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What is The minimum amount of tooth demineralization required for radiographic detection of dental caries is and explain in detail?
Lesions confined to enamel may not be evident until approximately 30% to 40% demineralization has occurred. For this reason, the actual depth of penetration of a carious lesion is often deeper than seen in the image.
 
Oral radiology by White & Pharaoh 7th ed. Page no 288