Abstract:
The present invention is, in one embodiment, a method for facilitating the analysis of computed tomographic (CT) images. The method includes steps of: acquiring attenuation data of a patient using a CT imaging system; reconstructing images of the patient using the acquired attenuation data; mapping intensity data from at least one reconstructed image into a color image using a color mapping indicative of physiological thresholds; and displaying the color image. 
     Embodiments of the present invention facilitate the assessment of images and differences between images for functional image data and for perfusion-related imaging data.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to methods and apparatus for facilitating computed tomographic (CT) image assessment, and more particularly to methods and apparatus for enhancing functional data on CT images. 
     In at least some computed tomography (CT) imaging system configurations, 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. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent the scintillator. 
     Computed tomography (CT) is an anatomical imaging modality, but recently there have been advances that give some functional imaging capabilities to CT. For example, one known image software package for CT perfusion provides facilities for a user to process dynamic image data and to generate functional information and images that include functional image data relating to perfusion. The software uses changes in image intensity as a function of time to generate the information and the images. However, CT has classically been a grayscale-only modality, and it is difficult to assess differences in data on such images. 
     There exist known embodiments of other imaging modalities, such as nuclear medicine and positron emission tomography (PET), that regularly utilize functional maps to show differences. Color is often utilized with these modalities to give better discrimination than grayscale maps in the assessment of the displayed data. 
     CT Perfusion is a functional imaging software and is similar to perfusion packages in nuclear medicine or MR but is a unique use for CT. 
     It would therefore be desirable to provide methods and apparatus for computed tomographic imaging that facilitated the assessment of images, and differences between images, especially for functional image data, and more particularly for perfusion-related imaging data. 
     BRIEF SUMMARY OF THE INVENTION 
     One embodiment of the present invention is therefore a method for facilitating the analysis of computed tomographic (CT) images. The method includes steps of: acquiring attenuation data of a patient using a CT imaging system; reconstructing images of the patient using the acquired attenuation data; mapping intensity data from at least one reconstructed image into a color image using a color mapping indicative of physiological thresholds; and displaying the color image. 
     Embodiments of the present invention facilitate the assessment of images and differences between images for functional image data and for perfusion-related imaging data. 
    
    
     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 representation of an anteroposterior projection of a head of a patient showing a potential location of a blockage. 
     FIG. 4 is a representation of a CT anatomical image 
     FIG. 5 is a representation of a CT parametric image. 
     FIG. 6 is a representation of a CT parametric image overlaid over a CT anatomical image. 
     FIG. 7 is a representation of the combination of several composite images or slices into a 3D (three dimensional) image showing 3D functional data. 
     FIG. 8 is a sample three-color parametric image. 
     FIG. 9 is a sample rainbow colored composite image. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In one embodiment of the present invention, computed tomographic images are used. 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, such as 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 object or 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 . In one embodiment, and as shown in FIG. 2, detector elements  20  are arranged in one row so that projection data corresponding to a single image slice is acquired during a scan. In another embodiment, detector elements  20  are arranged in a plurality of parallel rows, so that projection data corresponding to a plurality of parallel slices can be acquired simultaneously during a scan. 
     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 embodiment and referring to FIG. 3, blood flow parameters of an organ of patient  22  such as brain  50  are measured by injecting a substance (for example, one containing iodine) into patient  22  that produces a contrasting appearance on CT images. For example, blood flow, blood volume, and mean transit time can be measured by examining the perfusion of the contrasting substance, which allows blockages  52  to be recognized. A blockage in one embodiment of the invention causes a change in color because of the change in volume, flow, or mean transit time. 
     In one embodiment of the present invention, maps with different color schemes are used for CT perfusion parametric images to facilitate their assessment. For example, display  42  is a color display, and software in computer  36  uses a mapping of intensities to color to enhance images for analysis. The software compares reconstructed images denoting the same region at different times, the software denote differences in measurements, as a function of time, of quantities such as blood flow, blood volume, and mean transit time for blood containing the injected substance to move through a cell. Depending upon the intended purpose of the images selected for display, intensities or intensity differences are mapped onto a set of colors by the software and the images are displayed on color display  42 . One or more types of color maps are available for use in one embodiment. Examples of suitable maps include rainbow maps (i.e., maps in which a range of intensities or intensity differences are mapped into a range of colors having the same sequence as a rainbow spectrum), 3 colors, inverted rainbow, “Hot Iron” (in which colors range from yellow to red, with orange colors for intermediate intensities or intensity differences) and “Puh Thallium” (a color map in which the colors are somewhat darker than the rainbow colors). The two latter maps are similar to those used in conjunction with nuclear medicine imaging modality. In addition, colors representing intensities (or intensity differences) are mapped using threshold values that correspond to a physiological threshold. For example, to facilitate detection of tissues in which a stroke is occurring, blues are mapped to threshold levels selected to show where the stroke is occurring. On the other hand, greens are mapped to threshold areas of lower intensity in a reconstructed image representing healthy tissues, and red is mapped to areas having intensities characteristic of blood vessels. Such mappings are useful for assessment of blood flow. 
     In one embodiment, parametric data for functional imaging is used in the display of color maps. Thus, intensities in the reconstructed image correspond to the parametric data. Also, and referring to FIGS. 4,  5 , and  6  CT anatomical images  54  (images acquired beforehand, without injection of the perfusion substance) and parametric data images  56  (wherein each type of shading in FIG. 5 represents a different color in this example) are overlaid  58 . Overlaid image  58  facilitates correlation of the parametric data  56  with the anatomy  54  of patient  22 . For example, in one embodiment, the image data acquired beforehand is image data for brain structures  60 ,  62  of a patient, and a pre-processing and a post-processing image (i.e., one taken in the presence of perfusion) are overlaid. The opacity of the overlaid image  56  is made adjustable so that, in the case of overlaid parametric data, for example, more or less of the anatomical data can be seen beneath it, as desired. In one embodiment, maps for mean transit time for CT perfusion are used as the parametric data. 
     In another embodiment of the present invention and referring to FIG. 7, a plurality of composite images or slices  64 ,  66 ,  68  are combined  70  to produce a 3D image  72  (i.e., a perspective image) showing 3D functional data  74  in color. 
     In one embodiment of the present invention, a rainbow color map emphasizes ischemia areas (i.e., areas with a lower blood volume) as cold-colored areas. In another embodiment, a three-color color map allows a visual segmentation of damaged, healthy, and hyper-vascularized tissue. In yet another embodiment, an inverse rainbow color map emphasizes areas with longer transit time as cold-colored areas. These areas are surrogates of an ischemic process. 
     FIG. 8 is a sample three-color parametric image. FIG. 9 is a sample rainbow colored composite image. In each image, a mapping  76  of intensities or intensity differences to colors is shown. 
     These sample results show the results of a mapping of intensities of embodiments of the present invention and how such embodiments facilitate the assessment of images, and differences between images, for functional image data and for perfusion-related imaging data. 
     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.