A Study On Spatial Frequency Domain Optical Imaging For Tissue With Complex Surface Profile

Photodynamic therapy(PDT)has been well applied in treatment on many aspects such as port-wine stains,digestive cancer and skin cancer.However,the therapeutic effect of PDT highly relies on physicians'experience and individual differences between patients.Thus,an in-vivo method for quantification and evaluation of doses is urgently needed.Spatial frequency domain imaging(SFDI)has been emerging as a non-invasive and non-contact wild-field optical imaging method with the providing functional information in a high speed.Therefore,SFDI is considered as an appropriate method for in-vivo evaluation of PDT for large-area tissue(e.g.port-wine stains).Since SFDI relies on the measurement of light intensity,the differences in the distance between tissue and the reference lead to the differences on the sample and further result in the deviation of extracted optical parameters.Consequently,it is critical to recover the real profile of tissue and adopt the profile as a prior information in the reconstruction of optical properties.In this paper,we designed and built a SFDI system for PDT evaluation,and put forward a profile correction method for quantitative,wide-field measurement of tissue optical parameters.The method can modify remitted light intensity of tissue with complex surface profile and improve the reconstruction accuracy.In SFDI system,Digital micromirror device(DMD)is adopted as a spatial modulator to produce the precise projected of spatial modulation light.Electronic Multiplying Charge Coupled Device(EMCCD)is applied as the detector to rapidly and efficiently acquire the diffuse light intensity.DMD and EMCCD are controlled by host computer software based on X86 platform.Four-step phase shift method and package processing are employed to recover the complex surface profile of tissue,and correct the remitted light intensity from diffusive surface by the Lambertian formula method.In this way,it is expected to extract optical parameters of tissue more precisely.In this paper,experiments have been conducted to verify the correction of the SFDI system,the method of the three-dimensional surface profile recovery and the extraction of optical parameters.Results show that the distortion of the modulated light to standard sine wave is less than 7.41%,and the maximum relative error in measurement of height is3.57%,which meets the requirement of the system designation.In the preliminary experiments,we chose a solid phantom with an angle variation within 20° and height variation within 15mm to extracted optical parameters.The results show that the relative error of the extraction of absorption coefficient decreased from33%to 15%after correction of the three-dimensional surface profile.The SFDI system can be used for measuring the optical parameters of tissue with complex surface profile,and lays a foundation for quantification and evaluation of doses for PDT.