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 80Hz. 16 laser diode sources (8 at 785 and 8 at 850nm) 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.

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A portable topographic near-infrared spectroscopic (NIRS) imaging system has been developed to provide real-time temporal and spatial information about the cortical response to stimulation in unrestrained infants. The optical sensing array is lightweight, flexible, and easy to apply to infants ranging from premature babies in intensive care to children in a normal environment. The sensor pad consists of a flexible double-sided circuit board onto which are mounted multiple sources (light-emitting diodes) and multiple detectors (p-i-n photodiodes), all electrically encapsulated in silicone rubber. The control electronics are housed in a box with a medical grade isolated power supply and linked to a PC fitted with a data acquisition card, the signal acquisition and analysis being performed using LABVIEW™. The signal output is displayed as an image of oxy- and deoxyhemoglobin concentration ([HbO2], [Hb]) changes at a frame rate of 3 Hz. Experiments have been conducted on phantoms to determine the sensitivity of the system, and the results have been compared to theoretical simulations. The system has been tested in volunteers by imaging changes in forearm muscle oxygenation, following blood pressure cuff occlusion to obtain typical [Hb] and [HbO2] plots.