The vascular mean transit time: a surrogate for the penumbra flow threshold?

Area of Science:

• Neuroimaging
• Cerebrovascular Diseases
• Medical Physics

Background:

• Identifying salvageable brain tissue (penumbra) is crucial in acute stroke management.
• Perfusion imaging (CT/MRI) often uses time-based variables like mean transit time (MTT) as surrogates for cerebral blood flow (CBF).
• Direct validation of MTT against gold-standard positron emission tomography (PET) for penumbra threshold prediction is lacking.

Purpose of the Study:

• To directly validate mean transit time (MTT) as a predictor of the penumbra threshold.
• To compare MTT-derived metrics against gold-standard (15)O-PET measurements in two independent acute stroke cohorts.
• To assess the utility of absolute MTT, MTT delay, and MTT ratio in defining different penumbra thresholds.

Main Methods:

• Utilized (15)O-PET data from two independent acute stroke patient samples (N=7 and N=30).
• Derived receiver operating characteristic (ROC) curves to determine areas under the curve (AUCs), optimal thresholds (OTs), and 90%-specificity thresholds (90%-Ts).
• Evaluated absolute MTT, MTT delay, and MTT ratio against three penumbra definitions based on CBF and oxygen extraction fraction (OEF).

Main Results:

• AUCs for MTT parameters predicting penumbra thresholds ranged from 0.79 to 0.92, indicating good validity.
• Optimal thresholds (OTs) were established for absolute MTT (7.8–8.3s), MTT delay (2.8–4.7s), and MTT ratio (151%–267%).
• Findings were consistent across both independent cohorts, with no significant differences observed.

Conclusions:

• Mean transit time (MTT) is a highly effective surrogate for cerebral blood flow (CBF) in depicting the penumbra threshold.
• This validation using gold-standard PET confirms MTT's reliability in clinical stroke imaging.
• MTT-based imaging can confidently guide the identification of salvageable brain tissue in acute stroke.