The invention relates to computed tomography (CT) and, more particularly, to an apparatus and a method for use in a CT system for reconstructing an image with reduced artifacts.
Computed tomography (CT) is a technique that generally involves subjecting a patient to x-rays, acquiring digital x-ray data of a portion of the patient""s body, and processing and back-projecting the digital x-ray data to produce an image that is then displayed on a display monitor of the CT system. CT systems typically comprise a gantry, a table, an x-ray tube, an x-ray detector array, a computer and a display monitor. The computer sends commands to controllers of the gantry to cause the controllers to rotate the x-ray tube and/or the detector array at a particular rotational speed. The detector array is usually comprised of either a curved array (third generation CT system) of detector elements or a ring (fourth generation CT system) of detector elements. In the case where a ring of detector elements is used, only the x-ray tube is rotated.
In third and fourth generation CT systems, relative rotational motion is produced between the detector array and the x-ray tube about the patient""s body. As this relative rotational motion is produced, the computer controls the data acquisition process performed by the x-ray tube and the detector array to acquire digital x-ray radiographs. The computer then processes and back-projects the digital x-ray radiograph data by performing a reconstruction algorithm and displays the reconstructed CT image on the display monitor.
In current CT systems, in order to appropriately produce CT images of the coronary vasculature, it is necessary to acquire CT radiograph data while the heart is at a certain position that is substantially spatially stationary. This requires that the heart rate of the patient be extremely slow, which is not clinically viable, or that the speed of the gantry be extremely high, which is not technically viable. In the past, a few different techniques have been employed in attempts to solve this problem. One technique, known as prospective gating, uses the ECG (electrocardiogram) signal of the heart to trigger data acquisition by the detector array at points in time when the heart is fairly stationary (typically during diastole) so that the radiographs used to reconstruct the image correspond to instants in time when the heart is fairly stationary. Another technique, known as retrospective gating, measures the ECG signal while acquiring CT radiograph data and then retrospectively selects the data that corresponds to a point in time of the ECG signal when the heart is fairly stationary.
With both of these techniques, only CT radiograph data that corresponds to a certain time interval during which the heart is substantially spatially stationary is used in reconstructing the CT images. Therefore, during reconstruction, both techniques only use CT radiograph data corresponding to limited view angles, i.e., neither technique uses measured CT radiograph data at all view angles of the CT gantry. Also, both techniques use CT radiograph data obtained during a particular window in time as the heart is moving. Consequently, the CT reconstructions may suffer from motion artifacts and/or limited view angle artifacts.
Another disadvantage of these techniques is that they are limited to reconstructing an image of the heart at a particular time interval, which typically corresponds to diastole. Therefore, neither of these techniques are suitable for reconstructing a four-dimensional (AD) representation of a heart (i.e., 3-D spatial and 1-D temporal). It would be desirable to provide a technique by which a reconstructed image of the heart and coronary vasculature could be generated at any point in time during the cardiac cycle. It would also be desirable to provide a technique by which an image of the heart and coronary vasculature could be generated at several points in time during the cardiac cycle for all views of the gantry to thereby enable a 4-D representation of the heart to be generated.
Accordingly, a need exists for a method and apparatus for use in a CT system that enable CT radiograph data corresponding to all views of the gantry to be utilized in performing CT reconstruction so that the occurrence of limited view angle artifacts in reconstructed CT images can be reduced or eliminated. A need also exists for a method and apparatus for use in a CT system that enable reconstruction to be performed at any instant in time during the cardiac cycle so that the occurrence of motion artifacts in the reconstructed CT images can be reduced or eliminated. A need also exists for a method and apparatus for use in a CT system that will enable reconstruction to be performed at several points in time during the cardiac cycle to enable a 3-D or 4-D representation of the heart to be generated.
The present invention provides a method and apparatus for use with a computed tomography (CT) system that collects CT radiograph data for every view of the CT gantry so that a data set corresponding to all views of the gantry is available for use in reconstructing an image of a patient""s heart and coronary vasculature. For each view of the gantry, the CT radiograph data associated with the view is collected at different instants in time with respect to the period of the cardiac cycle in each revolution of the gantry.
Prior to data acquisition, the patient""s heart rate is measured and the period of the gantry is set such that data is acquired at a different time with respect to the period of the cardiac cycle for every view of the gantry and for each revolution of the gantry. Therefore, for each revolution of the gantry and for each view of the gantry, the instant in time in the period of the cardiac cycle at which any given detector element of the detector array is sampled will be different from the instant in time in the period of the cardiac cycle at which the same detector element was sampled in the previous revolution. After all of the CT radiograph data is collected, the radiographs are processed by an interpolation algorithm that interpolates radiographs to a selected instant in time with respect to the period of the cardiac cycle. A reconstruction algorithm is then used to process and back-project the interpolated radiographs to produce a three-dimensional (3-D) image of the heart and coronary vasculature. The interpolation algorithm may be performed repeatedly to interpolate radiographs back to more than one instant in time, and then corresponding reconstructions may be performed to generate a four-dimensional (4-D) image of the heart and coronary vasculature.