REVIEW ARTICLE |
https://doi.org/10.5005/jp-journals-10089-0046 |
Physics of Echocardiography, Knobology and Image Optimization
Department of Cardiac Anaesthesiology, Goa Medical College, Bambolim, Goa, India
Corresponding Author: Sushmit S Kamat, Department of Cardiac Anaesthesiology, Goa Medical College, Bambolim Goa, India, Phone: +91 9850173804, e-mail: sushmit007@rediffmail.com
Received: 08 March 2023; Accepted: 02 April 2023; Published on: 19 February 2024
ABSTRACT
Echocardiography is becoming a widely used tool for the diagnosis and treatment of cardiac conditions. Understanding the physics of echocardiography is very essential for its correct application. Doppler is an equally useful modality in echocardiography for the assessment of blood flow. The aim of this article is to outline the basic physics of echocardiography and Doppler and a brief description of various echocardiography machine controls used for the optimization of echocardiography images.
How to cite this article: Kamat SS. Physics of Echocardiography, Knobology and Image Optimization. J Acute Care 2023;2(3):101–109.
Source of support: Nil
Conflict of interest: None
Keywords: Doppler, Echocardiography, Knobology, Optimization, Physics, Ultrasound.
INTRODUCTION
Echocardiography involves imaging of the heart based on reflected ultrasound waves. The echocardiography machine measures the time delay between transmitted and reflected ultrasound waves. Since the speed of ultrasound is constant in a particular medium, the time delay measured helps to derive the exact distance of the cardiac structures from the transducer and produce an image displayed on the monitor.1
HISTORY OF ECHOCARDIOGRAPHY
Ultrasound waves have been used for ages by bats and dolphins in echolocation to hunt and travel. The discovery of piezoelectricity by Jacques and Pierre Curie in 1880 helped us harness the power of ultrasound. After World War II, military ultrasound technology, such as sound navigation and ranging, was made available for peaceful applications such as medical use. The first major use of cardiac ultrasound was contributed by Dr Inge Edler and Hellmuth Hertz in 1953 in Lund, Sweden, Scania Country. In 1956, Dr Satomura in Japan first utilized the Doppler principle to examine the movement of cardiac structures. Dr Harvey Feigenbaum published the first report of ultrasound diagnosis of pericardial effusion in 1965. Transesophageal echocardiography was first described with M mode by Frazin et al. in an article in circulation in 1976. This imaging modality has successfully evolved with the changing tides of time with the advent of three-dimensional (3D) echocardiography.2,3
PHYSICAL PROPERTIES OF ULTRASOUND
Sound is defined as a pattern of disturbance caused by movement of energy through a medium. As sound waves traverse through a medium, the particles of the medium vibrate producing compressions and rarefactions, which can be represented as a sine wave (Fig 1). Ultrasound has frequency above human audible range (>20,000 Hertz) (Fig 2) and share the same physical properties as sound waves. Hence ultrasound waves also require a source and a medium to propagate and cannot travel through vacuum. Frequency, wavelength, propagation speed, amplitude and intensity are physical properties of ultrasound waves.4
Fig. 1: Sound wave depicted as a sine wave pattern with marked λ and A
Fig. 2: Different soundwave bandwidths of frequencies in Hz
- Amplitude (A) denotes the height of the ultrasound wave and is an indirect measure of the loudness and power of ultrasound. It is measured in watts (W) or decibels.
- Intensity (I) of ultrasound is directly proportional to the square of amplitude and represents the power of ultrasound per unit area denoted in terms of W/cm2. It is related to the potential for tissue damage caused by the ultrasound. The United States Food and Drug Administration limits the maximum intensity of ultrasound systems to <720 W/cm.1
- Wavelength (λ) is the distance between the start of an ultrasound wave to the beginning of the next wave. It is measured in millimeters. Wavelength is inversely proportional to the frequency of the wave.
- Frequency (f) in Hz represents the number of cycles per second in an ultrasound wave. Medical ultrasound used frequencies in the range of 2–15 MHz. The practical implication of knowing the frequency of a particular ultrasound system is that a higher-frequency ultrasound has lower tissue penetration but offers superior image quality, and conversely, lower-frequency ultrasound waves have higher tissue penetration but poor image resolution.
