X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2003-05-21
2004-09-14
Bruce, David V (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S007000, C378S015000, C378S901000
Reexamination Certificate
active
06792067
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method of correcting the extrafocal radiation of an X-ray tube in image recordings with a computed tomograph, preferably with which measured data obtained from detector channels of at least one detector row in the computed tomograph are subjected to logarithmic manipulation in order to obtain the image recordings.
BACKGROUND OF THE INVENTION
A computed tomograph includes, inter alia, an X-ray tube, at least one detector row with individual X-ray detectors, also referred to as detector channels below, and a patient support table. The X-ray tube and X-ray detectors are arranged on a gantry which, during the measurement, rotates about the patient support table or an examination axis running parallel to the latter. Alternatively to this, the X-ray detectors can also be arranged on a stationary detector ring around the patient support table, only the X-ray tube moving with the gantry.
As a rule, the patient support table can be displaced along the examination axis relative to the gantry. The X-ray tube produces a beam widened in a fan shape in a layer plane at right angles to the examination axis. The boundary of this beam in the direction of the layer thickness is set by the size or the diameter of the focus on the target material of the X-ray tube and one or more aperture stops arranged in the beam path of the X-ray beam. In addition, the angle of the fan-like widening of the beam is defined by an aperture stop arranged in front of the X-ray tube. During image recordings with the computed tomograph, the X-ray beam passes in the layer plane through a layer of an object, e.g. a layer of the body of a patient, which is mounted on the patient support table, and strikes the X-ray detectors of the detector row lying opposite the X-ray tube. The angle at which the X-ray beam penetrates the layer of the body of the patient and, if appropriate, the position of the patient support table relative to the gantry change continuously during the image recording with the computed tomograph.
During the measurement with a computed tomograph, a number of sets of measured data are obtained, which correspond to different projections of the respective transilluminated layer. A set of projections which have been recorded at various positions of the gantry during the rotation of the gantry around the patient is designated a scan. The computed tomograph records many projections at different positions of the X-ray source relative to the body of the patient, in order to reconstruct an image which corresponds to a two-dimensional slice (sectional image) of the body of the patient. For this purpose, following its logarithmic manipulation, the measured data is initially convoluted with a convolution core which, taking into account the physical relationships and the measuring system, produces specific image characteristics and then transforms them into the Cartesian space of the image in order to reconstruct the two-dimensional slice. This technique is also designated filtered back projection. The convolution cores used in the convolution are set up in accordance with the desired image characteristics or are known for a large number of such image characteristics. These image characteristics can be, for example, high local resolution or good low-contrast detectability. Using a suitable convolution core, the desired image characteristic in the reconstructed slice can be achieved.
During image recordings with computed tomographs, however, image artifacts can occur, which can be attributed to the undesired extrafocal radiation produced by the X-ray tube. The extrafocal radiation is produced outside the focal spot of the X-ray tube by backscattered electrons which fall back onto the anode of the X-ray tube. During the generation of this extrafocal radiation in the environment of the focal spot or focus of the X-ray tube, this cannot be masked out by the aperture stop on the tube side for bounding the fan-like beam, and is concomitantly registered by the detector channels of the detector row. This extrafocal radiation, which depends on the X-ray tube used, leads for example to what is known as the cupping effect in cranial tomograms of adults or to what is known as the halo effect in cranial tomograms of children.
In order to suppress the undesired image artifacts caused by extrafocal radiation, hitherto the convolution cores used for the filtered back projection have been manipulated in a suitable way in order to obtain an improved image quality. The convolution cores differ, however, depending on the object or object region respectively examined. For example, depending on the region of the body examined (head or body) and the age of the patient (adult or child), they must be different. Furthermore, these convolution cores must be matched to each computed tomograph and, in addition, do not supply optimal image results with regard to the suppression of the extrafocal radiation.
SUMMARY OF THE INVENTION
An object of an embodiment of the present invention is to specify a method of correcting the extrafocal radiation of an X-ray tube in image recordings with a computed tomograph which supplies good image results and requires no distinction between individual convolution cores.
In the method of an embodiment of the application of correcting the extrafocal radiation of an X-ray tube during image recordings with a computed tomograph, the measured data obtained from the detector channels of at least one detector row of the computed tomograph are subjected to logarithmic manipulation and, if appropriate, filtered back projection, in order to obtain the image recordings of the object examined. In the method of an embodiment of the application, the measured data from each detector channel, before the logarithmic manipulation and back projection, is subjected to convolution with a detector-channel-dependent convolution core, which is derived from a distribution of the extrafocal radiation on at least one detector channel of the computed tomograph or a computed tomograph of the same type. This distribution of the extrafocal radiation on the detector channel can in this case be obtained from a measurement with the detector channels or else by other ways, for example via the blackening of an X-ray film.
By way of this convolution of the measured data before the logarithmic manipulation, with the aid of a previously determined convolution function, virtually complete correction of the extrafocal radiation is possible. In particular, this correction does not require any distinction between the convolution cores for different regions of the body and ages of the patients. Instead, following the logarithmic manipulation, the correction supplies logarithmically manipulated raw data which no longer has to be subjected to any further physical correction of the extrafocal radiation. The subsequent back projection or reconstruction is carried out with this corrected raw data and can be carried out with any desired known convolution cores without a correction component for the extrafocal radiation.
The method of an embodiment of the application is based on the finding that the extrafocal radiation produces detector-channel-specific errors and can be described by a convolution process. As a result, correction by way of convolution of measured data is approximately possible, as carried out by the method of an embodiment of the application. A precondition for the derivation of the convolution core is the determination of the distribution of the extrafocal radiation for each individual detector channel or region of detector channels. The intensity of the extrafocal radiation can be determined or measured for a combination of tube and CT device type and then supplies the correction and the convolution cores for all devices of this type combination.
To derive the convolution core for a detector channel, the impulse response of the extrafocal radiation of the computed tomograph is preferably determined first and Fourier transformed. The Fourier transform is then inverted and trans
Bruce David V
Harness & Dickey & Pierce P.L.C.
Siemens Aktiengesellschaft
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