Abstract:
a. Provided is a phantom for evaluating the accuracy of image registration software based on a result of matching tomograms of a predetermined position of the phantom, taken using two or more imaging apparatuses. Accordingly, it is possible to more efficiently evaluate the accuracy of the image registration software by comparing the tomograms with one another using a three-dimensional analysis. In addition, it is possible to facilitate the comparison of the tomograms with one another by installing a plurality of indicating bars in the phantom so that their cross sections can appear on each of the tomograms.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims the priority of Korean Patent Application No.2003-29296, filed on May 9, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a phantom that can be tomographed by various medical imaging apparatuses after being built in each of the medical imaging apparatuses, and more particularly, to a phantom for evaluating accuracy of image registration software by matching and comparing images, taken by various medical imaging apparatuses, with one another with the use of imaging registration software.  
         [0004]     2. Description of the Related Art  
         [0005]     Recent developments in medical science and technology have enabled many medical procedures and diagnoses that at one time were considered impossible. One of these developments is imaging apparatuses, such as a computed tomography (CT) apparatus, a magnetic resonance imaging (MRI) apparatus, a single photon emission computed tomography (SPECT) apparatus, and a positron emission tomography (PET) apparatus, which enable a detailed observation of the entire human body, and thus is very important for diagnosing or treating diseases such as tumors or cancer.  
         [0006]     Since the CT, MRI, SPECT, and PET apparatuses are based on different imaging principles and have different advantages and disadvantages, it is preferable to use the one that best fits the purpose of diagnosis.  
         [0007]     The CT apparatus is an imaging apparatus that uses differences in X-ray attenuation coefficients among parts of, for example, the entire human body, which are caused by electron density variations. The CT apparatus can provide detailed anatomical images with fewer distortions. In particular, the CT apparatus provides excellent imaging of a bone structure of the entire human body. In addition, the CT apparatus allows electron density information to be immediately applied to dose calculations, and thus can provide standard images for the planning of radioactive treatments.  
         [0008]     The MRI apparatus is an imaging apparatus that uses frequency conversion signals generated in the process of magnetizing and demagnetizing hydrogen atoms in the entire human body. The MRI apparatus provides anatomical images with high contrast and high resolution. However, there is a possibility of the MRI apparatus providing distorted images, which is mainly due to the irregularity of magnetic fields.  
         [0009]     The SPECT apparatus is an imaging apparatus that forms images of parts of the entire human body by injecting a reagent containing radionuclides that emit gamma rays into a desired part of the entire human body and detecting the gamma rays emitted from the radionuclides. The SPECT apparatus is generally used for analyzing metabolism and nervous functions of the desired part of the entire human body. However, the SPECT apparatus provides images with relatively low resolution, so it is rather difficult to obtain detailed anatomical information from SPECT images.  
         [0010]     The PET apparatus forms images of parts of the entire human body using the fact that a malignant tumor in the entire human body consumes more glucose than normal tissues. The PET apparatus makes it possible to provide early diagnosis of abnormal symptoms or diseases by visualizing degrees of sugar, oxygen, and protein metabolism in the entire human body. However, the PET apparatus cannot provide detailed information on, for example, where a tumor is located in the entire human body and how big the tumor is.  
         [0011]     The above-described imaging apparatuses have different advantages and disadvantages. Therefore, for more accurate and more effective diagnosis and treatment of diseases, it would be desirable to get an image of a desired part of the entire human body using as many imaging apparatuses as possible and analyze the resultant images taken by the different imaging apparatuses by comparing them with one another.  
         [0012]     For a more accurate comparative analysis of images taken by different imaging apparatuses, image registration, which is a technique of mapping the images on the same coordinate system, is necessary. The image registration operation indicates processes of mapping and overlapping various images of a desired part of the entire human body, taken by the different imaging apparatuses, on a given coordinate system, thus guaranteeing more accurate and more effective diagnosis and treatment of diseases.  
         [0013]     An image registration tool, namely, image registration software, matches images of a desired portion of the entire human body, taken by different imaging apparatuses, with one another. Therefore, unless accuracy of the image registration software is guaranteed, reliability of image registration results cannot be attained. Inaccurate image registration results inevitably lead to inaccurate diagnosis and inappropriate treatment of diseases.  
         [0014]     Therefore, research has been carried out on image registration, and development of image registration software that can provide very accurate image registration results in a more convenient manner is under way.  
         [0015]     In the meantime, a phantom is necessary for evaluating the general performance and accuracy of the image registration software. The phantom makes it possible to obtain multiple images and more accurately carry out error analysis. In short, the phantom can be tomographed using various imaging apparatuses and are used for evaluating the accuracy of the image registration software and other necessary procedures.  
         [0016]     Until now, no phantoms have been developed exclusively for evaluating the accuracy of image registration software. In other words, conventional phantoms have been mainly used to control the quality of imaging apparatuses or radioactive therapy equipment so that they can compare images at best two-dimensionally.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention provides a phantom for evaluating the accuracy of image registration software. The phantom is used for evaluating the accuracy of the image registration software by allowing a three-dimensional comparison of images taken using different medical imaging apparatuses. A plurality of indicating bars are included in the phantom such that their cross sections can appear on each of the images, thus facilitating the comparison of the images and the evaluation of the image registration software.  
