What is the brush sign on brain MRI, what does it indicate, and how should it be evaluated and managed?

MRI Brush Sign

The brush sign on MRI refers to prominent hypointensity of deep medullary veins and subependymal veins visible on susceptibility-weighted imaging (SWI) or T2-weighted sequences, indicating severe tissue hypoperfusion and impaired cerebral hemodynamics* 1.

What the Brush Sign Represents

The brush sign is a radiological biomarker of severe oxygen metabolic impairment in brain tissue. When present, it indicates:

• Increased conspicuity of deep medullary veins (DMVs) appearing as prominent dark linear structures radiating from the ventricles toward the cortex, resembling bristles of a brush 1, 2
• Severe reduction in cerebral metabolic rate of oxygen (CMRO2) within both the ischemic core (-52.9% vs -44.1%) and critically, within the penumbra (-31.0% vs -13.8%) 3
• Physiological adaptation to cerebral ischemia representing increased deoxyhemoglobin in dilated venous structures attempting to maintain tissue oxygenation 4

Clinical Conditions Where Brush Sign Appears

The brush sign manifests across multiple cerebrovascular diseases:

Acute Ischemic Stroke

• Most commonly seen in middle cerebral artery territory strokes 1
• Present in approximately 34-38% of patients with large vessel occlusion 4, 5
• Associated with younger age, higher baseline NIHSS scores (16 vs 13), and larger ischemic cores 4, 5

Moyamoya Disease

• Predicts postoperative infarction risk after surgical revascularization 2
• Defined as >5 conspicuous DMVs on SWI 2
• Independent risk factor for postoperative complications (OR 13.669) 2

Other Conditions

• Cerebral venous thrombosis 1
• Sturge-Weber syndrome 1
• Single small subcortical infarctions 6

Prognostic Significance

In Acute Stroke Treated with Thrombectomy

The brush sign has contradictory prognostic implications depending on the clinical context:

• Positive outcome predictor: When present pre-treatment, independently associated with functional independence at 3 months (OR: 2.234) 4, suggesting it represents viable tissue with preserved autoregulatory capacity
• Negative outcome predictor: Independently predicts large infarct growth >11.6 mL after successful thrombectomy (OR: 5.2) 5 and higher risk of not achieving functional independence (OR: 1.8) 3

This apparent contradiction likely reflects timing and reperfusion success—the brush sign indicates tissue at the edge of viability that benefits from successful early reperfusion but deteriorates with delayed or incomplete revascularization.

In Moyamoya Disease

• Strong predictor of postoperative infarction (OR 13.669) 2
• Correlates with higher modified Rankin Scale scores at discharge 2
• Indicates need for focused perioperative management and surgical risk stratification 2

Evaluation Algorithm

When you identify a brush sign on MRI:

1. Confirm the finding on SWI or T2-weighted sequences*

   • Look for >5 prominent hypointense linear structures radiating from ventricles
   • Distinguish from normal venous anatomy (which should be less conspicuous)

2. Assess the underlying vascular pathology

   • Obtain MR venography (MRV) or CT venography (CTV) to evaluate for venous thrombosis 7
   • Evaluate arterial patency with MR angiography or CT angiography
   • Consider moyamoya disease in appropriate clinical context

3. Quantify tissue at risk

   • Measure ischemic core volume on diffusion-weighted imaging (DWI)
   • Assess penumbra using perfusion imaging (Tmax ≥6s minus DWI lesion)
   • Evaluate collateral status using hypoperfusion intensity ratio (<0.4 indicates good collaterals) 5

4. Determine clinical severity

   • Document baseline NIHSS score
   • Assess time from symptom onset
   • Identify any hemorrhagic transformation on gradient-echo sequences

Management Approach

For Acute Ischemic Stroke with Brush Sign

Proceed urgently with mechanical thrombectomy if indicated by standard criteria, as the brush sign suggests tissue that may still be salvageable but is at high risk for infarct growth 4, 5, 3.

• Do not delay reperfusion therapy based on brush sign presence
• Anticipate larger infarct growth even with successful reperfusion 5
• Monitor closely for hemorrhagic transformation post-procedure
• Optimize blood pressure management to maintain cerebral perfusion

For Cerebral Venous Thrombosis

When brush sign accompanies venous thrombosis findings (absence of flow void, altered signal intensity in dural sinuses) 7:

• Initiate therapeutic anticoagulation immediately with low molecular weight heparin or unfractionated heparin 7
• Hemorrhagic venous infarction is NOT a contraindication to anticoagulation 7
• Confirm diagnosis with contrast-enhanced MRI and MRV 7
• Consider endovascular therapy only if absolute contraindications to anticoagulation exist or therapeutic anticoagulation fails 7

For Moyamoya Disease

When brush sign is present preoperatively 2:

• Stratify as high-risk for postoperative infarction
• Implement intensive perioperative blood pressure management
• Maintain aggressive hydration protocols
• Consider staged rather than simultaneous bilateral revascularization
• Increase frequency of postoperative neurological monitoring

Critical Pitfalls to Avoid

1. Do not confuse brush sign with normal venous anatomy—the key is abnormal prominence and number (>5 vessels) of DMVs 2

2. Do not assume brush sign always indicates poor prognosis—in the context of timely reperfusion, it may identify patients who benefit most from intervention 4

3. Do not withhold anticoagulation in venous thrombosis even when hemorrhagic transformation is present, as this is not a contraindication 7

4. Do not rely on brush sign alone—always integrate with clinical presentation, timing, and other imaging findings including collateral status and perfusion parameters 5, 3

5. Recognize that acute thrombus may mimic normal flow void on standard sequences, making contrast-enhanced imaging or venography essential for definitive diagnosis of venous thrombosis 7