On CT perfusion (CTP) imaging in acute ischemic stroke, which parameter best identifies the ischemic core (irreversibly infarcted tissue)?

Explanation

## CT Perfusion Parameters in Acute Stroke **Key Point:** The ischemic core is identified by **reduced cerebral blood volume (CBV) with prolonged mean transit time (MTT)**. This combination indicates tissue with severely compromised perfusion that is likely irreversibly infarcted. ### CTP Parameter Definitions | Parameter | Definition | Acute Stroke Significance | |-----------|-----------|---------------------------| | **CBF** | Cerebral blood flow (mL/100g/min) | Reduced in both core and penumbra | | **CBV** | Cerebral blood volume (mL/100g) | Reduced in core; may be preserved in penumbra | | **MTT** | Mean transit time (seconds) | Prolonged in both core and penumbra | | **TTP** | Time to peak | Delayed in ischemic tissue | ### Tissue Viability Classification **Ischemic Core (Irreversible Infarction):** - Markedly reduced CBV (typically <2 mL/100g) - Prolonged MTT - Reduced CBF - Represents tissue beyond salvage window **Ischemic Penumbra (Potentially Salvageable):** - Reduced CBF and prolonged MTT - **Relatively preserved or only mildly reduced CBV** - Tissue at risk if reperfusion not achieved **Normal Brain:** - Normal CBF, CBV, MTT values ### Pathophysiology: Why CBV Reduction Indicates Core 1. **Autoregulation failure** → cerebral vessels cannot maintain perfusion 2. **Capillary bed collapse** → reduced blood volume in tissue 3. **Irreversible ischemia** → neuronal death begins 4. **CBV reduction is the marker of no-reflow** — tissue that cannot be salvaged **High-Yield:** The **CBV threshold** for ischemic core is typically <2 mL/100g. Tissue with CBV >2 mL/100g but reduced CBF/prolonged MTT represents the penumbra and is a target for thrombolysis or thrombectomy. **Clinical Pearl:** CTP mismatch (perfusion deficit larger than CBV abnormality) identifies patients with salvageable penumbra — these patients benefit from reperfusion therapy even beyond the standard 4.5-hour thrombolysis window. **Mnemonic:** **"CBV = Core, CBF = Penumbra"** — When CBV is reduced, tissue is core (dead). When only CBF is reduced (CBV preserved), tissue is penumbra (salvageable). ### Why MTT Prolongation Matters MTT prolongation reflects: - Slowed circulation through collateral vessels - Impaired microvascular perfusion - Combined with CBV reduction → indicates severe, irreversible ischemia