CA1072175A - Apparatus for improving the radial homogeneity of axial magnetic fields - Google Patents
Apparatus for improving the radial homogeneity of axial magnetic fieldsInfo
- Publication number
- CA1072175A CA1072175A CA261,427A CA261427A CA1072175A CA 1072175 A CA1072175 A CA 1072175A CA 261427 A CA261427 A CA 261427A CA 1072175 A CA1072175 A CA 1072175A
- Authority
- CA
- Canada
- Prior art keywords
- polefaces
- magnetic field
- central axis
- coil
- flux
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F7/00—Magnets
- H01F7/06—Electromagnets; Actuators including electromagnets
- H01F7/20—Electromagnets; Actuators including electromagnets without armatures
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F27/00—Details of transformers or inductances, in general
- H01F27/34—Special means for preventing or reducing unwanted electric or magnetic effects, e.g. no-load losses, reactive currents, harmonics, oscillations, leakage fields
- H01F27/38—Auxiliary core members; Auxiliary coils or windings
Landscapes
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Magnetic Resonance Imaging Apparatus (AREA)
Abstract
APPARATUS FOR IMPROVING THE RADIAL
HOMOGENEITY OF AXIAL MAGNETIC FIELDS
Abstract of the Invention Apparatus for improving the radial homogeneity of axial magnetic fields obtained between polefaces is provided which includes a plurality of magnetic field generating means each located between said polefaces and at the outer end of equal length radii extending from the central axis of said polefaces. The plurality of magnetic field generating means are energized so that the magnetic lines of flux created thereby will be in the same direction as the magnetic lines of flux created by the polefaces.
The magnetic field generating means are positioned with respect to said polefaces so that the magnetic lines of flux created thereby are minimal at the center of said polefaces and increase away from the center thereof, thereby compensating for magnetic field drop-off away from the central axis of the polefaces.
HOMOGENEITY OF AXIAL MAGNETIC FIELDS
Abstract of the Invention Apparatus for improving the radial homogeneity of axial magnetic fields obtained between polefaces is provided which includes a plurality of magnetic field generating means each located between said polefaces and at the outer end of equal length radii extending from the central axis of said polefaces. The plurality of magnetic field generating means are energized so that the magnetic lines of flux created thereby will be in the same direction as the magnetic lines of flux created by the polefaces.
The magnetic field generating means are positioned with respect to said polefaces so that the magnetic lines of flux created thereby are minimal at the center of said polefaces and increase away from the center thereof, thereby compensating for magnetic field drop-off away from the central axis of the polefaces.
Description
19 fitatem~nt of the Invention Thls invention relates to lmprov~ng the rndlal homo-21 geneity of magnetic fields and, more partlcularly, relates 22 to apparatus for compensating for magnetic field drop-off 1 23 away from the central axis of the polefaces.
'i ~' ' ' ! 24 - Background of the Invention .j, .
j 25 It is well known that nuclear magnetic resonance (NMR) 'i 26 apparatus requires magnetic fields having a high degree 1 1, 27 of homogeneity. Any irregularities in the magnetic field . .. . ~ i . ~ .... .
- ~-i 28 are normally shimmed out by utilizing electric coils which - - . ..
; 29 arè located in a plane perpendicular to the poleface axis.
~` 30 ^ Controlling the current to thè coils can, correspondingly, , .
, . . . .
. . . :
` j - PO9-75-016 -1-!, ~
~ '.
~. ' ':
.
== I . ~
' ' ~` ' "' ' ' .:
', . .
- ` , ' ,': ' . ' ' ' . . ' .' ' ' .
', . ' , . . ' ' . ~ ' ' ' , ' ' . ~
.. . .
.' . " ~ ' ' . ' , .. , , .. ' . ' , , ' ' , ' ~ ' ' . .
` 1 control the magnetic field generated by the coils and, consequently, control the flux density in the area under control.
U.S. Patent 3,566,255, filed March 6, 1959 and issued February 23, 1971 discloses various coil arrangements for improving the homogeneity of magnetic fields and, in par-ticular, for removing substantially all of the first and second order gradients in the given magnetic field. Actually, all of the first order gradients can be removed by the provision of five sets of independent or mathematically : orthogonal coil geometries. U.S. Patent 3,582,779, filed January 20, 1966 and issued June 1, 1971, shows an arrange-ment having seven independent coils disposed about the ~ magnetic field within which it is desired to remove the - second order gradients. These patents set forth a corrective technique of controlling the magnetic field which appears to have the inhomogeneities, which is generally known as shimming, and which removes all the first and second order inhomogeneities. When applied between polefaces, these coils ~ 20 are mounted on the poleface perpendicular to the poleface ; axis. These patents ignore the radial drop-off in field ` away from the central axis of the polefaces.
