Abstract:
Provided is a highly reliable technique for the evaluation of ischemic conditions. A preferred embodiment is a nuclear medicine measurement protocol in which the administration of a radiopharmaceutical agent and radiation measurement are performed twice at rest and under stress. In the nuclear medicine measurement protocol, radiation collection is performed without radiopharmaceutical agent administration before the second radiopharmaceutical agent administration, and the result is used to correct the nuclear medicine measurement result after the second radiopharmaceutical agent administration.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
       [0001]    The present application relates to a method of analyzing myocardial nuclear medicine image data and an apparatus for analyzing myocardial nuclear medicine image data. 
       2. Description of the Related Art 
       [0002]    Nuclear medicine technology is used to yield various types of physiological and biochemical information about the heart in many cases. Specifically, a single-photon emission computed tomography (SPECT) examination has excellent characteristics including an easy load examination, a high examination success rate and low invasiveness. 
         [0003]    An application field of nuclear medicine imaging examination on the heart is the detection of ischemia. The detection of ischemia is performed by comparing a nuclear medicine image recorded at rest with a nuclear medicine image recorded under stress. 
         [0004]    The primary image obtained from nuclear medicine measurement is prepared by the imaging of radiation count values or tissue radioactivity concentrations. Pixels corresponding to the position at which a tracer is highly accumulated have a large pixel value and are displayed brightly. However, the radiation count value or the tissue radioactivity concentration is affected by various factors, and thus even when particular pixels have a pixel value different from those of the other positions, whether the corresponding tissue is abnormal is not necessarily evident. To address this uncertainness, attempts have been made to normalize pixel values in accordance with a certain rule so as to enable quantitative evaluation of the pixel values. As such a quantitative value, a standardized uptake value (SUV) is typically used. The SUV is determined in accordance with the following equation: 
         [0000]      SUV Tissue radioactivity concentration/{Administered radiation dose/Body mass of subject}. 
         [0005]    In other words, the SUV is calculated by normalization of a tissue radioactivity concentration by the administered radiation dose per body mass. In place of a simple body mass, a lean body mass is used in some cases (Non-Patent Document 1). 
         [0006]    [Non-Patent Document 1] Yoshifumi Sugawara, Kenneth R. Zasadny et al., “Reevaluation of the Standardized Uptake Value for FDG: Variations with Body Weight and Methods for Correction”, November 1999 Radiology, 213, 521-525. 
       SUMMARY OF THE INVENTION 
       [0007]    The existing SUV is determined on the assumption that a tracer is evenly distributed in the whole body or muscle. In the case of the nuclear medicine examination of the heart, a tracer is, however, accumulated mainly in myocardia, and thus the assumption of the existing SUV may be inappropriate. On this account, there is a demand for a novel technique for quantitatively evaluating tracer accumulation. 
         [0008]    Existing ischemia detection methods are reported to have a possibility of underestimating ischemic conditions of patients with a diffusely lowered myocardial blood flow, including patients with multivessel ischemic cardiomyopathy. There is thus a demand for a highly reliability technique for evaluating ischemic conditions. 
         [0009]    The invention disclosed in the present application is intended to solve at least one of the above-described problems. 
         [0010]    An embodiment of the invention described in the present application is intended to normalize image data obtained from myocardial nuclear medicine measurement, using a value relating to the size of the heart. 
         [0011]    In a preferred embodiment, the pixel value of each pixel of the myocardial nuclear medicine image data is converted into an SUV represented by the following equation: 
         [0000]      SUV=Tissue radioactivity concentration/(Administered radiation dose/Value relating to size of heart). 
         [0012]    The invention uses a value relating to the size of the heart in which a tracer is accumulated, as a normalization standard to normalize myocardial nuclear medicine image data. The normalization standard thus reflects actual conditions of a cardiac function diagnostic agent more correctly than in the related art. This improves the validity of a normalized value as compared with the related art and enables more appropriate image evaluation than ever. 
         [0013]    In the invention, the “value relating to the size of the heart” may be a heart weight, for example. The heart weight may be a myocardial weight, for example. The myocardial weight may be a value obtained by multiplying a myocardial volume by a density factor, for example. 
         [0014]    In the invention, the “tissue radioactivity concentration” may be a value obtained by multiplying a pixel value of the myocardial nuclear medicine image data by a becquerel calibration factor (BCF). The BCF is a factor for converting a radiation count value into a radioactivity concentration (for example, Bq/ml). The BCF can be determined by a known method. For example, a nuclear medicine image of a vial (or a syringe) containing a radiopharmaceutical agent having a known total radioactivity can be taken, and the BCF can be calculated in accordance with the following equation: 
         [0000]      BCF=Decay-corrected total radioactivity (Bq)/(Total count of all slices/Collection time (seconds)). 
         [0015]    To determine the BCF from the data obtained using a cylindrical phantom, the following equations may be used: 
         [0000]      Volume factor=Average count value per slice/(Volume of single pixel×Collection time (seconds))
 
         [0000]      BCF=Decay-corrected total radioactivity (Bq)/(Phantom volume×Volume factor).
 
         [0016]    In some embodiments, the BCF may be subjected to collection time correction. The collection time correction may be performed by multiplying {Volume of single pixel [cm 3 ]/Collection time [sec]} by BCF, for example. 
         [0017]    In some myocardial nuclear medicine image data, each pixel value itself may represent a radioactivity concentration. 
         [0018]    Needless to say, no conversion using BCF is needed in such a case. 
         [0019]    An embodiment of the invention includes the following method, which is the method for processing myocardial nuclear medicine image data.
       This method includes operating the apparatus as first means for storing a heart parameter serving as a value relating to a size of a heart and as second means for storing an administered radiation dose.   This method also includes converting pixel values of at least part of pixels of the image data using the values stored in the first means and the second means into SUVs in accordance with the following equation, and storing the SUVs:       
 
         [0000]      SUV=Tissue radioactivity concentration/(Administered radiation dose/Value based on heart parameter). 
         [0022]    In some embodiments, the heart parameter is a myocardial weight, and the value based on the heart parameter is also a myocardial weight. 
         [0023]    In some embodiments, the heart parameter is a myocardial volume, and the value based on the heart parameter is a myocardial weight calculated by multiplying the myocardial volume by a conversion factor. 
         [0024]    An embodiment of the invention includes a computer program including a program instruction configured to cause an apparatus to perform the above-described method when the computer program is executed by processing means in the apparatus. 
         [0025]    Another embodiment of the invention includes an apparatus including processing means and memory means. The memory means stores a program instruction, and the program instruction is configured to perform the above-described method when the program instruction is executed by the processing means. 
         [0026]    Another embodiment of the invention described in the present application is a nuclear medicine measurement protocol in which the administration of a radiopharmaceutical agent and radiation measurement are performed twice at rest and under stress. In the nuclear medicine measurement protocol, radiation collection is performed without radiopharmaceutical agent administration before the second radiopharmaceutical agent administration, and the result is used to correct the subsequent nuclear medicine measurement results after the second radiopharmaceutical agent administration. 
         [0027]    In the present description, the radiation collection performed before the second radiopharmaceutical agent administration is referred to as prescan. The prescan is performed immediately before later measurement between the measurement under stress and the measurement at rest and is performed immediately before the radiopharmaceutical agent administration for the later measurement. 
         [0028]    When the time interval between the measurement at rest and the measurement under stress is insufficient, the nuclear medicine data obtained through the later nuclear medicine measurement is affected by the radiopharmaceutical agent administration for the earlier nuclear medicine measurement. With the above-described invention, such an effect can be corrected by data collection results of the prescan. The pieces of nuclear medicine data are thus compared between at rest and under stress more accurately. In the existing examination, the second nuclear medicine measurement can be performed only after no effect of the first radiopharmaceutical agent administration is observed. The above-described invention can correct such an effect and thus shorten the time interval between the measurement at rest and the measurement under stress. In the existing examination, the measurement at rest and the measurement under stress need to be performed on different days, for example. The above-described invention allows the two measurements to be performed successfully on the same day. 
         [0029]    An embodiment of the invention includes the following method, which is the method of processing myocardial nuclear medicine image data for determining the increase rate of blood flow under stress relative to blood flow at rest. This method includes:
       storing at least part of first myocardial nuclear medicine image data collected earlier between myocardial nuclear medicine image data collected at rest and myocardial nuclear medicine image data collected under stress;   storing at least part of second myocardial nuclear medicine image data collected between the nuclear medicine image data collection at rest and the nuclear medicine image data collection under stress, the second myocardial nuclear medicine image data being collected before administration of a radiopharmaceutical agent for later data collection between the myocardial nuclear medicine image data at rest and the myocardial nuclear medicine image data under stress;   storing at least part of third myocardial nuclear medicine image data collected later between the myocardial nuclear medicine image data collected at rest and the myocardial nuclear medicine image data collected under stress;   correcting at least part of the third myocardial nuclear medicine image data using at least part of the second myocardial nuclear medicine image data to eliminate an effect of remaining radioactivity from the first myocardial nuclear medicine image data remaining in the third myocardial nuclear medicine image data; and   determining the blood flow increase rate using at least part of the first myocardial nuclear medicine image data and at least part of the third myocardial nuclear medicine image data after the correction.       
 
