X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1998-09-29
2001-04-10
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Reexamination Certificate
active
06215841
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to computed tomography (CT) imaging and more particularly, to three dimensional artifact reduction in a CT system.
In at least one known CT imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a “helical” scan may be performed. To perform a “helical” scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighing algorithms which weight the collected data as a function of view angle and detector channel index. Specifically, prior to filtered back projection, the data is weighted according to a helical weighing factor, which is a function of both the gantry angle and detector angle. The helical weighting algorithms also scale the data according to a scaling factor, which is a function of the distance between the x-ray source and the object. The weighted and scaled data is then processed to generate CT numbers and to construct an image that corresponds to a two dimensional slice taken through the object.
It often is desirable to generate three-dimensional (3D) images or multi-planar reformatted images (herein referred to as 3-D images) of the object. Known algorithms for generating such images further process the helically weighted and scaled data. However, the 3D images typically include noticeable artifacts. Particularly, as a result of a heterogenous object being constantly translated during data acquisition, the attenuation distribution inside the scan plane changes continuously. To correct for this deviation from the basic tomographic reconstruction assumption, helical reconstruction is utilized to suppress the image artifacts. Recent developments of various computer graphic techniques applied to helical computed tomography, however, have discovered additional artifacts. These artifacts appear as periodical rings or grooves superimposed on the surface of the 3D image.
It would be desirable to provide an algorithm which facilitates the reduction of artifacts in 3D images due to the heterogenous object. It also would be desirable to provide such an algorithm which does not significantly increase the processing time.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained in a system which, in one embodiment, includes a 3D image generation algorithm that reduces artifacts by determining object boundaries using an optimal intensity threshold. More particularly, and in accordance with one embodiment of the present invention, a threshold intensity level is selected, based on the intensity of the object as well as its background such that the 3D image artifact is minimized, to determine the composite boundary profile. This composite boundary profile, is then used to generate a 3D image of the object.
In accordance with another embodiment, the composite boundary profile is determined by determining the slope of the object boundary and a system edge response. This information, together with the weighting utilized in a helical reconstruction algorithm are used to determine a boundary error. Based on the boundary error information, adaptive filtering is then employed to reduce the inconsistency of the object boundary due to the helical reconstruction algorithm. More specifically, the object boundary profile for an axial scan is:
g
(
x
)
=
Γ
h
k
⊗
f
where:
Γ
h
k
(
x
)
=
{
0
x
≤
0
kx
h
0
<
x
<
h
k
1
Other
f is a edge response of an imaging system;
k is a slope of the object boundary in x-z plane;
h is a slice thickness;
x is an image index variable; and
{circle around (×)} represents a convolution.
For a helical scan, the composite boundary profile is:
g(x)=w
1
g
1
(x−x
1
)+w
2
g
2
(x−x
2
)
where:
g
1
, g
2
are object boundary profiles (if axial scan were taken), as shown above;
w
1
, w
2
are helical reconstruction weights associated with views that are perpendicular to the boundary which form a complimentary sampling pair;
x
1
, x
2
are object boundary locations at two views that are perpendicular to the object boundary.
By generating the composite boundary profile as described above, the reduction of artifacts in 3D helical image reconstruction may be achieved by either adaptive filtering of the composite boundary or by adjusting the boundary location during 3D image generation. Particularly, by generating the 3D image using the above described process, reduces, and possibly eliminates, artifacts. Such algorithm also does not significantly increase the processing time.
REFERENCES:
patent: 4712178 (1987-12-01), Tuy et al.
Carl Crawford and Kevin King, “Computed Tomography Scanning with Simultaneous Patient Translation,” Med. Phys. 17 (6), Nov./Dec. 1990, pp. 967-982.
Jiang Hsieh, “A general approach to the reconstruction of x-ray helical computed tomography,” Med. Phys. 23 (2), Feb. 1996, pp. 221-229.
Armstrong Teasdale LLP
Bruce David V.
Cabou Christian G.
General Electric Company
LandOfFree
Methods and apparatus for 3D artifact reduction does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Methods and apparatus for 3D artifact reduction, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Methods and apparatus for 3D artifact reduction will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2465161