Bone A&P and scintigraphy

Five functions of bone

Support (rigid framework), levers for movement (via muscle/tendon attachment), protection (e.g. skull, ribs), housing bone marrow (produces red cells, white cells, platelets), storage reservoir for calcium and phosphorus

Bone chemical composition

About 69% inorganic salts (hydroxyapatite), 22% organic component (collagen/proteins), 10% water

Hydroxyapatite

Rock-like crystals of calcium and phosphate combined with hydroxyl ions (~38% calcium); the inorganic component most important for Nuclear Medicine imaging because ions like phosphonates can substitute into its crystal lattice

Inorganic vs organic bone components

Inorganic salts make bone stiff and strong; organic components (collagen fibres, glycoproteins) give bone flexibility, ductility, and toughness, plus tensile/compressional strength

Four bone cell types

Osteogenic cells (stem cells that continuously produce osteoblasts, found in endosteum/periosteum), osteoblasts (bone-forming cells, synthesise matrix, do not undergo mitosis), osteocytes (former osteoblasts trapped in matrix, maintain bone homeostasis and calcium/phosphate levels), osteoclasts (bone-dissolving cells, derived from bone marrow stem cells, use acid phosphatase to dissolve bone)

Osteoblast secretion role

Osteoblasts can secrete osteocalcin, a protein hormone that causes the pancreas to release insulin, increases insulin sensitivity in fat cells, and limits fat tissue growth — controlling energy for bone growth/repair

Osteon (Haversian system)

The microscopic structural unit of compact bone, made of concentric rings called lamellae; the central (Haversian) canal running through it carries blood vessels, nerves, and lymphatics; perforating (Volkmann's) canals connect it to the periosteum and endosteum

Bone remodelling cycle phases

Resorption (~2 weeks, osteoclasts break down bone), reversal (4-5 weeks, surface prepared for new bone formation), formation (up to 4 months, osteoblasts lay down new bone)

Fracture repair sequence

Bleeding/hematoma forms, phagocytes clean up dead cells, osteoclasts remove dead bone fragments, fibrocartilaginous tissue (soft callus) forms via chondroblasts and fibroblasts, soft callus hardens into a bony callus, osteoclasts reabsorb excess tissue

Bone scan radiopharmaceuticals

Technetium-99m diphosphonates, most commonly Tc-99m MDP (medronate) or Tc-99m HDP (oxidronate)

Chemisorption mechanism

MDP and HDP are phosphate analogues that bind to Ca2+ ions in hydroxyapatite by chemisorption — covalent and hydrogen-absorption bonding that is instantaneous, avid, and essentially irreversible

What drives bone tracer uptake

Regional blood flow, osteoblastic activity, and the extraction efficiency of the radiopharmaceutical from the systemic blood supply; uptake localises in proportion to osteoblastic activity/bone remodelling, so increased uptake occurs with fractures, infection, and malignant disease

Typical bone scan administered activity and timing

740-1000 MBq (weight-dependent); bone uptake visible within 5-10 minutes of IV injection; delay to imaging is closer to 2 hours for HDP and closer to 4 hours for MDP

Three phases of a standard bone scan

Blood flow (perfusion, immediately post-injection, shows areas of active blood supply), blood pool (soft tissue tracer accumulation, detects soft tissue abnormalities), delayed imaging (several hours later, shows bone uptake patterns for fractures, infection, tumours)

Log vs linear intensity display in bone scans

Logarithmic scaling compresses the high-intensity range and expands the low-intensity range, enhancing contrast and subtle differences (also shows soft tissue borders); linear intensity shows raw/directly measured values, useful for a broad intensity range without compression

Primary Bone Tumours

Benign vs malignant bone tumours

Benign tumours are confined and don't spread, with well-defined borders and slow growth; malignant tumours invade tissue and metastasise, requiring more aggressive treatment

Primary vs secondary bone tumours

Primary tumours arise from cells within the bone itself and are usually localised; secondary tumours are metastases from cancer elsewhere in the body (e.g. breast, prostate) and are almost always malignant

The four primary bone tumours

Osteosarcoma (most common malignant, from bone-forming cells), chondrosarcoma (second most common malignant, from cartilage cells), Ewing's sarcoma (rare aggressive malignant, from primitive neuroectodermal cells), osteochondroma (benign, from growth plates)

