X-ray or gamma ray systems or devices – Beam control – Filter
Reexamination Certificate
1999-08-03
2001-03-06
Church, Craig E. (Department: 2876)
X-ray or gamma ray systems or devices
Beam control
Filter
C378S158000
Reexamination Certificate
active
06198806
ABSTRACT:
The invention relates to an X-ray examination apparatus for forming X-ray images of an object, including
an X-ray source for generating an X-ray beam,
an X-ray filter which is provided with filter elements, a wall of which is provided with an electrically conductive layer and an electrically insulating coating layer, the filter elements containing a first liquid, the quantity of which can be adjusted so as to adjust an intensity profile on the object, and
an X-ray detector for detecting X-ray images.
The invention also relates to an X-ray filter for use in such an X-ray examination apparatus.
A device of the kind set forth is described in international patent application WO 96/13040. The X-ray filter in the known X-ray examination apparatus is used to limit the dynamic range of an X-ray image, formed on the X-ray detector, of an object, for example a human or animal body to be examined. The filter elements of the X-ray filter are constructed as capillary tubes, one end of which communicates with the first liquid. The X-ray absorptivity of the liquid can be increased by using aqueous solutions of salts of, for example lead, cesium or tungsten. The adhesion of the first liquid to the electrically insulating coating layer is adjustable by way of an electric voltage which is applied across the electrically conductive layer and the first liquid. The filling of each of the capillary tubes can thus be adjusted by variation of the electric voltage, so that the X-ray absorption profile of the X-ray filter is adjusted within a short period of time, for example 0.4 seconds. A voltage adhesion characteristic of a capillary tube of the X-ray filter represents a relationship between the electrically insulating coating layer and the first liquid in the capillary tube and the voltage applied across the electric conductor and the first liquid. It is a drawback of the known X-ray filter that the voltage adhesion characteristic is liable to change so that, for example the service life of the X-ray filter is reduced.
It is an object of the invention to provide an X-ray examination apparatus in which the changing of the voltage adhesion characteristic is counteracted. To this end, an X-ray examination apparatus according to the invention is characterized in that the filter elements contain a second liquid at an interface of the electrically insulating coating layer with the first liquid, the second liquid being inert relative to the first liquid and having an electrical conductivity which deviates from that of the first liquid. Attractive embodiments of the X-ray examination apparatus are defined in the dependent Claims. The invention is based on the recognition of the fact that a three-phase system is thus formed in which the electrically insulating coating layer, the first liquid and the second liquid maintain their respective phase and the first and the second liquid constitute a thermodynamically stable two-phase system. Furthermore, the chemical interaction with the electrically insulating coating layer and the first liquid is reduced. The chemical interaction occurs because in comparison with the remainder of the capillary tube, a high electric field strength arises near the interface, for example a contact line between the first liquid and the electrically insulating coating layer. This high field strength is due to the small radius of curvature along the contact line. Because of the high field strength, the electric field induces a chemical reaction at the interface so that atoms or ions of the first liquid are left behind in the coating layer. Because the second liquid is inert relative to the coating layer, the chemical reaction induced by the electric field is less than in the known X-ray examination apparatus, without using the second liquid, and the changing of the voltage adhesion characteristic is counteracted. An advantage of a reduced changing of the voltage adhesion characteristic consists in that the service life of the X-ray filter can be prolonged to, for example 10
5
switching cycles. It is a further advantage of the presence of the second liquid that hysteresis in the voltage adhesion characteristic is reduced.
A special embodiment of the X-ray examination apparatus according to the invention is characterized in that the filter elements also contain a gas. When the filter elements are partly filled with, for example air, a lower flow resistance of the first liquid in the filter element is achieved, so that the response time of the X-ray filter is reduced.
A further embodiment of the X-ray examination apparatus according to the invention is characterized in that the first liquid contains a polar liquid with a solution of salts or that it contains a liquid metal. It is an advantage of polar liquids that salts are readily soluble therein, so that it is simply possible to adapt the electrical conductivity and to compose an X-ray spectrum so that the absorption spectrum of the first liquid can be adapted to the spectrum of the X-ray source or the spectrum of the X-ray detector. An example of a polar liquid is an aqueous solution of lead nitrate, Pb(NO
3
)
2
or lead perchlorate, Pb(ClO
4
)
2
. It is an advantage of liquid metals that they have a high density, and hence a high X-ray absorptivity, in comparison with, for example aqueous solutions. The use of liquid metals is described in European patent application EP 98201706.3.
A further embodiment of the X-ray examination apparatus according to the invention is characterized in that the second liquid contains an apolar non-fluoridized hydrocarbon compound or a silicon oil. The miscibility of these liquids with polar liquids is low; they are chemically inert and have a low electrical conductivity in comparison with the first liquid. In the context of the present application electrically conductive is to be understood to mean a specific conductivity &sgr;.
Another embodiment of the X-ray examination apparatus according to the invention is characterized in that the filter elements are filled exclusively with the first liquid and the second liquid, the X-ray absorptivity of the first liquid deviating from that of the second liquid. As a result of this step, the voltage adhesion characteristic of the filter elements is rendered less dependent on an orientation of the filter elements relative to the force of gravity, thus enabling the use of the X-ray filter in an X-ray examination apparatus comprising a C-arm.
A further embodiment of the X-ray examination apparatus according to the invention is characterized in that the second liquid also contains compounds containing iodine or bromium. Solution of compounds containing iodine or bromium yields filter elements whose electrical conductivity is lower relative to the first liquid whereas the X-ray absorptivity of the second liquid is higher than that of the first liquid. A second liquid having a lower electrical conductivity and a higher X-ray absorptivity offers the advantage that X-ray absorbing deposits from the second liquid onto the coating layer are counteracted, thus increasing the service life of the X-ray filter. An example of such a second liquid is hexadecane in which, for example Ch
2
I
2
or 1,3,5-tri-iodine benzene is dissolved.
A further embodiment of the X-ray examination apparatus according to the invention is characterized in that the electrically insulating coating layer contains two sub-layers, a first sub-layer being provided on the electrically conductive layer whereas the second sub-layer is provided on the first sub-layer, the first sub-layer containing parylene, silicon oxide, silicon nitride or an anodic oxide whereas the second sub-layer contains a hydrophobic material. The use of an electrically insulating coating layer comprising two sub-layers is known from the international patent application WO 97/03449. A combination of this kind has suitable electrically insulating properties and ensures a large contact angle between the wall and the first liquid so as to enable the draining of the liquid from the filter element. An example of an anodic oxide is alumin
Church Craig E.
U.S. Philips Corporation
Vodopia John F.
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