Title of Invention	"A GRADIENT COIL MANUFACTURING METHOD"
Abstract	The present invention aims to implement a pair of gradient coils low in magnetizing force with respect to pole pieces. Upon manufacturing a pair of gradient coils each of whish produces gradient magnetic fields by currents that flow through a plurality of concentric passes, the maximum radius of one of current passes for the gradient coil is set as the minimum value at which a gradient magnetic field having a magnetic field error within a predetermined allowable rainge can be produced.

Title of Invention

"A GRADIENT COIL MANUFACTURING METHOD"

Abstract

The present invention aims to implement a pair of gradient coils low in magnetizing force with respect to pole pieces. Upon manufacturing a pair of gradient coils each of whish produces gradient magnetic fields by currents that flow through a plurality of concentric passes, the maximum radius of one of current passes for the gradient coil is set as the minimum value at which a gradient magnetic field having a magnetic field error within a predetermined allowable rainge can be produced.

Full Text	BACKGROUND OF THE INVENTION The present invention relates to a gradient coil manufacturing method, a gradient coil and a magnetic resonances imaging system/ and particularly tp a gradient coil provided on a polar surface of a static magnetic field magnet, a method of manufacturing the same, and a magnetic resonance imaging system having such a gradient coil,. In a magnetic resonance imaging (MRI Magnetic Resonance Imaging) system/ a target to be shot or imaged is carried in an internal oore of a magnet system, i.e., a bore or space in which a static magnetic field ife formed. A gradient magnetic field and a high-frequency magnetic field are applied to produce a magnetic rescnance signal within the target. A tomogrum is produced (reconstructed) based on its,received si.gr.al. In a .rnagne system using permanent magne1;s.forrgeratingi.static magnetic fields,poie.preces for uniform!zing a magnetic fImx.di'stributibn in a static .magnetic field space ans rsspectively.provided at leading ends of a pair of the permanent magnets opposite to each other. Further, gradient coils for generating gradient magnetic fields are provided on their corresponding polar surfaces of nhe pole pieces. In the abovji-describedmagnet system, the pole pieces are magnetized by the gradient nu.gnetic fields since the gradient; coils are respecivs.ly c;lo3e to the pole pieces. Due co the residual oradient nagne.ic ields ffcirined by their immanent nagnetizacion, the phase of a spin is subjected to .such an infuere as though eddy currents extremely log in tir.s constant hp.d existed. Therefore, it would interfere with imaging made by for example, a fast spin echo (FSE) method or the like which needs accurate phase control. Statement Of Invention Atjradient coil manufacturing method comprising the steps of providing a pair of pole pieces each having a bottom plate portion and a peripheral edge portion protruding in a direction orthogonal to surfaces of said bottom plate portion, with said pole pieces being disposed opposite each other with the protruded peripheral edge portion formed with a space defined therebetween; manufacturing a pair of gradient coils having concentric passes and disposed along said surfaces of said bottom plate portion which produces a gradient magnetic field in said space by current that flows through said concentric passes; setting a maximum radius of one of said passes for each of said pair of gradient coils to a minimum value with a range for producing said gradient magnetic field having a magnetic field error within a predetermined range; and determining radii of said passes according to the following procedure (a) Setting measurement points Pi, wherein i=1-N, onto a maximum spherical surface supposed in an imaging area; (b) calculating magnetic fields Bit, wherein i=1-N, at measurement points, to be produced by said gradient coils; (c) setting a tolerance at for an error with respect to each magnetic field; (d) setting a value rO of maximum radius of a pass for said each gradient coil within a range not exceeding a limit value r00; (e) defining radii of said passes as M, r2,. . . rM, and max (r1, r2, . . . , rM) under the above restricted condition, and r=(r1, r2 , rM), with the above as a parameter as herein described, determining optimum values of rj, wherein j=1-M, using quadratic programming so that the last above equation of this step (e) is established, wherein the following is calculated using Biot-Savart's law as herein described; (f) calculating an error in magnetic field at each measurement point Pi according to the equation as herein described (g) determining rj when .αi ≤ αt is satisfied; (h) when αi ≤ αt is not satisfied, increasing the value rO to be within a range not exceeding said limit value r00; and (i) repeating the foregoing procedure subsequent to step (e). A magnetic resonance imaging system for forming an image, based on magnetic resonance signals acquired using a static magnetic field, a gradient magnetic field and a high-frequency magnetic field, the system comprising a pair of gradient coils configured as gradient coils each of which generates the high-frequency magnetic field, said pair of gradient coils being respectively provided along the surfaces of bottom plate portions lying inside peripheral edge portions of a pair of pole pieces having the bottom plate portions and the peripheral edge portions protruding in a direction orthogonal to surfaces of the bottom plate portions, with said pole pieces being disposed opposite each other with the protruded peripheral edge portion formed with a space defined therebetween, and which produces gradient magnetic fields in the space by currents that flow through a plurality of concentric passes, and wherein the maximum radius of one of said passes for each of said pair of gradient coils is set to a minimum value within a range for producing said gradient magnetic field having a magnetic field error within a predetermined range, the radii of said passes being determined according to the following procedure (a) Setting measurement points Pi, wherein i=1-N, onto a maximum spherical surface supposed in an imaging area; (b) calculating magnetic fields Bit, wherein i=1-N, at measurement points, to be produced by said gradient coils; (c) setting a tolerance at for an error with respect to each magnetic field; (d) setting a value rO of maximum radius of a pass for said each gradient coil within a range not exceeding a limit value r00; (e) defining radii of said passes as r1, r2, . .. rM, and max (r1, r2, . . . , rM) under the above restricted condition, and r=(r1,r2t. .. , rM), with the above as a parameter, as herein described determining optimum values of rj, wherein j=1-M, using quadratic programming so that the last above equation of this step (e) is established, wherein the following is calculated using Biot-Savart's law as herein descibed; (f) calculating an error in magnetic field at each measurement point Pi according to the following equation (g) determining rj when .αi ≤ αt is satisfied; (h) when αi ≤ αt is not satisfied, increasing the value rO to be within a range not exceeding said limit value r00; and (i) repeating the foregoing procedure subsequent to step (e). SUMMARY ,OF THE INVENTION Therefore, an object of the present invention is to implement a gradient coil low in magnetizing fcrco with respect to each pols piece, a method of manufacturing the same, aid n magnetic resonance imaging system having such a gradient coil. (1) The invention according to. one aspect for solving the above problems! is a gradient coil manufacturing method comprising the tep of, upon manufacturing a pair of gradient coils which is respectively provided along the surfaces of bottom plate portions lying inside peripheral edge portions of a pair of pole pieces having the bottom plate portions and the peripheral dge portions protrudir.g in the direction orthogonal to the surf.aces-.of che bottom plate portions, the pole pieces being opposed topach. gther w;th • the protruded, peripheraled'gevpbrtl;gn;s f drmsd with a spacerdef ined. therebetween,, and'which arduces.'gradient magne-tlc^f ieids in the space by currents that flow through a-plurality of concentric passes, setting the maximum radius of one of the passes for each of the gradient coils to the minimum value within a range for producing a gradient magnetic field having a magnetic field error lyinc within a predetermined allowable range. In the according to th.s aspect, the inurt radius of one pass for esc^ gradient coil is set tc the minimur. value at vhich a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the c'.istanca between the outermost pas.s and a protruded peripheral edge portion of each pole jjpiece increases. Therefore, the magnetizing force with respect to the protruded peripheral edge portion is '.ow and the residual magnetization is low. (2) The invention according to ar.other aspect for solving the above problems is the gradient coil manufacturing method described in (1) , wherein the radii of the plurality of passes are determined according to the following procedures. Note (a.) setting measurement points Pi (where i=l-N) onto the maximum epherical surface supposed in an imaging area. (b) calculating magnetic fields B..t (where i=l-N) at the measurement points, to be produced by the gradient coils. (c) setting a tolerance at for ein error with respect to each magnetic field. •(•d) setting an allowable value rO of the maximum radius of the pass ;f or .each gradient ; coil within a, -range- not.