X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2001-03-01
2002-06-25
Kim, Robert H. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S019000, 36, 36
Reexamination Certificate
active
06411671
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method and apparatus for reducing line artifacts in a CT image, wherein the CT image is produced by scanning with an X-ray source which is rotatable around an examination subject, with X-rays from the X-ray source, after being attenuated by an examination subject, being incident on a detector system.
2. Description of the Prior Art
CT devices are known which have an X-ray source, e.g. an X-ray tube, which direct a collimated, pyramid-shaped X-ray bundle through the examination subject, e.g. a patient, onto a detector system that is composed of a number of detector channels. Each detector channel has at least one detector element and one associated electronic element for reading out and amplifying the signal that is generated in the detector element as a result of the incident radiation. A number of detector elements can be allocated to one electronic element. The X-ray source and, depending on the construction of the CT device, the detector system as well are attached to a gantry that rotates around the examination subject. A support device for the examination subject can be displaced along the system axis relative to the gantry. The position along the system axis at which the X-ray bundle penetrates the examination subject, and the angle, under which the X-ray bundle penetrates the examination subject, are continuously modified as a result of the displacement and the rotation of the gantry. Each detector element of the detector system struck by the radiation produces a signal representing a measure of the overall transparency of the examination subject for the radiation proceeding from the X-ray source to the detector system. The set of output signals of the detector element, of the detector system, which is acquired for a specific position of the X-ray source, is referred to as a projection. A scan is composed of a set of projections, which are acquired at different positions of the gantry and/or at different positions of the support device. The CT device picks up a number of projections during a scan in order to be able to construct a two-dimensional tomogram of a slice of the examination subject. A number of slices can be picked up at the same time by a detector system that is composed of an array having a number of rows and columns of detector elements. Such planar-like detector systems, however, frequently contain detector channels which do not supply proper data. It may be that detector system contains faulty detector channels already after the production process, for example as a result of defects in fabrication caused by the high integration density of the electronic elements. Defects of individual detector channels also may arise during the operation of the CT device. Such defects cause circular structures in the acquired CT images, these circular structures being referred to as circle artifacts. Techniques referred to as “ring-balancing” methods are known from the literature for the purpose of attempting to correct or prevent such artifacts in CT images. Such methods are disclosed in U.S. Pat. No. 4,670,840 and in German OS 198 35 451 (corresponding to U.S. Pat. No. 6,047,039), for example.
A disadvantage of such known methods is that they insufficiently eliminate artifacts which arise in a CT device having a defective detector channel.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for reducing artifacts in a CT image, so that the obtainable image quality is improved in a CT device having at least one defective detector channel. It is also an object of the invention to provide a CT device for implementing the method.
The above object is achieved in accordance with the principles of the present invention in a method for reducing line artifacts in a CT image D
1
, as well as in an apparatus for implementing the method, wherein the image has been subjected to interpolated filtering for preventing or correcting faulty values of picture elements represented in a circle K
1
, and wherein the following steps are implemented. The picture elements of the CT image D
1
are subjected to a median filtering, orthogonal to the straight line extending through the respective picture element and the center of the circle K
1
, for producing an image M
1
. A difference value image F
1
is generated by subtracting the image M
1
from the CT image D
1
. Two resulting images G
1
1
and G
2
1
are produced by filtering the picture elements of the difference value image F
1
in the respective directions of tangents t
1
1
and t
2
1
to the circle K
1
extending through the respective picture element. Filtering is conducted along t
1
1
to produce the resulting image G
1
1
and is conducted along t
2
1
for producing the resulting image G
2
1
. A correction image D
2
is then obtained by subtracting both of the resulting images G
1
1
and G
2
1
from the image D
1
.
Defective detector channels of a detector system lead to faulty values for picture elements, which appear as a circle in a CT image acquired by the detector system. Such image errors therefore are referred to as circle artifacts. Defective detector channels are not only ones that fail to supply an output signal as a result of the defect, but also are channels with a measuring accuracy that exceeds a specific tolerance value. Various methods are known for correcting or preventing circle artifacts, as noted above. These methods are carried out on the measurement data or on the image data and are primarily based on interpolated filtering, and achieve a noticeable weakening of the circle artifacts in the acquired CT images. Such known methods have the disadvantage, however, that line-like image errors frequently arise in the resulting CT images after such a method has been implemented. These image errors are referred to as line artifacts. They increasingly occur in association with large signal unsteadiness caused by high-contrast areas of an examination subject. The inventive method is particularly advantageous for eliminating such line artifacts, which occur after circle artifacts have been eliminated and which appear as tangents to the circles in the CT image. The size and position data of the circles of the circle artifacts caused by the defective detector channels are assumed to be known. It is sufficient to know the position of the circle center, which is the same for all circles, and the radii of the circles.
The elimination of the line artifacts is initially described for the case of a single defective detector channel. The faulty values of picture elements caused by the defective detector channel are situated on a circle K
1
. These faulty values are corrected by a known ring-balancing method on the measuring data or on the image data. Line artifacts, which appear as tangents to the circle K
1
, arise in the resulting CT image D
1
. For producing an image M
1
, a median filtering is carried out for each picture element of the CT image D
1
situated outside of the circle K
1
, orthogonally to the straight line extending through the respective picture element and the center of the circle K
1
. The width of the median filtering can be modifiable. This width should be selected wider than the expected line width of the line artifacts. A median filter of the width
5
has proven to be beneficial. The sampling distance A
1
must be selected dependent on the convolution kernel used for the reconstruction. Ideally, the image M
1
no longer contains line artifacts.
A difference value image F
1
=D
1
−M
1
is generated by subtracting the image M
1
from the CT image D
1
. For producing two resulting images G
1
1
and G
2
1
, a filtering is carried out in each picture element of the difference value image F
1
in the respective directions of the tangents t
1
1
and t
2
1
to the circle K
1
extending through the picture element in question, i.e., the filtering is carried out along t
1
1
for the resulting image G
1
1
and the filtering is carried out along t
2
1
for the resulting image G
2
1
.
Bruder Herbert
Flohr Thomas
Stierstorfer Karl
Von Der Haar Thomas
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
Wang George
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