Patent Publication Number: US-9423479-B2

Title: Split gradient coil and hybrid systems using same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 13/489,491 filed Jun. 6, 2012 and since issued as U.S. Pat. No. 8,604,795 which is a divisional application of U.S. patent application Ser. No. 12/531,979 filed Sep. 18, 2009 and since issued as U.S. Pat. No. 8,334,697 which is a 371 application of PCT application number PCT/IB2008/050151 filed Jan. 16, 2008 which claims the benefit of U.S. provisional application Ser. No. 60/910,032 filed Apr. 4, 2007, all of which are incorporated herein by reference. 
    
    
     The following relates to the imaging arts. The following finds illustrative application to hybrid magnetic resonance scanning and positron emission tomography (PET) systems, and is described with particular reference thereto. The following finds more general application to magnetic resonance scanning systems with or without integrated radiation detectors for PET imaging. 
     Some existing magnetic resonance scanners include a generally cylindrical set of main magnetic field windings generating a main (B 0 ) magnetic field in at least an examination region disposed within the cylinder defined by the main magnet windings. A generally cylindrical gradient coil assembly is disposed coaxially inside the main magnetic field windings to selectively superimpose magnetic field gradients on a main magnetic field. One or more radio frequency coils are disposed inside the gradient coil assembly. These radio frequency coils can take various forms ranging in complexity from single-loop surface coils to complex birdcage coils. In some embodiments, a whole-body birdcage coil is provided, which is a cylindrical coil arranged coaxially inside of the gradient coil assembly. The gradient coil assembly and the radio frequency coil assembly are both whole-body cylindrical structures that are disposed at different radial positions, and as such they occupy a substantial amount of the cylindrical bore space. 
     Heid et al., U.S. Pat. No. 6,930,482, discloses a gradient coil having two separate halves that are separated by a central gap over which no windings pass. A short co-radial radio frequency coil is placed in the central gap so that the gradient coil and radio frequency coil are at about the same radius, thus making more efficient use of the valuable bore space. However, the efficiency of the gradient coil assembly decreases as the width of the central gap increases. For a central gap of more than about 10 centimeters, there is a substantial degradation of efficiency. The small achievable gap provides correspondingly short radio frequency coil rods or rungs, which reduces the field of view of the radio frequency coil. 
     There is also interest in multi-modality or hybrid scanners including both magnetic resonance and positron emission tomography (PET) capability. For example, Fiedler et al., WO 2006/111869 discloses various hybrid imaging systems. In some hybrid system embodiments disclosed in that reference, solid state PET detector elements are disposed between rungs of a whole-body birdcage coil in order to efficiently use the available cylindrical bore space. The gradient coil assembly of Heid et al., with its central gap, might also be considered as a promising candidate for use in a hybrid imaging system. However, the small central gap achievable using the gradient coil assembly of Heid et al. is likely to be too small to receive a practical assembly of PET detector elements. The usable portion of this already small gap is further reduced by spacing gaps that would be needed between the coil and the PET detectors to accommodate mechanical movement of the halves of the coil under the influence of Lorentz forces. 
     The following provides a new and improved apparatuses and methods which overcome the above-referenced problems and others. 
     In accordance with one aspect, a magnetic field gradient coil is disclosed, comprising a generally cylindrical set of coil windings defining an axial direction and including primary coil windings and shield coil windings at a larger radial position than the primary coil windings, the generally cylindrical set of coil windings having an arcuate or annular central gap that is free of coil windings, the central gap having an axial extent of at least ten centimeters and spanning at least a 180° angular interval, the generally cylindrical set of coil windings further including connecting conductors disposed at each edge of the central gap that electrically connect selected primary and secondary coil windings, the generally cylindrical set of coil windings being operable to superimpose a transverse magnetic field gradient on an axially oriented static magnetic field in a region of interest that is surrounded by the generally cylindrical set of coil windings responsive to electrical energizing of the generally cylindrical set of coil windings. 
     In accordance with another aspect, a magnetic resonance scanner is disclosed, comprising: a generally cylindrical set of coil windings defining an axial direction and including primary coil windings and shield coil windings at a larger radial position than the primary coil windings, the generally cylindrical set of coil windings having an arcuate or annular central gap that is free of coil windings, the central gap having an axial extent of at least ten centimeters and spanning at least a 180° angular interval, the generally cylindrical set of coil windings further including connecting conductors disposed at each edge of the central gap that electrically connect selected primary and secondary coil windings; and a main magnet disposed outside of the generally cylindrical set of coil windings and operable to generate an axially oriented static magnetic field in a region of interest surrounded by the generally cylindrical set of coil windings, the generally cylindrical set of coil windings being operable to superimpose a transverse magnetic field gradient on the axially oriented static magnetic field in the region of interest. 
