Abstract:
A phantom for co-registering a magnetic resonance image and a nuclear medical image is disclosed. The phantom includes a longitudinal member having a first end cap and a second end cap and a chamber contained within the longitudinal member. The chamber contains a fluid for producing a first image using a first imaging modality. The phantom further includes a first rod disposed within the chamber of the longitudinal member. The first rod contains a radioactive substance for producing a second image using a second imaging modality.

Description:
PRIORITY CLAIM TO RELATED APPLICATIONS 
     This application is a Divisional application of U.S. patent application Ser. No. 13/753,894, filed on Jan. 30, 2013, which is a Continuation-In-Part of U.S. patent application Ser. No. 13/552,962, filed on Jul. 19, 2012, which claims priority from U.S. Provisional Patent Application Ser. No. 61/509,182, filed Jul. 19, 2011, the disclosures of which are incorporated by reference in their entirety herein. 
    
    
     TECHNICAL FIELD 
     In general, the invention relates to nuclear medicine. More particularly, the invention relates to phantoms used for quality control measurement and calibration of nuclear medical imaging devices. 
     BACKGROUND 
     Magnetic resonance imaging (MRI) is primarily used in medical imaging to visualize anatomical structure of a patient&#39;s body. MRI technology can provide detailed images of the body in any plane. MRI has the ability to show soft tissue contrasts, which makes MRI scans especially useful in neurological, musculoskeletal, cardiovascular, and oncological imaging. MRI scans use a powerful magnetic field to align the magnetization of hydrogen atoms in the body. Radio waves are used to systematically alter the alignment of such magnetization, thereby causing the hydrogen atoms to produce a rotating magnetic field detectable by the MRI system. The resulting signal can be manipulated by additional magnetic fields to build up enough information to reconstruct an image of the body. 
     Positron emission tomography (PET) is a nuclear medicine imaging technique that produces a three-dimensional image or map of functional processes in a patient&#39;s body. A PET system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope, which is introduced into the body on a non-metabolically or metabolically active molecule. As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, the antimatter counterpart of an electron. After travelling up to a few millimeters, the positron encounters and annihilates with an electron, producing a pair of annihilation (gamma) photons moving in opposite directions, which are then detected when they reach a scintillator material in a scanning device of the PET system. Images of metabolic activity and other physiological and biochemical events in space are then reconstructed by computer analysis. 
     The most significant fraction of electron-positron decays result in two 511 keV gamma photons being emitted at almost 180 degrees to each other. Thus, it is possible to localize the source of the positron annihilation event along a straight line of coincidence (also referred to as a line of response (LOR)), and then an image reconstruction can be performed using coincidence statistics. For example, using statistics collected from a plurality of coincidence events, a set of simultaneous equations for a total activity of each parcel or bit of tissue (also called a voxel) along many LORs can be solved by a number of techniques, and thus a map of radio-activities as a function of location for parcels or bits of tissue can be constructed and plotted. The resulting map shows the tissues in which the molecular probe has become concentrated, and the resulting map can be interpreted by a physician and used for patient diagnosis and treatment. The parcels or bits of tissue can be quite small since the gamma ray event energy is large. This is one of the advantages of PET—a high signal to background ratio. 
     PET scans are commonly read alongside CT scans, in an attempt to produce a combination image by “co-registration” that gives the physician both anatomic and metabolic information about the patient&#39;s body. It is widely accepted that co-registration of anatomical information improves the diagnostic value of functional imaging, as can be seen in the success of hybrid scanners using PET and CT imaging. Recently, hybrid scanners using PET and MRI imaging have become available. The combination of PET and MRI may offer advantages, such as higher soft tissue contrast in the MRI anatomical images, real simultaneous acquisition, and minimum radiation exposure to the patient. 
     In general, a phantom is used to calibrate and/or verify the accuracy of nuclear medical imaging devices such as PET scanners or MR scanners. In essence, a phantom contains positron emitting activity in a known shape and uniform distribution of radiation activity throughout the body. Thus, by imaging the phantom with its known geometry and radiation distribution, the accuracy of the software used to assemble the various tomographic slices acquired by the imaging apparatus into three-dimensional representations of a patient&#39;s region of interest can be assessed and, if necessary, the various apparatus settings can be adjusted. 
     More specifically, the purpose of the alignment phantom is to measure the orientation and offset of the PET field of view with respect to the MR field of view. 
     Recently, a fully integrated magnetic resonance/PET scanner has been developed, which integrated scanner allows for simultaneous MR and PET imaging (see, U.S. Pat. No. 8,073,525, to Ralf Ladebeck et al. and incorporated herein by reference). Such a scanner requires a dedicated phantom which, in addition to facilitating quality control assessment of the PET scanning functionality of the machine, facilitates a determination of the extent to which operating the MR functionality of the machine influences the PET signals acquired by the machine. 
     A conventional phantom will not work for this purpose because the MR coil of the scanner has to experience a certain load in order to dampen the resonance peak sufficiently, even when maximum RF power is applied to the coil. Otherwise, if the MR coil is not loaded when maximum RF power is applied, too much current will be developed in the MR resonance circuit, and that current could potentially be large enough to destroy the MR coil itself. 
     Thus a need exists for a hybrid phantom that can accommodate a PET scanner, a MR scanner and/or a MR/PET scanner and overcome the problems in the prior art. 
     SUMMARY 
     Embodiments of the invention provide a phantom for co-registering a magnetic resonance image and a nuclear medical image is disclosed. The phantom includes a longitudinal member having a first end cap and a second end cap and a chamber contained within the longitudinal member. The chamber contains a fluid for producing a first image using a first imaging modality. The phantom further includes a first rod disposed within the chamber of the longitudinal member. The first rod contains a radioactive substance for producing a second image using a second imaging modality. 
     In a first aspect, an embodiment of the invention provides a method of aligning a Positron Emission Tomography (PET) Field of View (FoV) with a Magnetic Resonance Imaging (MR) FoV, the method includes providing a phantom, imaging the phantom with MR to provide an MR image, imaging the phantom with PET to provide a PET image, and applying voids detected in the MR image to align the PET FoV with the MR FoV. 
     In another aspect, the invention features a method of quality control testing an integrated multimodality medical imaging scanner, wherein magnetic resonance constitutes one of the modalities. The method includes disposing a phantom within the scanner, the phantom comprising a plastic housing having a plurality of rods containing a radioactive substance along an axis of the plurality of rods to provide an image using a second imaging modality. The method further includes operating magnetic resonance imaging features of the scanner, and simultaneously imaging the phantom using the second modality imaging feature of the scanner because the phantom allows both modalities to be imaged. The modalities can be imaged sequentially or simultaneously. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will now be described in greater detail with reference to the attached drawings, in which: 
         FIG. 1  is a combined positron emission tomography (PET) and magnetic resonance imaging (MRI) scanning system in which the PET scanner and the MRI scanner are axially offset and aligned; 
         FIG. 2  is a combined PET and MRI scanning system in which the PET scanner and the MRI scanner are provided in a single unit for simultaneous PET and MRI scanning, according to an exemplary embodiment; 
         FIG. 3A  is a schematic perspective view of a PET-MRI phantom for use in a process for providing image alignment for combined PET and MRI scanning, according to an exemplary embodiment; 
         FIG. 3B  is an exploded view of a rod in an opening of an end cap in accordance with an exemplary embodiment of the invention; 
         FIG. 4  is a view of a PET-MRI phantom depicting a plurality of rods for use in a process for providing image alignment for combined PET and MRI scanning, according to an exemplary embodiment; 
         FIG. 5  is a flow chart setting forth a process for providing data acquisition using the phantom for alignment for combined PET and MRI scanning, according to an exemplary embodiment; 
         FIG. 6  is a flow chart setting forth a process for providing data processing using the phantom for alignment for combined PET and MRI scanning, according to an exemplary embodiment; and 
         FIG. 7  is a block diagram of an exemplary computer system for implementing an alignment algorithm for a MR/PET imaging system using a single phantom in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A method and apparatus for providing image alignment for combined positron emission tomography (PET) and magnetic resonance imaging (MRI) are described. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the embodiments of the invention. It is apparent, however, to one skilled in the art that the embodiments of the invention may be practiced without these specific details or with an equivalent arrangement. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the embodiments of the invention. 
     Co-registration of anatomical information can greatly improve the diagnostic value of functional imaging. For example, the combination of PET and MRI images can offer numerous advantages, such as higher soft tissue contrast in the MRI anatomical images, real simultaneous acquisition, and minimum radiation exposure to the patient. Correlative imaging can open exciting new applications in oncology, neurology, and cardiology. Combining functional information from nuclear medical imaging with anatomical information from CT or MRI images has become of great interest since PET tracers are becoming more and more organ and tissue specific. MRI images have the additional advantage of providing physiological information. Image co-registration and fusion techniques are being developed and optimized for the interpretation of PET, CT, and MRI data. While medical imaging apparatuses that combine PET and CT systems in one hardware device have been used in oncology, the combination of PET with MRI exhibits several technical challenges. 
       FIG. 1  depicts one possible PET-MRI scanner system. In the PET-MRI scanner system shown in  FIG. 1 , a patient  10  is positioned on a pallet  112  that is movably supported by a table  114 . The pallet  112  is axially movable on the table  114  in a left-right direction as shown in  FIG. 1 . The PET-MRI scanner system in  FIG. 1  includes an MRI scanner  116  and a PET scanner  118  that are axially aligned along the moving direction of the pallet. Thus, the patient  10  can be moved through a patient scanning space in the MRI scanner  116  and through a patient scanning space in the PET scanner  118  to perform sequential imaging in each scanner. However, such sequential imaging can prove to be troublesome, since generating a combination image by co-registration of the PET image and the MRI image can be very difficult and inaccurate. For example, when PET and MRI datasets are acquired separately, as in such a sequential imaging arrangement, alignment inaccuracies can frequently occur between the PET image and the MRI image due, for example, to movement or repositioning of the patient during scanning or while transporting the patient from one modality to the next. 
     In order to reduce such alignment inaccuracies, another possible PET-MRI scanner system can be provided that performs simultaneous PET and MRI scans.  FIG. 2  depicts an embodiment of such a combined PET-MRI scanner system with simultaneous measurements. In the embodiment of  FIG. 2 , the patient  10  is positioned on a pallet  212  that is movably supported by a table  214 . The pallet  212  is axially movable on the table  214  in a left-right direction as shown in  FIG. 2 . The PET-MRI scanner system in  FIG. 2  includes a combined PET and MRI scanner  220  that houses both an MRI scanner and a PET scanner that can simultaneously scan the patient  10  as the patient  10  is axially moved through a patient scanning space in the combined PET and MRI scanner  220 . As the PET imaging and the MRI imaging are performed simultaneously along the patient&#39;s body, the alignment inaccuracies noted above can be reduced or eliminated. 
     In principle, a combined PET and MRI scanner will provide simultaneous functional and anatomical information with near perfect spatial registration. By fully integrating a PET scanner inside a magnet of an MRI scanner, true simultaneous imaging can be realized. However, both PET and MRI images are based on different coordinate systems (image domains). The PET image domain (x, y, z) coordinate system is fixed in “absolute” space and it is relative to the PET gantry within the scanner. Thus, the positioning of the PET coordinate system is known through engineering design. On the other hand, the MRI image domain (x m , y m , z m ) coordinate system is not absolute in space and will change based on several MRI specific tuning and shimming procedures, thereby making the alignment of both modalities difficult. Accordingly, a special alignment procedure is needed in order to achieve a valid image fusion (co-registration) of both a PET imaging modality and an MRI modality. 
     That is, in order for a clinical medical imaging system to acquire simultaneous MR/PET acquisitions in which the information in the two images can be correlated, the MR/PET system preferably measures the offset, orientation, and scale of the PET image coordinate system with respect to that of the MR system so that the images can be scaled, rotated and translated into alignment. The offset, rotation, and scale of the PET and MR coordinate systems can be measured with an alignment phantom that can be imaged by both the MR and PET subsystems as with the current invention. 
     The PET and MR image acquisitions need not be simultaneous if the position of the phantom remains unchanged between the PET and MR acquisitions and if a sufficient number of features in the two views of the phantom can be correlated. 
       FIGS. 3 and 4  depict embodiments of PET-MRI phantoms that can be used with an alignment procedure to align PET and MRI modalities in order to provide image registration, according to embodiments of the present invention. Such PET-MRI phantoms can be used with a fully automated alignment algorithm to provide an advantageous alignment procedure. Embodiments of the PET-MRI phantom meet requirements regarding visibility for both the PET scanning system and the MRI scanning system, in order to provide a dual-system phantom. 
     The design of the alignment phantom  300  enables a coordinate system to be determined in the MR system with respect to a location known in the MR Field of View (FoV), and determines the position of that same coordinate system in the PET FoV. The offset rotations, and scaling are computed from this information in order to fuse the PET and MR FoVs in the MR image display system. This fusion facilitates the correlation of the information in each type of image. 
     The phantom  300  comprises a first longitudinal member  302  that is cylindrical in shape and defines a chamber  308 . Longitudinal member  302  comprises a first end cap  304  and a second end cap  306 . The combination of longitudinal member  302 , first end cap  304  and second end cap  306  comprise a casing  310 . The casing  310  is preferably cylindrical, and transparent. First end cap  304  and second end cap  306  are attached to longitudinal member  302  via at least one of a pressure mount arrangement, screw on arrangement, adhesive, and the like. Preferably, at least one of the first end cap  304  or the second end cap  306  is attached to the longitudinal member  302  such that the end caps  304  and  306  can be reattached and removed at the convenience of a user. The longitudinal member  302 , first end cap  304  and second end cap  306  comprise, preferably, a plastic material. For example, The preferred material for the longitudinal member  302  is acrylic plastic, and the preferred material for the end caps  304  and  306  of the phantom  300  is ABS plastic. However, it should be appreciated by those skilled in the art that the end caps  304  and  306  and the longitudinal member may be constructed of any material that is compatible with the operating environment of a MR clinical imaging system. In addition, the construction material in the embodiment must not exhibit multiple MR images due to chemical shifts. 
     At least one of first end cap  304  or second end cap  306  includes a sealable opening  309  for filling the chamber  308  with a liquid or magnetic resonance material upon which the MRI scan acts to generate an MRI image. In an embodiment of the present invention, no sealable opening is required, a user simply removes either first end cap  304  or second end cap  306 , fills the chamber  308  with the liquid and reseals first end cap  304  or second end cap  306  to the longitudinal member  302 . 
     In an embodiment of the present invention, the phantom  300  is structured and arranged to enable the rods  312  to be held in place by gravity. In another embodiment the rods are held in place via clips or similar type retention mechanism above an orifice in the end caps  304  and  306  through which the rods are inserted. Preferably the ends of the rods  312  are tapered as well as the opening  309  in the end caps see  FIG. 3B . A groove  315  is placed in the opening  309  in the end caps. The O-rings  313  rest within the groove  309 . Once the O-ring  313  is in place, the assembly is epoxied via an epoxy  317  to provide a second seal. 
     The O-ring  313  ensures that the line source outer diameter and the rod outer diameter  311  are collinear at least on one end. The O-ring  313  allows the absolute diameter of the rod to vary while still keeping the line sources centered. The ends of the rods  312  can be left closed or kept open. For example a seal or screw at the ends of the rods  312  can be used to keep out fluids. 
     The chamber  308  can be filled with water having a dissolved salt such as NiSO 4 , MnCl, NACL, and the like, in order to reduce the MR imaging time. The water and salt therein represent a certain load for MRI transmit and receive coils of the MRI scanning device, and such that hydrogen inside the water compound delivers an MRI signal at a certain resonance frequency, which can be detected by the MR receive coil. In an embodiment of the invention, the chamber  308  is filled with an oil such as mineral oil. It should be appreciated by those skilled in the art that other types of oils can also be used without departing from the scope of the present invention. It should also be appreciated that a low viscosity substance such as a gel can also be used. 
     In another embodiment of the present invention, the chamber  308  is filled with silicone containing hydrogen as the fluid material. The content of silicone containing hydrogen provides the MRI scanning system with enough mass of hydrogen to allow for frequency and transmitter adjustment. During scanning, the chamber  308  containing the fluid material is generally only visible in the MRI scan and not in the PET scan. 
     The longitudinal member  302  includes a second longitudinal member  312  also known as a rod. The rod  312  is hollow in its center for the placement of PET line sources along the axis of the rod  312  and because the hollow rod  312  produces a void in the MR signal which allows the MR FoV to be measured and positioned. The rod  312  preferably has markings on its surface to indicate end points  314   1  and  314   3  and a center point  314   2  of the PET line sources within the rod  312 . The rod  312  containing the line sources is preferably ABS plastic, aluminum or another type of metallic material. 
     The rod  312  is mounted in a fluid such as salt doped water, water with a dissolved salt, or a mineral oil filled casing  310 . The PET scanner detects the radiation from the annihilation of the positrons emitted from the line sources, and the MR detects the signal void caused by the rods in the fluid. The alignment procedure requires performing image processing on the images in order to determine the centroids of the voids in each MR image and of the line sources in each PET image, fitting lines to the sets of MR and PET centroids, and computing the offset, rotation and scale of the PET coordinate system with respect to the MR coordinate system. 
     In an embodiment of the invention, the axis of an outside diameter  311  of the rod  312  is required to be collinear with the axis of the line source. 
     In an embodiment of the invention the phantom  300  includes a central mark  316  on its surface that can be aligned to the Field of View (FoV) center defined by the MR positioning lasers. MR imaging is used to visualize with MR the liquid filling the cylindrical chamber  308  outside the rods  312 . 
     In principle at least two rods determine a coordinate system. Two rods provide three vectors along each rod and along the shortest path between the rods allowing a 3D rotational correction to be made. The order and magnitude of translations and rotations provide a solution to bridge the coordinate system between PET and MR. The calculations can be determined via rod pairs or amongst individual rods. It should be appreciated by those skilled in the art that other combinations can be realized without departing from the scope of the invention. 
       FIG. 3A  depicts a single rod  312  for ease of viewing. However, a plurality of rods can be used with the invention. When more rods are used, then the coordinate system can be determined from the line segment along the shortest path between rod pairs. The measured length of these line segments in the MR images gives a quality measurement of MR scaling errors that could be the result of non-linear gradient fields. As the number of rods used increases, the accuracy of the FoV offset correction and scaling is improved. 
     In addition, with two rods the MR and PET coordinate systems can be scaled. The orthogonal distance between the rods can be used for checking the scaling in X, Y, Z directions with the proper rod placement. 
     As shown in  FIG. 4  four rods  412   1 ,  412   2 ,  412   3  and  412   4  pass from a first end cap  404  to a second end cap  406  with each of the rods  412  preferably tilted or skewed with respect to the axis of a longitudinal member  402  in planes perpendicular to the Y axis (top and bottom) and X axis (right and left). The tilt angle can be as little as 0.5 degrees. 
     Each of the rods  412  has a hole drilled through its axis. In this embodiment of the invention, the holes hold four line sources with activity that emits positrons for imaging with PET. The cladding of the line sources is a non-magnetic, non-porous material. In one embodiment of the invention, the line source activity is clad in a sheath of aluminum. 
     The phantom  400  includes a central mark on its surface that can be aligned to the FoV center defined by the MR positioning lasers. Specifically, the central mark is used to position the alignment phantom near the MR and PET FoV centers so that the measurement need not be repeated after re-positioning the phantom. MR imaging is used to visualize the liquid filling the cylindrical chamber  408  outside the rods  412 . 
     In a preferred embodiment of the invention, the diameter and the length of the phantom  400  allows for the phantom  400  to be in the FoV of the PET FoV and the MR FoV. An exemplary diameter for the phantom  400  is no greater than about 20 cm in order to sample the MR FoV where it has the least distortion due to gradient non-linearities. The rod length projected on the phantom axis should be about 30 cm in order to cover the PET FoV. However, it should be appreciated by those skilled in the art, that these ranges can vary based on the manufacturer and model of imaging machine used. For example, the dimensions are dependent on the MR magnet design. Another size magnet could allow a larger phantom or require a smaller one. 
     In an embodiment of the present invention, the phantom  400  is structured and arranged to enable the rods to be held in place by gravity. In another embodiment the rods are held in place via clips or similar type retention mechanism above an orifice in the end caps  404  and  406  through which the rods are inserted. In still another embodiment of the invention, the rods are held in place via O-rings residing in a groove in at least one end cap  404  or end cap  406 . Preferably, the O-rings reside in both end caps  404  and  406 . The O-ring ensures that the line source outer diameter and the rod outer diameter are collinear at least on one end. The O-ring allows the absolute diameter of the rod to vary while still keeping the line sources centered. The ends of the rods  412  can be left closed or kept open. For example a seal or screw at the ends of the rods  412  can be used to keep out fluids. 
       FIG. 5  is a flow chart setting forth a process  500  for providing data acquisition using the phantom for alignment for combined PET and MRI scanning, according to an exemplary embodiment. The process is initiated at step  502  where the phantom is prepared for the MR/PET alignment procedure by ensuring that the cavity surrounding the rods is filled completely with a fluid such as a salt dissolved in water or mineral oil and the like that can be imaged in an MRI. 
     At step  504 , the four positron emitting line shaped sources of activity that are visible with the aid of PET will be inserted into the holes bored through the rods of the phantom  300 . 
     At step  506 , the alignment phantom is positioned on the MR/PET patient table. The phantom is positioned with means such as mechanical, electrical or combination thereof, that ensure that the orientation and position of the alignment phantom with respect to the MR system is known. 
     At step  508 , the MR/PET patient table transports the alignment phantom to within approximately 1 cm of the centers of the overlapping MR FoV and PET FoV. 
     At step  510 , the phantom  300  is imaged with the MR system, and the MR images are reconstructed. The rods are visible in the MR images as oval areas where the MR signal is absent from the images. 
     At step  512 , simultaneously with the MR image acquisition, the phantom is imaged with the PET system. In an embodiment, the PET images are reconstructed without attenuation and scatter correction since information to align the attenuation map with the phantom is not known at this step. The PET positron line sources are visible as four, round, bright points in a dark background. 
     In another embodiment, the rods are non-metallic with a negligible chemical shift and the MR and PET data are acquired simultaneously. 
       FIG. 6  is a flow chart setting forth a process  600  for providing data processing using the phantom for alignment for combined PET and MRI scanning, according to an exemplary embodiment. At step  602  a study is retrieved. At step  604  a search is performed for the previously acquired PET images and MR images. A failure message is returned if one or more of the image data is missing. The process  600  proceeds to step  606 . 
     At step  606 , the MR and PET images are sorted in descending order. Memory is allocated and the PET images are read from the data files. 
     At step  608 , the PET image intensity is clipped significantly above the noise and set to a unitary value on a plane by plane basis in order to eliminate the bias introduced by the absent attenuation and scatter corrections. 
     At step  610 , A determination is made as to the intersection points of the line sources with the PET image planes by searching for the binary PET bright spots and computing the centroid of each bright spot. 
     At step  612 , Intersection points are grouped into sets or pairs with one set per line source. 
     At step  614 , the axes of the line sources are determined by fitting lines to the sets of line source centroids computed in each plane. 
     At step  616 , memory is allocated and the MR images are read from the data files. 
     At step  618 , the MR image gray scale is inverted so that the dark pixels become dark. Steps  608  through  612  are repeated for the rods visible as signal voids in the MR images. 
     At step  620 , the coordinate transformation e.g., offset, rotations, and scaling are calculated as described below in equations 1 and 2 below. The phantom  400  with four rods has four line sources having six independent line pairs and six pairs of independent calibration points where each calibration point pair is defined by end points of the shortest line segment between each line pair. 
     At step  622 , the calculated offsets are provided to the client. 
     In another embodiment of the invention, the process  600  could be iterated at least twice with the attenuation and scatter corrections applied to the PET data in the second and subsequent passes in order to determine the offset. 
     With reference to the alignment process. Equation 1 describes the computation of the rotation matrix R and the translation vector t required to align the MR and PET FoVs. If the singular value decomposition of the covariance matrix between the MR and PET calibration point sets has the form UΣV T , where U, Σ and V are matrices and where V T  is the transpose of matrix V, then R and t can be computed from the expressions:
 
 R=UV   T   (equation 1)
 
 T=μ   MR   −Rμ   PET   (equation 2)
 
Where μ PET  and μ MR  are the centroids of the MR and PET calibration points.
 
     The phantoms  300  and  400  provide the following advantages—the MR signal from the alignment object does not have shadows due to chemical shifts, provides very accurate alignments, provides sufficient information for alignment when imaging a partial volume of the phantom, employs components that can be used for other calibration measurements, provides an activity source that is easy to manufacture and store and can be used for other system calibrations, and all MR adjustments can be easily performed. 
     Given the above, a phantom as per embodiments of the invention can be used to verify the accuracy of an integrated MR/PET scanner by placing the phantom within the scanner, running the MR features of the scanner at full RF power, and conducting a PET scan of the phantom at the same time. The PET signals/images would then be analyzed to verify their accuracy. Thus, a phantom as per embodiments of the invention eliminates the need for a separate water-based (NaCl solution) phantom doped with FDG, and the hazards associated with such prior art phantoms can be eliminated. 
     Referring now to  FIG. 7 , according to an embodiment of the present invention, a computer system  700  for implementing the present invention can comprise, inter alia, a central processing unit (CPU)  702 , a memory  704  and an input/output (I/O) interface  706 . The computer system  700  is generally coupled through the I/O interface  706  to a display  708  and various input devices  710  such as a gamma camera, detector, mouse and a keyboard. The support circuits can include circuits such as cache, power supplies, clock circuits, and a communication bus. The memory  704  can include random access memory (RAM), read only memory (ROM), disk drive, tape drive, hard drive, USB drive, CD, DVD, etc., or combinations thereof. Methods for using invention can be implemented as a routine  712  that is stored in memory  704  and executed by the CPU  702  to process the signal from the signal source  714 . As such, the computer system  700  can be a general purpose computer system that becomes a specific purpose computer system when executing the routine  712  of the present invention or a PET/MR, PET or MR system, or combinations thereof. 
     The computer system  700  also includes an operating system and micro instruction code. The various processes and functions described herein can either be part of the micro instruction code or part of the application program (or combination thereof) which is executed via the operating system. In addition, various other peripheral devices can be connected to the computer platform such as an additional data storage device and a printing device. 
     It is to be further understood that, because some of the constituent system components and method steps depicted in the accompanying figures can be implemented in software, the actual connections between the systems components (or the process steps) may differ depending upon the manner in which the present invention is programmed. Given the teachings of the present invention provided herein, one of ordinary skill in the related art will be able to contemplate these and similar implementations or configurations of the present invention. 
     While the invention has been disclosed with reference to specific exemplary embodiments, modifications to and departures from the disclosed embodiments will occur to those having skill in the art. For example, while the invention has been described with reference to an integrated MR/PET system, the phantoms  300  and  400  also could be applicable to other integrated multimodality systems such as MR/SPECT and MR/CT. For example, the CT requires high-density (but not metal rods) rather than line sources and where the CT detects the metal rods in transmission images rather than in emission images. Accordingly, what is protected is defined by the scope of the following claims.