Patent Publication Number: US-7915578-B2

Title: Method and apparatus for correcting scattering in SPECT imaging

Description:
RELATED APPLICATION 
     This nonprovisional patent application claims priority benefit, with regard to all common subject matter, of earlier-filed U.S. provisional patent application titled “METHOD AND APPARATUS FOR CORRECTING SCATTERING IN SPECT IMAGING”, Ser. No. 60/917,278, filed May 10, 2007. The identified earlier-filed application is hereby incorporated by reference in its entirety into the present application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate to methods and apparatuses for correcting scattering in SPECT imaging. More particularly, various embodiments of the present invention correct scattering in SPECT I-123 images by utilizing an iterative algorithm and a Compton window. 
     2. Description of the Related Art 
     Single photon emission computed tomography (SPECT) imaging is a popular nuclear medicine imaging technique operable to generate three-dimensional information through the detection of gamma rays. SPECT imaging is often utilized to perform Myocardial perfusion imaging (MPI) to facilitate in the detection of cardiovascular defects. Various radioisotopes are typically employed with SPECT imaging, including Technetium-99m (Tc-99m) and Thalium-201 (Tl-201). 
     Iodine-123 (I-123) is being widely investigated for various cardiac molecular imaging applications as an alternative to Tc-99m and Tl-201. However, the low-energy high resolution (LEHR) collimators typically employed by cardiac laboratories are unable to stop high-energy (e.g., 529 keV) I-123 emissions, thereby resulting in photopeak scattering and the degradation of spatial uniformity. As such, the usage of I-123 in SPECT imaging is often significantly limited by photopeak scattering. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention solve the above-described problems and provide a distinct advance in the art of SPECT scattering correction. More particularly, various embodiments of the present invention correct scattering in SPECT I-123 images by utilizing an iterative algorithm and a Compton window and a number of finely sampled Compton scatter windows near the photopeak energy. 
     In various embodiments, a method of correcting scattering in SPECT I-123 imaging is provided. The method generally includes: accessing highly sampled photon energy data corresponding to a first SPECT I-123 image generated using a gamma camera; generating a raw energy spectrum for at least some of the pixels utilizing the highly sampled photon energy data; acquiring a gamma camera model corresponding to the gamma camera; utilizing the gamma camera model and an iterative algorithm to apply a first scattering correction to the raw energy spectrum; utilizing a Compton window to apply a second scattering correction to the raw energy spectrum; and generating a correction table with the corrected raw energy spectrum. In some embodiments, the correction table may be utilized to correct a second SPECT I-123 image. 
     Other aspects and advantages of the present invention will be apparent from the following detailed description of the embodiments and the accompanying drawing figures. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Embodiments of the present invention is described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a block diagram of a system constructed in accordance with various embodiments of the present invention; 
         FIG. 2  is a perspective view of a computing device coupled with a SPECT scanner; 
         FIG. 3  is a schematic diagram of an exemplary gamma camera operable to be employed by the SPECT scanner of  FIG. 1 ; 
         FIG. 4  is a graph of an exemplary raw I-123 energy spectrum; 
         FIG. 5  is a graph of the exemplary raw I-123 energy spectrum of  FIG. 3  showing portions of the spectrum in more detail; 
         FIG. 6A  is a graph showing various exemplary measurements of relatively pure energy sources; 
         FIG. 6B  is a graph showing an exemplary gamma camera model generated from the measurements of  FIG. 6A ; 
         FIG. 7  is a graph showing an exemplary corrected I-123 energy spectrum generated by various embodiments of the present invention; 
         FIG. 8  is an exemplary I-123 image that is uncorrected; 
         FIG. 9  is an exemplary I-123 image that has been corrected; and 
         FIG. 10  is a block diagram illustrating various functions operable to be performed by various embodiments of the present invention. 
     
