Note: Descriptions are shown in the official language in which they were submitted.
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.
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! 24 - Background of the Invention
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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,
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` 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.
PO9-75-016 -2-
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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-
072175
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
PO9-75-016 -4-
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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
PO9-75-016 -5-
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
PO9-75-016 -6-
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. .
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
P09-75-016 -7-
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~` 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
PO9-75-016 -8-
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'' '
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
P09-75-016 -9-
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- 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:
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PO9-75-016 -10-
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