Abstract:
In one aspect, the present invention is a method for producing CT images of a patient&#39;s heart suitable for calcification scoring, in which the heart has a cardiac cycle. The method includes steps of acquiring data representative of a first scout-scanned CT image of physical locations of the patient&#39;s body including at least a portion of the patient&#39;s heart at phases φ 1 (L) of the cardiac cycle, acquiring data representative of a second scout-scanned CT image of the physical locations of the patient&#39;s body including at least a portion of the patient&#39;s heart at phases φ 2 (L) of the cardiac cycle different from φ 1 (L) at physical positions L of interest, and determining a difference image from the acquired data representative of the first scout-scanned CT image and the acquired data representative of the second scout-scanned CT image data. It is not necessary that φ 1 (L) and φ 2 (L) be constant as a function of position L.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to methods and apparatus for cardiac CT imaging, and more particularly to methods and apparatus that minimize an impact of heart motion in collecting calcification data from coronary images. 
     In at least one known computed tomography (CT) 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 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 interests 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. 
     A main objective of cardiac CT applications is to perform calcification scoring, a diagnostic procedure in which an amount of calcification present in a patient&#39;s heart is estimated. At least one known CT imaging system requires about 0.5 s to complete data acquisition for an image. Although this speed is satisfactory for general imaging purposes, it is not fast enough to avoid motion-induced image artifacts in cardiac CT imaging, in which a typical cardiac cycle is about 1.0 s long. These artifacts present major problems for cardiac calcification scoring. 
     At least one other known CT imaging system reduces motion-induced image artifacts by acquiring data rapidly enough to effectively freeze cardiac motion. This imaging system employs a scanning electron beam to generate a moving source of x-rays rather than an x-ray source and detector on a rotating gantry. However, CT imaging systems employing scanning electron beams are quite expensive and are not available at many hospitals. 
     It would therefore be desirable to provide methods and apparatus that overcome motion-induced artifacts produced in images acquired by CT imaging systems having relatively slow scanning and detection systems such as rotating gantries. It would also be desirable to provide cardiac calcification scoring methods and apparatus utilizing such CT imaging systems. It would further be desirable to provide methods and apparatus that can readily identify and score calcification from the small incremental x-ray attenuation produced by small amounts of calcification. 
     BRIEF SUMMARY OF THE INVENTION 
     There is therefore provided, in one embodiment of the present invention, a method for producing CT images of a patient&#39;s heart suitable for calcification scoring, in which the heart has a cardiac cycle. The method includes steps of acquiring data representative of a first scout-scanned CT image of physical locations of the patient&#39;s body including at least a portion of the patient&#39;s heart at phases φ 1 (L) of the cardiac cycle, acquiring data representative of a second scout-scanned CT image of the physical locations of the patient&#39;s body including at least a portion of the patient&#39;s heart at phases φ 2 (L) of the cardiac cycle different from φ 1 (L) at physical positions L of interest, and determining a difference image from the acquired data representative of the first scout-scanned CT image and the acquired data representative of the second scout-scanned CT image data. It is not necessary that φ 1 (L) and φ 2 (L) be constant as a function of position L. 
     The above described embodiment overcomes motion-induced image artifacts by making calcification signals more readily observable as a change between images. Moreover, even small amounts of calcification are readily identifiable and qualifiable, because much larger variations in x-ray attenuations that would otherwise hide calcification deposits are canceled out. 
    
    
     
       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 a representative of a portion of the system illustrated in  FIG. 1  showing a patient translated by the table shown in  FIG. 1  while the x-ray source and detector remain stationary during a scout scan. 
         FIG. 4  is a representation of a scout image showing time relationships between columns of data that make up the scout image. 
         FIG. 5  is a simplified graphical representation of an electrocardiogram, showing times represented by columns in the scout image of  FIG. 4 and a  relationship between a first scout image and a second scout image in one embodiment of the present invention. 
         FIG. 6  is a simplified graphical representation of intensity vs. detector location in a column of a first scout scan. 
         FIG. 7  is a simplified graphical representation of intensity vs. detector location in a column of a second scout scan corresponding to the column represented in FIG.  6 . 
         FIG. 8  is a representation of a difference between intensities as a function of detector location between data such as that represented in FIG.  6  and  FIG. 7 , whereby a calcium signal is isolated in one embodiment of the present invention.  FIGS. 6 ,  7 , and  8  should not necessarily be assumed to be drawn to the same scale. 
         FIG. 9  is a representation of pixels of an image analyzed using image processing techniques in an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIGS. 1 and 2 , a computed tomography (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 an object  22 , for example a medical patient. Detector array  18  may be fabricated in a single slice or multi-slice configuration. 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 parameter 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 . Particular, table  46  moves portions of patient  22  through gantry opening  48  along a z-axis. In some of the embodiments described below, cardiac cycles are measured utilizing EKG machine  50 . 
