X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
2000-03-30
2001-12-25
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S110000, C378S156000
Reexamination Certificate
active
06333965
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an X-ray examination apparatus in which the influence of the X-ray filter on the adjustment of the X-ray examination apparatus is substantially mitigated.
2. Description of the Related Art
Such an X-ray examination apparatus is known from European patent application EP 0 629 105.
The known X-ray examination apparatus comprises an X-ray image intensifier and an image pick-up device. The X-ray image intensifier converts the X-ray image into an optical image. The image pick-up device derives the image signal from the optical image. The brightness control system of the known X-ray examination apparatus comprises an auxiliary photodetector which measures brightness values of a portion of the optical image. This portion is called the measuring field. The brightness control system is arranged to adjust the X-ray examination apparatus on the basis of the measured brightness values of the measuring field. In particular, the energy of the X-rays from the X-ray source is adjusted by the brightness control system. Although the measuring field of the known X-ray examination apparatus is adjustable, it has been found that it is difficult to avoid that a part of the X-ray filter is imaged in the measuring field. Brightness values in the measuring field which correspond to the X-ray filter lead to a sub-optimum adjustment of the X-ray examination apparatus.
The X-ray filter is, for example, adjusted so as to attenuate X-rays which are hardly attenuated by the object to be examined. Notably when a radiological examination of the patient's spinal column is performed, a part of the X-rays which pass through lung tissue are hardly attenuated by the lung tissue which contains a comparatively large amount of air. In such a radiological examination, the X-ray filter is positioned such that the X-rays that are directed towards the portion of the lungs of the patient are attenuated comparatively strongly by the X-ray filter and X-rays that are directed to the patient's spinal column are hardly or not at all attenuated by the X-ray filter. Accurate positioning of the X-ray filter is disclosed in European patent application EP 0 496 438. It has been found, however, that detrimental effects of the adjustment of the X-ray examination apparatus can occur despite of accurate positioning of the X-ray filter.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an X-ray examination apparatus in which the influence of the X-ray filter on the adjustment of the X-ray examination apparatus is substantially mitigated.
This object is achieved by means of an X-ray examination apparatus which includes: an X-ray source for emitting X-rays, an X-ray detector for deriving an image signal from an X-ray image, an X-ray filter which is placed between the X-ray source and the X-ray detector, and a brightness control system for adjusting the X-ray examination apparatus on the basis of the X-ray image, characterized in that the brightness control system is arranged to derive a threshold value from the energy of the X-rays and the composition of the filter, adjust the X-ray examination apparatus on the basis of a portion of the X-ray image having brightness values mainly larger than the threshold value, which threshold value represents the X-ray absorptivity of the X-ray filter.
According to the invention the brightness control system is arranged to adjust the X-ray examination apparatus on the basis of brightness values of the X-ray image that are larger than the threshold value. Brightness values which are less than the threshold value correspond to at least as strong X-ray absorption as caused by the X-ray filter. Such brightness values in the X-ray image that are less than the threshold value pertain to the X-ray filter and do not relate to image information of the object to be examined. Thus, by adjusting the X-ray examination apparatus on the basis of brightness values that are larger than the threshold, detrimental effects of the X-ray filter on the adjustment of the X-ray examination apparatus are substantially avoided. In particular, the adjustment of the X-ray examination apparatus concerns the setting of the energy of the X-rays from the X-ray source. The adjustment of the X-ray examination apparatus may also concern the setting of the gain factor of an amplifier whereto the image signal is applied. The threshold value is derived from the energy of the X-rays and from the composition of the X-ray filter. Hence, it is achieved that the threshold value takes into account the fact that the X-ray absorption by the X-ray filter is dependent on the energy of the X-rays and on the material and the thickness of the X-ray filter. Consequently, an accurate adjustment is achieved and in particular detrimental effects of the X-ray filter are avoided for various values of the X-ray energy. In particular, accurate adjustment is sustained when variations of the X-ray energy occur.
Preferably, the adjustment of the X-ray examination apparatus is performed on the basis of an average value of the brightness values larger than the threshold value. The average is notably taken over brightness values of a selected portion of the X-ray image, which brightness values also exceed the selected portion. The average value is less sensitive to noise, such as X-ray quantum noise, than the brightness values of the X-ray image. Hence, on the basis of the average value a more stable adjustment of the X-ray examination apparatus is achieved.
In a particular embodiment the X-ray detector comprises an X-ray image intensifier and an image-pick up apparatus, such as a television camera. The image intensifier derives an optical image from the X-ray image. Hence, the brightness values of the optical image correspond to brightness values of the X-ray image. Consequently, accurate adjustment of the X-ray examination apparatus is achieved on the basis of brightness values of the optical image larger than the threshold value. Portions of the optical image that pertain to the X-ray filter notably have brightness values less than the threshold value and hence are not taken into account for deriving the adjustment of the X-ray examination apparatus.
Preferably, the X-ray examination apparatus is provided with a photosensor to measure the light intensity in a selected portion of the optical image. The photosensor converts incident light into an electrical current and the photosensor generates an electrical photosensor signal that represents the light intensity in a selected portion of the optical image. For example, the photosensor signal corresponds to the average light intensity in the selected portion of the optical image. The intensity of the X-rays from the X-ray source is controlled by the electrical current applied to the X-ray source; this electrical current is often called the filament current. This filament current heats a cathode of the X-ray source which emits electrons to an anode of the X-ray source. The electrons are accelerated in the electrical field generated by a high voltage applied between the cathode and the anode. As the electrons strike the anode, X-rays are emitted, the intensity being dependent on the filament current and the energy being dependent on the high voltage. The ratio of the signal level of the photosensor signal to the filament current represents the average X-ray absorption in parts of the object to be examined relative to the X-ray absorption of portions of the X-ray filter which are imaged in the portion of the optical image at issue. The ratio of the signal level of the photosensor signal to the filament current is called the absorption ratio. At individual positions in the optical image, i.e. pixels of the optical image, the relative brightness value is the ratio of the brightness value to a reference brightness
Kim Robert H.
U. S. Philips Corporation
Vodopia John F.
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