Phantom for standardization in functional near-infrared spectroscopy, part 2: optical properties and Monte Carlo...

The international standard IEC 80601-2-71 defines requirements for the basic safety and essential performance of medical continuous wave functional near-infrared spectroscopy (fNIRS) equipment, including a set of tests based on a dedicated tissue-equivalent phantom consisting of switchable apertures sandwiched between two diffusing media.

In a companion paper, we described a first implementation of this phantom developed by AIST, its basic characterization of attenuation properties, and results of its application in the testing of commercial fNIRS devices. In the present paper, we provide the absorption and reduced scattering spectra of the AIST phantom and of alternative silicone phantom matrices, complemented with extensive simulations on the effect of the phantom optical properties and geometry on the features of the phantom relevant for the fNIRS standard test.

All spectral measurements were performed with time-domain diffuse optical spectrometers for accurate quantification of the absorption and reduced scattering coefficient, whereas simulations were obtained using a Monte Carlo code.

The POM-C matrix of the AIST phantom exhibits negligible absorption ( $<0.01{\mathrm{cm}}^{-1}$ ) and a monotonically decreasing reduced scattering coefficient (from 10.5 down to $8.5{\mathrm{cm}}^{-1}$ ) in the 650 to 850 nm range. Monte Carlo simulations explain the independence from wavelength, geometry, and partially also optical properties in the change in attenuation when switching between two apertures of 6 and 4 mm diameter within the phantom, with $<0.1\mathrm{dB}$ spectral variation. Extended spectral characterization of POM-C and silicone matrices in the 600 to 1100 nm range showed additional marked absorption peaks around 900 and 1020 nm.

We provide valid support for researchers and manufacturers to understand the operation of the phantom described in the fNIRS standard and to replicate it in their labs. In addition, we provide a basis for further evolution of the standard itself to encompass alternative probe geometries or functional tests.