Method and apparatus for automatic exposure control in ct scanning

A computed tomography apparatus comprising:

• an X-ray source which emits an X-ray beam from a focus, said X-ray source having a tube current which determines a radiation dose associated with said X-ray beam;
• a radiation detector on which said X-ray beam is incident, said X-ray source and said radiation detector being adapted to receive a subject therebetween so that said X-ray beam is incident on said radiation detector after being attenuated by said subject, said radiation detector generating electrical signals dependent on X-rays incident thereon;
• at least said focus being rotatable through at least one revolution around said subject to conduct a CT scan wherein said electrical signals form projection data for a slice of said subject;
• a computer supplied with said projection data for reconstructing an image of said slice therefrom, said image being composed of a plurality of pixels having pixel noise associated therewith;
• a dose monitor disposed in said X-ray beam on which said X-ray beam is incident before being attenuated by said subject, said dose monitor generating a monitor output signal dependent on X-rays in said X-ray beam incident thereon;
• a tube current controller connected to said X-ray tube for adjusting said tube current;
• and
• an exposure controller connected to said radiation detector, said dose monitor and said tube current controller, said exposure controller, during a first half of said revolution, calculating an actual attenuation profile of said slice from said monitor output signal and said electrical signals generated during said first half of said revolution, and calculating an extrapolated attenuation profile for a second half of said revolution from said actuation attenuation profile to achieve a target pixel noise in said second half of said revolution, said exposure controller supplying a control signal, at least at a beginning of said second half of said revolution, to said tube current controller for adjusting said tube current, and thereby modifying said radiation dose, during said second half of said revolution.

A method for operating a computed tomography apparatus comprising the steps of:

• emitting an X-ray beam from a focus of an X-ray source, said X-ray source having at least one operating parameter which determines a radiation dose associated with said X-ray beam;
• detecting said X-ray beam with a radiation detector on which said X-ray beam is incident;
• disposing a subject between said X-ray source and said radiation detector so that said X-ray beam is incident on said radiation detector after being attenuated by said subject, said radiation detector generating electrical signals dependent on X-rays incident thereon;
• rotating at least said focus through at least one revolution around said subject to conduct a CT scan wherein said electrical signals form projection data for a slice of said subject;
• supplying a computer supplied with said projection data and reconstructing an image of said slice therefrom;
• and
• in a control arrangement connected to said radiation detector and to said X-ray source, during a first half of said revolution, calculating an actual attenuation profile of said slice from said electrical signals generated in said first half of said revolution and calculating an extrapolated attenuation profile of said slice for a second half of said revolution from said actual attenuation profile;
• and
• adjusting said at least one operating parameter to modify said radiation dose during said second half of said revolution dependent on said extrapolated attenuation profile.

A method for operating a computed tomography apparatus comprising the steps of:

• emitting an X-ray beam from a focus of an X-ray source, said X-ray source having a tube current which determines a radiation dose associated with said X-ray beam;
• detecting said X-ray beam with a radiation detector on which said X-ray beam is incident;
• disposing a subject between said X-ray source and said radiation detector so that said X-ray beam is incident on said radiation detector after being attenuated by said subject, said radiation detector generating electrical signals dependent on X-rays incident thereon;
• rotating at least said focus through at least one revolution around said subject to conduct a CT scan wherein said electrical signals form projection data for a slice of said subject;
• supplying a computer supplied with said projection data and reconstructing an image of said slice therefrom, said image being composed of a plurality of pixels having pixel noise associated therewith;
• disposing a dose monitor in said X-ray beam on which said X-ray beam is incident before being attenuated by said subject, said dose monitor generating a monitor output signal dependent on X-rays in said X-ray beam incident thereon;
• adjusting said tube current with a tube current controller connected to said X-ray tube;
• and
• in an exposure controller connected to said radiation detector, said dose monitor and said tube current controller, during a first half of said revolution, calculating an actual attenuation profile of said slice from said monitor output signal and said electrical signals generated during said first half of said revolution, and calculating an extrapolated attenuation profile for a second half of said revolution from said actuation attenuation profile to achieve a target pixel noise in said second half of said revolution;
• and
• supplying a control signal, at least at a beginning of said second half of said revolution, from said exposure controller to said tube current controller for adjusting said tube current, and thereby modifying said radiation dose, during said second half of said revolution.