Multiwavelength wideband intensity-modulated optical spectrometer for near-infrared spectroscopy and imaging

摘要

Abstract: Quantitative determination of chromophore concentrations by near infrared spectroscopy (NIRS) is possible by using the differential pathlength of the detected light, measured noninvasively as the mean time delay of the impulse response function of the tissue. Experimentally this information in the time domain is also available in the frequency domain by intensity modulating the input light and sweeping the modulation frequency between zero and infinity. We describe an intensity modulated optical spectrometer which differs from previously described systems in using four different wavelengths, a wideband modulation frequency (1 MHz to 500 MHz), and the simultaneous measurement of three parameters the dc intensity, ac amplitude, and the phase shift. The measured dc intensity in conjunction with the ac phase shift data allows changes in absorption coefficient (and hence chromophore concentration) to be determined more accurately by correcting for real time path length variations. The ac phase shift in combination with the ac modulation depth theoretically allows for the absolute measurement of tissue absorption and scattering coefficient. Preliminary performance figures for the system suggest values of rms noise of 0.0006 OD, 0.0011 rad and 0.0008% for the measured attenuation, ac phase shift and modulation depth. Using a phantom of fixed geometry with known scattering and absorption properties, the ability of the system to reproduce the information content of the impulse response function for a homogeneous phantom is investigated. !16