X-ray or gamma ray systems or devices – Specific application – Computerized tomography
Reexamination Certificate
2002-06-13
2004-04-13
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Computerized tomography
C378S004000
Reexamination Certificate
active
06721387
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method of and system for reducing metal artifacts in images generated by x-ray scanning devices, and more particularly to a method of and system for manipulating an image generated by the x-ray CT scanner to remove metal artifacts.
BACKGROUND OF THE INVENTION
Computed Tomography (CT) inspection systems are utilized in many applications, including the fields of security and medicine. In the case of the security field, CT systems are used for scanning packages and baggage for the presence of explosives, weapons, and other contraband. While CT systems are very effective in detecting these types of objects, metal objects that may be present in the scanned items tend to generate streaks or artifacts in the resulting image, which degrade the quality of the image, causing the possibility of misread images. These artifacts appear in the form of dark and bright streaks that emanate from metal objects in the scanned item. These artifacts are caused because the metals objects greatly attenuate the x-ray beam and reduce the number of photons reaching the detector of the CT system, causing a low photon count at the detector. This results in a poor signal-to-noise ratio which causes high-frequency streak artifacts. Additionally, metal objects harden the x-ray beam by attenuating x-rays in an energy-specific manner. The resulting nonlinear changes in the projection data appear as low-frequency tail artifacts around the metal objects, as well as between the metal and other high-density objects.
Metal artifacts are problematic because they interfere with CT image analysis programs that are designed to identify features in the resulting image of the scanned item. For example, in programs that are designed to identify thin sheet objects which may be explosives, metal artifacts can cause the sheet object to appear distorted and possibly escape detection.
Prior art attempts at reducing metal artifacts in a CT image include removing the data associated with the metal objects from the scanned images, correcting the images by removing or reducing the metal artifacts and replacing the metal object data in the resulting images. However, in such systems, the integrity of any thin or sheet objects can be degraded, thus reducing the effectiveness of the detection system.
SUMMARY OF THE INVENTION
The present invention is directed to a method for correcting CT images for metal artifacts while preserving the integrity of thin objects in the images.
According to one aspect of the invention, a method of reducing metal artifacts in a computed tomography (CT) system includes:
A. generating a preliminary image from input projection data collected by the CT system;
B. identifying metal objects in the preliminary image;
C. generating secondary projections from the input projection data by removing projections of objects having characteristics that may cause the objects to be altered in a final artifact-corrected image.
D. extracting the projections of metal objects identified in step B from the secondary projection data generated in step C.
E. generating corrected projections by removing the projections of the metal objects extracted in Step D from the input projection data.
F. generating a final image by reconstructing the corrected projections generated in step E and inserting the metal objects identified in Step B into the final image.
Step B may include identifying objects in the preliminary image having intensities which are greater than a predetermined amount. Step B may further include selecting metal objects which generate artifacts. Step C may include identifying objects in the preliminary image having characteristics that may cause the objects to be altered in a final artifact-corrected image. Step C may further include generating a modified image by setting pixels in the preliminary image, which are not included in the objects identified in step C to a calculated value. The calculated value may be equal to mean value of the preliminary image determined using pixels other than the pixels included in the objects identified in step B. Step C may further include reprojecting the modified image and subtracting the reprojections from the input projection data to generate secondary projections which contain the input projection data with the projections of objects identified in step C removed. Step D may include identifying at least one region in the secondary projections which contain the projections of metal objects identified in step B. Step D may further include determining a baseline value of each of the at least one regions. The baseline value of each of the at least one region may be determined by linear interpolation between projection data surrounding the at least one region. Step D may further include subtracting each baseline value from the secondary projection values in its respective region to generate projection data corresponding to the metal projections in each region. The objects identified in step C may include objects in the preliminary image having at least one dimension which is less than a predetermined length. The objects identified in step C may include objects in the preliminary image having an intensity which differs from the mean intensity of the preliminary image by more than a predetermined threshold.
According to another aspect of the invention, a method of reducing metal artifacts in a computed tomography (CT) system includes:
A. generating a preliminary image from input projection data collected by the CT system;
B. identifying metal objects in the preliminary image which have generated metal artifacts in the preliminary image;
C. identifying objects in the preliminary image having characteristics which may cause the objects to be altered in a final artifact-corrected image;
D. generating a modified image by setting pixels in the preliminary image which are not included in objects identified in step C to a mean value of the preliminary image;
E. reprojecting the modified image;
F. subtracting the reprojections from the input projection data to generate secondary projections containing the preliminary image with the objects identified in step C removed;
G. identifying at least one region in the secondary image which contains projections of metal objects identified in step B;
H. determining a baseline value of each of the at least one regions;
I. subtracting each baseline value from the secondary projection values in its respective region to generate projection data corresponding to the metal object in each region;
J. subtracting the metal projection data from the input projection data to generate corrected projections;
K. reconstructing the final artifact-corrected image by:
a. reconstructing corrected projections; and
b. superimposing the metal objects identified in step B into the corrected image.
The objects identified in step C may include objects in the preliminary image having at least one dimension, which is less than a predetermined length. The objects identified in step C may include objects in the preliminary image having an intensity, which differs from a mean intensity of the preliminary image by more than a predetermined amount. In step G, the baseline value of each of the at least one region may be determined by linear interpolation between projection values surrounding the at least one region.
According to yet another embodiment of the invention, a system for reducing metal artifacts in a computed tomography (CT) system includes a tunnel having a platform disposed therein for receiving an object to be scanned, an x-ray source for directing x-rays at the object to be scanned, a detector array including a plurality of detectors for receiving the x-rays within the tunnel and processing means for reducing metal artifacts in images of the object reconstructed from the x-rays detected by the detectors. The processing means performs the steps of:
A. generating a preliminary image from input projection data collected by the CT system;
B. identifying metal objects in the preliminary image;
C. generating sec
Bechwati Ibrahim
Crawford Carl R.
Karimi Seemee S.
Naidu Ram
Simanovsky Sergey
Analogic Corporation
Bruce David V.
McDermott & Will & Emery
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