The present disclosure relates to systems and methods for reconstructing an image in an imaging system. The methods may include obtaining scan data representing an intensity distribution of energy detected at a plurality of detector elements and determining an image estimate. The methods may further include determining an objective function based on the scan data and the image estimate. The objective function may include a regularization parameter. The methods may further include iteratively updating the image estimate until the objective function satisfies a termination criterion, and for each update, updating the regularization parameter based on a gradient of an updated image estimate. The methods may further include outputting a final image based on the updated image estimate when the objective function satisfies the termination criterion.