Clinical Applications

• Hydroxyapatite


  A calcium-phosphate-hydroxyl ion compound that makes up crystals bonded to protein, mostly collagen, to form bone
• Bone remodeling


  1. Osteoclasts resorb old bone

  2. Osteoblasts lay down new bone

  3. To form and thicken normal bone to react to daily mechanical loads
• Skeleton


  • Axial skeleton contains head, spine, and shoulder girdle
  • Appendicular skeleton contains upper, pelvic, and lower components
• Bone-seeking radiopharmaceuticals


  Mimic hydroxyapatite calcium, hydroxyl groups, or phosphates
• Technetium-labeled diphosphonates (Tc-MDP)


  The most used radiopharmaceuticals for skeletal imaging
• Bone uptake mechanism

  1. Chemisorption onto the bone surface

  2. Absorption into hydroxyapatite
• Patient preparation and technique

  1. Administered intravenously with 20-25 mCi of technetium diphosphonate

  2. Scanned 2 to 4 hours later

  3. SPECT imaging may improve lesion detection

  4. 18F-NaF PET/CT scans use 20 mCi of intravenous activity, scanned 1 hour after injection
• Normal scan appearance

  • Varies significantly between children and adults
  • In children, areas of growth in the epiphyses show intense radionuclide accumulation
  • In adults, quality of the bone scan can be related to age, with poor-quality scans common as the patient gets older
• The skull is usually well-visualized, with relatively increased accumulation of activity in the nasopharynx
• Focal activity in the maxillary or mandibular alveolar ridge in adults is often due to dental disease
• Focal areas of increased activity in the lower cervical spine on anterior images are usually due to degenerative changes or the lordosis of the cervical spine
• Increased activity in the knees of older patients is relatively common due to arthritic changes
• The kidneys and bladder should be routinely scrutinized for focal space-occupying lesions
• Extravasation of the radiopharmaceutical at the injection site will result in slow resorption
• Inadvertent arterial injection may lead to localized increased soft-tissue or skeletal activity in the distal extremity (the glove phenomenon)
• Metastatic disease


  • Bone scans demonstrate metastatic lesions much earlier than other radiologic imaging
  • Cross-sectional and hybrid imaging with CT adds to sensitivity and specificity
• Multifocal areas of increased activity in noncontiguous ribs, especially in a linear configuration, suggest metastatic disease
• Multiple foci of linear activity in the vertebral bodies more likely suggest osteoporotic fractures, while lesions extending into the posterior elements or involving the pedicle are more likely metastatic
• Bone imaging may play a significant role in detecting metastatic disease in the absence of definitive 18F-FDG PET imaging
• Malignant bone tumors


  • Osteogenic sarcoma has increased, often patchy activity with photopenic areas
  • Ewing sarcoma has intense, homogeneous activity that may mimic osteomyelitis
• Benign osseous neoplasms


  • Osteoblastomas, osteoid osteomas, chondroblastomas, and giant-cell tumors usually have intense activity
  • Enchondromas can have moderately or occasionally intensely increased activity
  • Fibrous cortical defects and nonossifying fibromas are of normal or near-normal intensity
  • Bone islands, hemangiomas, and cortical desmoid tumors rarely show increased activity
• Fibrous dysplasia and Paget disease may be confused with multifocal metastatic disease
• Soft-tissue uptake


  • Calcifying soft-tissue neoplasms or their metastases may show activity
  • Malignant pleural effusions or ascites may demonstrate diffusely increased activity
  • Areas of recent infarction or amyloid infiltration may show soft-tissue activity
  • Calcification in trauma or myositis ossificans may also show increased activity
• Breast activity may be increased during menstruation, in breast carcinoma, mastitis, trauma, and various benign conditions
• Renal failure commonly produces delayed clearance and generalized increased soft-tissue activity
• Renal osteodystrophy may cause localized activity within the gastrointestinal tract, lungs, and kidneys
• Trauma


  • Fractures not apparent on radiographs may be detected with CT, MRI, or bone scanning
  • Skull fractures may not show increased activity on bone scan
  • Rib fractures almost always show intense activity
• Renal failure commonly produces delayed clearance of the radiopharmaceutical and generalized increased activity throughout the soft tissues caused by diminished excretion of the radiopharmaceutical
• Renal failure secondary to hyperparathyroidism (renal osteodystrophy) may cause localized activity within the walls of the gastrointestinal tract (particularly in the stomach), lungs, and kidneys, owing to excessive parathyroid hormone production and subsequent calcium deposition in these organs
• Fractures


  • Not apparent on routine radiographs may be readily detected with CT, MRI, or radionuclide bone scanning
  • When multifocal trauma (such as in child abuse) is suspected, bone scanning may be more effective
  • Detection of trauma at the ends of long bones may be complicated by the often intense activity in the physes of children
• Skull fractures constitute a major exception and may not show any increase in activity on bone scan
• Rib fractures


  • Almost always show intense activity and can often be recognized by their location in consecutive ribs
  • Single rib fractures are often difficult to distinguish from a metastasis, but eliciting a trauma history from the patient is often helpful
  • Multiple rib fractures typically present as several aligned punctate foci of increased activity in adjacent ribs, whereas neoplastic lesions, such as metastases, have a more random distribution among the ribs and an individual configuration following the long axis of each rib
• Three-phase bone scans are usually positive in all three phases at the time of clinical presentation and offer a means of early diagnosis and treatment
• Bone scans are an excellent way to diagnose both fatigue or insufficiency stress injuries
• Shin splints (medial tibial stress syndrome)


  • Normal blood flow and normal blood pool are seen on a three-phase bone scan
  • On delayed images, there is typically linear increased activity along at least one-third the length of the posteromedial tibial cortex at the insertion of the soleus muscle
• Stress fractures


  • Tend to be more focal or fusiform on delayed images and show increased activity on blood flow and blood pool phases of a three-phase bone scan
• While sensitive, radionuclide bone tracer studies are not specific in distinguishing among the major causes of prosthesis failure, namely mechanical (loosening) or septic (infection), regardless of the pattern of distribution of activity at the site of the prosthesis
• If the study is normal, the patient's symptoms are very unlikely to be related to the prosthesis
• If increased activity is seen either diffusely or focally in a location on a bone scan, the findings are further evaluation is warranted
• Cellulitis


  • Presents as increased blood flow (perfusion) and diffusely increased soft tissue on early images, with decreasing activity on later scans
  • No significant foci of increased bony activity should be seen on the delayed images in the area of concern