Whole-Body PET-CT: Pathophysiologic Basis and Non-Oncologic Role in Orthopaedics
DNB Orthopaedics • October 2024 • Paper IV • 10 Marks
Introduction
PET-CT combines Positron Emission Tomography (PET), which provides functional/metabolic information, with Computed Tomography (CT), which provides precise anatomical localization.
The most commonly used tracer is 18F-fluorodeoxyglucose (18F-FDG), a glucose analogue. Increased FDG uptake occurs not only in malignant tissue but also in infection, inflammation and metabolically active reparative processes.
Key concept
PET tells us “where metabolism is increased”, while CT tells us “exactly where the abnormality is anatomically located.”
A. Pathophysiologic Basis of Whole-Body PET-CT
1. Radiotracer Administration
A positron-emitting radionuclide is administered intravenously. The most commonly used tracer is:
18F-FDG = 18F-fluorodeoxyglucose
It behaves as a glucose analogue and accumulates preferentially in cells with increased glucose metabolism.
2. Cellular Uptake of FDG
FDG is transported into cells through glucose transporters (GLUT).
18F-FDG in blood
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Transport into cell via GLUT transporters
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Phosphorylation by hexokinase
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18F-FDG-6-phosphate
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Cannot proceed normally through glycolysis
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Intracellular metabolic trapping
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Accumulation in metabolically active tissue
Unlike glucose-6-phosphate, FDG-6-phosphate is not efficiently metabolized further through the glycolytic pathway and therefore becomes relatively trapped intracellularly.