- exceeding a, 'limit -value r00. (e) defining .'the radii of the. plurality of ' passes '.as rl, r2, . .., • rM . max (rl, r2, . . . , rM) with the above as a parameter, (Equation Removed) determining ths optimum values of rj (where j=l-M) using quadratic firogramraing so that the above equation 18 is established. Incidentally, the above is calculated using the Biot-Savart' s law. (i) calculating an error in macneuic field at each measurement point ti according tc the following equation. (g) determining rj when aiSat. iis satisfied. (h) when αi ≤ αt is not satisfied, .Increasing the allowable value r3 within a.range. jiot vexceeding'-,the 1 iir.i t.value r00;; and- (1) repeating the-procedures sub-sevquent to . (e) In the invention according to this aspect, .the maximum, radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance between tshe outermost pass and a protruded peripheral edge portion of each pole pdece increases. Therefore, the magnetizing force with resp-sct to the Dsrctrudec. periph'.sral edge portion is low and the residual magnetization is low. (3) The invention according to a further aspect for solving the; above problems is a pair of gradient coils which is respectively provided al.ong the surfaces of bottom plate portion? lying inside peripheral edge portions of a pair of pol pieces having the bottom plate portions and the peripheral edge portions protruding in the direction orthogonal to the surfaces of the bottom plate portions, the pola pieces being opposed to each other with the protruded peripheral edge portions formed with a space defined therebe-ween, and which produces gradient magnetic fields in the space toy currents that flow through a plurality of concentric passes, wherein the maximum radius of one of the possess for each of the gradient coils is set to the minimum value within a rungo for producing a gradient magnetic tfield having a magnetic field error lying within a predetermined allowable iSange, In the invention according to this aspect, the maximum radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable .range can be produced. Thus, the distance between tlha.aute-rmo.st pass. and a protruded peripheral/edge portion 'of"eah"'pole, piece increases.. Therefore; the magnetizing force with respect.to the-.protruded peripheral edge portion.is' lew -and the residualr.magneti.zation iis low. (4; The invention according to a still further aspect for s.olving the above problems is the pair of gradient coils described in (3) , wherein the plurality of passes respectively have radii determined according to the following procedures. Note (a) setting measurement points Pi (where i=l-N) onto the maximum spherical surface supposed in an imaging area. (b) calculating magnetic fields Bit (where i-l-N) at the measurement points, to be produced by the gradient coils. (c) setting a tolerance at for an error with respect to each magnetic field. (3) setting an allowable valua rD of the maximum radius of the pass for eaen gradient coil within a range not exceeding a limit value r00. (e) defining the radii of the plurality of passes as rl, c2, ..., rM. max (rl, r2, ..., rM) with the above as a parameter, (Equation Removed) determining .the optimum values o.f -rj where 'j=1-M) using-quadratic programming so that the above equation 23 is established.. Incidentally, the above is calculated using the Bict-Savart's lav;. (f) calculi tir.g an error in magneti c field at each ns asurerr.erv poin-?i accorc.ir.g to the following equation. (cj) determining rj when αi ≤ αt is satisfied. (h) when αi ≤ αt is not satisfied, increasing the allowable value rO Within £i range not exceeding the limit value r00, and (i) repeating the procedures subsequent to (e) . In the invention according tc this aspect, the maximum radius of one pass for each gradient coil is set. to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance oetween the outermost pass and a protruded peripheral edge portion of each pole i piece increases. Therefore, the magnetizing force with respect to the protruded peripheral edge portion is; low and the residual magnetization iis low. (5) The invention, according tc a-still,further, aspect for. jiolving tine above problems is a magnetic resoranceritiaging sys,tem f or forming an.image, based, on magnetic resonance signals acquired.,using a static •niagnetic filed, a gradient magneticfield and a high-frequency magnetic field, comprising, a pair of gradient ceils configured as gradient, coils each of which generates the high-frequency magnetic field, the pair of gradient coils birg respectively provided along the surfaces of bottom plate portions Ivir.g inside peripheral edge portions of a pair of pole pieces having tr.e c-tom plate portions and the peripheral edge portions protruding in the direction orchogonai. tc the surfaces of ths bo-coir plate portions, the pels pieces being opposad to each other with the protruded. peripheral edge portions formed with a space defined therebetween, and roauces gradient magnetic ields in the space by currents that flow through a plurality of concentric passes, and wherein the maximum radius of the pa.ss is set to the minimuit. value within a range for producing a gradient magnetic field having a magnetic field error lying within a predetermined allowable range. In the invention according to t'.iis aspect, the maximum radius of one pass for each gradient coil is se'i to the minimum value at which a gradient magnetic field having a magretic field error lying within a predetermined al-lowable range can be produced' Thus, the distance between the outermost pass and a protruded peripheral edge portion of ee.ch pole piece increases. Therefore, the magnetizing force with respect to the protruded peripheral edge portion i.s low and the residual magnetization is low. Thus, imaging on which the residual magnetization has a little efifect, can be carried out. (6) The invention according to a still further aspect for .solving the. -above. problems .is the...magnetic renonanca. imaging; system described. iit (5)., w'.ierein the plurality of passes raspectively have radii detormined according., to .the; following procedures. Note (a) setting measurement point 'where i=l-N) cnto the maximum spherical surface supposed in an imaging area, (b) calculi, ting magnetic fields Bi [where i=l-N) at the measurement points, to be produced by the gradient coils. (c) setting a tolerance at for an er ror with respect to each magnetic field. (d) setting an allowable value .0 (if the maximum radius of the pass for each gradient coil within a rangs not exceeding a limit valus r00. (e) defining the radii of thfis plurality of passes as rl, r2, ..., rW. max (rl, r2, . . , rM) with the above as a parameter, (Equation Removed) determining the optimum values of rj (where j=l-M) using quadratic programming so that the above equation i8 is established. Incidentally, above is . calculated -using the-Blot-Sava-rt's law. . (f) calculating an error in magneti.c field at each measuremen' point Pi according to the following equation. (Equation Removed) (g) determining rj when αi ≤ αt is satisfied. (h) when αi ≤ αt is not satisfied,, increasing the allowable value rO within a range not exceeding the limit value r00, and (i) repeating the procedures subsequent to (e) . In the invention according to this aspect, the maximum radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance between the outermost pass and a protruded pezipheral edge portion of e.ach pole piece increases. Therefore, the mitgnstizing force with respec- to the protruded peripheral edge portion is low and the residual magnetisation is low. Thus, imaging on which the re.sidual magnetization has a little Effect, can be carried out. According to the present invention, a gradient coil reduced in magnetiziing force with respect to a pcle piece, a manufacturing method therefor, and a magnetic resonance imacing system having such a gradient coil can be implemented. Further objects and advantages of the present invention will be Apparent from the foJ.lowingde-scrip.tion.of the preferred eir.bodiment.s.of. the invention as illustrated in the .accompanying drawings. BRIEF DESCRIPTION 0F THE DRAWINGS Fig. i is a block diagram of a system showing one example of an embodiment according to the present invention. Fig. 2 is a diagram showing one exanrtcle of a pulse a&quer.ce executed by the system shcwr. in Fig. 1. Fig , 5 is a diagram illustrating or.e example cf a pul.se sequence Executed by the system shown in Fig. 1. Fig. 4 is a typical diagram showing a cr.figuratior. of the neighborhood of a gradient coLl of a magnet system onoloyed in the system shown in FLg. 5 is a schematic illustration showing patterns for current passes of a gradient coil. Fig. 6 is a flowchart for descriaing the procedure of determining rajdii of. current passes. DERAILED DESCRIPTION OF THE INVENTION Preferred embodiments of the present invention will.hereinafter be described in detail with reference to the accompanying drawings . A block di&gram of a magnetic resonance imaging system is shown in Fig. 1. The present system is one example of an embodiment of the present invention. One example of an embodiment related to a system of the present invention is shown according to the configuration of the present system. As shown in Fig. If the present system has a magnetic system 100. The magnetic system 100 has main magnetic field magnet units 102, gradient coil units 106 and RP /radio frequency) coil-.units.10.8'..Any-of theise main-magnetic field magnet units 102 and respective coil.units comprises paired; onfcs oppoised to one another with a space inter-posed therebetween. Further, any of thorn has a substantially disc sshape and is placed with its central axis held in common. A target 300 is placed on a cradle 500 in an internal boye of tlie magnetic system 100 and carried in and out by unillustrated conveying means. Eac.iof thenainmagnetic f ieldmaijnetunits 1C2 forms a staticmaqnetic field in the internal bore of the magnetic system 100. The direction of the static magnetic field is approxinataly orthogonal to the direction of (the body axis of the target 300. Nanely, each of the main macnetic field magnet units 102 forms a so-called vertical magnetic field. Each of the main magnetic field magnet units 102 is configured using a permanent mignet or the like, for example. Incidentally, the main magnetic field magnet unit 102 is not limited to the permanent magnet and may of course be configured using a superconductiv= electromagnet or a normal conductive electromagnet or the like. The gradient coil units 106 p.codace gradient magnetic fie.'