Orthopaedic Surgery - Skeletal Scintigraphy

Basics

Skeletal scintigraphy, commonly called a:

Bone scan

is a nuclear medicine technique used to evaluate abnormalities of:

Bone metabolism

Blood flow

and

Osteoblastic activity.

It is particularly sensitive for detecting areas of:

Increased bone turnover

often before abnormalities become visible on:

Plain radiographs.

Basic Principle

Bone-seeking radiopharmaceuticals accumulate preferentially in areas with:

Increased perfusion

and

Active new bone formation.

Therefore, regions undergoing:

Fracture healing

Tumor-associated bone reaction

Infection

or other processes that stimulate osteoblastic activity may demonstrate:

Increased tracer uptake.

Advantages

Major advantages include:

High sensitivity for early skeletal disease

and the ability to perform a:

Whole-body survey

during a single examination.

Disadvantage

The principal limitation is:

Low specificity.

Many unrelated disorders can produce increased tracer uptake, so scintigraphic findings must be interpreted together with:

Clinical history

Physical examination

and

Anatomic imaging.

Indications

Whole-body skeletal scintigraphy may be used to evaluate:

Primary bone tumors

Bone metastases

Osteomyelitis

Painful joint prostheses

Occult fractures

Stress fractures

Medial tibial stress syndrome

Spondylosis

Complex regional pain syndrome

Fracture nonunion

Avascular necrosis

Unexplained musculoskeletal pain

Heterotopic ossification

Paget disease

Fibrous dysplasia

Three-Phase Skeletal Scintigraphy

Three-phase skeletal scintigraphy, or:

TPSS

adds early vascular and soft-tissue imaging to delayed bone-phase imaging.

It is particularly useful in selected cases of:

Osteomyelitis

Stress fracture

Complex regional pain syndrome

and

Osteoid osteoma.

Radiopharmaceutical

The most commonly used radiopharmaceuticals are:

Technetium-99m-labeled diphosphonate compounds.

These agents bind to the mineral phase of bone, particularly in regions of:

Active remodeling

and

Osteoblastic response.

Technique

Whole-Body Skeletal Scintigraphy

A typical adult dose is approximately:

20–30 mCi of technetium-99m diphosphonate

administered by:

Intravenous injection.

Whole-body delayed images are generally obtained approximately:

2–4 hours later.

Three-Phase Study

TPSS uses the same radiopharmaceutical but acquires images during several distinct phases.

Phase 1 – Flow Phase

Immediately after intravenous bolus administration, rapid sequential images are obtained, typically every:

1–3 seconds

for approximately:

60 seconds.

This phase evaluates:

Regional blood flow.

Flow-Phase Interpretation

Increased activity during this phase suggests:

Hyperemia

or increased vascular delivery to the region of interest.

Phase 2 – Blood-Pool Phase

Immediately after the flow phase, higher-count images are obtained.

This phase reflects tracer distribution within:

Blood pool

and

Extracellular soft tissues.

Blood-Pool Interpretation

Increased uptake may indicate:

Soft-tissue inflammation

Hyperemia

or active:

Synovitis.

Phase 3 – Delayed Bone Phase

Delayed images are obtained approximately:

2–4 hours after injection.

These assess tracer incorporation into:

Bone.

Delayed Bone-Phase Interpretation

Focal increased uptake generally indicates increased:

Bone turnover

or

Osteoblastic activity.

Optional Phase 4 – Very Delayed Imaging

Additional images may be obtained approximately:

24 hours after injection.

These are sometimes useful when tracer delivery or clearance is delayed, such as in patients with:

Poor peripheral perfusion

Diabetes

Peripheral vascular disease

or

Renal dysfunction.

SPECT

Single-photon emission computed tomography:

SPECT

provides:

Three-dimensional cross-sectional imaging

and improves:

Lesion contrast

and anatomic localization compared with planar imaging.

SPECT/CT

When combined with CT, SPECT can provide both:

Functional information

and

Anatomic localization.

This can substantially improve characterization of:

Focal skeletal abnormalities.

Pathological Findings

Primary Malignant Bone Tumors

Primary malignant bone tumors may demonstrate:

Hyperemia on flow images

and

Intense delayed tracer uptake

corresponding to areas of:

Reactive bone formation.

The exact appearance varies with tumor type.

