The present invention relates to a coincidence counting method of γ rays (gamma rays) in a nuclear medicine diagnostic apparatus, for example positron emission tomography (PET). 
Diagnostic methods and diagnostic apparatuses are known in which a radioactive agent such as fluorodeoxyglucose (FDG) is injected to a patient and annihilation γ rays, which are emitted simultaneously in pairs in opposite directions from the body, are detected to create a functional image (PET image).  In such diagnostic methods and diagnostic apparatuses, γ rays emitted in opposite directions from the body are simultaneously detected by two of a plurality of detectors provided in the diagnostic apparatus, so that the incident direction (emitting direction) of the γ ray is identified and a PET image used for a medical diagnosis is created.  Considering that a detection time is delayed in the detectors, processing is delayed in a detection circuit, and three or more γ rays are not spontaneously detected, an extremely small time window of, e.g., 8 nsecs (8×10−9 seconds) is provided.  When a subsequent γ ray is detected within 8 nsecs after the first γ ray is detected, the first detected γ ray and the subsequently detected γ ray are regarded as being emitted from the same source (γ rays emitted in a pair) and are counted (coincidence counting), and the detection results of the γ rays are used to create a PET image.  On the other hand, when a subsequent γ ray is not detected within 8 nsecs, a PET image is not created based on the detection result of the first detected γ ray (JP-A-11-72566, claims and 0021).  That is, a time window with a predetermined width is set and two γ rays detected in the time window are judged as γ rays generated at the same time. 
γ rays are highly penetrating and hardly interact with substances.  However, in some cases, γ rays interact with water and elements consisting of a detector and are scattered in a living body and the detector, and the γ rays lose some energy thereof (in vivo scattering, Compton scattering).  In this case, the detector does not detect γ rays with energy of 511 keV but detects γ rays with low energy of 200 keV or 400 keV, which is considerably lower than 511 keV. γ rays with low energy may be detected for other reasons.  Moreover, annihilation γ rays of fluorine 18 (18F) has energy of 511 keV.
Conventionally, it is not possible to correctly determine the emitting directions of the γ rays scattered with low energy (scattered ray, etc.)  and thus a threshold value is set to prevent the detection results of low energy γ rays from being used to create an image (JP-A-2003-4853, claims, 0021, FIG.  7).  However, considering that valuable detection results are also obtained from low energy γ rays and thus sensitivity has to be increased without placing a burden on patients and health care workers, attempts have been made to create a PET image using scattered ray in recent years. 
Coincidence counting is performed as described above also when scattered ray is used to create a PET image.  Experiments conducted by the present inventors proved a difference in detection time between a γ ray with normal energy (γ ray in a PP area, which will be described later) and a low energy γ ray scattered by Compton scattering (γ ray in a CS area, which will be described later).  When detection data with a difference in detection time on scattered ray are used, γ rays originally detected at the same time (within a predetermined time window) may be judged as γ rays detected at different times or γ rays not being detected at the same time (within the predetermined time window) may be judged as γ rays detected at the same time.  Further, it was found that when coincidence counting is performed over a wide range from high energy γ rays not being scattered (γ rays in the PP area) to low energy γ rays having been scattered (γ rays in the CS area), the low energy γ rays cannot be used due to the time difference unless the time window widened.  Moreover, a wider time window is not preferable because other γ rays with different sources are more likely to be detected spontaneously. 
Thus, it is an object of the present invention to solve the problems of coincidence counting using scattered ray of low energy.