Contrast to Noise Ratio (CNR)

Along with the uniformity areas marked in yellow above, the Tomophan™ phantom has a .2mm thick, 1cm square Aluminum foil insert marked in pink above. This Aluminum square can be used in the calculation of the Contrast to Noise Ratio (CNR) Measure the pixel values in regions below the Aluminum square, and in similar small regions of noise adjacent to the Aluminum square. The contrast is defined as the difference between the pixel value of the pixels in the Aluminum square and the pixel value just outside the square. This contrast can be scaled to the typical noise (standard deviation) found in the pixels adjacent to the Aluminum square to arrive at the CNR, Contrast to Noise Ratio. This can be plotted as a function of mAs (or scaled to automatic exposure control (AEC) levels) to look at Dose dependency of the system. The edge profile of the aluminum square can be used to calculate the line spread function and the corresponding MTF in both x and y directions.

Tomophan Manual