     In accordance with another aspect, a magnetic resonance scanner is disclosed, comprising: an annular ring of positron emission tomography (PET) detectors; a generally cylindrical set of coil windings including primary coil windings and shield coil windings at a larger radius than the primary coil windings, the generally cylindrical set of coil windings having an annular central gap receiving the annular ring of PET detectors, the generally cylindrical set of coil windings further including connecting conductors disposed at each edge of the annular central gap that electrically connect selected primary and secondary coil windings; and a main magnet disposed outside of the generally cylindrical set of coil windings and operable to generate an axially oriented static magnetic field in a region of interest surrounded by the generally cylindrical set of coil windings, the generally cylindrical set of coil windings being operable to superimpose a transverse magnetic field gradient on the axially oriented static magnetic field in the region of interest. 
     In accordance with another aspect, a magnetic field gradient coil is disclosed, comprising: a generally cylindrical set of coil windings including primary coil windings and shield coil windings at a larger radial position than the primary coil windings; and a second order shimset comprising second order shim windings at least a portion of which are disposed at a larger radial position than the shield coil windings. 
     In accordance with another aspect, a hybrid scanner is disclosed, comprising: a magnetic resonance scanner; positron emission tomography (PET) detectors disposed proximate to an isocenter of the magnetic resonance scanner; and an active shim system including shim coils and a shimset controller configured to control the shim coils to compensate a magnetic field inhomogeneity induced by the PET detectors. In some embodiments, the shimset controller is configured to control the shim coils to apply a first correction when the PET detectors are operational and to apply a second correction different from the first correction when the PET detectors are non-operational. 
     One advantage resides in providing a magnetic field gradient coil with an arcuate or annular gap having a width that is larger than heretofore achievable. 
     Another advantage resides in providing a magnetic field gradient coil having a central gap of a width sufficient to receive a PET detector array. 
     Another advantage resides in providing a magnetic field gradient coil having an arcuate gap comporting with an asymmetrical radio frequency coil. 
     Another advantage resides in providing a hybrid magnetic resonance/PET scanner having improved vibrational isolation for the PET detectors. 
     Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understand the following detailed description. 
    
    
     
         FIGS. 1 and 2  diagrammatically show perspective and end views, respectively, of a first illustrative transverse magnetic field gradient coil. 
         FIGS. 3 and 4  diagrammatically show perspective and end views, respectively, of a second illustrative transverse magnetic field gradient coil. 
         FIG. 5  diagrammatically shows a dielectric former for the second illustrative transverse magnetic field gradient coil including a stiffening brace. 
         FIG. 6  diagrammatically shows a magnetic resonance scanner including the second illustrative transverse magnetic field gradient coil and an annular array of positron emission tomography (PET) detectors disposed in a central gap of the second illustrative transverse magnetic field gradient coil. 
         FIG. 7  diagrammatically shows a perspective view of a third illustrative transverse magnetic field gradient coil. 
         FIG. 8  diagrammatically shows a dielectric former for the third illustrative transverse magnetic field gradient coil along with annular conductors of a radio frequency coil. 
         FIG. 9  diagrammatically shows a sectional view of a portion of a first embodiment of the annular array of PET detectors of  FIG. 6 . 
         FIG. 10  diagrammatically shows a sectional view of a portion of a second embodiment of the annular array of PET detectors of  FIG. 6 . 
         FIGS. 11 and 12  plot selected Golay coil type shim coil patterns over ½ of the azimuthal range, that is, between azimuthal values between φ=−90° and φ=90°, with the azimuthal dimension unrolled to provide 2-D plots. 
         FIG. 13  plots a side sectional view of the gradient coil assembly of the hybrid PET/magnetic resonance scanner of  FIG. 6  showing a suitable arrangement of second order shim coils on an outside of the split gradient coil and on a mechanical brace spanning the gradient coil portions. 
         FIG. 14  plots a side sectional view of the gradient coil assembly of a hybrid PET/magnetic resonance scanner showing a suitable arrangement of second order shim coils on an outer sheathing cylindrical former that also serves as structural reinforcement for the shim and gradient coils. 
     
    
    
     With reference to  FIGS. 1 and 2 , a magnetic field gradient coil includes a generally cylindrical set of coil windings  10  defining an axial direction D A  (indicated by a dashed arrow in  FIG. 1 ) and including primary coil windings  12  and shield coil windings  14  at a larger radial position than the primary coil windings. The generally cylindrical set of coil windings  10  has an arcuate or annular central gap  16  that is free of coil windings. The arcuate or annular central gap spans at least a 180° angular interval. In the embodiment shown in  FIGS. 1 and 2  the central gap  16  is an annular gap that spans a full 360° so as to space apart two sub-sets  20 ,  22  of the generally cylindrical set of coil windings each including primary coil windings and shield coil windings at a larger radial position than the primary coil windings. 
