Imaging of Red Blood Cell and Plasma Dispersion in the Brain Cortex

Abstract

Most of the methods for measuring regional or local blood flow cannot specify the respective fractions of the measured flow attributable to red blood cell (RBC) flow and plasma flow within the measured volume. It is obviously important to know this because of the decisively different microhaemodynamic behaviour of these two components in the microcirculation and their specialized roles in gas and solute exchange in tissue metabolism. Beyond obtaining a separate measure of RBC and plasma flow for a given volume of tissue, a separate assessment of the complexity of RBC and plasma transit routes within that volume seems equally important. This parameter reveals the dissimilarity between seemingly identical conditions when total volume flows are equal but are realized via vascular transit routes of different geometrical complexity, which could indicate the presence in one, and the absence in the other, of certain conditions such as highly focal oedema, capillary obstruction, vasomotion, etc. It has been repeatedly demonstrated, that the television densitometric method of Eke (1982, 1983, 1984) can yield separate images of RBC and plasma flow at high topographical and temporal resolution by monitoring the transit of RBC and plasma indicator pulses through an array of tissue areas and by analysing these indicator-dilution arrays for arrays of RBC and plasma mean transit time (t) and flow. This method has been recently supplemented by a statistical procedure adopted from Knopp and Bassingthwaighte (1969) to analyse the arrays of RBC and plasma indicator-dilution curves for the scatter of the individual transit times over the mean transit time, t (Eke, 1986). This analysis yields a parameter assessing the spatial complexity of indicator transit pathways confined by the monitored tissue volume and is analogous to the approach reported by Tomita et al. (1983). Their concept is based on some a priori assumptions of the distribution of parenchymal transit times (gamma distribution) and the calculation of the ‘gamma index’ as the spatial parameter of the gamma density function applied in the procedure to obtain the transport function of the parenchymal region. The major differences between their approach and mine, beyond the mathematics, are attributable to the facts that their indicator-dilution technique does not allow for imaging, requires invasion of the tissue and derives the gamma index for a cerebrocortical tissue volume of around 1–2 cm³ in contrast to the reflectometric measuring volume of 0.01 mm³.