X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
2002-10-21
2004-08-17
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S154000
Reexamination Certificate
active
06778632
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to an examination arrangement of the type having an X-ray detector or a gamma detector with detector elements arranged in a matrix in rows and columns that form a detector surface with detection regions that are sensitive to X-rays or gamma radiation and having insensitive intermediate regions, and a stray radiation grid or collimator of absorbent structure elements that is arranged over the detector surface.
2. Description of the Prior Art
In X-ray image technology, high demands are currently made of the image quality of the X-ray exposures. For making such exposures, particularly as implemented in medical X-ray diagnostics, a subject to be examined is transirradiated by X-rays of an approximately punctiform X-ray source, and the attenuation distribution of the X-rays is two-dimensionally acquired at that side of the subject opposite the X-ray source. A line-by-line acquisition of the X-rays attenuated by the subject also can be undertaken, for example in computed tomography installations. In addition to X-ray films and gas detectors, solid-state detectors are being increasingly utilized, these usually having a matrix-like arrangement of optoelectronic semiconductor components as optoelectrical receivers. Ideally, each picture element of the X-ray exposure should correspond to the attenuation of the X-rays by the subject on a straight-line axis from the punctiform X-ray source to a location at the detector surface corresponding to the picture element. X-rays that are incident on the X-ray detector that proceed on such a straight-line axis from the punctiform X-ray source are referred to as primary rays.
Due to unavoidable interactions, however, the X-rays emanating from the X-ray source are scattered in the subject, so that scattered rays, referred to as secondary rays, are also incident onto the detector in addition to the primary rays. These scattered rays, which can cause up to more than 90% of the overall signal modulation of an X-ray detector in diagnostic images dependent on properties of the subject, represent an additional noise source and therefore diminish the recognizability of fine contrast differences. This serious disadvantage of the stray radiation is due to a significant, additional noise component in the image exposure caused by the quantum property of the stray radiation.
For reducing the stray radiation incident on the detector, a stray radiation grid is introduced between the subject and the detector. Stray radiation grids are composed of regularly arranged structures that absorb X-rays and between which through channels or through slots are fashioned for the optimally unattenuated passage of the primary radiation. Given focused stray radiation grids, these through channels or through slots are directed toward the focus in conformity with the distance from the punctiform X-ray source, i.e. the distance from the focus of the X-ray tube. In unfocussed stray radiation grids, the through channels or through slots are arranged over the entire surface of the stray radiation grid perpendicular to the surface thereof. This, however, leads to a noticeable loss of primary radiation at the edges of the image exposure since a larger part of the incident primary radiation strikes the absorbent regions of the stray radiation grid at these locations.
Extremely high demands are made on the properties of X-ray stray radiation grids for achieving a high image quality. The scattered rays should be absorbed as well as possible; however, as much of the primary radiation as possible should pass through the stray radiation grid unattenuated. A reduction of the scattered rays incident on the detector surface can be achieved by a large ratio of the height of the stray radiation grid to the thickness or the diameter of the through channels or through slots, i.e. by a high shaft ratio. However, image disturbances due to absorption of a part of the primary radiation can occur because of the thickness of the absorbent structure or wall elements lying between the through channels or through slots. Especially given employment of solid-state detectors, inhomogeneities of the grid, i.e. deviations of the absorbent regions from their ideal position, lead to image disturbance due to an imaging of the grid in the X-ray image.
For minimizing image disturbances due to stray radiation grids, it is known to move the grids in the lateral direction during the exposure. Given extremely short exposure times of, for example, 1-3 ms, however, stripes still can occur in the image due to an inadequate motion velocity of the grids. Disturbing stripes due to the reversal of the motion direction of the grids during the exposure also can occur given very long exposure times.
The same problem arises in nuclear medicine, particularly in the employment of gamma cameras such as, for example, Anger cameras. Similar to X-ray diagnostics, care must also be exercised in the exposure technique to ensure that as few scattered gamma quanta as possible reach the detector. In contrast to X-ray diagnostics, the radiation source for the gamma quanta is located in the inside of the subject in nuclear diagnostics. The patient is injected with a metabolism preparation marked with certain unstable nuclides, which is metabolized organ-specifically. An image of the organ is then obtained by means of detecting the decay quanta emitted from the body. The time curve of the activity in the organ allows conclusions about its function to be made. A collimator that defines the projection direction of the image must be introduced in front of the gamma detector for obtaining an image of the interior of the body. In terms of function and structure, such a collimator corresponds to the stray radiation grid in X-ray diagnostics. Only the gamma quanta defined by the privileged direction of the collimator can pass the collimator; quanta incident at an angle thereto are absorbed in the collimator. Due to the higher energy of the gamma quanta compared to X-ray quanta, collimators must be implemented with multiply higher absorption capability than stray radiation grids for X-radiation.
Scattered quanta can thus be excluded by only quanta having a specific energy being taken into consideration in the image. However, every detected stray quantum causes a dead time of the gamma camera of, for example a microsecond during which no further events can be registered. Therefore, when a primary quantum arrives shortly after the registration of a stray quantum, it cannot be registered and is lost for the image. A similar effect also arises when a stray quantum coincides in time—within certain limits—with a primary quantum. Since the evaluation electronics then no longer can separate the two events, too high an energy is determined and the event is not registered. Both of these occurrences require that a highly effective scattered ray suppression be provided to achieve an improved quantum efficiency in nuclear diagnostics. Further, an improved image quality is obtained for a given dosage of the applied radio-nuclide, or a lower radio-nuclide dosage can be used to obtain a given same image quality, so that the radiation exposure of the patient can be lowered and shorter image exposure times can be achieved.
Different detectors are utilized for the registration of medical projection X-ray images as well as for the registration of gamma quanta in nuclear medicine. In particular, solid-state detectors with detector elements arranged in row and column directions have recently been playing an important part. The detector elements are also referred to as pixels below. These detectors also require a stray radiation grid or collimator that blanks out scattered X-ray or gamma quanta.
Conventional grids made of lead lamellae have the problem that they are coarser than the pixel structure and are also too non-uniform, so that disturbing Moiré effects occur. The use of moving grids is complicated and also leads to a high absorption of the primary radiation.
U.S. Pat. No. 6,021,173
Hoheisel Martin
Sklebitz Hartmut
Bruce David V.
Schiff & Hardin LLP
Siemens Aktiengesellschaft
Thomas Courtney
LandOfFree
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