How do you interpret a bone scan?

How to Interpret a Bone Scan

Bone scan interpretation requires systematic evaluation of three sequential phases (flow, blood pool, and delayed skeletal imaging), with abnormal uptake patterns analyzed in the context of clinical history, distribution, and intensity to differentiate between benign and malignant processes.

Technical Phases of Bone Scan Acquisition

A three-phase bone scan involves IV injection of Tc-99m methylene diphosphonate followed by imaging at three distinct time points 1:

• Flow/angiographic phase: Immediate dynamic imaging (2-5 seconds for 60 seconds) to detect asymmetric increased blood flow 1
• Blood pool/tissue phase: Static imaging immediately after the angiogram to identify increased extracellular fluid from soft-tissue inflammation 1
• Delayed/skeletal phase: Obtained 2-4 hours post-injection to demonstrate increased tracer activity at sites of active new bone formation 1

Fundamental Interpretation Principles

Normal vs. Abnormal Uptake Patterns

Increased uptake ("hot spots") indicates areas of accelerated bone turnover and may represent metastatic disease, infection, fracture, or benign bone remodeling 1. The sensitivity of bone scans depends on blood flow and rate of new bone formation, both of which are increased in pathologic conditions 1.

Key interpretation points:

• Metastatic disease: Diagnosed based on overall pattern of activity or in conjunction with anatomic imaging 1
• Infection: Classic finding is increased uptake on all three phases in the same location (positive three-phase bone scan) 1
• Reduced uptake: Rare finding requiring careful analysis; differential includes acute leukemia, avascular necrosis, or aggressive lytic lesions 2

Specificity Limitations

Bone scans are highly sensitive but not specific—increased uptake is a nonspecific finding that cannot reliably differentiate infection from aseptic loosening, fracture healing, or postoperative changes 1. The accuracy of bone scans for diagnosing complications ranges from 50-70% 1.

Clinical Context-Specific Interpretation

Oncologic Applications

For cancer staging, bone scans detect skeletal metastases with higher sensitivity than plain radiographs 3, 4:

• Prostate cancer: Indicated for T1 disease with PSA ≥20 ng/mL, T2 disease with PSA ≥10 ng/mL, Gleason score ≥8, or T3/T4 disease 1, 3
• Breast cancer: 15-25% of patients with normal radiographs have abnormal scans due to metastases 4
• Bone scans rarely positive in asymptomatic patients with PSA <10 ng/mL 1

The majority of bone metastases appear as hot spots and are easily recognized, though 5% of patients may have normal scans despite abnormal radiographs 5.

Infection Evaluation

For suspected spine infection, three-phase bone scan with Tc-99m-MDP has sensitivity of 73% and specificity of 81% 1. However:

• MRI without and with IV contrast is superior, with 96% sensitivity and 94% specificity for spine infection 1
• Bone scans should be reserved for cases where MRI is contraindicated or when evaluating abnormal radiographic/CT findings 1

Prosthetic Joint Evaluation

In the postoperative setting after arthroplasty 1:

• Increased uptake may persist as normal postoperative bone remodeling for up to 1-2 years after surgery 1
• Infection is more likely than aseptic loosening if increased uptake occurs on both blood-pool and delayed images 1
• No patient with infection had a negative three-phase bone scan in one series of 80 patients 1
• Patients with abnormal bone scans and suspected infection require additional assessment (WBC scan, FDG-PET/CT) to characterize the abnormality 1

Enhanced Imaging Techniques

SPECT/CT Integration

Adding SPECT or SPECT/CT improves contrast resolution and anatomic localization of radiopharmaceutical uptake 1, 6:

• Whole-body SPECT/CT has significantly higher sensitivity than targeted SPECT/CT for detecting bone metastases (p=0.0297) and extra-axial metastases (p=0.0266) 7
• SPECT/CT allows differentiation of soft-tissue involvement from bone involvement 1
• Blood pool SPECT/CT obtained immediately after static images further localizes foci of hyperemia 1

Complementary Imaging

When bone scan findings are equivocal 1:

• MRI provides superior soft-tissue characterization and should be obtained for spine infections or when evaluating extent of disease 1
• CT with metal artifact reduction techniques confirms fractures and evaluates osseous detail 1
• FDG-PET/CT shows 94.8% sensitivity and 91.4% specificity for spine infection when MRI is inconclusive 1

Critical Pitfalls to Avoid

• Do not interpret increased uptake in isolation—always correlate with clinical symptoms, laboratory values, and anatomic imaging 1
• Beware of the "flare phenomenon" after treatment initiation, which can falsely suggest progression 6
• Plain radiographs will not detect lesions until 50% of bone mineral content is lost or gained, making bone scan more sensitive for early detection 1, 5
• Solitary scan lesions in patients with known primary tumors have 20-64% incidence of metastases—require confirmation with additional imaging or biopsy 4
• Bone scans miss lymphatic or visceral metastases—comprehensive staging requires additional cross-sectional imaging 6