How is ischemic stroke volume measured on Computed Tomography (CT) or Magnetic Resonance Imaging (MRI) brain?

Measuring Ischemic Stroke Volume on CT or MRI

Ischemic stroke volume is measured using specialized software that performs region-of-interest analysis on contiguous brain slices, with the measurement technique varying by imaging modality: on CT, use 5mm contiguous non-helical slices for manual or semi-automated volumetric quantification; on MRI, diffusion-weighted imaging (DWI) provides the most accurate measurement of the ischemic core by identifying hyperintense regions that represent cytotoxic edema, which can be quantified using planimetric methods across all affected slices. 1, 2, 3

CT-Based Volume Measurement

Standard Non-Contrast CT (NECT)

• Obtain contiguous, discrete (non-helical) 5mm slice thickness images to enable accurate volumetric quantification of early infarct 1
• Manual planimetric measurement involves outlining the hypodense ischemic region on each slice and multiplying by slice thickness, though this has limited sensitivity in the hyperacute phase (detecting only 61-65% of strokes within 6 hours) 2
• The accuracy for detecting ischemic areas involving more than one-third of the middle cerebral artery territory is approximately 70-80% 1

CT Perfusion (CTP) for Core and Penumbra

• CTP provides quantitative maps of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT) that can be analyzed with region-of-interest measurements 1
• **The ischemic core is defined by regions with >75% reduction in CBF or CBV <2 mL/100g**, which has >95% positive predictive value for infarction despite recanalization 1
• Normalized (relative) CBF is the most robust parameter: >66% reduction in CBF indicates nonviable penumbra (95% specificity for infarction), while <50% reduction indicates benign oligemia (95% sensitivity for tissue survival) 1
• CTP has greater spatial resolution than MR perfusion and more readily lends itself to quantification 1

CTA Source Images (CTA-SI)

• CTA-SI provides qualitative cerebral blood volume maps that detect the infarct core and can be used for whole-brain assessment 1
• With early complete recanalization, CTA-SI lesion volume approximates final infarct size, whereas without recanalization there is significant lesion growth 1
• A CTA-SI lesion volume <100 mL predicts good/fair outcome, while >100 mL predicts poor outcome despite complete recanalization 1

MRI-Based Volume Measurement

Diffusion-Weighted Imaging (DWI) - Gold Standard for Core

• DWI is the most sensitive and specific technique for measuring acute infarct volume, with 88-100% sensitivity and 95-100% specificity 1, 2, 3
• Hyperintense regions on DWI represent cytotoxic edema in the ischemic core and can be measured using manual or semi-automated segmentation across all slices 1, 2
• Lesion volume on early DWI correlates strongly with final infarct volume (r=0.79, p=0.001) and clinical outcome 4, 5
• In first-ever stroke, DWI lesion volume ≤22 mL predicts good outcome with 75% sensitivity and 100% specificity 4

Apparent Diffusion Coefficient (ADC) Maps

• ADC maps must be used alongside DWI to eliminate T2 "shine-through" effect and increase specificity 3
• The mean ADC of ischemic lesions is typically 29% lower than normal brain tissue 4
• ADC values correlate with clinical outcome and help distinguish irreversible infarction from salvageable penumbra 3, 4

Perfusion-Weighted Imaging (PWI) for Total Ischemic Volume

• PWI demonstrates the total area of acute ischemia using dynamic susceptibility contrast-enhanced T2/T2-weighted imaging* 1, 2
• The same perfusion indices used for CT (CBF, CBV, MTT, time-to-peak) are calculated for MR perfusion 1
• The perfusion-diffusion mismatch (larger PWI defect surrounding smaller DWI lesion) identifies salvageable penumbra, with the mismatch volume correlating with infarct growth (r=0.699, p<0.001) 6
• The volume of hypoperfusion on initial PWI correlates strongly with final infarct size (r=0.827, p<0.001) 6

Practical Measurement Algorithm

For Acute Stroke (<6 Hours)

1. If using CT: Measure early ischemic changes on 5mm NECT slices, obtain CTP to quantify core (CBV <2 mL/100g or >75% CBF reduction) and penumbra (50-66% CBF reduction), and use CTA-SI for whole-brain assessment 1
2. If using MRI: Measure hyperintense lesion on DWI with ADC confirmation, obtain PWI to identify mismatch, and calculate volumes using planimetric methods 2, 3

For Extended Window (6-24 Hours)

• Multimodal imaging with perfusion assessment is essential 7
• Measure both core (DWI or CTP-CBV) and total hypoperfusion (PWI or CTP-MTT) to calculate salvageable tissue volume 1, 7

Critical Pitfalls to Avoid

• Never rely on visual estimation alone—use quantitative software for volumetric analysis, as visual assessment has only 70-80% accuracy 1
• Account for gray-white matter differences when using perfusion thresholds, as baseline CBF values differ significantly between tissue types 1
• Exclude large vascular structures from perfusion calculations, as vascular pixels contaminate mean CBF values 1
• Do not use standard T2-weighted or FLAIR sequences for acute volume measurement—they detect <50% of acute strokes and are relatively insensitive compared to DWI 1, 3
• CTP sensitivity is only 49.7% for all strokes but 92.2% for strokes >5cc, so small infarcts may be missed 8
• DWI may miss small brainstem or posterior fossa lesions if slice thickness is too large—use thin slices for these regions 1