Technical Quality and Tips

Optimizing Two-Dimensional Images

The most commonly used controls for optimizing two-dimensional (2D) images are summarized in Table 8.1 . These controls can be used to improve spatial, contrast, and temporal resolution. Spatial resolution refers to the ability of the ultrasound machine to detect structures that are anatomically separate and to display them as being separate. The best spatial resolution occurs at higher transducer frequencies, where the spatial pulse length (and wavelength) is smallest, and at the focal zone, where the ultrasound beam is narrowest. Therefore, the operator should use the highest possible transducer frequency that allows adequate depth penetration, and the focus should be positioned at the center of the region of interest. Because higher frequency transducers improve spatial resolution, the measurement of small structures is more accurate, and the distinction between closely related anatomic structures is more apparent ( Fig. 8.1 and Video 8.1 ).

Video 8.1 Effect of transducer frequency on spatial resolution. Both images are acquired from the parasternal long-axis view of the left ventricle in the same patient. An echo-free space is seen anterior to the right ventricle (RV), and a mesothelioma is seen posterior to the left atrium. In the left image (acquired using a transducer frequency of 3 MHz), the echo-free space anterior to the RV appears to be a pericardial effusion. In the right image (acquired using a transducer frequency of 5 MHz), the distinction between the pericardium and pleural space can be appreciated, thus identifying the echo-free space as a right pleural effusion rather than as a pericardial effusion.

Contrast resolution refers to the ability of the ultrasound machine to differentiate subtle differences in echogenicity between anatomic structures and to then display these as visually distinguishable structures. Controls that can be manipulated to improve contrast resolution include the overall gain, time-gain compensation (TGC), and dynamic range (DR) and the use of harmonic imaging. The overall gain increases the amplitude (or gain) of returning ultrasound signals. As echoes returning from deeper structures become progressively weaker as the ultrasound beam is attenuated, compensation for this effect is required. This is achieved via the adjustment of TGC whereby deeper echo signals are amplified more than shallower echo signals. The general aim of gain and TGC is to ensure that signals arising from tissues of similar acoustic properties are displayed at the same echo amplitude.

Dynamic range refers to the ratio of the maximum to minimum signal level; that is, the DR is the ratio of the strongest to the weakest echo. The aim of adjusting the DR is to provide the optimal amount of grayscale information so that the image is not too grainy, the contrast is not too high, and the image is not too hazy or soft. Essentially, the DR is increased when images are of a high quality (softens the images and increases the gray scale), and DR is decreased when image quality is poor (enhances the strongest echoes, eliminates the weaker signals, and reduces background noise). Harmonic imaging, which typically uses the second harmonic frequencies that arise from native tissue, has revolutionized 2D imaging by significantly reducing background noise and near-field artifacts and by improving endocardial border delineation. ^, Harmonic imaging should be used to improve bubble visualization in agitated-saline contrast studies, and harmonic imaging at a low or very low mechanical index should be used when performing studies using ultrasound enhancement agents.

Temporal resolution refers to the ability of the ultrasound machine to accurately show the position of moving structures at a particular instant in time. Temporal resolution is discussed in terms of the frame rate so that the higher the frame rate, the better the temporal resolution. High frame rates are desirable in echocardiography because of the dynamic nature of cardiac motion; in addition, high frame rates are required for advanced imaging modalities such as 2D speckle-tracking echocardiography. ^, The frame rate can be increased by decreasing the image depth (decreases the pulse travel time) and by narrowing sector width (decreases the number of scan lines).