         [0035]    In some embodiments, the above-described method may further include:
       operating the apparatus as first means for storing a heart parameter serving as a value relating to a size of a heart and as second means for storing an administered radiation dose;   converting pixel values of at least part of pixels of the image data using the values stored in the first means and the second means into SUVs in accordance with the following equation, and storing the SUVs:       
 
         [0000]      SUV=Tissue radioactivity concentration/(Administered radiation dose/Value based on heart parameter); and       determining the blood flow increase rate using at least part of the SUVs.         
         [0039]    In some embodiments, the heart parameter may be a myocardial weight, and the value based on the heart parameter may also be a myocardial weight. In some embodiments, the heart parameter may be a myocardial volume, and the value based on the heart parameter may be a myocardial weight calculated by multiplying the myocardial volume by a conversion factor. 
         [0040]    In some embodiments, the above-described method may further include converting the first to third myocardial nuclear medicine image data into two-dimensional array data or polar maps, and calculating the myocardial blood flow increase rate using the data after the conversion. 
         [0041]    An embodiment of the invention includes a computer program including a program instruction configured to cause an apparatus to perform the above-described method when the computer program is executed by processing means in the apparatus. 
         [0042]    Another embodiment of the invention includes an apparatus including processing means and memory means. The memory means stores a program instruction, and the program instruction is configured to perform the above-described method when the program instruction is executed by the processing means. 
         [0043]    Some embodiments of the invention of the present application thought to be preferred now are specified by the appended claims. However, the configurations specified by these claims do not necessarily completely encompass all the novel technical spirit disclosed in the description and the drawings of the present application. It should be noted that the applicant claims a right to the patent of all the novel technical spirit disclosed in the description and the drawings of the present application regardless of whether the technique is described in the present claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0044]      FIG. 1  is a diagram for explaining a hardware configuration of a system capable of performing the present invention; 
           [0045]      FIG. 2  is a chart for explaining nuclear medicine measurement protocols for obtaining data to be analyzed in the present application; 
           [0046]      FIG. 3  is a flowchart for explaining a preferred example of the processing for calculation of a myocardial blood flow increase rate; 
           [0047]      FIG. 4  is a flowchart for explaining a specific example of step  345  in  FIG. 3 ; 
           [0048]      FIG. 5  is a flowchart for explaining another specific example of step  345  in  FIG. 3 ; 
           [0049]      FIG. 6  is a flowchart for explaining still another specific example of step  345  in  FIG. 3 ; and 
           [0050]      FIG. 7  is a flowchart for explaining still another specific example of step  345  in  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0051]    Preferred embodiments of the technical spirit disclosed in the present application will now be described with reference to the attached drawings. 
         [0052]      FIG. 1  is a diagram for explaining a hardware configuration of a system  100  capable of performing the present invention. As illustrated in  FIG. 1 , the hardware configuration of the system  100  is substantially the same as those of conventional computers, and can include a CPU  102 , a main memory unit  104 , a mass storage unit  106 , a display interface  107 , a peripheral interface  108 , and a network interface  109 , for example. Similarly to conventional computers, the main memory unit  104  may be a high-speed random access memory (RAM), and the mass storage unit  106  may be an inexpensive, large-capacity hard disk or SSD. The system  100  may be connected to a display for displaying information via the display interface  107 . The system  100  may also be connected to user interfaces, such as a keyboard, a mouse, and a touch panel, via the peripheral interface  108 . The network interface  109  can be used to connect the system to other computers and the Internet via a network. 
         [0053]    The mass storage unit  106  stores an operating system (OS)  110 , a myocardial blood flow increase rate calculation program  120 , an alignment program  122 , and a contour extraction/volume calculation program  124 . The most basic function of the system  100  is provided through execution of the OS  110  by the CPU  102 . The myocardial blood flow increase rate calculation program  120  includes program instructions relating to the novel processing disclosed in the present application. Through execution of at least part of these instructions by the CPU  102 , the system  100  can perform the novel processing disclosed in the present application. 
         [0054]    The alignment program  122  includes instructions for adjusting the positions or the sizes of images to each other between a plurality of pieces of nuclear medicine image data. The existing commercially available positron emission tomography (PET) or SPECT apparatuses typically include such a program, and all or part of the program can be used as the alignment program  122 . 
         [0055]    The contour extraction/volume calculation program  124  includes instructions for extracting the myocardial contour. Some algorithms and software for myocardial contour extraction are known, and such an algorithm is disclosed by the present applicant in PCT International Publication (WO2013/047496A1), for example. In addition, QGS by Cedras-Sinai Medical Center, 4D-MSPECT by the University of Michigan, and pFAST by Sapporo Medical University are also disclosed as the algorithm or software for myocardial contour extraction. The program instructions included in the contour extraction/volume calculation program  124  may be configured to extract the myocardial contour using such an algorithm or software and to calculate the volume of the extracted myocardium. An embodiment of the invention disclosed in the present application can be operated together with various myocardial contour extraction algorithms, but the algorithm described in WO2013/047496A1 is preferably used to extract the myocardial contour because the algorithm has high extraction accuracy. 
         [0056]    The mass storage unit  106  can further store three-dimensional nuclear medicine image data  130 , three-dimensional nuclear medicine image data  132 , and three-dimensional nuclear medicine image data  134 . Such nuclear medicine image data is to be analyzed or operated by the programs  120 ,  122 , and  124 . The mass storage unit  106  can also store collection condition files  131 ,  133 , and  135  that store various data collection conditions relating to the nuclear medicine image data. These pieces of data will be specifically described later.  FIG. 1  also illustrates data  140 , data  150 , and data  152 , which will be specifically described later. 
         [0057]    The system  100  can also include typical components included in a common computer system, such as a power supply and a cooler, in addition to the units illustrated in  FIG. 1 . Known embodiments of the computer system can include various forms using various techniques such as distribution, redundancy, and virtualization of memory units, use of multiple CPUs, CPU virtualization, use of a processing-specific processor such as a DSP, and a combination of hardware for particular processing performed by a CPU. The invention disclosed in the present application can be installed on any computer system, and the type of computer system does not limit the scope of the invention. The technical spirit disclosed in the present description can be typically embodied as (1) a program including instructions configured to cause an apparatus or a system including processing means to perform various types of processing described in the present description when the program is executed by the processing means; (2) a method of operating an apparatus or a system implemented by the processing means executing the program; or (3) an apparatus or a system including the program and processing means configured to execute the program, for example. As described above, software processing may be partially made into hardware. 
         [0058]    It should be noted that the data  130  to the data  135 , for example, is not stored in the mass storage unit  106  in many cases while the system  100  is being produced and sold or is being started. Such data may be transferred from an external device to the system  100  via the peripheral interface  108  or the network interface  109 , for example. In some embodiments, the data  131 ,  133 ,  135 ,  140 ,  150 , and  152  may be formed through execution of the myocardial blood flow increase rate calculation program  120  by the CPU  102 . Depending on an installed alignment program  122  or an installed OS  110 , at least one of the data  131 ,  133 ,  135 ,  140 ,  150 , and  152  is not stored in the mass storage unit  106  but is stored only in the main memory unit  104  in some cases. It should be noted that the scope of the invention disclosed in the present application is not limited by whether the data is included. 
