X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-09-26
2003-10-28
McCall, Eric S. (Department: 2855)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S070000, C378S086000
Reexamination Certificate
active
06639964
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method of forming a computed X-ray tomogram of an object which is irradiated in a measuring device that consists of an X-ray source and a detector field, the radiation intensity measured in the detector field being subjected to scatter correction. The detector field may notably be a two-dimensional multi-cell field, a part of which that is not bounded at right angles is shielded from direct irradiation by the X-ray source. The invention also relates to an X-ray computed tomography apparatus which includes a measuring device with an X-ray source and a detector field, as well as a correction unit for performing a scatter correction on the radiation intensity measured in the detector field. The invention notably relates to an X-ray computed tomography apparatus in which the detector field is a two-dimensional multi-cell field which includes shielding means that are arranged in such a manner that they shield a part of the detector field that is not bounded at right angles.
BACKGROUND OF THE INVENTION
In order to form a computed tomogram, an object to be examined, notably the body of a patient, is irradiated by X-rays from an X-ray source and the radiation having traversed the object is detected in respect of its radiation intensity in a detector field on the other side of the object. For the formation of a cross-sectional image of the irradiated object, the absorption of the X-rays during their travel through the object, and hence also the optical density (measured in Hounsfield units) or the material composition of the object, are established on the basis of the radiation intensity measured in the detector field.
The (primary) radiation that directly traverses the object is responsible for generating the desired imaging information. However, in addition processes of scattering of the photons of the X-rays also occur in the object; during these processes the photons change their direction and possibly their energy. Part of the scattered radiation also reaches the detector and contributes to the radiation intensity measured thereby. However, because the scattered radiation does not reach the detector along a direct path from the radiation source, it does not contribute to the useful image information but is superposed instead on the information that can be derived from the direct radiation.
Because in a first approximation the scattered radiation increases linearly as a function of the irradiated volume of the object, its disturbing effect is greater as the slice thickness irradiated in the computed tomography (CT) apparatus is greater. Whereas nowadays slice thicknesses of from 0.8 to 3 mm are customarily examined in computed tomography apparatus, there is a growing tendency towards the use of multi-line computed tomography apparatus with a slice thickness of approximately 2 cm or even more. Consequently, the background due to scattered radiation and the associated deterioration of the image quality will increase.
Various methods are known for the reduction of the disturbing effects of the scattered radiation. For example, on the one hand it can be attempted to prevent the scattered radiation from reaching the detector in the first place. These methods utilize the sole relevant difference between primary photons and scattered photons, that is, the difference in the distribution of their angle of incidence. Whereas all primary photons arrive from the radiation source along a straight path, scattered photons have “oblique” angles of incidence that deviate therefrom. Therefore, it is known to utilize so-called anti-scatter grids (ASG) which consist of thin foils of a high-grade absorbing material such as, for example molybdenum or tungsten and are arranged so as to be aligned with the focal point of the X-ray anode. Therefore, the ASG suppresses primary photons only if they are incident on an end face of the foils whereas scattered photons are absorbed upon incidence on the foil surface. For computed tomography applications the suppression factor for the scattered radiation can thus readily reach high values of from 10 to 20. The primary radiation, however, is reduced by only a factor of from approximately 1.1 to 1.3. The anti-scatter grids, however, have the drawback that their manufacture is very expensive and that their application is highly complex. Furthermore, it is questionable whether the suppression factors achieved will indeed be adequate for future large slice thicknesses.
Moreover, scatter correction is normally performed by subtracting a constant scattered radiation background, conservatively estimated on the basis of the object size, from the measuring values. Image artefacts such as stripes and so-called pockets, however, remain visible.
U.S. Pat. No. 5,615,279 discloses a method for scatter correction in a computed tomography apparatus in which first measurements of the scattered radiation are performed on model bodies (phantoms) of different thickness, the measuring results being taken up in a table. When computed tomograms are formed from real patients, correction values are calculated for the measuring data, while taking recourse to the stored tables, in order to reduce the effect of the scattered radiation as much as possible. This method has the drawback that a large amount of experimental work is required for composing the tables with the correction values. Because of this large amount of work, usually only a few parameters that have an effect on the scattered radiation can be varied. Notably the object size of the phantom is one of these parameters.
Furthermore, DE 197 21 535 A1 discloses an X-ray computed tomography apparatus in which the detector field is formed by a plurality of adjacently situated rows that consist of concatenated detector cells. The measuring device, consisting of the X-ray source and the detector field as well as collimators, can be displaced relative to the longitudinal axis of the patient. The plurality of rows of the detector field are arranged so as to extend perpendicularly to the axis of displacement and parallel to one another, so that different slices of the body are successively imaged on the detector cells during a relative displacement between the measuring arrangement and the patient. The collimators can notably be adjusted in such a manner that the detector rows that are situated at the edge of the detector field are shielded from direct irradiation by the X-ray source. Consequently, only scattered radiation can be incident on these rows, so that the measuring signal acquired in these row forms an indication of the extent of the scattered radiation; this indication can be used for scatter correction of the primary measuring values. This arrangement has the drawback that specific rows of the detector field must be reserved for the scatter correction.
SUMMARY OF THE INVENTION
Considering the foregoing it is the object of the present invention to provide a method of generating an X-ray computed tomogram as well as an X-ray computed tomography apparatus which both enable improved scatter correction.
This object is achieved by means of a method in conformity with the characteristics of claim 1, by means of a method in conformity with the characteristics of claim 10, by means of an X-ray computed tomography apparatus in conformity with the characteristics of claim 7 as well as by means of an X-ray computed tomography apparatus in conformity with the characteristics of claim 18. Advantageous further embodiments are disclosed in the dependent claims.
According to a first version of a method in accordance with the invention for forming an X-ray computed tomogram of an object, notably the body of a patient, the object is irradiated in a measuring arrangement that consists of an X-ray source and a detector field and the radiation intensity measured in the detector field is subjected to scatter correction. The detector field is then conceived so as to be a two-dimensional multi-cell field, meaning that the individual cells of the detector are adjacently arranged in rows and columns in th
Lauter Josef
Schneider Stefan
Such Olaf
Wieczorek Herfried Karl
Koninklijke Philips Electronics , N.V.
McCall Eric S.
Vodopia John
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