Longitudinal prospective monocentric observational study in which patients to start Mirikizumab for normal clinical practice will be followed for 24 weeks. Based on the findings in the comparison between US (IUS and TPUS) and endoscopy, the study aim is to build up a modified MUC (Milan Ultrasound Criteria) scoring system taking each colonic segment and the rectum into account, developing non-invasive quantitative ultrasound-based criteria to identify patients with active UC and assess its severity.