Abstract:
One embodiment of the present invention is a method for imaging an object with a computed tomographic (CT) imaging system that includes steps of scanning an object with a beam of radiation from a CT imaging system to produce a view stream including attenuation data for the object being scanned; sensing one or more dynamic parameters relating to at least one of the object being scanned and the CT imaging system; and integrating information relating to the one or more sensed dynamic parameters into the view stream. 
     This embodiment integrates information necessary for compensating reconstructed images directly into the view stream, thereby making the necessary information more conveniently available for such compensation.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to computed tomography (CT) imaging, and more particularly methods and apparatus for producing dynamically compensated CT 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 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. 
     Known CT imaging system scans include acquisition information and view information. “Acquisition information” includes patient, scanning, and reconstruction information that is static in nature. “View information” is actual attenuation data collected by a detection system of the CT imaging system and is dynamic in nature. In known CT imaging systems, compensation for dynamic changes from a patient or a scanning environment cannot be performed from present, view stream information alone. Thus, blurring in reconstructed images sometimes occurs. For example, images in fluoro CT applications are blurred during tilting of the gantry. Also, helical scans of different portions of a body using different helical pitches now require separate scans, because it is difficult to produce good images during speed transitions or even to compute actual image locations. 
     It would therefore be desirable to provide convenient methods and apparatus for compensating CT images for dynamic changes from a patient or scanning environment. It would further be desirable to provide such methods and apparatus for utilizing a view stream to provide the compensation information. 
     BRIEF SUMMARY OF THE INVENTION 
     In one embodiment of the present invention, there is thus provided a method for imaging an object with a computed tomographic (CT) imaging system that includes steps of scanning an object with a beam of radiation from a CT imaging system to produce a view stream including attenuation data for the object being scanned; sensing one or more dynamic parameters relating to at least one of the object being scanned and the CT imaging system; and integrating information relating to the one or more sensed dynamic parameters into the view stream. 
     The above-described embodiment integrates information necessary for compensating reconstructed images directly into the view stream, thereby making the necessary information more conveniently available for such compensation. 
    
    
     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 block schematic diagram of an acquisition system embodiment of the present invention that integrates dynamic compensation information into a view stream. 
     FIG. 4 is an illustration of a compensated image being displayed in conjunction with corresponding temporally-related dynamic information. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a computed tomographic (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 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 . 
     In one exemplary embodiment of the present invention and referring to FIG. 3, a view stream modifier  50  is provided in a view stream path  52  between a rotating side portion  54  of DAS  32  and a data acquisition/recovery system  56  that includes a stationary side portion of DAS  32  and image reconstructor  34 . The division of components by slip ring  58  is convenient for implementation of this embodiment, but otherwise is only exemplary. View stream modifier  50  is in electrical communication with one or more dynamic parameter sensors  60  that provide electrical indications of data other than attenuation measurements. For example, sensors  60  include a gantry/table position sensor and a physiological data sensor in one embodiment. Other embodiments have different numbers of sensors  60  and/or different types or combinations of sensors  60 . In one embodiment, an object  22  is scanned with a beam of radiation  16  from CT imaging system  10  to produce a view stream communicated via view stream path  52 . One or more dynamic parameters related to either or both of object  22  or CT imaging system  10  are sensed. View stream modifier  50  integrates information relating to one or more of the sensed dynamic parameters into the view stream. 
     In one embodiment, CT imaging system  10  is used to capture physiology in a particular state without motion artifacts. For example, sensors  60  include an EKG sensor and a respiration sensor to sense an EKG parameter and a respiration parameter, respectively. These dynamic parameters are integrated into the view stream and temporally related with attenuation measurement data in the view stream. For example, dynamic parameters are sampled at particular times and multiplexed into the view stream in corresponding, predefined time slots, and/or explicit time indications are included with either or both of the dynamic information and the attenuation data. Stationary side acquisition/recovery system  56  receives sensor  60  information in the view stream along with attenuation measurements. Both the attenuation measurements and the temporally-related dynamic information are used by acquisition/recovery system  56  in a modified reconstruction algorithm to select particular segments of view data for image reconstruction. The view data segments selected are those that minimize motion artifacts in reconstructed images and therefore compensate for motions of a patient  22 . For example, in one embodiment, the reconstruction algorithm utilizes only segments of view data corresponding to a relatively stationary portion of a cardiac cycle to reconstruct an image of a heart. Useful segments for compensated reconstruction are readily determined by their temporal relationships to R-peaks in an EKG parameter cycle. In another embodiment, motion-induced artifacts resulting from patient respiration are reduced by utilizing view data having a specified relationship with portions of the patient&#39;s respiration cycle. 
     In one embodiment, patient information acquired by one or more sensors  60  is also displayed, for example, on CRT display  42 , in conjunction with corresponding reconstructed images displayed on the same (or a separate) display. 
     In another embodiment of the present invention, imaging system  10  is configured to provide helical scans having variable pitch and/or table  46  translation speed so that a radiologist is able to scan different locations of a body using different pitches. Because a dynamic parameter related to a table position parameter is integrated into a view stream by view stream modifier  50 , separate helical scans for these different body locations are not required. The integrated dynamic parameter and its temporal relationship with the attenuation data in the view stream are used by a reconstruction algorithm in stationary side acquisition/reconstruction system  56  to reconstruct images with compensation for speed changes. In addition to eliminating the need for separate helical scans, this embodiment also provides information to produce acceptable images during translation speed changes and to determine actual image locations. In one embodiment, and as shown in FIG. 4 these determined image locations  62  are displayed in conjunction with corresponding reconstructed images  64  on CRT display  42 . 
     In yet another embodiment, CT imaging system  10  is used in a fluoro application, and sensors  60  provide dynamic positional parameters relating to table  46  and gantry  12 . A real-time reconstruction algorithm is used by stationary-side acquisition/reconstruction system  56  to produce images as a patient is scanned. Table  46  and/or gantry  12  in this embodiment are manually moveable so that a radiologist is able to manually move table  46  and/or tilt gantry  12  during a scan. Stationary-side acquisition/reconstruction system  56  is configured to use the dynamic positional parameters, the attenuation data included in the view stream, and their temporal correlation to reconstruct compensated images. Thus, blurring is reduced during manual gantry  12  tilting and/or movement of table  46 . In one embodiment, gantry  12  tilt information and/or table location information is displayed in conjunction with a corresponding compensated image. 
     Although position and/or tilt sensors are used in some of the above embodiments to sense dynamic information parameters, sensors detecting changes or derivatives (including first or second derivatives) of these parameters are considered as being entirely equivalent for purposes of this invention. Changes or derivatives need only to be summed or integrated from known initial conditions (e.g., a starting position or tilt) to provide the same dynamic parameter information as the corresponding position and/or tilt sensors. In principle, changes and first or second derivatives of an EKG signal and/or a respiration signal could also be used instead of the signals themselves in other embodiments. 
     The above-described embodiments are intended to be exemplary only. However, it will be seen that these embodiments provide convenient methods and apparatus for compensating conventional CT images for dynamic changes in a patient or in a scanning environment, utilizing a view stream to provide the compensation information. 
     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 fall-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. Thus, while the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.