         [0059]    Next, the three-dimensional nuclear medicine image data  130 ,  132 , and  134  will be described in detail. These pieces of image data are obtained by nuclear medicine measurement performed for determining a myocardial blood flow increase rate. In the present example, the three-dimensional nuclear medicine image data is obtained using SPECT as a nuclear medicine measurement technique. To determine a myocardial blood flow increase rate, the nuclear medicine image data obtained through nuclear medicine measurement under stress is typically compared with the nuclear medicine image data obtained through nuclear medicine measurement at rest. To determine a myocardial blood flow increase rate, at least the two pieces of nuclear medicine image data are required. In the present example, the data  130  is data obtained through the SPECT data collection immediately after stress application, and the data  134  is data obtained through the SPECT data collection at rest. Hereinafter, the data  130  and the data  134  are also referred to as stress data and rest data, respectively. 
         [0060]    In the examples specifically described below, the image data  130 ,  132 , and  134  is image data in which each pixel value corresponds to a radiation count value. In some embodiments, the image data  130 ,  132 , and  134  may be image data in which each pixel value represents a tissue radioactivity concentration. 
         [0061]    The protocol of collecting nuclear medicine data to be analyzed in the present application is characterized by collecting radiation emitted from the body of a subject using a nuclear medicine apparatus without administration of a radiopharmaceutical agent between the measurement under stress and the measurement at rest. This radiation collection is referred to as prescan. The prescan is performed immediately before later measurement between the measurement under stress and the measurement at rest, and is performed immediately before a process of data collection in the later measurement. For example, as described later with reference to  FIG. 2 , the prescan is performed immediately before the application of stress when the later measurement is the measurement of stress data, and the prescan is performed immediately before the administration of a radiopharmaceutical agent when the later measurement is the measurement of rest data. In the present example, the nuclear medicine image data obtained through the prescan is indicated by sign  132 . The data to be analyzed in the present application accordingly includes at least three types of data: the stress data  130 , the prescan data  132 , and the rest data  134 . 
         [0062]    With reference to  FIG. 2 , the nuclear medicine measurement protocol for obtaining data to be analyzed in the present application will be described in further detail.  FIG. 2  schematically illustrates events performed in protocols on the time axis. As described above, to determine a myocardial blood flow increase rate, nuclear medicine measurement is required to be performed under stress and at rest. In a protocol, the measurement under stress is performed earlier. In another protocol, the measurement at rest is performed earlier. 
         [0063]    When the measurement under stress is performed earlier, stress is first applied to a subject ( 202 ). The stress may be exercise stress using an ergometer or a treadmill load or drug stress with adenosine or dipyridamole, for example. In the stress application, a radiopharmaceutical agent is administered (intravenous injection) ( 204 ). Next, the stress application is stopped, and a nuclear medicine apparatus is used to collect data of radiation emitted from the body of the subject ( 206 ). After a certain period of time (several hours), the measurement at rest is performed. Before the measurement at rest, a nuclear medicine apparatus is used to collect data of radiation emitted from the body of the subject without administration of a radiopharmaceutical agent ( 208 ). In other words, prescan is performed. Immediately after the completion of the prescan, a radiopharmaceutical agent for measurement at rest is administered ( 210 ), and a nuclear medicine apparatus is used to collect data of radiation emitted from the body of the subject ( 212 ). The image data obtained through the data collection  206  is the stress data  130 , the image data obtained through the data collection  208  is the prescan data  132 , and the image data obtained through the data collection  212  is the rest data  134 . 
         [0064]    When the measurement at rest is performed earlier, a radiopharmaceutical agent for the measurement at rest is first administered to a subject ( 222 ), and then a nuclear medicine apparatus is used to collect data of radiation emitted from the body of the subject ( 224 ). After a certain period of time (several hours), the measurement under stress is performed. Before the measurement under stress, a nuclear medicine apparatus is used to collect data of radiation emitted from the body of the subject ( 226 ) without administration of a radiopharmaceutical agent. In other words, prescan is performed. Immediately after the completion of the prescan, stress application is started ( 228 ). In the stress application, a radiopharmaceutical agent for the measurement under stress is administered ( 230 ). After the completion of the stress application, a nuclear medicine apparatus is used to collect data of radiation emitted from the body of the subject ( 232 ). The image data obtained through the data collection  232  is the stress data  130 , the image data obtained through the data collection  226  is the prescan data  132 , and the image data obtained through the data collection  224  is the rest data  134 . 
         [0065]    As described above, in the present example, the nuclear medicine apparatus used for the radiation data collection is an SPECT apparatus, and the radiopharmaceutical agent administered is accordingly a pharmaceutical agent suitable for the radiation data collection by the SPECT apparatus. Examples of known SPECT radiopharmaceutical agents suitable for the nuclear medicine imaging of myocardial blood flow include  201 TlCl (thallium chloride) injection solution, technetium ( 99m Tc) tetrofosmin injection solution, and 15-(4-iodophenyl)-3(R,S)-methylpentadecanoic acid ( 123 I) injection solution. The invention disclosed in the present application is applicable to any radiopharmaceutical agent suitable for the nuclear medicine imaging of myocardial blood flow. 
         [0066]    Next, the flow of myocardial blood flow increase rate calculation processing  300  of nuclear medicine image data disclosed in the present application will be described with reference to  FIG. 3 . The processing  300  may be performed by the system  100  in which the myocardial blood flow increase rate calculation program  120  is executed by the CPU  102 . In some embodiments, midway through the processing  300 , the alignment program  122  or the contour extraction/volume calculation program  124  may be called from the myocardial blood flow increase rate calculation program  120  and executed by the CPU  102  to perform certain processing. 
         [0067]    Step  305  indicates the start of processing. In step  310 , data to be processed by the myocardial blood flow increase rate calculation program  120  is loaded. In other words, all or part of each of the stress data  130 , the prescan data  132 , and the rest data  134  is read from the mass storage unit  106  and is stored in the main memory unit  104 . The stress data  130 , the prescan data  132 , and the rest data  134  may be directly imported from an external nuclear medicine apparatus into the main memory unit  104  via the network interface  109 . 
         [0068]    In step  315 , a data collection protocol is identified. In other words, a protocol in which the stress data  130 , the prescan data  132 , and the rest data  134  have been collected is identified as either the protocol in which the measurement under stress is performed earlier or the protocol in which the measurement at rest is performed earlier. The identification may be performed by comparing time information (for example, data collection start time) contained in the stress data  130  and the rest data  134 , for example. For example, when the collection start time of the stress data  130  is earlier than the collection start time of the rest data  134 , such a protocol can be identified as the protocol in which the measurement under stress is performed earlier. When the collection start time of the stress data  130  is later than the collection start time of the rest data  134 , such a protocol can be identified as the protocol in which the measurement at rest is performed earlier. In some embodiments, an operator can input protocol identification information into the system  100  to allow the system  100  to specify the data collection protocol. 
         [0069]    The determination of the data  130  as stress data may be made by identification information contained in the data  130 . Similarly, the determination of the data  134  as rest data may be made by identification information contained in the data  134 . 
         [0070]    In step  320 , various collection conditions of the stress data  130 , the prescan data  132 , and the rest data  134  are retrieved. The various collection conditions include the following information, for example.
       A radiation dose measured before administration of a radiopharmaceutical agent to a subject (radiation dose before administration). For example, a value obtained by measuring the radiation dose of a whole administration syringe containing a radiopharmaceutical agent to be administered   The measurement date and time of a radiation dose before administration   The date and time at the start of data collection   The data collection time   A radiation dose measured after administration of the radiopharmaceutical agent to the subject (radiation dose after administration). For example, a measurement value of the radiation dose remaining in the syringe after administration   The measurement date and time of a radiation dose after administration   The half-life of a tracer contained in the radiopharmaceutical agent   Becquerel calibration factor (BCF, a factor for converting a radiation count value into a radioactivity concentration (for example, Bq/ml))       
 
