What is the formula for calculating the contrast to noise ratio (CNR)?

The formula for calculating the contrast to noise ratio (CNR) is specifically designed to measure the ability to distinguish between different signal levels in an image while accounting for the level of noise present. The correct choice indicates that CNR is determined by the difference in signal between two areas — typically representing a region of interest and a background or lesser signal area — divided by the noise level present in the image.

In this formula, "Signal (area 1)" represents the intensity of the desired feature or object, while "Signal (area 2)" represents an area with a lesser intensity, often indicative of background tissues or noise. The subtraction of these two signals provides a contrast value, which reflects how well the feature stands out against the background.

By dividing this contrast by the noise level, CNR quantifies the effectiveness of the imaging technique in differentiating the desired signal from the background noise. A higher CNR indicates better image quality and a greater ability to identify the feature or pathology of interest, making this formula crucial for evaluating radiographic performance in fluoroscopic imaging and many other imaging modalities.