Extracorporeal motion (130) relative to a medical subject being imaged is detected, through the imaging or from motion detectors on the imaging probe, and either backed out of the medical images so that it can be determined whether lung sliding exists or measured to determine whether lung sliding detection is to be suspended due to excessive extracorporeal motion. Image sub-regions (164, 168) corresponding to respective ones of the images are selected for image-to-image comparison such that the selected sub-regions contain only body tissue that is, with respect to imaging depth in the acquiring of the images, shallower than an anatomical landmark within the images. Based on a result of the comparing, lung sliding detection that entails examining image data deeper than the landmark may be initialized. A motion sensor may detect the extracorporeal motion and, based on its output: pair-wise co-register (170) images to an extent of backing out the effect of the extracorporeal motion and/or determine whether to suspend deciding as to whether lung sliding is, during the respiration, occurring in the subject.