The present invention relates to a monitoring apparatus (100, 200) for monitoring a structural change in an object (280, 340), such as a heart wall tissue. Tissue at first and second locations exhibits first and second velocities. The monitoring apparatus (100, 200) comprises an ultrasound signal providing unit for providing ultrasound signals of the object (280, 340) for different times. The monitoring apparatus (100, 200) further comprises a discontinuity determination unit (120, 220) for determining a discontinuity of the first and second velocities based at least in part on a change in time of tissue velocity differences and of tissue velocity sums. From the discontinuity an ablation depth can be determined, which is an important parameter while performing an ablation procedure. For example, it can be used for determining the progress of ablation within the object and for determining when the ablation has reached a desired dimension.