Abstract: Aspects described herein estimate the pressure difference across a hollow region arising from fluid flow within the hollow region, based on an imaged fluid flow. The method utilises a complete description of fluid mechanical behaviour to derive an estimate of relative pressure or pressure difference over arbitrary flow segments. The method uses the concept of a virtual or arbitrary velocity field in the analysis of the work-energy of the fluid flow. Furthermore, the method uses statistical analysis to derive the acquired flow as a mean field and a related covariance quantity and uses this statistical description in the evaluation of virtual work-energy of the fluid flow. This assessment of virtual work-energy of the fluid flow is then used to derive an estimate of the pressure difference across any two given points (relative pressure) in the hollow region.