What are the indications for PET (Positron Emission Tomography) CT with contrast in vasculitis?

Indications for PET-CT with Contrast in Vasculitis

Primary Diagnostic Indications

FDG-PET/CT is indicated as an alternative imaging modality for diagnosing large vessel vasculitis when ultrasound or MRI are unavailable or inconclusive, particularly for detecting extracranial arterial inflammation and aortitis. 1

Giant Cell Arteritis (GCA)

• FDG-PET/CT may be used for detecting mural inflammation or luminal changes in extracranial arteries to support the diagnosis of large vessel GCA, especially when patients present with predominantly systemic symptoms rather than cranial symptoms 1

• PET is NOT recommended for assessment of cranial arteries in GCA due to lack of evidence, radiation exposure, and high resource utilization 1

• FDG-PET is particularly valuable in patients with unspecific symptoms to detect alternative causes of illness beyond vasculitis 1

• PET/CT enables simultaneous visualization of vessel wall inflammation and luminal changes across the entire aorta and its major branches in a single examination 2, 3

Takayasu Arteritis (TAK)

• FDG-PET may be used as an alternative imaging modality when MRI (the preferred first-line test) is unavailable or contraindicated 1

• PET/CT provides comprehensive assessment of the thoracic aorta and major branches, which are commonly affected in TAK and difficult to assess with ultrasound 1

Disease Monitoring Indications

Suspected Disease Relapse

• FDG-PET may be used to assess vessel abnormalities in patients with suspected relapse, particularly when laboratory markers of inflammation (ESR, CRP) are unreliable or discordant with clinical presentation 1, 3

• PET/CT can demonstrate disease activity even when ESR and CRP are nearly normal, making it valuable when clinical suspicion remains high despite negative serologic markers 3

• Imaging is not routinely recommended for patients in clinical and biochemical remission 1

Long-term Structural Monitoring

• CT angiography (the anatomical component of PET/CT) may be used for long-term monitoring of structural damage, particularly to detect stenosis, occlusion, dilatation, and aneurysms at sites of preceding vascular inflammation 1, 2

• The frequency of screening should be determined based on individual risk factors for vascular complications 1

Technical Requirements and Timing

Patient Preparation

• Blood glucose levels should be <7 mmol/L (126 mg/dL) preferred, <10 mmol/L (180 mg/dL) acceptable 1

• Interval between FDG infusion and image acquisition should be at least 60 minutes, preferably 90 minutes 1

• Patient positioning should be supine with arms down 1

Imaging Protocol

• Hybrid PET with low-dose CT or CT angiography should be used 1, 4

• Body coverage should extend from top of head to at least midthigh, preferably below the knees 1

• FDG uptake scoring should use qualitative visual grading (0-3 scale), comparing uptake to liver background when unclear 1, 4

Critical Timing Considerations

• Imaging should ideally be performed before glucocorticoid therapy initiation, or within the first few days of treatment, as glucocorticoids rapidly reduce detectability of vascular inflammation 2

• Treatment should NOT be delayed for imaging due to risk of irreversible complications, particularly vision loss in GCA 2

Clinical Context and Interpretation

When PET/CT Adds Most Value

• Patients with suspected large vessel GCA presenting with constitutional symptoms (fever, weight loss, malaise) rather than typical cranial symptoms 1

• Patients with discordant clinical and laboratory findings, where clinical suspicion remains despite normal inflammatory markers 3

• Assessment of disease extent when multiple vascular territories may be involved, as PET/CT provides whole-body evaluation 5, 6

Important Limitations and Pitfalls

• PET/CT cannot reliably distinguish between active inflammation and vascular remodeling in treated patients, limiting its utility for treatment response assessment 2

• False-positive FDG uptake can occur with atherosclerosis, infection, or other inflammatory conditions, requiring correlation with clinical and laboratory findings 1, 7

• Radiation exposure is significant, making it less suitable for repeated monitoring compared to ultrasound or MRI 1

• Ultrasound has limited access to the thoracic aorta, which is where PET/CT provides particular advantage 1

Operator Requirements

• Imaging examination must be performed by a trained specialist using appropriate equipment and standardized operational procedures 1, 2

• Interpretation requires expertise in recognizing patterns of vascular FDG uptake specific to vasculitis versus other conditions 4, 6