In a method for generating a tomographic image, at a first point in time, a first tomographic image is acquired of a layer of a patient and the region of cartilaginous tissue is determined in the first tomographic image. At a later second point in time a second tomographic image is acquired of the layer of the patient and the region of the cartilaginous tissue is determined in the second tomographic image. The tomographic image is generated from the first tomographic image and the second tomographic image, in which every pixel of the tomographic image is assigned a difference value between a corresponding pixel of the first tomographic image and a corresponding pixel of the second tomographic image. The validation number is determined by adding all the pixels in the region of the cartilaginous tissue in the tomographic image.