Osteosarcoma characteristics

Arises from osteoblasts; mainly affects children/young adults (15-25 yrs) with a second peak in the elderly; occurs most commonly in the metaphyseal region of long bones, most commonly the knee (~50%); good outcome if amputation possible, poor outcome if in shoulder/jaw/pelvis or metastatic

Osteosarcoma bone scan appearance

Localised increased uptake ("hot spot"), periosteal reaction (uptake along the bone surface), soft tissue extension in advanced cases, and detection of lung metastases (osteosarcoma's most common metastatic site)

Osteosarcoma imaging protocol

Three-phase bone scan with Tc-99m MDP or HDP — early dynamic (3s/frame over affected region), early blood pool statics, whole body blood pool; delayed imaging at 2.5-4 hours with statics, whole body, and SPECT/CT (64 stops at 15s/head)

Chondrosarcoma characteristics

Malignant cartilage cell cancer; develops in bones containing cartilage (pelvis 50%, proximal humerus 10%, ribs 15%); affects ages 30-70, more prevalent in males (3

Ewing's sarcoma characteristics

Arises from mesenchyme cells in bone marrow; most frequent in male teenagers (5-30 years, ratio 1.6

Osteochondroma characteristics

The most common benign bone tumour, typically arising in childhood/adolescence from growth plates; two types — sessile (attached directly to bone) or pedunculated (attached by a stalk); often asymptomatic and found incidentally; rarely requires removal unless problematic

Nuclear medicine physicians' use of multi-planar imaging

Fused SPECT/CT or PET/CT data can be viewed in axial, coronal, and sagittal planes to comprehensively understand the spatial distribution and extent of tracer uptake, since some pathologies show characteristic patterns better in specific orientations

Metastatic bone disease overview

About 60-70% of advanced cancer patients develop bone metastases; most frequent primary cancers that metastasise to bone are breast, lung, prostate, kidney, and thyroid

Osteolytic bone lesions

Areas of localised bone destruction/resorption, decreased bone density; weaken bone and increase fracture risk; associated with metastatic cancers (lung, breast, kidney), multiple myeloma, osteomyelitis, and benign tumours like giant cell tumours

Osteosclerotic bone lesions

Areas of increased bone density; caused by metastatic cancer (prostate, breast, lymphoma), Paget's disease of bone, osteopetrosis (marble bone disease, genetic), or rare infections

Most common osteolytic bone cancers

Multiple myeloma (plasma cell cancer, causes bone pain, fractures, elevated calcium), metastatic bone cancer (breast/lung/prostate/kidney cancer causing osteoclast-driven bone breakdown), primary bone lymphoma (rare non-Hodgkin lymphoma affecting bone)

Why WBBP imaging is used for thyroid, bronchus, lymphoma, and renal cell carcinoma primaries

These cancers are typically hyper-vascular with lytic bone metastases, so whole body blood pool imaging is essential to catch hyperaemic spots; historically used because PET wasn't available for these cancer types, and some centres (e.g. Peter MacCallum) still use it as a cheap, readily available alternative even with PET/CT available

Two-phase vs one-phase bone scan by primary cancer

Thyroid, bronchus, lymphoma, renal cell carcinoma primaries use a two-phase scan (early WB blood pool plus 3h delayed WB imaging); breast, prostate, or gastric primaries use a single-phase scan (WB at 2.5-4h delayed only); SPECT/CT is performed for any painful region that looks normal on WB imaging

Bone scan superscan

A pattern of uniform/near-uniform tracer uptake across the entire skeleton, giving the appearance of "hot spots" everywhere, typically due to widespread metastatic disease saturating the bone with tracer; a key feature is absence of kidney/bladder visualisation (normally seen in a standard scan)

Flare phenomenon (osteoblastic flare)

Interval appearance of sclerotic rims around previously lytic lesions, or newly visible small "hot" lesions, typically seen 2 weeks to 3 months after starting chemotherapy; represents a positive response to treatment (reparative mineralisation) rather than progression; if lesions continue increasing beyond 6 months, this indicates true disease progression instead

PSA (Prostate-Specific Antigen)

A protein produced by prostate cells, measured by blood test; normal range below 4.0 ng/mL, borderline 4.0-10.0 ng/mL, high above 10.0 ng/mL; elevated levels can indicate prostatitis, benign prostatic hyperplasia, or prostate cancer, but is not specific — further investigation (DRE, biopsy, or 68Ga-PSMA PET/CT) is needed

