X-ray or gamma ray systems or devices – Beam control – Filter
Reexamination Certificate
2000-12-08
2002-09-17
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Filter
C378S156000
Reexamination Certificate
active
06453012
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to the field of X-ray radiography, in particular to an X-ray apparatus with an X-ray source for producing a beam of X-ray radiation, an X-ray detector to detect the X-ray radiation and a filter comprising filter elements containing an X-ray absorbing medium and positioned between the X-ray source and the X-ray detector to absorb the X-ray radiation, in which the X-ray absorption properties of filter elements are individually controllable by changing a length of a column of the X-ray absorbing medium in a propagation direction of the X-ray beam, electrical means being provided, comprising a voltage supply means for the filter elements and an electrical measuring means to measure a quantity which is related to the X-ray absorption properties of individual filter elements.
An X-ray apparatus of this type provided with an X-ray equalization filter to be positioned between the X-ray source and the X-ray detector is known from American U.S. Pat No. 5,778,046. The known X-ray equalization filter comprises an X-ray absorbing mask housed within a shapeable membrane which is positioned in the primary X-ray beam, wherein the local thickness of the mask material can be varied. The known X-ray apparatus can be used in the field of radiodiagnostics, in particular to obtain X-ray transmission images of human body. It is well known that some regions of human body, in particular thorax, have structures of quite different X-ray absorption, which result in pixels on the X-ray detector of different blackening in the X-ray transmission image. The range of the blackening in one X-ray transmission image is defined as dynamic range. The function of the X-ray equalization filter is to compensate the patient regions of low electron density, matching the dynamic range of an X-ray imager in order to obtain a better quality of the X-ray transmission image.
In the known X-ray apparatus a matrix of mechanical pistons is used which can locally modify the length of the column of the X-ray absorbing mask and by doing this, perform a local alteration of the X-ray fluence. The pistons are driven by nickel-titanium wires, wherein the individual length of each nickel-titanium wire is controlled by an electric current applied to the wire. When the electric current is applied to the wire, the wire temperature increases and the wire length decreases. Therefore, there is a certain time necessary for a nickel-titanium wire to shrink or to resume an original length. Further, according to the logistics of the known X-ray apparatus, first a low exposure X-ray image is obtained to calculate the necessary mask profile, then the shaped mask is positioned in the primary X-ray beam. Therefore, there is a considerable time lost for the whole procedure. In the known X-ray apparatus the desired two-dimensional equalization profile of the X-ray absorbing mask is obtained within five seconds. In the field of radiodiagnostics the timing properties of the image acquisition play an important role, especially due to the image artifacts resulting from the movements of the thorax.
Further, the known X-ray equalization filter comprises a limited number of pistons, which is in the order of 8×8 or 16×16. However, for a high quality X-ray diagnostic image this degree of spatial resolution may not be sufficient. Next to this, for the quality assurance purposes and operation control of the X-ray equalization filter it is desirable to have means of a local length detection of the column of the X-ray absorption medium within the filter. In the known X-ray apparatus the position of each piston is sensed during potentiometers which are specifically constructed for that purpose. Due to the movement of the piston with respect to the potentiometer the local resistance in the corresponding electric circuit is varied, whereby it is assumed that the degree of the resistance variation has a direct correlation with the length of the column of the X-ray absorption mask.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide an X-ray apparatus with a dynamic beam attenuation filter with a high degree of spatial resolution and a high operation speed, where the length of column the X-ray absorption medium is directly sensed using a simple electrical circuit. This is achieved in the X-ray apparatus according to the invention, which is characterized in that the filter elements are tube-like structures with an inner volume for containing the X-ray absorbing medium being comprised in a fluid filling having electrically conducting and X-ray absorbing properties, the X-ray absorption of each filter element depending on the length of the column of the X-ray absorbing fluid in the inner volume, each filter element comprising a first electrode for supplying a first electric potential to its wall and a second electrode for supplying a second electric potential to the electrically conducting fluid, the first electrode being separated from the electrically conducting fluid by means of an isolator layer so that an electric capacitance is defined between the first electrode and the electrically conducting fluid, the electrical measuring means being further embodied to measure the electric capacitance in individual filter elements.
In the X-ray apparatus according to the invention the high degree of spatial resolution is achieved by constructing a filter comprising a plurality of tube-like filter elements assembled together, each individual filter element containing an X-ray absorption fluid with an individually controllable level of the X-ray absorption fluid within each filter element, the actual level of the X-ray absorption fluid within the filter element being sensed using simple electrical means. The operation of each filter element is based on the known physical effect of electrowetting.
To explain the operation of the filter element different compositions of the X-ray absorption fluid are to be considered. Firstly, the X-ray absorption fluid with electric conduction and X-ray absorption properties can be obtained by dissolving a salt into a liquid (for example, water). The resulting solution is a single component fluid with both electric conducting and X-ray absorbing properties. Apart from this, it is possible to produce a two-component fluid solution, the first fluid component having exclusively electric conducting properties and a negligible degree of X-ray absorption, the second fluid component having exclusively X-ray absorption properties and being an insulator. The resulting solution is a fluid with both electric conducting and X-ray absorbing properties. It is also possible to produce a two-component fluid from two mutually not mixable fluids, the first fluid having mainly electric conducting properties and a negligible degree of X-ray absorption, and the second fluid having mainly X-ray absorption properties and being an insulator. The resulting fluid will contain two fluid columns on top of each other with the corresponding physical properties attributed to a corresponding fluid component.
The essence of the phenomenon of electrowetting is the variation of the contact angle between the electrically conducting fluid on top of the metal and the metal and thus, the variation in the hydrophobicity of the contact surface. For that purpose each filter element comprises a first electrode located in the wall of the filter element in order to bring an electric potential to the wall. A second electrode is located in the inner volume of each filter element in order to bring a second electric potential to the electrically conducting fluid. The first electrode in separated from the inner wall by means of an isolator layer. For a given composition of the filter element there is a set of operation voltages, namely a ‘fill’ voltage to fill the filter element with the electrically conducting fluid, a ‘hold’ voltage to hold the meniscus of the electrically conducting fluid on the desired level and an ‘empty’
0
voltage to initiate the emptying process of the filter element. The
Dunn Drew A.
Koninklijke Philips Electronics , N.V.
Vodopia John
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