Advances in technology have led to the rapid emergence of new echo modalities such as real-time 3D echocardiography, myocardial mechanics, and contrast echocardiography, along with new applications for the echocardiographer, including interventional echocardiography, monitoring of mechanical circulatory support devices, and intracardiac echocardiography (ICE). A strong working knowledge of ultrasound physics, image formation, and recognition of artifacts is fundamental to excellence in echocardiography.
Considering the complexities of ultrasound physics, a detailed exposition of physical principles is beyond the scope of this chapter. Instead, we choose to explore essentials, laying a strong emphasis on practical application in instrumentation, rather than underlying theory. Where appropriate, we have included more technical details separately for the benefit of those with technology leanings. A glossary with common definitions has been included at the end of the chapter. Further, an attempt has been made in this chapter to communicate complex ideas employing simple examples from relevant day-to-day situations.
BASIC PRINCIPLES
High-frequency ultrasound has been abundantly exploited by nature, notably in the lives of certain marine and land species. In the ocean, dolphins and odontocetes employ high, single-frequency clicks both for the purposes of communication and echo-localization of prey among smaller marine life.1 On land, bats employ ultrasound to identify insects and maneuver potential obstructions along their path of flight, overcoming visual impairment by employing acoustic advantage.2 While vision and hearing are rarely confused, ultrasound promotes heightened perception in the animal world. In more recent times, we have employed sound to “see” in medical imaging as well.
Ultrasound is the region of the sound spectrum having a frequency of over 20 kilohertz, or 20,000 cycles per second. Each cycle is comprised of alternate zones of particle compression and rarefaction, transmitted along the line of propagation (Figure 1.1).3 An adult voice would typically range between 100 and 250 Hz, a standard ‘A’ concert pitch is 440 Hz, and the highest note on an 88-key piano is approximately 4100 Hz.
Ultrasound, however, is much higher than what the human ear can discern, and showcases advantages unavailable to lower frequencies. First, it can be focused and emitted as a beam. Second, it is governed by the physical laws of reflection, refraction and attenuation (Figure 1.2). Third, it allows a characterization of even small-sized reflectors, such as minute regions of tissue. Medical ultrasound generally employs frequencies between 1 and 20 mega hertz, roughly 100,000 times the frequency of the human voice.
SELECTING A TRANSDUCER
One of the first steps in performing an echocardiogram is to select a probe, also known in more technical terms as a transducer.
Contemporary echo machines are equipped with multiple transducers, and each broadband transducer is capable of emitting ultrasound within a range of frequencies. Given that the frequency of the transducer is directly related to the resolution of the echocardiographic image, it is desirable to use the maximum permissible frequency when performing a study. An inherent disadvantage, however, lies in the inverse relationship between frequency and wavelength, which, in-turn, governs penetration. Higher frequency signals suffer from greater attenuation as the ultrasound beam travels away from the transducer, resulting in weaker reflected signals.4 A simple example to substantiate this phenomenon would be that of listening to a concert from the far end of an open field. The bass of the drums and low-pitched instruments travel further to reach one's ear, as compared to the trebles of the high-pitched instruments, which are often faint and attenuated in transit. In a similar fashion, obese or technically challenging patient require the selection of a lower frequency that affords greater penetration.
Echocardiographic systems counter attenuation-related loss of intensity employing both overall gain and time gain compensation (TGC) functions. These functions can be compared with the volume button on a radio that amplifies the strength of the received audio signal. More specifically, TGC allows for a differential increase in signal strength from the far field, combating loss in signal strength owing to attenuation as the signal propagates through a medium. It is important to recognize that the gain functions augment the strength of the reflected signal, and not that of the emitted ultrasound beam.
The intensity of the transmitted signal, however, is controlled by the mechanical index (MI). This is generally pre-configured for each setting, and is rarely adjusted by the operator. Typical MI settings on commercial echocardiographic machines range between 1.0 and 1.4. In the US, the FDA mandates that MI be kept below 1.9. An exception is seen while performing contrast studies, where a lower MI, typically in the range of 0.1 to 0.3, is chosen to ensure the stability of the microbubble infusion during the study period.
UNDERSTANDING TRANSDUCER COMPONENTS
The ultrasound transducer performs the role of a speaker as well as a microphone, switching alternatively from the role of a generator to that of a receiver of ultrasound. Short pulses of ultrasound are emitted, followed by a period of quiescence during which the transducer “listens” for reflected signals. The time taken for ultrasound waves to travel to the target tissue and return provide information on depth of the structure of interest, and strength of the reflected signal provides information on characteristics of the reflector. Figure 1.3 provides a schematic of the most common components of a transducer.
