Afterglow artifact reduction

Claim 1

A method of reducing afterglow scintillation-caused artifacts in computerized tomographic X-ray images, the said method comprising the steps of: a) directing X-rays from an X-ray source towards a subject from a plurality of positions around the subject; b) using detectors, including scintillators, for converting the X-rays received from the source and traversing the subject during a plurality of measurements ranging from a first measurement to a last measurement n, to light photons both from direct scintillation and from afterglow; c) converting the light photons to measured electrical signals having measured values that are functions of both the direct scintillation and the afterglow; d) operating on the measured electrical signals using a backprojector to provide a tomographic image of the subject; e) wherein said operating step also includes filtering out electrical signals caused by said afterglow occurring in the scintillators using a recursive filter to obtain corrected electrical signals having corrected values, f) said step of filtering out said afterglow further comprises the steps of: g) determining afterglow intensity characteristics by applying a unit step function to the X-ray source to obtain a square wave intensity output from the detectors, said square wave output having an afterglow tail that has an original relative intensity amplitude AMP at the termination of the square wave and a decay time constant τ; h) determining values for the original relative intensity amplitude AMP and the decay time constant τ of the afterglow tail; and i) using the determined values as parameters to adjust the filter for removal of the electrical signals caused by the afterglow intensity to obtain the corrected electrical signals having the corrected values.

Claim 2

A method of reducing afterglow scintillation-caused artifacts in computerized tomographic X-ray images, the said method comprising the steps of: a) directing X-rays from an X-ray source towards a subject from a plurality of positions around the subject; b) using detectors, including scintillators, for converting the X-rays received from the source and traversing the subject during a plurality of measurements ranging from a first measurement to a last measurement n, to light photons both from direct scintillation and from afterglow; c) converting the light photons to measured electrical signals having measured values that are functions of both the direct scintillation and the afterglow; d) operating on the measured electrical signals using a backprojector to provide a tomographic image of the subject; e) wherein said operating step also includes filtering out electrical signals caused by said afterglow occurring in the scintillators using a recursive filter to obtain corrected electrical signals having corrected values, f) said step of filtering out said afterglow further comprises the steps of: g) determining afterglow intensity characteristics by applying a unit step function to the X-ray source to obtain a square wave intensity output from the detectors, said square wave output having an afterglow tail that has an original relative intensity amplitude AMP at the termination of the square wave and a decay time constant τ; h) determining values for the original relative intensity amplitude AMP and the decay time constant τ of the afterglow tail; and i) using the determined values as parameters to adjust the filter for removal of the electrical signals caused by the afterglow intensity to obtain the corrected electrical signals having the corrected values.

Claim 5

A system for reducing afterglow scintillation-caused artifacts in computerized tomographic x-ray images, the said system comprising: (a) a gantry for directing x-rays from an x-ray source towards a subject from a plurality of positions around the subject; (b) detectors including scintillators on said gantry for converting the x-rays received from the source and traversing the subject to light photons both from direct scintillation and from afterglow during a plurality of measurements ranging from a first measurement to a last measurement n, (c) a transducer for converting the light photons to measured electrical signals having measured values that are functions of both the light photons from the direct scintillations and the afterglow; (d) a backprojector for operating on the measured electrical signals to provide a tomographic image of the subject; (e) a recursive filter located prior to said backprojector for filtering said measured electrical signals to remove electrical signals caused by said afterglow occurring in said scintillators to obtain corrected electrical signals having corrected values; said recursive filter comprising: (f) a calibrator for determining afterglow characteristics by applying a unit step function to the x-ray source whereby the detectors provide a source wave intensity output having a large afterglow tail that has an intensity amplitude AMP at the termination of the square wave and a decay time constant, (g) said calibrator further determining the original relative afterglow intensity amplitude (AMP) and the decay time constant τ of the afterglow tail; and (h) a computer for using the determined afterglow intensity value (AMP) and the determined decay time for obtaining component values for said filter that assures that the filter removes the electrical signals caused by the afterglow tail and provides corrected electrical signals having corrected values.

Claim 6

A system for reducing afterglow scintillation-caused artifacts in computerized tomographic x-ray images, the said system comprising: (a) a gantry for directing x-rays from an x-ray source towards a subject from a plurality of positions around the subject; (b) detectors including scintillators on said gantry for converting the x-rays received from the source and traversing the subject to light photons both from direct scintillation and from afterglow during a plurality of measurements ranging from a first measurement to a last measurement n, (c) a transducer for converting the light photons to measured electrical signals having measured values that are functions of both the light photons from the direct scintillations and the afterglow; (d) a backprojector for operating on the measured electrical signals to provide a tomographic image of the subject; (e) a recursive filter located prior to said backprojector for filtering said measured electrical signals to remove electrical signals caused by said afterglow occurring in said scintillators to obtain corrected electrical signals having corrected values; said recursive filter comprising: (f) a calibrator for determining afterglow characteristics by applying a unit step function to the x-ray source whereby the detectors provide a source wave intensity output having a large afterglow tail that has an intensity amplitude AMP at the termination of the square wave and a decay time constant, (g) said calibrator further determining the original relative afterglow intensity amplitude (AMP) and the decay time constant τ of the afterglow tail; and (h) a computer for using the determined afterglow intensity value (AMP) and the determined decay time for obtaining component values for said filter that assures that the filter removes the electrical signals caused by the afterglow tail and provides corrected electrical signals having corrected values.