What are the Bosniak cyst categories and their criteria for CT?

I   Simple cyst-Thin <2mm smooth wall, <20 HU no septae, enhancement or calcification - benign
II  Complex cyst-Septations, or high density but no enhancement <1% malignant
III Indeterminate cyst-Thick septae or mural nodule but no enhancement - 60% malignant
IV  Likely malignant-Solid enhancing component ~95% malignant

IIF Follow up cyst-multiple septae, nodular calcification, >3 cm no enhancement - 5- 25% malignant