Figure 16b. Misregistration due to physiologic bladder activity in a 58-year-old woman with intermediate to poorly differentiated Sertoli-Leydig cell tumor with heterologous elements. (a) CT scan shows a cystic mass in the left side of the pelvis (arrow). (b) Fused PET-CT scan shows activity in the right side of the pelvis (arrow). (c) CT scan obtained inferior to a reveals that the pelvic activity seen in b represents bladder activity secondary to misregistration due to respiratory motion and bladder filling (arrow). The lesion in the left side of the pelvis (cf a) represents a postoperative fluid collection and shows no activity.