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Molecular imaging aims to better understand biochemical processes at the molecular level in living organisms. Among emerging imaging modalities, diffuse optical tomography (DOT) coupled with the use of fluorescent bio-markers proposes solutions to visualize such processes. The Universite de Sherbrooke is developing a 128-channel DOT scanner dedicated to small animal imaging. The scanner is based on ultra-fast time-resolved optical measurements using time-correlated single photon counting (TCSPC). This paper reports a preliminary design of a high resolution 4- channel time to amplitude converter (TAC) used for time of flight optical measurements, in CMOS 0.18 mum technology. The TAC achieves a 5 ps resolution in a 13 ns timing window, and has a 150 ns dead time. It includes all auxiliary electronics: D-flip-flop, current source, integrator, switches and fully balanced buffer able to drive an off-the-shelf free running 50 Msps analog to digital converter (ADC).