Preprocessing controls adjust the transmission and acquisition of the ultrasound signals. Preprocessing settings control the formatting of the ultrasound signal for conversion into an electric signal. Changes in the preprocessing controls affect the information that the scanner will acquire to create an image (1), and this information is used to create an image. Postprocessing settings affect how the formatted information is displayed on the monitor. Postprocessing defines the “cosmetic appearance” of the ultrasound data displayed on the monitor.

Transmit Power controls the amplitude (acoustic power) of the transmitted ultrasound signal. Modern echocardiography systems default to a high-power setting to maximize the signal-to-noise ratio. A theoretic concern is that high-power ultrasound can have deleterious effects on the tissue, particularly in fetal echocardiography. Federal standards restrict the maximal intensities allowed for Transmit Power settings on commercially available ultrasound systems. Typically, echocardiography systems default to the maximum Transmit Power. Proper adjustment of Transmit Power becomes critical when echo contrast studies are performed.

Increasing the gain increases the amplitude of the electric signal generated by returning ultrasound signals received at all depths. Unfortunately, any noise present is also amplified. Setting the gain too high or too low affects the ability to read the image correctly. When the gain is set too high, the image appears bright, and linear structures, such as the mitral valve, appear thickened. Increases in the gain also increase the amount of visible noise. For instance, with moderately excessive gain settings, the left ventricular (LV) cavity acquires a speckled appearance, which can make it difficult to differentiate the LV cavity from the myocardium. With further increases in the gain, the entire LV takes on a whitened appearance and the ability to differentiate structures is lost.

When the gain is set too low, only bright signals, such as those from the pericardium, are visible, and very-low-amplitude signals, such as the signal from an LV thrombus or “smoke” in the LV, are lost (2). Therefore, the gain should be adjusted to obtain an image with a gray scale ranging from low-amplitude (dark gray) to high-amplitude (white) signals. The gray scale, displayed as a bar graph on the right side of the image, is useful for guiding adjustments. Figure 23.1 (Video 23.1A-C) demonstrates the effect of three separate gain settings on the same mid-esophageal two-chamber view (Video 23.1A-C).

Attenuation results in the signals returning from the far field being weaker than signals from the near field. Therefore, the ability to selectively adjust the gain setting at each depth is essential to optimize the image and is commonly referred to as time gain compensation (2). For example, the echocardiographer can use the time gain compensation to amplify the weaker signals returning from the far field more than the signals returning from shallower depths (near field). The echocardiographer should be careful when adjusting the time gain compensation. If it is set too low, the elimination of true tissue signals is a risk. The time gain compensation should be used to eliminate gain-related artifacts and optimize far-field structures. The effects of time gain compensation settings on image quality are shown in Figure 23.2 (Video 23.2B,C).

Ultrasound beams transmitted through tissue side by side can be subject to different levels of attenuation. Lateral gain compensation is used to counter this effect by amplifying the weaker signals to a greater extent and ensures brightness is uniform across the entire width of the image display. The effects of lateral gain compensation can be used to amplify weaker lateral signals and aid in detection of endocardial borders. The effects of lateral gain compensation on image quality are shown in Figure 23.3 (Video 23.3B,C).

This control selects the maximal distance to be displayed. Increasing depth beyond the structure of interest has several negative consequences.

Therefore, to optimize the image display and temporal resolution, the depth should be set just beyond the structure of interest, as shown in Figure 23.4.

It must also be appreciated that the lateral resolution of the ultrasound system is inversely proportional to the depth. Therefore, it is practical to have the position of the probe as close as possible to the structure of interest. For example, when the leaflets of the aortic valve are being evaluated, the mid-esophageal (ME) aortic valve short-axis view is preferable to the deep transgastric (TG) long-axis view because the probe is closer to the aortic valve and lateral resolution is improved.

The focus control enables the operator to focus the ultrasound beam at a selected distance from the transducer. This is achieved by altering the sequences of electric impulses sent to the transducer elements. The
goal of focusing is to have the beam narrowest at the location of the structure being evaluated because a thinner beam improves lateral resolution (4). The user must be cognizant of the focus depth of the system, which is typically marked on the edge of the sector (Fig. 23.4; Video 23.4A-B).

If the focal zone is located too far from the area of interest, the image resolution may not be sufficient for proper evaluation. When the atrial septum is being evaluated for a patent foramen ovale, the focus should
be placed at this level. Remember that structures distal to the focal point lie in the far field and may appear “fuzzy” or abnormally thick. Avoid evaluating small structures distal to the focal point until the focal point is moved to that level.

A feature of modern TEE systems is that they are capable of multiple frequencies, so that the transmitted ultrasound frequency can be adjusted. This can be especially important in TEE applications. The basic principle is that higher-frequency beams maximize the length of the near field. When the structures being evaluated are in close proximity to the transducer (atria, aorta), higher frequencies are used to optimize resolution (5). When the structures being evaluated are farther from the transducer (deep TG views), higher frequencies may not be adequate because penetration is poor and attenuation is greater. In these situations, the frequency should be reduced until a satisfactory image is produced.