Abstract
Frequency-domain diffuse optical spectroscopy (FD-DOS) utilizes intensity-modulated light to characterize optical scattering and absorption in thick tissue. Previous FD-DOS systems have been limited by large device footprints, complex electronics, high costs, and limited acquisition speeds, all of which complicate access to patients in the clinical setting. We have developed a new digital DOS (dDOS) system, which is relatively compact and inexpensive, allowing for simplified clinical use, while providing unprecedented measurement speeds. The dDOS system utilizes hardware-integrated custom board-level direct digital synthesizers and an analog-to-digital converter to generate frequency sweeps and directly measure signals utilizing undersampling at six wavelengths modulated at discrete frequencies from 50 to 400 MHz. Wavelength multiplexing is utilized to achieve broadband frequency sweep measurements acquired at over 97 Hz. When compared to a gold-standard DOS system, the accuracy of optical properties recovered with the dDOS system was within 5.3% and 5.5% for absorption and reduced scattering coefficient extractions, respectively. When tested in vivo, the dDOS system was able to detect physiological changes throughout the cardiac cycle. The new FD-dDOS system is fast, inexpensive, and compact without compromising measurement quality.
Highlights
Diffuse optical spectroscopy (DOS) has emerged over the past three decades as an important noninvasive method to characterize optical scattering and absorption and, in turn, calculate chromophore concentrations in thick tissue.[1,2] Diffuse optical methodologies can be broadly categorized into three distinct measurement types: time domain, frequency domain (FD), and continuous wave (CW)
This work focuses on Frequency-domain diffuse optical spectroscopy (FD-DOS), which allows for the separation of optical scattering and absorption through the measurement of amplitude attenuation and phase delay of photon density waves generated by temporally intensity modulated light sources at the tissue surface
We recently demonstrated the feasibility of FD-DOS measurements using all digital signal generation and detection using direct digital synthesizer (DDS) integrated circuits and a 3.6-gigasample∕s two-channel analog-to-digital converter (ADC).[16]
Summary
Diffuse optical spectroscopy (DOS) has emerged over the past three decades as an important noninvasive method to characterize optical scattering and absorption and, in turn, calculate chromophore concentrations in thick tissue.[1,2] Diffuse optical methodologies can be broadly categorized into three distinct measurement types: time domain, frequency domain (FD), and continuous wave (CW). We present a new FD-DOS system that overcomes these past limitations by fully integrating digital signal synthesis and detection to rapidly acquire multiwavelength, multiplexed, broadband frequency sweep measurements at a repetition rate of up to 97 Hz. The digital DOS (dDOS) system was validated relative to a gold-standard network analyzer based DOS system and tested in vivo. The digital DOS (dDOS) system was validated relative to a gold-standard network analyzer based DOS system and tested in vivo This new high-speed, broad-bandwidth, digital, miniaturized DOS system minimizes analog circuitry while reducing cost and device footprint, potentially enabling access to more patients. In the clinic and expanding the capabilities of this technology, especially for monitoring fast physiological changes
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