Aliceinboots
Neuroimaging has many processes including fMRI, EEG, and many more, and each of these processes measure different types of brain activity, and how the brain reacts to various sensory stimuli.
ASD has a multiple set of underlying causes each of which can have different patterns of brain activity, the areas of no activity, areas of little activity areas of high activity, etc. And as yet they have not been able to identify all of these issues in relation to ASD mainly due to the highly complex nature of ASD, and the sensitivities of those who have ASD, which cause laboratory investigation to be difficult due to the invasive nature of the technology currently in use. Researchers are trying to improve the technology to be less invasive, to help them study children and infants who have ASD.
So in the long term future neuroimaging and genetic analysis will be the scientific method of diagnosing the various differences / deficits / disabilities which combine to cause the various forms of ASD.
Currently because they have not identified all of the areas of brain activity or inactivity which contribute to causing ASD, they are not able to identify what you term a "clear scan" which could be used to rule out ASD either.
As much as it would be ideal to be able to have such clear cut diagnostic processes, the current scientific technology and resulting understanding of the full nature of these issues is still some way in the future.
As One leading UK researcher in a related area, says the more intensely we research these issues, the more we realise that we actually understand less than we previously thought.