X-ray or gamma ray systems or devices – Beam control – Filter
Reexamination Certificate
2000-01-13
2001-07-31
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Beam control
Filter
C378S156000
Reexamination Certificate
active
06269147
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to X-ray examination apparatus in which an X-ray filter is provided between an X-ray source and an X-ray detector. An object to be examined, for example a patient, is positioned between the X-ray filter and the detector.
2. Description of Related Art
Apparatus of this type is described in WO 97/03450.
The filter used in such an apparatus enables an X-ray image to be obtained with a low dynamic range. This is desirable so that small details of the X-ray image, which occur as small variations in contrast, may be analysed. This is possible by using the filter to block signals which would give rise to a larger dynamic range. Some parts of the body, for example lung tissue, have very high X-ray transmittance whereas other parts of the body, for example bone tissue, can hardly be penetrated by X-rays. Without filtering, this difference gives rise to the large dynamic range. The reduction in dynamic range achievable using a two-dimensionally controllable X-ray filter simplifies image intensifying techniques for the detected image. The ability to shield certain areas of the body using the filter may also be desirable to protect certain types of body tissue from X-ray exposure.
The filter described in WO 97/03450 comprises an array of capillary tubes, each containing an X-ray absorbing liquid, and the height of the liquid in each tube is controllable to provide a different degree of X-ray absorption. WO 97/03450 describes a method of controlling the level of liquid in each capillary by applying a given fixed voltage signal over a different time period for different capillaries. The change in liquid level as a function of time is used as a control parameter. The capillaries are arranged in rows and columns, and the capillaries are addressed row by row. It has also been proposed that the filter adjustment should be divided into a number of time frames so that the liquid level of each row may be adjusted in partial stages. This has been considered desirable because the physical inertia of the liquid is considered to favour liquid movement in discrete steps. Furthermore, a smaller time difference then exists between the filling of capillaries in the first rows and those in the last rows. This may enable imaging to be started during the latter stages of the filter adjustment period
A problem which is not addressed by WO 97/03450 is the need to reduce the overall time required to adjust the X-ray filter from one configuration to a new configuration. The time required to change the liquid level in an individual filter element is not only a function of the physical inertia of the X-ray absorbing liquid, but also is a function of the effective electrical capacitance of the filter elements, which varies as the liquid level varies.
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
According to the present invention, there is provided an X-ray examination apparatus comprising an X-ray source, an X-ray detector and an X-ray filter arranged between the X-ray source and the X-ray detector, the X-ray filter comprising a plurality of filter elements each comprising a vessel containing an X-ray absorbing liquid, the liquid level in each vessel determining the X-ray absorptivity of the respective filter element, wherein control means is provided for applying electric voltages to the individual filter elements to change the liquid levels within an adjust time period, wherein the control means is arranged to apply a respective control voltage to each individual filter element a repeat number of times within the adjust time period, the repeat number being selected so as substantially to maximise the rate of change of liquid level of the filter elements.
The apparatus of the invention controls the signals driving the individual filter elements in such a way that the rate of change of liquid level of the filter elements may be maximised. This may enable the adjust time to be reduced so that sequential images (for example at different angles through the patient) may be taken with a shorter interval between images, thereby reducing the overall examination time.
Each filter element preferably comprises a capillary tube, with the liquid in each filter element coupled to a common reservoir. This common reservoir then acts as a virtual earth, and each capillary acts as a capacitor coupled to earth, with the capacitance being a function of the liquid level.
The adjust time period may be between 100 ms and 200 ms, and the repeat number may be between 20 and 50.
The invention also provides a method of adjusting an X-ray examination apparatus, comprising the adjustment of the X-ray absorptivity of the filter elements of an X-ray filter by controlling a quantity of X-ray absorbing liquid within individual filter elements, wherein control voltages are applied to the individual filter elements to control the level of the X-ray absorbing liquid within the filter elements during an adjust time period within which the X-ray filter is to be adjusted, wherein respective control voltages are applied to each individual filter element a repeat number of times during the adjust time period, the repeat number being selected so as substantially to maximise the rate of change of liquid level of the filter elements.
REFERENCES:
patent: 5148465 (1992-09-01), Mulder et al.
patent: 5559853 (1996-09-01), Linders et al.
patent: 5878111 (1999-03-01), Schulz
patent: WO97.03450 (1997-01-01), None
patent: WO-98/21729 (1998-05-01), None
Kiknadze Irakli
Kim Robert H.
U.S. Philips Corporation
Vodopia John F.
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