Primary Benign Bone Tumors

Tracer uptake in benign tumors is:

Variable.

Some lesions are very active, while others demonstrate little:

Radiopharmaceutical accumulation.

Osteoid Osteoma

Osteoid osteoma typically demonstrates:

Marked focal uptake

and is one of the most scintigraphically active:

Benign bone lesions.

Osteoid Osteoma Three-Phase Pattern

Findings may include:

Increased flow

Marked blood-pool activity

and

Intense focal delayed uptake.

This gives skeletal scintigraphy high sensitivity for lesion:

Detection and localization.

Radionuclide-Guided Surgery

Historically, radiopharmaceutical localization has also been used to assist:

Intraoperative identification

of small lesions such as:

Osteoid osteoma.

Osteomyelitis

Classic acute osteomyelitis may demonstrate a:

Three-phase positive bone scan.

Osteomyelitis – Flow Phase

There is:

Focal arterial hyperemia.

Osteomyelitis – Blood-Pool Phase

There is increased:

Regional soft-tissue and osseous activity.

Osteomyelitis – Delayed Phase

There is:

Focal increased bone uptake.

Cellulitis Without Osteomyelitis

Cellulitis usually produces increased:

Flow

and

Blood-pool activity

but lacks the corresponding focal increase in:

Delayed osseous uptake.

Cellulitis With Osteomyelitis

When both are present, early phases may show relatively:

Diffuse regional activity

while delayed images demonstrate more focal uptake within:

Bone.

Accuracy in Osteomyelitis

Three-phase bone scanning can be highly accurate in uncomplicated native bone.

Historical estimates approach approximately:

90% accuracy

when confounding factors are absent.

Factors Reducing Specificity

Specificity decreases substantially after:

Recent fracture

Recent surgery

Orthopaedic hardware placement

or other causes of active bone remodeling.

Importance of Clinical Correlation

Because many disorders produce a similar three-phase pattern, interpretation requires correlation with:

Clinical history

and

Plain radiographs.

Differential Diagnosis of a Three-Phase Positive Study

Conditions that may produce increased uptake during all three phases include:

Fracture

Gout

Osteoarthritis

Charcot arthropathy

Complex regional pain syndrome

Healing osteonecrosis

Primary malignant bone tumor

Recent osteotomy

and

Osteomyelitis.

False-Negative Osteomyelitis Studies

False-negative examinations may occur in:

Neonates

Very elderly patients

and individuals with markedly impaired:

Blood flow.

Poor Perfusion

Examples include patients with:

Diabetes mellitus

or

Peripheral vascular disease.

Reduced tracer delivery may limit uptake despite active:

Infection.

Antibiotic Therapy

Prior or ongoing:

Antibiotic treatment

may also reduce scintigraphic activity in some cases.

Septic Arthritis

Septic arthritis may also demonstrate a:

Three-phase positive pattern.

Septic Joint Appearance

Typical findings include increased tracer activity in:

Periarticular bone

and around the:

Joint space.

These findings are sensitive but not specific because inflammatory arthritis may produce similar:

Periarticular uptake.

Complementary Nuclear Medicine Studies

When infection remains uncertain, other nuclear medicine examinations may improve:

Specificity.

Radiolabeled Leukocyte Scintigraphy

Autologous leukocytes can be labeled with a radionuclide and reinjected.

The leukocytes migrate toward sites of:

Active inflammation and infection.

Limitation of Leukocyte Imaging

Leukocytes also normally localize within:

Bone marrow.

This can complicate interpretation, particularly around:

Prostheses

or areas where marrow distribution has been altered.

Leukocyte Plus Bone Marrow Imaging

Combined:

Radiolabeled leukocyte imaging

and

Bone marrow scintigraphy

can help differentiate true infection from normal or displaced:

Marrow activity.

Principle

A site showing increased labeled leukocyte uptake without corresponding uptake on the:

Sulfur colloid marrow scan

supports the diagnosis of:

Infection.

Accuracy

Historical reports have described diagnostic accuracy of approximately:

89–98%

for appropriately performed paired leukocyte/marrow studies.

Gallium-67 Imaging

Sequential:

Three-phase bone scanning

and

Gallium-67 scintigraphy

has historically been used in selected cases of suspected infection.

