| 摘要 |
An F-kV and an F-mA output from tube-voltage adjuster and tube-current adjuster, respectively, are supplied to exposure-rate controller. Controller compares an input entrance exposure rate determined by the F-kV and the F-mA with a preset exposure rate set in view of a patient's safety. When the former is equal to or less than the latter, controller supplies the input F-kV and F-mA to X-ray tube. When the former exceeds the latter, controller supplies an F-kV and an F-mA corresponding to the preset exposure rate to X-ray tube and a limit signal to auto-iris controller. X-rays output from X-ray tube are transmitted through patient to form an X-ray image. The X-ray image is converted into an optical image by image intensifier tube. After the light quantity of the optical image is reduced by auto-iris, the optical image is picked up by TV camera. TV camera outputs an image signal to TV monitor and brightness detector. Brightness detector detects the brightness of an image displayed on TV monitor, and outputs it to auto-iris controller. In the normal mode wherein no limit signal is supplied, auto-iris controller closes auto-iris based on the operation method and the input size of the field of view of the image intensifier tube. When a limit signal is input, auto-iris controller opens auto-iris such that an output from brightness detector becomes a predetermined value.
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