Both FD-DOS and TR-DOS can estimate the absolute values of μ s’ and μ a with good accuracy. Ĭontinuous-wave diffuse optical spectroscopy (CW-DOS) has limitations in retrieving the absolute values of μ s’ and μ a because this approach depends on one quantity (changes of light intensity) which only allows for estimating the change of μ s’ and μ a values. DOS measurements are made using three main approaches: continuous-wave (CW), frequency-domain (FD), and time-resolved (TR). Therefore, the OP of a homogeneous object or the distribution of OP for a heterogeneous object can be recovered. The detected signal using DOS can be analyzed using an inverse problem solver to retrieve the OP, particularly the reduced scattering coefficient ( μ s’) and the absorption coefficient ( μ a). Hence, re-emitted photons can be detected either in transmittance geometry (source and detectors not on the same side), or in reflectance geometry (source and detector on the same side of the object) for thin and thick objects. Light propagation in turbid media is well described by the radiative transfer equation (RTE) and its simplified version, the diffusion equation (DE). In this range of wavelengths, light can diffuse in the tissue and penetrate up to a few cm. DOS exploits the low absorption and high scattering of tissues in the optical window (also called biological window) at red and near-infrared wavelengths (650–900 nm). Also, real-time functional monitoring of human tissue such as muscles, breasts and newborn heads will be possible by integrating this detector with a time-to-digital converter (TDC).ĭiffuse optical spectroscopy (DOS), also called near-infrared spectroscopy (NIRS), is non-invasive and non-destructive technology to determine the optical properties (OP) of turbid objects such as tissues in which scattering dominates absorption. Overall, the results of this study show that our silicon CMOS-based SPAD detectors can be used to build a multichannel TR-DOS prototype. Following the MEDPHOT protocol, the absolute values of the optical properties for several homogeneous phantoms were retrieved with good accuracy and linearity using a best-fitting model based on the Levenberg-Marquardt method. Also, using light with optical power lower than the maximum permissible exposure for human skin, this prototype can acquire raw data in reflectance geometry for phantoms with optical properties similar to human tissues.
Following the basic instrumental performance protocol, our prototype had sub-nanosecond total instrument response function and low differential non-linearity of a few percent.
This prototype was validated using assessments from two known protocols for evaluating TR-DOS systems for tissue optics applications. To address the issues of low-cost, compact size and high integration capabilities, we have developed free-running (FR) single-photon avalanche diodes (SPADs) using 130 nm silicon complementary metal-oxide-semiconductor (CMOS) technology and used it in a TR-DOS prototype. For medical imaging applications, important features of new generation TR-DOS systems are low-cost, small size and efficient inverse modeling. Time-resolved diffuse optical spectroscopy (TR-DOS) is an increasingly used method to determine the optical properties of diffusive media, particularly for medical applications including functional brain, breast and muscle measurements.