Osteolytic bone imaging protocol

Two-phase bone scan; early imaging (EBP statics, which may show increased vascularity that appears cold on delayed images, plus WBBP); delayed imaging at 2.5-4 hours (whole body, statics, SPECT/CT); dynamic perfusion is not typically useful unless pain is localised to a specific area

Osteosclerotic bone imaging protocol

Similar to osteolytic lesions; early imaging (EBP statics, may show increased vascularity, WBBP) and delayed imaging (WB, statics, SPECT/CT); early imaging may not be needed unless the patient reports recent localised pain

Osteoid osteoma

A rare benign bone tumour affecting young individuals (10-30 years), about 10% of all benign bone tumours, unknown aetiology; classic presentation is localised night pain relieved by NSAIDs; shows a nidus with cortical thickening on CT/x-ray; treated with CT-guided radiofrequency ablation, usually successful though can recur

Osteoid osteoma bone scan imaging

Three-phase bone scan; nuclear medicine is very effective for early detection, showing intense activity at the lesion site even when radiographic images are difficult to evaluate; dynamic, early blood pool, delayed statics, and SPECT/CT around the affected site; WBBP/delayed WB imaging usually not needed since the disease rarely spreads

Fibrous dysplasia

A rare bone disorder from abnormal fibrous tissue growth within bone, typically caused by a genetic mutation (G-NAS gene) affecting osteoblasts; can be monostotic (single bone, 70-80% of cases) or polyostotic (multiple bones, more likely symptomatic); polyostotic form can be associated with McCune-Albright syndrome; symptoms include bone pain, difficulty walking, deformities (bowing, scoliosis), fractures, and hearing/vision loss if facial bones affected

Fibrous dysplasia treatment

Mild cases monitored; severe cases may need bisphosphonates (to strengthen bone) or surgery; denosumab (a monoclonal antibody also used for osteoporosis and bone metastases) has shown success in clinical trials at reducing bone turnover in fibrous dysplasia lesions

Fibrous dysplasia bone scan imaging

Two-phase bone scan with WBBP early imaging and full delayed imaging (WB, statics, SPECT/CT); scintigraphic appearance shows multiple areas of intense activity that can easily be confused with metastatic disease

SPECT/CT multi-bed acquisition

Each bed position covers 38.7 cm (detector Y-axis), but accounting for overlap needed to join/knit images together, each step effectively covers 36.5 cm; a bone scan covering vertex to knees including the elbows is the Medicare-rebatable standard, usually requiring 4-5 bed positions

Paget's disease of bone (osteitis deformans)

A chronic, idiopathic bone disorder with abnormal bone remodelling; affects 2-4% of adults aged 55+; commonly involves the pelvis, spine, skull, or long bones of the legs; diagnosed via elevated alkaline phosphatase (marker of bone activity) plus imaging (bone scan or x-ray)

Paget's disease types

Monostotic (single bone, most common) or polyostotic (multiple sites — pelvis, sacrum, skull, spine, tibia, humerus, scapula)

Paget's disease symptoms

Bone pain (most common, can mimic arthritis), headaches and hearing loss (if skull affected), tingling/numbness in limbs (enlarged vertebrae compressing spinal nerves), hip pain (if pelvis/femur affected)

Paget's disease treatment

Bisphosphonates (strengthen bone, slow remodelling), pain relievers, physical therapy to maintain joint function; surgery may correct deformities or fractures

Paget's disease imaging protocol

One or two-phase bone scan; many clinical sites only perform a single delayed-phase study (WB imaging at 2.5-4 hours plus/minus SPECT/CT), which is common and widely acceptable; since patients often return for follow-up, less activity may be injected with longer/slower scanning (e.g. 8cm/min)

Rheumatoid arthritis (RA)

A chronic autoimmune disease with joint inflammation from the immune system attacking the synovium, causing progressive destruction of articular cartilage; predominant in females (3

RA bone scan quantification

SPECT reconstruction allows semiquantitative SUV (standardised uptake value) measurement of joint uptake; a young healthy/fibromyalgia patient may show a low summed SUV (~5.1) uniformly, while an RA patient shows markedly higher SUV (~19.6) concentrated at proximal interphalangeal, metacarpophalangeal, and wrist joints

Maximum Intensity Projection (MIP)

An imaging technique widely used with CT, MRI, SPECT, and PET that creates a 2D projection image from a 3D dataset by highlighting the maximum intensity value along the line of sight for each pixel

