X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2000-03-28
2001-08-21
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S901000
Reexamination Certificate
active
06278762
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to medical imaging and more particularly, to the reconstruction of images from projection data acquired from a helical scan by a multislice scanner.
In at least one known medical imaging system typically referred to as a computed tomography (CT) system, 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 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 that 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 required for multiple slices, 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. An image reconstruction algorithm which may be utilized in reconstructing an image from data obtained in a helical scan is described in U.S. Pat. application Ser. No. 08/436,176, filed May 9, 1995, and assigned to the present assignee.
The projection data gathered with fan-beam helical scan can be denoted as P(&thgr;,&ggr;,z) where &thgr; is the angle of the central ray of the fan beam with respect to some reference (e.g., the y axis), &thgr; is the angle of a particular ray within the fan beam with respect to the central ray, and z is the axial gantry position at the time the measurement is made. For each location z
0
at which actual projection data is not obtained, a commonly used and known helical reconstruction algorithm produces raw data for a slice at location z
0
by using linear interpolation in the z direction. Specifically, to produce P(&thgr;, &ggr;, z
0
), projection data at the same &thgr; and &ggr; and as close as possible, but on opposite sides in z, to z
0
are used. For example, if z
1
and z
2
are the values of z for which P(&thgr;, &ggr;, z) are available, and for which z
1
≦z
0
≦z
2,
P(&thgr;, &ggr;, z
0
) may be estimated from P(&thgr;, &ggr;, z
1
) and P(&thgr;,&ggr;,z
2
) by linear interpolation using the following:
P
(
θ
,
γ
,
z
0
)
=
z
2
-
z
0
z
2
-
z
1
P
(
θ
,
γ
,
z
1
)
+
z
0
-
z
1
z
2
-
z
1
P
(
θ
,
γ
,
z
2
)
.
(
1
)
In a helical scan, since the same ray is measured twice in each 360° rotation, i.e. , P(&thgr;,&ggr;,z)=P(&thgr;+2&ggr;+180°,−&ggr;,z), the z sampling is effectively doubled. This increased sampling enables reducing the total scan time.
It is desirable, of course, to reconstruct images from the data obtained in a four beam helical scan in a manner which provides a high quality image with a low level or number of artifacts. It also is desirable to reduce the total time required to reconstruct such an image. Further, since data may not necessarily be obtained for every axial location, it would also be desirable to provide an algorithm to estimate such projection data in a manner which enables generation of a high quality image.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained in a system which, in one embodiment, generates projection space data arrays from projection data acquired by each fan beam in a four fan beam helical scan. Data in each array is then weighted by the system to correct for the translational motion of the patient and to offset data redundancy effects. An image is then reconstructed using the weighted data.
More specifically, in reconstructing an image, the system generates projection data arrays which correspond to data planes associated with the slice to be imaged. Weighting factors are then applied by the system to the data arrays to assign a weight to each particular data element. The weighted projection data arrays are then filtered and back projected to generate an image data array. The image data arrays are then summed to generate a slice image data array.
With respect to reconstructing a slice for a particular slice at a particular z
0
location at which projection data was not actually measured, and in one embodiment, the projection data for the z locations closest to, but on opposite sides of, the particular z
o
are identified. The projection data for the slice is then estimated using the projection data gathered at the identified z locations. The slice image can then be reconstructed using such estimated projection data.
Using a four beam helical scan of a patient provides the advantage that total patient scan time is reduced. Further, the image reconstruction algorithm described above provides the advantage that even though the patient table translation speed is increased, a high quality image slice may be generated.
REFERENCES:
patent: 5541970 (1996-07-01), Hu
patent: 5559847 (1996-09-01), Hu et al.
patent: 5974110 (1999-10-01), Hu
patent: 6055291 (2000-04-01), Hu
Armstrong Teasdale LLP
Bruce David V.
Cabou Christian G.
General Electric Company
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