1. A method for image quality improvement, comprising the steps of: providing an imaging device configured with a degradation function h(x, y) relating to each detection position (x, y) on a detection plane; using the degradation function h(x, y) to determine position variables (ta) as each of the position variables (ta) is corresponding to a degradation distance measured between each detection position (x, y) and a datum position on the detection plane; using the imaging device to image at least one emission source on the detection plane so as to generate a measured image g(x, y) composed of a plurality of pixels accordingly, while allowing the plural pixels to be arranged corresponding the detection positions (x, y) of the detection plane in respective and one of the plural pixels to be arranged corresponding to the datum position; determining the degradation distances for each detection position (x, y) based upon the distances measured respectively between each of the plural pixels and the pixel so as to be used for determining the position variables (ta) for their corresponding pixels; performing an image restoration operation on each pixel of the measured image g(x, y) based upon the measured image g(x, y) and the degradation function h(x, y) so as to generate an initial restored image f(x, y); and for each pixel in the measured image g(x, y), performing an iterated algorithm at the detection position (x, y) corresponding thereto using the following formula: f k + 1 ⁡ ( x , y ) = ( 1 - t ) ⁢ f k ⁡ ( x , y ) + t ⁡ [ h ⁡ ( - x , - y ) * g ⁡ ( x , y ) h ⁡ ( x , y ) * f k ⁡ ( x , y ) ] ⁢ f k ⁡ ( x , y ) , wherein, t is a function variable that is the multiplication of a volume variable tb with a position variable ta, and thus, obtaining an iterated restored image f k+1 (x, y) after k iterations.