Cerebral blood flow and modern approaches for clinical assessment & monitoring: a view to the future.

Cerebral blood flow (CBF) is under homeostatic control via cerebral autoregulation, maintaining a constant blood supply to brain parenchyma by integrating myogenic, metabolic, and neurogenic inputs across the neurovascular unit to stabilise perfusion despite physiological variations in cerebral perfusion pressure. While the mechanisms that underpin the control of CBF have been extensively investigated, this narrative review aims to holistically synthesise key findings for clinicians and researchers across healthy and diseased states. We first summarise autoregulatory inputs, cellular effectors, and typical stimuli/responses (with practical caveats including non-linearity, frequency dependence, and pressure-passivity). We then discuss monitoring approaches for assessing CBF and contrast cross-sectional techniques with portable bedside modalities, highlighting what each primarily measures (perfusion, velocity, oxygenation proxy, or flow index), their typical applications, and key constraints (radiation exposure, invasiveness, operator dependence, penetration depth, quantification). For historical context, we briefly note invasive monitors that remain selectively indispensable within multimodal neurocritical care. Looking forward, we outline emerging directions that prioritise non- and minimally-invasive solutions, including photoacoustic imaging, functional ultrasound, diffuse optical methods, speckle-based optics, and machine learning-enhanced post-processing, alongside hybrid multimodal integration. Collectively, these developments aim to improve accuracy, repeatability, and scalability of CBF monitoring and to support individualised decision-making across the clinical neurosciences.