Interpretation

The studies are compared for:

Location

and

Relative intensity of uptake.

Gallium activity that is disproportionately greater than corresponding bone-scan activity may support:

Active infection.

Limitations of Gallium Combination Imaging

A substantial proportion of studies may be:

Equivocal

which limits:

Sensitivity

and practical usefulness.

Vertebral Osteomyelitis

Vertebral osteomyelitis often causes:

Intense uptake in adjacent vertebral bodies.

Sensitivity

Historical sensitivity of delayed bone scintigraphy for vertebral osteomyelitis has ranged from approximately:

86–100%.

Bone/Gallium Combination

Combining bone scintigraphy with:

Gallium imaging

can improve specificity in selected cases.

Leukocyte Imaging in Vertebral Osteomyelitis

Radiolabeled leukocyte imaging is less useful for:

Vertebral osteomyelitis

because false-negative studies are relatively common.

Historical false-negative rates have approached:

40–50%.

Diabetic Foot

Bone scintigraphy may be useful in evaluating suspected:

Diabetic foot osteomyelitis, although specificity is limited by neuropathic and postoperative changes.

Negative Predictive Value

A negative three-phase bone scan has a relatively high:

Negative predictive value

for osteomyelitis.

Forefoot Infection

When a forefoot TPSS study is:

Positive or equivocal

labeled leukocyte imaging may improve diagnostic accuracy.

Midfoot and Hindfoot

Interpretation is more difficult in the:

Midfoot

and

Hindfoot

because Charcot neuroarthropathy may produce intense uptake even without:

Infection.

Charcot Foot

Neuropathic bone and marrow remodeling may attract:

Radiolabeled leukocytes

without true osteomyelitis.

Therefore, paired:

Labeled leukocyte

and

Bone marrow scintigraphy

may be particularly useful.

Arthritis

Arthritis commonly produces:

Diffuse periarticular tracer uptake.

There may also be focal increased activity in:

Subchondral bone.

Occult Fractures

Bone scintigraphy is highly sensitive for fractures that are not yet visible on:

Plain radiographs.

Timing of Fracture Positivity

Historical data indicate that approximately:

80%

of fractures may be visible scintigraphically within:

24 hours

and approximately:

95% by 72 hours

in patients younger than:

65 years.

Older Patients

In patients older than approximately:

65 years

maximum sensitivity may not occur until around:

7 days after injury.

Return to Normal After Fracture

Tracer uptake may persist long after:

Clinical fracture healing.

Nondisplaced Fractures

Historical data suggest normalization in approximately:

60–80% by 1 year

and approximately:

95% by 3 years.

Displaced Fractures

Some displaced fractures may remain scintigraphically:

Positive indefinitely

because of persistent remodeling or deformity.

Athletic Injuries

Stress Fractures

Stress fractures often become positive on skeletal scintigraphy approximately:

1–2 weeks before radiographic abnormalities appear.

Stress-Fracture Pattern

Typical uptake is:

Intense

Focal

and often:

Oval or fusiform

at the fracture site.

Shin Splints

Medial tibial stress syndrome generally has a different scintigraphic pattern.

Shin-Splint Pattern

Flow and blood-pool phases are usually:

Normal.

Delayed images may demonstrate:

Mild to moderate linear uptake

along the:

Posteromedial tibial cortex.

Bilateral Tibial Involvement

The pattern is commonly:

Bilateral

and extends longitudinally rather than appearing as the focal intense uptake seen in:

Stress fracture.

Painful Prosthesis

Bone scintigraphy may demonstrate increased uptake around a:

Painful joint replacement

but is generally unable to reliably differentiate:

Aseptic loosening

from

Infection.

Typical Loosening Pattern

Around a hip prosthesis, increased uptake may occur near:

Greater trochanter

Lesser trochanter

and the:

Distal tip of the prosthesis.

Normal Postoperative Uptake

Increased bone-scan activity may persist for approximately:

1 year after cemented arthroplasty

and as long as:

2–3 years after uncemented arthroplasty.

Therefore, postoperative uptake alone does not establish:

Infection or loosening.

Prosthetic Joint Infection

Historically, combined:

Indium-111-labeled leukocyte

and

Technetium-99m sulfur colloid marrow imaging

has been one of the more accurate nuclear medicine strategies for distinguishing:

Infection

from noninfectious prosthetic changes.

Diagnostic Performance

Historical studies have reported sensitivity and specificity above approximately:

90%

in selected settings.

Marrow Displacement Pitfall

One cause of false-positive leukocyte imaging is:

Displaced or redistributed marrow.

The sulfur colloid marrow scan helps distinguish this from:

True infection.

Bone Grafts

Bone scintigraphy can sometimes evaluate the vascularity of:

Bone grafts

particularly during the early postoperative period.

Early Postoperative Assessment

Within approximately:

1 week after surgery

bone-seeking tracer uptake may provide information about:

Graft perfusion and viability.

Vascularized Graft

A viable vascularized graft may show:

Normal

or

Diffuse increased uptake

with focal uptake at the:

Osteotomy site.

Failed Graft

A failed or poorly perfused graft may appear as a:

Photopenic defect

with little or no tracer accumulation.

Graft Imaging Pitfalls

Interpretive problems include:

New bone formation on a nonviable graft

Postoperative changes

and

Osteoradionecrosis.

Metastatic Bone Disease

Skeletal scintigraphy is widely used to survey for:

Bone metastases.

Patterns of Metastatic Disease

Metastatic disease may appear as:

Multiple randomly distributed lesions

A solitary focus

Diffuse skeletal involvement

or occasionally:

Photopenic lesions.

Osteoblastic Metastases

The examination is particularly sensitive for tumors that induce substantial:

Osteoblastic activity.

Reduced Sensitivity

Sensitivity is lower for predominantly:

Lytic tumors

that provoke little osteoblastic response.

Examples include some metastases or lesions associated with:

Multiple myeloma

Renal cell carcinoma

Thyroid carcinoma

and

Lymphoma.

Multiple Myeloma

Conventional bone scintigraphy may underestimate disease in:

Multiple myeloma

because many lesions are predominantly:

Osteolytic

without substantial reactive bone formation.

Spinal Surgery

Following spinal surgery, increased uptake at operative sites is commonly:

Normal

because of:

Healing and new bone formation.

Nonunion and Pseudoarthrosis

Persistent:

Focal intense uptake

at a fusion or fracture site may suggest ongoing motion or:

Nonunion.

SPECT in Spinal Nonunion

SPECT or SPECT/CT can improve localization and sensitivity when evaluating suspected:

Pseudoarthrosis.

Pediatric Considerations

Young children may require:

Sedation

if they cannot remain still during prolonged imaging.

The need for sedation depends on:

Age

Developmental level

and examination duration.

Pregnancy Considerations

Technetium-99m bone scintigraphy exposes the fetus to a relatively:

Low radiation dose.

Historical estimates for most routine studies are below approximately:

0.5 rad.

Fetal Risk

Radiation-associated fetal risk is considered low at doses far below approximately:

5 rad.

Nevertheless, nuclear medicine imaging during pregnancy should be performed only when:

The expected diagnostic benefit outweighs the potential fetal radiation risk.

Follow-Up Applications

Serial skeletal scintigraphy may be used to evaluate:

Stability or progression of metastatic bone disease

Residual or recurrent primary bone tumor

Response to cancer therapy

Response to treatment of infection

and

Persistent fracture nonunion.

Limitations of Follow-Up

Persistent tracer uptake may reflect:

Healing

rather than active disease.

Therefore, interval changes must be interpreted in the context of:

Symptoms

Other imaging

and the expected time course of:

Bone remodeling.

Key Principle

Skeletal scintigraphy is a highly sensitive nuclear medicine technique for detecting abnormalities of bone perfusion and osteoblastic activity, but it has:

Limited specificity.

Technetium-99m-labeled diphosphonates accumulate in areas of:

Active bone remodeling, allowing detection of abnormalities such as:

Stress fractures, osteomyelitis, occult fractures, tumors, metastases, nonunion, and prosthesis-related bone changes.

Three-phase scintigraphy adds assessment of:

Blood flow, blood-pool activity, and delayed bone uptake, making it particularly useful for:

Infection and stress-related injuries.

Because many different disorders can produce similar uptake patterns, bone-scan findings should always be interpreted together with:

Clinical information, radiographs, and—when needed—MRI, CT, SPECT/CT, or complementary leukocyte-based nuclear imaging.