X-ray apparatus including a filter with filter elements...

X-ray or gamma ray systems or devices – Beam control – Filter

Reexamination Certificate

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Details

C378S156000

Reexamination Certificate

active

06438211

ABSTRACT:

BACKGROUND OF INVENTION
The invention relates to an X-ray apparatus which includes 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 having a longitudinal direction and a circumference, wherein
each filter element has an internal volume for receiving a liquid filling which contains at least one electrically conductive and one X-ray absorbing liquid component, the X-ray absorptivity of said filter element being dependent on the quantity of X-ray absorbing liquid component present in the internal volume,
each filter element is provided with a first electrode for applying a first electric voltage to a wall of the filter element and a second electrode for applying a second electric voltage to the internal volume of the filter element,
the first electrode is electrically isolated from the internal volume of the filter element by means of an isolator layer in such a manner that an electric capacitance per unit of surface area of the filter element exists between the first electrode and the electrically conductive liquid component when a quantity of the electrically conductive liquid component is present in at least a part of the internal volume of the filter element,
the X-ray absorptivity of each filter element is adjustable by step-wise control of a surface level of the X-ray absorbing liquid component in the longitudinal direction of each filter element.
An X-ray apparatus with an X-ray filter of this kind is known from U.S. Pat. No. 5,666,396 (PHN 15.378). The known X-ray apparatus includes a filter with a plurality of filter elements, each having an individual absorptivity which is being dependent on the level of a liquid filling present in the filter element. The X-ray apparatus is used inter alia for medical diagnostic imaging where a patient to be examined is arranged between the X-ray source and the X-ray detector in order to image internal structures. Because of the fact that there are structures of different electron density present within the patient, areas of different density are observed in a resultant X-ray image. The difference in density between the extreme values of the density in one X-ray image is defined as the dynamic range. The filter serves to limit a dynamic range per X-ray image.
In order to limit the dynamic range of the object to be examined, the known X-ray apparatus includes a filter with filter elements provided with a bundle of tubes for receiving a liquid filling which is X-ray absorbing as well as electrically conductive; each tube is connected to a common supply duct. 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 electric voltage applied to the first electrode of the filter element influences the adhesion between the liquid filling and an inner wall of the filter element; this adhesion determines whether the relevant filter element is filled with the liquid filling. The relative quantity of the liquid filling in individual filter elements is controlled on the basis of the electric voltages applied to the individual filter elements. For example, for a first value of the electric voltage the adhesion to the inner wall for the liquid filling is increased and the relevant filter element is filled with the liquid filling from the supply duct. For a second value of the electric voltage the adhesion is reduced and the liquid filling is drained from the filter element to the supply duct. The filter elements are adjusted to a high X-ray absorptivity by filling them with the liquid filling; they are adjusted to a low X-ray absorptivity by keeping filter elements empty.
It is a drawback of the known device that the filling of each filter element is controlled by application of a sequence of electric voltage pulses to the first electrode of the filter element so that the filter element is electrically charged. The level of the electric charge determines the degree of filling of the filter element. It has been found that in the course of time the filling level of the filter element becomes poorly reproducible. In many practical cases it is desirable to have a reproducible filling with an a priori known degree of discretization so as to realize a reliable range of gray scale values.
SUMMARY OF INVENTION
It is an object of the invention to provide an X-ray apparatus which includes a filter provided with filter elements whose X-ray absorptivity can be controlled in steps.
An X-ray apparatus according to the invention is characterized in that the electric capacitance per unit of surface area of the wall of the filter element varies substantially in the longitudinal direction of the filter element.
The invention utilizes the known effect that a contact angle between an electrically conductive liquid and an electrode which is isolated therefrom is changed by creating a potential difference between the electrically conductive liquid and the electrode. This phenomenon is known as electrowetting. When electrowetting is applied to a tubular filter element which has an electrode provided in its wall and is filled with an electrically conductive liquid filling, the level of said liquid filling in the filter element can be influenced due to the fact that the electrowetting force is oriented in the longitudinal direction of the filter element so that the degree of filling of the filter element can be increased or decreased at option. In order to realize the potential difference between the liquid filling and the first electrode provided in the wall of the filter element, they are electrically isolated from one another by means of an isolator layer deposited on the inner wall of the filter element. In order to achieve a low wetting hysteresis, the isolator layer may also be covered by an inert cover layer so that the liquid filling directly contacts the cover layer. A capacitance per unit of surface area of the filter element which is due to the geometry of the filter element can then be defined. It is known that in electrowetting a balance exists between the capillary force, the force of gravity and the electric capillary force or electrowetting force. The relationship between the level of the electrically conductive liquid filling in the filter element and the relevant physical quantities can be derived from the equations for the energy balance in the filter element. There can be defined two dominant variables whose value determines the electrowetting force in the filter element. The first variable is the capacitance per unit of surface area which is averaged over the length of the contact edge between the meniscus of the electrically conductive liquid filling and the inert cover layer. The second variable is the potential difference between the electrically conductive liquid filling and the first electrode. The invention is based on the idea to realize the filter element filling in steps by step-wise varying the capacitance, and hence the minimum potential difference necessary for electrowetting to occur, in the longitudinal direction of the filter element.
The procedure for the step-wise filling of the filter element is as follows. The operation of the filter element will be described first of all for the case where the liquid filling contains two liquid components that can fully dissolve in one another, thus forming one electrically conductive and X-ray absorbing liquid. It is also assumed that the filter element is empty, that the capacitance variation profile in the longitudinal direction of the filter element is known a priori, and that no potential difference exists yet between the liquid filling and the first electrode. Finally, a distinction is made between a “fill” voltage for completely filling the filter element, a “hold” voltage for keeping the liquid filling in position, and a “drain” voltage for draining th

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