In an FDG PET-CT study, what does it indicate when a focus of increased FDG uptake is seen on both the non‑attenuation‑corrected (NAC) and attenuation‑corrected (AC) images?

Interpretation of Increased FDG Uptake on Both NAC and AC Images

When a focus of increased FDG uptake appears on both non-attenuation-corrected (NAC) and attenuation-corrected (AC) images, this indicates true pathologic uptake rather than an artifact, representing genuine metabolic activity in that location.

Why This Matters: Understanding the Distinction

The presence of uptake on both NAC and AC images is the key to distinguishing real pathology from technical artifacts:

True Pathologic Uptake (Present on Both NAC and AC)

• Represents actual metabolic activity from:
  • Malignancy
  • Infection/inflammation
  • Physiologic activity
• Requires clinical correlation with CT findings and patient history 1

Artifacts (Present Only on AC Images)

The most recent EANM/SNMMI guidelines (2025) specifically address this critical distinction: "In the presence of metallic hardware, when older PET/CT devices are used and in case of doubt, both non-attenuated (NAC) and attenuation corrected (AC) images should be reviewed to identify metal induced artifacts on AC images" 1.

Metal-induced artifacts appear as:

• False-positive "hot spots" on AC images only
• Absent or minimal activity on NAC images
• Located near metallic implants (pacemakers, orthopedic hardware, dental work)
• Result from overcorrection of dense objects during CT-based attenuation correction 2

Clinical Algorithm for Interpretation

When you identify increased FDG uptake:

1. First: Check both NAC and AC images

   • If present on both → Proceed to step 2
   • If present only on AC → Suspect artifact, especially near metal

2. Correlate with CT component

   • Assess for anatomic substrate (mass, inflammation, bone changes)
   • Look for metallic hardware that could cause artifacts
   • Evaluate for fractures, arthropathy, degenerative changes 1

3. Assess uptake characteristics

   • Pattern: focal, linear, diffuse, heterogeneous
   • Intensity: compare to background and mediastinal blood pool
   • Location: proximity to physiologic uptake sites 1

4. Consider clinical context

   • Patient symptoms
   • Known malignancy or infection risk
   • Recent procedures or interventions

Important Caveats

When NAC Images Are Superior

Research demonstrates that NAC images can actually provide higher contrast for lesion detection in certain scenarios:

• Small pulmonary nodules (<1 cm) may be better visualized on NAC images 3, 4, 5
• NAC images showed 100% sensitivity for solitary pulmonary nodules versus 91% for AC images 5
• Particularly valuable for lesions in the chest where attenuation correction can reduce contrast 6

Modern PET/CT Systems

The 2025 EANM guidelines note that metal artifacts are "infrequent with more recent PET/CT devices" 1. However, reviewing both image sets remains best practice when:

• Metallic hardware is present
• Older PET/CT equipment is used
• Any doubt exists about uptake authenticity

Quantification Considerations

According to the most recent guidelines 7:

• Metal implants prevent proper quantification at and near the implant site
• SUV measurements should be confirmed by inspecting NAC images
• Even with metal artifact reduction techniques, quantification remains unreliable near hardware

Bottom Line for Clinical Practice

Uptake visible on both NAC and AC images represents real metabolic activity requiring clinical action. The next steps are:

• Detailed CT review for anatomic correlation
• Integration with clinical presentation
• Consideration of biopsy or further imaging if malignancy is suspected
• Assessment for infection if clinical context suggests inflammatory process

The dual presence on both image sets effectively rules out the most common technical artifact (metal-induced overcorrection), allowing you to proceed with confidence that the finding represents true pathology 1, 2.