Skeletal Scintigraphy

Technitium Bone Scan

Tc-pyrophosphate (Tc-PYP, 15-25mCi) and newer Tc-methylene diphosphonate (Tc-MDP, 20-30mCi) has HL 6hrs, 140keV from isometric decay. Chemical adsorption to mineral phase of bone as it is synthesised by osteoblasts. Bladder receives most dose. Excreted by glomerular filtration; 50% by 4hrs and 80% by 24hrs. Decreased renal function degrades image quality due to poor soft tissue activity clearing. Best target-to-background ratio at 3-4hrs for delayed imaging. Resolution ~5mm in best conditions. Three/four phase techniques useful in evaluating vascular nature, separating soft tissue injury/infection. Phase 1 dynamic arterial; phase 2 static blood pool/soft tissue (spot views); phase 3 dalayed at 3-4hrs (anterior and posterior whole body with prn oblique, lateral, spot views); phase 4 next morning if better skeletal detail is required (eg poor renal function, diabetic foot). Spot views have better resolution than table-feed whole body. SPECT improves contrast resolution and anatomic depiction in spine, skull, knees, ankles. High resolution low-energy collimation usually used; ultrahigh resulition has minor improvement with geometric increased imaging time; pinhole images of limited areas eg wrist. Marker tracer dot is always placed on the right side.

Normal uptake is faily uniform and symmetric, greater in the axial skeleton with mild uniform soft tissue uptake (apart from slightly hotter kidneys and ureters/bladder). Immature skeleton shows hot epiphyseal plates.

Increased activity is from:

  • Increased delivery – Arterial injection, arteriovenous malformation, infection, tumor, localized inflammation caused by trauma, increased use of a limb, RSD, and apparent increased uptake with actual reduced uptake in the contralateral body part.
  • Increased osteoblastic activity – Growth plates (intense, symmetric), fracture, infection, benign or malignant tumours.
  • Increased dwell time – Constricting clothing, tourniquets, venous obstruction, lymphatic obstruction.

Joint prostheses have increased osteoblastic activity for ~6/12, but may be hot up to 2 years. After this activity may represent infection, loosening or heterotopic bone formation. Specificity for infection is increased with combination with WBC/Gallium scan. Toggle sign – hot spot at tip of prosthesis and two areas at proximal end (like toggle switch), indicates prosthetic loosening.

Diseases with calcification/ossification show as hot, including metastatic calcification (hyperparathyroidism), tumoral calcinosis, hypertrophic osteoarthropathy, systemic mastocytosis. Heterotopic bone can form during repair of soft tissue, seen after muscle crush injury (myositis ossificans), around joint prosthesis, paralysed limb or burn injuries.
Increased uptake in bone dysplasias such as Pagets (all phases, may eventually disappear if repair complete), fibrous dysplasia, enchondromas, exostoses.

Extra-osseous MDP uptake seen in myocardium (MI, pericarditis, myocarditis), lungs (sarcoidosis, hypercalcaemia, osteosarcoma metastases, microlithiasis), brain (infarct, neoplasm), kidneys (nephritis, obstruction), liver (metastases), muscles (myositis, haematoma, rhabdomyolysis), breast (normal tissue, carcinoma), calcified lesions (eg fibroids), atherosclerosis (esp femoral andcarotid arteries).

Fractures

Fracture detectable at 24h, 95% by 72hrs. Healing seen ~10days before XR/CT. Initial phase decreased or normal osteoblastic activity before becoming hot. At 2yrs, 90% of fractures are undetectable. Intense uptake >1yr suggests non-union.
Occult fracture (normal XR) typical in elderly femoral neck and carpal bones with similar sensitivity as MRI. Fractures show as linear activity, in the spine is horizontal. Rib fractures in linear array cf metastases.

Sacral insufficiency fracture causes vertical linear uptake through sacral ala, bridged by horizontal activity across body = ‘H’/’Honda’ sign; usually not evident on XR. Other insufficiency fractures include vertebral endplates (esp superior), proximal femur (linear transverse).

Medial tibial stress syndrome – Activity along posteromedial cortex of tibia. Other sites of stress fracture include medial femoral neck, calcaneus, navicular, talus, 2nd/3rd metatarsals, pars interarticularis, pubic rami, sesamoids.

