Would a plain ct of the brain show unruptured anuerysm or other life threatening things in a child with headaches?

A head CT scan would discover tumors of various sizes but not the fine structures of blood vessels. A contrast enhanced MRI might. A MRI takes longer & might require anesthesia at that age to prevent movement artefacts. The need for and type of study are determined by history/exam and clinical factors.

Resolution of a CAT scan without contrast would be inadequate to demonstrate a small aneurysm. Better results can be obtained via MR angiogram or CT angiogram.