50 Part I Chapter 2 Figure 1. Normal whole body scan. Scintigraphic criteria allowing assessment of the quality and interpretability of a whole body scan. 2. Bone abnormalities Both increases and decreases of tracer uptake have to be assessed and any abnormality can be either focal or diffuse. Focal or diffuse skeletal increased uptake can be objectively assessed by comparing with the contralateral bone or soft tissue. The localization, size, shape, intensity, and number of abnormal findings should be described. In comparison to the normal bone activity, an increased tracer uptake indicates an increased osteoblastic activity. Some osteolytic skeletal lesions appear as a region of reduced tracer uptake, either surrounded by a rim of increased tracer deposition or, conversely, with a punched-out appearance. Decreased uptake is less common than focally increased activity and sometimes hard to identify. Indeed, bone scintigraphy has a low diagnostic sensitivity for purely osteolytic lesions (e.g. multiple myeloma). Differential diagnosis can sometimes be based on the configuration, location and number of abnormalities, although most patterns are non-specific. Lesions detected on bone scintigraphy can take an extended time to normalize, reflecting the protracted course of bone healing which may take multiple months. Therefore, it is rarely useful to repeat the exam within 4 or 6 months. In oncology, a decrease