Echocardiography

The use of echocardiography as an imaging modality has increased substantially over the past decade. Cardiologists perform most echocardiography studies, with internists being the next most common providers of these studies (15% of all Medicare charges submitted for the procedure).[1] The objective of this article is to provide clinicians with a brief review of basic principles and instrumentation of echocardiography.

Neither bone nor air is a good transmission medium for ultrasound waves; accordingly, specific windows (eg, apical, parasternal, subcostal, and suprasternal) are used to image the heart.

Two-dimensional (2-D) and 3-D echocardiography provide real-time imaging of heart structures throughout the cardiac cycle. Doppler echocardiography provides information on blood movement inside cardiac structures and on hemodynamics.

Tissue Doppler imaging (TDI) provides information about movement of cardiac structures. The relation between the dynamics of cardiac structures and the hemodynamics of the blood inside these structures provides information about cardiac diastolic and systolic function. Echocardiography is continuously evolving and constantly being augmented by newer modalities, such as tissue harmonics, speckle tracking, tissue Doppler strain, and tissue characterization.

The advent of myocardial perfusion echocardiography (MCE) has allowed​ functional evaluation of the coronary microcirculation, including quantitative coronary blood flow and fractional flow reserve.[2] This has helped to improve "the definition of ischemic burden and the relative contribution of collaterals in noncritical coronary stenosis." Moreover, MCE identifies no-reflow within myocardial infarctions (MIs) and low-flow around MIs, predicts potential functional recovery of stunned myocardium with appropriate interventions, and appears to have a diagnostic performance comparable to positron emission tomography (PET) scanning in microvascular reserve/dysfunction in angina patients.[2]

Dedicated training for competent performance and interpretation of echocardiography is essential. The American College of Cardiology (ACC) and the American Heart Association (AHA) have recommended a set of minimum knowledge and training requirements for the performance and interpretation of echocardiography, including a minimum number of 150 performed and 300 interpreted examinations for basic level II (for level III: 300 performed/750 interpreted) in interpreting echocardiography.[3, 4, 5]  Similar guidelines have been developed in Canada.[6, 7]

Basic principles of echocardiography

Humans can hear sound waves with frequencies ranging from 20 to 20,000 cycles per second—that is, from 20 Hertz (Hz) to 20 kHz. Frequencies higher than 20 kHz are referred to as ultrasound. Diagnostic medical ultrasonography usually uses transducers with frequencies of 1-20 MHz. Ultrasound waves are described in terms of the following features:

• Frequency (Hz)

• Wavelength (mm)

• Amplitude, or loudness (decibels [dB])

• Velocity of propagation (m/sec) - This varies according to the type of tissue medium carrying the wave (it is approximately 1540 m/sec in blood)

These phenomena are the basis for many of the clinical applications and calculations of echocardiography.

Interaction of ultrasound waves with tissue results in reflection, scattering, refraction, and attenuation of the waves. 2-D echocardiographic imaging is based on reflection of transmitted ultrasound waves. Doppler analysis is based on the scattering of ultrasound waves from moving blood cells, with a resulting change in the frequency of the waves received at the transducer. Attenuation limits the depth of ultrasound penetration. Refraction, a change in the direction of the ultrasound wave, results in imaging artifacts.

Because wavelength (λ) times frequency (ƒ) equals the propagation velocity (с), or λ × ƒ = с, and the propagation velocity in the heart is 1540 m/sec, the wavelength for any transducer frequency can be calculated as follows:

λ (mm) = 1.54/ƒ (MHz)

Wavelength has clinical importance, in that image resolution cannot be greater than 1-2 wavelengths. That is to say, the higher the frequency, the shorter the wavelength and the better the image resolution.

The depth to which ultrasound waves penetrate the body is directly related to wavelength (ie, shorter wavelengths penetrate a shorter distance).[8] The depth of penetration for adequate imaging tends to be limited to about 200 wavelengths. Thus, an obvious tradeoff exists between image resolution and depth of penetration. Thus, a 1-MHz transducer has a penetration depth of 30 cm and resolution of 1.5 mm, whereas a 5-MHz transducer has a lesser penetration depth, only 6 cm, but a higher resolution, approximately 0.3 mm.

