Why is CT venography (CTV) preferred over CT angiography (CTA) for a young female with anorexia nervosa, presenting with headache and left lower extremity weakness, suspected of cerebral venous thrombosis (CVT)?

Why CTV Over CTA for Suspected Cerebral Venous Thrombosis

CT venography (CTV) is the correct imaging modality for suspected cerebral venous thrombosis because it visualizes the venous system, whereas CT angiography (CTA) visualizes only the arterial system—you cannot diagnose venous thrombosis by looking at arteries. 1

Fundamental Distinction Between Modalities

• CTV images the venous circulation (dural sinuses, cortical veins, deep cerebral veins) by timing contrast injection to capture venous phase filling, which is essential for detecting thrombosis in these structures 2, 1
• CTA images the arterial circulation (carotid arteries, circle of Willis, cerebral arteries) and would completely miss venous pathology 1
• This is analogous to asking why you need a venogram for DVT rather than an arteriogram—the pathology is in a completely different vascular system 3

Optimal Imaging Strategy for CVT

The preferred first-line imaging is MRI with MR venography (MRV), but CTV is an excellent alternative when MRI is unavailable, contraindicated, or in emergency settings. 1

When to Use CTV:

• Emergency department setting where rapid diagnosis is needed and MRI access is limited 1, 4
• MRI contraindications (pacemakers, severe claustrophobia, metallic implants) 1
• Unstable patients who cannot tolerate prolonged MRI scanning 2
• High clinical suspicion requiring immediate imaging—CTV can be performed rapidly with high diagnostic accuracy 5, 6

Diagnostic Performance:

• CTV demonstrates 95% sensitivity and 91% specificity compared to digital subtraction angiography 1
• CTV is as accurate as MRV for diagnosing cerebral venous thrombosis 1
• CTV shows excellent interobserver agreement (kappa=0.83 per patient, 0.76 per vessel) with 94% full agreement between radiologists 5
• CTV is superior to MRV in visualizing sinuses with slow flow and smaller cerebral veins 6

Clinical Context for This Patient

In a young female with anorexia nervosa presenting with headache and focal neurologic deficit (left lower extremity weakness), CVT is a critical consideration because:

• Anorexia nervosa is a hypercoagulable state due to dehydration, malnutrition, and hormonal abnormalities 4
• Young women are the highest-risk population for CVT, particularly under age 50 4
• Headache with focal neurologic deficit is the classic presentation pattern (headache in 60%, focal deficits as second most common symptom) 2, 7, 4

Why Not Non-Contrast CT Alone:

• Non-contrast CT is normal or shows only non-specific findings in 70% of CVT cases 8, 7
• The "hyperdense sinus sign" is present in only 30-33% of cases 1, 9
• A negative plain CT does not rule out CVT—venographic imaging (CTV or MRV) must follow 8

Critical Pitfall to Avoid

Never order CTA when you suspect CVT—this is a fundamental error that will miss the diagnosis entirely because arterial imaging cannot visualize venous pathology. 1 The American College of Radiology explicitly recommends venographic imaging (CTV or MRV) following plain CT when lateral sinus thrombosis is suspected, even if the plain CT is negative. 8

Imaging Algorithm for Suspected CVT:

1. If MRI/MRV is immediately available: Use MRI with both non-contrast time-of-flight MRV and contrast-enhanced MRV (optimal combination) 1
2. If MRI unavailable or contraindicated: Use CTV as equally accurate alternative 1
3. Never use CTA: This images arteries, not veins 1
4. Reserve catheter angiography (DSA): Only for endovascular treatment planning, not routine diagnosis 1, 7

Treatment Implications

Once CVT is diagnosed on CTV, immediate therapeutic anticoagulation with low molecular weight heparin or unfractionated heparin is indicated, even in the presence of intracranial hemorrhage, as benefits of preventing thrombus progression outweigh bleeding risks. 8 Early diagnosis via appropriate venographic imaging (CTV or MRV) enables timely anticoagulation, which improves outcomes in this potentially fatal condition. 7, 4