PART ONE: Physics CHAPTER Physics of Ultrasound 1 Christopher R.B. Merritt SUMMARY OF KEY POINTS • Quality imaging requires an understanding of basic capabilities of conventional gray-scale acoustic principles. imaging. • Image interpretation requires recognition and • Knowledge of mechanical and thermal bioeffects of understanding of common artifacts. ultrasound is necessary for prudent use. • Special modes of operation, including harmonic imaging, • High-intensity focused ultrasound has potential therapeutic compounding, elastography, and Doppler, expand the applications. CHAPTER OUTLINE BASIC ACOUSTICS Two-Dimensional Arrays Interpretation of the Doppler Spectrum Wavelength and Frequency Transducer Selection Interpretation of Color Doppler Propagation of Sound IMAGE DISPLAY AND STORAGE Other Technical Considerations Distance Measurement SPECIAL IMAGING MODES Doppler Frequency Acoustic Impedance Tissue Harmonic Imaging Wall Filters Reflection Spatial Compounding Spectral Broadening Refraction Three-Dimensional Ultrasound Aliasing Attenuation Ultrasound Elastography Doppler Angle INSTRUMENTATION Strain Elastography Sample Volume Size Transmitter Shear Wave Elastography Doppler Gain Transducer IMAGE QUALITY OPERATING MODES: CLINICAL Receiver Spatial Resolution IMPLICATIONS Image Display IMAGING PITFALLS Bioeffects and User Concerns Mechanical Sector Scanners Shadowing and Enhancement THERAPEUTIC APPLICATIONS: Arrays DOPPLER SONOGRAPHY HIGH-INTENSITY FOCUSED Linear Arrays Doppler Signal Processing and Display ULTRASOUND Curved Arrays Doppler Instrumentation Phased Arrays Power Doppler ll diagnostic ultrasound applications are based on the detec- empower ultrasound with its unique diagnostic capabilities. The Ation and display of acoustic energy reflected from interfaces user must understand the fundamentals of the interactions of within the body. These interactions provide the information acoustic energy with tissue and the methods and instruments needed to generate high-resolution, gray-scale images of the used to produce and optimize the ultrasound display. With this body, as well as display information related to blood flow. Its knowledge the user can collect the maximum information from unique imaging attributes have made ultrasound an important each examination, avoiding pitfalls and errors in diagnosis that and versatile medical imaging tool. However, expensive state- may result from the omission of information or the misinterpreta- of-the-art instrumentation does not guarantee the production tion of artifacts.1 of high-quality studies of diagnostic value. Gaining maximum Ultrasound imaging and Doppler ultrasound are based on benefit from this complex technology requires a combination the scattering of sound energy by interfaces of materials with of skills, including knowledge of the physical principles that different properties through interactions governed by acoustic 1 2 PART I Physics physics. The amplitude of reflected energy is used to generate is the hertz (Hz); 1 Hz = 1 cycle per second. High frequencies ultrasound images, and frequency shifts in the backscattered are expressed in kilohertz (kHz; 1 kHz = 1000 Hz) or megahertz ultrasound provide information relating to moving targets such (MHz; 1 MHz = 1,000,000 Hz). as blood. To produce, detect, and process ultrasound data, users In nature, acoustic frequencies span a range from less than must manage numerous variables, many under their direct control. 1 Hz to more than 100,000 Hz (100 kHz). Human hearing is To do this, operators must understand the methods used to limited to the lower part of this range, extending from 20 to generate ultrasound data and the theory and operation of the 20,000 Hz. Ultrasound differs from audible sound only in its instruments that detect, display, and store the acoustic information frequency, and it is 500 to 1000 times higher than the sound we generated in clinical examinations. normally hear. Sound frequencies used for diagnostic applications This chapter provides an overview of the fundamentals of typically range from 2 to 15 MHz, although frequencies as high acoustics, the physics of ultrasound imaging and flow detection, as 50 to 60 MHz are under investigation for certain specialized and ultrasound instrumentation with emphasis on points most imaging applications. In general, the frequencies used for ultra- relevant to clinical practice. A discussion of the therapeutic sound imaging are higher than those used for Doppler. Regardless application of high-intensity focused ultrasound concludes the of the frequency, the same basic principles of acoustics apply. chapter. Propagation of Sound In most clinical applications of ultrasound, brief bursts or pulses BASIC ACOUSTICS of energy are transmitted into the body and