Our experimental studies are performed with a CT-analogous photon counting system, as shown in Fig. 1. The system uses a diode laser set consisting of a 660-nm fiber-tailed diode (LPS-660-FC, Thorlabs), specially, for Cy5.5 dye with its peak excitation and emission wavelengths at 670 and 710 nm, respectively, and a laser/temperature control system (LTC100, Thorlabs). The excitation light, from the laser whose intensity is adjusted appropriately by a variable attenuator (FVA-3100, EXFO, Canada), is coupled into a source fiber with core diameter of 62.5 μm and numerical aperture (NA) of 0.22. Then, the output beam is collimated and impinges the boundary of the phantom. The transmitted light is collected by detection fibers of 500 μm core diameter and , evenly distributed from to opposite to the incidence position with their tips being 1 mm apart from the phantom surface, i.e., in a noncontact detection geometry, and coupled into an fiber-optic switch with its output collimated again for normal incidence to a successive motorized filter wheel (FW102B, Thorlabs) that houses a bandpass interference filter (Cy5.5-A Emitter, Semrock) with transmission of over the bandwidth from 696 to 736 nm and rejection ratio of outside the passband. The filtered light, finally, enters into a PMT photon-counting head (H7155-01, Hamamatsu, Japan) coupled with a counting unit (C8855, Hamamatsu, Japan) for the photon-counting detection. A 32-bit counter in the counting unit offers a large dynamic range of measurement that eliminates requirement for neutral density filters. The data acquisition is performed for the emission and excitation signals with and without the filter in place, respectively, as well as for dark signals with the PMT fully blocked. By rotating the phantom at an angular interval and translating it at a vertical displacement, three-dimensional (3-D) spatial sampling process can be achieved with a programmed pattern. The whole experimental set is placed in a dark environment to shield the stray light. In our experiments, the integration times of photon-counting unit were set to 100 and 500 ms for the excitation and emission measurements, respectively. 2-D data-set at middle height of the phantom was acquired with an angular interval of , which provides equally spaced projection angles. This leads to two sets of measurements at the excitation and emission wavelengths, respectively.