a body, a head and a top that are arranged in sequence;

wherein the body is composed of a plurality of body module rings, the head is composed of N head module rings, and the top is composed of a plurality of top PET detection modules; where N is a natural number and the number is at least 2;

each of the body module rings is composed of several body PET detection modules evenly distributed in a circumferential direction, and a detection surface of each body PET detection module is arranged facing an interior of the body; all the body module rings are arranged in an axial direction to form the body;

among the N head module rings, a size of the rings decreases in sequence from a first head module ring to a N^th head module ring, and the N head module rings are arranged in the axial direction in the order from the first head module ring to the N^th head module ring to form the head; each of the N head module rings is composed of several head PET detection modules; and

detection surfaces of the plurality of top PET detection modules are located in the same plane or approximately in the same plane, and the detection surfaces of all the top PET detection modules are oriented in a direction toward the head or the body,

wherein:

the installation of the body is implemented using a first support method or a second support method;

in the first support method, the body is installed between a left body support plate and a right body support plate, and a body guide column frame is installed between the left body support plate and the right body support plate; for the body guide column frame, several lateral horizontal guide columns and approximately circular guide columns connected end to end in a vertical plane are arranged cross each other to form an overall hollow cylindrical body guide column frame and several inter-column positions suitable for installation of body PET modules, and each body PET detection module is installed in the inter-column position;

in the second support method, the body is installed between a left body support plate and a right body support plate, and a body guide column frame is installed between the left body support plate and the right body support plate; for the body guide column frame, several lateral horizontal guide columns and approximately circular guide column loops connected end to end in a vertical plane are arranged cross each other to form an overall hollow cylindrical body guide column frame and several inter-column positions suitable for installation of body PET modules, and each body PET detection module is installed in the inter-column position; and at least one of the guide column loops is replaced by a body intermediate support plate;

the installation of the head is implemented using a third support method or a fourth support method;

in the third support method, the head is installed between a left head support plate and a right head support plate; the first head module ring is installed between the left head support plate and a first head intermediate support plate, a M^th head module ring is installed between a (M−1)th head intermediate support plate and a M^th head intermediate support plate, where 2≤M≤N−1 and M is an integer, and the N^th head module ring is installed between a (N−1)^th head intermediate support plate and the right head support plate; each head PET detection module is installed on two adjacent head guide columns, and each of the head guide columns is parallel to an axis of the head;

in the fourth support method, a right side of a M^th head module ring is perpendicularly installed on a M^th head intermediate support plate, where 1≤M≤N−1 and M is an integer, and a left side of the N^th head module ring is installed on a (N−1)^th head intermediate support plate; each head PET detection module is installed on two adjacent head guide columns, and each of the head guide columns is parallel to an axis of the head;

the installation of the top is implemented using a fifth support method or a sixth support method;

in the fifth support method, the top is fixed on one surface of a vertically arranged top support plate;

in the sixth support method, the top is fixed on a second plate, and the second plate is embedded with a nut seat which cooperates with a lead screw driven by a motor, so that the top can be moved in parallel when driven by the motor; and

except for the top support plate, all the other support plates have circular or approximately circular through holes.