1. A single photon emission computed tomography (SPECT) instrument, comprising: a platform; at least one detector, each disposed at one side of the platform; at least one beam stopper, each disposed at a position between the platform and the at least one detector; a signal processing device, electrically connected to the at least one detector for communicated with the same; and a computing device, electrically connected to the signal processing device for communicated with the same, wherein the at least one beam stopper is configured to absorb beams of radiation during proceeding of SPECT imaging, and wherein each beam stopper is formed as an assembly of two oppositely disposed cones with their tips connected to each other.