Hello. Please talk to your doctor about your findings. Was it headaches that prompted the MRI? And do you mean hyper INtense? That's a more common term. And it means an area of intensity seen on the imaging. It often implies a lesion and 8 x 18 mm is the size. Now, I'm not reading your results or an expert. This is what you need to ask your doctor about. They may be seeing an arachnoid cyst but these are usually asymptomatic but can cause headache. Since they say 'may be' means they are not sure. Not sure if they will treat 'as if' on that. And when you see incidental, it means it is a finding when an MRI is given for an unrelated reason. CSF is fluid associated with a cyst. When do you see your doctor? What symptoms are you having and why did you get the MRI (I know I already asked).