Local relationships between restricted water diffusion and oxygen consumption in the ischemic human brain

Area of Science:

• Neuroimaging
• Stroke research
• Cerebrovascular disease

Background:

• Magnetic Resonance (MR) imaging is crucial for identifying viable ischemic tissue in acute stroke.
• The precise relationship between apparent diffusion coefficient (ADC) and energy failure due to oxygen deprivation in humans remains unclear.

Purpose of the Study:

• To investigate the quantitative relationship between ADC and cerebral blood flow (CBF) and oxygen metabolism (CMRO2) in acute stroke patients.
• To assess how these relationships change over time after stroke onset.

Main Methods:

• Prospective study of acute carotid-territory stroke patients using diffusion tensor imaging and 15O-PET.
• Analysis focused on voxels with significant hypoxia (oxygen extraction fraction >0.70).
• Quantitative assessment of ADC against CBF and CMRO2.

Main Results:

• ADC remained stable until CBF dropped below approximately 20 mL/100g/min, then declined linearly.
• ADC was a less accurate predictor of CMRO2 compared to CBF, especially when CMRO2 was not severely reduced.
• The correlation between ADC and both CBF/CMRO2 became more pronounced over time, indicating greater severity at later time points.

Conclusions:

• Restricted water diffusion (ADC) reliably indicates oxygen deprivation severity below the penumbral threshold.
• ADC is less strongly correlated with metabolic disruption, potentially explaining its limitations in predicting tissue outcome.
• The severity of diffusion restriction for a given ADC value increases with time post-stroke, a critical factor for interpreting results and developing infarct prediction models.