     The generally cylindrical set of coil windings  10  further includes connecting conductors  24  disposed at each edge of the central gap  16  that electrically connect selected primary and secondary coil windings. The generally cylindrical set of coil windings  10  is operable to superimpose a transverse magnetic field gradient G y  (indicated diagrammatically by an arrow in  FIG. 2 ) on an axially oriented static magnetic field in a region of interest R (indicated diagrammatically by a dotted boundary line in  FIG. 2 , and axially centered on the central gap  16 ) that is surrounded by the generally cylindrical set of coil windings  10 . The transverse magnetic field gradient G y  is generated responsive to electrical energizing of the generally cylindrical set of coil windings  10 . The embodiment of  FIGS. 1 and 2  further includes connecting conductors  26  disposed at the ends of the coil windings  10  distal from the central gap  16 . The connecting conductors  26  also electrically connect selected primary and secondary coil windings the selected windings being potentially the same as, or different from, the selected windings connected proximate to the central gap  16  by the connecting conductors  24 . Moreover, some primary windings or secondary windings may be isolated windings that are not connected by any of the connecting conductors  24 ,  26 . The connecting conductors  26  provide a relatively larger and more uniform field of view, as disclosed for example in Shvartsman et al., U.S. Publ. Appl. 2006/0033496 A1 which is incorporated herein by reference in its entirety. 
     The connecting conductors  24  enable non-zero current densities immediately adjacent the central gap  16  that compensate for the lack of any magnetically operative current density in the central gap  16 . It is recognized herein that this compensation enables the central gap  16  to be made larger than would otherwise be possible while still maintaining acceptable coil efficiency and field quality. The central gap  16  has an axial extent W of at least ten centimeters, and more preferably at least about fifteen centimeters, and in some embodiments at least about twenty centimeters. Such a large central gap has various useful applications, such as providing space for transverse rungs or rods of a radio frequency coil, receiving components of a second imaging modality, or so forth. 
     The central gap  16  is free of coil windings, by which it is meant that there are no magnetically operative conductors disposed in the central gap  16 . It is to be understood that one or more current feed conductors (not shown) optionally cross the central gap  16 , for example to electrically connect the two sub-sets of conductors in series. Such current feed conductors, if included, are not magnetically operative conductors in that they are not designed to contribute in a substantial way, and do not contribute in a substantial way, to the magnetic field generated by the generally cylindrical set of coil windings  10 . 
     The generally cylindrical set of coil windings  10  shown in  FIGS. 1 and 2  is configured with the “fingerprints” of the primary and shield coil windings  12 ,  14  aligned vertically. This generates the transverse magnetic field gradient G y  along the vertical direction, corresponding to the conventional “y” coordinate of a typical magnetic resonance scanner. In a typical arrangement, a corresponding set of windings rotated 90° respective to the illustrated generally cylindrical set of coil windings  10  is provided to selectively generate a magnetic field gradient along an “x” direction transverse to the “y” direction. It will be noted that the generally cylindrical set of coil windings  10  is rotatable such that the illustrated magnetic field gradient can be aligned with the “y” direction as shown, or with the aforementioned “x” direction, or with any other direction transverse to the axial direction. The illustrated alignment along the “y” direction, or along the “x” direction as suggested here, is convenient in that it matches conventional Cartesian x-y-z coordinates sometimes used in magnetic resonance imaging; however, the generally cylindrical set of coil windings  10  can have any orientation. 
     The precise configuration of the generally cylindrical set of coil windings  10  is designed to provide good magnetic field gradient uniformity at least across the region R of interest. Such design is suitably performed using a stream function approach, as described for example in Peeren, “Stream Function Approach for Determining Optimal Surface Currents”, Journal of Computational Physics vol. 191 pages 305-21 (2003) and in “Stream Function Approach for Determining Optimal Surface Currents”, Doctoral Thesis of Gerardus N. Peeren (Eindhoven University of Technology 2003), both of which are incorporated herein by reference in their entirety. The stream function approach determines a continuos current density distribution, represented by a stream function, that provides a specified magnetic field distribution, and then discretizes the obtained stream function to obtain the coil windings distribution. 
     With reference to  FIGS. 3 and 4 , a second magnetic field gradient coil embodiment is similar to the coil embodiment of  FIGS. 1 and 2 , and includes a generally cylindrical set of coil windings  30  defining the axial direction D A  and including primary coil windings  32  and shield coil windings  34  at a larger radial position than the primary coil windings. The generally cylindrical set of coil windings has an arcuate or annular central gap  36  that is free of coil windings. The arcuate or annular central gap spans at least a 180° angular interval. In the embodiment shown in  FIGS. 3 and 4  the central gap  36  is an annular gap that spans a full 360° so as to space apart two sub-sets  40 ,  42  of the generally cylindrical set of coil windings  30  each including primary coil windings and shield coil windings at a larger radial position than the primary coil windings. The generally cylindrical set of coil windings  30  includes connecting conductors  44  disposed at each edge of the central gap  36  that electrically connect selected primary and secondary coil windings. The embodiment of  FIGS. 3 and 4  further includes connecting conductors  46  disposed at the ends of the coil windings  30  distal from the central gap  36 . The connecting conductors  46  also electrically connect selected primary and secondary coil windings the selected windings being potentially the same as, or different from, the selected windings connected proximate to the central gap  36  by the connecting conductors  44 . Moreover, some primary windings or secondary windings may be isolated windings that are not connected by any of the connecting conductors  44 ,  46 . 
