Spectral-encoding for parallel source implementation in NIR tomography Daqing Piao, Shudong Jiang, Subhadra Srinivasan, Phaneendra K. Yalavarthy, Xiaomei Song, Brian W. Pogue Thayer School of Engineering, Dartmouth College, Hanover NH 03755 Abstract: While near-infrared tomography has advanced considerably over the past decade, key technological designs still limit what can be achieved, especially in terms of imaging acquisition speed. One of these fundamental limitations is the requirement that the source light be delivered sequentially or through frequency encoding of the time signal. Sequential delivery inherently limits the speed at which images can be acquired. Modulation frequency-dependent encoding of the sources solves the problem by allowing sources near the same location to be turned on simultaneously, thereby improving the speed for acquisition, but suffers from dynamic range problems. In this study, we demonstrate an alternative parallel source implementation approach which uses spectral wavelength encoding of the source. This new technique allows many sources to be input into the tissue at the same time, as long as the spectrally encoded signals can be decoded at the output. To test the implementation of this approach, 8 single-mode laser diodes of wavelengths distributed within a narrow range of 10 nm are used, and the lights are all input into tissue phantom simultaneously. On the detection side, a high-resolution spectrometer is used to spatially spread out the signals to facilitate parallel detection of the signal from each spectrally-encoded source. This robust approach allows rapid parallel sampling of all sources at all detection locations. The implementation of this technique in a NIR tomography application is examined, and the preliminary results of video-rate imaging at 30 Hz is presented. 1. Introduction It is well-known that near-infrared (NIR) measurements of the attenuation in between blood and parenchymal tissue have extremely high contrast. Owing to the high degree of vascularity in tumors, there is an elevated hemoglobin content and hence high intrinsic optical contrast between tumor and normal tissue [1-4]. Since the functionality of the tumor vasculature is known to be different from that of normal tissue, acquiring time- resolved functional changes in tissue could provide a fundamentally new way to image tumors. The extremely high contrast of optical signal substantiates the principle that rapid measurements can be obtained for imaging the temporal changes in contrast such as blood volume and oxygenation. Current approaches to rapid imaging have been limited by the need to multiplex the source serially into each channel. Schmitz et al have developed an advanced tomographic NIR imaging system that provides acquisition of complete data sets at a rate of 3 frames per second, where the light is cycled by a rotating mirror into each of 24 channels [5]. While it is very fast, this design is still inherently limited in frame rate, is not significantly faster than the required frame rate needed for imaging blood pulsation. In principle, optical measurements can be acquired mush faster, yet currently there is no tomographic NIR imaging device capable of frame rates greater than a few Hertz. Approaches to parallelize the source light from different spectral lines by using separate heterodyne frequencies for each wavelength have been reported [6]. However, this type of multiplexing has typically been used to parallelize the spectral but not the spatial dimension of the source excitation. This is an important distinction for two reasons. First, as a parallelization paradigm the approach leads to a linear reduction in the dynamic range of the measurement system with each source added; hence, extending it to the spatial dimension which has many degrees-of-freedom in spectroscopic tomography is not Optical Tomography and Spectroscopy of Tissue VI, edited by Britton Chance, Robert R. Alfano, Bruce J. Tromberg, Mamoru Tamura, Eva M. Sevick-Muraca, Proc. of SPIE Vol. 5693 (SPIE, Bellingham, WA, 2005) · 1605-7422/05/$15 · doi: 10.1117/12.594061 129