51 The EANM practice guideline for Bone Scintigraphy in intensity of tracer uptake and in number of abnormalities often indicates improvement or may be secondary to focal therapy (e.g. radiation therapy). An increase in the intensity or the number of increased uptake foci on scans performed less than 6 months apart may represent disease progression, but can also be associated with a flare phenomenon [55]. 3. Soft tissues findings The renal system and urinary tracts are also normally visualized on the exam, as well as diffuse or focal tracer uptake in the soft tissues. Tracer uptake in the kidney can be focal or diffuse. A diffusely increased soft-tissue uptake can be caused by drug interference, failed 99mTc labeling, severe osteoporosis, renal failure, dehydration, or an insufficiently long interval between tracer injection and image acquisition. Conversely, a low or absent tracer uptake in the soft tissues may be caused by an excessive avidity for the tracer of osteoblasts populating the axial skeleton, resulting in a “super bone scan” appearance or excessively long interval between tracer injection and imaging. 4. Sources of error Focal soft tissue hot spots have a wide range of causes, as illustrated for striated muscle uptake in table 4, and this may lead to an incorrect diagnosis of skeletal disease on planar imaging. Attenuation artifacts caused by metal parts or motion artifacts are generally obvious. The same holds true for tracer extravasation at the injection site due to (partly) paravenous injection. Table 4. Causes of striated muscle increased uptake. • Repeated intramuscular injections of iron supplements • Hematoma/necrosis/sickle cell anemia • Rhabdomyolysis (mechanical, toxic, electrical, …) • Muscular abscess • Primary tumors (rhabdomyosarcoma, other sarcomas) • Metastases from solid tumors • Poly(dermato)myositis (many causes) • Severe renal insufficiency/hypercalcemia/malignant calcinosis/multiple myeloma • Myositis ossificans The most common artifacts are related to urinary tracer contamination in case of dilatation, stasis or anatomical variant of the urinary tract, especially after urologic surgery, or contamination during urination (table 5). 2