WO2005045447A2 - Gradient coil arrangement and method for using the same - Google Patents
Gradient coil arrangement and method for using the same Download PDFInfo
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- WO2005045447A2 WO2005045447A2 PCT/US2004/037054 US2004037054W WO2005045447A2 WO 2005045447 A2 WO2005045447 A2 WO 2005045447A2 US 2004037054 W US2004037054 W US 2004037054W WO 2005045447 A2 WO2005045447 A2 WO 2005045447A2
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/385—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using gradient magnetic field coils
Definitions
- This invention relates generally to magnetic resonance arrangement and method, and more particularly, to gradient coil arrangement for improving magnetic resonance imaging and to methods for providing such arrangements.
- linear magnetic field gradients can be used for spatial encoding.
- Gradient coils are generally used to produce the linear magnetic field gradients.
- the gradient coils can be designed to provide an imaging field-of-view that may be fixed in size.
- the gradient coil may be designed to produce sufficiently linear or uniform magnetic field gradients over a volume that is larger than e.g., the volume for a dedicated cardiac scanner. As the useful volume is decreased, the stored energy of the gradient coil is generally reduced, which may allow for a higher system performance, and particularly, provide higher peak gradient strengths and faster gradient coil switching.
- the magnetic field gradients produce image distortion.
- the gradient coils are heavy electromechanical devices, unlike most RF surface coils, which can be easily removed and replaced with different RF coils between imaging procedures.
- a gradient coil due to its high power nature and the high forces created when it is energized, is generally firmly fixed within an MRI system. As such, a dedicated gradient coil tends to make the MRI system a special- purpose imaging system, thus limiting its scope and use for clinical application. Thus, a given geometry gradient coil may result in a corresponding field of view.
- MRI systems and methods use the magnetic field strength dependence of the frequency of magnetic resonance, in conjunction with computer-controlled magnetic field gradients, to produce images based on the magnetic resonance signal.
- the mapping of position into frequency through the gradient can be used for a reconstruction of the image from signals that are detected in the presence of the gradients or after the gradients have been applied (e.g., using "frequency” or "phase” encoding, respectively).
- the excitation of the magnetic resonance in the presence of the gradient can be used for a selective excitation of a "slice" or "slab” to be imaged.
- gradients in MRI systems and methods include a suppression of unwanted signals by "spoiling" excited resonance and a sensitization of the signal to particular aspects of motion, such as a bulk velocity or a diffusion.
- conventional magnetic resonance imaging systems include magnetic field gradient-generating coils, such coils are generally designed to have very uniform magnetic field gradients (i.e., a very linear field variation with position) over a relatively large volume of the imaging system magnet bore. These coils allow for a flexibility of choice of structures to be imaged with faithful reproduction of the subject's geometry; however, there are some disadvantages associated therewith. First, the practically achievable strength of the gradient field is limited because of the high power demands that are generally used to fill a large region with a uniform gradient.
- the achievable image resolution and the ability to use strong gradients for other purposes, such as measuring diffusion are limited.
- the achievable gradient switching speeds are also limited, because of power limitations (which are greater for a larger region of uniform gradient) and because of the physiologic limits on the allowable local rate of change of the magnetic field (which are more restrictive for larger regions of uniform magnetic field gradient).
- the imaging speed and the ability to use the rapidly switched gradients for other purposes are limited.
- an exemplary embodiment of the present invention can improve the local MRI by imaging a restricted or predefined portion of the subject, when such imaging is desired.
- the continual need to use gradients that are uniform over large regions of the subject is reduced or even eliminated.
- the exemplary embodiment also overcomes several disadvantages, as are mentioned above, when gradients are designed to be uniform over large regions.
- the imaging speed can be significantly improved (e.g., due both to more rapid switching of the gradients achievable and to less problems due to aliasing), and the imaging resolution may be improved (e.g., due to higher strength gradients achievable in the more restricted region being imaged).
