gms | German Medical Science

Objective: The BOLD CVR response to a CO2 challenge of neurovascular patients is clinically relevant and has a complex temporal dynamic. This neurovascular response-function differs greatly from the commonly used canonical HRF. Therefore, we present here a novel model-free iterative analysis for evaluating transient phase duration parameters describing BOLD fMRI signal dynamics during a monitored CO2 pseudo- square wave challenge. We aim to optimize the CO2 arrival time to increase sensitivity and reliability of cerebrovascular reactivity analysis, test for its clinical relevance on patient data and gain further pathophysiological insight.

Methods: The algorithm was done using datasets of patients with unilateral internal carotid occlusion and healthy Controls. BOLD-fMRI derived normalized maps of 25 healthy subjects in MNI standard space are combined for reference atlases of all BOLD-derived parameters to better assess alterations within a single subject. All subjects underwent a standardized CO2 pseudo square wave increase of ±10mmHg of PetCO2 from a calibrated baseline of ±40mmHg PetCO2. The iterative algorithm was then applied to the data to calculate both the transient phase durations, temporal delay maps and dynamic and static CVR maps. We compared our algorithm to the state-of-heart methods used in the literature: maximum correlation method.

Results: BOLD time series vs CO2 time series are presented with different CO2 arrival times applied to the data. We determined the most optimal CO2 arrival time and found that the parametric decomposition resulted in the best description of the physiological data for white and grey matter. The linear fit of the BOLD vs. CO2 scatter plot shows that only our algorithm removes the transition phases between the two static states correctly. Our algorithm corrected the too long CVR response delay obtained with the maximum correlation method: WM: 40 s, GM: 10 s; to the more realistic delay of WM 18 s, GM: 2 s. The CVR maps obtained with our algorithm differs significantly from previous CVR maps obtained with not correct delays. The new parametric maps obtained have a good SNR and clinically plausible.

Conclusion: We determined the most optimal CO2 arrival time and found that the parametric decomposition resulted in a better description of the physiological data for white and grey matter. The linear fit of the BOLD vs. CO2 scatter plot shows clearly that only our algorithm removes the transition phases between the two static states correctly. Our algorithm corrected the too long CVR response delay obtained with the maximum correlation method: WM: 40 s, GM: 10 s; to the more realistic delay of WM 18 s, GM: 2 s. The CVR maps obtained with our algorithm differed significantly from previous CVR maps obtained with not correct delays. The new parametric maps obtained: DTP, DTB have a good SNR and clinically plausible.