Abstract
.Significance: Frequency domain diffuse optical spectroscopy (FD-DOS) uses intensity modulated light to measure the absorption and reduced scattering coefficients of turbid media such as biological tissue. Some FD-DOS instruments utilize a single modulation frequency, whereas others use hundreds of frequencies. The effect of modulation frequency choice and measurement bandwidth on optical property (OP) extraction accuracy has not yet been fully characterized.Aim: We aim to assess the effect of modulation frequency selection on OP extraction error and develop a high-speed look-up table (LUT) approach for OP estimation.Approach: We first used noise-free simulations of light transport in homogeneous media to determine optimized iterative inversion model parameters and developed a new multi-frequency LUT method to increase the speed of inversion. We then used experimentally derived noise models for two FD-DOS instruments to generate realistic simulated data for a broad range of OPs and modulation frequencies to test OP extraction accuracy.Results: We found that repeated measurements at a single low-frequency (110 MHz) yielded essentially identical OP errors as a broadband frequency sweep (35 evenly spaced frequencies between 50 and 253 MHz) for these noise models. The inclusion of modulation frequencies diminished overall performance for one of the instruments. Additionally, we developed a LUT inversion algorithm capable of increasing inversion speeds by up to , with and error when a single modulation frequency was used.Conclusion: These results suggest that simpler single-frequency systems are likely sufficient for many applications and pave the way for a new generation of simpler digital FD-DOS systems capable of rapid, large-volume measurements with real-time feedback.
Highlights
Diffuse optical spectroscopy (DOS) is an optical technique that utilizes multiply scattered light to measure biological tissues
We found that repeated measurements at a single low-frequency (110 MHz) yielded essentially identical optical property (OP) errors as a broadband frequency sweep (35 evenly spaced frequencies between 50 and 253 MHz) for these noise models
All the analysis presented here were performed using a set of 10,000 pairs of OPs that were randomly selected from a uniform distribution such that μs0 > 10μa to stay in the range where the P1 approximation is valid
Summary
Diffuse optical spectroscopy (DOS) is an optical technique that utilizes multiply scattered light to measure biological tissues. Only a single modulation frequency and single source–detector separation is required to disentangle μa from μs[0,9] and many instruments use a single modulation frequency, often with heterodyne detection to generate a beat frequency so lower-speed electronics can be used.[10] In the early 1990s, a network analyzer-based FDDOS measurement technique was developed that has since found frequent use for some clinical applications.[11] The network analyzer allowed the measurement of frequency sweeps with hundreds of modulation frequencies over a wide bandwidth (e.g., 50 MHz to 1 GHz) at the expense of acquisition speed In between these two extremes, other groups have used two,[12] or several dozen modulation frequencies to measure μa and μs0.13
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