We have developed a motion-resistant three-wavelength spatial frequency domain imaging (SFDI) system using an 8tap CMOS image sensor. We have successfully measured two wavelengths using the same sensor. However, to remove the effect of melanin, measuring at three wavelengths is needed. In this study, the Hilbert transform is introduced to reduce the number of captured images for each wavelength from three to two. In the experiments, a DMD was illuminated with LEDs with wavelengths of 660nm, 780nm, and 850nm. One plane and one sinusoidal pattern were projected on the specimen for each wavelength. In addition, one ambient light image was captured to extract only the reflection components for the projected patterns. Pattern projection and exposure were repeated multiple times at a faster frame rate than the video rates to suppress the motion artifact while keeping appropriate signal levels. Totally 7 images were captured at once. Using this system, we demonstrated suppression of motion artifacts and ambient light in capturing a moving wrist.
The decay behavior of specific intensity is studied for spatial-frequency domain imaging (SFDI). It is shown using the radiative transport equation that the decay is given by a superposition of different decay modes, and the decay rates of these modes are determined by spatial frequencies and Case’s eigenvalues. This explains why SFDI can focus on shallow regions. The fact that light with nonzero spatial frequency rapidly decays makes it possible to exclusively extract optical properties of the top layer of a layered medium. We determine optical properties of the top layer of a solid phantom. This measurement is verified with different layered media of numerical phantoms.
We are developing a method for simultaneous multi-band spatial frequency domain imaging (SFDI) and blood flow mapping by multi-exposure laser speckle contrast imaging (MELSCI) with a laboratory-designed 2x2-aperture 4-tap CMOS image sensor. Four-tap CMOS image sensor can capture four images synchronized with controlled illuminations and can implement flexible exposure. Although we have demonstrated SFDI with suppressing motion artifact and ambient light bias and efficient video-rate MELSCI based on a four-tap CMOS image sensor separately, simultaneous multi-modal imaging is effective for improving the accuracy of medical diagnosis. The image sensing area is divided into 2×2 regions, and each region is equipped with optical filters and an imaging lens. By using optical bandpass filters, multi-wavelength imaging and wavelength-division multiplexed imaging are realized. Using the fabricated image sensor, we observed a human wrist. 450, 550, 660nm LEDs were used for 3-band SFDI, and a 785nm LD was used for MELSCI. In order to measure a change in the blood flow speed, an examinee was measured before and after an exercise (squatting 30 times). The unit exposure duration for SFDI and MELSCI was 10ms. The pattern was generated by a DMD. The spatial frequency of the projected sinusoidal patterns was 0.1mm -1 . MDC and MAC maps for the three wavelengths by SFDI, and K 2 value maps by MELSCI were successfully obtained.
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