         [0018]     According to an aspect of the present invention, there is provided a phantom for evaluating the accuracy of image registration software based on a result of matching tomograms of a predetermined position of the phantom taken using two or more imaging apparatuses. The phantom includes a container, which can contain water therein; and a phantom main body, which is installed in the container, the phantom main body having an empty space therein that embodies a predetermined portion of the entire human body, the empty space being able to be filled with water.  
         [0019]     The phantom may further include a localizer, which is disposed between the phantom main body and an inner sidewall of the container and indicates the height in the axial direction of the phantom to which the tomograms correspond.  
         [0020]     The phantom main body may include a case, which can contain water therein; and a slice stack, which comprises a plurality of unit slices that are sequentially stacked in the case and has an empty space that embodies the predetermined portion of the entire human body.  
         [0021]     The unit slices may be plates with a predetermined thickness, stacked on a bottom surface of the case, holes may be formed in each of the unit slices so that they can represent cross sections of the predetermined portion of the entire human body, and the empty space inside the slice stack may be defined by the holes in each of the unit slices when the unit slices are stacked.  
         [0022]     The phantom may further include at least one vertical indicating bar, which extends vertically upward from a bottom surface of the container such that its cross section appears on each of the tomograms of the phantom.  
         [0023]     The localizer may include a frame, which comprises a main body, which has a cylindrical shape with a predetermined height and contains the phantom main body therein, and upper and lower rings, which have a predetermined width and are respectively fixed to upper and lower ends of the main body; and at least one N-shaped indicator, which is coupled to the upper and lower rings at both the upper and lower ends such that its cross section appears on each of the tomograms of the phantom, the at least one N-shaped indicator comprising three indicating bars, two of which extend vertically upward from the lower ring and are separated by a predetermined distance, and one of which is slanted between the two indicating bars such that its lower end is located in the vicinity of the lower end of one of the two indicating bars disposed vertically and its upper end is located in the vicinity of the lower end of the other indicating bar disposed vertically.  
         [0024]     At least two N-shaped indicators may be evenly distributed around the circumference of the phantom main body.  
         [0025]     Each of the indicating bars may include an acrylic tube, which is fixed to the upper and lower rings at both ends and has an empty space therein; and an inserting rod, which is disposed in the acrylic tube such that its cross section appears on each of the tomograms of the phantom.  
         [0026]     The phantom main body may include a slice stack, which comprises a plurality of unit slices that are sequentially stacked in the case and has an empty space therein that embodies the predetermined portion of the entire human body; and at least one indicating bar, which is vertically fixed between the slice stack and the inner sidewall of the container such that its cross section can appear in each of the tomograms of the phantom.  
         [0027]     The at least one indicating bar may include an acrylic tube, which has an empty space therein; and an inserting rod, which is disposed in the acrylic tube such that its cross section appears on each of the tomograms of the phantom.  
         [0028]     The unit slices may be plates with a predetermined thickness, which are stacked on a bottom surface of the case, holes may be formed in each of the unit slices so that they can represent a cross section of the predetermined portion of the entire human body, and the empty space inside the slice stack may be defined by the holes in each of the unit slices when the unit slices are stacked.  
         [0029]     The phantom main body may have at least one auxiliary hole vertically formed through therethrough. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0031]      FIG. 1  is an exploded perspective view of a phantom for evaluating accuracy of image registration software according to a first embodiment of the present invention;  
         [0032]      FIG. 2  is a cutaway view of a localizer of  FIG. 1 ;  
         [0033]      FIG. 3  is an exploded perspective view of a main body of the phantom of  FIG. 1 ;  
         [0034]      FIG. 4  is a plan view of one of a plurality of unit slices of the main body of  FIG. 3 ;  
         [0035]      FIG. 5  is a perspective view of the phantom of  FIG. 1 , from which a lid and a sealing cover are removed;  
         [0036]      FIG. 6  illustrates a tomogram of a predetermined position of the phantom of  FIG. 1  taken by a computed tomography (CT) apparatus;  
         [0037]      FIG. 7  is a diagram illustrating functions of the localizer of  FIG. 1 ;  
         [0038]      FIG. 8  illustrates tomograms of the phantom of  FIG. 1  taken by a CT apparatus and a single photon emission computed tomography (SEPCT) apparatus, and a result of matching the images with each other;  
         [0039]      FIG. 9  illustrates tomograms of the phantom of  FIG. 1  taken by a CT apparatus and a positron emission tomography (PET) apparatus, and a result of matching the images with each other;  
         [0040]      FIG. 10  illustrates tomograms of the phantom of  FIG. 1  taken by a magnetic resonance imaging (MRI) apparatus and a SPECT apparatus, and a result of matching the horizontal tomograms with each other;  
         [0041]      FIG. 11  illustrates tomograms of the phantom of  FIG. 1  taken by an MRI apparatus and a PET apparatus, and a result of matching the horizontal tomograms with each another;  
         [0042]      FIG. 12  is an exploded perspective view of a phantom for evaluating the accuracy of image registration software according to a second embodiment of the present invention;  
         [0043]      FIG. 13  is a cutaway view of a main body of the phantom of  FIG. 12 ;  
