Radionuclide Bone Scanning (Bone Scintigraphy)

Principle and Tracer

Bone scintigraphy uses a physiologic approach to imaging. An organic phosphate analogue - methylene diphosphonate (MDP) or hydroxymethylene diphosphonate (HDP) - is tagged with the radionuclide technetium-99m (⁹⁹ᵐTc), giving the combined tracer ⁹⁹ᵐTc-MDP or ⁹⁹ᵐTc-HDP. After intravenous injection, the tracer is:

Distributed throughout the body in proportion to regional perfusion
Adsorbed onto the surface of osteoid at sites of active mineralization (calcium phosphate formation)
Detected by gamma cameras that record photons emitted by the decaying radionuclide

The delayed bone images (3 to 24 hours) provide an indirect measure of osteoblastic activity, making bone scintigraphy effectively an imaging map of bone metabolism. ⁹⁹ᵐTc has a 6-hour half-life, precluding imaging beyond 24 hours. Tracer not incorporated into bone is eliminated by renal excretion.

Three-Phase Bone Scan

The classic technique involves three phases performed sequentially:

Phase	Timing	What it shows
Phase 1 - Perfusion (Flow)	Dynamic images for first 2 minutes post-injection	Arterial blood flow
Phase 2 - Blood Pool	2-5 minutes after injection	Relative blood pool distribution; diffuse increased activity in infection
Phase 3 - Static (Delayed)	3 hours post-injection	True tracer incorporation into osteoid; depicts osteoblastic activity

In infection, diffuse increased activity is seen on blood pool images; this activity becomes focally increased on delayed views, and may persist for months
In low-grade infection, phases 1 and 2 may be negative - only phase 3 may show changes
If all three phases are negative, infection is essentially excluded
At 24 hours, clearance of normal tracer amplifies retention in pathologic bone

Planar vs. Tomographic Imaging

Planar scintigraphy provides whole-body and spot views. It is analogous to plain radiography.

SPECT (Single-Photon Emission Computed Tomography) uses tomographic principles to generate cross-sectional images from multiple planar images, providing improved contrast and spatial resolution. It is more sensitive for detecting active facet arthritis or spondylolysis.

SPECT/CT is a fusion imaging technique acquiring both a CT and a nuclear SPECT study concurrently. It localizes areas of increased bone metabolism precisely, increasing overall sensitivity and specificity. Without CT, precise anatomic localization on conventional SPECT alone can be very difficult - for example, distinguishing facet arthritis from active spondylolysis in the posterior vertebral elements.

Clinical Applications

Oncology

Most sensitive method for detecting bone metastases (whole-body skeletal scintigraphy)
Screening tool to identify areas of increased bone activity that could represent metastatic spread
Important limitation: multiple myeloma and purely lytic metastases may be negative on bone scan, as these tumors do not stimulate significant osteoblastic response

Orthopaedic Infection

Relatively effective in identifying spinal infection (adjunct to MRI)
Used as a screening tool for osteomyelitis, spondylodiscitis
Low specificity - positive in infection but also in recent trauma or surgery, diabetic arthropathy, gout
Sensitivity ~90%, Specificity ~100%, Accuracy ~94% when combined with ⁶⁷Ga scan

Bone & Joint Disorders

Stress fractures
Paget's disease
Avascular necrosis (osteonecrosis)
Pars interarticularis defects
Pseudarthrosis (incomplete fusion)
Active bone lesions: osteoid osteoma, osteoblastoma
Discitis / vertebral body osteomyelitis
Transient osteoporosis

Arthritis (Rheumatology)

Bone scintigraphy shows characteristic distribution patterns in different arthritides:

Bone scintigraphy patterns: OA (A) shows focal DIP/PIP/first CMC uptake; RA (B) shows symmetric radiocarpal/MCP/PIP uptake; PsA (C) shows asymmetric ray distribution

Distributions of activity on planar bone scintigraphy: (A) Osteoarthritis - severe activity in first CMC joints bilaterally with DIP/PIP involvement. (B) Rheumatoid arthritis - symmetric activity in radiocarpal joints, MCP and PIP joints. (C) Psoriatic arthritis - bilateral asymmetric distribution with ray pattern (Firestein & Kelley's Rheumatology)

Bone scans are more sensitive than plain radiography because radionuclide uptake precedes morphologic bone changes visible on X-ray
A suggestive bone scan must be confirmed with further imaging (not diagnostic alone)

Other Radionuclide Agents

Agent	Use
⁶⁷Ga (Gallium-67) scan	Good adjunct to ⁹⁹ᵐTc for osteomyelitis; especially detects soft-tissue infection in spondylodiscitis. Rapidly changes with resolution of acute infection (useful to document clinical improvement). Does NOT distinguish pyogenic from granulomatous infection
¹¹¹In (Indium-111) WBC scan	Useful for detecting abscesses; differentiates noninfectious lesions (hematomas, seromas) from true infection; not reliable in acute infections; false-negatives occur in chronic infections
⁹⁹ᵐTc-labeled leukocytes	Useful for acute infection; localizes in areas of acute inflammation
¹⁸F-FDG PET	Highest sensitivity and specificity; delineates lesions at very early disease stages; FDG enters glucose cycle and concentrates in activated leukocytes/macrophages

Below is a bone scan of both feet showing focal intense uptake in the left ankle region (likely infection or active arthritis):

Bone scan of both feet showing focal intense uptake left ankle

Limitations and Pitfalls

Low specificity: positive in any condition causing increased osteoblastic activity (infection, tumor, fracture, arthritis, Paget's, etc.)
False negatives: multiple myeloma, purely lytic lesions, early osteonecrosis (cold scan), photopenic ("cold spot") metastases
False positives: recent trauma or surgery, diabetic arthropathy, gout, degenerative changes
Cannot show bone or joint detail well; distinguishing between bone, joint, or soft-tissue inflammation is often difficult on bone scan alone
Multifocal vertebral lesions may be detected but bone scintigraphy is limited in depicting detailed surgical anatomy, particularly compared with CT or MRI
Positive findings in the spine may be attributed to degenerative changes

Summary

Feature	Detail
Tracer	⁹⁹ᵐTc-MDP or ⁹⁹ᵐTc-HDP
Half-life	6 hours
Mechanism	Adsorption onto osteoid at sites of active mineralization
Sensitivity	High (whole-body; detects early metabolic changes)
Specificity	Low (non-specific; many causes of increased uptake)
Imaging window	Up to 24 hours post-injection
Best for	Bone metastases survey, osteomyelitis (adjunct), metabolic bone disease, stress fractures
Pitfall	Negative in myeloma/lytic mets; false-positive from degenerative disease

Sources:

Firestein & Kelley's Textbook of Rheumatology, 2-Volume Set, pp. 1152-1154
Rockwood and Green's Fractures in Adults, 10th Ed 2025, p. Block 9
Campbell's Operative Orthopaedics 15th Ed 2026, Blocks 9 & 23
Bailey and Love's Short Practice of Surgery 28th Ed
Grainger & Allison's Diagnostic Radiology

This is a shared conversation. Sign in to Orris to start your own chat.