    
    
     The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following detailed description of various embodiments of the invention references the accompanying drawings which illustrate specific embodiments in which the invention can be practiced. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     Methods consistent with the present teachings are especially well-suited for implementation by a computing element, such as the computer  10  illustrated in  FIGS. 1 and 2 . The computer  10  can include various analog and digital components operable to perform the various functions discussed herein. In some embodiments, the computer  10  can include a microprocessor, a microcontroller, a programmable logic device, an application specific integrated circuit, digital and analog logic devices, computing elements such as personal computers, servers, computing networks, portable computing devices, combinations thereof, and the like. Thus, the computer  10  can comprise a plurality of computing elements in some embodiments. The computer  10  may include or be associated with a memory  12 . The memory  12  may include memories of any form or configuration for storing computer programs and information, as is discussed in more detail below. Thus, the memory  12  can comprise a computer-readable medium. 
     Some embodiments of the present invention may provide a computer program. The computer program may comprise instructions for implementing functions in one or more computing devices such that the program is not limited to functioning and/or controlling only the computer  10 . The program may also comprise various code segments, which each may include one or more instructions, one or more instruction lists, only a portion of an instruction list, and/or only a portion of an instruction. Code segments may include overlapping lists of instructions—that is, a first code segment may include at least portions of instructions A and B, and a second code segment may include at least portions of instructions B and C. Each code segment may be embodied as human-readable source code or script, as machine-readable object code, and/or as one or more machine-executable files such as compiled source code. Further, the computer program may comprise one or more computer programs each including any number of code segments to perform any of the functions disclosed herein. 
     The computer program may be stored in or on at least one computer-readable medium accessible by one or more computing devices, such as the computer  10 , to enable one or more computing devices to implement the various functions of the computer program. In the context of this application, a “computer-readable medium” can be any element or combination of elements that can contain, store, communicate, propagate or transport at least a portion of the program for use by or in connection with one or more computing devices such as the computer  10 . 
     The computer-readable medium can be, for example, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semi-conductor system, apparatus, device, or propagation medium operable for use by the computer  10  or other devices. More specific, although not inclusive, examples of the computer-readable medium may include the following: an electrical connection having one or more wires, a portable computer diskette, a random access memory (RAM), a read-only memory (ROM), an erasable, programmable, read-only memory (EPROM or flash memory), an optical fiber, a portable compact disc (CD), a digital video disc (DVD, HD-DVD, Blu-Ray), and an electrical signal representing one or more portions of the computer program. The computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. In some embodiments, the computer program is stored on the memory  12  associated with the computer  10 . 
     However, embodiments of the present invention may be implemented in hardware, software, firmware, and/or combinations thereof and are not limited to the computer program discussed herein. The computer program and associated equipment are merely examples of a program and equipment that may be used to implement embodiments of the present invention and may be replaced with other software and/or equipment without departing from the scope of the present teachings. 
     As shown in  FIG. 1 , the computer  10  may be directly or indirectly coupled with a scanner  14  to facilitate I-123 scattering correction as is discussed in more detail below. For example, the computer  10  may be coupled with the scanner  14  utilizing a wired connection and/or by indirectly coupling with the scanner  14  through a communications network  13  such as the internet. Data utilized by the computer  10  may also be stored and retrieved from a database  15 , which may be accessed directly or through the communications network  13 . 
     The scanner  14  is a single photon emission computed tomography (SPECT) scanner including a gamma camera  16 , shown in  FIG. 2 , operable to detect gamma rays. The gamma camera  16  may be rotated at least partially around a source object, such as a patient, to detect gamma rays emitted by I-123 from a plurality of angles. In some embodiments, the gamma camera  16  may be operable to rotate 180 and/or 360 degrees around the patient. 
     An exemplary gamma camera  16  operable to be utilized by the scanner  14  is illustrated in  FIG. 3 . The exemplary gamma camera  16  includes a low-energy high resolution (LEHR) collimator  18 , a layer of sodium iodide crystal with thallium doping  20 , and a plurality of photodetectors  22 . The collimator  18  is positioned between the crystal  20  and the patient and is operable to at least partially limit the photons that may pass to the crystal  20  in a generally conventional manner. The crystal  20  and photodetectors  22  form a matrix of gamma detectors that are each operable to detect an emitted gamma ray. In some embodiments, the gamma camera  16  may present a 64×64 or 128×128 pixel configuration. However, embodiments of the present invention may utilize matrices and pixel configurations of any size. 
     Emitted gamma photons of the particular source orientation defined by the collimator  18  pass through the apertures of the collimator  18  and strike the crystal  20 . The crystal  20  scintillates in response to the gamma photons and produces a flash of light in general proximity to the location of the photon-crystal collision. The photodetectors  22  are positioned behind the crystal  20  and are operable to detect the intensity of the light and its corresponding pixel location. 