         [0044]      FIG. 14  is a plan view of one of a plurality of unit slices of the main body of  FIG. 13 ;  
         [0045]      FIG. 15  is a cross-sectional view of a slice stack of  FIG. 13 ;  
         [0046]      FIG. 16  is an exploded perspective view of vertical indicating bars installed in the slice stack of the phantom of  FIG. 12 ;  
         [0047]      FIG. 17  illustrates a horizontal tomogram of a predetermined position of the phantom of  FIG. 12  taken by a CT apparatus;  
         [0048]      FIG. 18  illustrates functions of the vertical indicating bars;  
         [0049]      FIG. 19  illustrates tomograms of the phantom of  FIG. 12  taken by a CT apparatus and a SPECT apparatus, and a result of matching the horizontal tomograms with each other;  
         [0050]     (1)  FIG. 20  illustrates tomograms of the phantom of  FIG. 12  taken by a CT apparatus and an MRI apparatus, and a result of matching the horizontal tomograms with each other;  
         [0051]      FIG. 21  is an exploded perspective view of a phantom for evaluating the accuracy of image registration software according to a third embodiment of the present invention;  
         [0052]      FIG. 22  is a perspective view of the phantom of  FIG. 21 , from which a lid is removed;  
         [0053]      FIG. 23  is a plan view of one of a plurality of unit slices of a slice stack of  FIG. 21 ;  
         [0054]      FIG. 24  is a cross-sectional view of the phantom of  FIG. 21 ; and  
         [0055]      FIG. 25  illustrates a tomogram of a predetermined position of the phantom of  FIG. 21  taken by a CT apparatus.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0056]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0057]     A phantom for evaluating the accuracy of image registration software according to the present invention is formed of acrylic resin. Acrylic resin rods or lead rods may be selectively used as inserting rods  17   b ,  32   b , and  57   d  of the phantom depending on the kind of imaging apparatus. Here, the acrylic resin can be polymethylacrylate, polymethylmetacrylate, a mixture thereof, a copolymer of methylacrylate or methylmetacrylate.  
         [0058]      FIG. 1  is an exploded perspective view of a phantom  11  for evaluating the accuracy of image registration software according to a first embodiment of the present invention. Referring to  FIG. 1 , the phantom  11  includes a container  13 , which can contain water therein, a localizer  15 , which has an empty space therein and is inserted into the container  13  such that the outer surface of the localizer  15  contacts the inner surface of the container  13 , a phantom main body  19 , which is inserted into the empty space of the localizer  15 , and a lid  21 , which hermetically seals the container  13  with the localizer  15  and the phantom main body  19  contained in the container  13 .  
         [0059]     The container  13  is cylindrical with a predetermined capacity. Supporting protrusions  13   a  and  13   b  are formed on the bottom surface of the container  13 . The supporting protrusion  13   b , which is located closer to the inner sidewall of the container  13  than the supporting protrusions  13   a , is fitted into a hole  15   e  formed in a lower ring  15   c  of the localizer  15 , and the supporting protrusions  13   a  are fitted into two holes ( 19   k  of  FIG. 3 ), respectively, formed at the bottom of the phantom main body  19 . The supporting protrusions  13   a  and  13   b  serve as stoppers that prevent the localizer  15  and the phantom main body  19  from undesirably moving in the container  13 .  
         [0060]     An O-ring  13   c  is disposed on top of the container  13  along the circumference of the container  13 . The O-ring  13   c  prevents water contained in the container  13  from leaking.  
         [0061]     The localizer  15  comprises a frame  15   a  and four N-shaped indicators  16 . The frame  15   a  comprises a main body  15   d , which is cylindrical and contacts the inner sidewall of the container  13 , and upper and lower rings  15   b  and  15   c , which are fixed to upper and lower ends, respectively, of the main body  15   d . The upper and lower rings  15   b  and  15   c  have the same width. An outer edge of each of the upper and lower rings  15   b  and  15   c  is fixed to the main body  15   d  so that the upper and lower rings  15   b  and  15   c  and the main body  15   d  form an empty space within.  
         [0062]     An upper end of each of the N-shaped indicators  16  is coupled with the upper ring  15   b , and a lower end of each of the N-shaped indicators  16  is coupled with the lower ring  15   c . Therefore, the N-shaped indicators  16  connect the upper and lower rings  15   b  and  15   c . The N-shaped indicators  16  are evenly distributed around the circumference of the localizer  15 . Each of the N-shaped indicators  16  comprises three indicating bars  17  forming an alphabet N. Each of the indicating bars  17  is represented by a point on an image of the phantom  11 , taken by an imaging apparatus, which will be described in greater detail later.  
         [0063]     Two of the three indicating bars  17  of each of the N-shaped indicators  16  extend vertically from the lower ring ( 15   c ) to connect the upper and lower rings  15   b  and  15   c , and the other indicating bar  17  is slanted between the two vertical indicating bars  17 . In other words, one end of the slanted indicating bar  17  between the two vertical indicating bars  17  is fixed to the upper ring  15   b  in the vicinity of one of the two vertical indicating bars  17 , and the other end of the slanted indicating bar  17  between the two vertical indicating bars  17  is fixed to the lower ring  15   c  in the vicinity of the other vertical indicating bar  17 .  
         [0064]     Therefore, when taking several horizontal tomograms of the phantom  11  along a Z direction, namely, along the axial direction of the phantom  1 , each tomograms have different distance patterns of point from one another according to the height of axial direction. In other words, the cross-sectional view of each of the N-shape indicators  16  varies from position to position along the axial direction of the phantom  11 , so the height of axial direction can be known based on the point distance pattern of each of the horizontal tomograms, which will be described in greater detail later with reference to  FIG. 7 .  
