Various antiangiogenic agents have a modest effect in prolonging overall survival in solid tumours. In colorectal cancer it is clear that there are some patients in whom bevacizumab significantly prolongs survival, but it is not effective in the majority of patients. Biomarker studies using tumour tissue and blood have failed to define a consistent biomarker that correlates with a beneficial effect of bevacizumab on survival. DCE-MRI can detect changes in tumour blood flow which, in early phase drug studies, correlated with subsequent tumour responses, but is too expensive and time consuming to be used in larger scale trials. DCE-US is a promising biomarker for use in this group of patients with antiangiogenic agents, as detailed above. The investigators wish to use this technique as a predictive biomarker for any effects Aflibercept has on OS and PFS in patients with metastatic colorectal cancer refractory to standard treatment.