X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
1998-08-25
2001-05-08
Bruce, David V. (Department: 2876)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S015000, C378S901000
Reexamination Certificate
active
06229869
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to computed tomography systems, and more particularly, to image reconstruction when performing a scan with a tilted gantry using a multislice computed tomography system.
In a typical CT 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 or cell of the array produces a separate electrical signal that is a measurement of the beam attenuation at that detector location. The attenuation measurements from all the detector cells are acquired separately to produce a transmission profile.
In typical 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 rotational 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 rotational angle is referred to as a “view.” A “scan” of the object comprises a set of views made at different gantry rotational angles, or view angles, during one revolution of the x-ray source and detector. In a scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. This process is called image reconstruction, and there are many image reconstruction techniques. One such 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 and hence produce the image.
Image slices typically are acquired so that each slice is perpendicular to a longitudinal axis of the patient, i.e., the slices are substantially parallel to each other and spaced in the z direction. It often is preferable to acquire slices at a non-perpendicular orientation, however, to better visualize certain structures within the patient. For example, slices at angles other than 90 degrees to the longitudinal axis of the patient, i.e., non-transverse slices, are preferable when studying certain internal organs.
To provide such an angular orientation, the gantry of the system is tilted so that the axis of rotation of the x-ray source, or the gantry axis, is inclined relative to the axis of the patient. The gantry is tilted about a pivot point so that the gantry may be positioned at different tilt angles. An encoder or some other transducer is coupled to the gantry to detect the tilt angle. The angular orientation (theta) of the gantry about the pivot point, as indicated by the encoder or transducer, is supplied to a control processor that is programmed to control operation of the system.
Utilizing a tilted gantry to obtain images is problematic, however, in a CT system having multiple rows of detectors in the z-direction, known as a “multislice” system. The projection data acquired by a multislice system comprises data having different centers of rotation, due to the multiple rows of detectors in the z-direction. Typical image reconstruction methods, however, assume the data has the same center of rotation. Upon reconstruction of the image from the projection data using current systems, the different centers of rotation cause the image center to be shifted up or down.
Further, for axial CT scans, also known as stop-and-shoot scans, further error is introduced into a multislice system by the current table indexing methods. Present systems advance the table for each slice by the thickness of the x-ray beam in the z-direction. With a tilted gantry and multiple rows of detectors, however, this table indexing results in uneven slice spacing.
Additionally, current image reconstruction techniques create problems for helical CT scans using a tilted gantry. In a helical CT scan, the table constantly moves the patient in the z-direction at a specified speed while the data for the prescribed number of slices is acquired. Such a scan generates a single helix from a one fan beam helical scan, or in the case of multislice systems, multiple helixes. The helix mapped out by a fan beam yields projection data from which images in each prescribed slice may be reconstructed. In a multislice system, portions of projection data from multiple fan beams may be utilized to reconstruct a single slice. The differing centers of rotation for each portion of the data used to reconstruct the image cause artifacts and other image errors for a multislice system. Further, some current multislice systems combine projection data prior to the filtered back projection operation, mentioned above, to increase reconstruction speed. Combining projection data having differing centers of rotation, however, causes artifacts and other image errors in the reconstructed image.
Thus, utilizing current reconstruction techniques with a multislice system having a tilted gantry results in artifacts and other unacceptable defects in the image. Yet, implementing special, new reconstruction techniques only for tilted gantry scans in a multislice system would be time consuming, expensive and would create exceptions to the current standard techniques. Therefore, it is desirable to maintain the current reconstruction techniques and integrate them with multislice systems to produce images of equivalent quality.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a system and method for the accurate reconstruction of tilted gantry images comprising projection data having different centers of rotation relative to a longitudinal patient axis. According to the present invention, a computerized tomography system comprises a gantry having an aperture adapted to encircle an imaging object, and including an x-ray source and an opposing detector array each rotateably mounted on opposite sides of the aperture. A plane between the x-ray source and the detector array forms a gantry plane that is positionable along a longitudinal axis of the object. The frame is adapted for tilt displacement of the gantry plane at a tilt angle relative to the longitudinal axis. The x-ray source is rotateable about an axis of rotation perpendicular to the tilt angle and is capable of emitting x-rays from a focal spot at a each of a plurality of views corresponding to a combination of a rotational angle value and an associated tilt angle value. The detector array has a plurality of detector elements forming a plurality of fan beam planes between each of the plurality of detector elements and the x-ray source. Each of the plurality of fan beam planes has a center of rotation value along the axis of rotation. Each of the plurality of detector elements is capable of receiving the emitted x-rays at each of the plurality of views and providing raw projection data indicating the intensity of the x-rays received at each of the plurality of views.
Additionally, the system includes a computer system in communication with the detector array. The computer system has a memory to store data including data representative of the raw projection data, and the rotational angle value, the tilt angle value, and the center of rotation value corresponding to the raw projection data. The computer system further includes program signals representative of an algorithm for constructing image data from the raw projection data. The algorithm shifts the center of rotation values associated with the image data by an offset amount that is a function of the tilt angle so that the image data is centered about the long
Bruce David V.
Cabou Christian G.
General Electric Company
Teasdale LLP Armstrong
LandOfFree
Tilted gantry image correction for a multislice computed... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Tilted gantry image correction for a multislice computed..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Tilted gantry image correction for a multislice computed... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2463709