Reference optical phantoms for diffuse optical spectroscopy Part 1 – Error analysis of a time resolved transmittance characterization method

Abstract

Development, production quality control and calibration of optical tissue-mimicking phantoms require a convenient and robust characterization method with known absolute accuracy. We present a solid phantom characterization technique based on time resolved transmittance measurement of light through a relatively small phantom sample. The small size of the sample enables characterization of every material batch produced in a routine phantoms production. Time resolved transmittance data are pre-processed to correct for dark noise, sample thickness and instrument response function. Pre-processed data are then compared to a forward model based on the radiative transfer equation solved through Monte Carlo simulations accurately taking into account the finite geometry of the sample. The computational burden of the Monte-Carlo technique was alleviated by building a lookup table of pre-computed results and using interpolation to obtain modeled transmittance traces at intermediate values of the optical properties. Near perfect fit residuals are obtained with a fit window using all data above 1% of the maximum value of the time resolved transmittance trace. Absolute accuracy of the method is estimated through a thorough error analysis which takes into account the following contributions: measurement noise, system repeatability, instrument response function stability, sample thickness variation refractive index inaccuracy, time correlated single photon counting system time based inaccuracy and forward model inaccuracy. Two sigma absolute error estimates of 0.01 cm(-1) (11.3%) and 0.67 cm(-1) (6.8%) are obtained for the absorption coefficient and reduced scattering coefficient respectively.

Experimental and analytical comparative study of optical coefficient of fresh and frozen rat tissues

“…Optical index and anisotropy of the Biomimic phantom were evaluated by the manufacturer to n ¼ 1.52 and g ¼ 0.62. 21 The optical properties of our phantom measured using the time resolved method by the manufacturer were μ a ¼ …”

Phantoms for diffuse optical imaging based on totally absorbing objects, part 2: experimental implementation

“…Clearly, the availability of well characterized, reliable, reproducible, and realistic phantoms is essential for this process. Concerning homogeneous phantoms, the status is quite advanced, with many tested options, [17][18][19][20][21][22][23][24] off-the-shelf solid phantoms with well characterized and documented optical properties, 25 and good agreement in liquid phantoms' characterization in multilaboratory studies. 26 Conversely, the situation of inhomogeneous phantoms is less consolidated due to many criticalities and large combinations of geometries and optical properties.…”

Phantoms for diffuse optical imaging based on totally absorbing objects, part 1: basic concepts

“…Solid homogenenous phantoms can also be accurately characterized and are commercially available with a stated uncertainty of about 11% for the absorption coefficient, μ a , and about 7% for the reduced scattering coefficient, μ 0 s (Ref. 31). Conversely, the production of inhomogeneous phantoms-mimicking, for instance, a focal activation in the brain cortex or a tumor mass within the breast-is more challenging, due to the added complexity of fabricating the structure by solid-solid, solid-liquid, or liquid-liquid phases.…”