This is the protocol for a review and there is no abstract. The objectives are as follows: To compare the accuracy of CEUS, CECT, MRI, and 18F-FDG PET-CT for diagnosing liver metastases in people with newly-diagnosed colorectal cancer. Potential sources of heterogeneity We will investigate the following potential sources of heterogeneity: the use of different reference standards. different ways of selecting the study populations, e.g. different inclusion and exclusion criteria. different locations of the study populations (country, state, region). age of participants in the study population. sex of participants in the study population. differences in clinician skills for the performance of CEUS.