Frequency-domain near infrared spectroscopy instrumentation: A simplified design using a high frequency lock-in amplifier

Summary form only given. Use of intensity-modulated lasers to probe tissue optical properties is a non-invasive technique in which the intensity of the laser output intensity is sinusoidally modulated (also called frequency modulated, phase modulated, phase resolved or frequency domain method). The detected transmitted light signal is quantified for its parameters such as DC and AC (at the carrier frequency) components attenuation and the phase shift of the AC signal, to analyze the absorption and scattering properties, and subsequently the oxygenation state, of the tissue sample. This methodology has been adopted for near infrared imaging, both transilluminationally and tomographically. This paper presents a new design in which an RF lock-in amplifier is used to simplify the phase detection system. In order for the instrument to be used in fetal and neonatal brain monitoring, penetration of 6 cm within tissue is required, hence 600 mW high power diode lasers, irradiating only for a short period of time, are used to ensure adequate signal-to-noise ratio without approaching the safety limit.