Propagation velocity (V) in m/second is the velocity with which the ultrasound waves travel in a particular medium. It depends on the stiffness and density of the particular medium. A dense medium, such as a bone, has a higher propagation velocity as compared to a less dense medium, such as air which poorly transmits ultrasound. The propagation velocity through soft tissue is approximately 1540 m/second (Fig. 3).5,7
Fig. 3: Propagation velocities of sound in different media
Propagation velocity, frequency, and wavelength are associated by the equation:
V = f × λ
Hence, as seen from the equation, frequency, and wavelength are inversely related (Fig. 4).
Fig. 4: Frequency of ultrasound and corresponding wavelengths in a soft tissue
TISSUE INTERACTIONS OF ULTRASOUND
When ultrasound waves emitted from an ultrasound transducer encounter a particular tissue medium interface, based on the difference in their acoustic properties, various following interactions are possible:
- Reflection—when ultrasound waves strike a smooth reflecting surface such as pericardium, specular reflection occurs, and such objects appear bright or hyperechoic on the screen. In contrast to this, large irregular surfaces such as myocardium lead to nonspecular reflection.
- Refraction—depending on the difference in acoustic properties between two tissues and the angle at which the ultrasound waves make with the tissue interface, ultrasound waves get reflected back or get transmitted through the interface in a different direction. The latter is known as refraction. Refraction is a common source of artifacts in echocardiography and should be minimized for good optimization of echocardiography images.
- Scattering occurs when ultrasound encounters small irregular surfaces such as clumped red blood cells.
- Attenuation—as ultrasound waves travel through a medium, loss of energy occurs by absorption and dispersion of the ultrasound waves. The energy of the ultrasound wave gets converted to heat due to friction leading to absorption.8,9
PIEZOELECTRIC EFFECT AND ULTRASOUND TRANSDUCER
Ultrasound is based on the phenomenon of piezoelectricity. Ultrasound waves are produced by certain materials such as quartz, topaz, etc., having piezoelectric properties, wherein they can transform electrical stimulation into mechanical oscillation (sound) and vice versa. At the core of each ultrasound machine is this piezoelectric crystal transducer which, when activated with electricity, emits ultrasound waves, and when it receives the ultrasound wave reflected back from an object, converts the ultrasound energy into electrical energy.10,11
The design of an ultrasound transducer includes the following parts:
- A piezoelectric crystal at the heart of the transducer, which acts as an emitter and receiver of ultrasound waves.
- Backing plate—dampens the crystal vibrations.
- Insulation—avoids excessive vibrations producing additional ultrasound waves.
- Electrodes—which transmit electricity to the crystal and measure the voltage of returning ultrasound waves.
- Faceplate—maintains acoustic contact with the body surface or esophagus base on the type of transducer.
Based on arrangement of piezoelectric crystals, transducers are termed as (Fig. 5):
Fig. 5: Different kinds of ultrasound transducers—curvilinear and linear
Linear array probes in which the piezoelectric elements are arranged in line.
- Curved array probes where the piezoelectric elements are arranged in a curve.
- Curvilinear—a combination of linear and curved.
- Matrix array (piezoelectric elements arranged in multiple rows) can be linear or curvilinear, or curved. They are used in 3D echocardiography transducers.
Phased array probes—phasing is when different rows of piezoelectric elements are fired at different time intervals giving the ability to steer the ultrasound beam. The piezoelectric elements at the ends of the array are stimulated before the ones in the center, allowing the operator to direct the ultrasound beam and focus at a selected distance from the transducer.12
RESOLUTION
Resolution is the ability of the ultrasound system to distinguish between two objects in close proximity. Image optimization techniques aim to improve the resolution of the displayed image. Based on the orientation of the said two objects, resolution can be of different types, such as:
- Axial resolution refers to the minimum distance between two structures that lie along the path of the ultrasound beam that allows visualization of the structures as separate and distinct in the displayed image. It is dependent on the length of the pulse (spatial pulse length) and pulse duration. Axial resolution can be improved by using an appropriately dampened transducer with a high frequency and broader frequency bandwidth which will ensure a short pulse duration.13
- Lateral (azimuth) and elevational resolution—this is the ability of the beam to detect single small objects across the width of the beam and resolve two adjacent structures that are oriented perpendicular to the beam axis as separate entities. When the objects are horizontally placed perpendicular to the ultrasound beam, the resolution is termed lateral or azimuth resolution. Similarly, when the objects lie perpendicularly vertical to the beam, it is termed an elevational resolution. Both of them are optimal when the ultrasound beam is narrow.
