In this CT scan of a horse skull, a large, space-occupying mass is destroying the occipital bone and compressing the brain stem. Other sections indicate impingement on the cerebellum as well. Although this mass was palpable and some periosteal reaction was evident on plain radiographs, the invasion of the cranial vault was not visible on those studies. Note the enhancement of the numerous large vessels in this area by IV iodinated contrast administration.