Using Positron Emission Tomography to Evaluate the Effects of Bevacizumab on Intra-tumoral Pharmacokinetics of 5-fluorouracil in Metastatic Colorectal Cancer

* This is not a treatment study. A treatment regimen has already been decided upon by the patient and doctor. * At the screening visit for this trial the following will be performed: a medical history, physical examination, CT scan to measure the amount and size of the tumor(s), and a PET scan using glucose that has been attached to a small amount of radioactivity to determine how well the tumor takes up glucose. * Patients will then be randomized into one of 2 treatment groups. Group 1 will receive both 5-FU and bevacizumab on the first day of chemotherapy. Group 2 will receive just 5-FU on the first day of chemotherapy. * Patients will undergo a PET scan that uses water that has been attached to a small amount of radioactivity to measure how well the tumor takes up water prior to the 5-FU adminstration. * After patients receive 5-Fu they will undergo a PET scan during which a small amount of 5-FU has been attached to a small amount of radioactivity to measure how well the tumor takes up 5-FU.