SUMMARY OF THE INVENTION
It is the main object of the present invention to provide apparatus capable of correcting for the radial drop-off in magnetic field strength.
It is another object of the present invention to provide apparatus which can be varied to adjust the com-pensating field which corrects for the radial drop-off - 30 in field strength.
~0~175 1 It is a further ohject of the present invention to
'i ~' ' ' ! 24 - Background of the Invention .j, .
j 25 It is well known that nuclear magnetic resonance (NMR) 'i 26 apparatus requires magnetic fields having a high degree 1 1, 27 of homogeneity. Any irregularities in the magnetic field . .. . ~ i . ~ .... .
- ~-i 28 are normally shimmed out by utilizing electric coils which - - . ..
; 29 arè located in a plane perpendicular to the poleface axis.
~` 30 ^ Controlling the current to thè coils can, correspondingly, , .
, . . . .
. . . :
` j - PO9-75-016 -1-!, ~
~ '.
~. ' ':
.
== I . ~
' ' ~` ' "' ' ' .:
', . .
- ` , ' ,': ' . ' ' ' . . ' .' ' ' .
', . ' , . . ' ' . ~ ' ' ' , ' ' . ~
.. . .
.' . " ~ ' ' . ' , .. , , .. ' . ' , , ' ' , ' ~ ' ' . .
` 1 control the magnetic field generated by the coils and, consequently, control the flux density in the area under control.
U.S. Patent 3,566,255, filed March 6, 1959 and issued February 23, 1971 discloses various coil arrangements for improving the homogeneity of magnetic fields and, in par-ticular, for removing substantially all of the first and second order gradients in the given magnetic field. Actually, all of the first order gradients can be removed by the provision of five sets of independent or mathematically : orthogonal coil geometries. U.S. Patent 3,582,779, filed January 20, 1966 and issued June 1, 1971, shows an arrange-ment having seven independent coils disposed about the ~ magnetic field within which it is desired to remove the - second order gradients. These patents set forth a corrective technique of controlling the magnetic field which appears to have the inhomogeneities, which is generally known as shimming, and which removes all the first and second order inhomogeneities. When applied between polefaces, these coils ~ 20 are mounted on the poleface perpendicular to the poleface ; axis. These patents ignore the radial drop-off in field ` away from the central axis of the polefaces.
SUMMARY OF THE INVENTION
It is the main object of the present invention to provide apparatus capable of correcting for the radial drop-off in magnetic field strength.
It is another object of the present invention to provide apparatus which can be varied to adjust the com-pensating field which corrects for the radial drop-off - 30 in field strength.
~0~175 1 It is a further ohject of the present invention to
2 provide apparatus which can be adjusted to compensate for lack of parallelism between the polefaces.
4 The invention comprises apparatus for Lmproving the radial homogeneiey of axial magnetic fields obtained from 6 polefaces, wherein a plurality of magnetic field generating 7 means are provided, each located between said polefaces and 8 at the outer end of equal length radii extending from the 9 central axis of the polefaces. The plurality of magnetic field generating means are energized so that the magnetic 11 lines of flux created thereby will be in the same direction 12 as the magnetic lines of flux between the polefaces. The ~: .
13 magnetic field generating means are positioned with respect 14 to the polefaces so thàt the magnetic lines of flux created thereby are minimal at the center of the poleface and maximum 16 away from the center, thereby compen3ating for any magnetic 17 field drop-off away from the central axis of the poleface.
18 The magnetic field generating means are independently 19 adjustable so that any lack of parallelism between the polefaces can be compensated for.
21 Déscription of the Drawings 22 Fig. 1 is a schematic diagram showing the poleface 23 assembly and the magnetic flux field therebetween.
24 Fig. 2 is a graphical representation of the variation in magnetic field B versus distance from the poleface 26 center R.
27 Fig. 3 is a graphical representation of the flux Bz 28 versus radius ~ for the coils of the invention.
. . .
Po~-75-n~6 -3-1 Fig. 4 is a schematic representation showing the two 2 loops of a coil and the variable current source thereof.
4 The invention comprises apparatus for Lmproving the radial homogeneiey of axial magnetic fields obtained from 6 polefaces, wherein a plurality of magnetic field generating 7 means are provided, each located between said polefaces and 8 at the outer end of equal length radii extending from the 9 central axis of the polefaces. The plurality of magnetic field generating means are energized so that the magnetic 11 lines of flux created thereby will be in the same direction 12 as the magnetic lines of flux between the polefaces. The ~: .
