We recently applied time domain near infrared diffuse optical spectroscopy (TD-NIRS) to monitor
hemodynamics of the cardiac wall (oxy and desoxyhemoglobin concentration, saturation,
oedema) on anesthetized swine models. Published results prove that NIRS signal can provide
information on myocardial hemodynamic parameters not obtainable with conventional diagnostic
clinical tools.1 Nevertheless, the high cost of equipment, acquisition length, sensitivity to
ambient light are factors limiting its clinical adoption.
This paper introduces a novel approach, based on the use of wavelength and code division
multiplexing, applicable to TD-NIRS as well as diffuse optical imaging systems (both topography
and tomography); the approach, called WS-CDM (wavelength and space code division mltiplexing),
essentially consists of a double stage intensity modulation of multiwavelength CW laser
sources using orthogonal codes and their parallel correlation-based decoding after propagation
in the tissue; it promises better signal to noise ratio (SNR), higher acquisition speed, robustness
to ambient light and lower costs compared to both the conventional systems and the more recent
spread spectrum approach based on single modulation with pseudo-random bit sequences
(PRBS).2 Parallel acquisition of several wavelengths and from several locations is achievable.
TD-NIRS experimental results guided Matlab-based simulations aimed at correlating different
coding sequences, lengths, spectrum spreading factor, with the WS-CDM performances
on such tissues (achievable SNR, acquisition and reconstruction speed, robustness to channel
inequalization, ...).
Simulations results and preliminary experimental validation confirm the significant improvements
that WS-CDM could bring to diffuse optical imaging (not limited to cardiac functional
imaging).
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