What are the typical findings of vasculitis on a Positron Emission Tomography (PET) scan?

Typical Findings of Vasculitis on PET Scan

The hallmark finding of vasculitis on PET scan is circumferential, segmental intense FDG uptake along large vessels, which is typically graded in relation to liver uptake, with grade 3 uptake (vascular uptake > liver uptake) being highly specific for active vasculitis. 1

Key Diagnostic Features on PET/CT

Visual Assessment Patterns

• Intensity grading system:

  • Grade 0: No vascular uptake
  • Grade 1: Vascular uptake < liver uptake
  • Grade 2: Vascular uptake equal to liver uptake (possible vasculitis)
  • Grade 3: Vascular uptake > liver uptake (consistent with vasculitis) 1, 2

• Distribution patterns:

  • Segmental involvement in Takayasu arteritis
  • Diffuse vascular involvement in Giant Cell Arteritis (GCA) 3
  • Multiple vascular segments with diffuse (rather than focal) FDG uptake pattern is highly suggestive of vasculitis 2

Quantitative Measurements

• SUV-based metrics:
  • SUVmax of affected vessels
  • Target-to-background ratios:
    • Aorta-to-liver ratio >1.03 has highest diagnostic accuracy (sensitivity 69%, specificity 92%) 2
    • Aorta-to-blood pool ratio also used 1

Vessel Involvement Patterns by Vasculitis Type

• Giant Cell Arteritis:

  • Temporal arteries, vertebral arteries, and abdominal aorta 1, 3
  • Higher SUVmax values in aortic segments compared to Takayasu arteritis 1
  • Sensitivity 90-92%, specificity 98% for diagnosis 1

• Takayasu Arteritis:

  • Aorta and its branches, particularly carotid arteries 1, 3
  • Segmental involvement of vessel walls 3
  • Sensitivity 87%, specificity 73% for diagnosis 1

• Polyarteritis Nodosa:

  • Diffuse vascular involvement of both lower extremities 3

• ANCA-Associated Vasculitis:

  • Primarily extravascular granulomatous inflammation 3

Clinical Correlation and Diagnostic Accuracy

• FDG uptake intensity correlates strongly with inflammatory markers:

  • Grade I: CRP ~4 mg/L, ESR ~6 mm/h
  • Grade II: CRP ~37 mg/L, ESR ~46 mm/h
  • Grade III: CRP ~172 mg/L, ESR ~90 mm/h 4

• Overall diagnostic performance:

  • Sensitivity: 60-95% (higher in active inflammation)
  • Specificity: 91-99.8%
  • Accuracy: 78.6% 2, 4
  • Sensitivity drops significantly in patients with low inflammatory markers (CRP <12 mg/L or ESR <12 mm/h) 4

Extravascular Findings

• Polymyalgia Rheumatica (PMR):

  • FDG uptake in periarticular and extra-articular structures 1
  • Many patients with isolated PMR may show large vessel vasculitis on PET/CT 1

• Other extravascular findings:

  • PET/CT can detect alternative diagnoses (malignancy, infection) in up to 54% of patients when vasculitis is ruled out 1

Important Pitfalls and Limitations

1. Glucocorticoid treatment effects:

   • Significantly reduces sensitivity (from 90-92% to 69-83%) 2
   • Diagnostic window: First 3 days after starting glucocorticoids 1
   • Glucocorticoids increase liver FDG uptake, affecting vessel-to-liver ratios 1

2. Technical considerations:

   • Spill from physiologic cerebral FDG uptake can affect head and neck vessel assessment 1
   • Arterial grafts may show FDG uptake not related to disease activity 1

3. Timing considerations:

   • Optimal scan time is 2.5 hours after FDG injection 1
   • Scanning during "pulseless phase" of Takayasu arteritis may miss inflammatory changes 1

Monitoring Disease Activity

• PET/CT can predict treatment response and risk of relapse:

  • Increased FDG intensity and extensive vessel involvement predict less favorable treatment response 1
  • Aortic inflammation on PET/CT is a risk factor for complications like aneurysm or dissection 1
  • SUVmax values decrease significantly during follow-up in patients responding to treatment 5

• Low concordance between PET findings and acute phase reactants during follow-up highlights PET's value in detecting subclinical disease activity 5

PET/CT provides crucial information about disease activity independent of systemic inflammation markers, making it particularly valuable for detecting clinically silent disease and guiding treatment decisions in vasculitis patients.