1. A method of estimating a noise map from reconstructed images, comprising: acquiring data for a predetermined first number of views; grouping the views into two groups with some of the views that are correlated between the two groups, one of the two groups having a predetermined second number of the views, the other one of the two groups having a predetermined third number of the views, either one of the predetermined second number and the predetermined third number being less than the predetermined first number of the views; reconstructing two images from the two groups of the views; and estimating variance based upon the two reconstructed images and a weight.