     The primary coil windings  32  of the generally cylindrical set of coil windings  30  are disposed at a non-constant smaller radial position over a selected angular interval θ T  to define an approximately planar surface S T . While the surface S T  is approximately planar, it may have some bowing or curvature as shown in  FIG. 3 . The effect is that the primary coils in the selected angular interval θ T  are moved upward to be closer to the subject. For spine imaging, for example, this closer positioning of the primary coils in the selected angular interval θ T  enhances radio frequency coupling with the spine when the subject lies on a generally planar subject support overlaying the primary coils in the selected angular interval θ T . 
     The generally cylindrical set of coil windings  30  of  FIGS. 3 and 4  is configured with the “fingerprints” of the primary and shield coil windings  32 ,  34  rotated by about 45° away from the vertical. This generates a transverse magnetic field gradient G y ′ oriented at about a 45° angle respective to the vertical. The windings are designed using the stream function approach so that the transverse magnetic field gradient G y ′ is substantially uniform at least within a region R′ of interest. Using this approach with the illustrated 45° rotation of the gradient field, a transverse gradient coil was designed to have the annular central gap  36  with width W of twenty centimeters. 
     With reference to  FIG. 5 , the two sub-sets  40 ,  42  of the generally cylindrical set of coil windings  30  are suitably supported by or in two respective dielectric formers  50 ,  52  that are spaced apart by about the gap width W. The two sub-sets  40 ,  42  of the gradient coil assembly  30  and their respective carriers  50 ,  52  are prone to mechanical canting caused by substantial Lorentz forces generated during operation of the generally cylindrical set of coil windings  30 . To combat this, a stiffening brace  54  optionally spans the annular central gap  36  to substantially rigidly connect the two spaced apart generally cylindrical dielectric formers  50 ,  52 . In the embodiment of  FIG. 5 , the brace  54  is arcuate and spans about 180°. In other embodiments, a complete annular brace is contemplated. In some embodiments to be described, components such as a radio frequency coil, an array of positron emission tomography (PET) detectors, or so forth are contemplated to be disposed in the central gap  36 . In some such embodiments, the optional brace  54  may include openings  56  providing pass-throughs for mounting members that provide independent support for such components. 
     With reference to  FIG. 6 , for example, the dielectric formers  50 ,  52  are disposed in a magnetic resonance scanner  60  that includes main magnet windings  62  disposed in a cryogenic housing  64  defining a main magnet producing the static axially oriented B 0  magnetic field in the region R′ of interest. An annular ring of positron emission tomography (PET) detectors  66  are disposed in the annular central gap  36  of the generally cylindrical set of coil windings  30 , that is, in the gap between the dielectric formers  50 ,  52  that support the coil windings  30 . A brace  54 ′, which in this embodiment is an annular brace, lies outside the dielectric formers  50 ,  52  and outside the annular ring of PET detectors  66 . Independently supported mounting members  68  pass through the openings  56  in the brace  54 ′ and openings  69  in the magnet housing  64  to support the annular ring of PET detectors  66  independently from the dielectric formers  50 ,  52  supporting the generally cylindrical set of coil windings  30  (not shown in  FIG. 6 ). Such independent support is advantageous because the gradient coils move and accelerate during operation due to Lorentz forces, and such movement, if transferred to the PET detectors  66 , would result in degradation of PET images acquired using the PET detectors  66 . In some embodiments, the dielectric formers  50 ,  52  and the brace  54 ′ define a stiff unit that is vibrationally isolated from the annular ring of PET detectors  66  and the mounting members  68 . In one suitable approach, the stiff unit  50 ,  52 ,  54 ′ is mounted to the magnet housing  64  which in turn is mounted to a floor of a room. The mounting members  68  are independently mounted to the floor, walls, and ceiling of the room. This provides the desired vibrational isolation because the floor of the room is massive enough that it absorbs vibrations generated in the stiff unit  50 ,  52 ,  54 ′ by the generally cylindrical set of coil windings  30  so that these vibrations are not transferred to the mounting members  68 . 
     The outer support (not shown) to which the mounting members  68  connect can be a sub-frame completely surrounding the magnetic resonance scanner  60 , a set of hard points on the walls of the room containing the scanner  60 , or so forth. In order to enable the mounting members  68  to pass through the magnet housing  64  while enabling the housing  64  to maintain vacuum and cryogenic reservoir integrity, the mounting members  68  suitably pass through openings  69  formed as tubular pass-through regions whose ends are sealed to maintain vacuum and helium can integrity. Optionally, a radio frequency screen (not shown in  FIG. 6 ) can extend into the openings  69  to provide RF isolation. Additional pass-through openings can be provided for electrical and other connections to the annular ring of PET detectors  66 . As the pass-through openings are relatively small, they can be interspersed amongst the main magnet windings  62  so that the magnetic design of the main magnet is substantially unaffected. 
     The generally cylindrical sets of coil windings  10 ,  30  have complete annular gaps  16 ,  36  which advantageously can receive an annular component such as the annular array of PET detectors  66  as shown in  FIG. 6 . Having the illustrated complete annular array of PET detectors  66  provides better image resolution and image quality as compared with a less complete array of PET detectors, such as an arcuate array that spans less than a complete 360°. However, better magnetic gradient uniformity and coil efficiency can be achieved by having an arcuate gap that does not extend the full 360°. 