         [0065]     The phantom main body  19  is installed in the empty space of the localizer  15  such that the outer circumference of the phantom main body  19  contacts the inner circumference of each of the upper and lower rings  15   b  and  15   c . The phantom main body  19  comprises a cylindrical case  19   a , which can contain water, a slice stack  19   p , which is disposed in the cylindrical case  19   a , and a sealing cover  19   b , which hermetically seals the cylindrical case  19   a.    
         [0066]     The slice stack  19   p  includes a plurality of unit slices ( 19   q  of  FIG. 3 ), which are disk-shaped. The slice stack  19   p  has an empty space copying a certain part of the entire human body, for example, an internal organ or the brain. In the present embodiment, the empty space of the slice stack  19  embodies the brain.  
         [0067]     An opening/shutting screw  19   d , which opens or shuts a water supply hole ( 19   v  of  FIG. 3 ), is disposed at the center of the sealing cover  19   b.    
         [0068]     A water supply hole  21   a  is formed in the middle of the lid  21 . The water supply hole  21   a  is opened or closed by the opening/shutting screw  21   b . When the water supply holes  21   a  and  19   v  are opened, the empty space of slice stack  19   p  can be filled with water supplied thereinto.  
         [0069]      FIG. 2  is a cutaway view of the localizer  15  of  FIG. 1 . Referring to  FIG. 2 , the localizer  15  includes the main body  15   d , which is cylindrical, and the upper and lower rings  15   b  and  15   c , which are fixed to the upper and lower ends, respectively, of the main body  15   d . The upper and lower rings  15   b  and  15   c  each have a predetermined width and a predetermined thickness. Each of the upper and lower rings  15   b  and  15   c  has female screw holes  15   f  such that the N-shaped indicators  16  can be fixed into the female screw holes  15   f.    
         [0070]     As described above, each of the N-shaped indicators  16  includes three indicating bars  17 .  
         [0071]     Each of the indicating bars  17  includes an acrylic tube  17   a , an inserting rod  17   b , which is inserted into the acrylic tube  17   a , and a sealing screw  17   c , which hermetically seals an upper end of the acrylic tube  17   a.    
         [0072]     Male threads are formed along the outer circumference of either end of the acrylic tube  17   a  so that the acrylic tube  17   a  can be fitted into female screw holes  15   f . The acrylic tube  17   a  contains the inserting rod  17   b  such that a cross section of the inserting rod  17   b  can be displayed on an image of the phantom  11  taken by an imaging apparatus. The upper end of the acrylic tube  17   a  is open and can be hermetically sealed by the sealing screw  17   c . Therefore, if necessary, the inserting rod  17   b  can be removed from the acrylic tube  17   a , and then other inserting rod can be inserted into the acrylic tube  17   a.    
         [0073]     The inserting rod  17   b  is a rod with a predetermined diameter. An acrylic rod or a lead rod could be used as the inserting rod  17   b  depending on the type of imaging apparatus. For example, when taking an image of the phantom  11  using a CT apparatus or an MRI apparatus, an acrylic rod is used as the inserting rod  17   b . On the other hand, when taking the image of the phantom  11  using a SPECT apparatus or a PET apparatus, a lead rod is used as the inserting rod  17   b  because an acrylic rod shot by the SPECT or PET apparatus does not appear on the tomogram of the phantom  11 .  
         [0074]      FIG. 3  is an exploded perspective view of the phantom main body  19  of  FIG. 1 . Referring to  FIG. 3 , the cylindrical case  19   a  of the phantom main body  19  includes a bottom plate  19   f , which is circular, and a sidewall  19   e , which is cylindrical and is firmly fixed to the bottom plate  19   f.    
         [0075]     Two opening holes  19   g  are formed in the bottom plate  19   f  together with the holes  19   k . The opening holes  19   g  can be hermetically sealed by an opening/shutting screw  19   h . Water contained in the phantom main body  19  can be quickly discharged through the opening holes  19   g.    
         [0076]     Four vertical supporting rods  19   m  are fixed to the bottom plate  19   f . The vertical supporting rods  19   m  have equal diameters and lengths and are perpendicular to the top surface of the bottom plate  19   f . Female screws  19   n  are formed with threads on the interior surface of an upper portion of each of the vertical supporting rods  19   m . Once the vertical supporting rods  19   m  are fitted into the slice stack  19   p , their upper portions protrude over the slice stack  19   p . Fixing bolts  19   c  are fitted into the female screws  19   n  passing through the sealing cover  19   b.    
         [0077]     As described above, the slice stack  19   p , which is disposed in the case  19   a , comprises the unit slices  19   q  that are sequentially stacked. Each of the unit slices  19   q  is disk-shaped. And an empty space is formed in the slice stack  19   q , copying shape of the brain. The empty space inside the slice stack  19   p  can be exposed to the outside so that it can be filled with water supplied into the slice stack  19   p  from the outside.  
         [0078]     Four through holes  19   s  are formed through the slice stack  19   p . The unit slices  19   q  can be neatly arranged in the cylindrical case  19   a  due to the four vertical supporting rods  19   m , which pass through the four through holes  19   s , respectively.  