.ds used for causing the intensity of the static magnetic field to have a gradient or slope. The produced gradient magnetic fields include three types of gradient magnetic fields of a slice gradient magnetic field, a read out gradient magnetic field and a phase ancode gradient magnetic field. The gradient coil unit 106 has unillustrat-sd 3-systematic gradient coils in association with these three types of gradient magnetic fields. The three-systematic gradient coils respectively produce three gradient magnetic fields for applying gradients to static magnetic fields respectively as viewed in three directions, orthogonal to or.e another. Qiie of., .the.'.thre(5.directions cor responds., to ..-the', direction •.•.(vertical ••••. .d-Jjcection) pfv,the -static'magnetic f ieia.. and is .normally, -.defined as .a z .direction.' Another onethereof corresponds to a.horizontal direction and .is normally defined as a y direction. The remaining one corresponds to. the direction orthogonal to the z and y directions and is normal.ly defined ae an x direction. The x direction is orthogonal to the z direction within the vertical plane and perpendicular to they direction within the horizontal plane. x, y and z are also called qradient axes below. Any of x, y and z can be set as an axis for a slice gradient, when any of them is ssst as the slice grad.en axis, one of the remaining two is set as an axi; for a phase encode gradient and the other thereof is set as an axis for a read out gradient. The 3-systematic gradient coils be explained again later. Each of the RF coil units 108 transmits an RF excitation signal for exciting a spir. in a body of the targst 300 to a static magnetic field apace. Further, the RF coil unit 108 receives therein a magnetic rsonance aignal which produces the excited spin. The RF coil unit 108 has unillustrated transmitting and recoivtng coils. The transmitting coil and the receiving coil share the use o the same coil or make use of dedicated Ooils r&spectively. A gradient driver 130 is connected to the gradient coil units 106. Tthe gradient driver 130 supplies a drive signal to each of the gradient aoil units 106 to generate a gradient magnetic field. The gradient, driver 130 has unillustrated 3-systematic dr.ve circuits in association with the 3-systematio gradient coils in the gradient coil unit 106. A F.F drivei 140 is connected to the RF coil units 106. The RF driver 140 supplies a drive signal to each .of v.he RF coil units 108 to transmit an RF-excita.tior signal, thereby., exert ing-the- spinin the body of ..the/. target 300. A data collector 150 is connected to .each of the RF coil units 108. The data collector 150 takes in or captures signals received by the RF coil units 108 and collects the same as view data (view data) . A controller 160 is connected to the gradient driver 130, the RF driver 140 and the data collector 150. The controller 160 controls the gradient driver 1.30 to data collector 150 respectively to execute shooting or imaging. The output side of the data co.Hector 150 is connected to A data processor 170. The data processor 170 is configured using a computer (computer) or the- like, for example. The data processor 170 has an •jjniflustrated memory. The memory £itoes a program and various data for tine data processor 170 therein. The function of the present system is implemented by allowing the data processor 170 to execute the program stored in the memory. The data processor 170 causes the memory to store the data captured from the data ccllector 150. A data space is defined in the memoy. The data space forms a two-dimensional Fourier space. The data processor 170 transforms these data in the two-dimensional Fourier space into two-dimensional inverse Fourier fom to thereby produce (reconstruct) an image for the- target 300. The two-dimensional Fourier space is also called a "k~spaoe", Ths data processor 170 is connected to the controller 160. The data ptocessor 170 ranks ahead of the controller 160 and generally controls it. Further/ a display unit 180 and an operation or control unit 190 are connected to the data processor 170. he display unit 18.0 .comprises a •graphic display, or-the .like- The operation unit 190'. comprises-a keyboard- or the like provided with a pointing cevise. • . The display unit 180 displays "a reconstructed image and various . information outputted from the data processor 170. --.The operaticn unit 190 is operated by an operator and inputs various commands and information or the like to the data processor 170. The operator controls the present system on an interactive basis through the display unit 160 and the operation unit 190. Fig. 2 ;,hovs one example of a pulse sequence used when ..maging cr shooting is done by the present system The present pulse sequence corresponds to a pulse sequence of a gradient echo (GRE) method. Nair.oly, (1) shows a sequence of a α° pulse for RF excitation employed in the GRE method. (2), (3), (4) and (5) similarly respectively show Sequences of a slice gradient Gs, a read out gradient Gr, a phase encode gradient Gp and a gradient echo MR. Incidentally, the α° pulse is -;ypif ied by a center signal. The pulse sequence- proceeds from left to right along 4 time axis t. As shown in the same drawing, α° excitation for the spin is carried out bas&d on the α° pulse. A flip angle α° is less than or equal to 90°. At this time, the slice gradient Gs is applied to effect selective excitation qn a predetermined slice. After the α° excitation, the spin is phase-encoded based on the phase encode gradient Gp. Next, the spin is first dephased based on the read out gradient Gr. Next, the spin is reohased to generate each gradient etcho MR. The signal strength of the gradient echo MR reaches a maximum aifter an echo time TE has elapsed since the excitation. The gradient echo MR. is collected as view data by the data collector 150. Such a.-.pulse . sentience is. repeated 64..to-512. 'timesin-a.cycle- TR-. .(repetition time) . Each- time it is..repeated, ;the .phase encode gradient Gp is changed'...ard different phase encodes are "carried out. .every time. Thus., view data f >r 64 to 512 views for filing in a.k space can be obtained. Another example of a pulse sequence for magnetic resonance imaging is shown in Fig, 3. The pulse sequence corresponds to a pulss sequence of a spin echo (SE) method. Namely, shows a sequence of a 90° pulse and a 190s pulst for RF excica-ior. employed in the SE method. (2), (3), (4' and (S) similarly respectively show sequences of a slice tjradient Gs, a read out gradient. Gj, a pha&e encode gradient Gp and a spir. acho MR. Incidentally, the 30° pulse and 130° pulse are respectively typified by center signals. The pijuse sequence proceeds from left to right along a time axis t. As shown .n the same drawing, 90° excitation for the spin is carried otlt basad on the 90° pulse. At this time, the slice gradient Gs is applied to effect selective excitation on a predetermined slice. After a predetermined time has elapsed since the 90° excitation, 180° excitation based on the 180° pulse, i.e., spir. inversion is carried out. Even at t£is time, the slice gradient Gs is applied to effect selective inversion oa the same slice. The read out gradient Gr and the phase encode gradient Gp are applied dwring a period between the 90° excitation and the spin reversal. The spin, is dephased based on the read out gradient Gr. Further, the spin is phase-encoded based on the phase encode gradient Gp. After the spin reversal, the spin is rephased based on the read out gradient Gr to produce each spin echo MR. The signal strength of the spin efcho MR..reaches ;a maximum after TE has elapsed-since the 90° excitation. .The..spin echo.MF.'is collected as view.-data ..by.the data collect or1 .150. Such a.1 pulse sequence .is repeated 6-4 .to 512..times .-ina -cycle -.TR. Each time .it .is repeated,- the phase encode gradient Gp is'Changed and different phase encodes are carried out every timis.- Thus, view data for 64 to. 512 views for filling in a k space can obtained. Incidentally, the pulse sequence used for imaging is not limited tie .he GR12 method or S3 method. The pulse sequence nay be other suitable techniques such as ar. FSE (Fast Spin Echo) method, a fast recovery F3E fiFas- Recovery Fiist Spin Echo) method, echc planar imaging (EPI) , etc. The data processor 1"/0 transforms ihe view data in the k space into two-dimensicinai inverse Fourier forn to thereby reconstruct a tonogram for the target 3CO . The reconstructed image is stored in its corresponding memory and displayed on the display unit 180. Fig. 4 typically shows the structure of the magnet system 100 located in the neighborhood of the gradient soil units 106 in the form of a cross-sectional view. In the same drawing, 0 indicates the center of a static magnetic field, i.e., a roacnev. center, and x, y and 2 indicate the aforementioned three directiors respectively. A spheric volume SV of a radius R with the magnet center 0 as the center is a shooting or imaging area. The magnet system 100 is configured so that the static magnetic field and gradient magnetic field respectively have a predetermined accuracy in the SV. A pair of main magnetic field magnet units 102 has a pair of pole pieces 202 opposed to each other. The pole piece 202 is composed of a magnetic material having high permeab.lity, such as a soft iror. or the like and serves so as to uniformize a magnetic flux distribution in a static magnetic field space. the polepieces. 