**2. Normal bone scintigraphy**

Bone scan is a diagnostic technique used to assess the presence of anomalies in the distribution pattern of bone formation. It has high sensitivity, but specificity is frequently variable or limited. Three-phase bone scintigraphy is currently the most used technique because it allows evaluating the degree of hyperemia (flow phase), increased of articular permeability (blood pool phase) and the presence of alterations in bone remodeling (bone tissue phase). Traditional technique is based on the biological properties of bisphosphonates marked with 99mTc, usually MDP, when they are integrated into the bone metabolism after intravenous administration. Typically, 30% of the injected dose of Tc-99m-MDP remains in the skeleton, and most of bone uptake occurs in the first hour. The remainder is eliminated from the tissues and blood by the kidneys and imaging is obtained 3 – 4 hours later. In general, uptake of the tracer depends on local blood flow, osteoblastic activity and extraction efficiency. Normal scintigraphy imaging depends on technical equipment and employees, but it is also significantly influenced by other factors such as age and constitution of the patient, intake of drugs, degree of hydration, renal function and/or the presence of impaired circulation. Therefore a whole body study is recommended, with anterior and posterior screenings that allows assessing the symmetry or asymmetry in the distribution of the drug. However, a located study may be sufficient in some cases, since provides greater image quality and requires less time and is less expensive. In other patients, especially when a spine study is necessary or avascular osteonecrosis located in the hip or knee is suspected, it will require a SPECT, that is more sensitive for detecting abnormalities and provides, combined with tomography, three-dimensional images.

It is necessary to have knowledge of normal variants and patterns of abnormality to minimize misinterpretation. Whole body bone scan shows normal variations in the uptake of the radiotracer, as this is higher in areas with high bone remodeling. The age of the patient has a fundamental role in the appearance of the scan, especially during the growth period and in the elderly. In children, as they have a growing skeleton, there is a diffuse bone uptake and a striking uptake at the growth plates of bones, especially in metaphysealepiphyseal areas of long bones and cranial sutures. This decreases over the years until complete fusion of the epiphyses takes place. On the other hand, bone scan images are often of poor quality in old people. Aging may be reflected in scintigraphic images by a diffuse reduction of bone uptake of the radioisotope, although diffuse uptake at the dome or symmetrical uptake in the peripheral joints (secondary to osteoarthritis) may be present. Associated degenerative processes may lead to increased uptake in the involved joints. Obese people also get lower quality images. In addition, insufficient hydration and/or renal failure hamper the radiopharmaceutical removal of soft tissues and modify the end result.

The sternum and the sacroiliacs joints are normal uptake areas in scintigraphic studies (**Fig. 1**). Other areas that may appear as normal increased uptake (**table 1**) are forewings of the iliac bone, coracoid process, tip of the scapula and, sometimes, costochondral junction, lower portion of the cervical spine, kneecaps and some muscle attachments. Thoracic kyphosis and lumbar lordosis may cause the parts of the column that are farther away, appear as less warm. In patients with scoliosis, concave side usually appears hotter than the convex. There is also a physiological uptake located at renal pelvis and the bladder, since radiotracers are eliminated by the kidneys (Murray, 1998; Schneider, 2006).
