CT Angiography of the Chest with Dynamic Imaging
For a patient with reversed flow in the left vertebral artery suggestive of subclavian steal syndrome, CT angiography (CTA) of the chest performed in both neutral and elevated arm positions is the best CT imaging modality to evaluate the subclavian artery stenosis or occlusion causing this phenomenon. 1
Optimal CT Protocol
CTA chest should be performed with the following key technical considerations:
- Imaging in multiple arm positions (neutral and abducted/elevated) to assess dynamic arterial compression and stenosis severity 1
- Sagittal reformations are essential because axial slices alone underestimate stenosis in 43% of cases, while sagittal reformations reduce this to only 10% 1
- Volume-rendered images should be used rather than 3D surface displays, as surface displays overestimate stenosis in 16% versus only 7% with volume rendering 1
- Evaluation on vascular workstations is strongly recommended to properly assess cross-sectional area reduction and diameter measurements 1
Why CTA is the Best CT Option
CTA provides excellent anatomic visualization of the subclavian artery stenosis or occlusion that causes the vertebral artery flow reversal you've already detected. 1 The European Society of Cardiology guidelines specifically state that CTA is an excellent imaging tool for supra-aortic lesions and can provide extravascular information about potential causative structures. 1
CTA demonstrates good correlation with operative findings and surgical decompression results, making it valuable for treatment planning. 1
Critical Diagnostic Pitfall to Avoid
Be aware of the "shallow vertebral artery" sign on CTA - in 73.7% of subclavian steal cases, the ipsilateral vertebral artery appears shallow or poorly opacified on CTA, which can be misdiagnosed as vertebral arteriopathy rather than recognized as reversed flow. 2 This occurs because contrast is flowing retrograde (away from the brain) in the affected vertebral artery, resulting in poor opacification during the arterial phase of imaging.
Alternative and Complementary Imaging
While you asked specifically about CT, it's important to note:
- MRA with contrast provides both anatomic and functional information and can distinguish anterograde from retrograde perfusion, making it an excellent alternative to CTA 1
- Duplex ultrasound has already confirmed your flow reversal and remains the primary screening tool, showing monophasic post-stenotic flow patterns with >70% proximal subclavian stenosis 1, 3
- Catheter angiography remains the gold standard but is typically reserved for cases where endovascular intervention is planned 1
What the CTA Should Demonstrate
The study should identify:
- Location and severity of subclavian artery stenosis/occlusion (typically proximal, before vertebral artery origin) 1
- Percentage of cross-sectional area or diameter reduction 1
- Causative lesions such as atherosclerotic plaque, cervical ribs, or other anatomic abnormalities 1
- Collateral vessel formation 1
The combination of your documented vertebral artery flow reversal on duplex ultrasound plus CTA demonstration of proximal subclavian stenosis/occlusion confirms the diagnosis of subclavian steal syndrome. 1, 3