List six causes of single or multiple focal areas of increased osseous uptake (hot lesions).

1. Arthritis
2. Metastatic disease (activity may increase in treated lesions for weeks to months because of the flare phenomenon)
3. Fracture or other trauma
4. Stress injury
5. Infection
6. Paget’s disease
Other causes include fibrous dysplasia, benign and malignant primary bone tumors, infarction (subacute phase owing to bone remodeling), and increased blood flow.