X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-02-08
2001-11-20
Dunn, Drew (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Reexamination Certificate
active
06320929
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for scanning an examination subject with a CT (computed tomography) device and to a CT device for implementing the method. The CT device is of the type having an X-ray source that emits an X-ray beam, and a detector system composed of several rows of detector elements on which the beam is incident, the X-ray beam being displaceable relative to a system axis, and the detector signals that are formed in the various projections being fed to a computer, which constitutes images of the examination subject therefrom.
2. Description of the Prior Art
Known radiographic CT devices have an X-ray source that directs a collimated pyramidal X-ray beam through the examination subject, for instance a patient, onto a detector system that is constructed of a two-dimensional array of detector elements, which is completely irradiated by the X-ray source. The X-ray source and, depending on the construction of the radiographic CT device, the detector system as well, are attached on a gantry that rotates around the examination subject. A support table for the examination subject can be moved, relative to the gantry. The longitudinal position from which the X-ray beam penetrates the examination subject and the angle at which the X-ray beam penetrates the examination subject are continuously modified as a consequence of the rotation of the gantry and this relative movement. Every detector element of the detector system that is struck by X-rays produces a signal, which represents a measure of the total transparency of the body of the examination subject for the X-rays emanating from the X-ray source on its path to the detector system. The set of output signals of the detector elements of the detector system that is obtained for a particular angular position of the X-ray source is referred to as a projection. A scan comprises a set of projections which are obtained at different positions of the gantry and/or at different positions of the support table. The radiographic CT device picks up a number of projections during one scan, in order to be able to construct a two-dimensional tomogram of a slice of the examination subject. Several slices can be picked up at once by the detector system that is formed by several detector rows.
Larger volumes of the examination subject are usually picked up by sequential scanning or spiral scanning.
In sequential scanning, the data are picked up during the rotational movement of the gantry, while the examination subject is situated in a fixed position, and so a plane slice is scanned. Between scans of consecutive slices, the examination subject is moved into a new position, in which the next slice can be scanned. This process is continued until all slices that were specified prior to the examination have been scanned.
In spiral scanning the gantry with the X-ray source rotates continuously around the examination subject, while the support table and the gantry are continuously displaced relative to one another along a system axis. The X-ray source thus describes a spiral (helical) path in relation to the examination subject, until the volume that was specified prior to the examination has been scanned. Images of individual slices are then computed from the spiral data obtained in this manner.
In conventional radiographic CT devices, the cross-section of the X-ray beam is set by means of a radiation diaphragm at the source prior to the scanning of an examination subject and is not modified during this scan (German OS 197 21 535). Because of this, regions of the examination subject that are not used for image reconstruction are frequently penetrated by X-rays, particularly in CT devices with a multiline detector system. This is also true of a radiographic CT system as taught in German OS 197 48 891, in which it is possible to modify the slice thickness during the scan according to the body region of the examination subject currently being scanned.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of the above type so as to prevent the application of a radiation dose in the region of the examination subject from which images are not reconstructed.
This object is inventively achieved in a method for scanning an examination subject and to a CT device wherein an X-ray source of the CT device emits an X-ray beam and wherein the beam is incident on a detector system of the CT device composed of several rows of detector elements (detector rows), the X-ray beam being displaced relative to a system axis for purposes of scanning the examination subject, and wherein detector signals that are formed for different projections in the scanning of the examination subject are fed to a computer, which reconstructs images of the examination subject therefrom; and wherein the cross-section of the X-ray beam is modified during the scan such that at all times substantially only the region of the examination subject that is used for the reconstruction of images is penetrated by the X-ray beam.
In the inventive method for scanning an examination subject, it is advantageous that the application of unused radiation dosage is prevented, above all in the margin regions of the examined volume. This is easily accomplished by a radiation diaphragm at the source side, the cross section of which is varied in the course of the scan such that the X-ray beam, being so gated, only penetrates the region of the examination subject from which measurement data that can be evaluated for the image reconstruction are acquired.
To achieve this, it is necessary to set the aperture of the radiation diaphragm at the source side so as to be intermittently asymmetric to the center of the detector system, which is not typical.
In an embodiment of the invention, the slice diaphragm that is already present in conventional CT devices is used as the radiation diaphragm at the source side.
According to another embodiment of the invention, the application of unused radiation dosage is prevented by, during the scan, continuously adapting the cross-section of the X-ray beam to the detector system such that at all times substantially only the detector elements of the detector system whose measurement data are used for image reconstruction are irradiated. This is accomplished by control of the radiation diaphragm at the tube side. In general, this diaphragm is opened so wide that the detector system is completely irradiated. Assuming a point focus of the X-ray source, the region of the examination subject situated within the pyramid situated between the focus and the edges of the detector system are thus penetrated by the X-radiation.
The adaptation of the x-ray beam to the detector system inventively occurs specifically by, during the scan, fading or occluding (blanking out) respective individual rows of the detector system. That is, by adjustment of the radiation diaphragm at the tube side, the cross-section of the X-ray beam is so varied during the scan so that only a part of the detector system is irradiated in an intermittent manner. This is detailed below with in the context of examples of spiral and sequential scans.
In a spiral scan there are regions at the beginning and end of a scan that are covered by only a part of the detector rows. The measurement data of the detector elements that are acquired in these regions either are not used for image reconstruction, or they lead to images having diminished image quality compared to images from the middle scan region. These images are generally unusable for diagnostic purposes and therefore are discarded.
The dose of X-radiation that is applied to the examination subject therefore can be used only partly for reconstructing images in conventional radiographic CT devices. The portion of unused dosage increases in spiral scanning with the number of detector rows and with decreasing pitch (shift of the support mechanism per 360° revolution of the radiation source (full revolution) around the examination subject, in relation to the extent of a detector
Dunn Drew
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
LandOfFree
Method for scanning an examination subject with a CT device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for scanning an examination subject with a CT device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for scanning an examination subject with a CT device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2572298