Residency · Residency · Diagnostic Radiology
Skeletal Scintigraphy: Bone Scan Interpretation
Introduction
Skeletal scintigraphy (bone scan) is a highly sensitive technique for detecting osteoblastic activity throughout the entire skeleton. Although partially supplanted by cross-sectional imaging and PET in some applications, it remains a workhorse study for evaluating metastatic disease, occult fractures, infection, and metabolic bone disease.
Radiopharmaceutical and Mechanism
Technetium-99m Methylene Diphosphonate (Tc-99m MDP)
Tc-99m MDP is the most widely used bone-seeking radiopharmaceutical. It binds to hydroxyapatite crystals in bone matrix at sites of active bone formation (osteoblastic activity). Uptake depends on two factors: regional blood flow and osteoblastic activity. The administered dose is approximately 20-25 mCi IV in adults. The three-phase bone scan consists of the flow phase (dynamic, first 60 seconds), blood pool phase (3-5 minutes), and delayed phase (2-4 hours). Standard delayed whole-body imaging at 2-4 hours allows clearance of soft tissue activity.
Image Acquisition
Whole-body planar images in anterior and posterior projections are the standard acquisition. Spot views of areas of interest provide improved resolution. SPECT or SPECT/CT offers improved localization for structures like the spine and temporomandibular joints. Pinhole collimator imaging is used for small structures such as the hips in children with suspected Legg-Calve-Perthes disease.
Normal Scan Appearance
A normal scan shows symmetric uptake throughout the axial and appendicular skeleton, with increased uptake at sites of high metabolic activity such as growth plates in children, sacroiliac joints, and sternoclavicular joints. The kidneys are normally visualized due to renal excretion; absent renal visualization suggests a superscan. Bladder activity is prominent, and post-void images may be needed for pelvic evaluation. Growth plates in children are symmetric and linear; asymmetry or focal irregularity warrants investigation.
Patterns of Abnormality
Focal Increased Uptake
Metastatic disease produces multiple random foci of increased uptake, most commonly in the axial skeleton (spine, pelvis, ribs). Fractures show linear or focal uptake at fracture sites, with rib fractures appearing in a linear pattern. Degenerative disease produces uptake at facet joints, acromioclavicular joints, and weight-bearing joints. Paget disease shows markedly increased uptake involving an entire bone or large segment with cortical expansion.
Diffusely Increased Skeletal Uptake (Superscan)
A superscan shows diffusely increased skeletal uptake with absent or faint renal visualization. A metabolic superscan results from hyperparathyroidism, renal osteodystrophy, or osteomalacia and shows relatively uniform uptake with prominent calvarium, mandible, and sternum involvement. A metastatic superscan from diffuse osseous metastatic disease (prostate, breast) is often more heterogeneous. The metabolic superscan often shows prominent periarticular uptake in a beading pattern.
Photopenic (Cold) Lesions
Photopenic areas represent decreased uptake relative to surrounding bone. Causes include early avascular necrosis, purely lytic metastases (multiple myeloma, renal cell carcinoma, thyroid carcinoma), radiation therapy, and metallic hardware artifact. Multiple myeloma often produces a normal or near-normal bone scan despite extensive disease, making skeletal survey or PET/CT preferred.
Flare Phenomenon
The flare phenomenon is a paradoxical increase in the number or intensity of lesions on bone scan 2-3 months after initiating effective therapy. It represents a healing response with increased osteoblastic activity at treatment sites and should not be confused with disease progression. It usually resolves on subsequent scans at 6 months.
Three-Phase Bone Scan
Indications
The primary indication is distinguishing osteomyelitis from cellulitis. | Phase | Osteomyelitis | Cellulitis |
| Flow (dynamic) | Positive (increased) | Positive (increased) | |
|---|---|---|---|
| Blood pool (3-5 min) | Positive (increased) | Positive (increased) | |
| Delayed (2-4 hrs) | Positive (focal uptake) | Normal |
Osteomyelitis is positive in all three phases (increased flow, blood pool, and delayed uptake). Cellulitis is positive in the first two phases (flow and blood pool) but shows a normal delayed phase. Sensitivity for osteomyelitis is approximately 95%, though specificity is reduced in the setting of fractures, neuropathic joints, or recent surgery.
Specific Applications
Stress fractures show focal delayed-phase uptake, and SPECT or MRI may be needed for confirmation. Complex regional pain syndrome (CRPS) shows increased periarticular uptake on delayed images in a characteristic pattern in the affected extremity. Shin splints versus stress fracture can be distinguished because shin splints show longitudinal cortical uptake while stress fractures show focal fusiform uptake.
Specific Clinical Scenarios
Metastatic Disease Screening
Bone scans are indicated for staging of prostate, breast, and lung cancer and for any cancer with bone pain or elevated alkaline phosphatase. Predominantly osteoblastic metastases are well detected (prostate, breast). Purely lytic metastases may be missed (myeloma, renal cell carcinoma), making PET/CT or CT preferred. A solitary rib lesion is more likely benign (fracture), while multiple random rib lesions suggest metastases.
Prosthetic Joint Evaluation
Loosening appears as increased uptake at the bone-prosthesis interface, particularly on delayed images. Infection shows increased uptake on all three phases; combined with In-111 WBC scan or Tc-99m sulfur colloid scan, specificity improves. Normal post-operative uptake around prostheses can persist for 12-18 months (cemented) to 2 years (uncemented).
Pediatric Applications
In Legg-Calve-Perthes disease, pinhole imaging shows a photopenic femoral epiphysis representing early avascular necrosis. In child abuse, bone scans may detect occult fractures not seen on radiographs, though skeletal survey remains the primary imaging study. Osteoid osteoma shows focal intense uptake with a characteristic double-density sign on SPECT.
Clinical Pearls
A superscan shows diffusely increased skeletal uptake with absent kidney visualization; distinguishing metabolic from metastatic causes is important. Bone scan sensitivity for purely lytic lesions (myeloma, RCC) is poor, and PET/CT or skeletal survey should be used instead. The flare phenomenon after therapy initiation mimics progression but actually indicates treatment response. Three-phase bone scan differentiates osteomyelitis (positive all three phases) from cellulitis (positive flow and blood pool only).
References
- Society of Nuclear Medicine Procedure Guideline for Bone Scintigraphy. J Nucl Med Technol. 2018;46(4):398-404.
- Skeletal Scintigraphy: An Update on Current Concepts and Applications. Radiographics. 2020;40(5):1430-1451.
- Bone Scan Interpretation: A Systematic Approach. Semin Nucl Med. 2015;45(1):3-15.
- The Flare Phenomenon on Bone Scintigraphy. Clin Nucl Med. 2019;44(5):390-396.