This remarkable ability to convert energy from one form to another—in this case, electrical-to-mechanical vibration and vice-versa—is made possible by the piezo-electric effect. At the core of each transducer is an arrangement of ceramic crystals that rapidly change shape when an alternating current is applied to them. This rapid expansion and contraction of material generates sound vibrations of specific frequencies. Importantly, the crystals are also deformed by reflected acoustic waves, and convert the resultant mechanical deformation into electrical signals that are communicated to the analyzer.5 Adjacent to the crystal, the transducer houses backing material to shorten the excitation response of the crystals. Similar to resting one's palm on a ringing bell, the backing layer shortens the ringing response and generated pulse, thereby improving range resolution. At the surface of the transducer, matching layers minimize the acoustic impedance between the crystals and body, facilitating maximal transmission of ultrasound.6 The use of ultrasound jelly is also used to minimize impedance at the probe-tissue interface during an echocardiographic study.
Another important component of the transducer is the lens. Assuming the face of the transducer is circular, an ultrasound beam takes the shape of a cylinder as it leaves the transducer. This region is referred to as the near field or Fresnel zone. At a certain length from the transducer, also termed as the far field of Fraunhofer zone, the beam diverges and assumes a conical shape (Figure 1.4).7 A reduction in intensity in this region results in poorer image generation, making a lengthening of the near field into an important aim of instrumentation. The lens plays an important role in this function. From a practical stand-point, a distinction between the near field and far field on the arc-shaped echocardiographic display is made using the focus button. Altering the focal plane allows a concentration of ultrasound intensity at a specific depth along the insonation beam, thereby ensuring maximal lateral resolution at this point. While older equipment provided for the delineation of a focal zone employing multiple focal points, newer equipment have adapted technology that are focus-free.
UNDERSTANDING IMAGE RESOLUTION
Resolution, defined as the ability to distinguish between two objects in close proximity, plays an important role in echocardiography. In the context of dynamic imaging techniques, three components of resolution are generally referred to: spatial, temporal and contrast.8 All these components can be optimized during the acquisition of images employing certain setting to the echo-machine settings.
Spatial resolution refers to the ability of the system to distinguish two closely spaced targets in space. The higher the spatial resolution, the smaller the distance that can be discerned. The system's ability to set apart two closely spaced speckles in the myocardium of the left ventricle is a suitable example. If these two points are aligned along the axis of the ultrasound beam, then one behind the other, the resolution capability is referred to as axial resolution (Figure 1.5A).3
Given that resolution at any point along the line of insonation is the same, axial resolution is not affected by depth, but is improved with higher frequency and shorter pulse length. The second form of spatial resolution refers to the ability to distinguish two points side by side, perpendicular to the ultrasound axis. This is referred to as lateral resolution (Figure 1.5B). In contrast, lateral resolution reduces with increasing depth, as the beam diverges in the far field. Further, a wider beam (e.g. a probe with a larger footprint) would also lead to a reduction in lateral resolution.
Temporal resolution refers to the ability to detect motion, and is synonymous with frame rate (frames per second for 2D echo, and volumes per second for 3D). Temporal resolution determines, how seamlessly dynamic images are displayed on a screen. In the late 1920s’ silent movies were made at a frame rate of 24 frames per second (fps), and the occasionally irregular display of images were limited by the speed at which the film roll was played. With the introduction of HDTV in more recent times, seamless, fluidic motion could be experienced on screen as images were relayed at double this speed. Typical frame rates in echocardiography range between 40–100 fps, and can be adjusted to even higher values when employing modalities such as tissue velocity imaging. Temporal resolution is of particular relevance when imaging fast-moving structures such as valves, or in the evaluation of endocarditis.
A number of methods can be employed to improve frame rate during image acquisition. These include: (a) narrowing the sector to decrease the time taken to scan a single frame; (b) decreasing the depth to decrease pulse repetition period; (c) decreasing the line density to reduce the number of scan lines; and (d) turning-off multi-focus, which decreases the number of pulses needed per line. In 3D echocardiography, temporal resolution can be improved by employing a multi-beat capture that integrates subvolumes across subsequent cycles to create a volumetric dataset.
Finally, contrast resolution refers to the ability of the system to distinguish different shades of gray, and is important to delineate borders, characterize tissue detail, and distinguish tissue signals from background noise.
