Multichannel photon counting instrument for spatially resolved near infrared spectroscopy

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Abstract
We have developed a multichannel photon counting instrument for near infrared spatially resolved spectroscopy. The instrument uses two laser diodes at 780 and 830 nm. The two wavelengths are time multiplexed at a rate of 5 MHz for virtually simultaneous measurements at the two wavelengths. The photon pulses are then multiplexed with an optical switch so that the incident optical signals are directed to different source positions. Eight time-correlated channels have been employed for simultaneous acquisition of spectroscopic data at multiple locations. Photon detection is performed by multichannel plate photomultiplier tubes. Robust and accurate data analysis tools to perform deconvolution, data fitting, and absorption change quantification are presented. The instrument has been tested with phantoms simulating tissue optical properties. Absolute optical properties, namely absorption and reduced scattering coefficient, have been determined with an accuracy of ±5%. Quantification of absorption changes can be achieved with an accuracy of 10−3 cm−1 and a sensitivity of the order of 10−4 cm−1. Finally spatially resolved measurements of primary motor-cortex activation have been performed and successfully coregistered with functional magnetic resonance imaging data.