2-D imaging depends on reflection of ultrasound waves. These waves are reflected at tissue boundaries and interfaces, with the quantity dependent on the relative change in acoustic impedance (ie, density) between the two tissues. Optimal return of reflected ultrasound waves occurs at a perpendicular angle (90°).

Doppler flow imaging depends on the scattering phenomenon, and its optimal flow angle is the opposite of that for 2-D imaging (ie, parallel to the flow of interest, rather than perpendicular). The Doppler effect may be simply stated as follows: A person moving toward a sound source will hear a tone with higher frequency than the emitted wave frequency, whereas a person moving away from the source of sound will hear the tone with a lower frequency than the emitted wave frequency.

Additional common terms and concepts employed in echocardiography include the following:

• Duty factor - The fraction of time during which a transducer is sending an ultrasound impulse

• Intensity - Power divided by area, expressed as watts (W)/cm2

• Attenuation - Reduction in intensity with travel, resulting from the effects of absorption plus those of scattering; in tissues, attenuation can be calculated by means of the following equation: 0.5 dB/cm-MHz × depth (cm) × transducer frequency (MHz)

• Round-trip travel of an ultrasound beam in tissue - 13 μsec/cm

• Variables affecting pulse repetition frequency (PRF) - PRF is limited by the following equation: Depth (cm) × number of foci × lines/frame × frames/sec < 77,000

The ACC, the AHA, and the American Society of Echocardiography (ASE) have published detailed practice guidelines for the clinical application of echocardiography.[9] More recently, these and other bodies have collaborated to establish appropriate use criteria for echocardiography.[10]

Briefly, indications of echocardiography may be divided into structural imaging and hemodynamic imaging. Indications for structural imaging include the following:

• Structural imaging of the pericardium (eg, to exclude pericardial effusion)

• Structural imaging of the left or right ventricle and their cavities (eg, to evaluate ventricular hypertrophy, dilatation, or wall motion abnormality; to visualize thrombi)

• Structural imaging of the valves (eg, mitral stenosis, aortic stenosis, mitral valve prolapse; see the first image below)

• Structural imaging of the great vessels (eg, aortic dissection)

• Structural imaging of atria and septa between cardiac chambers (eg, congenital heart disease, traumatic heart disease; see the second image below)

  Echocardiography. This is an M-mode image across the aortic valve as well as the left atrium plotted against time.

  Echocardiography. This is a two-dimensional (2-D) apical four-chamber image.

Indications for hemodynamic imaging through Doppler techniques include the following:

• Imaging of blood flow through heart valves (eg, valvular stenosis and regurgitation; see the image below)

• Imaging of blood flow through the cardiac chambers (eg, cardiac output calculation, assessment of diastolic and systolic function of the heart)

• Tissue Doppler imaging of heart structures - This, compared with Doppler imaging of blood flow through these structures, provides valuable information about the dynamics of heart function; for example, in diastolic dysfunction, an increased ratio of early transmitral Doppler blood flow to early mitral annulus tissue Doppler indicates that a high intra-atrial pressure is an important driving force

  Echocardiography. This color Doppler image across the mitral valve shows evidence of mitral regurgitation; the color Doppler scale shown on the left indicates the Nyquist limit.

Echocardiography has no contraindications. It should be kept in mind, however, that this modality may yield only limited information in patients at the extremes of adult body weight, because a thick chest wall (in markedly obese patients) or overcrowded ribs (in severely underweight patients) may limit the penetration of ultrasound waves.

