X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-03-12
2001-04-24
Church, Craig E. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S098800
Reexamination Certificate
active
06222901
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an X-ray examination apparatus, including an image sensor matrix for picking up an X-ray image, and a correction unit for correcting disturbances in the primary image signal.
2. Description of the Related Art
An X-ray examination apparatus of this kind is known from European Patent Application EP 0 642 264, which corresponds to U.S. Pat. No. 5,530,238.
The known X-ray examination apparatus includes a correction unit for counteracting disturbances in the electronic image signal which are caused by delayed reading of charge. Incident X-rays release charge carriers, notably photoelectrons in the image sensor matrix and the X-ray image is thus picked up. A part of said photoelectrons can be intercepted in a trap state, be retained therein for some time, and can escape at a later stage, for example due to thermal excitation, and enter the reading circuit as electric charges read with a delay. If the primary image signal with the disturbances were applied to a monitor for displaying the image information, not only the image information of the instantaneous image would be reproduced, but at the same time also image information of a previously picked up image. As a result, the observer will perceive after-images superposed on the instantaneous image. The correction unit of the known X-ray examination apparatus utilizes a physical model for the interception and subsequent escape of the photoelectrons so as to correct disturbances due to electric charges read in delayed fashion. It is a drawback that the known X-ray examination apparatus requires a rather large image memory capacity for storing image information of a number of preceding images and/or for storing correction values for a long series of X-ray images as well as for different circumstances in which such X-ray images are formed. It is a further drawback that the correction unit of the known X-ray examination apparatus requires a powerful arithmetic unit for executing the desired correction on the basis of the image information stored. Because elaborate and complex operations are performed on a large number of data in the known X-ray examination apparatus, a rather long period of time is required to correct the primary image signal in respect of after-images. The long processing time prolongs the time elapsing between the picking up of the X-ray image and the reproduction of the image information in the X-ray image, for example on a monitor. Therefore, the known X-ray examination apparatus is not suitable for non-disturbed reproduction of the image information in an X-ray image quickly after it has been picked up. This concerns notably disturbances due to the superposition of image information of a previously picked up X-ray image on the image information to be reproduced, said superposed information being reproduced as an after-image together with the image to be reproduced. Consequently, the known X-ray examination apparatus is notably not suitable for reproducing a series of non-disturbed X-ray images in rapid succession.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an X-ray examination apparatus which, in comparison with the known X-ray examination apparatus, requires only a short period of time for the correction of the primary image signal.
This object is achieved by means of an X-ray apparatus according to the invention which is characterized in that it includes a control circuit for controlling the image sensor matrix so as to form a dark signal, and that the correction unit is arranged to derive an electronic image signal from the primary image signal and the dark signal.
The x-ray examination apparatus comprises an X-ray source for emitting an X-ray beam in order to form the X-ray image of an object. The image sensor matrix further includes radiation-sensitive elements for converting X-rays into electric charges, and a reading circuit for deriving a primary image signal from the electric charges. The radiation-sensitive elements are, for example semiconductor elements, preferably made of &agr;-Si:H, in which electric charge carriers are released by absorption of X-rays, notably electron-hole pairs. Radiation-sensitive elements made of a photoconductor material sensitive to X-rays, such as selenium or lead-oxide, are also suitable. The individual radiation-sensitive elements may also include a photosensitive element, such as a photodiode or a phototransistor, in combination with a scintillator. The scintillator converts incident X-rays into longwave radiation, for example infrared or ultraviolet radiation or visible light. The longwave radiation is electromagnetic radiation of a wavelength which is substantially longer than that of X-rays. The construction of the scintillator is preferably chosen to be such that the sensitivity of the photosensitive elements to the longwave radiation is considerable and preferably as high as possible. The photosensitive element converts the long-wave radiation into electric charge. Use can also be made of a scintillator in the form of a scintillator layer which is common to a group of several photosensitive elements. Further details of an image sensor matrix suitable for picking up an X-ray image are disclosed in the European Patent Applications EP 0 440 282, EP 0 440 720, and French Patent Application FR 2 593 343.
The dark signal is formed by reading the image sensor matrix in the absence of incident X-rays. The signal level of the dark signal represents electric charges which have stayed behind in the radiation-sensitive elements, for example after a previous exposure, or charges which have been produced therein by thermal excitation. Correction of the primary image signal for disturbances caused by electric charge read with a delay is then comparatively accurately performed by subtracting the dark signal from the primary image signal. Correction is performed by deriving the electronic image signal from the dark signal and the primary image signal, which electronic image signal is substantially free from disturbances, notably disturbances due to electric charge read with a delay. The correction accuracy is higher as the period of time elapsing between the formation of the dark signal and the primary image signal is chosen to be shorter. The primary image signal is corrected not only in respect of electric charges read with a delay, but also in respect of dark current caused by thermal excitation. Moreover, the primary image signal is corrected in respect of perturbations that are caused by delayed release of longwave length radiation by the scintillator. Because correction requires only comparatively simple calculations which are executed on a comparatively limited number of data, only a short period of time is required to perform the correction of the primary image signal. Moreover, the correction requires neither a large storage capacity nor a complex arithmetic unit.
A preferred embodiment of an X-ray examination apparatus according to the invention is characterized in that the control circuit is arranged to control the image sensor matrix so as to form the dark signal prior to the X-ray pulse and to form the primary image signal after the X-ray pulse has ceased.
No X-rays are incident on the image sensor matrix just before the start of the X-ray pulse, so that it is achieved that the dark signal has a signal level which represents notably residual electric charge. Such a residual electric charge has been formed, for example during a previous X-ray pulse. It is notably if such a previous X-ray pulse had a high intensity and/or high energy that an electric charge will remain in the radiation-sensitive elements after the previous X-ray image formed by said previous X-ray pulse has been read from the image sensor matrix.
A further preferred embodiment of an X-ray examination apparatus according to the invention is characterized in that the control circuit is arranged to control the image sensor matrix so as to form dark signals separately prior to individual X-ray pulses and to f
Alving Peter L.
Luijendijk Johannes A.
Meulenbrugge Hendrik J.
Stouten Johannes J.
Church Craig E.
U.S. Philips Corporation
Vodopia John F.
LandOfFree
X-ray examination apparatus including an image sensor matrix... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with X-ray examination apparatus including an image sensor matrix..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and X-ray examination apparatus including an image sensor matrix... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2528800