The PSNR compares two images. You can't have an PSNR with just one image - you need a reference image. Let's say the reference image is the noise free image. So the PSNR of a noise-free image to a noise-free image (i.e. itself) is, of course, infinite. The PSNR of the noisy image to the noise-free image will be some number less than infinity.
The PSNR is computed by the psnr() function of the Image Processing Toolbox.
