This year marks the 20th anniversary
of functional near-infrared spectroscopy and imaging (fNIRS/fNIRI). As the vast majority
of commercial instruments developed until now are based on continuous wave technology, the aim
of this publication is to review the current state
of instrumentation and methodology
of continuous wave fNIRI. For this purpose we provide an overview
of the commercially available instruments and address instrumental aspects such as light sources, detectors and sensor arrangements. Methodological aspects, algorithms to calculate the
concentrations
of oxy- and
deoxyhemoglobin and approaches for data analysis are also reviewed.
From the single-location measurements of the early years, instrumentation has progressed to imaging initially in two dimensions (topography) and then three (tomography). The methods of analysis have also changed tremendously, from the simple modified Beer-Lambert law to sophisticated image reconstruction and data analysis methods used today. Due to these advances, fNIRI has become a modality that is widely used in neuroscience research and several manufacturers provide commercial instrumentation. It seems likely that fNIRI will become a clinical tool in the foreseeable future, which will enable diagnosis in single subjects.