13 magnetic field generating means are positioned with respect 14 to the polefaces so thàt the magnetic lines of flux created thereby are minimal at the center of the poleface and maximum 16 away from the center, thereby compen3ating for any magnetic 17 field drop-off away from the central axis of the poleface.
18 The magnetic field generating means are independently 19 adjustable so that any lack of parallelism between the polefaces can be compensated for.
21 Déscription of the Drawings 22 Fig. 1 is a schematic diagram showing the poleface 23 assembly and the magnetic flux field therebetween.
24 Fig. 2 is a graphical representation of the variation in magnetic field B versus distance from the poleface 26 center R.
27 Fig. 3 is a graphical representation of the flux Bz 28 versus radius ~ for the coils of the invention.
. . .
Po~-75-n~6 -3-1 Fig. 4 is a schematic representation showing the two 2 loops of a coil and the variable current source thereof.
3 Fig. 5 is a schematic representation showing a cross
4 sectional view taken along the center line of the polefaces ` showing à c ~ ~in~pl we inrthe alr gàp thé;rébetween.
6 Fig. 6 is a schematic representation showing the top 7 view of the polefaces and a pair of coils in place therebetween.
8 Fig. 7 is a schematic representation of one of the pole-9 faces as seen-along the control axis, showing four coils located with respect thereto.
11 Description of the Preferred Embodiments 12 of the Invention 13 Referring to Fig. 1, there i~ shown the poleface 14 assembly with a schematic representation of the flux field 12 existing between the polepieces. The arrangement is of the 16 type that is generally used in nuclear magnetic resonance 17 apparatus for analyzing a sample. The flux field 12 is 18 shown extending from the polepiece-10 to a polepiece 11, 19 and is shown as having it's greatest flux density on the central axis between the polefaces and a drop-off in flux density 21 away from the central axis. The arrowheads note the direction 22 of the flux field and the spacing of the arrows with respect 23 to one another represents the density. The closer the spacing 24 of the arrows the greater the density. Thus, the greater flux density is at the center of the polefaces and the lines 26 of flux extend parallel to this central axis, and the density 27 drops off along any radial line eliminating from the central 28 axis. This radial drop-off in flux density is undesirable 29 in a magnetic arrangement used in nuclear magnetic resonance work, since any inhomogeneity in the flux field tends to \~
'`:` ` ' 10~21qS
1 distort the resultant NM~ spectra, thereby limiting the i 2 effectiven~ss of the analyzer in which the magnetic assembly `" 3 is used. This undesired drop-off in field strength can best 4 be illustrated by a graphical representatiQn, such as shown in
6 Fig. 6 is a schematic representation showing the top 7 view of the polefaces and a pair of coils in place therebetween.
8 Fig. 7 is a schematic representation of one of the pole-9 faces as seen-along the control axis, showing four coils located with respect thereto.
11 Description of the Preferred Embodiments 12 of the Invention 13 Referring to Fig. 1, there i~ shown the poleface 14 assembly with a schematic representation of the flux field 12 existing between the polepieces. The arrangement is of the 16 type that is generally used in nuclear magnetic resonance 17 apparatus for analyzing a sample. The flux field 12 is 18 shown extending from the polepiece-10 to a polepiece 11, 19 and is shown as having it's greatest flux density on the central axis between the polefaces and a drop-off in flux density 21 away from the central axis. The arrowheads note the direction 22 of the flux field and the spacing of the arrows with respect 23 to one another represents the density. The closer the spacing 24 of the arrows the greater the density. Thus, the greater flux density is at the center of the polefaces and the lines 26 of flux extend parallel to this central axis, and the density 27 drops off along any radial line eliminating from the central 28 axis. This radial drop-off in flux density is undesirable 29 in a magnetic arrangement used in nuclear magnetic resonance work, since any inhomogeneity in the flux field tends to \~
'`:` ` ' 10~21qS
1 distort the resultant NM~ spectra, thereby limiting the i 2 effectiven~ss of the analyzer in which the magnetic assembly `" 3 is used. This undesired drop-off in field strength can best 4 be illustrated by a graphical representatiQn, such as shown in
5`~ Fig. 2 where the axial magnetic field Bz~between a pair of polefaces is plotted against the radial distance R from the 7 poleface center. It can be seen that the magnltude of Bz, plot ~: 8 13, drops off radially as the distance R from the poleface 9 center increases. The upper portion of the graph shows, in dash-line form, the desired plot 14. That is, a plot 11 which is a straight line as the radius increases showing that 12 the field is constant and no radial inhomogeneity exists.
