X-ray or gamma ray systems or devices – Electronic circuit – With display or signaling
Reexamination Certificate
1999-03-03
2001-05-01
Bruce, David V. (Department: 2882)
X-ray or gamma ray systems or devices
Electronic circuit
With display or signaling
C378S098200, C378S098110
Reexamination Certificate
active
06226350
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to an imaging system, and more particularly, to a reconstruction algorithm for generating images representing a moving heart.
In at least one known 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 at least one known type of imaging system, commonly known as a computed tomography (CT) system, a group of x-ray attenuation measurements, i.e., projection data, from the detector array 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.
With known CT systems, projection data is collected from a helical or axial scan to generate sequential frames of images of an area, or organ, within a patient. A frame corresponds to a two dimensional slice taken through the imaged object, e.g., the patient. Typically, an operator attempts to minimize the amount of time required to generate each image by increasing the frame rate while minimizing image degradation.
At least one known CT system collects data utilizing a large flat panel digital x-ray device, or detector, having a plurality of pixels arranged in rows and columns. Each pixel includes a photosensor, such as a photodiode, that is coupled via a switching transistor to two separate address lines, a scan line and a data line. During operation, x-ray beams passing through the object are incident on the imaging device. The radiation incident on a scintillator material and the pixel photosensors measure, by way of change in the charge across the diode, the amount of light generated by x-ray interaction with the scintillator. As a result, each pixel produces a digital electrical signal that represents the intensity of an impinging x-ray beam.
To detect coronary calcification in a patient, images of the patient's heart are generated and reviewed to identify calcium deposits. This can be accomplished by detecting the average calcium concentration in a set of CT images. However, as a result of the data collection rate and the movement of the heart and the blood, the heart images may be blurred. On the other hand, the fluoroscopy mode of the digital x-ray device is capable of generating 30 frames per second or higher image rate which is sufficient to overcome blurring due to heart motion. However, the images may be difficult to view as a result of the structures which overlap over the heart. For example, the images may include ribs, a lung, and other surrounding soft tissue. These overlapping structures cause difficulty in identifying areas of calcium deposits.
To reduce the blurring of the images, it is desirable to provide an imaging system which gathers data at sufficiently high rate so that the heart motion is minimized. It would also be desirable to provide such a system which removes the overlapping structures from the images to improve the quality of the heart images.
BRIEF SUMMARY OF THE INVENTION
These and other objects may be attained in a digital x-ray imaging system which, in one embodiment, includes an reconstruction algorithm which filters collected data to remove overlapping, or stationary, structures so that clear enhanced images of a heart are generated. The enhanced images are then used to identify cardiac calcification in the heart. More particularly, and in accordance with one embodiment of the present invention, the imaging system includes an x-ray source and a flat panel digital detector array for high speed collection of projection data.
In one embodiment, after collecting a sequence of projection data, an estimated background is determined. More specifically, the estimated background is determined by generating an average of the entire sequence of data. The average represents the structures remaining stationary during data collection and an averaged or blurred heart. The stationary structures are then filtered, or removed, from the projection data by subtracting the estimated background from the projection data.
In another embodiment, difference data is determined between a selected frame of projection data and the remaining frames of projection data. An average difference value is then determined, for the entire image, or a specific region of interest. In one embodiment, by monitoring the average difference, the phase of the heart may be determined. More specifically, where the average difference represents the cardiac cycle of the heart, data collected from an opposite phase of the heart may be excluded from the collected data to improve image quality. Particularly, a maximum difference value may be utilized to exclude or assign a lower weight to the projection data collected during an opposite phase of the heart.
In another embodiment, an ECG signal representative of the phase of the heart is utilized to identify data collected when the heart is significantly out of phase with a reference frame of data. As discussed above, the data collected during these out of phase periods are then either excluded or lower weighted.
In yet another embodiment, the imaging system includes multiple x-ray sources and multiple detector arrays. By positioning these sources and detectors at different orientations, or angles, around a patient and time synchronizing the data, differential images are generated which provide depth information to further localize calcification.
By collecting and filtering the projection data sets as described above, identification of cardiac calcification level in the heart may be achieved. Particularly, by removing the overlapping structures and generating images of the moving heart, the quantity and location of the calcification within the heart may be identified.
REFERENCES:
patent: 4720843 (1988-01-01), Haaker et al.
patent: 5930329 (1999-07-01), Navab
patent: 6005917 (1999-12-01), Andersson et al.
Armstrong Teasdale LLP
Bruce David V.
Cabou Christian G.
General Electric Company
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