Assessment of basic instrumental performance of time-domain optical brain imagers

To facilitate the design and optimization of instruments for time-domain optical brain imaging within the European project "nEUROPt", the performance of various instruments is assessed and compared. This type of instruments relies on picosecond lasers with high repetition rates, fast detectors and time-correlated single photon counting. The first step of the assessment included a number of basic tests that are related to parameters of the source, to the differential nonlinearity of the timing electronics and to the temporal instrument response function (IRF). An additional test has been devised to measure the responsivity of the detection system, i.e. the overall efficiency to collect and detect light emerging from tissue. Dedicated solid slab phantoms have been developed and quantitatively spectrally characterized to provide sources of known radiance with nearly Lambertian angular characteristics. The wavelength-dependent transmittance factor of these phantoms was of the order of 10(20)/(W s m(2) sr). Measurements of the responsivity of the detection systems of three time-domain optical brain imagers tested yielded similar values of the order of 0.1 mm(2) sr.