Crosstalk in the backside-illuminated multi-collection-gate (BSI-MCG) image sensor was analyzed by means of Monte Carlo simulation. The BSI-MCG image sensor was proposed to achieve the temporal resolution of 1 ns. In this sensor, signal electrons generated by incident light near the back side travel to the central area of the pixel on the front side. Most of the signal electrons are collected by a collecting gate, to which a higher voltage is applied than that of other collection gates. However, due to spatial and temporal diffusion, some of the signal electrons migrate to other collection gates than the collecting gate, resulting in spatiotemporal crosstalk, i.e., mixture of signal electrons at neighboring collection gates and/or pixels. To reduce the crosstalk, the BSI-MCG structure is modified and the performance is preliminarily evaluated by Monte Carlo simulation. An additional donut-shaped N type implantation at the collection-gate area improves the potential gradient to the collecting gate, which reduces the crosstalk caused by the spatial diffusion. A multi-framing camera based on the BSI-MCG image sensor can be applied to Fluorescence Lifetime Imaging Microscopy (FLIM). In this case, crosstalk reduces accuracy in estimation of the lifetimes of fluorophore samples. The inaccuracy is compensated in a post image processing based on a proposed impulse response method.