Abstract:
A method is described for evaluating substance scoring, the scoring based on imaging system-generated images of an object having regions of interest due to possible presence of the substance, the method including the steps of simulating the regions of interest using a phantom having a plurality of volumes, each volume having dimensions simulating dimensions of a region of interest, each volume having a density representative of a substance density; generating images of the phantom; scoring the substance based on the phantom images; and comparing results of the substance scoring to expected phantom-image results. The above-described phantom and method allow a scoring system user to verify substance scoring accuracy and to compare scores resulting from different imaging systems, scanning methods and reconstruction algorithms.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to imaging systems and, more particularly, to a phantom for use in evaluating substance scoring using imaging system-generated images. 
     Imaging systems include a source that emits signals (including but not limited to x-ray, radio frequency, or sonar signals), and the signals are directed toward an object to be imaged. The emitted signals and the interposed object interact to produce a response that is received by one or more detectors. The imaging system then processes the detected response signals to generate an image of the object. 
     For example, in computed tomography (CT) imaging, 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 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 backprojection 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 required for multiple slices, 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. In addition to reduced scanning time, helical scanning provides other advantages such as improved image quality and better control of contrast. 
     It is known to use imaging data to identify evidence of disease by detecting and quantifying, i.e. “scoring”, substances that may be present in a patient&#39;s system. One known software system, for example, analyzes CT images of the heart to quantify amounts of calcium in coronary regions of interest. Scoring is based upon the volume and Hounsfield unit of a calcified region. A number called the “calcium score” expresses the quantity of calcium present in the patient&#39;s arterial system. 
     It would be desirable to provide a method for verifying accuracy of substance-scoring systems. It also would be desirable to provide a method for measuring the validity, reproducibility and repeatability of a substance score for different imaging systems (e.g. CT single-slice or multi-slice), for different scanning methods (e.g. CT helical or axial), and for different image reconstruction algorithms. 
     BRIEF SUMMARY OF THE INVENTION 
     There is therefore provided, in one embodiment, a method for evaluating substance scoring, the scoring based on imaging system-generated images of an object having regions of interest due to possible presence of the substance, the method including the steps of simulating the regions of interest using a phantom having a plurality of volumes, each volume having dimensions simulating dimensions of a region of interest, each volume having a density representative of a substance density; generating images of the phantom; scoring the substance based on the phantom images; and comparing results of the substance scoring to expected phantom-image results. 
     The above-described phantom and method allow a scoring system user to verify substance scoring accuracy and to compare scores resulting from different imaging systems, scanning methods and reconstruction algorithms. 
    
    
     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 frontal view of a phantom for calcium scoring; 
     FIG. 4 is a diagram of shape and orientation for rods included in the phantom shown in FIG. 3; 
     FIG. 5 is a table of CT number ranges and corresponding group target CT numbers and positional angles for one embodiment of the phantom shown in FIG. 3; 
     FIG. 6 is a side view of the phantom shown in FIG. 3; and 
     FIG. 7 is a diagram of a mounting bracket for the phantom shown in FIG.  3 . 
    
    
     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 . X-ray beam  16  is collimated by a collimator (not shown) to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as an “imaging plane”. Detector array  18  is formed by detector elements  20  which together sense the projected x-rays that pass through an object  22  such as a medical patient. Detector array  20  may be a single-slice detector or a multi-slice detector. Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray 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  along a Z-axis through gantry opening  48 . 
     In one embodiment and referring to FIG. 3, a phantom  50  for use in calcium scoring simulates regions of the human coronary system. As shown frontally in FIG. 3, e.g. in an X-Y plane, phantom  50  is oval in shape, having, for example, a long axis  52  of 35 centimeters and a short axis  54  of  25  centimeters. Phantom  50  includes a cylindrical core  60  representing the heart and having a diameter  62 , for example, of 20 centimeters. Core  60  is made of a material having a CT number simulating that of heart muscle, for example, a plastic material having a CT number of 60 Hounsfield units at a source 14 voltage of 120 kilovolts. 
     Core  60  is located, e.g. centered, inside an elliptical ring  64  representing tissues surrounding the heart. Ring  64  is made of a material having a CT number simulating that of heart tissue, for example, a plastic material having a CT number of 60 Hounsfield units at a source 14 voltage of 120 kilovolts. As shall be described below, a plurality of rods (not shown in FIG. 3) are embedded in core  60  along lines  66  radiating from a phantom axis  58  (shown in FIG. 3 as coming out of the page, i.e. orthogonal to the X-Y plane shown in FIG.  3 ). Radial lines  66  extend at angles  68  from phantom axis  58 . 
