Abstract:

During cerebral revascularization surgery, it is imperative to examine the perfusion of the treated region to preserve patients from fatal consequences, done by measuring the volume flow in single blood vessels. Weichelt et al. suggested to quantify the volume flow from contact free recorded fluorescence angiography video data, multiplying the vessel cross section by the observed fluorophor velocity. Compared to reference measurements, the method overestimates the volume flow. Depending on the vessel diameter d the deviations range from 7% (given as k = 1,07,d = 1,6mm) to 58% (given as k = 1,58,d = 4mm) [1]. The observed deviations are investigated in recent research. There is a flow velocity profile over the vessel cross section. There are varying amounts of intensity contributing to the video data coming from different depths within the vessel due to radiative transfer in turbid media. These varying amounts should be considered in an optic probability density function. So, one approach integrates the local relative blood velocity, weighted by the optic probability density function over the vessel cross section to approximate k. If the deviations can be explained by a combination of information depth and local blood velocity, the approximated k match the observed ones. In previous work, the optical weighting was obtained applying a Monte Carlo Multi Layer model, analyzing the deepest penetration depth of each photon. This implies many assumptions, especially regarding model geometry, information source and illumination modelling. The approximated k do not match the observations. [2] This work investigates the influence of use of optical weights from a Fluorescence Multi Cylinder Monte Carlo simulation instead of a Monte Carlo Multi Layer to assess the validity of assumptions made by using the optical weighting factors from Monte Carlo Multi Layer. Three aspects of the optic model were reimplemented to obtain the optic weights: 1. the fluorescence location of each photon was assumed to be the source of information given by this photon instead of the deepest penetration location 2. the Multi Layer geometry was changed to a Multi Cylinder geometry 3. homogeneous illumination was simulated instead of single point illumination It was found, that there are clear differences in approximated k-factors, obtained from optical weights from the Fluorescence Multi Cylinder Monte Carlo model, compared to the optical weights from Monte Carlo Multi Layer model. The deviations coming from model geometry, information source interpretation and illumination show a Root Square Mean Error of up to 38%. The assumptions made in previous work are not met.