propagated through tissue. Acoustic pressure waves can travel in a direction perpen- Wavelength and Frequency dicular to the direction of the particles being displaced (transverse Sound is the result of mechanical energy traveling through matter waves), but in tissue and fluids, sound propagation is primarily as a wave producing alternating compression and rarefaction. along the direction of particle movement (longitudinal waves). Pressure waves are propagated by limited physical displacement Longitudinal waves are important in conventional ultrasound of the material through which the sound is being transmitted. imaging and Doppler, while transverse waves are measured in A plot of these changes in pressure is a sinusoidal waveform shear wave elastography. The speed at which pressure waves (Fig. 1.1), in which the Y axis indicates the pressure at a given move through tissue varies greatly and is affected by the physical point and the X axis indicates time. Changes in pressure with properties of the tissue. Propagation velocity is largely determined time define the basic units of measurement for sound. The distance by the resistance of the medium to compression, which in turn between corresponding points on the time-pressure curve is is influenced by the density of the medium and its stiffness or defined as the wavelength (λ), and the time (T) to complete a elasticity. Propagation velocity is increased by increasing stiffness single cycle is called the period. The number of complete cycles and reduced by decreasing density. In the body, propagation in a unit of time is the frequency (f) of the sound. Frequency velocity of longitudinal waves may be regarded as constant for and period are inversely related. If the period (T) is expressed a given tissue and is not affected by the frequency or wavelength in seconds, f = 1/T, or f = T × sec−1. The unit ofacoustic frequency of the sound. This is in contrast to transverse (shear) waves for FIG. 1.1 Sound Waves. Sound is transmitted mechanically at the molecular level. In the resting state the pressure is uniform throughout the medium. Sound is propagated as a series of alternating pressure waves producing compression and rarefaction of the conducting medium. The time for a pressure wave to pass a given point is the period, T. The frequency of the wave is 1/T. The wavelength, λ, is the distance between corresponding points on the time-pressure curve. CHAPTER 1 Physics of Ultrasound 3 Air 330 Fat 1450 Water 1480 Soft tissue 1540 (average) Liver 1550 Kidney 1560 Blood 1570 Muscle 1580 Bone 4080 1400 1500 1600 1700 1800 Propagation velocity (meters/second) FIG. 1.2 Propagation Velocity. n I the body, propagation velocity of FIG. 1.3 Propagation Velocity Artifact. When sound passes through sound is determined by the physical properties of tissue. As shown, a lesion containing fat, echo return is delayed because fat has a propaga- this varies considerably. Medical ultrasound devices base their measure- tion velocity of 1450 m/sec, which is less than the liver. Because the ments on an assumed average propagation velocity of soft tissue of ultrasound scanner assumes that sound is being propagated at the 1540 m/sec. average velocity of 1540 m/sec, the delay in echo return is interpreted as indicating a deeper target. Therefore the final image shows a mis- registration artifact n i which the diaphragm and other structures deep to the fatty lesion are shown in a deeper position than expected (simulated image). which the velocity is determined by Young modulus, a measure of tissue stiffness or elasticity. Fig. 1.2 shows typical longitudinal propagation velocities for propagation velocity of sound for the tissue is known. For example, a variety of materials. In the body the propagation velocity of if the time from the transmission of a pulse until the return of sound is assumed to be 1540 meters per second (m/sec). This an echo is 0.000145 seconds and the velocity of sound is 1540 m/ value is the average of measurements obtained from normal soft sec, the distance that the sound has traveled must be 22.33 cm tissue.2,3 Although this value represents most soft tissues, such (1540 m/sec × 100 cm/m × 0.000145 sec = 22.33 cm). Because tissues as aerated lung and fat have propagation velocities sig- the time measured includes the time for sound to travel to the nificantly less than 1540 m/sec, whereas tissues such as bone interface and then return along the same path to the transducer, have greater velocities. Because a few normal tissues have propaga- the distance from the transducer to the reflecting interface is tion values significantly different from the average value assumed 22.33 cm/2 = 11.165 cm. By rapidly repeating this process, a by the ultrasound scanner, the display of such