Phase-amplitude crosstalk in intensity modulated near infrared spectroscopy

Near infrared spectroscopy (NIRS) instruments that rely on phase sensitive detection suffer from what is called "phase-amplitude crosstalk," i.e., the phase measured is dependent on the average light intensity entering the de tector. Changes in detector rise time with input light intensity is the acc epted explanation of this phenomenon. It is concluded here that an addition al simple mechanism can cause phase-amplitude errors, particularly if the r atio of the ac component of the detected signal to the dc component is low. It is shown that the form of the phase distortion encountered during the d evelopment of a new phase sensitive NIR instrument can be modeled by assumi ng the presence of a synchronous interfering signal, due to rf coupling, at the detector output. This modeling allows a required margin between the de tected signal of interest, i.e., the signal from the tissue and the interfe ring signal to be set in order to achieve a measured phase accuracy necessa ry to derive sufficiently accurate clinical parameters. (C) 2000 American I nstitute of Physics. [S0034-6748(00)00405-6].