Comparative analysis of retinal and cerebral vascular responses to CO₂ using Doppler optical coherence tomography and transcranial Doppler ultrasound

Objective. Access to blood flow data in cerebral and retinal vascular beds is crucial for diagnosing cerebrovascular diseases. This study addresses two technological gaps: (1) simultaneous recording of vascular responses in the brain and eye by integrating transcranial Doppler ultrasonography (TCD) with Doppler optical coherence tomography (DOCT); (2) automation of DOCT image processing, including vessel segmentation, to enable quantification of flow in small retinal vessels as a proxy for cerebral microcirculation. The research evaluates vascular reactivity to CO₂ changes, measuring pulsatility index (PI), resistance index (RI), and systolic/diastolic ratio (S/D). Approach. Ten healthy volunteers were recruited. Blood flow was simultaneously recorded in the middle cerebral artery (MCA) using TCD and in retinal microarteries and microveins (diameters 50–130 µm) using DOCT (95 Hz sampling rate). A novel software framework was developed for automated DOCT analysis: B-scan preprocessing, alignment, vessel segmentation, filtering, and extraction of parameters (area, lumen diameter, axial velocity component). Data were analyzed across cardiac phases (systole/diastole) during normal breathing, apnea, and hyperventilation. Correlations were assessed statistically, comparing velocity changes and vascular indices. Main results. In the MCA, significant correlations were observed between flow velocity changes and breathing tests (increase during apnea, decrease during hyperventilation; p < 0.01). Retinal vessels showed no statistically significant correlations (p > 0.05), despite DOCT enabling precise measurement of PI, RI, and S/D comparable to TCD. Retinal vessels exhibited lower CO₂ reactivity, confirming fundamental differences in ocular and cerebral microcirculation regulation. Significance. TCD-DOCT integration enables noninvasive, synchronized vascular reactivity assessment, paving the way for studies on brain-eye microcirculation interactions. Results on healthy subjects highlight retinal vessels' uniqueness as a model but indicate limitations in directly mirroring cerebral changes, guiding future hybrid protocols. The automated framework eliminates subjective errors, facilitating scalable clinical analyses in pathologies like ischemic stroke or cerebral small vessel disease.