13 To counteract the actual radial drop-off in field strength 14 13, a compensating magnetic field is needed that will be weaker at the central axis of the poleface and will increase 16 in strength along a radius from the poleface central axis, 17 a6 shown in r~ . 3. The plot 15 shows the magnetic field 18 Bz lncreasing with increasing distance from the central axis 19 of the poleface along a radius R. It has been found that a 20 coil located in a plane which i8 perpendicular to the ~-21 poleface and parallel to the central axis of the poleface 22 and located at a distance R from the poleface central axis, 23 and perpendicular to the radius drawn from the poleface central 24 axis to the plane containing the coil, when properly energized, produces a magnetic field at the poleface central axis which 26 is in the direction of the main field and which is essentially 27 parallel to the poleface axis. The strength of the produced 28 field increases with the radial distance R from the poleface i 29 central axis. It should be appreciated that this variation in field is opposite to the uncompensated flux field variation l~q~l~S
_ 1 of the pole pieces and, therefore, should, when properly energized compensate for the magnetic field drop-off in the , central reqion where NMR analysis occurs.
.j - A schematic representation of a pair of loops of a coil is shown in Fig. 4. The coil may consist of a single conductor or a multi-conductor arrangement or even an etched circuit on a printed card. The important thing is that the coil produces the required compensating magnetic field when properly energized by a current. One coil pattern 17 con-- 10 sisting of a pair of loops 16 and 18, shown in Fig. 4, is energized by a suitable power source shown as battery 20.
The current flow is chosen as the conventional current flow from positive to negative. Thus, the current in loop 16 is shown to flow down the outside leg of the loop and up the ' inside leg of the loop as indicated by the arrows thereon.
- Similarly, the current flows down the outside leg of loop 18 and upward in the inner leg as indicated by the arrows thereon. The end of the coil 17 is returned to the power source 20 through a suitable ground. The current path includes a variable resistance element 24 by means of which the amount of current flowing in the coil 17 can be either !, automatically or manually adjusted. The automatic adjust-ment, of course, could be by a computer or other controller which adjusts the current flowing in the coil and con-sequently controls the compensating field.
Fig. 5 is a cross-sectional representation taken along the center line axis of the poleface showing one coil 26 of a pair of coils 26, 32 in position between the polefaces.
It can be seen that coil 26 is located in a plane which is parallel to the cross-sectional plane taken along the central 07~175 . .
1 axis of the poleface. However, the planes containing the 2 respective coil of the pair of coils 26 and 32 are each : 3 displaced from the plane through the central axis of the 4 poleface by a chosen radius R. Accordingly, coil 32 is ` ' 5~ locatéd ~cr~s fr'o~the c~ 1'26 at~the apposité side of the ` 6 central axis as shown in Fig. 6. ~ctually, Fig. 6 is a ; ~ 7 ~`top cross-section view of the entire pole and coil assembly 8 of Fig. 5. Looking down at the top of the coils along the 9 cross-section, the conductors are represented by a circle ' 10 indicating the wires of the four legs of each coil. The ',' 11 notation ~ represents the current into the page away from the ' 12 observer, and the ~ notation represents the current out of '- 13 the page toward the observer. As can be seen from Fig. 6, 14 the coil 26 has the current going into the page on the outer lS legs and coming out of the page on the inner legs. This 16 is the same' current directions as depicted in Fig. 4. Thus, ., ~" ~ 17 using the right hand rule for finding the direction of the 18 magnetic field, that i8, pointing the thumb of the right .. .~ ~ .
19 hand in the direction of the current results in the coiled fingers pointing in the direction of the flux field, it 21 can be seen that the flux field direction is out of the left 22 loop 18 of the coil toward the observer and the flux field 23 direction is into the right coil 16 away from the observer.
