What are alternative imaging modalities for diagnosing giant cell arteritis when ultrasound is not available?

Alternative Imaging Modalities for Giant Cell Arteritis When Ultrasound is Not Available

When ultrasound is unavailable for diagnosing giant cell arteritis (GCA), high-resolution MRI or FDG-PET should be used as the primary alternative imaging modalities, particularly for assessment of cranial arteries. 1

Recommended Imaging Hierarchy When Ultrasound is Unavailable

For Cranial GCA Assessment:

• High-resolution MRI is the preferred first alternative for evaluating cranial arteries when ultrasound is unavailable 1

  • Technical parameters: 1.5T (preferably 3.0T) MRI scanner with minimum 8-channel head-coil 1
  • T1-weighted spin echo, gadolinium contrast-enhanced, fat-suppressed, high-resolution imaging 1
  • Transversal slices angulated parallel to skull base 1

• FDG-PET can also be used as an alternative for assessment of cranial arteries 1, 2

  • Typically combined with low-dose CT or CTA 1
  • Can visualize superficial temporal, facial, maxillary, superficial occipital branches and vertebral arteries 1
  • Visual grading method with vascular uptake higher than liver uptake offers highest diagnostic accuracy (sensitivity 83%, specificity 91%) 3

For Extracranial/Large Vessel GCA Assessment:

• FDG-PET is the preferred alternative imaging modality for extracranial arteries 1

  • Should be performed within 72 hours of starting glucocorticoid therapy to maintain sensitivity 2
  • Sensitivity decreases significantly in patients on glucocorticoid treatment 3

• MRI can also be used for detection of mural inflammation or luminal changes 1

  • Body MRI technique: 1.5T (preferably 3.0T) scanner with appropriate coils 1
  • MR angiography from carotid bifurcation to iliac arteries in coronal acquisition 1
  • T1-weighted, fat-suppressed, contrast-enhanced, black blood imaging for assessment of mural inflammation 1

• CT/CTA is another alternative for extracranial arteries 1, 2

  • Enables visualization of vessel wall and luminal changes 1
  • Associated with significant radiation exposure (important consideration) 1
  • Particularly valuable in patients with unspecific symptoms to detect alternative causes of illness 1

Clinical Implementation Considerations

• Imaging should not delay initiation of treatment in suspected GCA due to risk of permanent visual loss 1
• Imaging examination must be performed by a trained specialist using appropriate equipment and standardized procedures 1
• In patients with high clinical suspicion and positive imaging, no additional tests are needed 1
• In patients with low clinical probability and negative imaging, GCA can be considered unlikely 1
• In all other situations (including inconclusive imaging), additional diagnostic efforts are necessary 1

Monitoring Considerations

• For suspected disease relapse, FDG-PET, MRI, or CT may be considered for assessment of vessel abnormalities 1
• For long-term monitoring of structural damage, MRA, CTA, or ultrasound of extracranial vessels may be used 1
• The frequency of screening and imaging method should be decided individually, particularly at sites of preceding vascular inflammation 1
• Conventional angiography is not recommended for GCA diagnosis as it has been superseded by other imaging modalities 1

Important Caveats

• Sensitivity of all imaging modalities is affected by glucocorticoid therapy - imaging should be performed within days of treatment initiation 4
• FDG-PET has limited ability to differentiate between active inflammation and vascular remodeling in treated GCA patients 2
• Diffuse (rather than focal) FDG uptake pattern is significantly more common in GCA patients without glucocorticoid use 3
• CRP levels may not correlate with positive or negative FDG-PET scans in GCA patients 3
• Aorta should be routinely assessed in all GCA patients at diagnosis and during follow-up 4