Tommi Noponen, Helsinki University of Technology

Instrumentation of diffuse optical imaging in the frequency domain

Diffuse optical imaging (DOI) is a new functional medical imaging modality, in which near-infrared light is delivered into the tissue and the transmitted light is measured.

A one-channel frequency-domain instrument for physiological measurements had been developed earlier in the DOI-project in the Laboratory of Biomedical Engineering. In the frequency-domain technique, intensity-modulated light is delivered into the tissue and the modulation amplitude and phase are measured from the transmitted light. When the light transport is modeled in tissue, the optical properties of the tissue can be estimated using the measured data.

Methods for expanding the one-channel instrument to an imaging device were developed in this work. First a commercial fiberoptic switch is presented for extending the number of source fibers from 1 to 16. A computer program was developed for controlling the switch.

The receiving optics was expanded using time multiplexing. A chopper shutter was built for this purpose. The shutter is controlled with a stepper motor so that a single measurement bundle out of a possible four can be selected active. The coupling of the active bundle is 85-90% and the leak of the passive bundles is approximately -26 dB. The shutter system can be used for topographical imaging applications.

In the second part of this work, a four-channel digital lock-in amplifier (LIA) was developed. The LIA is used to measure the amplitude and phase of the signal. The design is based on digital signal processor technology. A DSP program for the LIA was programmed and a four-channel analog-to-digital converter board was designed for the extension of the DSP board. The noise level of the A/D converter board was approximately -86 dB. The LIA was connected to a PC and the performance of the four-channel LIA was compared with a commercial digital LIA (Stanford Research Systems, SR810). The amplitude and phase noise of both devices were approximately the same.