     As shown in FIG. 4, phantom  50  includes a plurality of volumes  70 , e.g. rods, simulating a plurality of calcified coronary regions. Rods  70  differ from one another in length, diameter and density. Each rod  70  simulates, in dimensions and density, a calcified material typically found in patient coronary systems. More particularly and in one embodiment, thirty rods  70  are embedded in core  60  in six groups  72  of five rods  70  each. Each group  72  is arranged along a radial line  66  and has a target CT number (not shown in FIG. 4) as shall be described below. Rods  70  in each group  72  are separated from one another by a distance  84  of, for example, four millimeters and have diameters  74  of 2, 3, 4, 5 and 6 millimeters respectively, with diameters  74  increasing with distance from phantom axis  58 . Center  76  of smallest rod  70  in a group  72  is located, for example, a distance  86  of five millimeters from phantom axis  58  along the appropriate radial line  66 . Each rod  70  has, for example, a length  78  equal to its diameter  74  and is aligned lengthwise parallel to phantom axis  58 . All rods  70  are lengthwise-centered on a midplane  80  which bisects phantom  50 . 
     Each group  72  is made of a material having a CT number representative of a range of calcium densities as reflected in CT images through the CT number. CT numbers (and materials having such numbers) are selected for rods  70  based on, for example, a scoring algorithm used by a calcium scoring system with which phantom  50  is to be used. One such algorithm categorizes calcification according to CT number in calcium density ranges  90  as shown in FIG.  5 . For a 120-kilovolt source 14 voltage, ranges  90  include, for example, zero to 129 Hounsfield units, 130 to 199 Hounsfield units, 200 to 299 Hounsfield units, 300 to 399 Hounsfield units, and above and including 400 Hounsfield units. With one exception as shall be described below, a target CT number  92  is selected for each group  72  from the middle of the corresponding range  90 . A middle value is selected to prevent range  90  boundary crossing when system  10  is subjected to noise. An exception is a calibration group  94  that is used to verify imaging system  10  accuracy. Calibration group  94  has a target CT number  92  of zero while other groups  72  have target CT numbers  92  of, e.g. 110, 150, 250, 350 and 450 Hounsfield units respectively. Phantom  50  is fabricated such that actual target CT numbers  92  are within tolerances of +5 HU and −5 HU of nominal target CT numbers  92 . Thus nominal CT numbers are closely approximated without engendering fabrication difficulty. Groups  72  are positioned along radial lines  66 , for example, at angles  68  as shown in FIG. 5, i.e. at 0 degrees, 45 degrees, 135 degrees, 180 degrees, 225 degrees, and 315 degrees respectively. 
     As shown in FIG. 6, core  60  and ring  64  are cylindrical in shape along phantom axis  58  and have a length  82  of, e.g., five centimeters. Core  60  has an alignment region  100  extending, for example, three centimeters in the direction of phantom axis  58 . Phantom  50  includes a mounting bracket  102 , removably affixed to alignment region  100  and shown frontally in FIG.  7 . Phantom  50  is supported during imaging by a phantom holder (not shown), to which mounting bracket  102  is removably affixed. 
     In use, phantom  50  and the supporting phantom holder are placed on table  46 . A centroid of phantom  50  is calculated and, based on the calculated centroid, phantom  50  is aligned by, for example, extending alignment region  100  up to three centimeters in the direction of the imaging system  10  Z-axis (along which table  46  is moved during imaging). Rods  70  are aligned along the imaging system  10  Z-axis. 
     When phantom  50  is placed on table  46  and aligned for imaging in imaging system  10 , it simulates, for example, calcified coronary arterial regions of interest to the user of a calcium scoring system. The user then generates imaging system  10  images of the simulated calcified regions, calcium-scores the images, and compares results of the calcium scoring to expected phantom-image results. 
     The above-described phantom allows a user of a calcium scoring system to evaluate scoring system accuracy. The user also can evaluate different imaging systems (e.g. single-slice CT or multi-slice CT), different scanning methods (e.g. helical or axial), and different reconstruction algorithms relative to the calcium scoring system. and thereby determine whether a calcium score is valid, reproducible and repeatable. 
     Although an embodiment of phantom  50  is shown herein relative to a CT imaging system and for use with a calcium scoring system using a scoring algorithm, phantom  50  can also be used with other imaging systems, other calcium scoring systems and other scoring algorithms. Furthermore, phantom  50  is not limited to use with calcium scoring systems but can be used to quantify other substances besides calcium. Alternative embodiments of phantom  50  also can be used to evaluate patient regions of interest other than coronary arteries. 
     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.