Subtraction radiography is a highly sensitive and useful technique for detecting crestal alveolar bone changes in patients. Utilizing this technique, researchers have demonstrated that 9% of crestal sites lose bone over a six-month period in untreated subjects with periodontitis. On the order of 10-13% of crestal sites were found to lose bone three months post-periodontal therapy which included surgery. Non-surgical therapy resulted in 0.5-2% of crestal sites with bone loss. Subtraction requires radiographs which have closely approximating projection geometry. Presently available technology for taking standardized radiographs based on an occlusal stent system is adequate for obtaining interpretable subtraction images.