What does positron emission tomography (PET) provide that computed tomography (CT) cannot?

What PET Scans Provide That CT Cannot

PET scans provide metabolic and functional information about tissue activity, while CT scans only provide anatomical and structural information. 1

Core Functional Difference

PET imaging visualizes differences in glucose metabolism and cellular activity, allowing differentiation between benign and malignant tissue based on metabolic behavior rather than just size or appearance. 1 This is fundamentally different from CT, which relies solely on anatomical changes and morphology to identify disease. 1

Metabolic vs. Anatomical Information

• PET using FDG (fluorodeoxyglucose) detects increased glucose metabolism in neoplastic cells, which have much higher rates of glycolysis and glucose uptake compared to normal cells. 1
• CT provides excellent morphologic information but has significant limitations in differentiating between benign and malignant lesions in organs or lymph nodes based on appearance alone. 1
• PET complements anatomic imaging with "metabolic" information that reveals tissue function rather than just structure. 1

Clinical Advantages of PET Over CT Alone

Superior Diagnostic Accuracy

• PET is significantly more accurate than CT in distinguishing benign from malignant pulmonary nodules, with overall sensitivity of 96%, specificity of 79%, and accuracy of 91%. 1
• For indeterminate lung lesions as small as 1 cm, PET has proven significantly more accurate than CT in differentiating benign from malignant disease. 1

Lymph Node Staging

• PET detects lymph node metastases based on metabolic activity rather than size criteria, making it superior to CT which relies on size thresholds (typically >1 cm). 1
• PET provides significantly superior loco-regional lymph node staging compared to CT alone, with negative predictive value equal or superior to mediastinoscopy. 1

Detection of Distant Disease

• PET improves extrathoracic staging through detection of lesions missed at conventional imaging or characterization of lesions that remain equivocal on CT. 1
• Whole-body PET is the most sensitive technique for detecting extracranial metastatic disease, unexpected additional primary malignancies, and recurrence. 1

Specific Clinical Scenarios Where PET Excels

Tumor vs. Inflammation Differentiation

• PET can distinguish metabolically active tumor from peritumoral atelectasis or inflammatory changes, which may appear identical on CT. 1
• This is particularly important when CT shows extensive contact between tumor and mediastinum with loss of fat planes—findings that could represent either tumor invasion or inflammatory changes. 1

Early Treatment Response Assessment

• PET can assess changes in metabolic activity very early in the course of therapy, allowing treatment adjustment based on tumor chemosensitivity and radiosensitivity before anatomic changes are visible on CT. 1
• There is correlation between FDG uptake and prediction of tumor response and patient outcome early in therapy. 1

Important Limitations and Caveats

False Negatives on PET

• False-negative PET results occur in subcentimetric lesions because a critical mass of metabolically active malignant cells is required for detection. 1
• Bronchoalveolar cell carcinomas may exhibit little or no FDG uptake even when larger than 1 cm. 1
• Negative PET results do not preclude cancer diagnosis and further work-up is indicated. 2

False Positives on PET

• False-positive FDG uptake occurs in inflammatory conditions, granulomatous diseases, fungal infections, and benign tumors that have increased glucose metabolism. 1, 2
• Because specificity is only 79%, positive PET findings cannot replace biopsy to confirm cancer diagnosis. 1, 2
• In clinically suspicious cases with high FDG uptake, further investigation for infectious or granulomatous disorders is indicated before proceeding to resection. 1

Why Combined PET/CT Is Now Standard

Integrated PET/CT overcomes the limitations of both modalities by providing co-registered anatomic and metabolic information in a single examination. 1, 3

• PET/CT allows accurate localization of metabolic abnormalities to specific anatomic structures, which may be difficult or impossible with PET alone. 1, 4
• PET/CT is superior to PET alone, CT alone, and visual correlation of both techniques performed separately. 1
• The combination reduces scanning time and provides more rapid final diagnosis compared to separate studies. 1