202 are • respectivelyrshaped -substantially.-the -fornrof 'dis.cs but protrude in the direction (.2-'direction) ..in which .their ;• peripheral edge, portions are orthogonal to their plate "surfaces, -i.e., in the direction in which the pole p.ecss 202 are opposed to each other. Thus, the pole pieces 2 02 have hot torn plate portions and protruded per .pher a 1 edge portions. protruded peripheral, edge portions serve so as to make up for re?ductior.s in magnetic flux density at the peripheral sc.ges of the pole pieces 202. The cradier'; coil units 106 ae respectively provided in their corresponding concave portions of the pole pieces 202, which are defined inside the protruded peripheral edge porrions. -each of the gradient coil units 106 has an X coil 204, a Y co.l 206 and a Z208 coil. Of these, the Z coil 208 is one example of an embodiment of a gradient coil employed in the present invention. Any of the respective soils is shaped substantially intheformof a disc. The respective coils are mounted on a polar surface of the pole piece 202 by unillustrated suitable mounting means so that they are successively layered. Pc.tterns ;or current passes of tiie X coil 204, Y coil 206 and Z208 eoil are shown in Fig. 5 by a diagrarrmatic illustration. As showi in the $ame drawing, the X coil 204 has linear plural main current passes (main passes) parallel to a y direction at a portion near the center of a circle. these main passes are symmetric with espect to a y axis which passes through the center of the circle. Return passes for the main passes are formed along the circumference of the cicle. The radius of the outermost return pass, i.e., the outside diaireter of the X coil 204 is g.ven as r00. The; Y coil 206 has linear plural naii passes parallel to an x direction ,data portion near the center of a circle These main- passes are-.-symmetri-c •. vfith respect .co. c.n x-.axis which parsses.thro.ugh?-the center'of the circle.'. Ue.turn .passes .fo the main passes are formed-along, -the -circumference of." the-circle. Tha radium of the outermost return pass, . the outside diameter of the Y coil 206 is given as r00. The Z coil 206 has s. plurality of current passes which forr, concentric Circles respectively. These current passes are all main passes . The radii of the respective main passes are given as rl, r2, ..., rM in order from inside. H is defined as the outer diarr.eter of the Z coil 2 OS. Since the Z coil 208 has no returr. passes, it is ensy to generate a gradient, magnetic field good in linearity as compared with those having return passes as in the case of the X coil 204 and the Y coil 206. Thus, even if the outside diameter rM of the Z coil 209 is set smaller than the outside diameters r00of the X coil and the Y coil 20(5, the Z coil 206 can produce a gradient magnetic field having linearity equivalent to that of each of the X coil and the Y coil 206. Since the distance from the outur periphery of the Z coil. 208 to each peripheral protruded portion oi' the pole piece 202 is increased when the outside diameter rM of the Z coil 208 is reduced, the magnetizing force of each protruded portion by the Z coil 208 becomes weak in proportion to the square of the distance. Accordingly, the residual magnetization Of each pole pifcce 202 can be lowerad by reducing the outside diameter tM of the 2 coil 208. Insidentally/ the magnetization of each protruded portion by the X coil 204 and the Y coil 206 produces; a little influence as compared vfith tha-; by the Z coil because the nagietization intensities developed by the main passes .and' return passes ast uo as to be opposite-to one another if IT." polarity. The use o.f si.uch a.magjiet system small in residual-magnetization makes it.possible to properly perform, even .-shotting or.'imagirtg by, fcrvexample, the FSE trethud o;r che like which requires accurate .phase control on the spin. A method of manufacturing such a 2 coil 208 will next be described. The radii rl, r2, . . . , rM of the plurality of current passas that constitute the main passes, i.e., .he concentric circles respectively., are determined upon manufacturing rr.s Z coil 209. Fig. 6 shows, a flow chart for describing the procedure! of determining the radii of the main passes. As shov.n in the sane drawing, in Step 6C2, N points -o be measured PI (xl, yl, l) , ?2 (x2, y2, z2}, ..., PH {xN, yN, zN) are set onto the maximum spherical surface of an imaging area, i.e., the surface of SV shown in rig. 4. Niaxt, in Step 604, magnetic fielc.s Bit, B2t, . . ., BNt to be produced by each Z coil 208 at the measurement points PI, P2, . . . , PN are calculated. The following equation is used to calculate the magnetic fields. (Equation Removed) where, g magnetic field gradient Next, in Step 606, an allowable value or tolerance at on an error with respect to each magnetic field is set. at is set so as to be equal to a tolerance on an error with res;peot to each of the X coil .204 and the Y coil 206. Next,. inStep 608, a tolerance or c.llowable .-.value rO for the outside diameter of the Z. coil 208 is set .• .The rO is -set within a range not exceeding a limit -.value r00 The. limit value r00 corresponds -to- each ..of the outside diameter.of the -X-rcoil 204 and the Y coil.2.0S Next, in Step 610, the optimum vai.-ues-.of the radii rl, r2, . . ., rM of the plural main passes for the Z ceil 208 are determined. Namely, max (rl, r2, . . . , rM) the above is used as a parameter. (Equation Removed) The optimum values of rj (where j=L-K) are determined using quadratic programming so that the last above equation is established. Incidentally, the above is calculated using the Biot-savart's law. Next, in Step 612, an error in magnetic field at a measurement point Pi is calculated according to the following equation. (Equation Removed) Next, it is determined in Ste;? 614 whether αi ≤ αt is satisfied.. If the answer is found to be negative, then the allowable Vetlue rC is incremented by Ar within the range nat exceeding the limit value r00 in Step 616, and the routine procedure subsequent to Step 610 is repeated. When αi ≤ αt is satisfiea, rl, x2, . . ., rM are determined in Step 618. The Z coii'. 208 having tihe plurality o.f m.ain passes, which ccns'iituce the concentric circles having the deterninec radii is manufactured. Siroe the allowabie value of error kith respect to the; magnetic field is set to the sarr.e allowable value as each of the X coil 204 and the Y coil 20(5, the maximum radius rM obtained according to the above procedure results in a minimum value at which a magnetic field having an accuracy of an allowable range can be produced, This value results in one smaller than the outside diameter r00 of each of the X coil 204 and the Y coil 206. Many widely different embodiments of the invention may be cor figured without departing from the spirit and tie scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims. We Claim: 1. A gradient coil manufacturing method comprising the steps of: providing a pair of pole pieces each having a bottom plate portion and a peripheral edge portion protruding in a direction orthogonal to surfaces of said bottom plate portion, with said pole pieces being disposed opposite each other with the protruded peripheral edge portion formed with a space defined there between; manufacturing a pair of gradient coils having concentric passes and disposed along said surfaces of said bottom plate portion which produces a gradient magnetic field in said space by current that flows through said concentric passes; setting a maximum radius of one of said passes for each of said pair of gradient coils to a minimum value with a range for producing said gradient magnetic field having a magnetic field error within a predetermined range; and determining radii of said passes as herein described 2.The gradient coil manufacturing method as claimed in claim 1, wherein said pair of gradient coils are manufactured so that each comprises a first and a second flat coils placed on top of each other with said passes being placed in orthogonally disposed directions, and wherein a maximum value of each said first and second flat coils is within a minimum radius. S.The gradient coil manufacturing method as claimed in claim 2, wherein said pair of gradient coils bits are manufactured so that each comprises a third flat coil which has concentrically disposed windings. 4.The gradient coil manufacturing method as claimed in claim 3, wherein said third flat coil is manufactured so as to be placed to be on top of said first and second flat coils. 5.The gradient coil manufacturing method as claimed in claim 2, wherein said first and second flat coils are manufactured so as to be placed to be on top of each other and disposed within said peripheral edge portions. 6.The gradient coil manufacturing method as claimed in any of the preceding claims wherein each of said pair of gradient coils comprising a first and a second flat coils placed on top of each other with passes being placed in orthogonally disposed directions, and a third flat coil which has concentrically disposed windings; and a maximum radius of each of said first and second flat coils is set to be within a maximum radius of said third flat coil. 7.The method as claimed in claim 6, wherein said first and second flat coils are placed to be on top of each other and disposed within said peripheral edge portions. 8.The method o as claimed in claim 6, wherein said third flat coil is placed on top of said first and second flat coils. 9.The gradient coil manufacturing method as claimed in claim 1 for use in the magnetic resonance imaging system for forming an image, based on magnetic resonance signals acquired using a static magnetic field, a gradient magnetic field and a high-frequency magnetic field 10.A gradient coil manufacturing method substantially as herein described with reference to accompanying drawings.

Full Text

BACKGROUND OF THE INVENTION
The present invention relates to a gradient coil manufacturing method, a gradient coil and a magnetic resonances imaging system/ and particularly tp a gradient coil provided on a polar surface of a static magnetic field magnet, a method of manufacturing the same, and a magnetic resonance imaging system having such a gradient coil,.
In a magnetic resonance imaging (MRI Magnetic Resonance Imaging) system/ a target to be shot or imaged is carried in an internal oore of a magnet system, i.e., a bore or space in which a static magnetic field ife formed. A gradient magnetic field and a high-frequency magnetic field are applied to produce a magnetic rescnance signal within the target. A tomogrum is produced (reconstructed) based on its,received si.gr.al.
In a .rnagne system using permanent magne1;s.forrgeratingi.static magnetic fields,poie.preces for uniform!zing a magnetic fImx.di'stributibn in a static .magnetic field space ans rsspectively.provided at leading ends of a pair of the permanent magnets opposite to each other. Further, gradient coils for generating gradient magnetic fields are provided on their corresponding polar surfaces of nhe pole pieces.
In the abovji-describedmagnet system, the pole pieces are magnetized by the gradient nu.gnetic fields since the gradient; coils are respecivs.ly c;lo3e to the pole pieces. Due co the residual oradient nagne.ic ields ffcirined by their immanent nagnetizacion, the phase of a spin is subjected to .such an infuere as though eddy currents extremely log in tir.s constant hp.d existed. Therefore, it would interfere with imaging made by for example, a fast spin echo (FSE) method or the like which needs accurate phase control.
Statement Of Invention
Atjradient coil manufacturing method comprising the steps of
providing a pair of pole pieces each having a bottom plate portion and a peripheral edge portion protruding in a direction orthogonal to surfaces of said bottom plate portion, with said pole pieces being disposed opposite each other with the protruded peripheral edge portion formed with a space defined therebetween;
manufacturing a pair of gradient coils having concentric passes and disposed along said surfaces of said bottom plate portion which produces a gradient magnetic field in said space by current that flows through said concentric passes;
setting a maximum radius of one of said passes for each of said pair of gradient coils to a minimum value with a range for producing said gradient magnetic field having a magnetic field error within a predetermined range; and
determining radii of said passes according to the following procedure
(a) Setting measurement points Pi, wherein i=1-N, onto a maximum
spherical surface supposed in an imaging area;
(b) calculating magnetic fields Bit, wherein i=1-N, at measurement points,
to be produced by said gradient coils;
(c) setting a tolerance at for an error with respect to each magnetic field;
(d) setting a value rO of maximum radius of a pass for said each gradient
coil within a range not exceeding a limit value r00;
(e) defining radii of said passes as M, r2,. . . rM, and
max (r1, r2, . . . , rM) under the above restricted condition, and
r=(r1, r2 , rM),
with the above as a parameter as herein described,
determining optimum values of rj, wherein j=1-M, using quadratic programming so that the last above equation of this step (e) is established, wherein the following is calculated using Biot-Savart's law as herein described;
(f) calculating an error in magnetic field at each measurement point Pi
according to the equation as herein described
(g) determining rj when .αi ≤ αt is satisfied;
(h) when αi ≤ αt is not satisfied, increasing the value rO to be within a range not exceeding said limit value r00; and
(i) repeating the foregoing procedure subsequent to step (e).
A magnetic resonance imaging system for forming an image, based on magnetic resonance signals acquired using a static magnetic field, a gradient magnetic field and a high-frequency magnetic field, the system comprising
a pair of gradient coils configured as gradient coils each of which generates the high-frequency magnetic field, said pair of gradient coils being respectively provided along the surfaces of bottom plate portions lying inside peripheral edge portions of a pair of pole pieces having the bottom plate portions and the peripheral edge portions protruding in a direction orthogonal to surfaces of the bottom plate portions, with said pole pieces being disposed opposite each other with the protruded peripheral edge portion formed with a space defined therebetween, and which produces gradient magnetic fields in the space by currents that flow through a plurality of concentric passes, and wherein the maximum radius of one of said passes for each of said pair of gradient coils is set to a minimum value within a range for producing said gradient magnetic field having a magnetic field error within a predetermined range, the radii of said passes being determined according to the following procedure
(a) Setting measurement points Pi, wherein i=1-N, onto a maximum spherical
surface supposed in an imaging area;
(b) calculating magnetic fields Bit, wherein i=1-N, at measurement points, to be
produced by said gradient coils;
(c) setting a tolerance at for an error with respect to each magnetic field;
(d) setting a value rO of maximum radius of a pass for said each gradient coil
within a range not exceeding a limit value r00;
(e) defining radii of said passes as r1, r2, . .. rM, and
max (r1, r2, . . . , rM) under the above restricted condition, and r=(r1,r2t. .. , rM),
with the above as a parameter, as herein described
determining optimum values of rj, wherein j=1-M, using quadratic programming so that the last above equation of this step (e) is established, wherein the following is calculated using Biot-Savart's law as herein descibed;
(f) calculating an error in magnetic field at each measurement point Pi
according to the following equation
(g) determining rj when .αi ≤ αt is satisfied;
(h) when αi ≤ αt is not satisfied, increasing the value rO to be within a range not exceeding said limit value r00; and
(i) repeating the foregoing procedure subsequent to step (e).
SUMMARY ,OF THE INVENTION
Therefore, an object of the present invention is to implement a gradient coil low in magnetizing fcrco with respect to each pols piece, a method of manufacturing the same, aid n magnetic resonance imaging system having such a gradient coil.
(1) The invention according to. one aspect for solving the above problems! is a gradient coil manufacturing method comprising the tep of, upon manufacturing a pair of gradient coils which is respectively provided along the surfaces of bottom plate portions lying inside peripheral edge portions of a pair of pole pieces having the bottom plate portions and the peripheral dge portions protrudir.g in the direction orthogonal to the surf.aces-.of che bottom plate portions, the pole pieces being opposed topach. gther w;th • the protruded, peripheraled'gevpbrtl;gn;s f drmsd with a spacerdef ined. therebetween,, and'which arduces.'gradient magne-tlc^f ieids in the space by currents that flow through a-plurality of concentric passes, setting the maximum radius of one of the passes for each of the gradient coils to the minimum value within a range for producing a gradient magnetic field having a magnetic field error lyinc within a predetermined allowable range.
In the according to th.s aspect, the inurt radius of one pass for esc^ gradient coil is set tc the minimur. value at vhich a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the c'.istanca between the outermost pas.s and a protruded peripheral edge portion of each pole
jjpiece increases. Therefore, the magnetizing force with respect to the protruded peripheral edge portion is '.ow and the residual magnetization is low.
(2) The invention according to ar.other aspect for solving the above problems is the gradient coil manufacturing method described in (1) , wherein the radii of the plurality of passes are determined according to the following procedures. Note
(a.) setting measurement points Pi (where i=l-N) onto the maximum epherical surface supposed in an imaging area.
(b) calculating magnetic fields B..t (where i=l-N) at the measurement
points, to be produced by the gradient coils.
(c) setting a tolerance at for ein error with respect to each magnetic
field.
•(•d) setting an allowable value rO of the maximum radius of the pass ;f or .each gradient ; coil within a, -range- not.- exceeding a, 'limit -value r00.
(e) defining .'the radii of the. plurality of ' passes '.as rl, r2, . .., • rM .
max (rl, r2, . . . , rM) with the above as a parameter,
(Equation Removed)
determining ths optimum values of rj (where j=l-M) using quadratic firogramraing so that the above equation 18 is established. Incidentally,
the above is calculated using the Biot-Savart' s law.
(i) calculating an error in macneuic field at each measurement point ti according tc the following equation.
(g) determining rj when aiSat. iis satisfied.
(h) when αi ≤ αt is not satisfied, .Increasing the allowable value r3 within a.range. jiot vexceeding'-,the 1 iir.i t.value r00;; and- (1) repeating the-procedures sub-sevquent to . (e)
In the invention according to this aspect, .the maximum, radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance between tshe outermost pass and a protruded peripheral edge portion of each pole pdece increases. Therefore, the magnetizing force with resp-sct to the Dsrctrudec. periph'.sral edge portion is low and the residual magnetization is low.
(3) The invention according to a further aspect for solving the; above problems is a pair of gradient coils which is respectively provided al.ong
the surfaces of bottom plate portion? lying inside peripheral edge portions of a pair of pol pieces having the bottom plate portions and the peripheral edge portions protruding in the direction orthogonal to the surfaces of the bottom plate portions, the pola pieces being opposed to each other with the protruded peripheral edge portions formed with a space defined therebe-ween, and which produces gradient magnetic fields in the space toy currents that flow through a plurality of concentric passes, wherein the maximum radius of one of the possess for each of the gradient coils is set to the minimum value within a rungo for producing a gradient magnetic tfield having a magnetic field error lying within a predetermined allowable iSange,
In the invention according to this aspect, the maximum radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable .range can be produced. Thus, the distance between tlha.aute-rmo.st pass. and a protruded peripheral/edge portion 'of"eah"'pole, piece increases.. Therefore; the magnetizing force with respect.to the-.protruded peripheral edge portion.is' lew -and the residualr.magneti.zation iis low.
(4; The invention according to a still further aspect for s.olving the above problems is the pair of gradient coils described in (3) , wherein the plurality of passes respectively have radii determined according to the following procedures. Note
(a) setting measurement points Pi (where i=l-N) onto the maximum
spherical surface supposed in an imaging area.
(b) calculating magnetic fields Bit (where i-l-N) at the measurement
points, to be produced by the gradient coils.
(c) setting a tolerance at for an error with respect to each magnetic field.
(3) setting an allowable valua rD of the maximum radius of the pass for eaen gradient coil within a range not exceeding a limit value r00.
(e) defining the radii of the plurality of passes as rl, c2, ..., rM.
max (rl, r2, ..., rM) with the above as a parameter,
(Equation Removed)
determining .the optimum values o.f -rj where 'j=1-M) using-quadratic programming so that the above equation 23 is established.. Incidentally,
the above is calculated using the Bict-Savart's lav;.
(f) calculi tir.g an error in magneti c field at each ns asurerr.erv poin-?i accorc.ir.g to the following equation.
(cj) determining rj when αi ≤ αt is satisfied.
(h) when αi ≤ αt is not satisfied, increasing the allowable value rO Within £i range not exceeding the limit value r00, and (i) repeating the procedures subsequent to (e) .
In the invention according tc this aspect, the maximum radius of one pass for each gradient coil is set. to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance oetween
the outermost pass and a protruded peripheral edge portion of each pole
i piece increases. Therefore, the magnetizing force with respect to the
protruded peripheral edge portion is; low and the residual magnetization iis low.
(5) The invention, according tc a-still,further, aspect for. jiolving tine above problems is a magnetic resoranceritiaging sys,tem f or forming an.image, based, on magnetic resonance signals acquired.,using a static •niagnetic filed, a gradient magneticfield and a high-frequency magnetic field, comprising, a pair of gradient ceils configured as gradient, coils each of which generates the high-frequency magnetic field, the pair of gradient coils birg respectively provided along the surfaces of bottom plate portions Ivir.g inside peripheral edge portions of a pair of pole pieces having tr.e c-tom plate portions and the peripheral edge portions protruding in the direction orchogonai. tc the surfaces of ths bo-coir plate portions, the pels pieces being opposad to each other with the protruded. peripheral edge portions formed with a space defined therebetween, and

roauces gradient magnetic ields in the space by currents that flow through a plurality of concentric passes, and wherein the maximum radius of the pa.ss is set to the minimuit. value within a range for producing a gradient magnetic field having a magnetic field error lying within a predetermined allowable range.
In the invention according to t'.iis aspect, the maximum radius of one pass for each gradient coil is se'i to the minimum value at which a gradient magnetic field having a magretic field error lying within a predetermined al-lowable range can be produced' Thus, the distance between the outermost pass and a protruded peripheral edge portion of ee.ch pole piece increases. Therefore, the magnetizing force with respect to the protruded peripheral edge portion i.s low and the residual magnetization is low. Thus, imaging on which the residual magnetization has a little efifect, can be carried out.
(6) The invention according to a still further aspect for .solving the. -above. problems .is the...magnetic renonanca. imaging; system described. iit (5)., w'.ierein the plurality of passes raspectively have radii detormined according., to .the; following procedures. Note
(a) setting measurement point 'where i=l-N) cnto the maximum
spherical surface supposed in an imaging area,
(b) calculi, ting magnetic fields Bi [where i=l-N) at the measurement
points, to be produced by the gradient coils.
(c) setting a tolerance at for an er ror with respect to each magnetic
field.
(d) setting an allowable value .0 (if the maximum radius of the pass
for each gradient coil within a rangs not exceeding a limit valus r00.
(e) defining the radii of thfis plurality of passes as rl, r2, ...,
rW.
max (rl, r2, . . , rM) with the above as a parameter,
(Equation Removed)
determining the optimum values of rj (where j=l-M) using quadratic programming so that the above equation i8 is established. Incidentally,
above is . calculated -using the-Blot-Sava-rt's law. .
(f) calculating an error in magneti.c field at each measuremen' point Pi according to the following equation.
(Equation Removed)
(g) determining rj when αi ≤ αt is satisfied.
(h) when αi ≤ αt is not satisfied,, increasing the allowable value rO within a range not exceeding the limit value r00, and (i) repeating the
procedures subsequent to (e) .
In the invention according to this aspect, the maximum radius of one pass for each gradient coil is set to the minimum value at which a gradient magnetic field having a magnetic field error lying within a predetermined allowable range can be produced. Thus, the distance between the outermost pass and a protruded pezipheral edge portion of e.ach pole piece increases. Therefore, the mitgnstizing force with respec- to the protruded peripheral edge portion is low and the residual magnetisation is low. Thus, imaging on which the re.sidual magnetization has a little Effect, can be carried out.
According to the present invention, a gradient coil reduced in magnetiziing force with respect to a pcle piece, a manufacturing method therefor, and a magnetic resonance imacing system having such a gradient coil can be implemented.
Further objects and advantages of the present invention will be Apparent from the foJ.lowingde-scrip.tion.of the preferred eir.bodiment.s.of. the invention as illustrated in the .accompanying drawings.
BRIEF DESCRIPTION 0F THE DRAWINGS
Fig. i is a block diagram of a system showing one example of an embodiment according to the present invention.
Fig. 2 is a diagram showing one exanrtcle of a pulse a&quer.ce executed by the system shcwr. in Fig. 1.
Fig , 5 is a diagram illustrating or.e example cf a pul.se sequence Executed by the system shown in Fig. 1.
Fig. 4 is a typical diagram showing a cr.figuratior. of the neighborhood of a gradient coLl of a magnet system onoloyed in the system shown in
FLg. 5 is a schematic illustration showing patterns for current passes of a gradient coil.
Fig. 6 is a flowchart for descriaing the procedure of determining rajdii of. current passes.
DERAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention will.hereinafter be described in detail with reference to the accompanying drawings . A block di&gram of a magnetic resonance imaging system is shown in Fig. 1. The present system is one example of an embodiment of the present invention. One example of an embodiment related to a system of the present invention is shown according to the configuration of the present system.
As shown in Fig. If the present system has a magnetic system 100. The magnetic system 100 has main magnetic field magnet units 102, gradient coil units 106 and RP /radio frequency) coil-.units.10.8'..Any-of theise main-magnetic field magnet units 102 and respective coil.units comprises paired; onfcs oppoised to one another with a space inter-posed therebetween. Further, any of thorn has a substantially disc sshape and is placed with its central axis held in common. A target 300 is placed on a cradle 500 in an internal boye of tlie magnetic system 100 and carried in and out by unillustrated conveying means.
Eac.iof thenainmagnetic f ieldmaijnetunits 1C2 forms a staticmaqnetic field in the internal bore of the magnetic system 100. The direction of the static magnetic field is approxinataly orthogonal to the direction of (the body axis of the target 300. Nanely, each of the main macnetic field magnet units 102 forms a so-called vertical magnetic field. Each
of the main magnetic field magnet units 102 is configured using a permanent
mignet or the like, for example. Incidentally, the main magnetic field magnet unit 102 is not limited to the permanent magnet and may of course be configured using a superconductiv= electromagnet or a normal conductive electromagnet or the like.
The gradient coil units 106 p.codace gradient magnetic fie.'.ds used for causing the intensity of the static magnetic field to have a gradient or slope. The produced gradient magnetic fields include three types of gradient magnetic fields of a slice gradient magnetic field, a read out gradient magnetic field and a phase ancode gradient magnetic field. The gradient coil unit 106 has unillustrat-sd 3-systematic gradient coils in association with these three types of gradient magnetic fields.
The three-systematic gradient coils respectively produce three gradient magnetic fields for applying gradients to static magnetic fields respectively as viewed in three directions, orthogonal to or.e another. Qiie of., .the.'.thre(5.directions cor responds., to ..-the', direction •.•.(vertical ••••. .d-Jjcection) pfv,the -static'magnetic f ieia.. and is .normally, -.defined as .a z .direction.' Another onethereof corresponds to a.horizontal direction and .is normally defined as a y direction. The remaining one corresponds to. the direction orthogonal to the z and y directions and is normal.ly defined ae an x direction. The x direction is orthogonal to the z direction within the vertical plane and perpendicular to they direction within the horizontal plane. x, y and z are also called qradient axes below.
Any of x, y and z can be set as an axis for a slice gradient, when any of them is ssst as the slice grad.en axis, one of the remaining two is set as an axi; for a phase encode gradient and the other thereof is set as an axis for a read out gradient. The 3-systematic gradient coils
be explained again later.
Each of the RF coil units 108 transmits an RF excitation signal for exciting a spir. in a body of the targst 300 to a static magnetic field apace. Further, the RF coil unit 108 receives therein a magnetic rsonance aignal which produces the excited spin. The RF coil unit 108 has unillustrated transmitting and recoivtng coils. The transmitting coil and the receiving coil share the use o the same coil or make use of dedicated Ooils r&spectively.
A gradient driver 130 is connected to the gradient coil units 106. Tthe gradient driver 130 supplies a drive signal to each of the gradient aoil units 106 to generate a gradient magnetic field. The gradient, driver 130 has unillustrated 3-systematic dr.ve circuits in association with the 3-systematio gradient coils in the gradient coil unit 106.
A F.F drivei 140 is connected to the RF coil units 106. The RF driver 140 supplies a drive signal to each .of v.he RF coil units 108 to transmit an RF-excita.tior signal, thereby., exert ing-the- spinin the body of ..the/. target 300.
A data collector 150 is connected to .each of the RF coil units 108. The data collector 150 takes in or captures signals received by the RF coil units 108 and collects the same as view data (view data) .
A controller 160 is connected to the gradient driver 130, the RF driver 140 and the data collector 150. The controller 160 controls the gradient driver 1.30 to data collector 150 respectively to execute shooting or imaging.
The output side of the data co.Hector 150 is connected to A data processor 170. The data processor 170 is configured using a computer (computer) or the- like, for example. The data processor 170 has an
•jjniflustrated memory. The memory £itoes a program and various data for tine data processor 170 therein. The function of the present system is implemented by allowing the data processor 170 to execute the program stored in the memory.
The data processor 170 causes the memory to store the data captured from the data ccllector 150. A data space is defined in the memoy. The data space forms a two-dimensional Fourier space. The data processor 170 transforms these data in the two-dimensional Fourier space into two-dimensional inverse Fourier fom to thereby produce (reconstruct) an image for the- target 300. The two-dimensional Fourier space is also called a "k~spaoe",
Ths data processor 170 is connected to the controller 160. The data ptocessor 170 ranks ahead of the controller 160 and generally controls it. Further/ a display unit 180 and an operation or control unit 190 are connected to the data processor 170. he display unit 18.0 .comprises a •graphic display, or-the .like- The operation unit 190'. comprises-a keyboard-
or the like provided with a pointing cevise. •
. The display unit 180 displays "a reconstructed image and various . information outputted from the data processor 170. --.The operaticn unit 190 is operated by an operator and inputs various commands and information or the like to the data processor 170. The operator controls the present
system on an interactive basis through the display unit 160 and the operation unit 190.
Fig. 2 ;,hovs one example of a pulse sequence used when ..maging cr shooting is done by the present system The present pulse sequence corresponds to a pulse sequence of a gradient echo (GRE) method.
Nair.oly, (1) shows a sequence of a α° pulse for RF excitation employed
in the GRE method. (2), (3), (4) and (5) similarly respectively show Sequences of a slice gradient Gs, a read out gradient Gr, a phase encode gradient Gp and a gradient echo MR. Incidentally, the α° pulse is -;ypif ied by a center signal. The pulse sequence- proceeds from left to right along 4 time axis t.
As shown in the same drawing, α° excitation for the spin is carried out bas&d on the α° pulse. A flip angle α° is less than or equal to 90°. At this time, the slice gradient Gs is applied to effect selective excitation qn a predetermined slice.
After the α° excitation, the spin is phase-encoded based on the phase encode gradient Gp. Next, the spin is first dephased based on the read out gradient Gr. Next, the spin is reohased to generate each gradient etcho MR. The signal strength of the gradient echo MR reaches a maximum aifter an echo time TE has elapsed since the excitation. The gradient echo MR. is collected as view data by the data collector 150.
Such a.-.pulse . sentience is. repeated 64..to-512. 'timesin-a.cycle- TR-. .(repetition time) . Each- time it is..repeated, ;the .phase encode gradient Gp is changed'...ard different phase encodes are "carried out. .every time. Thus., view data f >r 64 to 512 views for filing in a.k space can be obtained.
Another example of a pulse sequence for magnetic resonance imaging is shown in Fig, 3. The pulse sequence corresponds to a pulss sequence of a spin echo (SE) method.
Namely, shows a sequence of a 90° pulse and a 190s pulst for RF excica-ior. employed in the SE method. (2), (3), (4' and (S) similarly respectively show sequences of a slice tjradient Gs, a read out gradient. Gj, a pha&e encode gradient Gp and a spir. acho MR. Incidentally, the 30° pulse and 130° pulse are respectively typified by center signals. The
pijuse sequence proceeds from left to right along a time axis t.
As shown .n the same drawing, 90° excitation for the spin is carried otlt basad on the 90° pulse. At this time, the slice gradient Gs is applied to effect selective excitation on a predetermined slice. After a predetermined time has elapsed since the 90° excitation, 180° excitation based on the 180° pulse, i.e., spir. inversion is carried out. Even at t£is time, the slice gradient Gs is applied to effect selective inversion oa the same slice.
The read out gradient Gr and the phase encode gradient Gp are applied dwring a period between the 90° excitation and the spin reversal. The spin, is dephased based on the read out gradient Gr. Further, the spin is phase-encoded based on the phase encode gradient Gp.
After the spin reversal, the spin is rephased based on the read out gradient Gr to produce each spin echo MR. The signal strength of the spin efcho MR..reaches ;a maximum after TE has elapsed-since the 90° excitation. .The..spin echo.MF.'is collected as view.-data ..by.the data collect or1 .150. Such a.1 pulse sequence .is repeated 6-4 .to 512..times .-ina -cycle -.TR. Each time .it .is repeated,- the phase encode gradient Gp is'Changed and different phase encodes are carried out every timis.- Thus, view data for 64 to. 512 views for filling in a k space can obtained.
Incidentally, the pulse sequence used for imaging is not limited tie .he GR12 method or S3 method. The pulse sequence nay be other suitable techniques such as ar. FSE (Fast Spin Echo) method, a fast recovery F3E fiFas- Recovery Fiist Spin Echo) method, echc planar imaging (EPI) , etc.
The data processor 1"/0 transforms ihe view data in the k space into two-dimensicinai inverse Fourier forn to thereby reconstruct a tonogram for the target 3CO . The reconstructed image is stored in its corresponding
memory and displayed on the display unit 180.
Fig. 4 typically shows the structure of the magnet system 100 located in the neighborhood of the gradient soil units 106 in the form of a cross-sectional view. In the same drawing, 0 indicates the center of a static magnetic field, i.e., a roacnev. center, and x, y and 2 indicate the aforementioned three directiors respectively.
A spheric volume SV of a radius R with the magnet center 0 as the center is a shooting or imaging area. The magnet system 100 is configured so that the static magnetic field and gradient magnetic field respectively have a predetermined accuracy in the SV.
A pair of main magnetic field magnet units 102 has a pair of pole
pieces 202 opposed to each other. The pole piece 202 is composed of a
magnetic material having high permeab.lity, such as a soft iror. or the
like and serves so as to uniformize a magnetic flux distribution in a
static magnetic field space.
the polepieces. 202 are • respectivelyrshaped -substantially.-the -fornrof 'dis.cs but protrude in the direction (.2-'direction) ..in which .their ;• peripheral edge, portions are orthogonal to their plate "surfaces, -i.e., in the direction in which the pole p.ecss 202 are opposed to each other. Thus, the pole pieces 2 02 have hot torn plate portions and protruded per .pher a 1 edge portions. protruded peripheral, edge portions serve so as to make up for re?ductior.s in magnetic flux density at the peripheral sc.ges of the pole pieces 202.
The cradier'; coil units 106 ae respectively provided in their corresponding concave portions of the pole pieces 202, which are defined inside the protruded peripheral edge porrions. -each of the gradient coil units 106 has an X coil 204, a Y co.l 206 and a Z208 coil.
Of these, the Z coil 208 is one example of an embodiment of a gradient coil employed in the present invention. Any of the respective soils is shaped substantially intheformof a disc. The respective coils are mounted on a polar surface of the pole piece 202 by unillustrated suitable mounting means so that they are successively layered.
Pc.tterns ;or current passes of tiie X coil 204, Y coil 206 and Z208 eoil are shown in Fig. 5 by a diagrarrmatic illustration. As showi in the $ame drawing, the X coil 204 has linear plural main current passes (main passes) parallel to a y direction at a portion near the center of a circle. these main passes are symmetric with espect to a y axis which passes through the center of the circle. Return passes for the main passes are formed along the circumference of the cicle. The radius of the outermost return pass, i.e., the outside diaireter of the X coil 204 is g.ven as r00.
The; Y coil 206 has linear plural naii passes parallel to an x direction ,data portion near the center of a circle These main- passes are-.-symmetri-c •. vfith respect .co. c.n x-.axis which parsses.thro.ugh?-the center'of the circle.'. Ue.turn .passes .fo the main passes are formed-along, -the -circumference of." the-circle. Tha radium of the outermost return pass, . the outside diameter of the Y coil 206 is given as r00.
The Z coil 206 has s. plurality of current passes which forr, concentric Circles respectively. These current passes are all main passes . The radii of the respective main passes are given as rl, r2, ..., rM in order from inside. H is defined as the outer diarr.eter of the Z coil 2 OS.
Since the Z coil 208 has no returr. passes, it is ensy to generate a gradient, magnetic field good in linearity as compared with those having return passes as in the case of the X coil 204 and the Y coil 206.
Thus, even if the outside diameter rM of the Z coil 209 is set smaller than the outside diameters r00of the X coil and the Y coil 20(5, the Z coil 206 can produce a gradient magnetic field having linearity equivalent to that of each of the X coil and the Y coil 206.
Since the distance from the outur periphery of the Z coil. 208 to each peripheral protruded portion oi' the pole piece 202 is increased when the outside diameter rM of the Z coil 208 is reduced, the magnetizing force of each protruded portion by the Z coil 208 becomes weak in proportion to the square of the distance. Accordingly, the residual magnetization Of each pole pifcce 202 can be lowerad by reducing the outside diameter tM of the 2 coil 208.
Insidentally/ the magnetization of each protruded portion by the X coil 204 and the Y coil 206 produces; a little influence as compared vfith tha-; by the Z coil because the nagietization intensities developed by the main passes .and' return passes ast uo as to be opposite-to one another if IT." polarity.
The use o.f si.uch a.magjiet system small in residual-magnetization makes it.possible to properly perform, even .-shotting or.'imagirtg by, fcrvexample, the FSE trethud o;r che like which requires accurate .phase control on the spin.
A method of manufacturing such a 2 coil 208 will next be described. The radii rl, r2, . . . , rM of the plurality of current passas that constitute the main passes, i.e., .he concentric circles respectively., are determined upon manufacturing rr.s Z coil 209.
Fig. 6 shows, a flow chart for describing the procedure! of determining the radii of the main passes. As shov.n in the sane drawing, in Step 6C2, N points -o be measured PI (xl, yl, l) , ?2 (x2, y2, z2}, ..., PH {xN,
yN, zN) are set onto the maximum spherical surface of an imaging area, i.e., the surface of SV shown in rig. 4.
Niaxt, in Step 604, magnetic fielc.s Bit, B2t, . . ., BNt to be produced by each Z coil 208 at the measurement points PI, P2, . . . , PN are calculated. The following equation is used to calculate the magnetic fields.
(Equation Removed)
where,
g magnetic field gradient
Next, in Step 606, an allowable value or tolerance at on an error with respect to each magnetic field is set. at is set so as to be equal to a tolerance on an error with res;peot to each of the X coil .204 and the Y coil 206.
Next,. inStep 608, a tolerance or c.llowable .-.value rO for the outside diameter of the Z. coil 208 is set .• .The rO is -set within a range not exceeding a limit -.value r00 The. limit value r00 corresponds -to- each ..of the outside diameter.of the -X-rcoil 204 and the Y coil.2.0S
Next, in Step 610, the optimum vai.-ues-.of the radii rl, r2, . . ., rM of the plural main passes for the Z ceil 208 are determined. Namely,
max (rl, r2, . . . , rM) the above is used as a parameter.

(Equation Removed)
The optimum values of rj (where j=L-K) are determined using quadratic
programming so that the last above equation is established.
Incidentally, the above is calculated using the Biot-savart's law.
Next, in Step 612, an error in magnetic field at a measurement point Pi is calculated according to the following equation.
(Equation Removed)
Next, it is determined in Ste;? 614 whether αi ≤ αt is satisfied..
If the answer is found to be negative, then the allowable Vetlue rC is incremented by Ar within the range nat exceeding the limit value r00 in Step 616, and the routine procedure subsequent to Step 610 is repeated.
When αi ≤ αt is satisfiea, rl, x2, . . ., rM are determined in Step 618. The Z coii'. 208 having tihe plurality o.f m.ain passes, which ccns'iituce the concentric circles having the deterninec radii is manufactured.
Siroe the allowabie value of error kith respect to the; magnetic field is set to the sarr.e allowable value as each of the X coil 204 and the Y

coil 20(5, the maximum radius rM obtained according to the above procedure results in a minimum value at which a magnetic field having an accuracy of an allowable range can be produced, This value results in one smaller than the outside diameter r00 of each of the X coil 204 and the Y coil 206.
Many widely different embodiments of the invention may be cor figured without departing from the spirit and tie scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.

We Claim:
1. A gradient coil manufacturing method comprising the steps of:
providing a pair of pole pieces each having a bottom plate portion and a peripheral edge portion protruding in a direction orthogonal to surfaces of said bottom plate portion, with said pole pieces being disposed opposite each other with the protruded peripheral edge portion formed with a space defined there between;
manufacturing a pair of gradient coils having concentric passes and disposed along said surfaces of said bottom plate portion which produces a gradient magnetic field in said space by current that flows through said concentric passes;
setting a maximum radius of one of said passes for each of said pair of gradient coils to a minimum value with a range for producing said gradient magnetic field having a magnetic field error within a predetermined range; and
determining radii of said passes as herein described
2.The gradient coil manufacturing method as claimed in claim 1, wherein said pair of gradient coils are manufactured so that each comprises a first and a second flat coils placed on top of each other with said passes being placed in orthogonally disposed directions, and wherein a maximum value of each said first and second flat coils is within a minimum radius.
S.The gradient coil manufacturing method as claimed in claim 2, wherein said pair of gradient coils bits are manufactured so that each comprises a third flat coil which has concentrically disposed windings.
4.The gradient coil manufacturing method as claimed in claim 3, wherein said third flat coil is manufactured so as to be placed to be on top of said first and second flat coils.
5.The gradient coil manufacturing method as claimed in claim 2, wherein said first and second flat coils are manufactured so as to be placed to be on top of each other and disposed within said peripheral edge portions.
6.The gradient coil manufacturing method as claimed in any of the preceding claims wherein each of said pair of gradient coils comprising a first and a second flat coils placed on top of each other with passes being placed in orthogonally disposed directions, and a third flat coil which has concentrically disposed windings; and
a maximum radius of each of said first and second flat coils is set to be within a maximum radius of said third flat coil.
7.The method as claimed in claim 6, wherein said first and second flat coils are placed to be on top of each other and disposed within said peripheral edge portions.
8.The method o as claimed in claim 6, wherein said third flat coil is placed on top of said first and second flat coils.
9.The gradient coil manufacturing method as claimed in claim 1 for use in the magnetic resonance imaging system for forming an image, based on magnetic resonance signals acquired using a static magnetic field, a gradient magnetic field and a high-frequency magnetic field
10.A gradient coil manufacturing method substantially as herein described with reference to accompanying drawings.

Documents:

444-del-2001-abstract.pdf

444-del-2001-claims.pdf

444-del-2001-correspondence-others.pdf

444-del-2001-correspondence-po.pdf

444-del-2001-description (complete).pdf

444-del-2001-drawings.pdf

444-del-2001-form-1.pdf

444-del-2001-form-18.pdf

444-del-2001-form-2.pdf

444-del-2001-form-3.pdf

444-del-2001-form-5.pdf

444-del-2001-gpa.pdf

444-del-2001-pa.pdf

444-del-2001-petition-137.pdf

444-del-2001-petition-138.pdf

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Patent Number

231584

Indian Patent Application Number

444/DEL/2001

PG Journal Number

13/2009

Publication Date

27-Mar-2009

Grant Date

06-Mar-2009

Date of Filing

30-Mar-2001

Name of Patentee

GE MEDICAL SYSTEMS GLOBAL TECHNOLOGY COMPANY LLC

Applicant Address

3000 NORTH GRANDVIEW BOULEVARD, WAUKESHA, WISCONSIN 53188, U.S.A.

Inventors:

#	Inventor's Name	Inventor's Address
1	GOTO TAKAO	7-127, ASAHIGAOKA, 4-CHOME, HINO-SHI, TOKYO 191-8503, JAPAN.

PCT International Classification Number

G01R 33/385

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2000-117881	2000-04-19	Japan