         [0079]     Brain section holes  19   r  are formed in a central portion of each of the unit slices  19   q  such that they represent a shape of brain. More specifically, the brain section holes  19   r  are formed by referring to tomograms of different positions of the read brain, which are taken at predetermined intervals along the axial direction.  
         [0080]     When the four vertical supporting rods  19   m  are fitted into each of the unit slices  19   q  through the four through holes  19   s , respectively, empty spaces, which embody the entire brain, are defined by the brain section holes  19   r  of each of the unit slices  19   q . The slice stack  19   p  can be filled with water supplied thereinto such that the shape of the brain embodied by the empty spaces is filled with water.  
         [0081]     First two unit slices  19   q  of the top of the slice stack  19   p  are provided so that the bottom surface of the sealing cover  19   b  and the unit slice  19   q  can be separated from each other. Since the first two unit slices  19   q  from the top of the slice stack  19   p  have a through hole  19   u  in their centers, they do not interfere with the flow of water supplied into the slice stack  19   p.    
         [0082]     The water supply hole  19   v , which can be sealed by the opening/shutting screw  19   d , is formed in the center of the sealing cover  19   b , and four bolt holes  19   t  are formed around the water supply hole  19   v . The fixing bolts  19   c  are fitted into the female screws  19   n , passing through the four bolt holes  19   t.    
         [0083]      FIG. 4  is a plan view of one of the unit slices  19   q  of the slice stack  19   p  of  FIG. 3 . Referring to  FIG. 4 , four through holes  19   s  are disposed a predetermined distance from the outer boundary of a unit slice  19   q , and the brain section hole  19   r  is formed through the unit slice  19   q . The brain section hole  19   r  embodies a cross section at a predetermined position of the brain. Cross sections of the brain, each embodied on each of the unit slices  19   q  of the slice stack  19   p , are different from one another.  
         [0084]      FIG. 5  is a perspective view of the phantom  11  of  FIG. 1 , from which the lid  21  and the sealing cover  19   b  are removed. Referring to  FIG. 5 , the localizer  15  is disposed in the container  13 , and the phantom main body  19  is disposed in the localizer  15 . The vertical supporting rods  19   m  are fitted into the slice stack  19   p  of the phantom main body  19  such that the slice stack  19   p  is supported by the vertical supporting rods  19   m.    
         [0085]     The upper ends of the vertical supporting rods  19   m  protrude over the slice stack  19   p . The empty space of the slice stack  19   p  is filled with water by supplying water into the slice stack  19   p  through the through hole  19   u  of the uppermost unit slice  19   q . Thereafter, the phantom  11  is hermetically sealed, and tomograms of different sections of the phantom  11  are taken using a desired imaging apparatus.  
         [0086]      FIG. 6  illustrates a tomogram of a predetermined position of the phantom  11  of  FIG. 1 , taken by a computed tomography (CT) apparatus. The inserting rod  17   b  in the acrylic tube  17   a  of the phantom  11  is an acrylic rod.  
         [0087]     Referring to  FIG. 6 , ring-shaped images representing the cross sections of the container  13  and the sidewall  19   e  of the cylindrical case  19   a  of the phantom main body  19  are shown on the tomogram of the phantom  11 , and a cross-sectional image of a predetermined position of the brain is shown in the ring-shaped image representing the sidewall  19   e.    
         [0088]     Points A 1 , A 2 , and A 3  between the two ring-shaped images of the container  13  and the sidewall  19   e  represent the three inserting rods  17   b  of each of the N-shaped indicators  16 . Locations of the points A 1  and A 3  on the tomogram at a predetermined position of the phantom  11  in the axial direction of the phantom  11  are fixed regardless of the height of the unit slice  19   q  of the phantom  11 , and thus, the points A 1  and A 3  serve as fixed reference points. The points A 1  and A 3  are located a predetermined distance W apart from each other. The point A 2 , unlike the points A 1  and A 3 , is an indicating point whose location is variable between the fixed points A 1  and A 3  according to the height in the axial direction of the phantom  11  to which the tomogram corresponds.  
         [0089]     Therefore, the height of the interesting position in the phantom  11  from the bottom surface of the case  13  in the axial direction can be obtained by calculating a distance S between the points A 1  and A 2 .  
         [0090]     In other words, as shown in  FIG. 7 , the height Z in the axial direction of the phantom  11  to which the tomogram (R) corresponds can be expressed by the following equation: Z=(H*S)/W where H represents the height of the vertical inserting rod  17   b  and S represents a distance between the points A 1  and A 2 .  
         [0091]     Thus the height at the position of interest in the phantom  11  displayed on a computer monitor can be determined by knowing the distances between the fixed points A 1  and A 3  and the moving point A 2  on each tomogram.  
         [0092]      FIG. 8  illustrates tomograms of the phantom of  FIG. 1  taken by a CT apparatus and a single photon emission computed tomography (SPECT) apparatus, and a result of matching the tomograms with each other using an image registration technique. Referring to  FIG. 8 , the tomogram taken by the SPECT apparatus is converted into a 256*256 pixel size image, and the tomogram taken by the CT apparatus, which is originally a 512*512 pixel size image, is also converted into a 256*256 pixel size image. Thereafter, the two converted images are superposed to carry out an image registration.  
         [0093]     The two tomograms subjected to the image registration should represent the same position at the same height in the axial direction of the phantom  11 . So after getting a CT image that represents the position of the same height in the axial direction as a SPECT image, based on calculating distances between fixed points and an indicating point, the image registration can be carried out.  
