Towards a wearable near infrared spectroscopic probe for monitoring concentrations of multiple chromophores in biological tissue in vivo

Abstract

The first wearable multi-wavelength technology for functional near-infrared spectroscopy has been developed, based on a custom-built 8-wavelength light emitting diode (LED) source. A lightweight fibreless probe is designed to monitor changes in the concentrations of multiple absorbers (chromophores) in biological tissue, the most dominant of which at near-infrared wavelengths are oxyhemoglobin and deoxyhemoglobin. The use of multiple wavelengths enables signals due to the less dominant chromophores to be more easily distinguished from those due to hemoglobin and thus provides more complete and accurate information about tissue oxygenation, hemodynamics, and metabolism. The spectroscopic probe employs four photodiode detectors coupled to a four-channel charge-to-digital converter which includes a charge integration amplifier and an analogue-to-digital converter (ADC). Use of two parallel charge integrators per detector enables one to accumulate charge while the other is being read out by the ADC, thus facilitating continuous operation without dead time. The detector system has a dynamic range of about 80 dB. The customized source consists of eight LED dies attached to a 2 mm × 2 mm substrate and encapsulated in UV-cured epoxy resin. Switching between dies is performed every 20 ms, synchronized to the detector integration period to within 100 ns. The spectroscopic probe has been designed to be fully compatible with simultaneous electroencephalography measurements. Results are presented from measurements on a phantom and a functional brain activation study on an adult volunteer, and the performance of the spectroscopic probe is shown to be very similar to that of a benchtop broadband spectroscopy system. The multi-wavelength capabilities and portability of this spectroscopic probe will create significant opportunities for in vivo studies in a range of clinical and life science applications.

Fig. 1

A schematic diagram of the multi-wavelength NIRS system.

Fig. 2

The transient signals observed during switching of the LEDs (volt/div a = 2 V, b = 1 V, c = 0.2 V; time/div =100 ns).

Fig. 3

The multi wavelength LED source and the optical interface. (a) Top view, (b) side view.

Fig. 4

The assembled NIRS probe. (a) The source and detector boards. (b) The internal surface of the assembled probe that is in optical contact with the scalp. (c) The external surface.

Fig. 5

(a) The circuit used for calibration of a red LED employed for data integrity testing of the detector system. (b) The sinusoidal signal received by the detector. (c) The Fourier transform modulus of the received signal. The peak in the spectral intensity occurs at 0.103 Hz.

Fig. 6

The spectral characteristics of the eight LEDs. The peaks of the emission spectra occur at wavelengths of 778, 808, 814, 841, 847, 879, 888, and 898 nm.

Fig. 7

A solid tissue-equivalent phantom containing a rod which can be manually translated back and forth. The rod incorporates a region with ten times the absorption of the surrounding rod and block.

Fig. 8

(a) The variation in the attenuation of received light for each LED as the high absorbing region of the rod is translated through the midpoint of the cavity in the phantom, below the midpoint of the line joining the source and the detector. (b) The observed maximum change in attenuation due to the high absorbing region as a function of wavelength.

Fig. 9

The results obtained from the visual stimulation paradigm. This shows Δ[HbO₂], Δ[HHb], and Δ[oxCCO] derived from the block averaged broadband (a) and (b) 8-wavelength data. The average EEG data recorded over the same interval is shown in (c), and the residuals obtained from a 2-component and 3-component fit to the data acquired with the 8-wavelength system are shown in (d).