To validate a new backpropagation algorithm for three-dimensional (3-D) tumor localization in breast tissue with a two-dimensional measurement, we performed experiments on breast phantoms using a heterodyne, frequency-domain photon migration system. The near-infrared (680 or 780 nm) laser diode used to provide illumination was intensity modulated at both megahertz (20-60 MHz) and kilohertz (10 kHz) frequencies. The breast phantoms were made of plastic resin with a variety of simulated tumors imbedded inside and then scanned with the system in a planar geometry. With megahertz modulation signals, both of the amplitude and phase of the transmitted light were used in the data reduction process, whereas with kilohertz modulation signals only the amplitudes were used for the reconstruction. In both cases, the backpropagation reconstruction algorithm was used to accomplish 3-D localization of the imbedded tumors. In all cases, the reconstructed locations for the hidden objects are in good agreement with the true values. Our current work demonstrates the possibility and potential of developing low-cost optical tomographic instruments.
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