wherein the number of detection collimation units in each detection collimation unit layer are fixedly connected and are distributed around a detected object in a shape of any of a circle, a polygon, an arc and a partial polygon;

wherein the number of detection collimation unit layers are arranged in an incident direction of a gamma ray emitted by a radioactive source and are spaced from each other, and the number of detection collimation unit layers are staggered in a direction perpendicular to the incident direction of the gamma ray emitted by the radioactive source, so that at least one scintillation crystal is contained in the detection collimation unit of the last detection collimation unit layer, and a gamma photon emitted from a point of an imaging field does not pass through any photoelectric device or circuit board material in a transmission path along which the gamma photon is incident and reaches the scintillation crystal of the last detection collimation unit layer,

wherein each of the number of detection collimation units, comprises: a scintillation crystal array configured to receive the gamma photon emitted by a radioactive source in the detected object; and a number of photoelectric devices configured to receive the gamma photon and convert the gamma photon into a digital signal;

wherein the scintillation crystal array comprises a number of scintillation crystals, the number of scintillation crystals are arranged substantially in parallel and are spaced from each other, and each scintillation crystal has a side face configured to receive a ray emitted by the radioactive source and an end face; and

wherein the number of photoelectric devices are coupled to the end faces of the number of scintillation crystals.