- Temporal resolution (frame rate) is the property to differentiate timebound events and distinguish between rapidly moving objects. It depends on the depth of scanning, sector scan angle, and scanline density.14
ELECTRICAL PROCESSING AND IMAGE FORMATION
Ultrasound waves which are reflected and returned back to the transducer, are converted to a radiofrequency electric signal, and a digital scan converter converts the electric signal to analog video format for display. This process provides us with two types of controls that can be used for image optimization.
Preprocessing controls affect transmission and acquisition of ultrasound and the information that the scanner will access to create the image before it is saved.
Postprocessing controls affect the conversion of the analog video format, hence the cosmetic appearance of the image after it is saved.15
KNOBOLOGY AND IMAGE OPTIMIZATION
The knowledge of various controls on ultrasound machine control panels (Fig. 6) and the process of optimizing their settings help to obtain high-quality echocardiography images with the highest resolution. Accurate image optimization also aids in minimizing artifacts and avoiding pitfalls of echocardiography. Following is a brief description of display modes and commonly used controls for image optimization.
Fig. 6: Typical echocardiography machine control panel—Philips CX50 ultrasound system (Philips, United States of America)
Two-dimensional (2D) Echocardiography
The most primary control or display mode is 2D echocardiography which is the mainstay of echocardiographic examination. The original display format was A mode, where the A of returning signals was plotted on a vertical axis. This evolved into brightness (B) mode, where amplitudes were represented as pixels of varying brightness and higher amplitude as brighter pixels. 2D echocardiography is a modification of B mode. The transducer sequentially directs ultrasound pulses across a 30–90° sector of anatomy and then repeats the process to update the image and capture motion. Each image created by a sector is called a frame. Frame rate is the frequency at which the sector is rescanned per second, which is 30–60 frames/second for 2D echocardiography. The temporal resolution of the image depends on the frame rate; hence image optimization techniques aim to increase the frame rate and improve temporal resolution.16
Since the image quality depends on the amount of ultrasound waves reflected back to the transducer, a particular structure of interest is seen best when it lies perpendicular to the ultrasound beam. Accordingly, the appropriate view or window is to be selected to visualize the structure best. For example, in Video 1, a midesophageal four-chamber view is seen, in which the mitral subvalvular apparatus is not seen as it lies parallel to ultrasound waves. Video 2 shows the appropriate view to be selected if the mitral subvalvular apparatus is the structure of interest, that is, the transgastric two-chamber view.
M Mode
Motion (M) mode displays a series of collected B mode images from a single scan line at a very high frame rate of >2,000 frames per second. It provides an “ice-pick” view of the heart and displays superior dynamic motion and axial resolution. It is best for examining the timing of cardiac events with electrocardiography (Fig. 7).17,18
Fig. 7: M mode echocardiography
Gain
Increasing gain increases the amplitude of returning ultrasound waves at all depths. It should be set at a minimal level and adjusted incrementally, as too high gain can reduce image resolution and increase the chance for the artifact. A trick of the trade is eliminating the operating room, or ambient light prevents us from setting unnecessary higher gain (Fig. 8).
Figs 8A to C: Transgastric midpapillary LV short-axis view with gain setting. (A) Normal; (B) Too high; and (C) Too low
Time Gain Compensation
Selectively adjusts gain at specific depth levels from the transducer by a series of sliding controls. The upper controls affect the near field, and the lower controls affect the far field. Higher settings are ideally set for the far-field structures. Figure 9A is a midesophageal four-chamber view wherein the far-field structures, such as the tricuspid valve and right ventricle, are not visualized. In Figure 9B, increasing the time gain compensation (TGC) setting in the far field helps to visualize these structures more clearly.
Figs 9A and B: Applying TGC settings in the far field of the view
Lateral Gain Compensation
The lateral gain compensation (LGC) also uses a series of sliding controls. The left controls affect the left of the image sector, while the right controls affect the right. This control can help us to delineate myocardial borders better.9 In Figure 10A, a transgastric midpapillary left ventricle (LV) short-axis view is shown where the septal wall of the LV cavity is not visualized accurately. Increasing the LGC setting on the left side of the screen in Figure 10B, the septal wall can be seen better.
