Tracking a moving point source using triple gamma imaging

With positron emission tomography (PET), the positron of a β + emitter radioisotope annihilates with a nearby electron producing a pair of back-to-back 511 keV gamma rays that can be detected in a scanner surrounding the point source. The position of the point source is somewhere along the Line of Response (LOR) that passes through the positions where the 511 keV gammas are detected. In standard PET, an image reconstruction algorithm is used to combine these LORs into a final image. This paper presents a new tomographic imaging technique to locate the position of a β + emitting point source without using a standard PET image reconstruction algorithm. The data were collected with a Proof-of-Concept (PoC) PET scanner which has high spatial and energy resolutions. The imaging technique presented in this paper uses events where a gamma undergoes Compton scattering. The positions and energies deposited by the Compton scattered gamma define the surface of a Compton cone (CC) which is the locus of all possible positions of the point source, allowed by the Compton kinematics. The position of the same point source is also located somewhere on the LOR. Therefore, the position of the point source is defined by the 3 gammas and is given by the intersection point of the LOR and the Compton cone inside the Field of View (FOV) of the scanner. We refer to this method as CC×LOR. This new technique can locate the point source with an uncertainty of about 1 mm, after collecting a minimum of 200 CC×LOR events.