What Is Molecular Imaging?
Molecular imaging is a type of medical imaging that visualizes biological processes at the molecular and cellular levels inside the body, rather than just anatomy.
It helps physicians detect cancer early, understand how tumors behave, and assess how well treatments are working.
PET-CT: The Core of Molecular Imaging in Oncology
What is PET-CT?
PET-CT (Positron Emission Tomography–Computed Tomography) combines:
PET: Functional imaging that detects metabolic activity using radioactive tracers.
CT: Provides detailed anatomical structure of tissues.
Together, PET-CT gives both metabolic (function) and structural (location) information—making it a powerful tool in cancer care.
How PET-CT Works
Radiotracer injection
A small amount of radioactive tracer (commonly 18F-FDG) is injected into the patient’s vein.
FDG = fluorodeoxyglucose, a glucose analog.
Cancer cells consume more glucose → tracer accumulates there.
PET scan detects emitted positrons as the tracer decays, showing hot spots (areas of high metabolism).
CT scan maps exact locations of those hot spots.
Combined PET-CT images are fused by software → 3D visualization of tumor activity and anatomy.
Role of PET-CT in Cancer Management
A. Detection and Diagnosis
Detects tumors that may not be visible on CT/MRI due to their metabolic activity.
Differentiates between benign and malignant lesions based on tracer uptake.
B. Staging of Cancer
Accurately identifies primary tumor size, lymph node involvement, and distant metastases.
Commonly used in lung, lymphoma, colorectal, breast, and head-neck cancers.
C. Treatment Planning
Helps radiation oncologists delineate target volumes more precisely.
Guides biopsy to the most metabolically active tumor regions.
D. Response to Therapy
Evaluates early response to chemotherapy, radiotherapy, or immunotherapy.
Metabolic response often appears before structural changes, enabling early therapy adjustments.
E. Recurrence and Surveillance
Differentiates between post-treatment scar tissue and active disease.
Detects recurrence earlier than conventional imaging.
Quantitative Parameters in PET-CT
SUV (Standardized Uptake Value): Measures tracer uptake intensity.
Higher SUV = higher metabolic activity → potential malignancy.
SUVmax, SUVmean, and Total Lesion Glycolysis (TLG) are used for prognosis and treatment monitoring.
Clinical Applications by Cancer Type
Cancer Type | Clinical Use of PET-CT
Lung Cancer | Diagnosis, staging, detecting recurrence
Lymphoma | Gold standard for staging and therapy response (Deauville score)
Breast Cancer | Detects distant metastasis, recurrence
Colorectal Cancer | Detects recurrence, metastases (esp. liver/lung)
Prostate Cancer | PSMA PET-CT for detection of recurrence or metastasis
Head & Neck Cancers | Differentiates tumor from post-radiation changes
Brain Tumors | 11C-Methionine PET or FET-PET for tumor grading and recurrence
