Echocardiography Technique

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 (F_T) and the scattered signal received by the transducer (F_r) is the Doppler shift:

Doppler shift = F_r – F_T

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

Blood flow velocity = c (F_r – F_T)/2 F_T (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