X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1999-11-29
2001-10-23
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S008000, C378S901000
Reexamination Certificate
active
06307909
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for image reconstruction in a computed tomography (CT) device having a radiation source that can be displaced relative to an examination subject and a detector system for radiation emanating from the radiation source that registers projections, in different angular positions of the radiation source relative to the examination subject, of a region of the examination subject located within the measuring field, whose extent exceeds the measuring field, a projection being represented by a series of measuring points, each of which is characterized by a channel number and a measurement value.
2. Description of the Prior Art
In CT imaging systems the geometry of the measuring arrangement consisting of the radiation source, for instance an x-ray source, and the detector system defines a cylindrical measuring field centered on a rotational axis, around which the radiation source and possibly the detector system rotate in order to register projections. If parts of the examination subject that is to be projected are situated outside this measuring field, sharp image artifacts arise in the form of extensive bright regions and streaks at the margin of the measuring field near where the measuring field is exceeded. These artifacts are caused by measurement values at the beginning and/or the end of the projections that differ greatly from zero. The measuring field usually lies centered in a gantry opening. The circumstances of the measuring field being exceeded thus can be caused by either unusually large subjects or by mis-positioned subjects.
There are known dedicated methods for image reconstruction from segmented projections, such as iterative methods or wavelet methods, but these are characterized by high computing outlay.
It is also taught by U.S. Pat. No. 5,640,436 to counteract artifacts that are caused by the subject exceeding the measuring field by applying measuring data acquired by means of X-radiation of different energy levels.
In addition, U.S. Pat. No. 5,299,248 and European Application 0 030 143 teach methods to expand projections in which the measuring field is traversed by extrapolation such that each series of measurement points representing a detected projection is complete, beginning and ending with a measurement point whose measurementvalue is approximately zero. Despite this improvement, the image quality frequently leaves something to be desired.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of the above-descrbed type that permits correction of the image artifacts that arise due to the subject exceeding the measuring field and that offers a good quality of the reconstructed images.
The above object is achieved is achieved in accordance with the principles of the present invention in a method for image reconstruction in a CT device wherein a radiation source, or at least the focus thereof, is rotated around a system axis and an examination subject so as to irradiate the examination subject from different projection angles, and a detector system receives the attenuated radiation and generates signals which form respective data sets (projections) for each projection angle, wherein the projections are used to generate an image of a region of the examination subject located in the measuring field, with the examination subject also exceeding (extending out of) the measuring field, and wherein each projection is represented by a series of measurement points, each of which is characterized by a channel number and a measurement value. Projections wherein the examination subject exceeds the measuring field are detected (identified), and extrapolated measurement points, representing the identified projections, are added to the series of “actual” measurement points in order to complete each series of actual measurement points, so that each series of measurement points representing a detected projection begins and ends with a measurement point having a measurement value which is substantially zero, The extrapolation can occur by means of at least one of two techniques. In the first technique, the extrapolation ensues so that measurement points which are to be added are acquired by mapping the measurement points representing the projection in a rectangular coordinate system having a measurement value axis and a channel number axis, and reflecting a number of successive measurement points at the beginning and/or at the end of the series of measurement points representing the projection at a line proceeding through the first or last measurement point respectively of that series which is parallel to the measurement value axis. A subsequent reflection is also made along a line proceeding through the first or last measurement point respectively of the series of points representing the projection parallel to the channel number axis of the rectangular coordinate system. In a second extrapolation technique, the extrapolation is accomplished using reference data for the measuring field that is present without gating of the radiation.
The inventive method is based on the expansion and extrapolation of the measured projections. The expansion and extrapolation can be limited to those projections having detectable exceeding of the measuring field and should ensure a drop in the measurement values to zero at the beginning and end of the expanded series of measurement points representing the projection.
By the inventive method it is possible to maintain the diagnostic value of even those images which were registered with the subject exceeding measuring field and to avoid a repetition of the examination with a modified position of the examination subject, such as may be necessary due to inadequate image quality. Besides the high quality of correction that can be achieved, the described method is distinguished by the possibility of simple realization and by low computing outlay.
The use of the inventive method permits an effective enlargement of the measuring field defined by the geometry of the CT device and is particularly appropriate for “compact” CT devices having relatively small measuring fields, but also for systems having larger measuring fields.
Particularly good results are achieved with the inventive method, because the extrapolation, a symmetric extrapolation according to a first altemative, occurs such that the added measurement points are acquired by reflecting a number of successive measurement points at the beginning or the end of the series of measurement points that represent the projection, mapped in a rectangular coordinate system having a measurement value axis and a channel number axis, at a line proceeding through the first and last measurement points of the series of points representing the projection, parallel to the axis of the coordinate system corresponding to the measurement value. A subsequent reflection also occurs at a line proceeding through the first and last measurement points of the series of points representing the projection parallel to the axis of the coordinate system corresponding to the channel number. In this way, the projections are expanded by realistic data, with the result that the noise behavior also is successfully maintained for the extrapolated regions of the projections.
A high image quality is likewise achieved when, according to a second inventive alternative, wherein the extrapolation is accomplished using reference data for the measuring field that is present without gating of the radiation, since it is then also guaranteed that the data added in the course of the extrapolation will very closely approximate the actual relations.
According to a variation of the invention, projections encompassing an exceeding of the measuring field, these projections being used to perform the extrapolation, are detected by considering threshold values. If the average value of the measurement values of a number of successive measurement points N
th,sco
preferably in immediate succession at least at the beginning or at the end of
Flohr Thomas
Ohnesorge Bernd
Schwarz Karl
Bruce David V.
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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