Figs 10A and B: Applying LGC settings on the left side of the image sector
Depth
This control sets the depth of field visualized by the transducer. Depth should be set just beyond the structure of interest. Increasing depth decreases resolution by reducing the frame rate (Fig. 11).
Figs 11A and B: (A) The transgastric mid short-axis view with too much depth; (B) The transgastric mid short-axis view with depth correctly set
Sector Width
The setting of the sector width should be kept as small as required to visualize the structure of interest so as to allow higher frame rates (Fig. 12).
Figs 12A and B: (A) A midesophageal aortic valve short-axis view with a normal sector width; (B) Same view with sector width narrowed. Note the increase in frame rate from 46 to 99 Hz
Focus
This control helps to converge the ultrasound beam at a selected depth from the transducer marked at the edge of the sector. The goal is to have the beam narrowest at the structure of interest to improve lateral and elevational resolution. This is achieved by using phase array crystals where crystals at one end are stimulated before those at the center, creating an ultrasound wave directed toward the focus point.19
DOPPLER PRINCIPLES AND ECHOCARDIOGRAPHY
Two-dimensional (2D) echocardiography provides information about the anatomy and function of the heart but is unable to visualize and study the blood flow. By applying Doppler techniques, the direction, and velocity of the blood flow in different parts of the heart and blood vessels can be estimated. The Doppler effect was first described by Austrian physicist Christian Doppler in 1842. It describes how an observer perceives a change in the wavelength or frequency of a sound (or light) wave if the source is moving relative to them. Doppler echocardiography measures blood flow velocities on the basis of the Doppler effect. When an ultrasound beam with known frequency (FT) is transmitted to the heart, it is reflected by red blood cells. The frequency of the reflected ultrasound waves (FR) increases when the red blood cells are moving toward the source of the ultrasound beam and decreases when the red blood cells are moving away. This is similar to the volume of the siren of an ambulance or train, which appears to be louder as the ambulance or train approaches us and decreases when it is traveling in a direction away from us.1
The difference in frequency between transmitted sound and reflected sound is the frequency shift or Doppler shift (FD): (FD = FR − FT), which helps to detect the direction of the blood flow.
The velocity of blood flow is calculated by a mathematical equation:
V = FD × C/2 × FT × Cos θ
Here, C is the speed of ultrasound in soft tissue (1540 m/second), and θ is the Doppler angle (angle between the ultrasound beam and the blood flow). As the Doppler angle approaches 90°, Cos θ becomes 0, and when the Doppler angle is 0, Cos θ is 1; hence the greatest FD is when the ultrasound beam is parallel to the blood flow being interrogated. For accurate determination of velocities, the Doppler angle should be <20°.
The FD calculated by the ultrasound system falls within the audible range of the human ear and is amplified and broadcasted by the system. This can help to guide the operator to correctly align the Doppler beam along the blood flow based on the pitch and loudness of this audible FD. The harshness of the pitch can also give a rough idea about the severity of the lesion, such as a low-pitch soft broadcast indicating a laminar flow, whereas a harsher coarse pitch usually suggests a severe valvular lesion or turbulent flow.
The other presentation of Doppler data is in the form of a time–velocity graph known as spectral Doppler. The returning ultrasound beam from which the FD is calculated contains a spectrum of frequencies. A mathematical technique called fast fourier transform is applied to perform spectral analysis. A spectral Doppler then displays conventionally plotted frequency shifts (shown as velocities) on the vertical axis against time on the horizontal axis. A zero line is drawn horizontally, and flow toward the transducer is plotted above the line, and flow away from the transducer is plotted below the line. For each time point, the gray pixels show the blood flow velocity detected, and the density of the signal (the shade of gray plotted at each point in the spectrum) represents the amplitude.20,21
DOPPLER TECHNIQUES
Pulsed Wave Doppler
Pulsed wave Doppler (PWD) uses a single crystal to emit and receive the ultrasound waves. Velocities at a specific depth or location, which can be set by the operator, are evaluated, known as the sample volume. The length of the sample volume can also be adjusted. The rate at which these pulses are emitted per second is known as pulse repetition frequency (PRF). Since PRF is limited, the maximum velocity which can be measured is limited, known as the Nyquist limit, which is equal to half of PRF (Fig. 13).