     An amount of calcification present in the cardiac system of patient  22  is reliably estimated from scout images taken with CT imaging system  10  in one embodiment of the present invention. Patient  22  is instructed to hold his or her breath while images of the heart of patient  22  are scanned by CT imaging system  10  in a scout imaging mode of operation. Because patient  22  is holding his or her breath, the only moving object within the scan field of view is the heart of patient  22 . Data for two scout scans are obtained, and a difference between the data for the two images is used to remove non-moving body structure and highlight calcification, as explained below. 
     In one embodiment of the invention and referring to  FIG. 3 , scout-scanned data of patient  22  is acquired. Scout-scanned data is acquired by moving table  46  through gantry opening  48  in a z-direction while gantry  12  is held stationary. Thus, x-ray source  14  and detector array  18  are stationary, while data such as that represented in  FIG. 4  is acquired. The acquired data represents a CT image of a region of the body of patient  22 . Each column of scout image  52  is representative of x-ray attenuation data obtained at a certain instant of time. In one embodiment, each column of data, such as column  54  and column  56 , is acquired in approximately 1 millisecond. Thus, each successive column of acquired data is sampled at a slightly different time. In  FIG. 4 , for example, column  54  is acquired at item t and at a displacement p from a starting position of the scout scan in a z-axis direction. Column  56 , which is immediately adjacent to column  54 , is acquired at time t+Δt, where Δt is approximately 1 millisecond. Column  56  is located at displacement p+Δp from the start of the scout scan. An entire scout scan image  52  sufficient for the present embodiment is taken in about two or three seconds. Data representing physical locations of a portion of the body of patient  22  including at least a portion of heart  58  is acquired and used for cardiac calcification scoring. 
     Data representing a second scout image (not shown) is also acquired. The second scout scan image is acquired in a manner that ensures that corresponding columns of the second scout scan are taken at times during which heart  58  is in a different phase of first scout image  52 . In this manner, data representative of a first and a second scout-scanner CT image of physical locations of the body of patient  22  are obtained. Data for each physical location is obtained at different phases of the cardiac cycle in the two images. 
     For example, referring to  FIG. 5 , column  54  of first scout image  52  is acquired at a time corresponding to phase  60  just prior to systole  62  of EKG signal  64 . (EKG signal  64  is obtained by monitoring heart  58  of patient  22  using EKG machine  50 .) Column  56  of scout image  52  is acquired at a time corresponding to phase  66 . The second scout image is taken a few seconds after first scout image  52 . By selection of a start time for its acquisition, corresponding columns of the second scout image are acquired at phases  68  and  70 , immediately after systole  72 . Both scout images are acquired at the same rate starting from the same position of patient  22  and table  46  moves at the same speed and in the same direction for each scout image acquisition in this embodiment. Thus, starting each scan at a different phase of EKG signal  64  is sufficient to ensure that corresponding columns in the two scans represent different phases of heart  58 , assuming heart  58  is beating at a constant rate. This assumption is applicable because the entire procedure is completed in only a few seconds. 
     In one embodiment, scout scans are manually started. For example, phases of EKG signal  64  from EKG machine  50  are manually monitored to determine trigger times to begin each scout-scanned data acquisition. In another embodiment, scans are started automatically. For example, computer  36  of CT imaging system  10  is configured to receive and monitor EKG signal  64  or an equivalent to determine trigger times. 
     During scanning, patient  22  holds his or her breath and remains as still as possible to minimize differences between the first and the second scout images other than those related to heart movement. It is reasonable to request patients to hold their breath during the scanning and feasible for patients to comply with such requests due to the brevity of the procedure. 
     It will be observed that data for each scout image, for example, image  52 , is a composite representing different phases of heart  58 . Each physical location L represented by data of the first scout image is acquired at a phase φ 1 (L) of the cardiac cycle. For the second scout image, data for location L is acquired at a phase φ 2 (L), where φ 1 (L)≠φ 2 (L). Because of the amount of time taken by table  46  to travel from one end of each scout scan to the other, neither φ 1  nor φ 2  are constant across each scout image. However, their difference at any location L is constant, or nearly so. The present invention advantageously uses this difference to highlight cardiac calcification. 
