1. A method for extending the effective FOV of a CT scanner, in which the first step is a), b) or c): a) defining an image mask to be used in the CT reconstruction of transmission data where the mask encompasses the region outside the CT FOV; b) defining an image mask to be used in the CT reconstruction of transmission data where the mask encompasses the region outside the CT FOV between the maximum and minimum extent of the patient from top to bottom and from right to left, wherein the image mask and an image center comprises one boundary; c) defining an image mask to be used in the CT reconstruction of transmission data where the mask encompasses the region outside the CT FOV and inside the outer boundary of the patient as defined by a PET or SPECT image, wherein the image reconstruction calculation employs the image mask and is performed by an iterative reconstruction data calculation, and wherein said iterative reconstruction data calculation comprises solving an equation in the form of: λ n + 1 = λ n ⁢ m ⁢ 1 BP ⁡ ( N ) ⁢ BP ⁡ ( y FP ⁡ ( λ n ) ) + λ 0 ⁡ ( 1 - m ) for n=0 to a finite number of iterations, and where λ n+1 is the calculated estimate of the image, λ n is the previously calculated estimate of the image, λ 0 is the original CT image, m is the image mask, N is the normalized projection data mask, y is the measured projection data, BP is the back projection operator, and FP is the forward projection operator.