X-ray or gamma ray systems or devices – Beam control – Filter
Reexamination Certificate
2001-04-12
2002-09-17
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Filter
C378S156000
Reexamination Certificate
active
06453013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an X-ray apparatus which is provided with an X-ray source for producing X-rays, an X-ray detector for detecting the X-rays, and a filter which is arranged between the X-ray source and the X-ray detector and includes a plurality of tubular filter elements for receiving an X-ray absorbing, electrically conductive liquid filling, first means being provided for applying an electric voltage to individual filter elements, an X-ray absorptivity of the individual filter elements being adjustable by control of a quantity of X-ray absorbing liquid filling present within the individual filter elements.
The invention also relates to a filter for use in the X-ray apparatus.
2. Description of Related Art
An X-ray apparatus of this kind is known from U.S. Pat. No 5,625,665 (PHN 15.044). The known X-ray apparatus includes a filter which includes a plurality of filter elements having an individual absorptivity, said absorptivities being dependent on a level of an X-ray absorbing and electrically conductive absorption liquid that is present within the filter element. The X-ray apparatus is used inter alia for medical diagnoses during which a patient to be examined is arranged between the X-ray source and the X-ray detector in order to image internal structures. Thanks to the fact that the patient has structures of different electron density, regions of different density are observed in a resultant X-ray image. The degree of difference in density between the regions of the extremes of the density in one X-ray image is defined as the dynamic range. The filter serves to limit the dynamic range per X-ray image.
In order to limit the dynamic range of the X-ray image, the known X-ray apparatus includes a filter with a matrix of filter elements for receiving an absorption liquid. The known filter consists of a matrix of filter elements, the filter elements being connected to a corresponding, common supply duct for the supply of an absorption liquid to the corresponding filter elements. The known apparatus utilizes the phenomenon that a contact angle between an electrically conductive liquid and an electrode that is isolated therefrom changes when a potential difference is applied between the electrically conductive liquid and the electrode. This phenomenon is known as electrowetting. In order to realize this intended functional property, each filter element is provided with a first electrode which is arranged in a wall of the filter element in order to apply an electric voltage to the wall of the filter element. A second electrode is in contact with the liquid filling. The known apparatus operates as follows: in the presence of a first value of the electric voltage (filling voltage) the adhesion of the liquid filling to the inner wall is increased so that the relevant filter element is filled with the liquid filling from the supply duct. In response to a second value of the electric voltage (drain voltage) the adhesion decreases and the liquid filling is drained from the filter element to the supply duct. Filter elements are adjusted to a high X-ray absorptivity by filling with the X-ray absorbing liquid filling; filter elements are adjusted to a low X-ray absorptivity by keeping them empty.
The mediocre reproducibility of the height of the column of the liquid filling constitutes a drawback of the known filter. The reproducibility of the height of the column of the liquid filling in a filter element is influenced by the intrinsic properties of a filter element, for example, by varying material properties of the filter element itself, and also by external factors, for example an orientation of the filter as a whole. It has been demonstrated that after a large number of electrical switching operations (more than 1000) a change occurs in the material properties of the first electrode in the filter element, which change is relevant to the height of the column of the liquid filling in the filter element.
Furthermore, the known filter is mounted on a gantry of the X-ray examination apparatus. It is known that the exposures of a patient take place while the gantry of the X-ray apparatus is in a rotated position; the gantry thus encloses different angles relative to the direction of the force of gravity. As a result of these rotated positions, changes occur in the hydrostatic pressure in the common liquid supply duct, which changes affect the reproducibility of the height of the column of the absorption liquid in the internal volume of the filter elements.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide an X-ray apparatus in which the reproducibility of the height of the column of the liquid filling in the filter elements is ensured.
The X-ray apparatus in accordance with the invention is characterized in that the filter is also provided with a hydrostatic pressure control system for controlling a hydrostatic pressure in the filter elements, measuring means being provided for measuring a physical quantity related to the height of a column of the X-ray absorbing liquid filling within the filter elements, and with control means for controlling the hydrostatic pressure control system, the height of the column of the X-ray absorbing liquid filling in an individual filter element being determined by the combination of the hydrostatic pressure and the electric voltage applied to the filter element.
The filter in accordance with the invention is provided with a hydrostatic pressure control system in order to enable complete compensation of a change of the hydrostatic pressure in the common liquid supply duct due to a change of the orientation of the filter as a whole. A hydrostatic pressure control system is known per se from WO 99/38172. The hydrostatic pressure control system in the known device, however, serves a different purpose, that is, the addressing and filling of the filter elements with the X-ray absorbing liquid. The filter in accordance with the invention is also provided with second means which are arranged to measure a physical quantity that is related to the height of the column of the X-ray absorbing liquid filling within the filter elements. This step is taken so as to render the reproducibility of the height of the column of the X-ray absorbing liquid in a filter element independent of the secondary effects influencing the operation of the filter as a whole. Such secondary effects are, for example, a change of the physical properties of filter elements that is caused by the number of switches of the electric voltage applied to the first electrode, and also an orientation of the filter as a whole relative to the force of gravity. The influence of such secondary effects on the filter can be compensated by measuring a physical quantity that is related to these secondary effects. To this end, a first embodiment of the filter in accordance with the invention is characterized in that the measuring means are arranged to measure a reference electric voltage in at least one reference filter element. It has been demonstrated that a relationship exists between a resultant height of the column of the X-ray absorbing liquid in the filter element and the corresponding electric voltages applied to the filter element. In order to compensate the above-mentioned variation of the material properties, use is made of one or more reference elements that are representative of the entire system of filter elements. It is possible to select a filter element within the matrix of filter elements and to calibrate it so as to define a relationship between the electric voltage applied to the filter element and the resultant height of the column of the X-ray absorbing liquid in the filter element in a reference situation, for example, for a gantry angle amounting to zero degrees.
As the material properties of filter elements change, the relationship between the resultant column height of the liquid filling in the filter element and the electric voltage applied to the filter element and determined a priori als
Dunn Drew A.
Koninklijke Philips Electronics , N.V.
Vodopia John
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