     Columns of intensity (or equivalently, attenuation) data is obtained by detector array  18  while table  46  moves to obtain a scout scan. Each column, for example column  54  shown in  FIG. 4 , represents data obtained simultaneously by different detector elements  20  of detector array  18 .  FIG. 6  represents a plot of intensity data received for a column in a first scout scan as a function of detector element position in the column. (Arrow A is shown in  FIGS. 6 ,  7 , and  8  to provide a directional reference with respect to FIG.  4 . However, it should not be assumed that  FIGS. 6 ,  7 , and  8  are necessarily representative of the image shown in  FIG. 4 , nor should it be assumed that  FIGS. 6 ,  7 , and  8  are drawn to the same scale.) Although cardiac calcification data is present in  FIG. 6 , a calcification signal is not immediately evident.  FIG. 7  shows a similar plot of a column in a second scout scan of patient  22  containing data representative of the same physical positions of patient  22 , but at a different phase of the cardiac cycle of heart  58 . An example of differences between two column signals such as those of FIG.  6  and  FIG. 7  is plotted in FIG.  8 . Because the body of patient  22  is essentially motionless except for beating heart  58  (disregarding motion of table  46 ), overlaying, non-moving body structures of patient  22  are removed by computing differences between the two scout images. As a result, the signals shown in  FIG. 8  represent essentially only moving heart  58 . Because calcification signals are stronger than those of soft tissue and because calcification deposits move with heart  58 , signals from calcification deposits such as peak  74  are very apparent. Thus, when a difference image is determined between the two images that include the columns represented in  FIGS. 6 and 7 , peaks such as peak  74  are easily seen. Peak  74  is thus readily identified as a calcification deposit on a portions of the image corresponding to a moving body structures of patient  22 . In one embodiment, computer  36  computes difference images and displays the computed difference images on CRT display  42 . Calcification scoring is readily accomplished using these computed difference images, either manually using an image on CRT display  42  or automatically, using image processing techniques. 
     In one embodiment, image processing techniques are used by computer  36  to further isolate, identify, and score calcification peaks such as peak  74 . For example, intensities of small groups of pixels  76  of a difference image  78  shown in part in  FIG. 9  are compared to intensities of neighboring small groups of pixels  80 , where a “small group of pixels” refers either to one pixel or a few pixels in a cluster. When a difference is determined to be greater than a predetermined threshold indicative of calcification, sites represented by pixels  76  are identified as calcification sites for further study. In one embodiment, results of the intensity comparison are used directly for scoring an amount of calcification in accordance with differences in image intensities. The scoring results are used as a guideline for further examination. 
     In one embodiment, a difference image is enhanced by image processing to enhance the appearance of calcification  74  utilizing, for example, contrast enhancement algorithms. Differencing or other image processing procedures needed for contrast enhancement are implemented, for example, in hardware, software, or firmware of image reconstructor  34  or computer  36 , or both. In one embodiment, computer  36  is programmed both to display a difference image on CRT  42  and to automatically recognize and score calcification  74  by analysis of the difference image. 
     In one embodiment, scans of the two scout images are triggered by EKG signal  64  from EKG machine  50 . The EKG signal is supplied to computer  36 , which controls scanning and acquisition of image data in CT imaging system  10 . Computer  36  ensures that the two scout images taken are images of the same region of the body of patient  22  by controlling movement of table  46 . Computer  36  also ensures that the heart is in a different cardiac phase by staring the scans at different points in a cardiac cycle. 
     In an embodiment in which CT imaging system  10  is a multi-slice imaging system having more than one row of detector elements  20 , similar procedures for movement of table  46  are followed. However, a plurality of difference images are obtained, one for each row of detector  18 . 
     In another embodiment, multiple detector rows of a detector  16  in a multi-slice CT imaging system  10  are used in a single pass to generate a difference image. Computer  36  adjusts a rate of movement of table  46  during acquisition of data so that a small time lag occurs between acquisition of image data of the same body portions patient  22  by different rows of detector array  18 . Computer  36  selects an amount of time lag in accordance with a heart rate of patient  22  determined, for example, from EKG signal  64 . The amount of time lag is selected to ensure that image data is acquired by different rows of detector  18  during different portions of a cardiac cycle. In this manner, image data acquired from two different rows of a multi-slice detector  18  obtained during a single pass of a scout scan is used to obtain two suitable scout images. A difference image for scoring is computed from those portions of the two scout images that include at least a portion of heart  58  and that represent the same physical locations of the body of patient  22 . Portions of each image acquired by the two rows of detector  18  that do not overlap are simply ignored. 
     In another embodiment utilizing a multi-slice CT imaging system  10  having more than two rows of detectors, additional information for estimating background noise in obtained. For example, three or more rows of detectors obtain three or more scout images, including two for computing a difference image, and noise estimation information including at least a third scout image. Background noise in the difference image is estimated and reduced utilizing the noise estimation information and standard signal processing techniques. 
     From the preceding description of various embodiments of the present invention, it is evident that the problem of motion-induced artifacts in CT imaging systems is overcome, especially for calcification scoring purposes. Moreover, by reducing or eliminating non-moving body parts in a difference image, scoring of calcification is readily accomplished, even though only small incremental x-ray attenuation is produced by calcification. 
     Although particular embodiments of the invention have 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. Accordingly, the spirit and scope of the invention are to be limited only by the terms of the appended claims and legal equivalents.