X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
2000-11-22
2003-07-15
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S098800
Reexamination Certificate
active
06594339
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an X-ray examination apparatus which includes an X-ray source, an X-ray detector, and an exposure control system for controlling the X-ray source and the X-ray detector, which exposure control system is arranged to control the X-ray source so as to carry out a test exposure with a low X-ray dose while producing a control signal by the X-ray detector, and to control the X-ray source on the basis of the control signal in order to carry out an X-ray exposure with a high X-ray dose and to acquire an X-ray image by the X-ray detector during this exposure.
2. Description of the Related Art
An X-ray examination apparatus known in the art is German Offenlegungsschrift DE 43 30 787.
An object to be examined during a radiological examination, for example a patient to be examined, is exposed to an X-ray dose in order to form one or more X-ray images of the object. The X-ray dose required for individual patients to be examined is customarily adjusted individually. The X-ray dose is adjusted by the exposure control system.
The known X-ray examination apparatus carries out the test exposure first with a comparatively low dose, thus forming a test image. During the formation of this test image, the patient to be examined is briefly exposed to X-rays with a predetermined intensity and energy. Because X-rays are applied only during a short period of time during the formation of the test image, the X-ray dose for forming the test image remains comparatively small. The test image is read out from the X-ray detector and digitized in digital grey scale values with, for example, a bit depth of 10 bits. The exposure control system of the known X-ray examination apparatus derives the X-ray dose which is required to form the X-ray image during the X-ray exposure from the distribution of the digital grey scale values. The X-ray dose required for the formation of the X-ray image is large in comparison with the X-ray dose required to form the test image. In the known X-ray examination apparatus, the larger X-ray dose is obtained by using a longer exposure time with the same intensity and energy of the X-rays as used during the test exposure. The known X-ray detector is provided with a sensor matrix having a large number of sensor elements.
It is a drawback of the known X-ray examination apparatus that the reading out of the X-ray detector after the test exposure requires the same amount of time as the reading out after the X-ray exposure whereby the X-ray image is formed. Consequently, the adjustment of the X-ray source for the X-ray exposure requires a comparatively long period of time.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an X-ray examination apparatus enabling faster adjustment of the X-ray source.
This object is achieved by an X-ray examination apparatus according to the invention which is characterized in that the exposure control system is arranged to adjust the X-ray detector to a coarse spatial resolution during the execution of the test exposure, and to adjust the X-ray detector to a fine spatial resolution during the execution of the X-ray exposure.
As the X-ray detector is adjusted to a coarser resolution during the test exposure, fewer signal values need be read out from the X-ray detector. Consequently, the time required to read out the X-ray detector is shorter. Moreover, only a smaller number of signal values, for example brightness values, need be processed in order to derive the control signal therefrom; consequently, less time will also be required to form the control signal. Because the X-ray examination apparatus according to the invention requires only a short period of time for the adjustment of the X-ray examination apparatus on the basis of the test exposure, the X-ray image will become available sooner. More specifically, only a short period of time elapses between the test exposure and the X-ray exposure. Consequently, changes are less likely to occur between the test exposure and the X-ray exposure, for example due to motion in or of the object to be examined. The X-ray examination apparatus according to the invention is thus capable of quickly forming an X-ray image while ensuring that the X-ray image has a high diagnostic quality. This means that small details of low contrast are clearly reproduced nevertheless in the X-ray image. The adjustment of the X-ray examination apparatus concerns notably the adjustment of the energy and the intensity of the X-rays emitted by the X-ray source.
These and other aspects of the invention will be described in detail with reference to the following embodiments, limited only the scope of the claims appended hereto.
Preferably, the invention is used in an X-ray examination apparatus provided with an X-ray detector having a sensor matrix with a large number of sensor elements. The individual sensor elements convert incident X-rays into electric charges. The sensor matrix also includes read-out lines via which the electric charges formed in the sensor elements can be read out or detected. During the test exposure, electric charges are formed in the sensor elements. The separate signal levels of the control signal are composed of the electric charges in a comparatively small number of or in comparatively large groups of sensor elements. Such a large group contains a large number of sensor elements, i.e. more than one sensor element in any case. During the execution of the X-ray exposure, electric charges are formed again in the sensor elements. Any residual charges due to the test exposure are removed from the sensor elements. Such removal of electric charges is also called “electric reset” and is performed, for example by simultaneously reading out and draining the electric charges via all read-out lines. It is to be noted that the electric reset is known per se from U.S. Pat. No. 5,905,772 in which it is applied to counteract ghost images caused by electric charges left behind in the sensor matrix. After the X-ray exposure, the electric charges are read out again so as to form the image signal. The separate signal levels of the image signal are derived from a rather large number of or from comparatively small groups of sensor elements. Such a small group of sensor elements contains only a small number of sensor elements; for example, such a group contains only a single sensor element. The small groups of sensor elements contain fewer, preferably far fewer, sensor elements than the large groups of sensor elements. It is to be noted, however, that the sensor elements of the large groups as well as those of the small groups preferably form part of the whole assembly of sensor elements of the sensor matrix. Sensor elements may form part of a large as well as a small group. Furthermore, the sensor elements of the large as well as those of the small groups preferably have the same construction. For example, the sensor elements are constructed as photodiodes. The sensor matrix is provided with, for example, 1000×1000 or even 4000×4000 sensor elements. For example, a small group contains only a single sensor element or two neighboring sensor elements. For example, 4×4 or 32×32 large groups of sensor elements are used for the test exposure. When electric charges are combined from large groups of sensor elements, less time will be required for the reading out of the sensor matrix. The spatial resolution of the X-ray detector is coarser as electric charges from more sensor elements are combined during reading out. The spatial resolution represents the dimensions of the smallest detail that is faithfully detected by the X-ray detector. It has been found that it is not necessary for the control signal that very small details are accurately detected. The advantage of the shorter period of time that is required for the reading out of the electric charges from the large groups outweighs the loss of spatial resolution. Moreover, the signal-to-noise ratio of the control signal is higher as larger groups of sensor elements are u
Alving Peter Lex
Faber Albert Louw
Joosten Johannes Henricus Maria
Dunn Drew A.
Kiknadze Irakli
Vodopia John
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