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Functional imaging of the brain using a portable NIR instrument

Near Infrared Spectroscopy (NIRS) is a powerful method for non-invasive mapping of cerebral functional activation. We have developed an NIRS instrument that is portable, inexpensive and lightweight consisting of an array of light emitting diodes (LEDs) and photodiodes (PDs) mounted on a flexible printed circuit board (PCB). The flexibility and portability of the instrument makes it easy to apply to subjects ranging from premature babies in intensive care to adults. The flexible PCB array consists of 48 LEDs operating at two different wavelengths (780nm & 880nm) together with 14 photodiodes. Transimpedence amplifiers for each of the PDs are located on the flexible pad to minimise noise pick up. The LEDs are pulsed (10ms) at a peak optical power of 20mW, while a sample and hold circuit monitors the voltages at all of the PDs. The array is also encapsulated in black silicon rubber, except for the regions directly above the LED's and PD's, which have a clear silicone rubber cover. The sensor array is attached to the electronics by two one metre long flexible ribbon cables. The monitoring circuit provides medical grade electrical isolation between the patient and computer. Studies have been conducted on phantoms to test the penetration depth of the sensor array for two different separations of LEDs and PDs (11mm and 27mm). The maximum depths that can be probed are 5mm and 11mm respectively. This makes it suitable for studying cortical activation in babies.