     With reference to  FIG. 7 , a third magnetic field gradient coil embodiment is similar to the second gradient coil embodiment of  FIGS. 3 and 4 , and includes a generally cylindrical set of coil windings  80  defining the axial direction D A  including primary coil windings  82  and shield coil windings  84  at a larger radial position than the primary coil windings. The generally cylindrical set of coil windings has an arcuate or annular central gap  86  that is free of coil windings. The arcuate or annular central gap spans at least a 180° angular interval. In the embodiment shown in  FIG. 7  the central gap  86  is an arcuate gap of axial extent W that spans an angular interval of greater than 180° but less than 360°, there being coil windings disposed over the complementary angular interval θ C  not spanned by the central gap  86 . The generally cylindrical set of coil windings  80  includes connecting conductors  94  disposed at each edge of the central gap  86  that electrically connect selected primary and secondary coil windings. The embodiment of  FIG. 7  further includes connecting conductors  96  disposed at the ends of the coil windings  80  distal from the central gap  86 . The connecting conductors  96  also electrically connect selected primary and secondary coil windings the selected windings being potentially the same as, or different from, the selected windings connected proximate to the central gap  86  by the connecting conductors  94 . Moreover, some primary windings or secondary windings may be isolated windings that are not connected by any of the connecting conductors  94 ,  96 . 
     The primary coil windings  82  of the generally cylindrical set of coil windings  80  are disposed at a non-constant smaller radial position over the complementary angular interval θ C  to define an approximately planar surface S C . While the surface S C  is approximately planar, it may have some bowing or curvature as shown in  FIG. 7 . The effect is that the primary coils in the selected angular interval θ C  are moved upward to be closer to the subject. For spine imaging, for example, this closer positioning of the primary coils in the complementary angular interval θ C  enhances radio frequency coupling with the spine when the subject lies on a generally planar subject support (not shown) overlaying the primary coils in the complementary angular interval θ C . The arcuate gap  86  is advantageous for spine imaging because it does not extend under the spine—rather, gradient coil windings of the primary and shield coil windings  82 ,  84  are continuous under the spine for typical spinal imaging arrangements in which the subject lies supine during the spinal imaging. In  FIG. 7 , a dielectric former  100  supporting the coil windings  82 ,  84  is shown in a wire frame representation. 
     Preliminary calculations for a coil configured in accordance with the design of  FIG. 7  have shown that for a gradient gap width W of twenty centimeters in the upper region and a gradient bore size of seventy-two centimeters, a stored energy of less than 4.4 J can be expected at 10 mT/m. In this coil design, the zero-level of the magnetic field gradient is vertically offset from the mechanical z-axis or isocenter of the surrounding cylindrical (B 0 ) magnet by about 10-15 centimeters. This offset is recognized herein as improving the efficiency of the gradient coil. In contrast, if the zero-level of the magnetic field gradient precisely coincides with the mechanical z-axis or isocenter of the cylindrical (B 0 ) magnet, then relatively more windings are included in the upper part of the coil and fewer in the complementary angle interval θ C , but overall more ampere-turns are included and the stored energy is higher. These considerations also apply to the design of the generally cylindrical set of coil windings  30  in which the central gap  36  is annular. 
     The illustrated arcuate gap  86  has a constant width W (except over the complementary angular interval θ C  where the gap is absent). However, it is contemplated for the width of the gap to vary with angular position to trade off between magnetic field gradient performance of the magnetic field gradient coil (improved by having a smaller gap) and RF performance of the radio frequency coil (improved by having a larger gap and concomitantly axially longer upper conductors). 
     With continuing reference to  FIG. 7  and with further reference to  FIG. 8 , a further advantage of the arcuate gap  86  is that it comports with a radio frequency coil  110  that is designed for spine imaging. In  FIG. 8 , the dielectric former  100  is shown, along with axially oriented conductors of the radio frequency coil  110 . To conserve bore space, upper axially oriented conductors  112  are disposed in the arcuate central gap  86  and are substantially axially coextensive with the axial extent W of the arcuate central gap  86 . This arrangement efficiently uses the available bore space by placing the coil windings  82 ,  84  and the upper axially oriented conductors  112  at about the same radial position. Moreover, the upper axially oriented conductors  112  are located relatively far away from the spine, that is, relatively far away from the region of interest for spinal imaging. On the other hand, lower axially oriented conductors  114  are positioned in the region of the complementary angular interval θ C  where the primary coil windings  82  are raised to conform with a generally planar subject support (not shown). In this region, the lower axially oriented conductors  114  are positioned above the primary coil windings  82  and are substantially longer than the axial extent W of the central gap  86 . The longer lower axially oriented conductors  114  advantageously provide a larger and more uniform field of view of the proximate spinal region of interest. In some contemplated embodiments, the lower axially oriented conductors  114  of the radio frequency coil  110  are configured as a SENSE-capable receive array. For this purpose, the lower axially oriented conductors  114  can optionally be segmented along the axial direction. Such a configuration is suitably operated using multi-point excitation, for example by a power splitter or with multiple power amplifiers. 
