Does a head ct scan help diagnose multiple sclerosis? if a ct scan is normal, is it still likely that multiple sclerosis can be present?

Maybe: CT is helpful in diagnosing MS, though MRI is better. While a CT showing MS is pretty accurate, a CT NOT showing MS doesn't mean it's impossible that you have it, just less likely. Often a spinal tap or other testing will be done to help confirm or rule out MS.

Takes time to show: "Multiple lesions over time and space" is how med students learn about MS. It evolves over time. Sometimes subtle MRI findings can suggest it, but are non-specific ("unidentified bright objects"). But it can take a long time before MS presents itself. If seriously concerned, see a neurologist.

Not recommended: CT scan is a rather lousy way to assess MS, when the MRI is far more sensitive and specific. A normal CT scan does not exclude MS, rather rules out larger areas of acute inflammations.