Noise

noise Noise is defined as a random disturbance that obscures or reduces clarity. In a radiographic image, this translates into a grainy or mottled appearance of the image.

Signal-to-Noise Ratio (SNR) One way to describe noise in digital image acquisition is the concept of signal-to-noise ratio (SNR). The number of x-ray photons that strike the receptor (mAs) can be considered the “signal.” Other factors that negatively affect the final image are classified as “noise.” A high snR is desirable in imaging, in which the signal (mAs) is greater than the noise, so that low-contrast soft tissue structures can be demonstrated. A low snR is undesirable; a low signal (low mAs) with accompanying high noise obscures soft tissue detail and produces a grainy or mottled image.

High SNR Although a high SNR is favorable (Fig. 1-158), technologists must ensure that exposure factors used are not beyond what is required for the projection so as not to overexpose the patient needlessly. Overexposed images are not readily evident with digital processing and display, so checking the exposure indicator as described on the previous page is the best way to determine this.

Low SNR When insufficient mAs is selected for a projection, the receptor does not receive the appropriate number of x-ray photons, resulting in a low SNR and a noisy image (Fig. 1-159). This mottle may not be readily visible on the lower resolution monitor of the technologist’s workstation, but the exposure indicator, as checked for each projection, can aid in determining this. The technologist may check for noise at the workstation by using the magnify feature and magnifying the image to determine the level of noise present within the image. In the event that noise is clearly visible in the image without any magnification, the image should be reviewed by the radiologist to determine if the image needs to be repeated.

Scatter radiation leads to a degradation of image contrast that can be controlled by the use of grids and correct collimation, as was described previously.

A secondary factor related to noise in a radiographic image is electronic noise. Although a comprehensive discussion of electronic noise is beyond the scope of this text, electronic noise typically results from inherent noise in the electronic system, nonuniformity of the image receptor, or power fluctuations.

Fig. 1-158 Good-quality image—acceptable SNR.

Fig. 1-159 Poor-quality image, “noisy” (grainy)—low SNR.