     With returning reference to  FIG. 6  and with further reference to  FIG. 9 , a suitable embodiment of the annular ring of PET detectors  66  is further described.  FIG. 9  shows a sectional view of a portion of the PET detectors  66  disposed in the central gap  36  of the generally cylindrical sets of coil windings  30 , between the inner surface of the cylindrical magnet housing  64  and a radio frequency screen  116  of an optional birdcage-type radio frequency coil (a rung  118  of which is visible in the diagrammatic sectional view of  FIG. 9 ). At least in the area of the PET detectors  66 , the radio frequency coil components  118  are made of thin copper strips without capacitors to reduce the scattering of gamma particles. For example, copper strips of thickness five or six times greater than the RF skin depth (e.g., about six microns for  1 H magnetic resonance in a 3-Tesla scanner) are suitable. The radio frequency screen  116  is similarly made of a thin conductive foil or mesh that is substantially transparent to gamma particles. 
     The PET detectors  66  include an array of scintillators  120  that are viewed by an array of photodetectors  122 . In some embodiments, the photodetectors  122  are silicon photomultipliers (SiPM&#39;s). Some suitable SiPM devices are described in Frach et al., WO 2006/111883 A2 and in Fiedler et al., WO 2006/111869 A2, both of which are incorporated herein by reference in their entireties. The photodetectors  122  are electrically connected with time domain conversion (TDC)/analog-to-digital conversion (ADC) electronics  124  that convert radiation detection events into digital data including digitized intensity information corresponding to the detected particle energy and a digital timestamp indicating the detection time. In some embodiments, SiPM detectors  122  and TDC/ADC electronics  124  are monolithically integrated on common silicon substrates. In some other embodiments, some or all of the TDC and/or ADC processing is disposed remotely away from the scanner. In yet other contemplated embodiments, the annular ring of PET detectors disposed in the gap  36  includes only the scintillators  120  and coupled optical fibers that transmit scintillation light off of the scanner to remotely located photodetectors and associated remotely located TDC/ADC electronics. 
     The radiation detection hardware  120 ,  122 ,  124  is disposed in light shielding  130  (indicated by a solid line) to avoid spurious detection of light photons, and inside of a galvanic isolation container  132  (indicated by a dashed line) such as a radio frequency screen to suppress radio frequency interference. The galvanic isolation container  132  provides broadband RF shielding, whereas the radio frequency screen  116  is a low pass filter that provides RF shielding at the magnetic resonance frequency and allows the pulsed magnetic field gradients to be substantially unaffected by the RF shielding. Power and communication cabling  134  is suitably run outside of the radio frequency screen  116  to keep these cables outside of the high RF field. 
     To suppress interaction of the PET detectors  66  with the magnetic field gradients, the stiff brace  54 ′ that secures the generally cylindrical set of coil windings  30  also defines a thick copper shield  54 ′. This shield  54 ′ is mechanically connected with the magnet housing  64 , and includes extensions  140  into the central gap  36  of the generally cylindrical set of coil windings  30  to enhance shielding of the PET detectors  66  against the generated magnetic field gradients. The thick copper shield  54 ′ is either left open in front of the PET detectors  66  to avoid blocking gamma particles, or includes a thinned front portion  142  that is substantially transmissive to gamma particles. The various shielding components can be variously combined—for example, it is contemplated in some embodiments to integrate the thinned front portion  142  of the gradient shield with the galvanic isolation container  132 . Moreover, selected shielding components are optionally omitted (possibly at the cost of higher interaction between the magnetic resonance and PET components). As noted previously with reference to  FIG. 6 , the mounting members  68  pass through openings  69  in the magnet housing  64  and openings  56  in the shielding and mechanical bracing component  54 ′ to independently support the PET detectors  66 . 
     With reference to  FIG. 10 , in an alternative embodiment vibrational isolation of the PET detectors  66  is achieved by using compensatory piezo-actuators  150  disposed between the annular ring of PET detectors  66  and the support (e.g., the dielectric formers  50 ,  52  and the stiff brace  54 ′) of the generally cylindrical set of coil windings  30  to support the PET detectors  66  while vibrationally isolating the PET detectors  66  from the generally cylindrical set of coil windings  30 . The piezo-actuators  150  are operatively coupled with acceleration sensors  152 , such as MEMS-based accelerometers, and are configured in a feedback loop to adjust the piezo-actuators  150  to minimize acceleration of the PET detectors  66  as indicated by the acceleration sensors  152 . In some embodiments, the piezo-actuators  150  and the acceleration sensors  152  are monolithically integrated as a single unit, for example formed into or on a silicon substrate. Because the acceleration levels are large but the displacements are of order a few millimeters or less, tolerances of a as small as a few microns between edges of the central gap  36  and the PET detectors  66  are contemplated. In some embodiments, it is contemplated to include openings in the containment formed by the brace  54 ′ and the walls of the central gap  36  to provide fluid communication to avoid air compression during rapid acceleration. Additionally or alternatively, larger tolerances can be used to provide air cushioning. The array of piezo-actuators  150  preferably provides acceleration suppression along three displacement degrees of freedom (e.g., along the three orthogonal coordinates of a Cartesian system) and three rotational degrees of freedom. Advantageously, by using the piezo-actuators  150  or vibration isolation it is possible to eliminate the independent mounting members  68  and corresponding openings  56 ,  69  in the stiff brace  54 ′ and magnet housing  64 . 
