A lipoma is a benign soft tissue tumour composed entirely of fat. They are of identical signal to subcutaneous fat on all sequences, i.e. bright on T1 and suppress (black) on STIR images.
In the following example a lipoma is seen between the biceps and brachialis muscles. Due to its size, it is causing displacement of the brachial artery and median nerve. The patient had sensory symptoms along the course of the median nerve on elbow extension, presumably due to stretching of the nerve around the mass. MR allows diagnosis of the lesion and depicts its anatomical relationships prior to excision.