Nuclear Medicine — Bone Scan & PET Basics

Nuclear medicine imaging uses radioactive tracers (radiopharmaceuticals) administered to the patient, which accumulate in specific tissues based on physiological activity; the emitted gamma radiation is detected by a gamma camera to produce images reflecting metabolic and physiological processes rather than anatomy alone. In orthopaedics, nuclear medicine has three principal applications: the technetium-99m (Tc-99m) bone scan for identifying areas of increased bone turnover, the labelled white cell scan (WBC scan) for detecting infection, and positron emission tomography (PET) — increasingly combined with CT (PET-CT) or MRI (PET-MRI) — for tumour staging, infection diagnosis, and monitoring treatment response.

• Technetium-99m (Tc-99m): the workhorse radionuclide in nuclear medicine; a metastable nuclear isomer of techium-99; half-life = 6 hours (short enough to minimise radiation dose, long enough for imaging); emits a 140 keV gamma ray (ideal for gamma camera detection); produced from a molybdenum-99 generator (`the cow`) at regular intervals; the most widely used radionuclide in clinical nuclear medicine; for bone scans, Tc-99m is combined with a bisphosphonate — methylene diphosphonate (MDP) — forming Tc-99m MDP which accumulates in areas of high osteoblastic activity (bone turnover)
• Principle of bone scan uptake: Tc-99m MDP accumulates preferentially in areas of increased osteoblastic activity and blood flow; the more active the bone turnover, the greater the uptake (`hot spot`); areas of reduced blood flow or pure osteoclastic activity without osteoblastic response may appear cold (`cold spot` = photopenic); the key concept is that the bone scan reflects metabolic activity, NOT anatomy — it detects pathology before plain X-ray changes appear

• Osteomyelitis vs cellulitis on three-phase bone scan: Phase 1 and 2 are both positive in osteomyelitis AND cellulitis (both show increased soft tissue uptake in the flow and blood pool phases); the KEY differentiator is Phase 3 — in osteomyelitis, there is FOCAL BONY uptake in Phase 3 corresponding to the infected bone; in cellulitis, Phase 3 shows DIFFUSE soft tissue uptake without focal bony uptake; positive Phases 1 + 2 + 3 (focal) = osteomyelitis; positive Phases 1 + 2 + diffuse 3 = cellulitis

• PET (Positron Emission Tomography): uses positron-emitting radiotracers; the most common tracer is 18F-fluorodeoxyglucose (18F-FDG); FDG is a glucose analogue that accumulates in metabolically active cells (particularly rapidly dividing tumour cells and activated inflammatory cells); as the positron from 18F decay encounters an electron, two 511 keV gamma rays are emitted in opposite directions; coincidence detection of these paired photons allows precise 3D localisation; PET is combined with CT (PET-CT) to provide simultaneous metabolic and anatomical information in the same session
• FDG uptake: SUV (standardised uptake value) — a measure of how much FDG accumulates in a lesion compared to background; SUVmax >2.5 is generally considered suspicious for malignancy (though this threshold varies by tissue); highly metabolically active tumours (osteosarcoma, Ewing`s sarcoma, lymphoma, metastases) have high SUVmax; benign lesions have low FDG uptake; however, infection and inflammation also have high FDG uptake — PET cannot always distinguish infection from malignancy by uptake alone; clinical context and combined PET-CT interpretation is required
• Orthopaedic applications of PET-CT: (1) Staging of primary bone and soft tissue sarcomas — identifies distant metastases (lung, bone) and lymph node involvement; evaluates response to neoadjuvant chemotherapy (reduction in SUVmax after treatment = good response — correlates with histological necrosis rate); (2) Identifying the primary tumour in metastatic bone disease of unknown origin — FDG-PET-CT whole-body surveys detect occult primaries; (3) Suspected prosthetic joint infection — FDG-PET has high sensitivity for PJI; (4) Lymphoma staging (including bone involvement); (5) Fever of unknown origin with suspected musculoskeletal focus; NaF-PET (sodium fluoride PET) — NaF accumulates in bone like Tc-99m MDP but with superior sensitivity; superior to conventional bone scan for metastasis detection

• Tc-99m MDP bone scan: reflects osteoblastic activity; three phases — flow (blood flow), blood pool (soft tissue), delayed/bone (bone turnover); whole body at 2–4 hours; detects pathology before X-ray changes (3–5 days vs 10–21 days)
• Osteomyelitis vs cellulitis: three-phase bone scan — both positive in phases 1 and 2; KEY = phase 3: osteomyelitis = focal BONY uptake; cellulitis = diffuse soft tissue uptake only; focal bone phase uptake = osteomyelitis
• Myeloma = COLD on bone scan (purely lytic = no osteoblastic activity); do NOT use bone scan for myeloma staging; use whole-body MRI or PET-CT; myeloma is the most important `cold scan` diagnosis in orthopaedics
• Osteoid osteoma: intense focal hot spot at the nidus; the most reliably intense focal hot spot on bone scan; confirm with CT (nidus = small lytic lesion with sclerotic rim); night pain + aspirin relief + intense bone scan = osteoid osteoma
• Superscan: diffusely uniform increased skeletal uptake + absent kidney uptake = extensive metastatic infiltration (usually prostate or breast mets); so much uptake in bone that kidneys are not visualised; extremely poor prognosis sign
• PET-CT (18F-FDG): glucose analogue; accumulates in metabolically active cells (tumour, infection, inflammation); SUVmax >2.5 suspicious for malignancy; used for sarcoma staging, response to chemotherapy (falling SUVmax = good response), PJI diagnosis, unknown primary detection
• Labelled WBC scan: white blood cells labelled with 99mTc-HMPAO or 111In-oxine; accumulate at sites of infection (neutrophil migration to infected tissue); combined bone scan + WBC scan has highest accuracy for PJI; SPECT-CT adds anatomical detail
• Bone scan radiation dose: ~4–6 mSv (Tc-99m MDP); comparable to CT head; MRI preferred when equivalent diagnostic quality — zero radiation; pregnancy = avoid nuclear medicine (radiopharmaceuticals cross placenta and concentrate in foetal thyroid)