Could a mri/ct scan detect all brain injurys?

Nope: Sometimes you can see evedence of bleeding. With traumatic brain injury sufficient to cause changes on ct typically survival is markedly reduced. Certain types of MRI done at the right time following a brain injury can sometimes show evidence of cerebral contusion. Much of the damage done with traumatic brain injury is microscopic (too small even with diffuse axonal injury) to be seen on mri.