Fig. 2. Axial T1-weighted gadolinium-enhanced magnetic resonance image of the lumbar spinal cord before and after initiation of morphine (12.5 mg/ml at 40 μl/h) infusion for 10 days. Small masses (  arrows ) were reliably observed by 3 days and were substantial at 10 days. At 10 days, the pump was refilled with saline, and infusion continued for 35 more days. By day 7–8 of saline (day 17), the mass was reduced in size. By day 14 of saline (day 24), the mass observed on day 10 was no longer visible.