1. A method of improving the resolution of a medical nuclear image of a body volume by combining it with an ultrasound image of the same body volume, the method comprising for various assumed relative positions of the nuclear medical image and the ultrasound image each time an evaluation index concerning the correspondence of a nuclear sub-image and an ultrasound sub-image of the same region of the body volume is calculated, and the relative position having the optimum value of the evaluation index is selected as the spatial registration position of the nuclear sub-image and the ultrasound sub-image.