What is the preferred imaging modality between MRI (Magnetic Resonance Imaging) and PET CT (Positron Emission Tomography Computed Tomography) for diagnosing large vessel vasculitis?

MRI versus PET-CT in Large Vessel Vasculitis

The preferred imaging modality depends on the specific type of large vessel vasculitis and clinical context: for Takayasu arteritis (TAK), MRI should be the first-line imaging test, while for giant cell arteritis (GCA), ultrasound is preferred first-line, with FDG-PET and MRI serving as alternative modalities for extracranial vessel assessment. 1

For Takayasu Arteritis (TAK)

MRI is the recommended first imaging test for suspected TAK to investigate mural inflammation and luminal changes, assuming high expertise and prompt availability. 1 This recommendation is based on:

• Absence of radiation exposure, which is critical given the young age of most TAK patients who require serial imaging over time 1
• Comprehensive vessel assessment including simultaneous evaluation of multiple vessels and the aorta 1
• High diagnostic accuracy with one study showing 100% sensitivity and specificity using conventional angiography as reference 1

FDG-PET, CT, or ultrasound may serve as alternative imaging modalities when MRI is unavailable or contraindicated. 1 However, ultrasound has significant limitations for assessing the thoracic aorta, which is frequently affected in TAK. 1

For Giant Cell Arteritis (GCA)

Cranial Vessel Assessment

Ultrasound of temporal and axillary arteries is the first-line imaging modality for investigating mural inflammatory changes in suspected GCA. 1

High-resolution MRI or FDG-PET can be used as alternatives for cranial artery assessment when ultrasound is unavailable or inconclusive. 1 The diagnostic performance is comparable:

• MRI pooled sensitivity: 73%, specificity: 88% 1
• Direct comparison showed MRI and ultrasound with similar sensitivity (69% vs 67%) and identical specificity (91%) 1

Extracranial Vessel Assessment (Large Vessel GCA)

FDG-PET is the preferred modality for detecting extracranial large vessel involvement, with MRI or CT as alternatives. 1 This is particularly important for:

• Patients with predominantly systemic symptoms 1
• Detection of aortitis and involvement of major branch vessels 1
• Situations where ultrasound has limited access (thoracic aorta) 1

Critical Timing Considerations

Imaging must be performed before or within 72 hours of initiating glucocorticoid therapy to avoid false-negative results. 1 Evidence shows:

• FDG-PET sensitivity remains equal to pretreatment scans at 72 hours but drops to 36% after 10 days of glucocorticoids 1
• Ultrasound shows significantly higher intima-media thickness when performed within 1 day versus after 1 week of treatment 1

However, imaging should never delay treatment initiation in patients with high clinical suspicion, particularly those with ischemic manifestations like transient visual loss or jaw claudication. 1

Comparative Advantages

MRI Advantages:

• No radiation exposure (critical for young TAK patients and serial monitoring) 1
• Simultaneous multi-vessel assessment including aorta and branch vessels 1
• Standardized data acquisition with less operator dependency 1
• Assessment of both vessel wall and lumen 1

FDG-PET Advantages:

• Comprehensive whole-body assessment of vascular inflammation 2, 3
• Detection of alternative diagnoses in patients with nonspecific symptoms 1
• Quantifiable disease activity using standardized uptake values 4
• Usually combined with CT/CTA allowing simultaneous luminal evaluation 1

MRI Limitations:

• Restricted availability and longer acquisition times 1
• Higher costs compared to ultrasound 1
• Potential adverse effects from contrast agents 1
• May not be feasible for emergency referrals in all settings 1

FDG-PET Limitations:

• Radiation exposure (less suitable for young patients requiring serial imaging) 1
• High resource utilization and cost 1
• Cannot distinguish between active inflammation and vascular remodeling in treated patients 5

Disease Monitoring

For suspected relapse, particularly when laboratory markers are unreliable (e.g., patients on IL-6 inhibitors), ultrasound or FDG-PET are preferred, with MRI as an alternative. 1

For long-term structural damage monitoring, MRA, CTA, or ultrasound may be used to detect stenosis, occlusion, dilatation, or aneurysms at sites of preceding inflammation. 1 The frequency and modality should be individualized based on disease severity and prior vascular involvement. 1

Key Clinical Pitfalls

• Conventional angiography is not recommended for diagnosis as it has been superseded by cross-sectional imaging 1
• Imaging should not be routinely performed in patients in clinical and biochemical remission 1
• All imaging must be performed by trained specialists using appropriate equipment and standardized protocols 1
• False-positive findings can occur with FDG-PET in atherosclerosis, infection, or other vasculitides 1