What is the purpose of Positron Emission Tomography (PET) Computed Tomography (CT)?

Purpose of PET-CT

PET-CT is a hybrid imaging modality that combines metabolic information from positron emission tomography with anatomical detail from computed tomography, primarily used for cancer diagnosis, staging, treatment response assessment, and detection of recurrence, while also having emerging roles in cardiovascular disease, neurological disorders, and infection/inflammation. 1

Core Functional Principles

PET-CT merges two complementary imaging technologies into a single examination 2, 3:

• PET component: Uses radioactive glucose analogue (18F-FDG) that accumulates in areas of high glucose metabolism, including tumors, infection, and inflammation 1
• CT component: Provides cross-sectional anatomical imaging using x-rays to create three-dimensional images reflecting tissue density variations 1
• Integration advantage: Allows accurate localization of metabolic activity to specific anatomic structures, which is difficult or impossible with PET alone 2, 3

The key distinction from conventional imaging is that PET-CT displays and quantifies molecular processes throughout the entire body in a single noninvasive examination with molecular sensitivity far exceeding conventional modalities 1.

Primary Clinical Applications in Oncology

Cancer Staging and Initial Diagnosis

PET-CT serves multiple critical roles across cancer types 1:

• Lung cancer: Superior for T3/T4 staging, delineating tumors with atelectasis, and detecting mediastinal nodal disease 1
• Lymphoma: Recommended for staging and response evaluation in Hodgkin's and diffuse large B-cell lymphoma, with ability to change stage and therapy decisions 1
• Melanoma: Most sensitive method for diagnosing distant metastases in stage 3-4 disease, detecting unrecognized metastases in 10-19% of patients 1
• Head and neck cancer: Identifies at least 30% of primary tumors not detected by conventional means in unknown primary cases 1
• Colorectal cancer: Benefits surgical decision-making for liver metastases when CT/MRI is equivocal 1

Treatment Response and Recurrence Detection

PET-CT is the first-choice modality for suspected cancer recurrence when conventional imaging is negative or equivocal 1:

• Colorectal cancer: Recommended as first choice when suspicion of recurrence exists 1
• Cervical cancer: Superior for treatment efficacy evaluation and predicts event-free and overall survival 1
• Ovarian cancer: Recommended when CA-125 is elevated but CT/MRI are negative 1
• Whole-body capability: Most sensitive technique for detecting extracranial metastatic disease, unexpected additional primary malignancies, and recurrence 1

Specific Advantages Over Conventional Imaging

PET-CT demonstrates superiority to PET alone, CT alone, and visual correlation of both techniques separately 1:

• Eliminates need for biopsy: Negative PET-CT in suspected adrenal metastases may obviate biopsy 1
• Differentiates malignant from benign: Achieves sensitivity of 88.2% and specificity of 92.9% for malignant pleural disease 1
• Guides biopsy targets: Identifies areas of high metabolic activity representing most diagnostic biopsy sites 1

Emerging and Expanding Applications

Beyond Oncology

PET-CT will play increasing roles in non-cancer conditions 1:

• Infections of unknown origin 1
• Inflammatory conditions 1
• Neurological disorders including dementias, epilepsy, and movement disorders 1, 4
• Cardiovascular disease: Likely to replace myocardial scintigraphy 1

Technical Innovations

Future developments include 1:

• New tracers: Beyond FDG for specialized functions and molecular imaging development 1
• Solid-state detectors: Improved sensitivity for detecting smaller lesions and higher patient throughput 1
• Radiotherapy planning: Integration into treatment planning protocols 1

Important Clinical Caveats

Limitations to Recognize

• False positives: Sufficiently frequent in mediastinal nodal disease to require tissue sampling in some positive cases 1
• FDG non-specificity: Accumulates in benign processes including infection and inflammation, not exclusively malignant tissue 1, 2
• Metallic artifacts: Metallic objects and contrast agents can alter quantitative measurements and create PET image artifacts 3

Optimal Utilization Strategy

The vast majority of PET-CT scans should be performed on indications with good evidence support 1. The actual clinical use data shows high agreement with evidence-based recommendations, with most scans performed for staging (18-62% depending on cancer type), suspected recurrence (15-45%), and response evaluation (3-36%) 1.