Broad bandwidth frequency domain instrument for quantitative tissue optical spectroscopy

Near-infrared (NIR) optical properties of turbid media, e.g., tissue, can b e accurately quantified noninvasively using methods based on diffuse reflec tance or transmittance, such as frequency domain photon migration (FDPM). F actors which govern the accuracy and sensitivity of FDPM-measured optical p roperties include instrument performance, the light propagation model, and fitting algorithms used to calculate optical properties from measured data. In this article, we characterize instrument, model, and fitting uncertaint ics of an FDPM system designed for clinical use and investigate how each of these factors affects the quantification of NIR absorption (mu(a)) and red uced scattering (mu(s)') parameters in tissue phantoms. The instrument is b ased on a 500 MHz, multiwavelength platform that sweeps through 201 discret e frequencies in as little as 675 ms. Phase and amplitude of intensity modu lated light launched into tissue, i.e., diffuse photon density waves (PDW), are measured with an accuracy of +/- 0.30 degrees and +/- 3.5%, while phas e and amplitude precision are +/- 0.025 degrees and +/- 0.20%, respectively . At this level of instrument uncertainty, simultaneous fitting of frequenc y-dependent phase and amplitude nonlinear model functions derived from a ph oton diffusion approximation provides an accurate and robust strategy for d etermining optical properties from FDPM data, especially for media with hig h absorption. In an optical property range that is characteristic of most h uman tissues in the NIR (5 x 10(-3) < mu(a) < 5 x 10(-2) mm(-1), 0.5 < mu(s )' < 2 mm(-1)), we theoretically and experimentally demonstrate that the mu ltifrequency, simultaneous-fit approach allows mu(a) and mu(s)' to be quant ified with an accuracy of +/- 5% and +/- 3%, respectively. Although excepti onally high levels of precision can be obtained using this approach (< 1% o f the estimated absorption and scattering values), we show that the absolut e accuracy of optical property measurements is highly dependent on specific factors associated with instrument performance, model function relevance, and details of the fitting strategy used to calculate mu(a) and mu(s)'. (C) 2000 American Institute of Physics. [S0034-6748(00)00106-4].