UNDERSTANDING TISSUE HARMONIC IMAGING
The frequency of the ultrasound beam transmitted by the transducer is referred to as fundamental frequency. Non-linear interactions of this frequency with tissue leads to the creation of new frequencies, termed as harmonic frequencies, which are reflected and return to the transducer combined with the fundamental frequency. Harmonic frequencies are generally integer multiples of the original frequency, and are weak close to the chest wall, but strongest at a depths of 4 to 8 cm. Similar to a wave that approaches the shore, harmonic signals undergo a constant change in intensity as they penetrate tissue. From an application standpoint, these harmonic frequencies are important as they reduce near field clutter and improve signal-noise ratio, enhancing endocardial delineation in the process. However, they also make valves appear thicker and should be used with caution in the assessment of valvular pathologies. It is recommended to include both fundamental and harmonic imaging in an echocardiographic examination, keeping in mind the advantages and pitfalls of each of these applications.
UNDERSTANDING ARTIFACTS
An important reason to gain an essential understanding of ultrasound physics is to distinguish an artifact from actual pathologic presentations. Ultrasound artifacts are commonly encountered in clinical practice, and it is important to recognize them to avoid a false diagnosis. We present the most common types seen in an echocardiographic examination.
Reverberation
When an ultrasound beam encounters two strong parallel reflectors, the beam goes back and forth, or reverberates, between these reflectors. The transducer interprets the sound waves received from the reverberation as deeper structures given the extended time taken to return to the probe. A suitable example from echocardiography would be the illusion of a secondary structure at a greater depth owing to reflections from the posterior pericardium in the parasternal long axis view (Figure 1.6). Reverberation artifacts can be improved by changing the angle of insonation to minimize reverberations between strong parallel reflectors.
Acoustic Shadowing
When an ultrasound beam encounters a structure that strongly absorbs or reflects ultrasonic waves, it creates an area of signal void directly behind it. This happens most commonly while imaging solid structures with high attenuation capacities. A good example would be in the setting of prosthetic valves or during the imaging of native structures that are highly calcified, such as the aortic valve (Figure 1.7).
Side Lobes
Side lobe artifacts are generated by low-amplitude beams that project radially from the main beam axis.4
Strong reflectors present along the path of these low-energy, off-axis beams generate echoes that the transducer eventually picks up. These reflected signals are often mistaken as originating from the central beam, and displayed on the echo screen, as if located along the central axis. Given that the laterally radiating signals are much weaker than the primary signal, these reflections also create low-intensity echoes to the sides of the display. Echoes generated by the posterior mitral annulus and atrioventricular groove, for example, can create an impression of a mass within the left atrium (Figure 1.8).
Near Field Clutter
Also referred to as ring down artifact, near field clutter can be attributed to high-amplitude oscillations emitted by the transducer, resulting in artefactual appearances in the near field region of the screen. These are often confused for mural thrombi in the LV or RV apex. Technological advances in instrumentation have minimized the occurrence of these artifacts in current-day equipment (Figure 1.9).
Putting it All Together: Image Optimization in 5 Easy Steps
A sound knowledge of the physical principles of ultrasound can be applied to obtain high-quality images when performing an echocardiographic examination. However, complex instrumentation and innumerable functions on current-day consoles may overwhelm the early career echocardiographer. We suggest 5 essential steps to optimizing 2D images during an echocardiographic study.
Step 1: Choose the Right Transducer Frequency
Choose higher frequencies for pediatric applications or to image structures closer to the chest wall. Choose lower frequencies to image thick chested, obese or technically challenging patients.
Step 2: Adjust Depth and Sector Width
Adjust depth such that the image occupies approximately two-thirds of the sector. Likewise, narrow sector width to improve visualization of the structure of interest. These measures enhance lateral resolution and temporal resolution (Figures 1.10A and B).
Step 3: Employ Tissue Harmonics for Better Endocardial Delineation
Choose harmonic frequencies over fundamental frequencies when imaging myocardial segments and assessing regional motion abnormalities. Use harmonic imaging prudently when imaging valves, however, as they can make them look thicker than they actually are (Figures 1.11A and B).5
Step 4: Adjust Receiver Gains and TGC
Overall gains and TGC compensates for depth-related attenuation by amplifying received ultrasound signals. Maintain an optimal gain to provide for adequate visualization of the structure of interest. Excessive or inadequate gain can distort images (Figures 1.12A and B).
Step 5: Focus
Select an appropriate focal plane to optimize of lateral resolution at a given depth. Exercise caution when choosing multiple focal points in older systems, as this deteriorates temporal resolution (Figures 1.13A and B).6
CONCLUSION
A sound understanding of the basic principles of ultrasound is fundamental to performing a high-quality echocardiographic study. Practical aspects of understanding these principles result in a clearer distinction between artefact and pathological abberrations, superior quality diagnostic images, and greater control over instrumentation and image settings.
