Modulation frequency selection and efficient look-up table inversion for frequency domain diffuse optical spectroscopy

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 >300 MHz diminished overall performance for one of the instruments. Additionally, we developed a LUT inversion algorithm capable of increasing inversion speeds by up to 6×, with 1000 inversions∕s and ∼1% 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.

A scalable, multi-wavelength, broad bandwidth frequency-domain near-infrared spectroscopy platform for real-time quantitative tissue optical imaging

“…Second, we implemented an ultra-high-speed knn lookup table method into hardware, further reducing downstream computation and maximizing inversion/s speed, while also allowing chromophore computation to be done within the embedded SoC. Our knn lookup table method resulted in a 12 -1,900x (in SoC) and 170 -11,000x (modern CPU) improvement over previously reported literature (39). The lookup table approach was chosen as it can be significantly faster (1,000x) than iterative linear optimization solutions typically used to find inverse model solutions without loss in accuracy (17).…”

“…These parameters can be optimized specifically for the desired application all while providing real-time display, similar to the various tissue modes of an ultrasound device. For example, some applications and tissue types can benefit from a range of modulation frequencies, while others require only one (30, 39). We also showed that the large amount of streaming data can be averaged to provide an order of magnitude improvement in phase precision at the expense of speed if desired.…”

Investigation of effect of modulation frequency on high-density diffuse optical tomography image quality

“…Additionally, in general, the development of system design for FD systems is complex and ongoing with various options that each lead to drastically different noise spectra across modulation frequencies. 51,63 While a full discussion of the hardware challenges with possible solutions is beyond the scope of the present work, we show herein the potential for FD imaging given further developments in FD system design that can lower the noise in high bandwidth systems. In addition, more elaborate models of the physiological noise component, as could be explained using the local covariance of each source-detector pair measurement, may further improve management of noise and therefore improve image quality.…”

“… 23 , 48 , 49 Additionally, it is known that the noise level of FD measurements varies with the source-detector separation distance and is also affected by modulation frequency. 50 , 51 Given that we are modeling source-detector measurements separated by up to 47 mm, the data are in the high light level regime and so we considered the physiological and shot noise based variance as dominating the thermal noise. Thus, we generated herein a noise model that combines separate empirical noise models as functions of source-detector separation and modulation frequency.…”

High optode-density wearable diffuse optical probe for monitoring paced breathing hemodynamics in breast tissue

“…Uncertainties for these (and subsequent) FD-DOS measurements are ∼0.001 mm −1 , based on our prior work. 37 SNR was calculated according to E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 2 ; 1 1 6 ; 6 0 7…”