Pt tells Dr
.Chronic means "old", acute means "recent". The patient is who tells the dr. For how long he/she has had the pain or symptoms. Said that you may be asking how can i know if the lesion I have in my back is recent or old? The radiologist have some criteria to read images like "swelling in the lesion ( acute) and callous formation (old)in a bone to determine the age of a bony lesion.