Partial thrombosis of the corpus is rare and often well-defined (as in this case) due to the segmental nature of the corpus whereby there appears to be a septum dividing it into proximal and distal portions (proximal always seems to be involved).  Microtrauma (at intercourse) is believed to be responsible for the development of thrombus.  Treatment is expectant and usually successful with spontaneous resolution.  MRI is helpful for diagnosis (as in this case) due to the presence of T1 hyperintensity = methemoglobin and absence of enhancement as might occur in a solid mass lesion.