Imaging will be performed twice in the nuclear medicine department of Day 1, and once on either Day 2 or 3. On day 8 (7 days after the first dose of rituximab), patients will receive a second dose of rituximab. This will immediately be followed by a dose of Ibritumomab tiuxetan given by vein over ten minutes. Patients will receive diphenhydramine (Benadryl) by vein and mouth and acetaminophen (Tylenol) by mouth before each dose of rituximab. This is done to prevent fever and chills. All treatments will be given in an outpatient setting.