Echocardiography Periprocedural Care

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.

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• 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.