     Thus, for each position of the gamma camera  16 , the scanner  14  is operable to generate data corresponding to the pixel location and intensity of each detected gamma ray. Further, as should be appreciated by those skilled in the art, the gamma camera  16  is operable to detect a plurality of gamma rays (counts) at each position. The scanner  14  is operable to generate highly sampled photon energy data, such as list mode data, corresponding to the detected gamma rays at one or more positions of the gamma camera  16 . For example, for each of the 64×64 pixels, the generated list mode data may represent the number of gamma rays detected and their corresponding energies. Thus, in some embodiments, the scanner  14  is operable to generate list mode data for each pixel and camera position in form of (x, y, E) where (x, y) represents a pixel coordinate and E represents gamma photon energy (in keV for example). 
     It should be appreciated that the computer  10  and scanner  14  need not be directly coupled as the scanner  14  may be operable to store list mode data on an intermediate device, such as a second computer or internal medium, from which the computer  10  may access the data. Further, it should be appreciated that the computer  10  and the scanner  14  may be integral with each other such that the computer  10  may be operable to provide conventional transmission, diagnosis, computing, and processing functions in addition to the methods and functions disclosed herein. 
     A flowchart of steps that may be utilized by embodiments of the present invention to correct SPECT I-123 scattering is illustrated in  FIG. 10 . Some of the blocks of the flow chart may represent a module segment or portion of code of the computer program of embodiments of the present invention which comprises one or more executable instructions for implementing the specified logical function or functions. 
     In some alternative implementations, the functions noted in the various blocks may occur out of the order depicted in  FIG. 10 . For example, two blocks shown in succession in  FIG. 10  may in fact be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order depending upon the functionality involved. Further, any combination of the steps illustrated in  FIG. 10  may be performed by embodiments of the present invention and one or more of the steps may be omitted by various embodiments of the present invention. 
     The steps illustrated in  FIG. 10  generally include: accessing highly sampled photon energy data, referenced as step  100 ; generating a raw energy spectrum  24 , referenced as step  102 ; acquiring a gamma camera model  26 , referenced as step  104 ; applying a first scattering correction, referenced as step  106 ; applying a second scattering correction, referenced as step  108 ; generating a correction table, referenced as step  110 ; and correcting an image, referenced at step  112 . 
     In step  100 , the computer  10  accesses highly sampled photon energy data for a plurality of pixels corresponding to a first SPECT Iodine-123 (I-123) image generated using the gamma camera  16 . The steps of the method disclosed herein may apply to highly sampled photon energy data other than list mode data that may be generated by other cameras or other pieces of equipment. Multiple window energy sampling may be used to generate highly sampled photon energy date. However, to illustrate the steps of the method, list mode data is used as an example. “List mode data,” as utilized herein, refers to data that indicates the pixel location and energy of a detected gamma ray. As discussed above, the scanner  14  may generate the list mode data in a generally conventional manner by detecting gamma rays emitted by I-123 disposed within a patient or any other source object. However, the list mode data accessed in step  100  may be generated in any fashion to indicate the pixel location and energy of a detected gamma ray. 
     In some embodiments, the computer  10  may access the list mode data by accessing the memory  12 . Additionally or alternatively, the computer  10  may access the list mode data by directly or indirectly coupling with the scanner  14  and acquiring data therefrom. The computer  10  may also acquire the list mode data from other sources, such as from another computing device  10 , database  15 , or data repository. For example, the computer  10  may access a database  15  through the internet or another communications network  13  to access the list mode data in step  100 . The list mode data accessed by the computer  10  in step  100  may be stored within the memory  12  for later access and processing by the computer  10 . 
     In step  102 , the computer  10  generates a raw energy spectrum  24  utilizing the list mode data accessed in step  100 . In various embodiments, the computer  10  generates a raw energy spectrum  24  for each pixel corresponding to the SPECT I-123 list mode data accessed in step  100 . Thus, for example, where the gamma camera  16  presents a 64×64 pixel matrix, the computer  10  generates a raw energy spectrum  24  for each of the 64×64 pixels based on the accessed list mode data. To generate the raw energy spectrum  24  for each pixel, the computer  10  may aggregate the number of gamma rays (counts) detected at each energy value. Exemplary raw energy spectrums  24  for one pixel are illustrated in  FIGS. 4 and 5 , where the 159 keV peak associated with I-123 is clearly visible. 
     The exemplary raw energy spectrums  24  illustrated in  FIGS. 4 and 5  include the effects of photopeak scattering and down scattering. Photopeak scattering is caused by high-energy (approximately 556 keV) I-123 gamma rays penetrating the collimator  18 , regardless of the configuration of the collimator&#39;s apertures, and striking the crystal  20  to create a flash of light. These stray high-energy I-123 gamma rays originating from angles different than those defined by the collimator  18  produce energy spectrums that erroneously indicate counts and associated energy levels. In a situation while scattered photons contribute a substantial portion of overall counts, image uniformity and contrast can be highly degraded and diagnostic accuracy can be lost. Down scattering is caused by the deflection of 159 keV I-123 gamma rays off of electrons in the patient and reduces the energy levels detected by the gamma camera  16 , thereby also producing energy spectrums that erroneously indicate counts and associated energy levels. 
