When the er doc is doing a ct scan on head to check for tumor and or aneurysm , do they have to use contrast? or can i get it without contrast?

Yes; no: For tumor, usually with and without contrast and with contrast for aneurysm.

For the best study: For brain tumors, a ct scan of the head without and with contrast best delineates the tumor. To check for an aneurysm, contrast is needed for the ct-angiogram. If you are concerned about radiation and contrast, and if it is an elective evaluation for brain tumor or aneurysm, you might consider an mri/mra.