         [0079]    In some embodiments, these collection conditions may be included in the stress data  130 , the prescan data  132 , and the rest data  134 . In such a case, the system  100  may read the information from the data  130  to  134  and store the information in the main memory unit  104  or the mass storage unit  106 . 
         [0080]    In some embodiments, the system  100  may be configured to create and display a user interface (for example, a dialog box) to which an operator inputs these collection conditions. When an operator inputs intended collection conditions, the system  100  may store these collection conditions in the main memory unit  104  or the mass storage unit  106 . 
         [0081]    In an embodiment in which each pixel value of image data  130  to the data  134  represents a tissue radioactivity concentration, the BCF is not used and thus is not required to be retrieved. 
         [0082]    Of the above-described collection conditions, information required may vary with embodiments. Some embodiments of step  345  will be specifically described later, and information required varies with these embodiments. Information required may differ among the stress data  130 , the rest data  134 , and the prescan data  132 . The system  100  may create and display different user interfaces depending on collection condition data to be input, (i.e., depending on whether the data is the stress data  130 , the prescan data  132 , or the rest data  134 ), and the respective user interfaces may be configured to be convenient for inputting specific data required for the data. 
         [0083]    As described above, the system  100  may be configured to store the retrieved collection condition information in the main memory unit  104  or the mass storage unit  106 . In the present example, the collection condition information for the stress data  130  is considered to be stored in a collection condition file  131 , the collection condition information for the prescan data  132  is considered to be stored in a collection condition file  133 , and the collection condition information for the rest data  134  is considered to be stored in a collection condition file  135 , for example. 
         [0084]    In step  325 , whether data  132  is the prescan data is determined, as needed. In some embodiments, the determination may be made by identification information contained in the data  132 . In some embodiments, the determination may be made by time information (for example, data collection start time) contained in the data  132 . For example, for a data collection protocol in which the measurement under stress is performed earlier, the data  132  can be identified as the prescan data when the data collection start time indicated by the data  132  is between the data collection completion time of the stress data  130  and the data collection start time of the rest data  134 . For a data collection protocol in which the measurement at rest is performed earlier, the data  132  can be identified as the prescan data when the data collection start time indicated by the data  132  is between the data collection completion time of the rest data  134  and the data collection start time of the stress data  130 . 
         [0085]    In step  330 , the alignment of the stress data  130 , the prescan data  132 , and the rest data  134  is performed. In other words, positions or sizes are adjusted in such a manner that the positions or sizes of myocardial images made from these pieces of data will coincide with each other. The processing may be performed through execution of the alignment program  122  by the CPU  102 . The myocardial blood flow increase rate calculation program  120  may be executed by the CPU  102  calling the alignment program  122  in step  330 . 
         [0086]    For a data collection protocol in which the measurement under stress is performed earlier, the alignment is preferably performed on the basis of the rest data  134 . For a data collection protocol in which the measurement at rest is performed earlier, the alignment is preferably performed on the basis of the stress data  130 . 
         [0087]    Step  335  is optional processing. In this step, the stress data  130  after alignment and the rest data  134  after alignment are each subjected to myocardial contour extraction. The processing in step  335  is particularly required when processing  600  or processing  700  is performed as an embodiment of step  345 . 
         [0088]    The processing in step  335  may be performed through execution of the contour extraction/volume calculation program  124  by the CPU  102 . As described above, some algorithms and software for myocardial contour extraction are known, and such an algorithm is disclosed by the present applicant in PCT International Publication (WO2013/047496A1), for example. The program instructions included in the contour extraction/volume calculation program  124  may be configured to use the algorithm to extract the myocardial contour. 
         [0089]    In some embodiments, the contour extraction/volume calculation program  124  may be configured to use the extracted contour to calculate the myocardial volume. For example, the number of pixels present between the intima and the adventitia of the extracted myocardium may be multiplied by a pixel-volume conversion factor (for example, volume per pixel) to give a myocardial volume. 
         [0090]    In some embodiments, the prescan data  132  may be subjected to the myocardial contour extraction. In the prescan, however, the data collection time may be short, and the data collection is performed without the administration of a radiopharmaceutical agent. The pixel value (radiation count value) of each pixel in the prescan data  132  is thus low in many cases. On this account, the myocardial contour extraction of the prescan data  132  is failed in some cases. In such a case, the myocardial contour information of the prescan data  132  may be considered to be the same as the myocardial contour information of the stress data  130  or the rest data  134 . In particular, the myocardial contour information of the prescan data  132  may be considered to be the same as the myocardial contour information of later-collected data of the stress data  130  and the rest data  134 . 
         [0091]    Step  340  is also an optional step. In some embodiments, all the pixels or all the myocardial pixels of the stress data  130  and the rest data  134  may be used to calculate a myocardial blood flow increase rate. In some embodiments, part of the pixels can be used to calculate a myocardial blood flow increase rate. For example, the data  130  to the data  134  can be converted into array data or polar maps (polar coordinate display), which are often used in the technical field of the present application, and each pixel can be used to calculate a myocardial blood flow increase rate. In other words, array data or polar maps for a myocardial blood flow increase rate may be prepared. The array data is prepared as follows: each short axis tomogram in a certain range is radially scanned at every certain angle from the image center to determine a maximum pixel value, and the determined values are used to prepare a two-dimensional map in which one axis represents positions of the short axis tomograms and the other axis represents angles from the image center. The polar maps are prepared as follows: each short axis tomogram in a certain range is radially scanned at every certain angle from the image center to determine a maximum pixel value, and the determined values are plotted at concentric polar coordinates. 
         [0092]    By converting the data  130  to the data  134  into array data or polar maps, the processing load in the next step  345  can be reduced, and the visibility of the results can be improved. 
         [0093]    When step  340  is performed, each of the stress data  130 , the prescan data  132 , and the rest data  134  is converted into array data or a polar map. It should be understood that when the conversion is performed, each of the stress data  130 , the prescan data  132 , and the rest data  134  has been converted into array data or a polar map in the following description. 
         [0094]    In step  345 , a myocardial blood flow increase rate is calculated. The step typically includes four embodiments. With reference to  FIGS. 4 to 7 , each embodiment will be described next. 
         [0095]      FIG. 4  is a flowchart for explaining processing  400  that is an embodiment of step  345  in  FIG. 3 . As with the processing illustrated in  FIG. 3  and  FIGS. 5 to 7 , the processing  400  is processing for calculating a myocardial blood flow increase rate and is performed by the system  100  in which the myocardial blood flow increase rate calculation program  120  is executed by the CPU  102 . 
         [0096]    The processing  400  is performed when a data collection protocol is identified in step  315  as the protocol in which the measurement under stress is performed before the measurement at rest. The processing  400  is performed when a myocardial blood flow increase rate is calculated without SUV conversion of the image data  130  or the data  134 , which is described later. 
         [0097]    Step  402  indicates the start of processing. In step  404 , the radiation dose administered to a subject for the measurement under stress (dose under stress) is calculated. The information required for the calculation of an administered radiation dose is the following information.
       A radiation dose measured before administration of a radiopharmaceutical agent to a subject (radiation dose before administration)   The measurement date and time of a radiation dose before administration   The date and time at the start of data collection   A radiation dose measured after administration of the radiopharmaceutical agent to the subject (radiation dose after administration)   The measurement date and time of the radiation dose after administration   The half-life of a tracer contained in the radiopharmaceutical agent
 
In the present example, the information is retrieved in step  320  and is stored in the collection condition file  131 . The system  100  may thus retrieve the information from the collection condition file  131  in step  404 .
       
 
         [0104]    Subsequently, the dose under stress is calculated in accordance with the following equations: 
         [0000]      Decay time 1 (seconds)=|Measurement date and time of radiation dose before administration−Date and time at start of data collection|
 
         [0000]      Decay time 2 (seconds)=|Measurement date and time of radiation dose after administration−Date and time at start of data collection|
 
         [0000]      Decay coefficient=LN (2.0)/Half-life (seconds) (LN: natural logarithm to the base  e ) 
         [0000]      Dose under stress={Radiation dose before administration×Exp (−Decay coefficient×Decay time 1)}−{(Radiation dose after administration×Exp (−Decay coefficient×Decay time 2)}.
 
