MRI. As a screening exam, a CT cannot use contrast. In this context, a CT scan has a problem differentiating tumors. This is because a CT focuses on density. The density of a tumor is not greatly different from the density of a benign lesion such as a cyst, but has a vastly different significance. A CT scan can miss many tumors and equally concerning, can more often lead to additional testing when it is unnecessary. Importantly, CT scans cannot accurately assess breast tumors or liver tumors. CT scans also use radiation, while MRI does not.