40 Part I Chapter 2 5. Hematopoietic growth factors: increased spinal tracer uptake, possible increased tracer uptake in the appendicular skeleton 6. Iron: increased renal tracer uptake, increased tracer uptake at site of intramuscular injection, diffuse hepatic tracer uptake 7. Methotrexate: diffuse hepatic tracer uptake 8. Nephrotoxic chemotherapy: increased renal tracer uptake and reduced skeletal tracer uptake 9. Nifedipine: reduced skeletal tracer uptake Radiopharmaceuticals 1. Physical characteristics of the radionuclide 99mTc decays by isomeric transition with a half-life of 6.02 hours to 99Tc by emitting one 140.5 keV gamma ray per decay. 2. Characteristics of the tracer molecule The molecules most commonly used for performing bone scintigraphy are bisphosphonates: methylene diphosphonate (MDP), hydroxymethylene diphosphonate (HMDP) or hydroxyethylene diphosphonate (HDP), and 2,3-dicarboxypropane-1,1diphosphonate (DPD). 3. Pharmacokinetics The injected radiolabeled bisphosphonates adsorb to the surface of hydroxyapatite crystals in proportion with local bone vascularization and osteoblastic activity. After intravenous administration, the plasma clearance of bisphosphonates is biexponential and a function of skeletal uptake and urinary elimination. Four hours after injection, approximately 50 to 60% of the injected amount will be fixed to the skeleton, the unbound fraction (34%) is excreted in the urine, and only 6% remains in circulation. Tracer elimination through the gastro-intestinal tract is insignificant. Maximum bone accumulation is reached 1 hour after tracer injection and remains practically constant up to 72 hours. 4. Radiopharmaceutical preparation and storage The bisphosphonates listed above are commercially available and supplied as a vial containing the respective bisphosphonate, a stannous reducing agent and other excipients in the form of a powder ready for labeling. Vials containing the sterile nonpyrogenic lyophilisate should be stored at 4 to 8 °C or room temperature, as required by the manufacturer. These kits can be used until the expiry date of the batch (one to two years after the date of manufacture).