         [0094]     The image registration is carried out using image registration software in order to test the accuracy of the image registration software.  
         [0095]     The tomograms taken by the CT and SPECT apparatuses are superposed, and the accuracy of the image registration software is evaluated based on the degree to which the fixed points on one of the tomograms match with their respective counterparts on the other tomogram. For example, if the fixed points on one of the tomograms exactly match with their respective counterparts on the other tomogram, the image registration software is determined to operate normally. Otherwise, it is determined that the image registration software needs to be corrected.  
         [0096]      FIG. 9  is a tomogram of the phantom of  FIG. 1  taken by a CT apparatus and a PET apparatus, and a result of matching the tomograms with each other based on the image registration. Referring to  FIG. 9 , the tomograms taken by the CT and PET apparatuses are converted into 256*256 pixel size images. Thereafter, the converted images are resliced in order to get tomograms, which are used for image registration, represent the same height in the axial direction.  
         [0097]     Thereafter, superpose one tomogram upon the other tomogram using the image registration software, and then the accuracy of the image registration software is evaluated based on the degree to which the fixed points on one of the tomograms match with their respective counterparts on the other tomogram.  
         [0098]      FIG. 10  is a tomogram of the phantom of  FIG. 1 , taken by a MRI apparatus and a SPECT apparatus, and a result of matching the tomograms with each other based on the image registration, and  FIG. 11  presents tomograms of the phantom of  FIG. 1  taken by MRI and PET apparatuses, and a result of matching the tomograms with each another based on the image registration. Referring to  FIGS. 10 and 11 , the accuracy of image registration software is evaluated by matching the tomograms taken by the MRI and SPECT apparatuses with each other or the tomograms taken by the MRI and PET apparatuses with each other with the use of the image registration software. The basic principles and method of evaluating the accuracy of the image registration software have already been described above with reference to  FIGS. 8 and 9 .  
         [0099]      FIG. 12  is an exploded perspective view of a phantom  30  for evaluating the accuracy of image registration software, according to a second embodiment of the present invention. Hereinafter, the same reference numerals represent the same elements, and thus their descriptions will be omitted here. Referring to  FIG. 12 , the phantom  30  is the same as the phantom  11  of  FIG. 1  except for a phantom main body  31 .  
         [0100]      FIG. 13  is a cutaway view of the phantom main body  31  of  FIG. 12 . Referring to  FIG. 13 , the phantom main body  31  includes a case  31   a , which includes an empty space therein and can contain water supplied from the outside, a slice stack  31   h , which is disposed in the case  31   a , and a sealing cover  31   b , which covers and hermetically seals the case  31   a.    
         [0101]     The case  31   a  comprises a disc type bottom plate  31   f  with a predetermined thickness and a sidewall  31   e  fixed to the bottom plate  31   f . A plurality of female screw holes are formed in the top portion of the sidewall  31   e  such that fixing bolts  31   c  can be screwed thereinto. Four vertical supporting rods  31   g  are fixed on the top surface of the bottom plate  31   f . The vertical supporting rods  31   g  have the same measurements and serve the same functions as the vertical supporting rods  19   m  of  FIG. 3 .  
         [0102]     The slice stack  31   h  comprises a stack of a plurality of unit slices  31   k  stacked sequentially. The slice stack  31   h  has an empty space, which embodies a shape of the entire brain, inside. In order to take an image of the brain embodied in the slice stack  31   h , the empty space in the slice stack  31   h  must be filled with water, similar to the slice stack  19   p  of  FIG. 3 . In order to form the empty space inside the slice stack  31   h , a brain section hole  31   m  is formed in each of the unit slices  31   k  that embody the entire brain when stacked, similar to the slice stack  19   p  of  FIG. 3 .  
         [0103]     An opening hole  31   n  is formed in the center of the sealing cover  31   b . Water is supplied into the slice stack  31   h  through the opening hole  31   n , and the opening hole  31   n  can be sealed by an opening/shutting screw  31   d . A plurality of holes are formed along the circumference of the sealing cover  31   b  so that the fixing bolts  31   c  can be fixed into the sidewall  31   e  through the sealing cover  31   b.    
         [0104]      FIG. 14  is a plan view of one of the plurality of unit slices  31   k  of the slice stack  31   h  of  FIG. 13 . Referring to  FIG. 14 , a unit slice  31   k  assumes a simpler shape than the unit slice  19   q  of  FIG. 4 . While the brain section holes  19   r  on the unit slice  19   q  of  FIG. 4  represent the white and gray matter of the brain at a 1:1 ratio, the brain section hole  31   m  on the unit slice  31   k  represents outlines of a predetermined portion of the brain. Each of the unit slices  31   k  represents a horizontal cross section of the brain. The unit slices  31   k  have different shapes of brain section holes  31   m.    
         [0105]     A plurality of vertical indicating bars  32  of  FIG. 15  are disposed in the empty space inside the slice stack  31   h.    
         [0106]      FIG. 15  is a cross-sectional view of the slice stack  31   h  of  FIG. 13 . Referring to  FIG. 15 , the slice stack  31   h  includes the plurality of unit slices  31   k  that are stacked on one another, and an inner space  31   p , which embodies a shape of the brain, is formed inside the slice stack  31   h . The inner space  31   p  is defined by the brain section hole  31   m  in each of the unit slices  31   k.    
