Nuclear medicine diagnostics of the bones and joints: spot static and whole body scintigraphy, dynamic scintigraphy. Application in metastatic disease and benign bone diseases. Application of SPECT-CT
Bone scintigraphy (bone scan) is a functional nuclear medicine imaging technique that evaluates:
- Bone metabolism
- Bone blood flow
- Osteoblastic activity
- Bone remodeling
Unlike X-ray, CT or MRI, which mainly evaluate bone anatomy, bone scintigraphy detects physiological changes in bone metabolism, often before structural changes become visible.
It is one of the most sensitive methods for detecting:
- Bone metastases
- Fractures
- Osteomyelitis
- Avascular necrosis
- Arthritis
- Bone tumors
Principle of Bone Scintigraphy
Bone is constantly undergoing:
- Bone formation
- Bone resorption
These processes together are called Bone remodeling.
Whenever osteoblastic activity increases,
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More radiopharmaceutical accumulates.
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The area appears as a Hot spot (hyperfixation).
Therefore, Bone scintigraphy mainly reflects Osteoblastic activity rather than bone density.
Bone Physiology (Important)
Bone contains two major cell types involved in remodeling:
Osteoblasts
Function:
- Bone formation.
- Mineralization. Increase tracer uptake.
Osteoclasts
Function:
- Bone resorption.
They do not directly increase tracer uptake.
The scan mainly reflects osteoblastic response.
Mechanism of Tracer Uptake
After IV injection, the radiopharmaceutical circulates through the bloodstream.
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It reaches bone.
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It binds to Hydroxyapatite crystals at sites of active bone formation.
Therefore:
Greater osteoblastic activity
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Greater tracer uptake
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Hot lesion.
Radiopharmaceuticals
Most commonly used: Tc-99m MDP (Methylene Diphosphonate)
This is the standard tracer for bone scintigraphy.
Other tracers include:
- Tc-99m HMDP (Hydroxymethylene diphosphonate)
- Tc-99m HDP (Hydroxydiphosphonate)
Depending on local protocols.
Characteristics of Tc-99m MDP
- Gamma emitter.
- Half-life = 6 hours.
- Photon energy = 140 keV.
- Rapid blood clearance.
- High affinity for bone.
- Excellent image quality.
- Low radiation dose.
Normal Distribution of Tracer
Normal uptake is seen throughout the skeleton.
Higher physiological uptake occurs in:
- Vertebral column
- Pelvis
- Sternum
- Ribs
- Skull
- Shoulder joints
- Hip joints
- Knee joints
Children normally show increased uptake at: Growth plates.
Physiological Excretion
Tc-99m MDP is excreted by the kidneys.
Therefore normal uptake is also seen in:
- Kidneys
- Ureters (occasionally)
- Urinary bladder
These are physiological findings, not disease.