Controlling Contrast with Dynamic Range, Compression and Reject
©Walter Rasmussen, R.D.C.S.
Dynamic Range, Compression and Reject are separate methods of controlling contrast on the ultrasound image. The sonographer learns by experience that each of these controls has a slightly different effect on contrast and uses them almost interchangeably, tending to favor the use one over the others. Some ultrasound machines do not offer all of these adjustments. The overall goal of using these tools is to produce an image which reveals as much diagnostic information as possible without making it too gray or too dark.
Dynamic Range refers to the range of ultrasound intensities that can be displayed by the ultrasound machine. The dynamic range adjustment can make an image look either very gray or very black and white. Dynamic Range acts more on the weaker echoes and has a lesser effect on the stronger reflections. Weak echoes can be attributed to both small important reflectors that convey texture and noise that is of no diagnostic value. While it is desirable to eliminate some noise which can obscure the image, it should not be adjusted to the point of eliminating texture.
Left: Maximum Dynamic Range
Left: Minimum Dynamic Range
Reject increases contrast by filtering-out weaker echoes, making the image look darker. This adjustment helps to reduce a foggy appearance which at higher levels can obscure the image.
Compression alters the display of the range of echo intensities by compressing them into fewer shades of gray.
Left: Normal Compression
Left: Maximum Compression
Under normal circumstances, the sonographer seeks a balance of range of intensities in order to see relevant structures. There are however, times when it is useful for the image to have an extremely high contrast appearance for specific diagnostic purposes. For example, in the evaluation of pericardial effusion, these adjustments can be used to remove almost all of the weak echoes in order to emphasize both the pericardium and the pericardial fluid for better-assessment of the volume of fluid present.