a standard reference sound level, I is the intensity and Ir is a standard reference intensity.
More simply expressed, each doubling of the sound pressure equals a gain of 6 dB. The U.S. Food and Drug Administration (FDA) limits the maximum intensity output of cardiac ultrasound systems to be less than 720 W/cm2 due to concerns with possible tissue and neurologic damage from mechanical injury (resulting from cavitation or microbubbles caused by rarefaction) and thermal effects. The ALARA principle recommends that clinicians limit exposure levels to that which is As Little As Reasonably Achievable to protect patients.
Frequency and Wavelength
Sound waves are also characterized by their frequency (f), or pitch, expressed in cycles per second, also known as Hertz (Hz), and by their wavelength (λ). These attributes have a significant impact on the depth of penetration of a sound wave in tissue and the image resolution of the ultrasound system.
FIGURE 1.1 Sound wave. Vibrations of the ultrasound transducer create cycles of compression and rarefaction in adjacent tissue. The ultrasound energy is characterized by its amplitude, wavelength, frequency, and propagation velocity. In this example, four sound waves are shown in a period of 0.5 ms. The frequency can be calculated as four cycles divided by 0.5 ms and equals 8 MHz.
The travel velocity or propagation velocity of sound (v) is determined solely by the medium through which it passes. For example, the speed of sound in soft tissue is approximately 1,540 m/s. Velocity can be calculated as the product of wavelength and frequency:
v = λ × f
It becomes apparent that the wavelength and frequency are necessarily inversely related:
λ = v × 1/f
λ = (1,500 m/s)/f
lists the corresponding sound wavelengths and frequencies commonly used in clinical ultrasonography.
TABLE 1.1 Association Between Sound Frequency and Wavelength in Soft Tissue
WHAT IS SO SPECIAL ABOUT ULTRASOUND?
Several favorable physical properties of ultrasound explain its usefulness in clinical imaging. Ultrasound is sound with frequencies above those in the audible range for humans (>20,000 Hz). In clinical echocardiography, frequencies of 2 to 10 MHz are used. These high-frequency, short-wavelength ultrasound beams can be more easily manipulated, focused, and directed to a specific target. Image resolution also increases when higher-frequency sound waves are used (see later).
TRANSDUCER DESIGN AND BEAM FORMATION
The transducers used in echocardiography systems create a brief pulse of ultrasound that is transmitted into the tissue (Fig. 1.5
). To achieve this goal, most TEE transducer designs use the following components:
A ceramic piezoelectric crystal, which acts as an ultrasonic vibrator and receiver
Electrodes, which both conduct electric energy to stimulate the piezoelectric crystal and record the voltage signal from returning echoes
Backing, which acts to rapidly dampen the vibrations of the crystal
Insulation, which prevents unwanted vibration of the transducer from standing waves or extraneous incoming waves
A faceplate, which optimizes the acoustic contact between the piezoelectric crystal and the esophagus. The faceplate may also include an acoustic lens to focus the beam
The following sections detail the inner workings of the modern ultrasound transducer and their effects on the transmitted sound beam and the echocardiographic image.
Formation of Ultrasound Waves: The Piezoelectric Crystal
The heart of the transducer consists of a piezoelectric crystal, which contains polarized molecules trapped within a matrix. The formation of the sound wave used in echocardiography is based on the principle of piezoelectricity. When stimulated by alternating electric current, polarized particles within the crystal matrix vibrate, generating ultrasound. Conversely, when an ultrasound wave strikes the crystal, the resulting vibrations of the polarized particles generate an alternating electric current. Therefore, a piezoelectric crystal can function as both a transmitter and a receiver of ultrasound. This process is the hallmark of piezoelectricity—that is, the transformation of electric energy into mechanical energy and the reverse transformation of mechanical energy into electric energy.
FIGURE 1.5 Transducer components: Creating a sound pulse. A brief transmission of alternating current from the electric connector causes charged particles within the matrix of the piezoelectric crystal to vibrate. The backing material helps to dampen the crystal vibrations quickly, keeping the pulse length short; in this example, it is four wavelengths. An acoustic lens aids in focusing the sound energy. The faceplate contains layers of material that match the acoustic impedance of the esophagus, to avoid unwanted reflections and ensure excellent sound transmission. Epoxy filler secures the working components to the probe.
FIGURE 1.6 Effect of pulse length on axial resolution. A: The transducer emits a long sound pulse. Since the length of this pulse is greater than the length of the atrial septal defect (arrows), the reflections from the two tips of atrial septum are smeared and the defect cannot be resolved. Consequently, the resulting 2D echocardiographic display (right) does not show the abnormality. B: The pulse length has been shortened and is now less than the length of the atrial septal defect. The reflections from each interface are clearly identifiable, and the resulting display (right) shows the defect.
For imaging purposes, the transducer emits a brief burst of ultrasound. Typically, 2D transducers emit a sound pulse of two to four wavelengths. As illustrated in Figure 1.6
, the shorter the length of the sound pulse, the better the axial resolution of the system. Therefore, the shorter the wavelength, the shorter the resulting pulse length and the greater the axial resolution.
The Three-Dimensional Ultrasound Beam
Near and Far Fields
The ultrasound transducer emits a three-dimensional (3D) ultrasound beam similar to the beam of a flash-light (Fig. 1.7
Video 1.1). The physical dimensions of this beam determine the following:
The specific area of the heart examined
The intensity distribution of ultrasound energy
The lateral (side-to-side) and elevational (top-to-bottom) resolution of the system