- Other benefits of the more rapidly switched and stronger localized gradients can also be realized, such as a better diffusion weighting to study diffusion, and a more effective homogeneity spoiling to suppress an unwanted magnetization excitation.
- the cost of the improved performance may include additional image processing so as to remove distortion, but that can be readily and rapidly carried out.
- MRI may be performed using the gradient coils that may be designed to produce a strong gradient e.g., only over a relatively restricted region, (e.g., a "ZOnally
- OptiMized ZOOMTM gradient coil may result in a distorted image when implemented with conventional image reconstruction methods to obtain a good resolution, e.g., only in the region of the strong gradient.
- the portion of the subject which it is desired to image can be positioned within this region, the distortion of further remote regions would likely not pose a practical problem for the image interpretation.
- the spatial distribution of the gradient strength is previously known (e.g., can be determined by the geometry of the coil), and thus the resulting image distortion of the further remote regions is known, the images can be corrected for this distortion, and the correct image geometry can be recovered even if the effective local spatial resolution is still non-uniform.
- the nominal plane of the image may be distorted due to the nonuniformity of the gradient in conjunction with a selective excitation.
- the prior knowledge of the gradient configuration can be used to effectively re-sample the reconstructed region into corrected images using flat planes. Due to the fact that a gradient field that is strong only over a restricted region requires less power to generate, and can be switched more rapidly, images may be acquired more rapidly or with a higher resolution than would otherwise be achievable using conventional highly uniform gradient coils.
- the gradient coil can be used with relatively limited regions of gradient uniformity (e.g., the linear variation of the axial component of the magnetic field) so as to produce a locally-stronger gradient strength than could be achieved with conventional coils, which may be designed to have large regions of gradient uniformity when used in MRI.
- These coils can be used in conjunction with tailored image processing techniques which are designed to correct for the resulting distortions in image geometry, while retaining the potential for locally greater image resolution.
- a gradient coil may be provided for a production of a strong magnetic field gradient along the axis of an MRI system, which can include two "solenoidal-type" coils with oppositely directed current flows (and corresponding oppositely directed local longitudinal magnetic fields), coaxially arranged and axially offset with a short gap therebetween.
- the coil arrangement maybe in a form of either with a helical conductor arrangement or with an arrangement of approximately circular arcs of the conductor, as described herein below.
- a gradient coil can be provided for a generation of a magnetic field gradient that is transverse to the axis of the MRI system, such gradient coil can include at least two sets of approximately parallel arcs with oppositely directed current flows, which can effectively act as a split longitudinally solenoidal coil, with oppositely directed currents in the two fractions.
- this exemplary design of the coil can provide a very homogeneous transverse gradient field across the width of the MRI system, which may have advantages over the conventional transverse gradient coil designs.
- a method and arrangement for adjusting the effective geometry of the current-carrying components of a gradient coil using electrically switchable conducting elements are provided. Such arrangement may permit the adjustment of the characteristics of the locally optimized gradient field. Such arrangement may also permit the use of the same physical coil, in either a conventional mode (with a large homogeneous gradient region) or in the locally concentrated gradient mode as described above.
- Fig. 1 shows an isometric view of a conventional solenoid coil.
- Fig. 2A shows an isometric view of a first exemplary embodiment of a coil arrangement according to the present invention.
- Fig. 2B shows an isometric view of a second exemplary embodiment of the coil arrangement of the present invention.
- Fig. 3 A shows an isometric view of a third exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 3B shows an isometric view of a fourth exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 4A shows an isometric view of a fifth exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 1 shows an isometric view of a conventional solenoid coil.
- Fig. 2A shows an isometric view of a first exemplary embodiment of a coil arrangement according to the present invention.
- Fig. 2B shows an isometric view of a second exemplary embodiment of the coil arrangement of the present invention.
- FIG. 4B shows a graph illustrating a magnetic field associated with the coil arrangement of Fig. 4A.
- Fig. 5A shows a simulation generated by MATLAB software associated with the coil arrangement of Fig. 4A.