24 This is schematically represented by the arrows which repre-sent lines of flux extending across coil 17 in Fig. 4.
26 Referring again to coil 26 in Fig. 6 it can be seen that the 27 flux field caused by the current flowing in the coils is 28 from left to right as shown by the arrows. This direction 29 is the same as the direction which is indicated by the center arrow 30 and which is along the central axis of the ! ~
~` l()q21'~5 ~ 1 poleface. The flux field generated by coil 26 extends to the 2 center axis depicted by arrow 30, where the field i8 essen-3 tially a minimum. This flux density increases outwardly 4 from the central axis to the location of the field generating i~ ` ` 5 ' co~il''whèr`è'"'thè'fiel~,` of'course,`is a maximum. Thus,-a
13 To counteract the actual radial drop-off in field strength 14 13, a compensating magnetic field is needed that will be weaker at the central axis of the poleface and will increase 16 in strength along a radius from the poleface central axis, 17 a6 shown in r~ . 3. The plot 15 shows the magnetic field 18 Bz lncreasing with increasing distance from the central axis 19 of the poleface along a radius R. It has been found that a 20 coil located in a plane which i8 perpendicular to the ~-21 poleface and parallel to the central axis of the poleface 22 and located at a distance R from the poleface central axis, 23 and perpendicular to the radius drawn from the poleface central 24 axis to the plane containing the coil, when properly energized, produces a magnetic field at the poleface central axis which 26 is in the direction of the main field and which is essentially 27 parallel to the poleface axis. The strength of the produced 28 field increases with the radial distance R from the poleface i 29 central axis. It should be appreciated that this variation in field is opposite to the uncompensated flux field variation l~q~l~S
_ 1 of the pole pieces and, therefore, should, when properly energized compensate for the magnetic field drop-off in the , central reqion where NMR analysis occurs.
.j - A schematic representation of a pair of loops of a coil is shown in Fig. 4. The coil may consist of a single conductor or a multi-conductor arrangement or even an etched circuit on a printed card. The important thing is that the coil produces the required compensating magnetic field when properly energized by a current. One coil pattern 17 con-- 10 sisting of a pair of loops 16 and 18, shown in Fig. 4, is energized by a suitable power source shown as battery 20.
The current flow is chosen as the conventional current flow from positive to negative. Thus, the current in loop 16 is shown to flow down the outside leg of the loop and up the ' inside leg of the loop as indicated by the arrows thereon.
- Similarly, the current flows down the outside leg of loop 18 and upward in the inner leg as indicated by the arrows thereon. The end of the coil 17 is returned to the power source 20 through a suitable ground. The current path includes a variable resistance element 24 by means of which the amount of current flowing in the coil 17 can be either !, automatically or manually adjusted. The automatic adjust-ment, of course, could be by a computer or other controller which adjusts the current flowing in the coil and con-sequently controls the compensating field.
Fig. 5 is a cross-sectional representation taken along the center line axis of the poleface showing one coil 26 of a pair of coils 26, 32 in position between the polefaces.
It can be seen that coil 26 is located in a plane which is parallel to the cross-sectional plane taken along the central 07~175 . .
1 axis of the poleface. However, the planes containing the 2 respective coil of the pair of coils 26 and 32 are each : 3 displaced from the plane through the central axis of the 4 poleface by a chosen radius R. Accordingly, coil 32 is ` ' 5~ locatéd ~cr~s fr'o~the c~ 1'26 at~the apposité side of the ` 6 central axis as shown in Fig. 6. ~ctually, Fig. 6 is a ; ~ 7 ~`top cross-section view of the entire pole and coil assembly 8 of Fig. 5. Looking down at the top of the coils along the 9 cross-section, the conductors are represented by a circle ' 10 indicating the wires of the four legs of each coil. The ',' 11 notation ~ represents the current into the page away from the ' 12 observer, and the ~ notation represents the current out of '- 13 the page toward the observer. As can be seen from Fig. 6, 14 the coil 26 has the current going into the page on the outer lS legs and coming out of the page on the inner legs. This 16 is the same' current directions as depicted in Fig. 4. Thus, ., ~" ~ 17 using the right hand rule for finding the direction of the 18 magnetic field, that i8, pointing the thumb of the right .. .~ ~ .
19 hand in the direction of the current results in the coiled fingers pointing in the direction of the flux field, it 21 can be seen that the flux field direction is out of the left 22 loop 18 of the coil toward the observer and the flux field 23 direction is into the right coil 16 away from the observer.
24 This is schematically represented by the arrows which repre-sent lines of flux extending across coil 17 in Fig. 4.
26 Referring again to coil 26 in Fig. 6 it can be seen that the 27 flux field caused by the current flowing in the coils is 28 from left to right as shown by the arrows. This direction 29 is the same as the direction which is indicated by the center arrow 30 and which is along the central axis of the ! ~
~` l()q21'~5 ~ 1 poleface. The flux field generated by coil 26 extends to the 2 center axis depicted by arrow 30, where the field i8 essen-3 tially a minimum. This flux density increases outwardly 4 from the central axis to the location of the field generating i~ ` ` 5 ' co~il''whèr`è'"'thè'fiel~,` of'course,`is a maximum. Thus,-a
6 compensating field has been introduced which compensates for
7 the magnetic field drop-off away from the central axis of the
8 polefaces in the direction of the field generating '3 coil. Similarly, a second coil 32 i~ ~hown in Fi(l. 6 ~ te(~
t, ` 10 oppoBite to the irYt coil 26 and in a plane parallel thereto.
