A SOC Based high resolution and portable near infrared spectroscopy (NIRS) system combined with MIMO and CDMA technologies

Abstract

The NIRS produces functional brain images by analyzing the hemodynamic responses measured from neighboring pairs of sources and detectors. The spatial resolution of the NIRS is determined by the distance between the source and detector, which should be at least 2cm in order to detect the light passing deep into the cerebral layer. This being the case, the resolution of the conventional NIRS, in which a single channel source and detector are placed alternately, is limited by the minimum separation requirement of the sources and detectors. The placement of multiple source and detector pairs in a 2cm region with alternating extractions of the data in the time domain can improve the spatial resolution. However, the spatial resolution trade-offs with the temporal resolution in this case. We propose an efficient data extraction method for increased spatial resolution in the NIRS without sacrificing the temporal resolution. The performance improvement is attributed to high-signal-to-noise-ratio (SNR) receivers, a modulation scheme, and a MIMO based data extraction algorithm without hardware overhead. The proposed system shows a 2x over increment in the figure of merit (FOM) compared to conventional designs and the experimental results support the validity of the proposed system. In circuit implementation, we use simplified structures to decrease IC size with SNR improvement. The single channel detecting circuit is composed by low noise TIA with AC coupling passive components; the matched filter with current DAC based the programmable gain amplifier (PGA) and the single slope ADC. Each structure is differential structure to eliminate common noise term. The matched filter synchronized at input signal to increase SNR in gaussian channel. The integrator in the matched filter integrates charge in the large output capacitor proportional to input signal. This integrated charge is continuously discharged by cur-rent substractor as the counter count the discharge time until t...