         [0105]    In subsequent step  406 , the radiation dose administered to the subject for prescan (prescan dose) is calculated. The information required for the calculation is the following information.
       A radiation dose measured before administration of a radiopharmaceutical agent to the subject under stress (radiation dose before administration)   The measurement date and time of a radiation dose before administration   The date and time at the start of data collection   A radiation dose measured after administration of the radiopharmaceutical agent to the subject under stress (radiation dose after administration)   The measurement date and time of a radiation dose after administration   The half-life of a tracer contained in the radiopharmaceutical agent
 
In other words, the information required is the same as that fox calculation of a dose under stress.
       
 
         [0112]    As described above, no radiopharmaceutical agent is administered for only prescan. As described in the section of the protocol in which the measurement under stress is performed earlier in  FIG. 2 , the prescan in the embodiment is radiation data collection performed after the measurement under stress without the administration of a radiopharmaceutical agent. The above-described information is thus the same as that on the measurement under stress except the date and time at the start of data collection. In other words, the information is the same as that used in step  404 . Only for the date and time at the start of data collection, the information of the date and time at the start of prescan data collection is needed. 
         [0113]    In some embodiments, the information other than the date and time at the start of data collection, of the above-described information may be retrieved from the collection condition file  131  for the stress data  130 . The date and time at the start of data collection may be retrieved from the collection condition file  133  for the prescan data  132  (the date and time at the start of prescan data collection has been retrieved in step  320 ). In some embodiments, the system  100  may be configured to automatically copy the information other than the date and time at the start of data collection from the collection condition file  131  into the collection condition file  133 . In such a case, the system  100  may be configured to retrieve all the information from the collection condition file  133  in this step. 
         [0114]    After the retrieval of the information, the system  100  calculates a prescan dose in a similar manner to that under stress, in accordance with the equations. 
         [0000]      Decay time 1 (seconds)=|Measurement date and time of radiation dose before administration−Date and time at start of data collection|
 
         [0000]      Decay time 2 (seconds)=|Measurement date and time of radiation dose after administration−Date and time at start of data collection|
 
         [0000]      Decay coefficient=LN (2.0)/Half-life (seconds) (LN: natural logarithm to the base  e ) 
         [0000]      Prescan dose={Radiation dose before administration×Exp (−Decay coefficient×Decay time 1)}−{Radiation dose after administration×Exp (−Decay coefficient×Decay time 2)}.
 
         [0115]    In step  408 , the radiation dose administered to the subject at rest (dose at rest) is calculated. The information required for the calculation is also the same as that when the dose under stress is calculated, and is the following information.
       A radiation dose measured before administration of a radiopharmaceutical agent to the subject at rest (radiation dose before administration)   The measurement date and time of a radiation dose before administration   The date and time at the start of data collection radiation dose measured after administration of the radiopharmaceutical agent to the subject at rest (radiation dose after administration)   The measurement date and time of a radiation dose after administration   The half-life of a tracer contained in the radiopharmaceutical agent
 
In the present example, the information has been retrieved in step  320  and has been stored in the collection condition file  135 . The system  100  may thus retrieve the information from the collection condition file  135  in this step.
       
 
         [0121]    The system  100  may be configured to then calculate a dose at rest in a similar manner to that under stress, in accordance with the equations. 
         [0000]      Decay time 1 (seconds)=|Measurement date and time of radiation dose before administration−Date and time at start of data collection|
 
         [0000]      Decay time 2 (seconds)=|Measurement date and time of radiation dose after administration−Date and time at start of data collection|
 
         [0000]      Decay coefficient=LN (2.0)/Half-life (seconds) (LN: natural logarithm to the base  e ) 
         [0000]      Dose at rest={Radiation dose before administration×Exp (−Decay coefficient×Decay time 1)}−{Radiation dose after administration×Exp (−Decay coefficient×Decay time 2)}.
 
         [0122]    In step  410 , each pixel value of the prescan data  132  is multiplied by the following correction factor to perform dose correction: 
         [0000]      Dose correction factor for prescan data=Dose under stress/Prescan dose. 
         [0123]    In step  412 , each pixel value of the rest data  134  is multiplied by the following correction factor to perform dose correction: 
         [0000]      Dose correction factor for rest data=Dose under stress/Dose at rest. 
         [0124]    In step  414 , each pixel value of the prescan data  132  after dose correction is multiplied by the following correction factor to perform collection time correction: 
         [0000]      Collection time correction factor for prescan data=Stress data collection time/Prescan data collection time. 
         [0125]    In the present example, the stress data collection time and the prescan data collection time have been retrieved in step  320  and have been stored in the collection condition files  131  and  133 , respectively. The system  100  may thus be configured to retrieve the information about the stress data collection time and the prescan data collection time from these files and to calculate the collection time correction factor for prescan data. 
         [0126]    In step  416 , each pixel value of the rest data  134  after dose correction is multiplied by the following correction factor to perform collection time correction: 
         [0000]      Collection time correction factor for rest data=Stress data collection time/Rest data collection time. 
         [0127]    In the present example, the rest data collection time has been retrieved in step  320  and has been stored in the collection condition file  135 , and thus the system  100  may be configured to retrieve information required from the collection condition file  135  and to calculate the collection time correction factor for rest data. 
         [0128]    In step  418 , the prescan data  132  after dose correction and collection time correction is used to perform count correction of the rest data  134  after dose correction and collection time correction. The correction is performed by subtracting, from the pixel value of each pixel of the rest data  134  after the correction (dose correction (step  412 ) and collection time correction (step  416 )), the pixel value of the pixel at a corresponding position in the prescan data.  132  after the correction. By performing the count correction, the effect of a radiopharmaceutical agent administered under stress is eliminated from the rest data  134 . 
         [0129]    In step  420 , the stress data  130  and the rest data  134  after count correction are used to calculate a myocardial blood flow increase rate. The myocardial blood flow increase rate of a pixel ij (IncMap ij ) is determined from the pixel, value of each pixel in the stress data  130  (Stress ij ) and the pixel value of the pixel at a corresponding position in the rest data  134  after count correction (Rest ij ) in accordance with the equation: 
         [0000]      IncMap ij [%]=(Stress ij −Rest ij )/(Rest ij )×100.
 
         [0130]    The calculated increase rate data may be stored as increase rate data  140  in the mass storage unit  106 , for example (see  FIG. 1 ). The increase rate data  140  can be three-dimensional image data in which the pixel value of each pixel represents an increase rate, for example. When step  340  in FIG. is performed, the increase rate data may be two-dimensional array data or a two-dimensional polar map in which the pixel value of each pixel represents an increase rate. 
         [0131]    By performing the count correction in step  418 , the effect of a radiopharmaceutical agent administered under stress is eliminated from the rest data  134 . Accordingly, the pixel value of each pixel in the rest data  134  after the correction reflects cardiac functions at rest more correctly. Calculation using such data enables determination of myocardial blood flow increase rates more correctly than in the related art. 
         [0132]    Step  422  indicates the end of the processing. 
         [0133]      FIG. 5  is a flowchart for explaining processing  500  that is an embodiment of step  345  in  FIG. 3  and is for the calculation of a myocardial blood flow increase rate. The processing  500  is performed when a data collection protocol is identified in step  315  as the protocol in which the measurement at rest is performed before the measurement under stress. The processing  500  is performed when a myocardial blood flow increase rate is calculated without SUV conversion of the image data  130  or the data  134 , which is described later. 
         [0134]    Step  502  indicates the start of processing. In step  504 , the radiation dose administered to a subject in the measurement at rest (dose at rest) is calculated. The information required for the calculation of an administered radiation dose is the following information. 
         [0135]    A radiation dose measured before administration of a radiopharmaceutical agent to a subject (radiation dose before administration)
       The measurement date and time of a radiation dose before administration   The date and time at the start of data collection   A radiation dose measured after administration of the radiopharmaceutical agent to the subject (radiation dose after administration)   The measurement date and time of a radiation dose after administration   The half-life of a tracer contained in the radiopharmaceutical agent
 
In the present example, the information has been retrieved in step  320  and has been stored in the collection condition file  135 . The system  100  may thus retrieve the information from the collection condition file  135  in step  502 .
       
