How can an old infarct be differentiated from the core of a new infarct on CT perfusion imaging?

Differentiating Old Infarct from Core of New Infarct on CT Perfusion

On CT perfusion, an old infarct shows matched reduction in both cerebral blood volume (CBV) and cerebral blood flow (CBF), while the core of a new infarct demonstrates severely reduced CBF (<30% of contralateral) with variable CBV depending on collateral flow and timing of imaging.

Key Distinguishing Features

Old Infarct Characteristics

• Matched perfusion defect: Both CBV and CBF are proportionally reduced in the infarcted territory 1
• No active hypoperfusion: Time parameters (MTT, TTP, Tmax) are typically normal or only mildly elevated
• Encephalomalacia: Visible on non-contrast CT as established hypodensity with volume loss
• No penumbra: Absence of surrounding tissue with perfusion-diffusion mismatch

New Infarct Core Characteristics

• Severely reduced CBF: Absolute CBF <30% of contralateral hemisphere is the most accurate predictor of irreversible injury 2, 3
• Variable CBV pattern:
  • Decreased CBV indicates absent collateral flow and poor prognosis 1
  • Normal or elevated CBV suggests good collateral flow 1
• Elevated time parameters: Delayed TTP, increased MTT, and prolonged Tmax surrounding the core 1
• Surrounding penumbra: Presence of perfusion-diffusion mismatch indicating salvageable tissue 4

Critical Timing Considerations

The reliability of CTP core estimates is time-dependent and can significantly overestimate true infarct core in early presenters (<3 hours). This "ghost infarct core" phenomenon occurs when:

• Imaging is performed <170-185 minutes from symptom onset 5, 6
• Severely hypoperfused tissue appears as core but remains salvageable with successful reperfusion
• Up to 38% of patients may have >10 mL overestimation of core volume 5

For patients presenting <3 hours, interpret CTP core estimates cautiously and consider non-contrast CT ASPECTS score as the primary decision tool 5, 6.

Practical Imaging Algorithm

Step 1: Non-Contrast CT Assessment

• Identify established hypodensity with volume loss (old infarct)
• Assess for subtle early ischemic changes (new infarct)
• Calculate ASPECTS score for middle cerebral artery territory strokes 7

Step 2: CTP Parameter Analysis

Prioritize CBF over CBV for core determination 2, 3:

• Core threshold: rCBF <30-31% of contralateral hemisphere 2, 8
• Note: Thresholds vary by postprocessing software (RAPID vs. vendor-specific) 3, 9

Step 3: Collateral Flow Assessment

• Good collaterals: Lower peak, delayed TTP, increased MTT, decreased flow, but normal or elevated CBV 1
• Poor collaterals: Decreased CBV indicating high risk for infarction 1

Step 4: Temporal Context Integration

• <3 hours: CTP may overestimate core; rely more heavily on non-contrast CT and clinical assessment 5, 6, 8
• 3-24 hours: CTP core estimates more reliable for treatment decisions 7, 4

Common Pitfalls to Avoid

1. Recanalization artifact: Spontaneous recanalization before CTP acquisition can cause gross underestimation of actual infarct core, as perfusion appears normal despite established injury 10. Always correlate CTP with non-contrast CT findings.

2. White matter false positives: Low CBF/CBV in non-infarcted white matter, particularly with leukoaraiosis or reduced contrast bolus intensity, can mimic core 2. This relates to low signal-to-noise ratio in CTP maps.

3. Software variability: Optimal thresholds vary significantly between postprocessing platforms 3, 9. RAPID uses CBF <30% within Tmax >6 seconds, while conventional software uses CBV reduction within elevated MTT/TTP regions.

4. Early time window overestimation: In patients presenting <170 minutes, consider liberalizing rCBF thresholds (potentially up to rCBF <38%) to avoid excluding patients who may benefit from intervention 8.

Integration with MRI

When MRI is available, diffusion-weighted imaging (DWI) remains the gold standard for identifying acute infarct core within minutes of onset 4. DWI allows differentiation of acute from chronic stroke through temporal evolution of diffusion characteristics 1. The combination of DWI (core) with perfusion-weighted imaging (penumbra) provides superior tissue characterization compared to CT-based methods 4.

For old infarcts, FLAIR sequences show hyperintensity in chronic infarcts, while acute infarcts may show subtle FLAIR changes only after several hours 4.