Performance assessment of time-domain optical brain imagers: a multi-laboratory study

Novel protocols were developed and applied in the European project “nEUROPt” to assess and compare the performance of instruments for time-domain optical brain imaging and of related methods of data analysis. The objective of the first protocol, “Basic Instrumental Performance”, was to record relevant basic instrumental characteristics in a direct way. The present paper focuses on the second novel protocol (“nEUROPt” protocol) that was devoted to the assessment of sensitivity, spatial resolution and quantification of absorption changes within inhomogeneous media. It was implemented with liquid phantoms based on Intralipid and ink, with black inclusions and, alternatively, in two-layered geometry. Small black cylinders of various sizes were used to mimic small localized changes of the absorption coefficient. Their position was varied in depth and lateral direction to address contrast and spatial resolution. Two-layered liquid phantoms were used, in particular, to determine depth selectivity, i.e. the ratio of contrasts due to a deep and a superficial absorption change of the same magnitude. We introduce the tests of the “nEUROPt” protocol and present exemplary results obtained with various instruments. The results are related to measurements with both types of phantoms and to the analysis of measured time-resolved reflectance based on time windows and moments. Results are compared for the different instruments or instrumental configurations as well as for the methods of data analysis. The nEUROPt protocol is also applicable to cw or frequency-domain instruments and could be useful for designing performance tests in future standards in diffuse optical imaging.