     The raw energy spectrums  24  may be represented by the computer  10  in a listing or table form, such as within a listing of the number of counts associated with each energy value. Thus, the raw energy spectrums  24  are not necessarily represented in graphical format by the computer  10  as the spectrum illustrated in  FIGS. 4 and 5  is merely exemplary. The raw energy spectrums  24  for each of the pixels may be stored by the computer  10  in the memory  12  for later access and processing. Additionally or alternatively, the computer  10  may provide the raw energy spectrums  24  to other devices and systems through a communications network  13  or direct connections. 
     Although it is generally desirable to maximize the number of pixels utilized to form the list mode data and raw energy spectrums  24 , in some embodiments, the computer  10  may generate raw energy spectrums  24  for only some of the pixels presented by the gamma camera  16  to conserve computing and memory resources. 
     Further, in some embodiments, the computer  10  may generate the raw energy spectrums  24  by accessing raw energy spectrums  24  previously generated by the computer  10  or other devices. For example, the computer  10  may access the memory  12  and/or other databases  15  and computing devices to retrieve the various energy spectrums. Thus, a first computing device could perform step  100  to access the list mode data and a second computing device could generate the raw energy spectrums  24  by accessing the data retained by the first computing device. 
     In step  104 , the computer  10  acquires a gamma camera model  26  corresponding to the gamma camera  14  utilized to generate the list mode data accessed in step  100 . The gamma camera model  26  represents the response of the gamma camera  14  to the measurement of relatively pure energy sources. As is discussed in more detail below, the gamma camera model  26  is utilized to perform the first scattering correction. 
     An exemplary gamma camera model  26  is presented by  FIGS. 6A and 6B , which illustrate the response of the gamma camera  14  to a plurality of gamma rays having energies in the range of 30 keV to 230 keV. In some embodiments, the gamma rays may be measured utilizing the energy impulse response (EIR) conventionally provided by the gamma camera  14  to directly measure the energy response of the gamma camera  14  to various energy sources. To acquire the gamma camera model  26 , the individual EIR in various energy channels may be sampled and the counts of each channel may be normalized to the total number of counts. 
     Direct measurement of EIRs is generally limited to few energies because only a finite number of energy sources are available in the desired energy range (e.g. from 50 to 200 keV). To overcome this problem, the measured EIRs  28  may be extrapolated to generate other EIRs and form a complete model  26 . For example, seven sources with nine different energies may be utilized as initial EIR measurements: Am-241(60 keV), Ba-133(81 keV), Ga-67(94 keV, 184 keV), Co-57(122 keV), Tc-99m (140 keV) I-123 (159 keV) and In-111(171 keV, 245 keV). The response by the camera  16  to each energy level is measured to generate the EIR  28  and plotted as shown in  FIG. 6A . To complete the model  26 , additional data is extrapolated from the measured data and plotted as shown in  FIG. 6B . The measured EIRs may be extrapolated utilizing various mathematical methods, including the normalization discussed above, to enable the gamma camera model  26  to represent an entire spectrum of energies. 
     The gamma camera model  26  generated in step  104  will be generally similar for all cameras of the same model. For example, if first and second gamma cameras are both SIEMENS ECAM SPECT systems, the gamma camera model  26  for the first and second gamma cameras will be generally similar. Thus, in some embodiments, step  104  may only be performed once for any particular camera model and the resulting gamma camera model  26  may be stored within the memory  12  for later use. 
     In particular, in some embodiments a database  15  of gamma camera models  26  may be formed corresponding to commonly used gamma cameras such that the gamma camera model  26  may be acquired by the computer  10  in step  104  by accessing the database  15 . As such, it is not necessary in all embodiments to generate the gamma camera model  26  by directly measuring EIRs. However, it may be desirable in some embodiments to generate a gamma camera model  26  for each utilized gamma camera in the event a particular gamma camera includes defects or provides non-conforming functionality. 
     In step  106 , a first scattering correction is applied to at least some of the raw energy spectrums  24  generated in step  102 . The first scattering correction is applied by the computer  10  by utilizing the gamma camera model  26  generated in step  104  and an iterative algorithm. The iterative algorithm utilized in this step may be any one of a family of mathematical algorithms that operate on energy spectrum data to correct for scattering. The iterative algorithm, such as the maximum likelihood optimizer described below, provides an iterative energy spectrum deconvolution that is operable to improve the energy resolution of the gamma camera  14  to correct for scattering. 
     The raw energy spectrums  24  may be regarded as a linear combination of energy components that are from photopeak and scatter photons. Thus, if this spectrum is sampled by energy channels, the counts in each sample point can be assumed as a series of energy components convolved with probability functions which correlate energy components to energy channels. Such an assumption is given as: 
                       S   i     =       ∑   j     ⁢           ⁢       P   ij     ⁢     e   j           ,           EQ   .           ⁢   1               
where S i  represents counts of an energy spectrum sampled in ith energy channel, e j  is jth energy component and {P ij } is the probability of jth energy component correlating to ith energy channel.
 
