Near infra-red spectroscopy: a low cost device

In vivo near infra-red spectroscopy ( NIRS) is based on interrogation of tissue with light, a non invasive measurement. Penetration with near infra-red wavelengths in tissues is greater than with visible light, and specific absorption by compounds relevant for diagnosis and monitoring enable safe and convenient in vivo measurement.. NIRS can be used for the early diagnosis of cerebral pathologies of vascular origin, cortical blood flow monitoring and the analysis of cortical activity. Recent advances in microelectronics make it possible to build small, portable, low-cost NIRS instruments capable of measuring chemical compounds relevant for diagnosis and monitoring. We have developed a low cost NIR module to be used for spectroscopy and imaging. This device is based on emitter-detector modules using laser diodes and PIN photodiodes, overcoming disadvantages of vacuum photomultipliers, and obviates the need for optical fibre connection. We have used intensity modulation with spatially resolved measurements. For measurements of (O2Hb) an (HHb) two wavelengths are sufficient. For each of them we measure demodulation and phase shift and hence the absorption coefficient and the reduced scattering coefficient, from which chromophore concentrations can be determined. Detector signal analysis and sequentially controlled switching is achieved using a Pentium III computer. The module has a cost below a few hundreds US dollars and it is quite small, and can be used for cortical oxygenation maps.