Fig. 13: Doppler spectral display—blood flow through the mitral valve captured by using PWD directed from the midesophageal four-chamber view
As a result of this limitation, we cannot use PWD to evaluate high-velocity turbulent flows. If the velocity studied exceeds the Nyquist limit, aliasing occurs where the direction of flow may be shown as the opposite of the actual, and on spectral Doppler, this can be seen as a wraparound display on the other side of the baseline (Fig. 14).
Figs 14 A and B: (A) Aliasing (wraparound) appears in PWD spectral display once velocities exceed the Nyquist limit; (B) The baseline has been adjusted to the upper portion of the display, which increases the Nyquist limit to 60 cm/second for flow away from the transducer and captures the spectral signal without aliasing
To reduce aliasing, an appropriate echocardiography window or view is selected, where sample volume is closest to the probe as this increases PRF, and on spectral Doppler, the baseline is set so as to provide a maximum range in the direction of interest to avoid wraparound display (Video 3 and 4).21
Continuous Wave Doppler
Continuous wave Doppler (CWD) uses two separate crystals, one for continuously emitting the Doppler signals and the other one for continuously receiving them. Therefore, the maximal frequency shift and velocity recorded with CWD are not limited by the PRF or Nyquist phenomenon. Turbulent flows with velocity in excess of 7m/second can be measured using CWD. The only drawback of CWD is that it evaluates the highest velocity along the beam path, but the exact location cannot be determined, and this is termed range ambiguity (Video 5 and Fig. 15).1
Fig. 15: Continuous wave spectral signal, beam positioned across the aortic valve flow from the deep transgastric long-axis view
Color Flow Doppler
Color flow Doppler (CFD) is a combination of 2D echo and PWD. Doppler data based on velocity and direction are color-coded and superimposed on a 2D echocardiography image. Laminar flow toward the transducer is coded red, and flow away is coded as blue. Turbulent velocities coded yellow and green. Since PWD is used, CFD also is subjected to aliasing. When flow velocity is higher than the Nyquist limit, color aliasing occurs and is depicted as a color reversal. Turbulence is characterized by the presence of variance, which is the difference between the mean velocity of flow and individual flow velocities. Since turbulent flow has increased flow velocity differences, the variance is depicted as green color on the screen.22
Tissue Doppler Imaging
Tissue Doppler imaging records the motion of tissue or other structures with a velocity or frequency shift much lower than that of blood flow. Velocities of myocardial tissue are much lower (<30 cm/second) with larger amplitudes than those produced by blood. Therefore, PWD is modified to record the low velocities of myocardial tissue and to reject the high velocities generated by blood flow.22
CONCLUSION
Echocardiography has changed the practice of cardiology over the last decade. It is becoming a widely used tool for the diagnosis and management of cardiac conditions. A thorough understanding of physics and Doppler principles are very essential for the correct application of this useful modality. The detailed knowledge of extensive control options of modern echocardiography systems helps the operator obtain high-quality images. With a good understanding of the various controls available, the echocardiographer can optimize the images displayed so as to prevent any pathologic finding from going undetected.
SUPPLEMENTARY MATERIAL
The supplementary videos 1 to 5 are available online on the website of https://www.jacutecare.com/journalDetails/JAC
Video 1: A midesophageal 4-chamber view is seen, in which the mitral subvalvular apparatus is not seen as it lies parallel to ultrasound waves.
Video 2: The appropriate view to be selected if mitral subvalvular apparatus is the structure of interest, i.e. Transgastric 2-chamber view.
Video 3: Pulsed wave Doppler directed perpendicular to the flow across aortic valve in midesophageal AV short axis view fails to detect flow and shows a poor audible broadcast and spectral Doppler.
Video 4: In the same patient, Doppler beam directed parallel to flow across aortic valve using deep transgastric view results in louder pitch of broadcast and better capture of flow velocities on spectral Doppler.
Video 5: Continuous wave Doppler directed parallel to flow across aortic valve using deep transgastric view in a patient with severe aortic stenosis shows harsher pitch and higher velocities on spectral Doppler.
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