         [0107]     A total of 8 vertical indicating bars  32  are vertically fixed in the inner space  31   p . Functions of the vertical indicating bars  32  will be described later. Two supporting plates  31   q  vertically fix the vertical indicating bars  32  in the inner space  31   p.    
         [0108]     The supporting plates  31   q  having a predetermined thickness are interposed among the unit slices  31   k  in parallel with each other. The supporting plates  31   q  are formed of the same material of the unit slices  31   k , and through holes  31   s  are formed in each of the supporting plates  31   q  such that water can pass through the supporting plates  31   q.    
         [0109]      FIG. 16  is an exploded perspective view of the supporting plates  31   q  and the vertical indicating bars  32  coupled thereto. Referring to  FIG. 16 , the two supporting plates  31   q  are parallel with each other, and through holes  19   s  are formed in each of the supporting plates  31   q  near the boundary of each of the supporting plates  31   q  such that the vertical supporting rods  31   g  of  FIG. 13  can be inserted therethrough. The through holes  31   s  are formed in the supporting plates  31   q  such that water supplied into the slice stack  31   h  from the outside can pass through the supporting plates  31   q.    
         [0110]     The vertical indicating bars  32  are fixed in the middle of the supporting plates  31   q . The vertical indicating bars  32 , like the N-shaped indicators  16 , are provided such that their cross sections can be displayed on an image of the phantom  30 . The vertical indicating bars  32  may have different lengths from one another depending on the geometrical shape of the inner space  31   p  they are disposed.  
         [0111]     Each of the vertical indicating bars  32 , which are vertically fixed into the supporting plates  31   q , includes an acrylic tube  32   a , which has an empty space therein, an inserting rod  32   b , which is inserted into the acrylic tube  32   a , and a sealing screw  32   c , which hermetically seals the acrylic tube  32   a . The vertical indicating bars  32  comprises the same elements as the N-shaped indicators  16 .  
         [0112]     When taking an image of the phantom  30  using a CT or MRI apparatus, an acrylic rod is used as the inserting rod  32   b . And when taking an image of the phantom  30  using a SPECT or PET apparatus, a lead rod is used as the inserting rod  32   b . Since the acrylic tube  32   a  can be opened or sealed using the sealing screw  32   c , the inserting rod  32   b  inserted in the acrylic tube  32   a  can be easily replaced by the other one.  
         [0113]      FIG. 17  illustrates a tomogram of a predetermined position of the phantom  30  of  FIG. 12  taken by a CT apparatus. Referring to  FIG. 17 , cross sections of the container  13  and the sidewall  31   e  appear on the tomogram of the phantom  30 , and a cross-section of the brain, corresponding to the predetermined position of the phantom  30 , is shown in the middle of the tomogram. Eight points C 1  through C 8 , which respectively represent eight inserting rods  32   b , are marked on the tomogram of the position of the brain. The eight fixed points C 1  through C 8  serve as benchmarks for determining whether the cross section of the predetermined position of the phantom  30  represented by the tomogram is parallel to the bottom surface of the container  13 , which will be described later.  
         [0114]     Points A 1 , A 2 , and A 3 , which respectively represent cross sections of the three inserting rods  17   b  of each of the N-shaped indicators  16 , are marked on the tomogram between the cross-sectional images of the container  13  and the sidewall  31   e . The height of the interesting position of the phantom  13  from the bottom surface of the container  13  can be obtained by using distances between the points A 1 , A 2 , and A 3 .  
         [0115]     When the fixed points C 1  through C 8  are mapped on an XY coordinate system such that C 4 , C 5  and C 6  are disposed along the X-axis, and C 2  and C 5  along the Y-axis, and C 5  at the origin, the coordinates of the other fixed points, for example C 3 , are obtained. The distance from the bottom surface of the container  13  to the point C 3  can be obtained using the distances between the points A 1 , A 2 , and A 3 .  
         [0116]      FIG. 18  illustrates functions of the vertical indicating bars  32 . More specifically,  FIG. 18  illustrates a result of transferring the tomogram of  FIG. 17  on an XYZ coordinate system. Referring to  FIG. 18 , supposing that an intersection point between the bottom surface of the container  13  and a line perpendicular to the bottom surface of the container  13  passing through C 5  is set as the origin of the XYZ coordinate system, coordinates of C 3  can be obtained using the above-mentioned method, and a vector representing C 3  with respect to the origin can be obtained using the coordinates of C 3 .  
         [0117]     Vectors respectively representing the other fixed points with respect to the origin can also be obtained using their coordinates. Thus, tomograms at a position of interest of the phantom  30 , taken by different imaging apparatuses, such as CT and MRI apparatuses or SPECT and PET apparatuses, can be three-dimensionally compared with each other. In other words, if the images at the same position in the axial direction of the phantom, taken using each imaging apparatus, are obtained, the accuracy of the image registration software can be evaluated two-dimensionally by observing the degree to which the fixed points C 1  through C 8  on one of the tomograms match with their respective counterparts on the other tomogram. In addition, the accuracy of the image registration software can also be evaluated three-dimensionally by calculating vectors of the fixed points on each of the tomograms and comparing them.  