Avascular necrosis (AVN/osteonecrosis)

Bone tissue death from lack of blood supply, typically affecting the epiphysis of long bones at weight-bearing joints (most common sites

AVN symptoms

Persistent joint pain, stiffness (especially after rest), limited range of motion, joint instability/collapse with shape changes, crepitus (grinding/popping)

AVN bone scan appearance

Can show a photopenic (cold) area in the affected bone (early avascularity) surrounded by increased osteoblastic activity (reparative response); bone scans can detect early AVN stages by showing areas of decreased blood flow

AVN imaging protocol

Three-phase bone scan; WBBP imaging is important to determine if there are multiple sites of disease; early dynamic and blood pool statics, then delayed statics, whole body, and SPECT/CT around the primary site

Osteomyelitis

A bone infection, typically caused by bacteria; bone scanning identifies areas of infection, assesses extent/severity, and monitors treatment response; main radiotracers are Tc-99m MDP/HDP (bone scan), Tc-99m-HMPAO labelled WBC, or 18F-FDG PET/CT

Acute vs chronic osteomyelitis

Acute is the initial stage with rapid onset of pain, swelling, warmth, and redness, requiring prompt treatment; chronic develops if acute osteomyelitis is inadequately treated, with persistent infection, bone necrosis, and abscess formation

Osteomyelitis by route of infection

Hematogenous (bacteria travel via bloodstream from a distant site, often affects long bones in children and vertebrae/pelvis in adults) versus contiguous (infection spreads from adjacent infected tissue, e.g. open fractures, infected joint replacements, diabetic foot ulcers)

Osteomyelitis by causative organism

Bacterial (most common, e.g. Staphylococcus aureus including MRSA, Streptococcus species, Gram-negative bacteria) versus fungal (less common, especially in immunocompromised individuals, often Candida or Aspergillus species, harder to diagnose and treat)

Osteomyelitis vs cellulitis distinction on bone scan

Osteomyelitis (bone infection) typically shows increased tracer uptake in the affected bone due to increased blood flow and bone remodelling; cellulitis (skin/soft tissue infection not involving bone) usually shows normal or decreased bone tracer uptake unless there is secondary osteomyelitis — an important distinction for diabetic patients where it affects whether amputation or antibiotic management is chosen

Osteomyelitis bone scan imaging protocol

Three-phase bone scan (dynamic, early blood pool statics, WBBP) early imaging, then delayed statics, whole body, and SPECT/CT around the primary affected site; advantages are that it's non-invasive, widely available, and can detect infection earlier than other imaging

Infection-specific radiopharmaceuticals (if bone scan positive)

Tc-99m-colloid WBC, Tc-99m-HMPAO WBC, In-111-Oxine WBC, Ga-67 citrate; the first three require labelling white blood cells (like labelling RBCs); Ga-67 citrate is injected directly; Tc-99m-labelled WBC agents must be imaged same-day (short half-life), while In-111-Oxine WBC (67h half-life) and Ga-67 citrate (78h half-life) can be imaged 24-48 hours later; a positive osteomyelitis white cell scan shows lesions matching the positive bone scan

Prosthetic loosening

Detachment or movement of an artificial joint (hip/knee replacement) from the bone it was implanted in; causes include wear and tear, inadequate fixation, infection, bone resorption around the implant, mechanical stress/misalignment, or falls/accidents; symptoms (pain, swelling, instability, decreased range of motion, difficulty with activities) reduce quality of life

Prosthetic loosening imaging protocol

Three-phase bone scan (dynamic, early blood pool statics, WBBP early imaging; delayed statics, whole body, SPECT/CT); same advantages as standard bone scanning — non-invasive and can detect issues earlier than other imaging

Moore's prosthesis

An early type of hip implant developed by Sir John Charnley, introduced in the 1950s, consisting of a metal femoral stem inserted into the upper femur plus a metal or plastic acetabular cup component, used to restore mobility in severe osteoarthritis or hip fracture

Gonarthrosis

Wearing down of the knee joint, often seen on the contralateral (non-prosthetic) side due to compensatory limping and added weight-bearing stress after a knee replacement complication on the other side

Sensitivity

The true positive rate — the proportion of actual positives correctly identified by a test; tells how well a test identifies individuals who have the disease/condition

Specificity

The proportion of actual negatives correctly identified by a test; tells how well a test identifies individuals who do not have the disease/condition