5-Fluorouracil (5FU), is a primary chemotherapeutic agent for the treatment of colorectal cancer. Bevacizumab is an antibody directed against NEGF (Vascular Endothelial Growth Factor) known to improve response rate and duration of response when given together with chemotherapy for this disease. Whether the activity bevacizumab is mediated by increasing drug delivery and concentration to tumors or by non-specific effects on vasculature remains an important question. We hypothesize that bevacizumab allows more 5FU to enter liver metastases, and will test that hypothesis by directly measuring intra-tumoral 5FU concentration by 19F-MRS in patients treated with 5FU alone or 5FU and bevacizumab in a cross over design.