Characterization of homogeneous tissue phantoms for performance tests in diffuse optics

Solid homogeneous turbid phantoms can be employed to mimic the attenuation and angular distribution of light emerging from tissue, e.g., to assess the responsivity of the detection system of diffuse optics instrumentation and to support standardized performance tests of functional near-infrared spectroscopy devices. We present three methods to quantify the wavelength-dependent diffuse transmittance, relying on (1) measurement of radiance exiting the phantom by a detector far from the exit aperture, (2) simple recording of radiance by a power meter close to the exit aperture and correction for the finite distance between phantom surface and detector, (3) determination of the reduced scattering and absorption coefficients by time-resolved diffuse transmittance measurements and forward calculation of the time-integrated diffuse transmittance based on the diffusion model. The implications of the different approximations related to these approaches are discussed. The various methods were applied to characterize solid slab phantoms, and the results were compared. Specifically, for an epoxy-resin based phantom having a thickness of 2 cm, a reduced scattering coefficient of about 0.5/mm and an absorption coefficient of about 0.01/mm, the diffuse transmittance values obtained by the three different methods were found to agree within about 10%.