Feasibility of direct digital sampling for diffuse optical frequency domain spectroscopy in tissue

Darren Roblyer,Thomas D O’Sullivan,Robert V Warren,Bruce J Tromberg

doi:10.1088/0957-0233/24/4/045501

Abstract

Frequency domain optical spectroscopy in the diffusive regime is currently being investigated for biomedical applications including tumor detection, therapy monitoring, exercise metabolism and others. Analog homodyne or heterodyne detection of sinusoidally modulated signals has been the predominant method for measuring phase and amplitude of photon density waves that have traversed through tissue. Here we demonstrate the feasibility of utilizing direct digital sampling of modulated signals using a 3.6 gigasample/second 12 bit analog to digital converter. Digitally synthesized modulated signals between 50 MHz and 400 MHz were measured on tissue-simulating phantoms at six near-infrared wavelengths. An amplitude and phase precision of 1% and 0.6° were achieved during drift tests. Amplitude, phase, scattering and absorption values were compared with a well-characterized network analyzer-based diffuse optical device. Optical properties measured with both systems were within 3.6% for absorption and 2.8% for scattering over a range of biologically relevant values. Direct digital sampling represents a viable method for frequency domain diffuse optical spectroscopy and has the potential to reduce system complexity, size and cost.

Highlights

Frequency domain diffuse optical spectroscopy instruments have been utilized for a number of biological applications including breast cancer detection [1, 2], chemotherapy monitoring [3,4,5,6], cerebral hemodynamic monitoring [7] and others
Direct comparisons with other published instruments are difficult due to differences in experimental setup, the amplitude and phase precision and accuracy for the digital system (1.02% and 0.59◦ precision, 1.44% and 0.32◦ accuracy) was comparable with other frequency domain optical systems used for biological applications in which these parameters were documented, most of which are approximately 1% for amplitude and 1◦ for phase [7, 18,19,20]
The absorption and scattering precision measured during drift tests was approximately 1.5% and 0.5% respectively

Summary

Introduction

Frequency domain diffuse optical spectroscopy instruments have been utilized for a number of biological applications including breast cancer detection [1, 2], chemotherapy monitoring [3,4,5,6], cerebral hemodynamic monitoring [7] and others. Several groups that have developed clinical or preclinical frequency domain diffuse optical instruments employ heterodyne techniques [2, 10, 12,13,14,15,16] Several of these systems utilize commercially available heterodyne (vector) network analyzers to determine phase and amplitude. Some groups have utilized homodyne detection with CCD imaging by modulating an image intensifier with a reference signal to produce phase sensitive images [19, 20] For all of these methods, one or more modulation frequencies can be used depending on the specifics of the instrumentation and several groups scan through a range of modulation frequencies in attempts to improve fitting and increase signal-to-noise [10]

Methods

Results

Conclusion