Giovanni G. Guizzetti

Microcontroller-based digital front-end for near-infrared spectroscopy

Near IR Spectroscopy (NIRS) can be employed to noninvasively and continuously measure in-vivo local changes in haemodynamics and oxygenation of human tissues. Monitoring of these parameters is particularly useful both for basic research and during surgery, when a continuous and real-time measurement can help to avoid permanent damage to the tissues. We present a modular acquisition system in which each subsystem, from the case to the single acquisition front-end is designed to meet all the requirements of a research-grade instrument, dedicated to intraoperatory measurements. Part of the modules of the acquisition system has been dedicated to multipoint NIRS. A module prototype has been developed, which is able to control four LED sources and two detectors. On each front-end a RISC microcontroller performs source and detector multiplexing with a digital correlation technique. A number of such modules can be independently addressed through a bus by a PC-based workstation for data collection, processing and visualization. Preliminary test of the prototype on tourniquet-induced forearm ischemia show adequate detectivity and time response. The operating parameters derived from the prototype will be employed in the design of a high channel count module, which will exploit the capabilities of a digital signal processor, for spatially mapped brain oxygenation monitoring.

Simple instrument for the characterization of diffuse reflectance

An apparatus for the measurement of the radial dependence of the continuous-wave diffuse reflectance of a tissue is described. It consists in a probe, which is applied on the tissue, and a detection unit. By employing an array of incoherent semiconductor light sources (LEDs) and continuous- wave detection, probe structure is considerably simpler than other devices described in literature, allowing moreover an efficient coupling of the emitted light towards the tissue. The high responsivity so obtained permits fast and accurate measurements. Measurement speed, probe compactness and accuracy are potentially sufficient for the in-vivo application of the method to surgically exposed tissue. A preliminary set of data, measured on a scattering phantom and on non-exposed in-vivo tissue, is presented. Even though available models fitted to the measured data give the correct order of magnitude for light transport coefficients, in order to extract reliable absolute values they should be corrected for probe nonidealities. The availability of extensive high- quality in-vivo data is to this regard stimulating for further theoretical investigations.

Near-infrared spectroscopy study of tourniquet-induced forearm ischaemia in patients with coronary artery disease

Near-Infrared Spectroscopy (NIRS) can be employed to monitor local changes in haemodynamics and oxygenation of human tissues. A preliminary study has been performed in order to evaluate the NIRS transmittance response to induced forearm ischaemia in patients with coronary artery disease (CAD). The population consists in 40 patients with cardiovascular risk factors and angiographically documented CAD, compared to a group of 13 normal subjects. By inflating and subsequently deflating a cuff placed around the patient arm, an ischaemia has been induced and released, and the patients have been observed until recovery of the basal conditions. A custom NIRS spectrometer has been used to collect the backscattered light intensities from the patient forearm throughout the ischaemic and the recovery phase. The time dependence of the near-infrared transmittance on the control group is consistent with the available literature. On the contrary, the magnitude and dynamics of the NIRS signal on the CAD patients show deviations from the documented normal behavior, which can be tentatively attributed to abnormal vessel stiffness. These preliminary results, while validating the performance of the IRIS spectrometer, are strongly conducive towards the applicability of the NIRS technique to ischaemia analysis and to the endothelial dysfunction characterization in CAD patients with cardiovascular risk factors.