         [0118]     If the vectors representing the fixed points on one of the tomograms match with their respective counterparts on the other tomogram, the image registration software is determined to operate normally. Otherwise, it is determined that cross sections of the phantom  30  represented by the tomograms are not parallel with each other, which means the image registration software is not accurate.  
         [0119]      FIG. 19  illustrates tomograms of the phantom of  FIG. 12  taken by a CT apparatus and a SPECT apparatus, and a result of matching the tomograms with each other using image registration software, and  FIG. 20  illustrates tomograms of the phantom of  FIG. 12  taken by a CT apparatus and an MRI apparatus, and a result of matching the tomograms with each other using image registration software.  
         [0120]      FIG. 21  is an exploded perspective view of a phantom  50  for evaluating the accuracy of image registration software according to a third embodiment of the present invention. Referring to  FIG. 21 , the phantom  50  includes a container  53 , which can contain water therein and includes four vertical supporting rods  53   c , a phantom main body  51 , which is disposed in the container  53 , and a lid  59 , which hermetically seals the container  53 .  
         [0121]     The container  53  includes a bottom plate  53   a , which is disk-shaped, and a sidewall  53   b , which is cylindrical. The vertical supporting rods  53   c  are fixed on the bottom plate  53   a . The vertical supporting rods  53   c  are acrylic rods that pass through through holes  19   s  of a slice stack  55  so that they can support the slice stack  55 .  
         [0122]     The phantom main body  51  comprises the slice stack  55  and an indicator  57 .  
         [0123]     The slice stack  55  comprises a plurality of unit slices  55   a . Lung section holes  55   c , which embody a cross section of the lungs, are formed in the unit slices  55   a . The slice stack  55  has an empty space embodying the lungs.  
         [0124]     Auxiliary holes  55   b  are further formed in the slice stack  55  near the outer boundary of the slice stack  55  such that they can be filled with water. The auxiliary holes  55   b  filled with water are represented by points on a tomogram of the phantom  50 .  
         [0125]     The indicator  57  comprises a supporting slice  57   a , which covers the top surface of the slice stack  55 , and four vertical indicating bars  57   b , which are fixed to the bottom surface of the supporting slice  57   a  and extend vertically downward from the bottom surface of the supporting slice  57   a . The vertical indicating bars  57   b  have the same functions as the vertical indicating bars  32  in the second embodiment of the present invention.  
         [0126]     Each of the vertical indicating bars  57   b  comprises an acrylic tube  57   c , an upper end of which can be exposed to the outside over the supporting slice  57   a , an inserting rod  57   d , which is disposed in the acrylic tube  57   c , and a sealing screw  57   e , which hermetically seals the upper end of the acrylic tube  57   c . As described above, an acrylic or lead rod can be selectively used as the inserting rod  57   d . Two of the four vertical indicating bars  32  are shorter than the other two vertical indicating bars  32 .  
         [0127]     A lid  59  hermetically seals the container  53  with the phantom main body  51  disposed in the container  53 . A plurality of holes are formed near the boundary of the lid  59  so that fixing bolts  59   b  can be fixed into the container  53  passing through the lid  59 .  
         [0128]     A water supply hole  59   c  is formed in the middle portion of the lid  59 . The water supply hole  59   c  can be sealed by an opening/shutting screw  59   a.    
         [0129]      FIG. 22  is a perspective view of the phantom  50  of  FIG. 21 , from which the lid  59  and the indicator  57  are removed. Referring to  FIG. 22 , the unit slices  55   a  can be neatly stacked in the container  53  due to the vertical supporting rods  53   c . The circumferential boundary of the slice stack  55  does not contact the inner sidewall of the container  53  such that an empty space is formed therebetween. The empty space is filled with water.  
         [0130]      FIG. 23  is a plan view of one of the plurality of unit slices  55   a  of the slice stack  55  of  FIG. 21 . Referring to  FIG. 23 , four through holes  19   s  and four auxiliary holes SSb are formed in a unit slice  55   a  near the boundary of the unit slice  55   a . Lung section holes  55   c  on the unit slice  55   a  represent a cross section of a predetermined position in the axial direction of the lungs.  
         [0131]      FIG. 24  is a cross-sectional view of the phantom  50  of  FIG. 21 . Referring to  FIG. 24 , the plurality of unit slices  55   a , which are stacked sequentially, has an inner space  31   p , which embodies a shape of the lungs. The inner space  31   p  is defined by the lung section holes  55   c  formed through each of the unit slices  55   a.    
         [0132]     The vertical indicating bars  57   b  are disposed between the sidewall  53   b  and the slice stack  55 . Before tomographing the phantom  50 , the inner space  31   p  and a space where the vertical indicating bars  57   b  are filled with water.  
         [0133]      FIG. 25  illustrates a tomogram at a position of interest of the phantom  50  of  FIG. 21  taken by a CT apparatus. Referring to  FIG. 25 , cross sections of the sidewall  53   b  and the inserting rod  57   b  of each of the vertical indicating bars  57   b  are represented by points. Points S 1  represent cross sections of pillars of water filling the auxiliary holes  55   b . Accuracy of image registration software is evaluated by using the points S 1  and other points S 2 . In other words, tomograms of the predetermined position of the phantom  50  taken by different imaging apparatuses are superposed on one another, and then the accuracy of the image registration software is evaluated depending on the degree to which fixed points, such as the points S 1  and S 2 , on one of the tomograms match with their respective counterparts on another tomogram.  
         [0134]     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.