IMPROVEMENT OF DEPTH SENSITIVITY TO CEREBRAL HEMODYNAMICS W/ TIME GATED SYSTEM

Massachusetts General Hospital, Boston MA

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Abstract

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. By recording the distribution of times of flight of photons, Time Domain (TD) systems intrinsically provide more information that continuous wave (CW) ones. In particular, they enable depth discrimination at a single source-detector (SD) separation. This is of particular interest for functional brain imaging, where cortical activation is often hidden by superficial systemic response. Our TD device is based on a Ti:Sapphire pulsed laser and an intensified CCD camera (ICCD). We have developed a probe combining depth sensitivity and 2D imaging. It consists in 2 halves [unreadable]one per hemisphere [unreadable]each with 4[unreadable]4 sources and 3[unreadable]3 detectors in a square geometry (SD = 2.5 cm). Each detector consists of 7 fibers of different lengths, for parallel detection at 7 delays. All 126 fibers are imaged in parallel on the ICCD array. The 32 sources are illuminated sequentially, with 4 sets of 8 sources that are turned on during the same CCD frame without causing significant cross-talk. This source time-multiplexing and the parallel detection allow for an imaging frequency of almost 2 Hz for the whole head.