ACKNOWLEDGMENTS
The authors acknowledge the kind contributions of Jonas Johnson (PhD) towards illustrations and Monica Vinesh Dillikar (MBBS Dip. Cardiology) and Chandrappa Annappa, Senior Sonographer, towards echocardiographic images used in this chapter.
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GLOSSARY
Absorption: The conversion of sound to heat because of its interaction with tissue.
Acoustic enhancement: An increase in reflection amplitude that occurs when a wave strikes a medium that is located behind a tissue of weak acoustic impedance.
Acoustic impedance: The resistant properties of a medium to sound transmission.
Amplification: Increasing small echoes into larger ones.
Amplitude: The maximal variation within a sound wave.
Anechoic: Any medium that does not produce an echo.
Artifact: Any phenomenon that affects the acquisition or interpretation of an ultrasound image.
Attenuation: The decreasing amplitude of a sound wave as it propagates through tissue.
Attenuation coefficient: The degree of attenuation measured per unit length of wave travel.
Axial resolution: The ability of an ultrasound system to distinguish separate structures that are in close proximity to each other along the same axis.
B-mode: Creation of an ultrasound image in which the display records brightness for each echo based on the strength and time, the echoes are received.
Beam: The sum of all the sound waves generated by the transducer.
Coupling medium: Gel used to provide transmission of sound between the transducer and skin.
Cycle: One complete compression and rarefaction in a sound wave.
Damping: Material placed behind the transducer elements to reduce pulse variation and duration.
Diffuse reflector: (Also called scattering) Returning echoes that are forced to deviate from a straight-line trajectory due to small, localized non-uniformities in the tissue.
Echogenicity: The degree of reflection caused by varying degrees of acoustic impedance within a tissue.
Far Field zone or Fraunhofer zone: The region of the ultrasound beam after divergence
Focal region: The area where an ultrasound beam is at minimum diameter and area.
Focus: The ability to concentrate a sound beam in an area where it normally would not occur.
Frame: The single image that results from one complete scan of the sound beam.
Frame rate: The number of frames that can be displayed per unit of time.
Frequency: The number of cycles that occur per second, measured in Hertz (Hz).
Gray scale: The complete range of brightness between white and black that is displayed in a B-mode image.
Hertz (Hz): Unit of frequency, one cycle per second; unit of pulse repetition frequency, one pulse per second.
Hyperechoic: “Hyper” echoes; those tissues that cause increased reflection.
Incident angle: The angle created between the incident sound beam and a line drawn perpendicular to the medium.
Incident beam: The ultrasound beam that originates from the transducer.
Interface: The boundary between two tissues with different acoustic impedances.
Lateral resolution: The ability of an ultrasound system to differentiate between two objects that are perpendicular to a sound beam.
Longitudinal wave: Movement of particles in the same direction as the direction of wave propagation.
Matching layer: Material placed in front of the transducer elements to reduce the acoustic impedance between the transducer and skin.
Megahertz (MHz): One million hertz.
Mirror-image artifact: The duplication of an object on the opposite side of a strong reflector
Near zone or Fresnel zone: The region of the sound beam near the transducer with high spatial resolution.
Period: The amount of time required to complete one cycle.
Piezoelectric effect: The conversion of electrical energy to mechanical energy and vice versa.
Propagation velocity: The speed through which a wave will travel in a particular medium.
Pulse: A few cycles of a sound wave.
Pulse-echo ultrasound: Imaging and flow measurement that utilize the transmission of pulses to generate a display.
Pulse repetition frequency: The number of pulses per unit of time.
Reflection: The mirror-like redirection and return of a propagating sound wave towards the transducer that follows a standard law of reflection; for example, specular reflection results in the reflected angle being equal to the incident angle of the energy propagation.
Refraction: A change in the direction of wave propagation when traveling from one medium to another with different propagation speeds according to Snell's law.
Resolution: The ability to distinguish between two structures that lie close to one another.
Reverberation: Multiple reflections of the same object that creates the illusion of many objects.
Scattering: See diffuse reflection.
Signal: With regards to sound, it is the acoustic conveyance of information.
Sound: A longitudinal, mechanical wave of acoustic variables.
Sound wave: Traveling variation of acoustic variables.
Specular reflection: Return of echoes in a singular direction after contacting a medium that has a large smooth surface (e.g. bone).
Time gain compensation (TGC): The ability of an ultrasound system to equalize differences in reflection amplitude caused by attenuation and reflector depth.
Transducer: A device that converts energy from one form to another.
Ultrasound: Sound frequencies greater than 20 kHz.
Velocity: The sound speed and direction of motion specified.
Wavelength: The distance over which the acoustic disturbance repeats itself at any instant in time during a cycle.