 
         [0141]    Subsequently, a dose at rest is calculated in accordance with the equations. 
         [0000]      Decay time 1 (seconds)=|Measurement date and time of radiation dose before administration−Date and time at start of data collection|
 
         [0000]      Decay time 2 (seconds)=|Measurement date and time of radiation dose after administration−Date and time at start of data collection|
 
         [0000]      Decay coefficient=LN (2.0)/Half-life (seconds) (LN: natural logarithm to the base  e ) 
         [0000]      Dose at rest={Radiation dose before administration×Exp (−Decay coefficient×Decay time 1)}−{Radiation dose after administration×Exp (−Decay coefficient×Decay time 2)}.
 
         [0142]    In subsequent step  506 , the radiation dose administered to the subject for prescan (prescan dose) is calculated. The information required for the calculation is the following information.
       A radiation dose measured before administration of a radiopharmaceutical agent to the subject in the nuclear medicine image data collection at rest (radiation dose before administration)   The measurement date and time of a radiation dose before administration   The date and time at the start of data collection   A radiation dose measured after administration of the radiopharmaceutical agent to the subject in the nuclear medicine image data collection at rest (radiation dose after administration)   The measurement date and time of a radiation dose after administration   The half-life of a tracer contained in the radiopharmaceutical agent
 
In other words, the information required is the same as that for calculation of a dose at rest.
       
 
         [0149]    As described above, no radiopharmaceutical agent is administered for only prescan. As described in the section of the protocol in which the measurement at rest is performed earlier in  FIG. 2 , the prescan in the embodiment is radiation data collection performed after the measurement at rest without the administration of a radiopharmaceutical agent. Hence, the above-described information is the same as that on the measurement at rest except the date and time at the start of data collection. In other words, the information is the same as that used in step  504 . Only for the date and time at the start of data collection, the information of the date and time at the start of prescan data collection is needed. 
         [0150]    In some embodiments, the information other than the date and time at the start of data collection, of the above-described information may be retrieved from the collection condition file  135  for the rest data  134 . The date and time at the start of data collection may be retrieved from the collection condition file  133  for the prescan data  132  (the date and time at the start of prescan data collection has been retrieved in step  320 ). In some embodiments, the system  100  may be configured to automatically copy the information other than the date and time at the start of data collection from the collection condition file  135  into the collection condition file  133 . In such a case, the system  100  may be configured to retrieve all the information from the collection condition file  133  in this step. 
         [0151]    After the retrieval of the information, the system  100  calculates a prescan dose in a similar manner to that at rest, in accordance with the equations. 
         [0000]      Decay time 1 (seconds)=|Measurement date and time of radiation dose before administration−Date and time at start of data collection|
 
         [0000]      Decay time 2 (seconds)−|Measurement date and time of radiation dose after administration−Date and time at start of data collection|
 
         [0000]      Decay coefficient=LN (2.0)/Half-life (seconds) (LN: natural logarithm to the base  e ) 
         [0000]      Prescan dose={Radiation dose before administration×Exp (−Decay coefficient×Decay time 1)}−{Radiation dose after administration×Exp (−Decay coefficient×Decay time 2)}.
 
         [0152]    In step  508 , the radiation dose administered to the subject under stress (dose under stress) is calculated. The information required for the calculation is also the same as that when the dose at rest is calculated, and is the following information.
       A radiation dose measured before administration of a radiopharmaceutical agent to the subject (radiation dose before administration)   The measurement date and time of a radiation dose before administration   The date and time at the start of data collection   A radiation dose measured after administration of the radiopharmaceutical agent to the subject (radiation dose after administration)   The measurement date and time of a radiation dose after administration   The half-life of a tracer contained in the radiopharmaceutical agent       
 
         [0159]    In the present example, the information has been retrieved in step  320  and has been stored in the collection condition file  131 . The system  100  may thus retrieve the information from the collection condition file  131  in this step. 
         [0160]    The system  100  may be configured to then calculate a dose under stress in a similar manner to that at rest, in accordance with the equations. 
         [0000]      Decay time 1 (seconds)=|Measurement date and time of radiation dose before administration−Date and time at start of data collection|
 
         [0000]      Decay time 2 (seconds)=|Measurement date and time of radiation dose after administration−Date and time at start of data collection|
 
         [0000]      Decay coefficient=LN (2.0)/Half-life (seconds) (LN: natural logarithm to the base  e ) 
         [0000]      Dose under stress={Radiation dose before administration×Exp (−Decay coefficient×Decay time 1)}−{Radiation dose after administration×Exp(−Decay coefficient×Decay time 2)}.
 
         [0161]    In step  510 , each pixel value of the prescan data  132  is multiplied by the following correction factor to perform dose correction: 
         [0000]      Dose correction factor for prescan data=Dose at rest/Prescan dose. 
         [0162]    In step  512 , each pixel value of the stress data  130  is multiplied by the following correction factor to perform dose correction: 
         [0000]      Dose correction factor for stress data=Dose at rest/Dose under stress. 
         [0163]    In step  514 , each pixel value of the prescan data  132  after dose correction is multiplied by the following correction factor to perform collection time correction: 
         [0000]      Collection time correction factor for prescan data=Rest data collection time/Prescan data collection time. 
         [0164]    In the present example, the rest data collection time and the prescan data collection time have been retrieved in step  320  and have been stored in the collection condition files  135  and  133 , respectively. The system  100  may thus be configured to retrieve the information about the rest data collection time and the prescan data collection time from these files and to calculate a collection time correction factor for prescan data. 
         [0165]    In step  516 , each pixel value of the stress data  130  after dose correction is multiplied by the following correction factor to perform collection time correction: 
         [0000]      Collection time correction factor for stress data=Rest data collection time/Stress data collection time. 
         [0166]    In the present example, the stress data collection time has been retrieved in step  320  and has been stored in the collection condition file  131 , and thus the system  100  may be configured to retrieve information required from the collection condition file  131  and to calculate the collection time correction factor for stress data. 
         [0167]    In step  518 , the prescan data  132  after dose correction and collection time correction is used to perform count correction of the stress data  130  after dose correction and collection time correction. The correction is performed by subtracting, from the pixel value of each pixel of the stress data  130  after dose correction (step  512 ) and collection time correction (step  516 ), the pixel value of the pixel at a corresponding position in the prescan data  132  after the correction. By performing the count correction, the effect of a radiopharmaceutical agent administered for the measurement at rest is eliminated from the stress data  130 . 
         [0168]    In step  520 , the stress data  130  after count correction and the rest data  134  are used to calculate a myocardial blood flow increase rate. The myocardial blood flow increase rate of a pixel ij (IncMap ij ) is determined from the pixel value of each pixel in the stress data  130  after count correction (Stress ij ) and the pixel value of the pixel at a corresponding position in the rest data  134  after count correction (Rest ij ) in accordance with the equation: 
         [0000]      IncMap ij [%]=(Stress ij −Rest ij )/(Rest ij )×100.
 
         [0169]    The calculated increase rate data may be stored as increase rate data  140  in the mass storage unit  106 , for example (see  FIG. 1 ). The increase rate data  140  can be three-dimensional image data in which the pixel value of each pixel represents an increase rate, for example. When step  340  in FIG. is performed, the increase rate data may be two-dimensional array data or a two-dimensional polar map in which the pixel value of each pixel represents an increase rate. 
         [0170]    By performing the count correction in step  518 , the effect of a radiopharmaceutical agent administered at rest is eliminated from the stress data  130 . Accordingly, the pixel value of each pixel in the stress data  130  reflects cardiac functions under stress more correctly. Calculation using such data enables determination of myocardial blood flow increase rates more correctly than in the related art. 
         [0171]    Step  522  indicates the end of the processing. 
         [0172]      FIG. 6  is a flowchart for explaining processing  600  that is an embodiment of step  345  in  FIG. 3  and is for the calculation of a myocardial blood flow increase rate. The processing  600  is performed when a data collection protocol is identified in step  315  as the protocol in which the measurement under stress is performed before the measurement at rest. The processing  600  is performed when the image data  130  and the data  134  are subjected to characteristic SUV conversion disclosed in the present application and then a myocardial blood flow increase rate is calculated. 
         [0173]    Step  602  indicates the start of processing. In step  604 , the radiation dose administered to a subject in the measurement under stress (dose under stress) is calculated. The calculation method of the dose under stress is the same as in step  404  described for the processing  400  in  FIG. 4  and thus is not described. 
         [0174]    In step  606 , the pixel value of each pixel in the stress data  130  is converted into an SUV. The existing SUV conversion uses the body weight of a subject for normalization. In contrast, the SUV conversion of the present embodiment is performed in accordance with equation 1. 
         [0000]      SUV=Tissue radioactivity concentration/(Administered radiation dose/Myocardial weight)  [Equation 1]
 