     The scanner of  FIG. 6  is suitably operated as a hybrid scanner. Because the annular array of PET detectors  66  is disposed in the central gap  36  of the generally cylindrical magnetic field gradient windings  30 , the field-of-view (FOV) for PET imaging is substantially centered at the same position as the FOV for magnetic resonance imaging, although some PET/MR FOV offset in the axial direction and/or transverse to the axial direction is contemplated. The FOV for PET imaging can be the larger, smaller, or the same size as the FOV for magnetic resonance imaging. In some approaches, magnetic resonance and PET imaging are done in succession, or PET and magnetic resonance imaging periods are interleaved in time. In other embodiments, it is contemplated to perform PET imaging and magnetic resonance imaging simultaneously. 
     In the illustrated embodiments, the annular ring of PET detectors  66  advantageously provides a full 360° angular coverage for data collection. As is known in the art, less than full 360° coverage tends to lead to image artifacts resulting from missing lines of response due to the missing angular span of detectors. However, it is contemplated to use an arcuate set of PET detectors that spans at least a 180° angular interval but less than a full 360°. For example, such an arcuate set of PET detectors may be inserted into the arcuate gap  86  of the generally cylindrical sets of coil windings  80 . The missing lines of response can be compensated by acquiring additional information via time-of-flight localization along the lines of response. Other configurations of PET detectors are also contemplated, such as a plurality of interrupted angular spans of PET detectors that collectively provide at least 180° of angular coverage. 
     In some embodiments, it may be desired to include shim coils for correcting the magnetic field gradients for loading effects on the static (B 0 ) magnetic field. While first order magnetic field shims are advantageous, second order shim sets provide more control for shimming the (B 0 ) magnetic field. In hybrid embodiments, the annular ring of PET detectors  66  has the potential to produce further magnetic field inhomogeneity which may also be correctable using second order active coil shim sets. 
     With reference to  FIGS. 11 and 12 , some second order shim coils include portions passing through the scanner center (that is, the shim coils cross the axial plane denoted z=0 in  FIGS. 11 and 12 ).  FIGS. 11 and 12  plot selected Golay coil type shim coil patterns over ½ of the azimuthal range, that is, between azimuthal values between φ=−90° and φ=90°, with the azimuthal dimension unrolled to provide 2-D plots. For example,  FIG. 11  plots Golay coil type shim sets for the zx second order shim coil set ZX and for the z 2  second order shim coil set Z 2 . The ZY second order shim coil set ZY is also indicated in dashed lines, and is identical to the ZX second order shim set ZX except that it is rotated 90° in the azimuthal (φ) direction.  FIG. 12  plots the z 2  second order shim coil set Z 2  and also the (X 2 -Y 2 ) second order shim coil set X 2 -Y 2 . It will be noted that the (X 2 -Y 2 ) and Z 2  second order shim sets have a central gap at Z=0; accordingly, these shim coils could conceivably be mounted on the dielectric former portions  50 ,  52  of  FIG. 6  as long as the central gap is small enough. However, the ZX and ZY second order shim sets have no central gap and are centered on and cross the z=0 plane. Accordingly, the ZX and ZY second order shim sets cannot be mounted on the dielectric former portions  50 ,  52 . 
       FIG. 13  plots a side sectional view of the gradient coil assembly of the hybrid PET/magnetic resonance scanner of  FIG. 6 , showing one suitable arrangement of second order shim coils. In this embodiment the second order shim coils are divided into first and second groups  200 ,  202  that are disposed on or in the two dielectric former portions  50 ,  52 , respectively, between the primary and shield gradient coil windings  32 ,  34 . This arrangement is similar to that of second order shim coils in existing magnetic resonance scanners, in which the second order shim coils are disposed between the primary and shield gradient coil windings. To span the central gap, a central third group  204  of second order shim coil windings are disposed on the mechanical brace  54 ′ that provides support for the dielectric former portions  50 ,  52  and maintains the spacing and relative positioning of the dielectric former portions  50 ,  52  in the presence of Lorentz forces induced by energizing of the gradient windings. The shim set  200 ,  202 ,  204  can be similar to a Golay coil type shimset for second order gradients. The shim coil conductors of the central group  204  are suitably locally perturbed or routed to avoid the openings  56  in the mechanical brace  54 ′ that provide access for the mounting members  68  (shown in  FIG. 6 ). Additionally or alternatively, the openings  56  can be located to avoid the shim conductors of the third shim coils group  204 . The shimset in the embodiment illustrated in  FIG. 13  has certain shim coils located between the primary and shield gradient coil windings  32 ,  34 ; alternatively portions of the shim windings may be disposed outside of the shield gradient coil windings  34 . 
     The brace  54 ′ overlaps the dielectric former portions  50 ,  52  in order to provide for a secure connection of the brace  54 ′ to the former portions  50 ,  52 . In some embodiments, this overlap and the axial extent W of the central gap is such that the ZX and ZY second order shimming windings can be disposed entirely in the central group  204 , and the Z 2  and (X 2 -Y 2 ) second order shim windings can be disposed entirely in the first and second groups  200 ,  202 . In this case, the first and second groups  200 ,  202  and the third group  204  are advantageously operationally separate. On the other hand, in some embodiments the overlap of the brace  54 ′ with the dielectric former portions  50 ,  52  may be too small, or the axial extent W of the central gap may be too wide, to enable such a convenient separation of the shim coil sets. In this latter case, jumper conductors (not shown) electrically connect windings of the first and second groups  200 ,  202  and the third group  204 , for example to interconnect portions of a ZX shim coil residing in part on each of the three shimset groups  200 ,  202 ,  204 . The difference in radial positions of the first and second groups  200 ,  202 , on the one hand, and the central third group  204  on the other hand is relatively small (e.g., of order equal to the combined thicknesses of the shield gradient windings layer  34  and the thickness of the brace  54 ′), and so the electrical jumpers can be made relatively short. The shimset  200 ,  202 ,  204  is a three-dimensional shimset, and can be designed using stream function approaches as described in the Peeren references already cited and incorporated herein by reference. 
     As yet another approach (not illustrated herein), it is contemplated to design the second order shimset coils with the central gap (that is, with no windings extending into the axial extent W of the central gap) and to design the coils to provide the desired second order magnetic shimming fields using the stream function approach. If the axial extent W of the central gap is small enough, this approach is expected to be feasible even for ZX and ZY shim coils. By defining the ZX or ZY shim coil to include windings on two spaced apart radial surfaces connected at the edge of the central gap by connectors similar to the connectors  44  for the gradient windings, the ZX or ZY shimming current at the edge of the central gap can be made nonzero, thus providing flexibility in the stream line design optimization to compensate for the windings missing in the central gap. 
       FIG. 14  plots a side sectional view of the gradient coil assembly of the hybrid PET/magnetic resonance scanner of  FIG. 6 , modified in that the two dielectric former portions  50 ,  52  are replaced by a single cylindrical dielectric former  50 ″ that supports the primary and shield gradient coil windings  32 ,  34  and has a central annular indentation or slot that receives the annular ring of PET detectors  66 . In this arrangement the brace  54 ′ can be omitted in its entirety. Alternatively, if the central annular indentation or slot receiving the PET detectors  66  makes the dielectric former  50 ″ too mechanically weak, then a reinforcing outer cylindrical brace  54 ″ can be disposed around the dielectric former  50 ″. Second order shimset windings  210  can be disposed on the outer surface of the dielectric former  50 ″ or on the outer surface of the optional reinforcing outer cylindrical brace  54 ″. In this embodiment, standard Golay second order shim windings can be used, optionally with some rerouting or distortion of the windings in the vicinity of the openings  56  receiving the supports PET detectors ring  66 . 
     With continuing reference to  FIGS. 13 and 14 , a shimset controller  220  applies electrical current to selected second order shims of the shimset to produce desired second order shimming. The shimsets  200 ,  202 ,  204 ,  210  can be configured and energized by the shimset controller  220  to correct for subject loading inhomogeneities, either in a static fashion or dynamically during magnetic resonance acquisition pulse sequences. Additionally, the shimsets  200 ,  202 ,  204 ,  210  can be configured and energized by the shimset controller  220  to correct for inhomogeneities introduced by the annular ring of PET detectors  66 . These latter inhomogeneities may depend upon the operational state of the PET detectors. The non-operational PET detectors can be expected to introduce some magnetic field inhomogeneities due to the presence of electrically conductive components in the PET detectors that may have a weak or residual effect on the static magnetic field. During PET acquisition, the operating PET detectors are electrically biased and electrical currents flow in the PET detectors and related circuitry. These operational aspects can introduce additional magnetic field inhomogeneities. Accordingly, in some embodiments the shimset controller  220  applies shim currents that are calibrated for the operational and nonoperational states of the PET detector ring  66 , respectively, and the appropriate shim currents calibration is used during simultaneous MR/PET or MR-only imaging, respectively. 
     Further, the calibration examination volume may be differently selected for the operational and nonoperational states of the PET detector ring  66 , respectively. For example, if the PET system has a smaller field of view than the magnetic resonance scanner, then the shims calibration for the simultaneous MR/PET operation may shim the magnetic field to be uniform within a smaller examination region sized to match the relatively small PET examination region. By calibrating the shimming for the smaller PET examination volume, improved magnetic field uniformity is expected to be achievable, albeit only in the PET examination region. This spatial limitation is acceptable for simultaneous PET/MR imaging, since typically only the region that is imaged by both PET and MR is of interest in this case. On the other hand, during MR-only operation it may be advantageous for the MR-only shims calibration to shim the entire larger magnetic resonance examination volume. 
     The illustrated shimsets  200 ,  202 ,  204 ,  210  are configured to comport with the split gradient coil in which the central gap is an annular gap completely dividing the gradient windings into two separate sections. However, the illustrated shimsets are readily adapted for use in conjunction with a coil such as that shown in  FIG. 7  in which the central gap is an arcuate but not a complete annular gap. 
     The invention has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.