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.

Histologic diagnosis of colorectal adenocarcinoma

1.      Measurable  liver metastases from CRC by conventional CT or MRI imaging scanning.

2       No prior radiation to the liver

3        Prior therapy is allowed, including prior therapy with5FU.  Bevacizumab should not have been given for 8 weeksprior to study,though 6 weeksmay be acceptable if clinically indicated and approved by Pl.

4        Only patients for whom bevacizumab is clinically indicated are to enrolled

5.      Adequate organfunction with:

Absolute Neutrophil Count (ANC)1000/ul platelet count50,000/ul

bilirubin1.5 mg/dl

SGOT, alk phos5x  upper limit of normal

Serum creatinine1.5x upper limit of normal

6        ECOG performance status 0 or 1.

7        Anticipated  life expectancy of at least3 months.

8.    Forwomen of childbearing  potential: Not pregnant or lactating.

9 .   Women of childbearing potential and sexually activemales must use an acceptedand effective method of contraception.

10.    NoMRI contraindications.

11.    Signed, written informed consentobtained prior to study participation