         [0175]    Each parameter will be briefly described below. 
         [0176]    Tissue radioactivity concentration: the value obtained by multiplying the pixel value of each pixel in the stress data  130  by a becquerel calibration factor (BCF). The BCF is a factor for converting a radiation count value into a radiodensity (for example, Bq/ml). In the present example, the BCF has been retrieved in step  320 . In an embodiment in which the pixel value of each pixel in stress data  130  represents a tissue radioactivity concentration, the pixel value is not multiplied by the BCF, but the pixel value itself can be used as the tissue radioactivity concentration. 
         [0177]    Administered radiation dose: in this step, it is a radiation dose administered to a subject in the measurement under stress. In other words, it is the dose under stress determined in step  604 . 
         [0178]    Myocardial weight: it is calculated on the basis of the myocardium contour data of the stress data obtained in step  335 . For example, the number of pixels presents between the intima and the adventitia of the extracted myocardium may be multiplied by a pixel-volume conversion factor to give a myocardial volume, and the myocardial volume may be multiplied by a myocardial volume-myocardial weight conversion factor (density factor) to give a myocardial weight. The density factor can be known literature data and may be 1.05, for example. The myocardial weight may be calculated in step  335  or in the present step. In some embodiments, the myocardial weight calculation algorithm may be installed in the contour extraction/volume calculation program  124  or in the myocardial blood flow increase rate calculation program  120 . The calculated myocardial weight may be stored in the main memory unit  102  or the mass storage unit  106 . In some embodiments, the myocardial weight may be stored in a register of the CPU  102 . 
         [0179]    The stress data after conversion of the pixel value of each pixel into an SUV may be stored as stress SUV data  150  in the mass storage unit  106 , for example (see  FIG. 1 ). 
         [0180]    The BCF can be determined by a known method. For example, a nuclear medicine image of a vial (or a syringe) containing a radiopharmaceutical agent having a known total radioactivity can be taken, and the BCF can be calculated in accordance with the following equation: 
         [0000]      BCF=Decay-corrected total radioactivity (Bq)/(Total count of all slices/Collection time (seconds)). 
         [0181]    To determine the BCF from the data obtained using a cylindrical phantom, the following equations can be used: 
         [0000]      Volume factor=Average count value per slice/(Volume of single pixel×Collection time (seconds))
 
         [0000]      BCF=Decay-corrected total radioactivity (Bq)/(Phantom volume×Volume factor).
 
         [0182]    In some embodiments, the BCF may be subjected to collection time correction. The collection time correction may be performed by multiplying {Volume of single pixel [cm 3 ]/Collection time [sec]} by BCF, for example. 
         [0183]    In step  608 , the radiation dose administered to the subject in the measurement at rest (dose at rest) is calculated. The calculation method of the dose at rest is the same as in step  408  described for the processing  400  in  FIG. 4  and thus is not described. 
         [0184]    In step  610 , each pixel value of the prescan data  132  is multiplied by the following correction factor to perform collection time correction: 
         [0000]      Collection time correction factor=Rest data collection time/Prescan data collection time. 
         [0185]    In the present example, the rest data collection time and the prescan data collection time have been retrieved in step  320  and have been stored in the collection condition files  135  and  133 , respectively. The system  100  may thus be configured to retrieve the information about the rest data collection time and the prescan data collection time from these files and to calculate the collection time correction factor. 
         [0186]    In step  612 , each pixel value of the prescan data  132  after collection time correction is multiplied by the decay correction factor determined as follows to perform decay correction: 
         [0000]      Decay time (seconds)=Date and time at start of prescan data collection−Date and time at start of rest data collection,
 
         [0000]      Decay coefficient=LN (2.0)/Half-life (seconds) of tracer (LN: natural logarithm to the base  e ), and 
         [0000]      Decay correction factor=Exp (Decay coefficient×Decay time).
 
         [0187]    In step  614 , the prescan data  132  after collection time correction and decay correction is used to perform count correction of the rest data  134 . The correction is performed by subtracting, from the pixel value of each pixel of the rest data  134 , the pixel value of the pixel at a corresponding position in the prescan data  132  after correction. By performing the count correction, the effect of a radiopharmaceutical agent administered under stress is eliminated from the rest data  134 . 
         [0188]    In step  616 , the pixel value of each pixel in the rest data  134  after count correction is converted into an SUV. The conversion equation is the same as equation 1 described above. However, the “tissue radioactivity concentration” in the equation is the pixel value in the rest data  134  and is obtained by multiplying the pixel value after the count correction described in step  614  by the above-described BCF. In the equation, the “administered radiation dose” is the dose at rest determined in step  608 . In the equation, the “myocardial weight” is calculated on the basis of myocardial contour data of the rest data (for example, the data obtained in step  335 ). 
         [0189]    The rest data after conversion of the pixel value of each pixel into an SUV may be stored as rest SUV data  152  in the mass storage unit  106 , for example (see  FIG. 1 ). 
         [0190]    In step  618 , the stress SUV data  150  and the rest SUV data  152  are used to calculate a myocardial blood flow increase rate. The myocardial blood flow increase rate of a pixel ij (IncMap ij ) is determined from the pixel value of each pixel in the stress SUV data  150  (Stress_SUV ij ) and the pixel value of the pixel at a corresponding position in the rest SUV data  152  (Rest_SUV ij ) in accordance with the equation: 
         [0000]      IncMap ij [%−]=(Stress_SUV ij −Rest_SUV ij )/(Rest_SUV ij )×100.
 
         [0191]    The calculated increase rate data may be stored as increase rate data  140  in the mass storage unit  106 , for example (see  FIG. 1 ). The increase rate data  140  can be three-dimensional image data in which the pixel value of each pixel represents an increase rate, for example. When step  340  in  FIG. 3  is performed, the increase rate data may be two-dimensional array data or a two-dimensional polar map in which the pixel value of each pixel represents an increase rate. 
         [0192]    In the present example, the weight of the myocardium in which a tracer is accumulated is used as a standard to normalize myocardial nuclear medicine image data. The normalized value thus reflects actual conditions of cardiac functions more correctly than in the related art. Such normalized values are used to calculate myocardial blood flow increase rates, and thus the comparability of myocardial blood flow increase rates is improved as compared with the related art. In other words, the present technique facilitates the comparison with past examination results or the comparison with examination results of another subject. In some embodiments, the myocardial volume can be used to perform normalization in place of the myocardial weight. Alternatively, another index relating to the heart size can be used to perform normalization. 
         [0193]    By performing the count correction in step  614 , the effect of a radiopharmaceutical agent administered under stress is eliminated from the rest data  134 . Accordingly, the pixel value of each pixel in the rest data  134  reflects cardiac functions at rest more correctly. Calculation using such data enables determination of SUVs and myocardial blood flow increase rates more correctly than in the related art. 
         [0194]    Step  620  indicates the end of the processing. 
         [0195]      FIG. 7  is a flowchart for explaining processing  700  that is an embodiment of step  345  in  FIG. 3  and is for the calculation of a myocardial blood flow increase rate. The processing  700  is performed when a data collection protocol is identified in step  315  as the protocol in which the measurement at rest is performed before the measurement under stress. The processing  700  is performed when the image data  130  and the image data  134  are subjected to characteristic SUV conversion disclosed in the present application and then a myocardial blood flow increase rate is calculated. 
         [0196]    Step  702  indicates the start of processing. In step  704 , the radiation dose administered to a subject in the measurement at rest (dose at rest) is calculated. The calculation method of the dose at rest is the same as in step  408  described for the processing  400  in  FIG. 4  and thus is not described. 
         [0197]    In step  706 , the pixel value of each pixel in the rest data  130  is converted into an SUV. The conversion equation is the same as equation 1 disclosed in the section of step  606 . However, the “tissue radioactivity concentration” in the equation is obtained by multiplying the pixel value in the rest data  134  by the BCF. In the equation, the “administered radiation dose” is the dose at rest determined in step  704 . In the equation, the “myocardial weight” is calculated on the basis of myocardial contour data of the rest data. 
         [0198]    The rest data after conversion of the pixel value of each pixel into an SUV may be stored as rest SUV data  152  in the mass storage unit  106 , for example (see  FIG. 1 ). 
         [0199]    In step  708 , the radiation dose administered to a subject in the measurement under stress (dose under stress) is calculated. The calculation method of the dose under stress is the same as in step  404  described for the processing  400  in  FIG. 4  and thus is not described. 
         [0200]    In step  710 , each pixel value of the prescan data  132  is multiplied by the following correction factor to perform collection time correction: 
         [0000]      Collection time correction factor=Stress data collection time/Prescan data collection time. 
         [0201]    In the present example, the stress data collection time and the prescan data collection time have been retrieved in step  320  and have been stored in the collection condition files  131  and  133 , respectively. The system  100  may thus be configured to retrieve the information about the stress data collection time and the prescan data collection time from these files and to calculate the collection time correction factor mentioned above. 
         [0202]    In step  712 , each pixel value of the prescan data  132  after collection time correction is multiplied by the decay correction factor determined as follows to perform, decay correction: 
         [0000]      Decay time (seconds)=Date and time at start of prescan data collection−Date and time at start of stress data collection,
 
