Can lesions from MS show up on a CT brain scan with or without contrast?

Usually not. The typical ms lesions, especially early in the course of the disease, do not show on a standard ct scan, but are very clearly seen on an MRI of the brain. (magnetic resonance image).
Yes, but. The MRI is far more sensitive and precise, and the cat will miss many of the smaller lesions. In fact, research and diagnostics almost always refer to MRI data, and the cat being a far better study for sinuses and bony structures could well provide false negative findings which do not really aide a diagnosis.