~-" 11 It should be noted that this second coil 32 generate~ a flux 12 field, which is in the same direction as the main flux field 13 generated by the poles, and has a distribution wherein the ` 14 minimum flux density is located along the central axis of the . .~
polefaces and is at a maximum at the radial distance where the "'~ 16 coils are located. The coil 32 has the current reversed with ~` 17 respect to coil 26. That is, the oueer legs of the coil have 18 the current extending out of the page as indicated by the proper 19 notation, and has the current extending into the page on the :,:..~
~ 20 inner conductors. This current direction in the legs of the .
21 coil 32 produces the required direction of the magnetic 22 ' field. This change in current with respect to the coils can 23 be obtained by reversing the polarity of the current source 24 applied to the coil or can be simply obtained by inserting the coil 32 upside down with respect to the coil 26. It 26 should be appreciated that the upside down effect is obtained 27 by inserting one of the coils between the polefaces from the 28 other side with respect to the insertion of the other coil.
29 The invention is not limited to the two coil embodiment, shown in Fig. 6, but may also be embodied in a plurality of 10~175 '' ' 1 coils each located at the respective radii about the central 2 axis of the polefaces. An embodiment showing four sets of 3 coils is schematically depicted in Fig. 7. The cross-sectional 4 view is taken along one of the polefaces looking toward the ~ . . - . .
other poleface and, thus, shows thè-coils 36, 37, 38 and 40 as 6 straight lines, since we are viewing the edge thereof. ~ach 7 coil i8 located at right angles with respect to the adjacent 8 coil, thus, forming the four sides of a square. The field
t, ` 10 oppoBite to the irYt coil 26 and in a plane parallel thereto.
~-" 11 It should be noted that this second coil 32 generate~ a flux 12 field, which is in the same direction as the main flux field 13 generated by the poles, and has a distribution wherein the ` 14 minimum flux density is located along the central axis of the . .~
polefaces and is at a maximum at the radial distance where the "'~ 16 coils are located. The coil 32 has the current reversed with ~` 17 respect to coil 26. That is, the oueer legs of the coil have 18 the current extending out of the page as indicated by the proper 19 notation, and has the current extending into the page on the :,:..~
~ 20 inner conductors. This current direction in the legs of the .
21 coil 32 produces the required direction of the magnetic 22 ' field. This change in current with respect to the coils can 23 be obtained by reversing the polarity of the current source 24 applied to the coil or can be simply obtained by inserting the coil 32 upside down with respect to the coil 26. It 26 should be appreciated that the upside down effect is obtained 27 by inserting one of the coils between the polefaces from the 28 other side with respect to the insertion of the other coil.
29 The invention is not limited to the two coil embodiment, shown in Fig. 6, but may also be embodied in a plurality of 10~175 '' ' 1 coils each located at the respective radii about the central 2 axis of the polefaces. An embodiment showing four sets of 3 coils is schematically depicted in Fig. 7. The cross-sectional 4 view is taken along one of the polefaces looking toward the ~ . . - . .
other poleface and, thus, shows thè-coils 36, 37, 38 and 40 as 6 straight lines, since we are viewing the edge thereof. ~ach 7 coil i8 located at right angles with respect to the adjacent 8 coil, thus, forming the four sides of a square. The field
9 generated by each coil would be a minimum at the center of the poleface and maximum at the coil. Of course, the direction 11 of the magnetic lines of flux would be parallel to the magnetic 12 lines of flux generated by the poles between which the coils 13 are located.
14 Since the coils are individually energized and are - `
individually ad~u~table, an adjustment can be made in oppo~ite 16 coils, that iB coils on the uame diameter of the polefaces, 17 to compensate for axial flux density differences introduced 18 by a lack of parallelism with respect to the polefaces. Por , .
~ 19 example, if the polefaces along one half of the diameter are ::
closer together and the polefaces along the other half of the 21 diameter are further apart because of the lack of parallelism 22 between the polefaces, the corresponding axial flux density 23 will increase in the smaller spacing and decrease in the larger 24 spacing. Accordingly, the pair of coils lying along this particular diameter can,be adjusted to not only compensate 26 for the radial flux density inhomogeneity but can be adjusted 27 to compensate for the axial flux density differences due to 28 the lack of parallelism.
29 While the invention has been particularly shown and described with reference to the embodiment thereof, it will ' `
.
