I have a cavity that is very small and only visible with flash.is it possible to arrest that cavity with high ppm flouride? how can i known if the cavity has reached dentin?

You do not have the ability to determine the extent of the carious lesion, but your dentist does. Take care of the lesion while it is small and have a small restoration placed. Fail to diagnose it properly and try to treat it yourself and potentially subject yourself to root canal therapy and a crown. The choice is yours.

If you have a small cavity, than have it filled and remove any bacterial contents. You don't want it to get any larger and into the dentin. Fluoride does not arrest decay but helps in preventing decay. Home treatment is not recommended, professional treatment is.

Doesn't matter how many times you ask this question. Answer will always be the same. No, Fluoride will not fill a hole in a tooth. You will not know if cavity has reached dentin. Your Dentist, however, with proper imaging (equipment you do not have nor do you have licensure to operate), or by exploring the hole physically with instruments not available you. Can't fix w computer clicks. See Dentist