- Fig. 5B shows a graph of the magnetic field in an magnetic resonance imaging (MRI) machine along the z axis of a simulation of Fig. 5 A.
- Fig. 5C shows a graph of the z gradient along the z axis, produced by the simulation of Fig. 5A.
- Fig. 6A shows an isometric view of a seventh exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 6B shows a graph illustrating the magnetic field associated with the coil arrangement of Fig. 6A.
- FIG. 6C shows an isometric view of an eighth exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 7A shows a simulation generated by the MATLAB software associated with the coil arrangements of Fig. 6A.
- Fig. 7B shows a graph of the magnetic field in the MRI machine along the z axis of the simulation of Fig. 7A.
- Fig. 7C shows a graph of the z gradient along the z axis, produced by the simulation of Fig. 7A.
- Fig. 8 A shows an isometric view of a ninth exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 8B shows an isometric view of an tenth exemplary embodiment of the coil arrangement according to the present invention.
- FIG. 9 shows an isometric view of a eleventh exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 10A shows a simulation generated by the MATLAB software associated with the coil arrangement of Fig. 9.
- Fig. 10B shows a graph of the magnetic field in the MRI machine across the axis, of the simulation of Fig. 10A.
- Fig. IOC shows a graph of the x gradient along the x axis, produced by the simulation of Fig. 10A.
- Fig. 11 shows an isometric view of an twelfth exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 12 shows an isometric view of a thirteenth exemplary embodiment of the coil arrangement according to the present invention.
- Fig. 10A shows a simulation generated by the MATLAB software associated with the coil arrangement of Fig. 9.
- Fig. 10B shows a graph of the magnetic field in the MRI machine across the axis, of the simulation of Fig. 10A.
- FIG. 13A shows a simulation generated by the MATLAB software associated with the coil arrangement of Fig. 12, with asymmetric arcs.
- Fig. 13B shows a graph of the magnetic field in the MRI machine across the axis, of the simulation of Fig. 13 A.
- Fig. 13C shows a graph of the x gradient along the x axis, produced by the simulation of Fig. 13 A.
- gradient coils designed with restricted linear regions, as described herein, to similarly produce primary gradient directions along the nominal x, y, and/or z directions of the MRI system.
- conventional gradient coils are generally designed to produce large regions of a homogeneous gradient (i.e., a very linear variation of magnetic field with position along a particular direction over a large region)
- different coil arrangements may generate more restricted regions of the homogeneous gradient (i.e., magnetic fields that vary strongly with respect to position only over a more restricted region).
- These coil arrangements can vary for coils designed to produce gradients along the axis of the magnet and gradients that are perpendicular to the axis.
- Fig. 1 depicts a conventional solenoid coil 10.
- Figs. 2A to 7 depict a first exemplary embodiment of a coil arrangement according to the present invention.
- Each turn 26 of the coil arrangement 20 is provided in parallel with the turn on either side thereof, rather than being arranged in series as provided in the conventional arrangement illustrated in Fig. 1.
- the second section 28b may be located on a side of the coil arrangement that is opposite to the first section 24. Because each turn 26 of the coil arrangement 20 is arranged in parallel, for certain applications it may be desirable to monitor the relative resistance of parallel circuit branches.
- One exemplary method for monitoring the relative currents in each conducting path is to provide each such path with a small pickup coil (not shown herein). If an alternating-current test signal is injected into the main circuit, the resulting alternating voltage induced in each pickup coil may be proportional to the current (and inversely proportional to the net resistance) in the corresponding path being monitored. If the geometry of each such pickup coil is properly adjusted, equal currents may result in equal monitoring signals. Furthermore, any difference detected in the monitor signals may be used to adjust any trim resistors which can be provided (not shown herein) in each path in order to make the net resistances (and thus the net currents) in each path at least approximately equal.
- Fig. 3 A depicts a third exemplary embodiment of a coil arrangement 30 according to the present invention.
- Current may enter a first section 34 of the coil arrangement 30 in the direction of an arrow 32.
- the current may then pass through switches 39, which may be located between the first section 34 and a turn 36.
- the switches may be turned “on” thus allowing current to pass through them, or may be turned “off,” so that current is prevented from passing through them.
- Neighboring switches may be in different states, and this may allow a user (using a computer 35 or other suitable arrangement) to control the effective length and spacing of the turns of the coil arrangement 30.
- a particular switch 39 is turned “on,” current may then pass through the turn 36 of the coil ai ⁇ angement 30 associated with the switch 39, and may exit a second section 38 of the coil arrangement in the direction of the arrow 33.
- the second section 38 may be located on the side of the coil arrangement 30 that is opposite to the first section 34.
- a computer arrangement 35 is shown here in Fig. 3 A but may be applicable in all figures.
- Fig. 4A depicts a fifth exemplary embodiment of a coil arrangement according to the present invention.
- Two coils 40, 50 of this arrangement may be placed in an end - to - end configuration with opposite polarity (e.g., the direction of current that passes through turns 46 of the first coil 40 is opposite to the direction of current that passes through turns 56 of the second coil 50).
- a relatively localized region of varying field strength along the axis can be produced by constructing a composite longitudinal coil using the coil arrangement of Fig.4A. The generation of this field may result in a strong local field gradient in the longitudinal direction, without high fields being provided at a distance.
- two coils provided on a common axis e.g., aligned with the axis of the MRI system's magnet), with current flowing in opposite directions around them (positioned so that their ends are relatively close together), can produce such result.
- each separate component of the coil arrangement there may be produced an approximately uniform (and oppositely directed) local magnetic field.
- the field strength may vary rapidly with respect to a position along the axis thereof. However, the variation may be linear only in the center of this region.
- the illustration of such field strength is depicted in Fig. 4B.
- a point 45 of Fig. 4A at least approximately corresponds to an area 55 of the graph of Fig. 4B.
- Fig. 5A shows a simulation (created using a MATLAB product of the potential implementation) of the exemplary coil arrangement of Fig. 4A, illustrated with a gap 43 of 0.2.
- Fig. 5B depicts the magnetic field along the z axis of the coil arrangement shown in Fig. 5 A. Arrows depict the direction of the current flowing.
- the separation of the two coil elements 40, 50 can be varied to control the strength of the gradient and the effective length of its region in space.
- Fig. 5C depicts an exemplary z gradient along the z axis produced by the coil arrangement shown in Fig. 5A.
- Modified arrangements which include conventional solenoid coil, can also include localized gradient coil arrangements.
- two conventional solenoid coils provided on a common axis can produce similar results to those shown in Figs. 5B and 5C.
- Fig. 6A depicts a seventh exemplary embodiment of the coil arrangement according to the present invention.
- Two coils 60, 70 may be placed in an end - to - end configuration with an opposite polarity. This means that the direction of the current passing through turns 66 of a first coil 60 is opposite the direction of current passing through turns 76 of a second coil 70.
- the turns 66, 76 of both coils may have corresponding switches 69, which may either permit or prevent current from passing there through. Neighboring switches may be provided in different states, and thus allowing a user (using a computer or another suitable arrangement) to control an effective distance 63 between the innermost activated turns, the quantity of turns of activated coils, and/or the effective length of the coils.
- Fig. 6A depicts a seventh exemplary embodiment of the coil arrangement according to the present invention.
- inner turn 61 of the first coil/solenoid 60 and inner turn 71 of the second coil/solenoid 70 are shown as not being activated, and thus in this exemplary embodiment, the current does not pass therethrough because switches 67 and 77 are shown as being open.
- This arrangement may effectively produce a larger gap between the coils 60, 70.
- the field strength may vary rapidly with position along the axis, although the variation may be approximately linear only in the center of this region, as shown in Fig. 6B.
- a point 65 in Fig. 6A corresponds to an area 75 of the graph of Fig. 6B.
- Fig. 6C depicts an eighth exemplary embodiment of the coil arrangement according to the present invention which uses a conventional solenoid coil with switches 168 located so as to either permit or prevent current from passing there through.
- the arrows 162 depict the current flowing through the coil arrangement.
- Internal elements 164 of the switches 168 are shown schematically in Fig. 6C to illustrate how the current may pass through the windings of the coil arrangement.
- the inner windings 165 are shown as being not activated, and thus the results in the effective offset 163 as shown therein.
- a conventional solenoid coil can be used to dynamically control the effective offset and produce the magnetic field of the embodiment shown in Fig. 6A.
- Exemplary simulations of the implementations of the coil arrangements according to the present invention are shown in Figs. 7A, 7B, and 7C.
- Fig. 7 A is a simulation created in MATLAB of the embodiment of Fig. 6A, with a gap (63 in Fig. 6A) of 1.7. Arrows depict the direction of the current flowing.
- Fig. 7B depicts the magnetic field along the z axis of the exemplary coil arrangement of Fig. 7 A.
- Fig. 7C depicts the z gradient along the z axis, produced by the exemplary coil arrangement of Fig.
- FIG. 8A depicts an ninth exemplary embodiment of the coil arrangement 30 according to the present invention.
- Current may enter a first section 84 of a coil segment 80 in the direction of an arrow 82, pass through turns 86 of the coil arrangement 80, and may exit at a second section 88 of the coil arrangement 80 in the direction of an arrow 83.
- the second section 88b may be located on the side of the coil arrangement opposite to the first section 84.
- Fig. 9 depicts a eleventh exemplary embodiment of the coil arrangement according to the present invention.
- Two coil segments 90, 100 may be placed adjacent to one another with an opposite polarity, i.e., the direction of current passing through turns 96 of a first coil segment 90 is opposite the direction of current passing through turns 106 of a second coil segment 100.
- Such configuration may result in the generation of transverse gradients that are uniform throughout the lengths of the coil segments of the coil arrangement.
- the longitudinal gradient (in the Z direction) can be approximately zero for this coil arrangement.
- Figs. 10A, 10B, and IOC Exemplary simulations of the implementations of the coil arrangement according to the present invention are shown in Figs. 10A, 10B, and IOC.
- Fig. 10A is a simulation created in MATLAB of the embodiment of Fig. 9, with symmetric arcs.
- Fig. 10B depicts the magnetic field across the axis of the exemplary arrangement of Fig. 10 A.
- Fig. 10C depicts the x gradient along the x axis, produced by the exemplary arrangement of Fig. 10A.
- Fig. 11 depicts an twelfth exemplary embodiment of the coil arrangement according to the present invention.
- Two coil segments 110, 120 may be placed adjacent to one another with an opposite polarity i.e., the direction of current passing through turns 116 of a first coil segment 110 is opposite the direction of current passing through turns 126 of a second coil segment 120.
- the turns 116, 126 of both coils may have corresponding switches 119, which may either permit or prevent current from passing therethrough.
- Neighboring switches may be provided in different states, and thus allowing a user (through a computer or another suitable arrangement) to control a quantity of turns of activated coils and/or an effective length of the coils.
- cylindrical coil formers with ladder- like current-carrying segments of conductors including approximately circular arcs connected by approximately longitudinal segments, arranged on their surface, can achieve this result.
- the magnetic field may be fairly uniform and may be oriented along the axis depending on the direction of the flow of current. If a composite coil is constructed from close apposition of two such cylindrical segments on opposite sides of the axis (with current flowing in opposite directions in the segments), the superimposition of the fields may result in a generation of a transverse gradient of the magnetic field in the region between the segments. When the current-carrying arcs of the coils are of similar angular extent, the resulting gradient field can be approximately uniform across the bore of the coil, which may be a desirable feature according to the present invention, e.g., to be incorporated into conventional MRI systems (see Figs. 10B and IOC).
- a more localized (and likely locally stronger) gradient may be produced, possibly strongest in the region of the gap between the two coil element sets (see e.g., Figs. 13B and 13C, and the description hereafter).
- asymmetric arc segment coil sets may be useful for viewing areas that are radially offset from the z-axis of the MRI machine.
- the physical size of the subject may prevent a transverse displacement of the subject to be positioned within the con-esponding "sweet spot" of the transverse gradient coils.
- coils can be constructed with multiple connecting segments of conductors that can be electrically switched open or closed. In this manner, the effective size and location of the separate coil components can be varied. The location of the junction between two opposite magnetic fields can thus be correspondingly shifted to match the region to be imaged.
- the longitudinal gradient coil set can be constructed with switchable conducting elements so as to permit for the adjustment of the effective relative gap between the two coil element sets, and thus allow for the adjustment of the strength of the gradient and of the length of its effective region.
- switchable conducting elements so as to permit for the adjustment of the effective relative gap between the two coil element sets, and thus allow for the adjustment of the strength of the gradient and of the length of its effective region.
- FIG. 12 depicts a thirteenth exemplary embodiment of the coil arrangement according to the present invention which uses asymmetric coil segments of approximately 108 degrees and 252 degrees.
- a first coil segment 120 is of smaller angular extent than the second coil segment 130 of the coil arrangement.
- Exemplary simulations are shown in Figs. 13 A, 13B and 13C.
- Fig. 13 A shows an exemplary simulation generated using MATLAB software performed on the coil arrangement of Fig. 12, with asymmetric arc segments.
- Fig. 13B depicts the magnetic field across the axis associated with the simulation of Fig. 13 A.
- Fig. 13C depicts the x gradient along the x axis produced by the simulation of Fig. 13 A.
- the aforementioned exemplary coil arrangements may be constructed with additional elements surrounding them so as to minimize the electromagnetic interaction with external structures to further improve performance. The foregoing merely illustrates the principles of the invention.
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Citations (5)
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US4833410A (en) * | 1986-04-24 | 1989-05-23 | Commissariat A L'energie Atomique | System of coils for producing magnetic field gradients of very uniform polarization in an imaging or spectroscopy installation by NMR |
US6285188B1 (en) * | 1998-02-25 | 2001-09-04 | Kabushiki Kaisha Toshiba | Self-shielded coil with non-inductive winding |
US6479998B1 (en) * | 2000-05-23 | 2002-11-12 | Kabushiki Kaisha Toshiba | Active shield type of elliptic cylinder-shaped gradient coil for magnetic resonance imaging |
US6493572B1 (en) * | 1999-09-30 | 2002-12-10 | Toshiba America Mri, Inc. | Inherently de-coupled sandwiched solenoidal array coil |
US6958608B2 (en) * | 2003-06-10 | 2005-10-25 | Hitachi, Ltd. | Nuclear magnetic resonance equipment |
-
2004
- 2004-11-05 WO PCT/US2004/037054 patent/WO2005045447A2/en active Application Filing
Patent Citations (5)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4833410A (en) * | 1986-04-24 | 1989-05-23 | Commissariat A L'energie Atomique | System of coils for producing magnetic field gradients of very uniform polarization in an imaging or spectroscopy installation by NMR |
US6285188B1 (en) * | 1998-02-25 | 2001-09-04 | Kabushiki Kaisha Toshiba | Self-shielded coil with non-inductive winding |
US6493572B1 (en) * | 1999-09-30 | 2002-12-10 | Toshiba America Mri, Inc. | Inherently de-coupled sandwiched solenoidal array coil |
US6479998B1 (en) * | 2000-05-23 | 2002-11-12 | Kabushiki Kaisha Toshiba | Active shield type of elliptic cylinder-shaped gradient coil for magnetic resonance imaging |
US6958608B2 (en) * | 2003-06-10 | 2005-10-25 | Hitachi, Ltd. | Nuclear magnetic resonance equipment |
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