         [0000]      Decay coefficient=LN (2.0)/Half-life (seconds) tracer (LN: natural logarithm to the base  e ), and 
         [0000]      Decay correction factor=Exp (Decay coefficient×Decay time).
 
         [0203]    In step  714 , the prescan data  132  after collection time correction and decay correction is used to perform count correction of the stress data  130 . The correction is performed by subtracting, from the pixel value of each pixel of the stress data  130 , the pixel value of the pixel at a corresponding position in the prescan data  132  after correction. By performing the count correction, the effect of a radiopharmaceutical agent administered in the measurement at rest is eliminated from the stress data  130 . 
         [0204]    In step  716 , the pixel value of each pixel in the stress data  130  after count correction is converted into an SUV. The conversion equation is the same as equation 1 above. However, the “tissue radioactivity concentration” in the equation is the pixel value in the stress data  130  and is obtained by multiplying the pixel value after the count correction described in step  714  by the above-described BCF. In the equation, the “administered radiation dose” is the dose under stress determined in step  708 . In the equation, the “myocardial weight” is calculated on the basis of myocardial contour data of the stress data as described in step  606 . 
         [0205]    The stress data after conversion of the pixel value of each pixel into an SUV may be stored as stress SUV data  152  in the mass storage unit  106 , for example (see  FIG. 1 ). 
         [0206]    In step  718 , the stress SUV data  150  and the rest SUV data  152  are used to calculate a myocardial blood flow increase rate. The processing in this step is the same as in step  618 , and thus is not described. 
         [0207]    In the present example, the weight or the volume of the myocardium in which a tracer is accumulated is used as a standard to normalize myocardial nuclear medicine image data. The normalized value thus reflects actual conditions of cardiac functions more correctly than in the related art. Such normalized values are used to calculate myocardial blood flow increase rates, and thus the comparability of myocardial blood flow increase rates is improved as compared with the related art. In other words, the present technique facilitates the comparison with past examination results or the comparison with examination results of another subject. 
         [0208]    By performing the count correction in step  714 , the effect of a radiopharmaceutical agent administered in the measurement at rest is eliminated from the stress data  130 . Accordingly, the pixel value of each pixel in the stress data  130  reflects cardiac functions at stress more correctly. Calculation using such data enables determination of SUVs and myocardial blood flow increase rates more correctly than in the related art. 
         [0209]    Step  720  indicates the end of the processing. 
         [0210]    Embodiments of step  345  in  FIG. 3  have been described. 
         [0211]    In each embodiment, the data obtained from measurement performed later between the measurement under stress and the measurement at rest is corrected by the data obtained through prescan. This corrects the effect of radioactivity that has been administered in the earlier measurement, remaining in the data obtained through the measurement performed later. This correction improves the validity of data and increases the reliability of the myocardial blood flow increase rate to be calculated. This technique can markedly shorten the time interval required between the measurement under stress and the measurement at rest as compared with the related art, and can reduce the measurement burden on both an operator and a subject. 
         [0212]    In the embodiment exemplified in the processing  600  or the processing  700 , the pixel values of nuclear medicine image data are converted into SUVs using the myocardial weight, which is the normalization on a more appropriate supposition for the accumulation of a tracer than in the related art, resulting in improved validity or reliability when pieces of data are compared between different measurement dates and times or between different subjects. 
         [0213]    The flowchart in  FIG. 3  will be described again. In step  350 , the calculation result of the myocardial blood flow increase rate is displayed. The display may be made in various manners. For example, when the increase rate data  140  storing the results is three-dimensional image data in which the pixel value of each pixel represents an increase rate, calculated myocardial blood flow increase rates may be displayed as differences in brightness or color tone where short axis tomograms are displayed side by side. Such a displaying manner enables detailed observation of how the myocardial blood flow increase rate changes at respective sliced positions. 
         [0214]    For example, when the increase rate data  140  storing the results is two-dimensional array data or a two-dimensional polar map in which the pixel value of each pixel represents an increase rate, the result can be displayed as the two-dimensional array or the two-dimensional polar map without any processing. The variation of the myocardial blood flow increase rate with positions can be easily observed in a single chart. In some embodiments, the increase rate data  140  may be displayed together with the stress data  130  or the rest data  134  converted into two-dimensional array data or a two-dimensional polar map. Alternatively, the increase rate data  140  may be displayed together with the stress SUV data  150  or the rest SUV data  152  converted into two-dimensional array data or a two-dimensional polar map. In such a case, the arrangement of maps in the order of a map at rest, a map under stress, and an increase rate map facilitates the observation of increase rates under stress, which is advantageous. 
         [0215]    The invention of the present application has been specifically described with reference to preferred examples. The description and the attached drawings are not intended to limit the scope of the invention of the present application, but are intended to satisfy the requirements of the law. Embodiments of the invention of the present application include various variations in addition to the above-exemplified embodiments. For example, various numerical values shown in the description or the drawings are illustrative values and are not intended to limit the scope of the invention. Individual features included in the various examples that have been described in the description or the drawings are not limited to usage with examples in which these features are explicitly explained to be included, but may be used in combination with other examples that have been described herein or various specific examples that have not been described. In particular, the processes presented in the flowcharts do not necessarily need to be performed in the described order. According to the preference of an executor, the processes may be performed in a changed order or in parallel, or as a plurality of blocks integrally implemented, or in a loop as appropriate. These variations are all included in the scope of the invention disclosed in the present application. The form of implementing processes does not limit the scope of the invention. The order of the description of the processes defined in the claims does not necessarily specify the mandatory order of the processes. For example, an embodiment specifying a different order of the processes and an embodiment that executes the processes in a loop are also included in the scope of the invention according to the claims. 
         [0216]    For example, an embodiment of the myocardial blood flow increase rate calculation program  120  can include a single program, a program group including a plurality of independent programs, and a program integrated with all or part of the alignment program  122  or the contour extraction/volume calculation program  124 . A program can be installed in various manners, which are well known, and all the various manners are included in the scope of the invention disclosed in the present application. 
         [0217]    It should be noted that novel SUVs disclosed in the present application are not solely for deriving myocardial blood flow increase rates as disclosed in the present application. The novel SUV disclosed in the present application is characterized using the weight or volume of a myocardium for normalization, and thus the SUV of the present application can be used in all the fields in which the normalization is appropriate, such as various nuclear medicine examinations of the heart. It should be noted that the applicant claims to possess the right to have a patent granted on all the embodiments not deviating from the spirit of the invention regardless of whether a patent is claimed in the current set of attached claims.