Histogram is a two dimensional graphical representation of  Digital Image data. The histogram of a Digital Image with gray levels in the X-axis is in range [0, L-1] and Y-axis is the number of pixels n in the image correcsponding to each gray value. The histogram is normalized by dividing each value n by the total number of pixels. Here it gives an estimate of the probability of the occurrence of the gray level , and the sum of all the components of a normalized histogram is equal to 1. Figure  1 shows the histogram of the MR image data, where the gray values vary from 0 to 4095. The histogram plays a key role in most of the image processing operations.