  • Shin splint – Diffuse activity >1/3 of tibial shaft. XR may show cortical thickening, periosteal reaction.
  • Stress fracture – Focal fusiform activity <1/3 of shaft length, positive on all 3 phases (1st 2 phases may normalise by 2-4/52). Grade I <25% of cortex; II 25-50%; III 50-75%; IV >75%. XR may show fracture line.

Enthesopathy – Focal uptake at site of tendon insertion.

Infection and Inflammation

Joint inflammation increases activity via increased blood flow, bounded by synovial capsule; seen with toxic synovitis, septic arthritis, early degenerative disease, connective tissue arthropathy. Early OA may show subchondral activity on high-resolution bone scan. Neuropathic joints show intense activy long before XR/CT.

Osteomyelitis and discitis early arterial flow, increased blood pool and intense delay; more difficult to read and less specific with small bones. May take months before bone scan normalizes after sterilisation (white cell scan more useful). Cellulitis has increased arterial and blood pool phases with little in bone on delayed.

Malignancy

Primary bone tumours with osteoblastic or chondroblastic activity (osteosarcoma, Ewing’s sarcoma, chondrosarcomas) and metastases (including soft tissues) are hot. MDP also avid in normal osteoblasts reacting adjacent to tumour. Extremely destructive tumour may destroy bone more quickly than repair, hence cold defect (eg high grade sarcomas). Osteomas/bone islands neutral or not seen. Osteoid osteomas have increased early phase and intense delayed with “double-density” sign due to hot nidus.

Metastatic bone tumours mostly axial skeleton due to erythropoietic marrow (except bronchial carcinoma typically distal appendicular). Usually multiple at time of discovery. Osteolytic lesions (esp RCC, thyroid) have photopenic defects with osteoblasic leading edge. Solitary rib lesions more likely due to trauma. Interval scans 3-6 months for tumour spread. Increased labelling reflects status of bone repair, not status of the metastases with increased intensity reflecting tumour becoming static and surrounding osteoblasts active (flare response, common with successful chemotherapy, may show as new areas unmasking undetected disease; follow-up shows resolution); increased number and size reflecting increased tumour load.

Super Scan

Diffuse increased tracer uptake with absence of renal uptake. Concentration in axial skeleton with armless/legless appearance. From diffuse metastases (prostate, breast), metabolic (HPT, osteomalacia) or Paget’s disaese. Automatic controls in the scan may reduce apparent intensity.

Other Bone Lesions

  • Osteoid osteoma – Intense 3 phase uptake with double density.
  • Enchondroma – Normal to mildy increased uptake.
  • Fibrous cortical defect – Normal to mildly increased uptake.
  • ABC – Doughnut sign with central photopenia.
  • GCT – 3-phase uptake with doughnut sign.
  • FIbrous dysplasia – Variable
  • Paget’s disease – Intense uptake in pelvis, femurs, tibia, spine, skull with bony expansion. Articular surface into shaft with flame-shaped leading edge, bone expansion, bowing deformity. Photopenic regions reflect early disease or sarcomatous transformation. Monostotic Paget’s = single bone involvement.
  • Reflex sympathetic dystrophy (RSD, algodystrophy, Sudek’s atrophy, complex regional pain syndrome – Pain, vasomotor disturbance, soft tissue swelling and skin changes from limb subjected to trauma. Periarticular uptake on all 3 phases esp blood pool. Increased delayed uptake infers good prognosis. As the disease progresses there is reducing activity in the earlier phases. Atypical reflex sympathetic dystrophy causes vasospasm and decreased flow.
  • Avascular necrosis – Initial photopenia then later increased uptake with new bone formationand healing. Sensitivity greater than XR, less than MRI.
  • Hypertrophic osteoarthropathy (HOA) – From lung/pleura CA, benign pleural tumour, chest and GI diseases causes diffuse increased uptake along margins of long bones.
  • Lesions with normal bone scans include bone islands, haemangioma, myeloma, lytic processes, EG.

Other Bone Imaging

PET bone scan uses fluorine-18 (F-18) 5-15mCi, HL 110min, 511keV from positron decay. Hydroxyl ion analog, binding to hydroxyapatite crystals in bone. F-18-FDG also capable of identifying bony tumours.

Radiotherapy uses strontium-89 (Metastron, pure beta decay; calcium analog binds to hydroxyapatite crystals of bone) or samarium-153 (Quadramet, beta and gamma 103keV decay).