Abstract:
An imaging system which, in one embodiment, includes a timing algorithm which identifies the proper projection data to be used and a modified halfscan image reconstruction algorithm which provides improved image quality along with the benefits of an enhanced temporal response, is described. In an exemplary embodiment, the timing algorithm includes the steps of determining a diastolic period of a patient&#39;s heart and corresponding projection data during the diastolic period. The modified halfscan algorithm includes the steps of identifying redundant data and unequally weighting the data. The resulting images are used for coronary calcification detection.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to imaging and more particularly, to coronary calcification detection using an imaging system. 
     In at least one known imaging system generally referred to as a computed tomography (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 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, 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. 
     To reduce the total scan time, 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. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed. 
     With known CT system, 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 frame to minimize motion related image degradation. 
     To detect coronary calcification in a patient, images of the patient&#39;s heart are generated and reviewed to identify calcium deposits. However, as a result of the movement of the heart and the blood, the heart images may be blurred. The blurring causes difficulty in identifying the areas of calcium deposits. 
     To reduce the blurring of the images, it is desirable to provide an imaging system which gathers data as the heart motion is minimized. It would also be desirable to provide such a system which weights redundant data having different amounts motion with different weights to improve image temporal resolution. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other objects may be attained by an imaging system which, in one embodiment, gathers image data during the relatively motion free period and includes a modified halfscan image reconstruction algorithm which weights redundant data to provide acceptable image quality along with the benefits of an enhanced temporal response. In an exemplary embodiment, the imaging system utilizes an EKG signal to determine a diastolic period of the heart. The diastolic period is then utilized to determine an ending point of the projection data to be used to reconstruct an image. The minimum data duration is then subtracted from the ending point to determine a beginning point of the collected projection data. 
     In one embodiment, the modified halfscan image reconstruction algorithm unequally weights redundant projection data. More specifically, a higher weight is applied to data collected during a period of less motion of the heart. 
     The above described imaging system uses projection data during the period of time when heart motion is minimized. In addition, the system unequally weights redundant data having different amounts motion to improve temporal resolution of the images. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of a CT imaging system. 
     FIG. 2 is a block schematic diagram of the system illustrated in FIG.  1 . 
     FIG. 3 is an EKG signal waveform. 
     FIG. 4 is a radon space diagram for a modified halfscan. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a computed tomograph (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of gantry  12 . Detector array  18  is formed by detector elements  20  which together sense the projected x-rays that pass through a medical patient  22 . Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . 
     Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detector elements  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 . 
     Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 . 
     The timing and image reconstruction algorithms described herein typically are implemented by image reconstructor  34 . Such algorithms, however, could be implemented in other components of the imaging system such as in computer  36 . Also, it should be understood that system  10  is described herein by way of example only, and the following described timing and image reconstruction algorithms can be practiced in connection with many other types of imaging systems. 
     In operation, imaging system  10  is configured to generate at least one image of an object in a defined condition, or state. In one embodiment, system  10  is used to generate a series of images of a patient&#39;s heart to assist in the detection of coronary artery calcification (CAC) . Specifically, after collecting projection data and corresponding heart data, system  10  generates image data by selecting a portion of the projection data corresponding to a relatively motion free period of the heart. The selected projection data is then weighted in accordance with a halfscan weighting function. 
     More specifically and in one embodiment, system  10 , utilizing a timing algorithm, detects the condition, or state, of the heart of patient  22  by measuring, or determining, the state of an electrocardiography (EKG) signal. The EKG signal is coupled to system  10 , e.g., computer  36 , and represents the electrical activity associated with the heart muscle versus time. Referring to FIG. 3, the EKG signal waveform illustrates one cardiac cycle including a systole condition, or period, and a diastole condition, or period of the heart. The portion of the EKG signal which is labeled Q, R and S is referred to as the QRS complex, in which the R-feature, or R-wave, is the most prominent, highest amplitude, feature of the entire EKG signal. The cardiac cycle is typically defined as beginning with a R-wave and continuing until the occurrence of the next R-wave. 
     Heart functions are characterized by two distinct periods called systole and diastole. In systole, the heart muscle is contracting the volume of the left ventricle to pump the contents out through the aortic valve. During the diastole, or diastolic period, the left ventricle is filling through the mitral valve. At the end of the systole, the left ventricle has its smallest volume since it has been contracted to pump blood out. The end of the diastole is the point at which the left ventricle has its largest volume since it is filled with blood ready to be pumped out. During the diastolic period the heart is relatively motion-free allowing images generated from data collected during this period to be clearer as a result of the limited movement. 
     Particularly, corresponding projection data and heart data, i.e., the EKG signal, specifically, the R-wave, are continuously collected using system  10 . In one embodiment, projection data corresponding to the heart being in a diastolic condition, as determined by the state of the EKG signal, is used in image reconstruction. More specifically, as a result of the cardiac cycles being fairly constant, the R-wave portion of the EKG signal is utilized to determine an end of data period, or time, to be used in reconstruction. The end of data point defines the end of the portion of projection data to be used and is determined with respect to the time when the R-wave transitions from a first state, e.g., a low voltage level, to a second state, e.g., a value above a defined, or selected, voltage level. The R-wave second state helps to indicate the end of the diastolic condition of the heart. 
     A portion of the data collected using system  10  is then selected based on certain scan variables, including the scan speed, using, for example, computer  36 . A begin, or start, of data point, or time, to be used in reconstruction is then determined by subtracting a minimum data duration from the end of data period. 
     For example and in one embodiment, for a 1 second gantry rotational speed scan, the minimum data duration is approximately 0.6 second. If the end of data point is designated as time E, the begin of data point equals the quantity (E-0.6). In one embodiment, the value of E is derived relative to the R-wave transition, i.e., a low to high level transition, by adding or subtracting an offset, or error. 
     After the projection data to be used is determined, object images are generated using the halfscan weighting function, or algorithm. As shown in FIG. 4 illustrating a Radon space representation of the projection data, the upper and lower shaded triangles represent the beginning and ending portions of the projection data. More specifically, the upper and lower shaded triangles represent a respective first data set and second data set of redundant data. To improve temporal resolution of the images and reduce motion susceptibility, a modified halfscan algorithm is utilized. In one embodiment, the modified halfscan algorithm unequally weights the projection data. More specifically, after identifying the redundant data, the modified halfscan algorithm applies unequal weights to the redundant data. Particularly, a first weight is applied to the first data set and a second weight is applied to the second data set. More specifically and in one embodiment, where the first data set includes data experiencing less motion of the heart, the first weight is greater than the second weight. 
     After weighting the projection data in accordance with the modified halfscan algorithm, the images are generated in accordance with known image generation algorithms, e.g., filtered backprojection. Utilizing the images, an operator may identify, or detect coronary artery calcification of the heart of patient  22 . 
     The above described imaging system uses projection data during the period of time when the heart motion is minimized. In addition, the system unequally weights redundant data having different amounts motion to improve resolution of the images. As a result of the images being less motion susceptible, coronary artery calcification detection is improved. 
     From the preceding description of various embodiments of the present invention, it is evident that the objects of the invention are attained. Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is intended by way of illustration and example only and is not to be taken by way of limitation. In addition, the CT system described herein is a “third generation” system in which both the x-ray source and detector rotate with the gantry. Many other CT systems including “fourth generation” systems wherein the detector is a full-ring stationary detector and only the x-ray source rotates with the gantry, may be used if individual detector elements are corrected to provide substantially uniform responses to a given x-ray beam. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims.