Abstract
The design of inhomogeneous phantoms for diffuse optical imaging purposes using totally absorbing objects embedded in a diffusive medium is proposed and validated. From time-resolved and continuous-wave Monte Carlo simulations, it is shown that a given or desired perturbation strength caused by a realistic absorbing inhomogeneity of a certain absorption and volume can be approximately mimicked by a small totally absorbing object of a so-called equivalent black volume (equivalence relation). This concept can be useful in two ways. First, it can be exploited to design realistic inhomogeneous phantoms with different perturbation strengths simply using a set of black objects with different volumes. Further, it permits one to grade physiological or pathological changes on a reproducible scale of perturbation strengths given as equivalent black volumes, thus facilitating the performance assessment of clinical instruments. A set of plots and interpolating functions to derive the equivalent black volume corresponding to a given absorption change is provided. The application of the equivalent black volume concept for grading different optical perturbations is demonstrated for some examples.
Highlights
The investigation of the human body by diffusely propagated light is a highly appealing quest due to the intrinsic noninvasiveness of light at low power and the wealth of information carried by absorption and scattering
The work toward standardization requires, on one hand, the definition of shared protocols, identifying the key physical quantities to be measured and the procedures for their assessment[14,15,16,17,18] and, on the other hand, the description and production of phantoms mimicking some paradigmatic in vivo problems. For those problems that concern the protocols, several successful multilaboratory attempts have been initiated at the European Union (EU) level—fostered by EU-funded projects—such as the MEDPHOT protocol for the assessment of diffuse optics instruments on homogeneous media[15] or, more recently, for characterization of time-domain brain imagers, the Basic Instrumental Performance protocol[16] as well as the nEUROPt protocol,[17] the latter being related to localized absorption changes
By using numerical solutions of the radiative transfer equation reconstructed with an Monte Carlo (MC) code, in Sec. 4, we have investigated the validity of the equivalence relation beyond the diffusive regime
Summary
The investigation of the human body by diffusely propagated light is a highly appealing quest due to the intrinsic noninvasiveness of light at low power and the wealth of information carried by absorption (related to chemical composition) and scattering (linked to microstructure). Common standardization tools help in comparing results obtained in different clinical studies, permit For those problems that concern the protocols, several successful multilaboratory attempts have been initiated at the European Union (EU) level—fostered by EU-funded projects—such as the MEDPHOT protocol for the assessment of diffuse optics instruments on homogeneous media[15] or, more recently, for characterization of time-domain brain imagers, the Basic Instrumental Performance protocol[16] as well as the nEUROPt protocol,[17] the latter being related to localized absorption changes. We will show that the effect of a localized variation of the absorption coefficient within a finite volume can be adequately reproduced using a small totally absorbing object with a proper volume (equivalence relation) This concept can be useful in two ways. This protocol has been agreed to by a consortium of 17 partners all over Europe and has been applied to the clinical brain imagers and additional laboratory instruments developed by four groups of this consortium
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