.` lOqZ175 :` ~
- 1 be understood by those skilled in the art that the foregoing 2 and other changes in form and detail may be made therein 3 without departing from the spirit and scope of the invention.
.` 4 What is claimed is:
' ' ' ' ' . .
`'~
.; ' r .
;,~
. . .
' ,'' . .
14 Since the coils are individually energized and are - `
individually ad~u~table, an adjustment can be made in oppo~ite 16 coils, that iB coils on the uame diameter of the polefaces, 17 to compensate for axial flux density differences introduced 18 by a lack of parallelism with respect to the polefaces. Por , .
~ 19 example, if the polefaces along one half of the diameter are ::
closer together and the polefaces along the other half of the 21 diameter are further apart because of the lack of parallelism 22 between the polefaces, the corresponding axial flux density 23 will increase in the smaller spacing and decrease in the larger 24 spacing. Accordingly, the pair of coils lying along this particular diameter can,be adjusted to not only compensate 26 for the radial flux density inhomogeneity but can be adjusted 27 to compensate for the axial flux density differences due to 28 the lack of parallelism.
29 While the invention has been particularly shown and described with reference to the embodiment thereof, it will ' `
.
.` lOqZ175 :` ~
- 1 be understood by those skilled in the art that the foregoing 2 and other changes in form and detail may be made therein 3 without departing from the spirit and scope of the invention.
.` 4 What is claimed is:
' ' ' ' ' . .
`'~
.; ' r .
;,~
. . .
' ,'' . .
Claims (10)
1. Apparatus for improving the radial homogeneity of magnetic fields obtained from polefaces comprising;
a plurality of magnetic field generating means each located between said polefaces and at the outer end of equal length radii extending from the central axis of said polefaces;
means for energizing said plurality of magnetic field generating means so that the magnetic lines of flux created thereby will be in the same direction as the magnetic lines of flux of the polefaces;
said magnetic field generating means being positioned with respect to said polefaces so that the magnetic lines of flux created thereby are minimal at the central axis of said polefaces and maximum away from the central axis thereof, thereby compensating for magnetic field drop-off away from the central axis of the polefaces.
a plurality of magnetic field generating means each located between said polefaces and at the outer end of equal length radii extending from the central axis of said polefaces;
means for energizing said plurality of magnetic field generating means so that the magnetic lines of flux created thereby will be in the same direction as the magnetic lines of flux of the polefaces;
said magnetic field generating means being positioned with respect to said polefaces so that the magnetic lines of flux created thereby are minimal at the central axis of said polefaces and maximum away from the central axis thereof, thereby compensating for magnetic field drop-off away from the central axis of the polefaces.
2. Apparatus according to Claim 1, wherein said plurality of magnetic field generating means each comprise a coil having two loops located adjacent one another in the same first plane and having a longer edge of each loop ad-jacent and parallel to one another and close enough to provide a common magnetic field therebetween.
P09-75-016 Claims 1 and 2
P09-75-016 Claims 1 and 2
3. Apparatus according to Claim 2, wherein said first planes each including a coil extends perpendicular to said polefaces and perpendicular to said associated radius which extends from the central axis of said polefaces;
said first planes being equidistant along said radii, the distance being a function of the field strength created by said coils such that the magnetic lines of flux created at said central axis of said polefaces are minimal and the lines of flux increase in density with distance from the central axis to a maximum at the coil position.
said first planes being equidistant along said radii, the distance being a function of the field strength created by said coils such that the magnetic lines of flux created at said central axis of said polefaces are minimal and the lines of flux increase in density with distance from the central axis to a maximum at the coil position.
4. Apparatus according to Claim 2, wherein said means for energizing said plurality of magnetic field generating means includes a variable DC current source for energizing each of said loops of a coil so that the current flows in opposite directions in each of said loops and in the same direction in the adjacent edges of the loops to produce a resultant magnetic field in the direction of the magnetic field generated by said polefaces.
5. Apparatus according to Claim 4, wherein a variable resistance is included in said variable DC current source, said resistance being adjustable to provide adjustment of the amount of current supplied to said coils and consequently the flux density.
6. Apparatus according to Claim 1, which includes at least one pair of magnetic field generating means each located between said polefaces and at the outer ends of oppositely extending radii from said central axis of said polefaces.
P09-75-016 Claims 3, 4, 5 and 6
P09-75-016 Claims 3, 4, 5 and 6
7. Apparatus according to Claim 6, wherein each magnetic field generating means of each of said pair of magnetic field generating means comprises a coil having two loops located in the same first plane having a longer.
edge adjacent and parallel to one another and close enough to provide a common magnetic field therebetween.
edge adjacent and parallel to one another and close enough to provide a common magnetic field therebetween.
