| 摘要 |
PURPOSE:To reduce the missing of simultaneous counting otherwise causing non-linearity in the accidental simultaneous counting rate and the counting rate of a positron CT apparatus as probable noise component by decreasing a part of a dissipated gamma ray from the perimeter of a view having position information of a ray source with a member for absorbing gamma ray with a larger wavelength than it inserted between scintillators. CONSTITUTION:The chart shows changes in the simultaneous counting rate when the length of a tungsten plate is varied with a gamma ray incidence angle theta=25 deg. for example, employing tungsten for a gamma ray absorbing member. The simultaneous counting rate was reduced by about 10% when the tungsten plte was set at the reference length +4mm. in the length and 1mm. in the thickness and about 40% when the length of the tungsten plate exceeds the reference length +12mm. as compared with the case without the tungsten plate. The reference length +12mm. is found to be the optimum length of the tungsten plate. When the length of the tungsten plate was less than the reference length +4mm., the reduction rate in the simultaneous counting rate is small in the perimeter of the view. No substantial advantage was noted. The tungsten plate proved appropriate when it was 0.5-2mm. in the thickness and projected by more than 4mm. from the surface of scintillators in the length. Even in the case of a positron CT, the projection of the tungsten plate by more than 4mm. from the surface of the scintillators is expected to yield an effect of reducing the simultaneous counting rate in the perimeter of the view. |
