A positron CT apparatus, i.e. a PET (Positron Emission Tomography) apparatus, is constructed to detect a plurality of gamma rays generated by annihilation of positively charged electrons (positrons), and reconstruct an image of the patient only when a plurality of detectors detect gamma rays at the same time (that is, only when coincidences are counted).
A coincidence counting circuit is used to count coincidences. However, a signal time lag occurs in a signal channel from each detector to the coincidence counting circuit. This time lag is varied with each signal channel. It is therefore necessary to carry out timing correction for adjusting a delay time of each signal channel, so that the gamma rays will have the same timing of arrival at the coincidence counting circuit.
So, in carrying out such timing correction, calibration data is acquired using a radiation source (external radiation source) for calibration or simulated signals, and based on the calibration data, temporal variations in signal transmission are adjusted (see Patent Documents 1-3, for example). In recent years, a technique using time lag information (flight time) (TOF: Time Of Flight) has been proposed for localizing positron pair annihilation events (see Patent Document 4, for example). TOF is a technique for determining pair annihilation events, using that annihilation radiation is at light speed, by converting a time lag in arriving at detectors from the points of pair annihilation event into a distance difference from the points of pair annihilation event to light source generating positions by scintillator elements of the detectors.
The signal timing correction method described in Patent Document 1 noted above is as follows. The detectors detect radiation emitted from a radiation source, and timing signals indicating radiation incidence timing of radiation incident on the detectors are inputted to a coincidence counting circuit through delay adjusting circuits. In response to the input of these timing signals, outputs of the coincidence counting circuit are measured, to measure a sensitivity (that is, a count) of the radiation for each signal channel. Subsequently, the above sensitivity is measured while varying delay amounts adjusted by the delay adjusting circuits, to obtain a sensitivity distribution relative to the delay amount variations. The signal time lag is corrected by using in the delay adjusting circuit having a delay amount resulting in the highest measured sensitivity.
The signal timing correction method described in Patent Document 2 noted above is as follows. A radiation source (external radiation source) for calibration is installed in the field of view (FOV) of a PET apparatus. Here, a plurality of detectors are arranged in a ring (annularly). Regarding a certain detector as reference, timing values of a plurality of detectors sharing a field of view of the reference detector are averaged, and the averaged timing value is determined as a time delay value relative to the reference detector. A detector adjoining the reference detector is regarded as a new reference, and a time delay value is determined similarly. A difference between the time delay value determined first and the time delay value determined next is determined to serve as a reference correction value. Timing correction is carried out by uniforming times using the reference correction value. Subsequently, similar computations are carried out for successively adjoining detectors, to complete the timing correction when one circuit has been made of the ring.
The signal timing correction method described in Patent Document 3 noted above is as follows. Simulated signals outputted from a simulated signal generating device are inputted to a plurality of signal processing devices (signal processing units), respectively. Based on output of each signal processing device, calibration data is generated to carry out timing correction.
In Patent Document 4 noted above, a TOF type PET apparatus has incorporated therein DOI detectors which can discriminate light source positions having caused an interaction in a depth direction (DOI: Depth of Interaction). The DOI detectors are constructed of respective scintillator elements stacked in the depth direction of radiation (gamma rays here). Coordinates information in the depth direction in which the interaction has occurred and a transverse direction (direction parallel to the plane of incidence) is derived from centroid computations. Detection time correction information corresponding to this coordinates information is written to and stored in a table. By referring to the detection time correction information, information accuracy of flight time differences is improved.
[Patent Document 1]
Patent Publication H6-19436
[Patent Document 2]
Specification of U.S. Pat. No. 3,343,122
[Patent Document 3]
Unexamined Patent Publication No. 2006-90827
[Patent Document 4]
Unexamined Patent Publication No. 2008-51701