8. Apparatus according to Claim 7, wherein each of said first planes extend perpendicular to said polefaces and perpendicular to said associated radius which extends from the central axis of said polefaces, said first planes being equidistant from said center axis of said polefaces in opposite directions along respective radii, the distance being a function of the field strength created by said coils such that the magnetic lines of flux created at said central axis of said polefaces are minimal and the lines of flux increase in density as a function of distance from the central -axis to a maximum at the coil positions.
9. Apparatus according to Claim 7, which includes a variable DC current source for energizing each of said pair of loops of a pair of coils so that the current flows in opposite directions in each of the loops of a pair of loops and in the same direction in the adjacent edges of the loops to produce a resultant magnetic field in the direction of the magnetic field generated by said polefaces.
P09-75-016 Claims 7, 8 and 9
P09-75-016 Claims 7, 8 and 9
10. Apparatus according to Claim 9, wherein a variable resistor is included between the DC current source and each coil of said pair of coils to individually adjust the current flowing thru each of said coils of said pair of coil, to generate a magnetic field having a minimum density along said central axis of said polefaces and increasing in density as a function of distance from said central axis, said variable resistor associated with each coil of said pair of coils being located at the outer end of equal and opposite radii and being adjustable to correct the field variations due to non-parallelism of said polefaces in the direction of the opposite radii as well as the radial drop-off in flux density away from said central axis of the polefaces.
P09-75-016 Claim 10
P09-75-016 Claim 10
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US61501875A | 1975-09-19 | 1975-09-19 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1072175A true CA1072175A (en) | 1980-02-19 |
Family
ID=24463670
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA261,427A Expired CA1072175A (en) | 1975-09-19 | 1976-09-17 | Apparatus for improving the radial homogeneity of axial magnetic fields |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JPS5238995A (en) |
| CA (1) | CA1072175A (en) |
| FR (1) | FR2325167A1 (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4682111A (en) * | 1985-03-05 | 1987-07-21 | Kabushiki Kaisha Toshiba | Magnetic resonance imaging magnet |
-
1976
- 1976-08-10 FR FR7625011A patent/FR2325167A1/en active Granted
- 1976-08-20 JP JP51098818A patent/JPS5238995A/en active Pending
- 1976-09-17 CA CA261,427A patent/CA1072175A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| FR2325167B1 (en) | 1980-07-25 |
| JPS5238995A (en) | 1977-03-25 |
| FR2325167A1 (en) | 1977-04-15 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US6504462B1 (en) | Magnet structure having a passively controllable homogeneous field | |
| US7330031B2 (en) | Matrix shim system with grouped coils | |
| EP0371775A2 (en) | Permanent magnet arrangement | |
| EP0192331A1 (en) | Electromagnet | |
| KR920008203B1 (en) | Electromagnet and method of forming same | |
| EP0984461A2 (en) | Low eddy current and low hysteresis magnet pole faces in MR imaging | |
| JPS6458247A (en) | Uniform magnetic field coil | |
| US4816962A (en) | Uniform magnetic field coil | |
| JPH0378592B2 (en) | ||
| KR100550704B1 (en) | Magnetic field homogenizing method and apparatus, and magnetic field apparatus | |
| US3566255A (en) | Apparatus for improving the homogeneity of magnetic fields | |
| CA1072175A (en) | Apparatus for improving the radial homogeneity of axial magnetic fields | |
| JP2602513B2 (en) | Method of performing passive shim action of magnet and passive shim assembly for magnet | |
| US10726986B2 (en) | Apparatus and method for magnetic field compression using a toroid coil structure | |
| EP0526513B1 (en) | Magnetic field generating assembly | |
| US4095202A (en) | Coil for producing a homogeneous magnetic field in a cylindrical space | |
| GB2315555A (en) | NMR gradient coil with parallel paths | |
| EP2265969A1 (en) | Gradient system for a magnetic resonance apparatus | |
| JP7407857B2 (en) | Shim iron for magnetic resonance equipment | |
| GB2442750A (en) | Shimming of Magnet Systems | |
| JP2570044B2 (en) | Superconducting magnet assembly for MRI and MRI diagnostic device | |
| JP2958549B2 (en) | Static magnetic field drift correction coil and static magnetic field drift correction method | |
| EP1521095B1 (en) | Superconducting magnet apparatus | |
| US6031440A (en) | Thermally controlled magnetic reluctor structures and methods | |
| Abele | Generation and confinement of uniform magnetic fields with surface currents |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |