The invention relates to a method of improving the precision of a medical nuclear SPECT or PET image in which notably a nuclear sectional image (n) can be superposed on a two-dimensional ultrasound image (u). For the superposition that nuclear sectional image (n.sub.0) which exhibits the best correspondence with a free-hand ultrasound image (u) is chosen from a three-dimensional nuclear image (2). In order to determine this sectional image, an evaluation index f(n,u) is calculated, on the basis of an estimated value (1), for differently situated nuclear sectional images (n). This index assigns a positive value to positions of structures (heart muscle tissue etc.) which correspond in the two images, a neutral value to structures which are represented only in the ultrasound image (u) and a negative value to structures which are represented only in the nuclear sectional image (n). The nuclear sectional image (n.sub.0) having the highest value of the evaluation index is subsequently displayed so as to be superposed on the ultrasound image. In a generalized form the method can also be carried out in conjunction with three-dimensional ultrasound images.