Optimization of spatial frequency domain imaging technique for estimating optical properties of food and biological materials

Authors: HU, DONG - Zhejiang University; Lu, Renfu; YING, YIBIN - Zhejiang University

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 7/31/2017
Publication Date: 8/1/2017
Citation: Hu, D., Lu, R., Ying, Y. 2017. Optimization of spatial frequency domain imaging technique for estimating optical properties of food and biological materials. ASABE Annual International Meeting. doi: 10.13031/aim.201700717.

Interpretive Summary: Absorption and scattering coefficients are two fundamental optical property parameters, which are useful for assessing chemical and structural properties of food and biological materials. Spatial frequency domain imaging (SFDI) is an emerging technique for determining and/or mapping the optical properties of food and biological tissues. However, accurate measurements of the optical properties by SFDI are still challenging due to errors associated with signal acquisitions and the complex parameter estimation algorithm. This study was therefore aimed at optimizing the parameter estimation algorithm for improved estimation of the optical absorption and scattering parameters. Different data smoothing methods for preprocessing the SFDI data were compared, and the optimal spatial frequency resolution and range were determined by using the data generated by Monte Carlo simulation, a stochastic statistical method that provides accurate simulations of light transfer in food and biological tissues. Finally, a new, improved parameter estimation method was proposed and validated through experiments with liquid samples of known optical properties. Results showed that, compared with the conventional method, the proposed parameter estimation method, coupled with the optimal data smoothing method and the optimal spatial frequency resolution and range, improved the estimations of optical absorption and scattering properties by 37.5% and 9.8%, respectively. Hence implementation of the proposed method will enable more accurate measurement of optical properties, and thus better quality assessment, of horticultural and food products.

Technical Abstract: Spatial frequency domain imaging technique has recently been developed for determination of the optical properties of food and biological materials. However, accurate estimation of the optical property parameters by the technique is challenging due to measurement errors associated with signal acquisitions and the complex inverse parameter estimation algorithm for the diffusion model. This research was therefore aimed at optimizing the inverse algorithm for estimating the optical absorption and reduced scattering coefficients from spatial frequency domain diffuse reflectance generated by Monte Carlo simulations. Sensitivity analysis showed that the reduced scattering coefficient could be estimated more accurately than the absorption coefficient. Six data smoothing methods were then compared, with the results indicating that the ‘moving’ method with a span of 5 data points was the most effective in improving the parameters estimation. Further studies were conducted to determine the optimal frequency resolution and start and end frequencies in terms of the reciprocal of mean free path (1/mfp’). The results showed that the optimal frequency resolution increased with the reduced scattering coefficient and remained stable when it was larger than 2 (1/mm). The optimal end frequency decreased from 0.3/mfp’ to 0.16/mfp’ with the reduced scattering coefficient ranging from 0.4 to 3 (1/mm), while the optimal start frequency remained at 0. A two-step method was proposed based on the optimized frequency parameters, which improved estimation accuracies by 37.5% and 9.8% for the absorption and reduced scattering coefficients, respectively, compared with the conventional method. Experimental validation with seven liquid optical phantoms showed that the optimized technique resulted in the mean absolute errors of 15.4%, 7.6%, 5.0% for the absorption coefficient and 16.4%, 18.0%, 18.3% for the reduced scattering coefficient at the wavelengths of 675 nm, 700 nm, and 715 nm, respectively. Hence, implementation of the optimized technique can result in better measurement of optical properties of horticultural and food products.