US 20020171425 A1 Abstract A gradient coil for magnetic resonance imaging (MRI) has an elliptic cylindrical first coil and a second coil arranged coaxially to the first coil. The second coil is located outside the first coil in the radial direction thereof. The second coil is either elliptic-cylindrical or cylindrical. Winding positions of wires of the second coil are determined so that a magnetic field created outside the second coil by the first coil is cancelled out. Practically, circumferential current densities of the first and second coils are expressed by weighted even and odd functions. Ratios between weights of the second and first coils are then determined on values derived from the number of waves in the coil axial direction and a flatness rate of the ellipse. The winding positions of wires of the second coil are determined on the values.
Claims(10) 1. A gradient coil for MRI comprising:
a first coil forming an elliptic cylinder; and a second coil disposed coaxially to the first coil, wherein a coil wire of the second coil is positioned in winding so that a magnetic field created by the first coil outside the second coil is cancelled. 2. The gradient coil for MRI according to 3. The gradient coil for MRI according to 4. The gradient coil for MRI according to 5. The gradient coil for MRI according to 6. A magnetic resonance imaging (MRI) system comprising a gradient coil for applying pulsed magnetic gradients to an object placed in a static magnetic field in order to perform magnetic resonance imaging of the object, the gradient coil comprising:
a first coil forming an elliptic cylinder; and a second coil disposed coaxially to the first coil, wherein a coil wire of the second coil is positioned in winding so that a magnetic field created outside the second coil by the first coil is cancelled. 7. A method of designing a gradient coil used for magnetic resonance imaging (MRI) and provided with both of a first coil shaped into an elliptic cylinder and a second coil arranged coaxially to the first coil, comprising the steps of:
expressing current density in a circumferential direction of each of the first and second coils by using weighted even functions and weighted odd functions, first determining ratios between weights for the second coil and weights for the first coil by using values based on the number of waves in a coil-axis direction and a flatness rate of an ellipse; and second determining wound positions of the coil wire of the second coil based on the values, thereby a magnetic field created outside the second coil by the first coil being cancelled. 8. The method of designing the gradient coil according to 9. The method of designing the gradient coil according to 10. A computer-readable program for obtaining the steps of:
expressing current density in a circumferential direction of each of first and second coils by using weighted even functions and weighted odd functions, the first coil shaped into an elliptic cylinder and a second coil arranged coaxially to the first coil, the first and second coils being included into a gradient coil used for magnetic resonance imaging; first determining ratios between weights for the second coil and weights for the first coil by using values based on the number of waves in a coil-axis direction and a flatness rate of an ellipse; and second determining wound positions of the coil wire of the second coil based on the values, thereby a magnetic field created outside the second coil by the first coil being cancelled. Description [0001] 1. (Technical Field) [0002] The present invention relates to a gradient coil incorporated in a magnetic resonance imaging system in order to give magnetic field gradients to an examination space formed in a bore of the system, and in particular, to an active shield type of elliptic cylindrical gradient coil. [0003] 2. (Related Art) [0004] A medical MRI system uses a magnetic resonance phenomenon of nuclear spins within an object to acquire tomographic images or measure NMR spectrums. [0005] The MRI system has a gantry that has an examination space shaped into for example a substantial cylinder, into which an object to be examined is inserted. The gantry has a static magnet, which forms the examination space, for generating a magnetic static field, a gradient coil for generating magnetic field gradient pulses superposed on the static field, and an RF coil for transmitting and receiving RF pulse signals (including MR signals) to and from the object. [0006] A gradient amplifier, which is coupled with the gradient coil, is driven in response to instructions given from a sequencer to the gradient amplifier, so that the gradient coil creates magnetic filed gradient pulses. Currently, it is required that the magnetic field gradient pulses be switched at a switching time of less than 1 msec. Especially, in the case of ultra-fast imaging techniques that have