     As a scintillation system, the detection provided by the gamma camera  14  and in some embodiments, the emission of gamma photons can both be modeled as Poisson process, thus a log likelihood function of system can be created from the energy spectrum. This function is given as: 
     
       
         
           
             
               
                 
                   
                     
                       
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     By utilizing the gamma camera model  26  generated in step  104 , the iterative algorithm may utilize the knowledge of the energy components of an energy spectrum to perform scatter correction by excluding scatter components out of a narrow energy window (several keVs), which is equivalently to include energy components near I-123 photopeaks only. By applying E-step (expectation) and M-step (maximization), the iterative algorithm can be derived as: 
                       e   j   new     =         e   j   old         ∑   i     ⁢           ⁢     P   ij         ⁢       ∑   i     ⁢           ⁢       P   ij     ⁢       S   i         ∑   k     ⁢           ⁢       P   ik     ⁢     e   k   old                   ,           EQ   .           ⁢   4               
where S i  represents counts of an energy spectrum sampled in an ith energy channel, e j   old  is an initial jth energy component, P ij  is the probability of jth energy component correlating to ith energy channel, P ik  is the probability of kth energy component correlating to ith energy channel, and e j   new  is a corrected jth energy component. Thus, the iterative algorithm represents counts in an energy channel as a linear combination of energy components and the iterative algorithm solves the inverse problem to estimate energy components sampled in various energy channels. As the algorithm iteratively moves forward, each energy component eventually converges to a fixed value. Additional information concerning application of the iterative algorithm is provided by the following materials.
 
     In step  108 , a Compton window is utilized to apply a second scattering correction to the raw energy spectrum. In some embodiments, the Compton window of step  108  may be applied after the first correction of step  106 . However, in other embodiments, step  108  may be performed before or concurrently with step  106 . 
     Generally, the Compton window includes window A  30  and window B  32 , as seen in  FIG. 7 . Window A  30  is centered around the I-123 159 keV peak +/−3% keV. Window B  32  is a fixed 4 keV window. The second correction is applied by taking window A  30  minus the product of C and Window B  32 , where C is a scale factor. 
     In step  110 , a correction table is generated with the corrected raw energy spectrum resulting from steps  106  and  108 . The correction table embodies the corrections made in steps  106  to  108  to the raw energy spectrum  24  based on the gamma camera model  26  and allows the computer  10  to correct additional SPECT I-123 images as is discussed below in step  112 . 
     In step  112 , the correction table generated in step  110  is utilized to correct one or more SPECT I-123 images, as shown in  FIGS. 8 and 9 .  FIG. 8  shows an uncorrected SPECT I-123 image  34 , while a corrected image  36  is seen in  FIG. 9 . The correction table may be utilized to correct the SPECT I-123 image that was utilized to generate the raw energy spectrum  24  and/or to other SPECT I-123 images to correct for photopeak scattering and down scattering. For example, the raw energy spectrum  24  may be generated from a first SPECT I-123 image and the correction table may be utilized to correct any number of additional SPECT I-123 images. Thus, in some embodiments, steps  100 - 110  may be performed once to generate the correction table and step  112  may be repeated for every additional SPECT I-123 image generated by the scanner  14 . 
     It is believed that embodiments of the present invention and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes.