Abstract:
A hybrid medical imaging system comprises a nuclear medicine imaging subsystem for capturing an image of a target region of a subject, a magnetic resonance imaging (MRI) subsystem for capturing an MRI image of the target region based on at least one MRI parameter and processing structure communicating with the subsystems. The processing structure processes the MRI image to estimate attenuation within the target region and uses the estimated attenuation to correct the image captured by the nuclear medicine imaging subsystem.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/095,382 filed on Sep. 9, 2008 entitled “System And Method For Correcting Attenuation In Hybrid Medical Imaging”, the content of which is incorporated hereby reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to medical imaging and in particular, to a system and method for attenuation correction in hybrid MRI medical imaging. 
       BACKGROUND OF THE INVENTION 
       [0003]    Hybrid imaging technologies of nuclear medicine imaging and computer tomography such as Positron Emission Tomography/X-ray Computed Tomography (PET/CT) and more recently Single Photon Emission Computed Tomography/CT (SPECT/CT) are well-known. PET/Medical Resonance Imaging (PET/MRI), however, offers several important advantages over PET/CT. Unlike PET/CT where imaging is sequential, PET/MRI enables true simultaneous multi-modality measurement (although there are PET/MRI solutions utilizing field-cycled MRI offering temporally-interleaved PET and MRI). Compared with CT imaging, the technology of magnetic resonance imaging offers much larger breadth in measurement parameters. The well-established measurements of longitudinal and transverse magnetization relaxation (R1, R2) and proton density offer superior soft tissue contrast compared with CT. 
         [0004]    Although MRI has difficulties imaging bones due to lack of water content, the marrow can be seen and the bones easily inferred. In MRI, pulse sequences allow for functional imaging (fMRI). In the brain the functional imaging encompasses the detection of blood oxygenation level dependent (BOLD) tissue changes as described in “Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging” authored by Ogawa et al. (Proc Natl Acad Sci USA 89 5951-5, 1991) and quantitative measurements of blood flow using arterial spin labeling (ASL) as described in “Effects of the apparent transverse relaxation time on cerebral blood flow measurements obtained by arterial spin labeling” authored by St. Lawrence et al. (Magn Reson Med 53 425-33, 2005). In the heart, the functional imaging encompasses contrast agent bolus tracking and wall motion studies as described in “Myocardial blood flow quantification with MRI by model-independent deconvolution” authored by Jerorsch-Herold M et al. (Med Phys 29 886-897, 2002). Further, MRI can measure water diffusion amplitude (“coefficients”) as described in “Current status of body MR imaging: fast MR imaging and diffusion-weighted imaging” authored by Koyama et al. (Int J Clin Oncol 11 278-285, 2006), tensors enabling tractographic imaging as described in “In vivo Fiber Tractography using DT-MRI Data” authored by Basser et al. (Mag Reson Med 44 625-632, 2000), and spectroscopy to measure biochemistry. Another advantage of MRI over CT concerns the ionizing radiation association with the latter (typical dose from helical CT of the chest is about 8 mSv, resulting in a 0.03% increased risk of stochastic induction of fatal cancer). Although a typical PET study results in about a 14 mSv dose, the lack of MRI ionizing radiation is of value in serial and pediatric studies. This is not to say, however, that hybridization of PET with MRI is without challenges. 
         [0005]    To date, efforts to hybridize modalities has resulted in popular clinical PET/CT and SPECT/CT platforms. There is substantial overlap between SPECT and PET clinical capabilities, with each modality having advantages and disadvantages. SPECT has the advantage of being able to image multiple isotopes simultaneously. Single photon emitting radioisotopes generally have longer radioactive half-lives than their PET counterparts which has implications for radiopharmaceuticals with slow binding kinetics as described in “A reappraisal of the relative merits of SPET and PET in the quantification of neuroreceptors: the advantages of a longer half-life!” authored by Lassen (Eur J Nucl Med 23 1-4, 1996). On the other hand, clinical PET images have superior resolution and sensitivity compared with clinical SPECT, although research-grade pre-clinical SPECT systems (small field-of-view systems for imaging small animals) can outperform pre-clinical PET systems due to positron range issues as described in “Instrumentation for Molecular Imaging in Preclinical Research: Micro-PET and Micro-SPECT” authored by Chatziioannou (Proc Am Thoracic Soc 2 533-536, 2005). 
         [0006]    PET, like SPECT, suffers from the physical phenomena of gamma-ray attenuation and scattering. These emission tomography modalities rely on detection of gamma-rays emitted via decay of a radioisotope (bound to a radiopharmaceutical) that is injected into the subject to be imaged. From their point of isotropic emission, the gamma-rays travel through the subject interacting with the tissues of the subject (specifically, the electrons) along the way. During travel, some gamma-rays are absorbed by tissue or are scattered away from the detectors and hence are lost or “attenuated”, or are spatially mis-positioned (“scattered”). The obstacles of attenuation and scatter have received much attention over the past decades. Arguably, these obstacles have been overcome for specific circumstances in SPECT and PET, with increasingly sophisticated techniques offering diminishing returns. Attenuation compromises the quantitative accuracy of PET, and correcting for attenuation is a major area of research. In principal, the effects of attenuation can be corrected if the distribution of attenuation coefficients is known. In the past, this was accomplished by hybridizing PET with X-ray CT, yielding the PET/CT apparatus. This hybridization was convenient because CT measurements are easily related to electron density and hence the distribution of attenuation coefficients could be provided with relative ease. In Hybrid MRI/PET, however, there is significant challenge to deriving the distribution of attenuation coefficients, since MRI measurements using contemporary clinical MRI are not easily related to electron density. 
         [0007]    The unification of different imaging modalities into a single Hybrid platform is disclosed in “Myocardial Perfusion Imaging with a Combined X-ray CT and SPECT System” authored by Kalki et al. (J Nucl Med 38 1535-1540, 1997) for SPECT/CT and for Positron Emission Tomography (PET) PET/CT in “A Combined PET/CT Scanner for Clinical Oncology” authored by Beyer et al. (J Nucl Med 41 1369-1379, 2000). In both cases, X-ray CT data guided attenuation correction of the gamma-ray measurements. 
         [0008]    U.S. Pat. No. 5,672,877 to Liebig et al. discloses a method of coregistering medical image data of different modalities. In the method, an emission scan of an object is performed using a nuclear medicine imaging system to acquire single-photon emission computed tomography (SPECT) image data. A transmission scan of the object is performed simultaneously with the emission scan using the same nuclear medicine imaging system in order to acquire nuclear medicine transmission image data. The emission scan is performed using a roving zoom window, while the transmission scan is performed using the full field of view of the detectors. By knowing the position of the zoom windows for each detection angle, the nuclear medicine transmission image data can be coregistered with the SPECT emission image data as a result of the simultaneous scans. Image data of a modality other than SPECT, such as x-ray computed tomography (x-ray CT) data, magnetic resonance imaging (MRI) data, or positron emission tomography (PET) data, is also provided, which it is desired to have coregistered with the SPECT emission data. The nuclear medicine transmission image data is therefore coregistered with the image data of the different modality. As a result, the image data of the different modality becomes coregistered with the SPECT image data. 
         [0009]    U.S. Pat. No. 7,286,867 to Schlyer et al. discloses a combined PET/MRI scanner including a magnet for producing a magnetic field suitable for magnetic resonance imaging, a radiofrequency (RF) coil disposed within the magnetic field produced by the magnet and a ring tomograph disposed within the magnetic field produced by the magnet. The ring tomograph includes a scintillator layer for outputting at least one photon in response to an annihilation event, a detection array coupled to the scintillator layer for detecting the at least one photon outputted by the scintillator layer and for outputting a detection signal in response to the detected photon and a front-end electronic array coupled to the detection array for receiving the detection signal. The front-end array has a preamplifier and a shaper network for conditioning the detection signal. 
         [0010]    U.S. Pat. No. 6,927,406 to Zyromski discloses a multimodal source for imaging with at least one of a gamma camera, a positron emission tomography (PET) scanner and a single-photon-emission computed tomography (SPECT) scanner, and at least one of a computed tomography (CT) scanner, magnetic resonance imaging (MRI) scanner and optical scanner. The multimodal source has radioactive material permanently incorporated into a matrix of material, at least one of a material that is a target for CT, MRI and optical scanning, and a container which holds the radioactive material and the CT, MRI and/or optical target material. The source can be formed into a variety of different shapes such as points, cylinders, rings, squares, sheets and anthropomorphic shapes. The material that is a target for gamma cameras, PET scanners and SPECT scanners and/or CT, MRI and/or optical scanners can be formed into shapes that mimic biological structures. 
         [0011]    U.S. Pat. No. 7,378,660 to Case et al. discloses a computer program, method, and system to facilitate hybrid CT attenuation correction. In one embodiment, the method generally includes acquiring data from a scanner, utilizing an ordered subset expectation maximization-bayesian algorithm to reconstruct the acquired data, and forward projecting the reconstructed data. Such a configuration minimizes the computing resources required for reconstruction and improves attenuation correction accuracy. 
         [0012]    U.S. Pat. No. 7,348,564 to Wollenweber et al. discloses a method for correcting emission data from Positron Emission Tomography (PET) or SPECT and includes generating a plurality of computed tomography (CT) image data, selecting a portion of the CT image data, processing the selected CT data to generate a plurality of attenuation correction (CTAC) factors at the appropriate energy of the emission data, weighting the CTAC factors to generate an emission attenuation correction map, wherein the weights are determined based on the axial location and the slice thickness of the CT data and the axial location and the slice thickness of the PET or SPECT data, and utilizing the generated attenuation correction map to generate an attenuation corrected PET image. 
         [0013]    U.S. Pat. No. 6,628,983 to Gagnon discloses a nuclear imaging system comprising a transmission radiation source that radiates at a plurality of energy levels within a specified energy range. The energy range is divided into two or more energy subranges. Detectors detect the position or trajectory and energy of transmitted radiation and emitted radiation. A sorter sorts the detected radiation into the appropriate energy subrange. Data for each subrange is stored in a plurality of transmission data memories. Reconstruction processors generate a transmission image representation representative of each energy subrange. A combine processor weights each energy subrange image representation with an assigned weighting factor to provide enhancement of at least one feature when the images are combined to generate weighted image representations. The plurality of transmission images are also combined with equal weighting to generate an image representation used to generate attenuation correction factors for correcting the emission data. A reconstruction processor generates a corrected emission image representation. The emission image can be combined with one of the feature-enhanced structural images using a combiner and displayed, allowing the functional emission image to be located with respect to structural or anatomical features. Also, a feature-enhanced structural image, can be used to register the emission image with an image from another modality, such as a computed tomography (CT) image. 
         [0014]    U.S. Pat. No. 5,750,991 to Moyers et al. and U.S. Pat. No. 6,040,580 to Watson et al. disclose a method and apparatus for producing radioactive transmission measurements to form multi-dimensional attenuation correction data with a point source of radiation, such as required in positron emission tomography applications. This involves the passing of the point source proximate the face of a selected each of the tomograph units for the formation of a 3-D image, or a selected portion of the tomograph units for a 2-D image. As such, attenuation data, transmission data, detector performance data, etc., can be obtained. This point source of radiation, in one embodiment, is rapidly circulated through a conduit that passes across each detector face under the influence of a transport fluid in, for example, an oscillatory motion to achieve a selected radiation field whereby calculation of transmission measurements within a body positioned within the tomograph scanner is achieved. When not being circulated, the radiation source is held within a shield. Circulation of the transport fluid, typically a hydraulic fluid, is typically accomplished using a positive displacement pump. Position sensors are used to monitor the movement of the source in the conduit as well as its position within the shield. Disconnect units permit removal of the radiation source, as contained in the shield, from the system without accessing any other portions of the system. 
         [0015]    Although the above references discloses various imaging techniques, improvements in hybrid medical imaging systems are desired. It is therefore an object of the present invention at least to provide a novel system and method for attenuation correction in hybrid medical imaging. 
       SUMMARY OF THE INVENTION 
       [0016]    A novel technique is presented for facilitating non-uniform attenuation correction in hybrid medical imaging systems such as for example, PET/MRI or SPECT/MRI imaging systems by estimating the distribution of attenuation directly from MRI parameters. The technique uses one or more MRI parameter measurements, such as longitudinal and transverse magnetization weighted imaging, to estimate the distribution of different tissue types in a subject. Subsequently, known energy-dependent attenuation coefficients are assigned to those tissue types, allowing the creation of a non-uniform attenuation map. This attenuation map is then used to guide the reconstruction of the SPECT or PET projection data resulting in attenuation corrected images. 
         [0017]    Accordingly, in one aspect there is provided a hybrid medical imaging system comprising a nuclear medicine imaging subsystem for capturing an image of a target region of a subject; a magnetic resonance imaging (MRI) subsystem for capturing an MRI image of said target region based on at least one MRI parameter; and processing structure communicating with said subsystems, said processing structure processing the MRI image to estimate attenuation within said target region and using the estimated attenuation to correct the image captured by said nuclear medicine imaging subsystem. 
         [0018]    According to another aspect there is provided a method for hybrid medical imaging comprising producing an image of a target region of a subject based on a radioactive contrast agent in said subject; measuring at least one magnetic resonance parameter of said target region; and estimating attenuation in said target region using said at least one magnetic resonance parameter; and correcting degradations in said image using the estimated attenuation. 
         [0019]    According to yet another aspect there is provided a computer readable machine embodying computer program code for causing processing structure of a hybrid medicine imaging system to estimate attenuation in a target region using at least one magnetic resonance parameter of an MRI image; and correct degradations in a nuclear medicine image using the estimated attenuation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Embodiments will now be described more fully with reference to the accompanying drawings in which: 
           [0021]      FIG. 1  is a schematic side view of a hybrid magnetic resonance imaging-positron emission tomography system; 
           [0022]      FIG. 2A  shows an MRI image captured by the system of  FIG. 1 ; 
           [0023]      FIG. 2B  shows a PET image captured by the system of  FIG. 1  corresponding to the image shown in  FIG. 2A ; 
           [0024]      FIG. 3  shows MRI images based on different MRI parameters that are segmented and combined to yield a combined segmented MRI image; 
           [0025]      FIG. 4  shows a refined combined segmented MRI image derived from a PET image and the combined segmented MRI image of  FIG. 3 ; 
           [0026]      FIG. 5  shows an attenuation coefficient image derived from an attenuation coefficient lookup table and in situ MRI measurements and the refined combined segmented MRI image of  FIG. 4 ; 
           [0027]      FIG. 6  shows a re-scaled attenuation coefficient image derived from measured and calculated line integrals through the attenuation coefficient image of  FIG. 5 ; 
           [0028]      FIGS. 7A to 7F  show sample MRI images based on different MRI parameters and attenuation coefficient images; and 
           [0029]      FIGS. 8A to 8D  show CT and MRI attenuation maps together with line integral data. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]    Turning now to  FIG. 1 , a hybrid medical imaging system is shown and is generally identified by reference numeral  10 . In this example, hybrid medical imaging system  10  comprises a nuclear imaging subsystem such as for example, a positron emission tomography (PET) subsystem  12  and a magnetic resonance imaging (MRI) subsystem  14 . The PET subsystem  12  is intermediate the two ends of the MRI subsystem  14 . The PET and MRI subsystems  12 ,  14  are coupled to a processing unit  16 , such as for example a personal computer (PC) or other suitable processing device which executes programm code structure in suitable memory. The processing unit  16  in this embodiment comprises, for example, a processor, a system memory (volatile and/or non-volatile memory), other non-removable or removable memory (eg. a hard disk drive, RAM, ROM, EEPROM, CD-ROM, DVD, flash memory, etc.) and a system bus coupling the various components to the processor. The processing unit can include a network connection to access shared or remote drives, one or more networked computers, or other networked devices. The extent of an object, for example, a patient, animal or other subject, enclosed by the PET subsystem  12  can be imaged by both the PET subsystem  12  and the MRI subsystem  14  simultaneously. A patient bed  20  is moveable axially within the MRI subsystem  14  to position the subject lying on the bed at the desired location relative to the PET and MRI subsystems  12  and  14 . Images acquired simultaneously by the PET subsystem  12  and MRI subsystem  14  are conveyed to the processing unit  16 . The processing unit  16  in turn performs attenuation coefficient mapping techniques in order to yield corrected hybrid images. 
         [0031]      FIG. 2A  shows an MRI image  20  captured by the MRI subsystem  14 . As can be seen, MRI image  20  is an axial cross-section through the chest of a mammal, in this case a canine, revealing the mediastinum, lungs, and surrounding musculature and fat. As is known, the MRI subsystem  14  is capable of imaging the distribution of anatomy in the subject, which can include, for example, the distribution of muscle, fat, and nervous tissues as well as distortions to that anatomy resulting from disease or injury. The MRI subsystem  14  is said to image endogenous contrast since these tissue types can be distinguished by the MRI subsystem  14  due to differences in their physical properties. 
         [0032]    Conversely, the PET subsystem  12  images the distribution of a radionuclide introduced into the subject.  FIG. 2B  shows a PET image  22  captured by the PET subsystem  12  corresponding to the MRI image  20  of  FIG. 2A . As can be seen, the PET image  22  shows the distribution of a radiopharmaceutical that is designed to be taken up by myocardial tissue. The radionuclide is bound to a molecule, and the resulting construct termed a radiopharmaceutical. The radiopharmaceutical is designed to have a specific distribution in the subject, for example distributing according to cardiac blood flow or malignancy. However, radiopharmaceuticals often have some non-specific uptake, which can be used as spatial reference information in the absence of corresponding anatomical imaging. As mentioned above,  FIGS. 2A and 2B  correspond to each other, and are acquired simultaneously by the hybrid medical imaging system  10  as illustrated in  FIG. 1 . 
         [0033]      FIG. 3  shows three (3) MRI images  30 ,  32  and  34  captured by the MRI subsystem  14  based on different MRI parameters and the processing of the MRI images carried out by the processing unit  16  in order to derive a combined segmented MRI image  36 . In this example, the three (3) MRI images comprise a proton-density weighted MRI image  30 , a T1 weighted (longitudinal relaxation weighted on T1w) MRI image  32  and a proton-density weighted with water suppression MRI image  34 . During processing by the processing unit  16 , the proton-density weighted MRI image  30  is segmented to identify soft tissue  38 , the T1 weighted MRI image  32  is segmented to identify hard tissue  38 , and the proton-density weighted with water suppression MRI image  34  is segmented to identify tissue boundries  40 . There are a variety of methods for segmenting information from the MRI images. In this embodiment, the processing unit  16  employs the k-means algorithm to segment the MRI images  30  to  34 . The information identified through segmentation of the MRI images  30  to  34  is then combined by the processing unit  16  to yield the combined segmented MRI image  36 . 
         [0034]    The quality of the combined segmented MRI image  36  is then refined by utilizing additional information that is derived from the PET subsystem  12 , which acquires measurements of the distribution of radiopharmaceutical inside the subject. This refined segmentation assists in determining a more accurate estimate of the attenuation coefficient distribution.  FIG. 4  shows a PET image  50  that is subjected to a preliminary tomographic reconstruction  52  by the processing unit  16  from which the contours, or support (i.e. anatomy), of the subject can be inferred via voxel-by-voxel thresholding of the reconstruction with some spatial smoothing. Other techniques for inferring anatomy, or identifying different tissue types inside the anatomy may however be employed such as for example, the techniques disclosed in support “Scatter and Attenuation Correction for Brain SPECT using Attenuation Distributions Inferred from a Head Atlas” authored by Stodilka et al. (J Nucl Med 41 1569-1578, 2000) and in U.S. Pat. No. 6,740,883 to Stodilka et al. After the PET image  50  has been processed to infer anatomy, the processing unit  16  combines the resultant information with the combined segmented MRI image  36  to yield a refined combined segmented MRI image  54 . The quality of the refined combined segmentated MRI image  54  may be enhanced by providing information that cannot be discerned from MRI as easily, such as the location of bone or variations of attenuation coefficient in tissues that are visualized as having uniform MRI parameters, or tissues with the same attenuation coefficients that are visualized as having different MRI parameters. 
         [0035]    Once the refined combined segmented MRI image  54  has been generated, the processing unit  16  assigns attenuation coefficients to the various tissue types of the refined combined segmented MRI image to yield an attenuation coefficient image  60  as shown in  FIG. 5 . In this embodiment, the processing unit  16  derives attenuation coefficients for some tissue of the subject from a lookup table  62 , and derives attenuation coefficients for other tissue via in situ MRI or Magnetic Resonance Spectroscopy (MRS) measurements of the subject  64 . If desired, all of the attenuation coefficients may be derived from the lookup table  62  or all of the attenuation coefficients may be derived from the in situ measurements  64 . 
         [0036]    Alternatively, the MRI subsystem  14  can make measurements of the subject from which some or all of the attenuation coefficients  52  can be derived. For example, MRI techniques have been demonstrated for measuring bone density as described in “Biophysic evaluation of bone quality—application of Fourier transform infrared spectroscopy and phosphorus-31 solid-state nuclear magnetic resonance spectroscopy” authored by Takata et al. (J Med Invest 51 133-138, 2004) and “Magnetic Resonance Imaging Measurements of Bone Density and Cross-Sectional Geometry” authored by Hang et al. (Calcif Tissue Int 66 74-78, 2000). MRI techniques have also been demonstrated for measuring the attenuation coefficients for cortical and trabecular bone as described in “The Importance of Bone Attenuation in Brain SPECT Quantification” authored by Stodilka et al. (J Nucl Med 39 190-197, 1998). Information from these two sources can be combined with the refined combined segmented MRI image to yield the attenuation coefficient image. 
         [0037]    Attenuation in PET occurs along the line connecting the origin of the gamma ray emissions and the detector element where those gamma rays are detected. Attenuation correction relies upon knowing the values of the line integrals on the attenuation coefficient images between those same two points (emission and detection) to determine the magnitude of attenuation correction that must be applied. In the case of PET imaging, this requirement is simplified in that it is sufficient to know only the line integrals between opposing detectors. 
         [0038]      FIG. 6  shows the attenuation coefficient image  60  and a line integral  64  measured with a radioisotope transmission system  62 . The corresponding line integral  66  in the attenuation coefficient image  60  is calculated by the processing unit  16 . Based on the differences between the measured line integral  64  and calculated line integral  66 , the processing unit  16  recalculates and scales the attenuation coefficient image  70  such that the calculated line integral matches the measured line integral. With the attenuation coefficient image  70  re-scaled, the attenuation coefficient image is used by the processing unit to correct attenuation in the PET image. 
         [0039]    In one example, a healthy canine is anaesthetized, immobilized on a wooden platform, and injected with 18F-FDG. Sixty minutes later whole-body PET/CT images (Discovery LS, General Electric Healthcare, Waukesha, Wis.) are acquired. The canine, on the platform, is then moved to an MRI suite, and T1-weighted, T2-weighted, and proton density whole-body MRI images are acquired at 1.5T (Avanto, Siemens Medical Solutions, Erlangen, Germany). Subsequently, all MRI data are spatially registered to the CT data. In  FIGS. 7A to 7F , resulting images are shown. In particular,  FIGS. 7A to 7F  show transaxial images through the canine at the thoracic level.  FIG. 7A  is a T1-weighted MRI image.  FIG. 7B  is a T2-weighted MRI image.  FIG. 7C  is a proton-density weighted MRI image.  FIG. 7D  is a X-ray CT image.  FIG. 7E  an MRI-derived attenuation map from 511-keV attenuation, and  FIG. 7F  is a PET image. In this example, multi-parameter MRI data is collected, and that data is mapped into attenuation coefficients suitable for use with PET. In PET/CT and SPECT/CT, the mapping from CT to attenuation coefficients is relatively straightforward since both are related to electron density—unlike multi-parameter MRI images (T1 weighted, T2 weighted, proton-density weighted). 
         [0040]    CT data are converted to 511-keV attenuation coefficients, and registered to MRI. MRI data are segmented via k-means to classify tissue types for each MRI parameter (six (6) clusters per parameter), and known 511-keV attenuation coefficients are assigned to clusters to yield MRI-derived attenuation maps. Also, clusters from multiple MRI parameters are combined to yield a multi-parameter MRI-derived attenuation map. Single- and multi-parameter MRI-derived attenuation maps are evaluated by comparing radon transforms [Cho Z H, Jones J P, Singh M 1993 (line integrals) of those maps with radon transforms of the CT-derived attenuation maps as described in “Foundations of Medical Imaging” authored by Cho et al. (John Wiley &amp; Sons Inc pg 73-74, 1993).]. 
         [0041]      FIG. 8A  shows a CT-derived attenuation map (axial slice at the thoracic level).  FIG. 8B  shows the corresponding MRI-derived attenuation map using T1, proton-density, and proton-density with water suppression MRI parameters.  FIG. 8C  shows the correlation of line integrals and line of best fit.  FIG. 8D  shows the results of line integral correlation, for thoracic and abdominal slices (not shown). 
         [0042]    In this example, the proton-density MRI image contributed most to identifying soft tissue and lung and the proton-density with water suppression MRI image contributed most to edge and marrow detection. Multi-parameter segmentation outperformed single-parameter segmentation (see  FIG. 8 ). The best combination of MRI parameters in this example are T1 weighted, proton-density and proton-density with water suppression. Segmentation encounters challenges in regions near bone and cartilaginous structures, and performed best in abdominal axial slices with less bone. 
         [0043]    The program code executed by the processing unit  16  may comprise program modules including but not limited to routines, programs, object components, data structures etc. and may be embodied as computer readable program code stored on a computer readable medium. The computer readable medium is any data storage device that can store data, which can thereafter be read by a computer system. Examples of computer readable medium include for example read-only memory, random-access memory, CD-ROMs, magnetic tape and optical data storage devices. The computer readable program code can also be distributed over a network including coupled computer systems so that the computer readable program code is stored and executed in a distributed fashion. 
         [0044]    Those skilled in the art recognize that PET is only one modality capable of performing nuclear medicine imaging, and the T1 weighted, proton-density weighted and proton-density with water suppression MRI parameters are only some of the MRI parameters that can be measured with MRI. For example, single photon emission computed tomography may be employed. Also, T2 weighted (transverse relaxation weighted or T2w) MRI or other parameter based MRI images can be used. As will be appreciated, different MRI parameters provide different contrasts in the subject being imaged. Imaging with certain MRI parameters may improve contrasts in tissues with relatively large amounts of water. Also, data acquisition in the MRI subsystem  14  can be modified to suppress contrast from other materials besides water as in the proton-density with water suppression MRI images. 
         [0045]    The MRI parameters that are selected for imaging are chosen to suit the imaging environment and permit suitable distribution of attenuation coefficients. As will be appreciated, certain MRI parameters yield superior segmentation results for certain tissues although segmentation can depend on the segmentation algorithm employed and/or the MRI image acquisition sequence. 
         [0046]    Although the hybrid medical imaging system  10  is described as acquiring PET and MRI images simultaneously, those of skill in the art will appreciate that the PET and MRI images may be acquired sequentially or the PET and MRI images may be acquired alternatively. 
         [0047]    Although preferred embodiments have been described with reference to the drawings, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope thereof as defined by the appended claims.