The following considerations should be kept in mind in the performance of echocardiography:

• Transducer selection - Although each transducer has a stated frequency, with each burst it actually emits a range of different ultrasound wave frequencies, and this range is called its frequency bandwidth; a wider emitted bandwidth yields better resolution of distant structures and also allows a short pulse, thus avoiding overlap between the reflected ultrasound signals; a smaller transducer "footprint" or aperture size is an advantage, in that it facilitates use in narrow intercostal spaces

• Depth adjustment - To improve resolution, depth should be decreased to the minimum needed to visualize the structure of interest; in an average adult, a depth of 16 cm is usually adequate for the apical view, and a depth of 12 cm suffices for the parasternal view

• Transducer frequency - This should adjusted to the maximum that permits adequate depth penetration[11]

• Time gain compensation (TGC) - This function should be set in the midrange, with lower gain in the near field and higher setting in the far field to compensate for attenuation of the beam at higher depths[11]

• Tissue harmonics - Harmonic imaging can result in a false appearance of a thickened valve leaflet; if there is doubt about the actual valve thickness, the tissue harmonics feature should be turned off and a reassessment made

• Transmit gain/output - This should be adjusted to optimize image brightness; if emitted energy is too low, everything appears black, but if it is too high, a whiteout results; it should initially be set to high and then adjusted downward[11]

• Dynamic range/compress function - If image is quality is poor, this function should be decreased to produce better contrast images; on the other hand, increasing the setting of this function softens the images[11]

• Focus function - This is used to clarify a structure of specific interest (eg, an apical thrombus in the left ventricle); when the beam is focused, it is adjusted to be narrowest at the area of interest, resulting in better resolution

• Electrocardiography (ECG) lead placement - Because digital echocardiography stores few cycles for each view, ectopic beats can result in a false impression about wall motion or ventricular function; for example, a postectopic beat compensatory period may falsely increase the ejection fraction or Doppler transvalvular gradient; in general, store beats without ectopic activity, and in atrial fibrillation, record at least 7-10 beats

• Contrast agents - These should be considered for improving endocardial delineation

• Timing - Images should be obtained at end-expiration, when the heart is closer to the transducer

• A good continuous Doppler signal should yield a Doppler spectral signal that has a smooth contour with well-defined edge, onset, and end of flow; the display is "filled in" because lower-velocity signals throughout the pathway of the beam are captured by the continuous beam; the audible signal is tonal and smooth

• In continuous-wave Doppler, although the maximum frequency shift depends on the intercept angle between the Doppler beam and the flow of interest, the amplitude (grayscale intensity) and shape of spectral display are less dependent on the intercept angle; thus, a good Doppler signal may be recorded at a nonparallel intercept angle, resulting in underestimation of flow velocity[8] ; accordingly, the flow of interest should be examined from multiple windows to discover the highest value, which is then assumed to represent the parallel flow

• In Doppler measurement, the wall filter should be set to low to avoid overestimation of the low velocities

• In Doppler imaging, the gate width should be adjusted according to the flow of interest: 1-2 mm for mitral left ventricular outflow, 3-4 mm for pulmonary venous flow, and 5-10 mm for TDI

• In TDI, a sample size of 6-8 mm should be used, the Doppler scale should be decreased (velocities here are much lower than blood flow velocities), power and gain should be decreased (the amplitude are strong enough), the filter should be decreased (otherwise the lower velocities of tissue will be filtered out), and the reject and compress functions should be increased

• Digital echocardiography is now state-of-the-art; to enable interoperability between laboratories, all echocardiography machines should use Digital Imaging and Communications in Medicine (DICOM) standards for processing and storing information

The equipment required for echocardiography includes an echocardiography machine, a suitable transducer, and, for contrast examinations, contrast material. Proper adjustment of the settings on the echocardiography machine is crucial.

2-D echocardiography instrument settings

The following settings can be adjusted in most echocardiography machines:

• Power output - This adjusts the total ultrasound energy emitted by the transducer

• Time-gain compensation (TGC) - These controls allow differential adjustment of gain at different image depths; near-field gain can be set at a lower value (since reflected signal is strong) with gradual increase at midfield and higher gain in far field; usually, the degree of TGC amplification is displayed beside the echocardiographic image

• Depth - A tradeoff exists between depth of image, number of lines per sector (affecting lateral resolution), and number of frames per second (temporal resolution); this control allows the operator to adjust the depth according to body habitus and the structure of interest

• Sector width - This control can be used to increase lateral or temporal resolution by decreasing the number of ultrasound lines in each frame

• Grayscale/dynamic range (also called compression) - This postprocessing feature allows adjustment of the gray/white scale in relation to received signal intensity; increasing the compression results in a softer picture that may enhance lower-level signals, and decreasing it results in higher-quality contrast images in which weaker signals are eliminated, noise is reduced, and the strongest echo signals are enhanced; a variation of this function uses color intensity for each amplitude level

• Focusing - The focal zone in a beam is the zone where the beam is narrowest and spatial resolution is therefore best; this feature improves visualization of the structure of interest at the expense of distant structures

Tissue harmonics imaging

Standard ultrasound imaging is based on capturing the reflected ultrasound beam from tissue interfaces, which have the same frequency as the transmitted beam. Tissue harmonics imaging is based on the harmonic frequency energy generated as the ultrasound signal propagates through the tissue. These harmonic frequencies result from the nonlinear effects of the interaction of ultrasound with tissues, resulting in new waveforms of higher frequency that are multiples of the baseline frequency.

Tissue harmonics imaging has 2 important properties. The first is that the strength of the harmonic signal increases with the depth of propagation; this increased strength can overcome the limitation of traditional ultrasonography in which deeper structures are less well penetrated by ultrasound waves. Thus, harmonic imaging reduces near-field artifacts and improves far-field visualization.

The second property is that stronger fundamental frequencies produce stronger harmonics. Because valves and other planar objects may appear thicker than normal with harmonic imaging, most examiners use both standard and harmonic imaging as needed throughout the examination.

Doppler instrument settings

The following settings are adjustable on most Doppler instruments:

• Power output - This is the electrical energy transmitted to the transducer

• Filter - This function eliminates specific frequencies in order to enhance other frequencies that may be of specific interest (eg, elimination of low-frequency Doppler signals due to motion of myocardium and valves in order to yield a cleaner envelope of valvular blood flow)

• Baseline shift - This moves the midline horizontal axis upward or downward to allow displaying of a full-spectrum pulsed wave form

• Velocity range - This expands or compresses the vertical axis scale

• Postprocessing options (compression or dynamic range) - This function changes the grayscale in relation to the received signal

• Sample volume depth in pulsed Doppler - This adjusts the depth of the sample volume

• Sample volume length in pulsed Doppler - Typically, a sample volume length of 5 mm is used, but sample volume length can range from 20 mm to 1 mm; a larger sample size can capture weak signals at the risk of displaying more than 1 flow (eg, a large Doppler sample near the mitral valve may capture both left ventricular outflow and mitral inflow; see the image below); increasing the sample volume length means increasing the time in which the transducer receives returning Doppler signals

  Echocardiography. This is a spectral Doppler image of mitral valve inflow.

• Grayscale/dynamic range (also called compression) - This postprocessing feature allows adjustment the gray/white scale in relation to received signal intensity; for example, decreasing the dynamic range/compress function can improve poor quality images

Color Doppler instrument settings

In addition to depth and sector scan width, instrument settings on color Doppler instruments may include low-pass filter settings, gain, power output, and compression functions. Most instruments provide several choices of the color map used to develop velocity information, which is usually displayed on the echocardiography screen alongside the image. The general recommendation is to adjust color gain until noise appears in the color.

Because color Doppler is a pulsed Doppler technique, aliasing is an important problem (see Technique). The flow velocity resulting in aliasing (the Nyquist limit) can be adjusted on the machine. Typically, it is adjusted to 50-60 cm/sec. This means that a blood flow velocity equal to 50-60 cm/sec will be properly displayed superimposed on the 2-D picture using the correct colors for the blood direction and velocity. On the other hand, the lowest velocity that is displayed on the color map can be calculated from the following equation:

Minimal displayed velocity = Nyquist limit × 2/32

Therefore, decreasing the Nyquist limit increases the lowest velocity displayed, which increases the size of a jet area and increases the likelihood of aliasing.

In contrast to 2-D echocardiography, in which a higher transducer frequency allows better resolution, a lower transducer frequency in color Doppler imaging permits measurement of higher velocities. This is related to the Nyquist limit and the Doppler shift equation (see Technique).

A higher transducer frequency has a lower Nyquist limit. This explains why transesophageal echocardiography (TEE) transducers (which usually have a higher frequency so as to achieve better 2-D resolution) produce a larger area of flow disturbance than transthoracic echocardiography (TTE) transducers.

The wall filter excludes low-velocity/high-amplitude signals from myocardial motion. A typical initial setting is 400 Hz.

Minimizing the color sector width and depth increases the frame rate and, consequently, maximizes the color resolution.

Agents for contrast echocardiography

Contrast material is helpful when the endocardial surface of cardiac structures is difficult to visualize. It is also useful in elucidating the presence of intracardiac shunts: it can detect passage of blood from one chamber to another even when the defect itself cannot be clearly seen.

The contrast material used in echocardiography is not nephrotic. The simplest contrast material is agitated saline (saline mixed with some blood drops, exchanged quickly between 2 syringes across a 3-way stopcock). Because the bubbles of agitated saline are cleared by the lungs, this material is used to elucidate right-heart structures or to rule out right-to-left shunts (ie, contrast material appearing in the left side of the heart).

Commercially available contrast agents consist of microbubbles of perfluorocarbon gas encased in shells of albumin or synthetic phospholipid. These stable microbubbles reflect ultrasound and opacify intracardiac chambers, resulting in clear delineation of endocardial borders. Contrast agents are utilized in stress echocardiography, where clear visualization of endocardial borders is vital for correct reporting.

Anesthesia is generally not necessary for TTE examinations. For most of the echocardiographic examination, the patient should be placed in the left lateral decubitus position, with the left arm extended behind the head. However, for the subcostal view, the patient may be placed in the recumbent position.

Key technical aspects of 2-D echocardiography include the transducer type, the shape and size of the ultrasound beam, and the resolution available.

Transducer type

A detailed description of each different type of transducer is beyond the scope of this brief review. The main element in transducers is the piezoelectric crystal (titanate ceramic or quartz) that both emits and receives ultrasound waves. The frequency of a transducer is related to the nature and thickness of the piezoelectric crystal.

Characteristics of ultrasound beam

The shape and size of the ultrasound beam can vary, depending on several factors, including the design of the transducer and the inherent characters of ultrasound waves. Initially, the beam is columnar, but it gradually becomes divergent (less focused), and this divergence results in deterioration of the image quality. The length of the focused beam is directly related to the diameter of the transducer and the ultrasound beam frequency.

The shape of the beam can be altered to have a focal depth (narrowest point) by changing the surface of the piezoelectric crystal to be concave or by adding an acoustic lens. To optimize the picture, the focal length of the beam should be located at the area of imaging interest. Some transducers allow manipulation of the focal zone during the examination.

Axial, lateral, and elevational resolution

The ultrasound beam may be thought of as a quadrilateral pyramid with its head at the transducer. It has resolution in 3 dimensions: axial resolution (ie, resolution along the length of the beam), lateral resolution (ie, resolution from side to side across the 2-D image), and elevational resolution (ie, resolution across the thickness of the tomographic slice).

Of the 3 resolutions, axial resolution is the most precise. Accordingly, quantitative measurements are made most reliably by using data derived from a perpendicular alignment of the beam on the structure of interest. To improve axial resolution, a shorter wavelength and a wider bandwidth should be used.

Lateral resolution varies between different depths of the scanned image, being most dependent on beam width or focusing at each depth. With a focused beam at the point of interest, lateral resolution may approach axial resolution; however, with the same image, points at greater depths may be seen in lower resolutions as the beam starts to diverge.

The lack of lateral resolution at greater depths results in blurring of images at far field. This phenomenon is responsible for known echocardiographic artifacts (eg, a point of calcification at a deep structure may appear as a linear abnormal structure). Other factors that affect lateral resolution include transducer frequency, transducer aperture or footprint, and bandwidth.

Elevational resolution depends on transducer design, beam focusing in the elevational dimension, and transducer frequency.[8, 12] It is the most difficult type of resolution to appreciate, though it can be a source of many artifacts in echocardiography.

In general, echocardiography has a “slice thickness” of 3-10 mm, which may vary with the depth of the image. Accordingly, a strong reflector within the slice (eg, a calcified structure or prosthetic material) may appear to be within another structure that is being visualized on the image. Examples include a eustachian valve that appears as a linear shadow within the middle of the right atrium and a linear echo in the aortic lumen from an adjacent calcified atheroma that appears as a lumen dissection.

A fourth type of resolution, temporal resolution, is simply resolution across time, which is primarily related to frame rate (expressed in frames/sec).

Image procession and production

In 2-D echocardiography, the ultrasound beam sweeps across the tomographic plane, emitting bursts of ultrasound wave impulses and receiving the reflected signals from different structures. Accordingly, each ultrasound line in the sector consumes a finite amount of time (which varies with the depth of interest), and each full tomographic scan is composed of many ultrasound lines.

The time needed to acquire a full image is directly related to the width of the sector (ie, the number of lines) and the depth of the sector (ie, the length of each line). Moreover, because 2-D echocardiography shows real-time images, multiple frames must be acquired every second to yield a real-time display. A tradeoff exists between scan line density and the number of images per second. A high frame rate (> 30 frames/sec) is desirable for accurate display of cardiac movement.

As the ultrasound beam sweeps through the tomographic plane, information is received by the transducer and forwarded for processing. Each ultrasound signal received by the piezoelectric crystal generates an electric signal with an amplitude that is proportional to the acoustic impedance and a timing that is proportional to the distance from the transducer.

The complex process of linking each received signal to a specific point on the sector on the basis of its timing and marking that point with a specific brightness level on the basis of its intensity in comparison to the emitted wave is called image processing. Typically, this process includes signal amplification, time-gain compensation, filtering (to reduce noise), compression, and rectification. Some of these functions (eg, filtering level or postprocessing) may be manually controlled by the operator to modify the image.[8, 12]

As ultrasound waves strike moving structures (eg, flowing blood cells), the frequency of the reflected sound changes in relation to the moving target velocity and direction. The difference in frequency between the transmitted frequency (FT) and the scattered signal received by the transducer (Fr) is the Doppler shift:

Doppler shift = Fr – FT

The Doppler equation is subsequently used to estimate intracardiac velocities as follows:

Blood flow velocity = c (Fr – FT)/2 FT (cos θ)

where c is the speed of sound in blood (known to be 1540 m/sec) and θ is the intercept angle between the ultrasound beam and the direction of blood flow. The 2 is added because the beam has to travel both to and from the target. As the cosine θ of 0° and 180° is 1, the best scenario for this calculation is to have the Doppler beam parallel to the blood flow (the opposite of the scenario for 2-D imaging).

As the transducer receives the back-scattered sound waves, the frequencies of the received waves are analyzed and compared with the transmitted signal frequencies. The display generated in this manner is termed a spectral analysis.

The spectral analysis display shows time on a midline horizontal axis and frequencies in the vertical axis, so that frequency shifts indicating blood flow toward the transducer are above the midline horizontal axis and frequency shifts indicating blood flow away from the transducer are below the midline horizontal axis. Each frequency signal is displayed as a pixel on the vertical axis, with the grayscale indicating the amplitude (or loudness) and the position on the vertical axis indicating the blood flow velocity (or frequency shift).

Continuous-wave Doppler ultrasonography

Continuous-wave Doppler ultrasonography uses 2 piezoelectric crystals, of which one continuously emits ultrasound and the other continuously receives backscattered signals. Because, unlike pulsed-wave Doppler (see below), it sends and receives waves continuously, continuous-wave Doppler cannot sample back-scattered waves at any specific point along its pathway. Rather, the returning ultrasound waves represent all ultrasound frequencies reflected by moving bodies in the path of the continuous Doppler beam.

The major advantage of this Doppler modality is that very high velocities can be accurately measured because sampling is continuous. The major disadvantage is that it cannot sample a specific point along its pathway.

Pulsed-wave Doppler ultrasonography

Pulsed-wave Doppler imaging allows sampling of movement velocity from a specific intracardiac depth (at a specific point of interest, width can be varied by adjusting a sample length function). A pulse of ultrasound is transmitted, and after a specific time interval (determined by the depth of interest), the transducer briefly samples the back-scattered signals. This transmit-wait-receive cycle is repeated at an interval termed the pulse repetition frequency (PRF).[8]

Because the wait time is determined by the depth of interest (ie, it represents the time needed for the ultrasound signal to travel to and from the point of interest), PRF is depth-dependent. That is, the PRF is high for a nearby point of interest and low for a distant one.

The ability of pulsed-wave Doppler to sample from a specific point is hampered by a phenomenon called signal aliasing. For accurate determination of wavelength, a wave form must be sampled at least twice in each cycle. An analogy may be made with viewing a rotating fan. As long as the fan is rotating at a speed below the eye’s natural sampling speed, it appears to be rotating clockwise. However, when the fan accelerates so that is rotating faster than the eye’s natural sampling speed, it appears to be rotating counterclockwise.

In pulsed-wave Doppler, signal aliasing results in a wave form that may appear to be moving in the opposite direction to its actual direction (eg, a signal that should be toward the transducer may appear on the opposite side of the horizontal midline axis).

The Nyquist limit is the maximum frequency shift detectable by pulsed-wave Doppler. Because a wave needs to be sampled at least twice for accurate determination of its wavelength, the Nyquist limit is one-half the PRF.

If the velocity of interest exceeds the Nyquist limit by a small degree, signal aliasing is seen with the signal cutoff at the edge of the display and the top of the waveform appearing on the reverse side of the horizontal midline. If the velocity is much higher than the Nyquist limit, the wave wraps around the whole spectral display, so that determining its original direction may be difficult (see the image below).

Several techniques can be used to mitigate aliasing in pulsed-wave Doppler ultrasonography. If aliasing is minimal, shifting the baseline to allow longer spectrum is the easiest solution. Alternatively, continuous Doppler can be used instead; because this modality continuously emits and receives, aliasing does not occur. Other methods include increasing the PRF for that depth (the greater the depth, the lower the PRF) or using a lower-frequency transducer.

Doppler color flow imaging

Doppler color flow imaging is based on the principles of pulsed Doppler echocardiography. However, rather than evaluating one sample volume depth along a single beam, it evaluates multiple sample volumes along multiple sampling lines. Moreover, instead of grayscale spectral analysis, the received backscatters are displayed on a 2-D image in a color-coded pattern, with flow toward the transducer shown in red and flow away from the transducer shown in blue.

Because Doppler color flow imaging is a pulsed Doppler modality, aliasing occurs, and velocity above the Nyquist limit would show as bright yellow/white with color reversal.

Tissue Doppler imaging

Tissue Doppler imaging (TDI) is a pulsed Doppler technique that has led to improved understanding of the relation between the hemodynamics of blood flow and the movement of myocardial structures. Tissue motion creates Doppler shifts that are approximately 40 dB higher than Doppler signals from blood flow, whereas tissue velocities rarely exceed 20 cm/sec. Thus, TDI is based on adjusting the pulsed Doppler filter and gain to include low-velocity/high-amplitude myocardial motion instead of high-velocity/low-amplitude blood flow.

In regular Doppler, filtering is increased to eliminate the low-velocity signal; in TDI, filtering is decreased to allow detection of low-velocity signals. In addition, the Doppler transmit gain is decreased to exclude low-amplitude blood signals, mainly allowing high-amplitude tissue motion recording.[13]

TDI measurements of tissue strain and strain rate can be used to assess the dynamics of contraction and relaxation. Strain rate is the difference in myocardial velocity between 2 axial points (usually separated by 1 cm) along the path of the ultrasound beam; strain is an integral of strain rate over time. The attractiveness of these measures of ventricular function is their relative load independence.[14]

Other TDI-based indices include myocardial performance index, quantification of regional wall motion, and intersegmental delay in peak systolic longitudinal motion between segments.[15]

3-D echocardiography

Whereas computed tomography (CT) and magnetic resonance imaging (MRI) are capable of producing 3-D images of cardiac structure by reconstructing slices of images together, 3-D echocardiography can acquire 3-D images directly. The 3-D echocardiography transducer transmits a 3-D beam and receives ultrasound waves in a 3-D pattern.

Other newer modes of echocardiography

Newer modes of imaging include tissue characterization and tissue tracking. Tissue characterization is based on quantitation of the back-scattered signals from the myocardium. Tissue tracking (or speckle tracking) involves identification of multiple unique patterns of echocardiographic pixel intensity that are automatically tracked throughout the cardiac cycle. Speckle tracking provides a unique capability for evaluation of myocardial torsion, which can then be used in the assessment of systolic and diastolic functions.[15, 16]

Digital echocardiography

Echocardiographic images were initially stored on videotapes; however, digital imaging is now the state of the art in echocardiography and is endorsed by the American Society of Echocardiography (ASE).[17] Advantages of digital echocardiography include more efficient reading, easy comparison between studies, and higher image quality. However, two important challenges faced digital echocardiography: managing the size of the electronic files stored and standardizing the storage format to enable interoperability between laboratories.

Typical echocardiography cine loops consists of 480 rows and 640 columns, with 24 bits used to represent the color of each pixel. The typical frame rate is 30 Hz. Multiplying these numbers together yields an enormous storage requirement: 221,184,000 bits/sec, or more than 16 gigabytes of storage for a 10-minute study. To accommodate these storage requirements, a combined strategy of clinical compression (storage of only a single cardiac cycle or a few cycles for a given view) and digital compression is required.

To facilitate interoperability between laboratories, the Digital Imaging and Communications in Medicine (DICOM) image formatting standard was adopted as the standard for echocardiographic imaging. DICOM is simply a set of rules to specify how images and other data should be exchanged between compliant pieces of equipment. An in-depth review of the DICOM standard for echocardiography is available elsewhere.[18]

Handheld echocardiography

In the past two decades, advances in technology have allowed the development of small, compact, handheld ultrasonographic devices that use batteries and have reasonably good image quality.[19]  The advent of these devices has allowed quick clinical decision making because of their ease of use, availability, and transportability, as well as their relative low cost—all of which has also allowed for use among various clinicians and nonechocardiographers at the point-of-care and in resource-poor regions. However, training in performance and interpretation is required for proficiency.[19]  

It has previously been shown that evaluation of cardiac pathology by point-of-care ultrasonography performed by a novice examiner is comparable to that of a cardiology specialist.[20]

Theoretically, ultrasonographic waves have a thermal effect (ie, increased tissue temperature) and cavitation effects (ie, small gas-filled bodies created or vibrated by the ultrasonographic beam). In practice, however, current diagnostic ultrasonography systems have minimal thermal or cavitation effects. No evidence has suggested that echocardiography has any significant adverse effects at current ultrasonographic output levels.[8]

In 1-year prospective study (2017) of complications related to perioperative transesophageal echocardiography (TEE) in 28 centers across the United Kingdom and Ireland comprising 22,314 examinations, major complications (palatal injury or gastroesophageal disruption) occurred in 17 patients, directly leading to 7 deaths.[21] The complications corresponded to a 0.08% incidence (1 in 1300 examinations) and a 0.03% incidence of mortality (1 in 3000 examinations). The majority of the complications occurred in patients without known risk factors for TEE-associated gastroesophageal injury. The investigators suggested clinicians and institutions review their procedural guidelines, with particular focus on probe insertion techniques, as well as the information communicated to patients regarding the procedural risks and benefits.[21]

Limitations to the above study included an understimate of the numbers of TEE examinations owing to noncontribution by 6 of 34 centers and exclusion of the private sector.[21, 22]  Strategies to prevent perioperative TEE-associated complications include careful consideration of risks/benefits on a case-by-case basis; review of safety aspects of TEE probe use, as well as consent and procedures, in TEE training/courses; and continued research.[22]

The use of a contrast agent to enhance intraventricular viewing has several relative and absolute contraindications. In October 2008, the US Food and Drug Administration (FDA) mandated the addition of a black box warning to echocardiography contrast agents that listed new contraindications for the use of these agents (other than allergy to the contrast agent). These new contraindications, which remain the subject of considerable debate, include the following[23] :

• Clinical instability or recent worsening of congestive heart failure

• Acute coronary syndromes

• Respiratory failure, severe emphysema, or pulmonary embolism

• Serious ventricular arrhythmia or significant QT-interval prolongation

• Right-to-left, bidirectional, or transient right-to-left cardiac shunts