We demonstrate 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. The proposed imaging device is composed of an array of sub-image sensors. This multi-aperture configuration realizes multi-wavelength imaging to estimate chromophore concentrations and wavelength-division-multiplexed imaging for multi-band SFDI and MELSCI. For SFDI, 450, 550, 660nm LEDs were used as the light sources of a DMD. For MELSCI, a 785nm LD was used for flood illumination. Reflectance and K2 maps of a human arm before and after an exercise was measured.
We demonstrate three-wavelength spatial frequency domain imaging (SFDI) of a moving arm under a room light with suppressing motion artifact and biased reflectance based on an 8-tap CMOS image sensor developed in our research group. The images for two projected patters for three wavelengths and that for only ambient light are captured. Three LEDs with the wavelengths of 660nm, 780nm, and 850nm were utilized to decompose the oxy-/ deoxyhemoglobin and melanin concentrations. The exposure time for each frame was 70ms. The total hemoglobin, tissue oxygen saturation, and scattering coefficient maps were obtained without any significant motion artifact.
In this study, spatial frequency domain imaging (SFDI) and temporal frequency domain imaging (TFDI) are combined to observe superficial and deep tissues simultaneously using a time-resolving CMOS image sensor. SFDI is an established non-invasive wide-field imaging method for superficial or shallow tissues. On the other hand, time-resolved spectroscopy based near-infrared spectroscopy (TRS-NIRS) is suitable for deep tissue measurement while it is based on point or multipoint measurement. Recently, time-resolving CMOS image sensors based on the single-photon avalanche diode and charge modulators have emerged. To take advantage of their area-imaging capability, we propose a spatio-temporal frequency domain imaging (STFDI=SFDI+TFDI) method, where pulsed binary stripe patterns with a pitch, p, and a duty ratio of 1/N are sequentially projected onto the tissue. While the projected pattern is shifted N times with a step of p/N, time-resolved images are captured for every shift. SFDI is conducted by performing fast Fourier transform (FFT) at each pixel after integrating them in time for each shift. For TFDI, the detected light in the middle of the stripes is analyzed by FFT in time for each shift. Based on the obtained amplitude and phase for specific harmonics orders, the absorption and scattering coefficients are estimated. This concept was verified by a GPU-based Monte Carlo simulator, MCX, with a two-layer skin model. We also experimentally confirmed the difference in the measured reflectance and phase for SFDI and TFDI when the thickness of the first layer was changed.