A frequency multiplexed near-infrared topography system for imaging functional activation in the brain

We have developed a novel near-infrared optical topography system that can acquire images of functional activation in the human brain at 10 frames per second using 32 detectors. The image acquisition rate is inversely proportional to the number of detectors, so the maximum acquisition rate using four detectors is 80 Hz. 16 laser diode sources (8 at 785 and 8 at 850 nm) are illuminated simultaneously, and each of 8 avalanche photodiode detectors records light from several sources at the same time. The contribution from each source is demultiplexed in software using fast Fourier transforms. This allows for a more flexible, smaller, and less complex system than is achievable using traditional hardware demodulation techniques, such as lock-in amplifiers. The system will eventually incorporate a total of 64 sources and 32 detectors, enabling the entire adult cortex to be imaged. The system is designed to be as flexible as possible, and to be applicable to a wide variety of experimental and clinical needs. To this end, it can operate in two distinct modes: As a frequency multiplexed system and as a time multiplexed system. We describe phantom and in vivo investigations that have been undertaken using the new instrument in its frequency multiplexed operating mode. (c) 2005 American Institute of Physics.