drawn much attention, the switching time of less than 0.3 msec. is required. To realize this, it has been desired that a gradient coil assembly of higher energy efficiency be developed, together with a high-power gradient amplifier. [0007] Conventionally, for MRI systems of which magnetic static field is directed horizontally, cylindrical gradient coil assemblies have been used. To increase the energy efficiency of this cylindrical gradient coil assembly, a first measure is to reduce the coil radius. This is based on the nature that inductance that is a parameter to decide the switching time of coils is proportional to a coil radius to 5 [0008] One solution to this problem is proposed for example by U.S. Pat. No. 4,820,988. This publication discloses a gradient coil assembly of which coils are formed into an elliptic cylinder in place of a cylinder. The coil radius is shortened in the vertical direction (normally, in the Y-axis direction) in such a manner that a section perpendicular to the axis through the coils is formed into an elliptic shape. This makes it possible that a higher rate of energy efficiency is held and the whole-body examination for adults is performed. Practically, disclosed coil shapes include a coil shape in which a half cylinder is decentered in the Y-axis direction and placed face to lace and a second coil shape in which a cylindrical coil is flattened in a certain direction to the surface so as to form an elliptic cylindrical gradient coil. [0009] An alternative measure is proposed by Japanese Patent Laid-open 5-269100, in which disclosed are an elliptic gradient coil assembly formed into an elliptic cylindrical coil and a detailed design method of such coil assembly. [0010] One significant management item for the gradient coil assembly is to suppress eddy current that occurs in pulsating the assembly to a lower value of current. The eddy current occurs in transition manners when a time-dependent magnetic field penetrates a thermal shielding plate of the static magnet, thereby a magnetic eddy field being caused in a region necessary for imaging. This causes irregularities in intensity of an MR image, deeply deteriorating image quality. To prevent this problem, an active (self-) shielded type of gradient coil assembly (ASGC: Actively Shielded Gradient Coil) is used, as shown in for example U.S. Pat. No. 4,733, 189 and No. 4,737,716). In such coils, a shield coil is placed outside the gradient coil assembly to suppress or shield a magnetic field leaking out of the gradient coil serving as a main coil. The ASGC has coil assemblies in charge of generating magnetic fields in the individual X-, Y- and Z-channels of an MRI system, and each coil assembly is equipped with a main coil and a shield coil. This shield structure almost prevents, channel by channel, magnetic field gradients from leaking out [0011] Practical design techniques for an ASGC of which coils are formed into a cylindrical shape are proposed by Mansfield et al., “J. Phys. E. Sci. Inst. 19, 540-545 (1986),” Turner et al., “J. Phys. D: Appl. Phys. Vo.19, L147-L151,” and others. [0012] Thus, an elliptic cylindrical gradient coil should be an actively shielded type of coil. To meet this demand requires a design technique that permits magnetic field gradients to be generated at a higher accuracy. However, such a practical way of designing for the elliptic cylindrical gradient coil has yet to be proposed. There are therefore difficulties in designing an active shield type of elliptic cylindrical gradient coil in terms of such work as computation of a current distribution of shield coils and computation of a desired magnetic performance (such as a maximum gradient strength and linearity in the magnetic field). The active shield type of elliptic cylindrical gradient coil has not been put into practical uses. [0013] The present invention, which has been made with consideration of the above problem that the conventional techniques face, is directed to an object of proposing in detail how to design an active shield type of elliptic cylindrical gradient coil, thereby such coil being able to be put into practical uses. [0014] To realize this object, a gradient coil for magnetic resonance imaging (MRI) according to the present invention comprises a first coil forming an elliptic cylinder; and a second coil disposed coaxially to the first coil, wherein a coil wire of the second coil is positioned in winding so that a magnetic field created by the first coil outside the second coil is cancelled. [0015] Preferably, current density in a circumferential direction of each of the first and second coils is expressed by weighted even functions and weighted odd functions, ratios between weights for the second coil and weights for the first coil are determined by values based on the number of waves in a coil-axis direction and a flatness rate of an ellipse, and wound positions of the coil wire of the second coil are determined based on the values. [0016] More preferably, the second coil is located outside the first coil in a radial direction of the first coil. In this structure, the second coil is formed into an elliptic cylinder and an elliptic cylindrical plane formed by the second coil and an elliptic cylindrical plane formed by the first coil possess in common a focus located at almost the same position. By way of example, the second coil is formed into a cylinder. [0017] According to these structures, there can be provided how to design winding positions of an MRI gradient coil in which the elliptic-cylindrical or cylindrical second coil (shield coil) is disposed outside the elliptic cylindrical first coil (main coil) with the same center axis given. Thus, an active shield type of elliptic cylindrical gradient coil can be provided and put into a practical use, in which a higher rate of energy efficiency is kept, while still enabling the whole-body of an object to be imaged and eddy current to surely be suppressed to reduce artifacts. [0018] In addition, the present invention provides a magnetic resonance imaging system in which the above gradient coil is incorporated, a design technique for the gradient coil, and a computer-readable program preferable to the design. [0019] In the accompanying drawings: [0020]FIG. 1 is an outlined schematic view of a gantry of an MRI system according to embodiments of the present invention; [0021]FIG. 2 is a conceptual view illustrating wire arrangement of a Z-coil included in an elliptic cylindrical ASGC according to a first embodiment; [0022]FIGS. 3A and 3B show unrolled views of winding patterns of a symmetric type of Z-coil; [0023]FIGS. 4A and 4B stow unrolled views of winding patterns of an asymmetric type of Z-coil; [0024]FIGS. 5A and 5B show unrolled views of winding patterns of a symmetric type of X-coil; [0025]FIGS. 6A and 6B show unrolled views of winding patterns of an asymmetric type of X-coil; [0026]FIG. 7 is a view illustrating winding patterns of a Y-coil of an elliptic cylindrical gradient coil (non-shielded type) which is shown as a prior technique; [0027]FIG. 8 illustrates an elliptic coordinate; [0028]FIG. 9 is an outlined flowchart showing processing necessary for determining winding positions of the coils of the elliptic cylindrical ASGC; and [0029]FIG. 10 is a conceptual view illustrating wire arrangement of a Z-coil included in an elliptic cylindrical ASGC according to a second embodiment. [0030] Referring to the accompanying drawings, preferred embodiments of the present invention will now be described. [0031] (First Embodiment) [0032] Referring to FIGS. [0033] The MRI system has a gantry [0034]FIG. 1 outlines a section along the axial direction of the gantry [0035] The gantry [0036] The static magnet [0037] The elliptic cylindrical ASGC [0038] Practically, the elliptic cylindrical ASGC [0039]FIG. 2 outlines a pictorial configuration of the Z-coil [0040] The Z-coil [0041] Wigs. [0042] The X-coil [0043]FIGS. 5A and 5B and FIGS. 6A and 6B show examples of the winding patterns of the X-coil. Of these examples, FIGS. 5A and 5B illustrate winding patterns of the X-coil of a symmetric type of elliptic cylindrical ASGC (½-unrolled view), whilst FIGS. 6A and 6B do those of an asymmetric type of elliptic cylindrical ASGC (½-unrolled view). In addition, FIGS. 5A and 6A show the winding patterns of the main coil, while FIGS. 5B and 6B do those of the shield coil. The winding positions of the wires are analytically obtained on a desired distribution of magnetic flux. [0044] The Y-coil [0045] Though the foregoing Z-coil, X-coil and Y-coil have the winding patterns similar to those of the conventional cylindrical ASGC, spacings between winding positions of the wires are designed differently from the conventional in that, as will be described later, the wires are arranged around an elliptic cylindrical surface so as to obtain a magnetic field performance required for magnetic resonance imaging. [0046] As shown in FIG. 7, in the case of a conventionally known elliptic cylindrical gradient coil (non-shield type) disclosed by Japanese Patent Laid-open No. 5-269100, its one coil loop always includes an oppositely-wound loop. Similarly to FIG. 7 showing the Y-coil, each of the X-coil and the Z-coil includes such oppositely-wound loop. In contrast, for each coil of the elliptic cylindrical ASGC according to the present invention, such oppositely-wound loop is not necessarily required, with the result that the winding patterns according to the present invention also differs from those of the conventional. [0047] How to design the foregoing elliptic cylindrical ASGC will now be described in a quantitative manner. In other words, a technique for locating windings of coil wires of the elliptic cylindrical ASGC will now be detailed. The locations of the windings may be determined through processing executed by a host computer in the imaging system [0048] An elliptic cylindrical surface around which coil wires of the main coil are wound is generally expressed by the following equations (1) that use the elliptic coordinate system (refer to FIG. 8). [0049] where (±c, 0) shows the coordinates of focuses of the ellipses. Further, an elliptic cylindrical surface around which the coil wires of the shield coil are wound can be expressed by the following equations (2) in the similar manner to the above. [0050] Incidentally, the main coil has the same focuses as those for the shield coil. [0051] Then, described will be how to determine winding positions of coil wires of the shield coil under the condition that the winding positions of wires of the main coils are given. [0052] A η-directional current density function j [0053] In cases a η-directional current density function for the shield coil is written by
[0054] the relationships of
[0055] is accomplished, because the gradient coil is formed into the ASGC. Hence the current density function for the shield coil can be calculated. Calculating equations (3) and (5) enables current density in the circumferential direction of each of the main coil and the shield coil to be expressed by weighted even functions and weighted odd functions. And ratios between weights for the shield coil and weights for the main coil are determined with the number of waves in the coil axial direction and a flatness rate of the ellipse. [0056] After calculating the current density function, a stream function is calculated on the current density function by a known technique, as will be described later. Based on the stream function, discrete winding positions of wires of the shield coil are determined. These relationships can be applied to all the X-, Y- and Z-coils as well. [0057] All the terms in the relationships shown by the equations (5) can be written as follows, using Mathieu functions.
[0058] Z-components of a magnetic field created by each of the main and shield coils within its inner space are given by the following equation (10).
[0059] in which
[0060] In addition, inductance values Lc, Ls and L [0061] In the above Eq. (12), there are provided
[0062]FIG. 9 shows a flow of coil-position determination processing and winding operations for manufacturing the elliptic cylindrical ASGC according to the present invention on the basis of the foregoing qualitative analysis. Practically, first, a current density function is given to the main coil, as shown in equation (3) (step S [0063] These calculated results (the values of the magnetic fields and inductance) are inputted into a specified estimate function (step S [0064] In the present embodiment, the processing from steps S [0065] As stated above, to practically design the elliptic cylindrical ASGC, it is preferable that magnetic gradients be set to their desired intensities, the linearity in the magnetic field which is able to cover an imaging region be secured, and the inductance values be as small as possible. The winding positions of the coil wires are determined on the above qualitative analysis, so that the elliptic cylindrical ASGC can be designed for actual applications. Therefore, characteristics inherently given to the elliptic cylindrical ASGC, that is, higher energy efficiency and the whole-body examination for adults can be attained. [0066] (Second Embodiment) [0067] Referring to FIG. 10, a second embodiment of the present invention will now be described. [0068]FIG. 10 shows a Z-coil [0069] In the second embodiment, the ASGC coil is designed such that a current density function is given to the main coil, and a current density function of the shield coil corresponding to, that of the main coil is calculated based on known techniques such as a finite element method. The obtained current density function of the shield coil is then expressed by superposition of analytic formulas. Weight coefficients of those analytic formulas are specified as values dependent on the major axis, minor axis, and axis length, and a coil radius of the shield coil. The weight coefficients are given as known values, then inductance values and a magnetic field to the ASGC are calculated using equations that express inductance values and a magnetic field to a cylindrical coil. This provides an elliptic cylindrical ASGC of which main coil is shaped into an elliptic cylinder alone, with the ASGC having desired magnetic gradient intensities and a desired linearity in the magnetic field. [0070] The present invention is not necessarily limited to the configurations described in the above embodiments, but the ASGC according to the present invention is able to employ other various configurations within the scopes derived from the descriptions in the appended claims. Referenced by
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