Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2001-04-23
2002-10-22
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C335S299000
Reexamination Certificate
active
06469604
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a coil for a magnet and to a method of manufacturing a coil for a magnet. More particularly, it relates to a gradient coil and to a method of manufacturing a gradient coil and, in particular to a gradient coil for use in a magnetic resonance imaging (MRI) system.
BACKGROUND OF THE INVENTION
MRI systems are used today for investigating a large variety of body parts. These systems are based on nuclear phenomena displayed by atomic nuclei having a non-zero magnetic moment (or “spin”). When such nuclei are placed in a static, uniform magnetic field, the nuclear spins are aligned by the magnetic field so as to be either aligned with or against the static magnetic field. The nuclear spins are not stationary, but precess around an axis defined by the magnetic field The frequency at which the spins precess is known as the “Larmor frequency” &ohgr;
0
. The Larmor frequency is given by &ohgr;
0
=&ggr;B
0
where &ggr; is the gyromagnetic ratio of the nucleus and B
0
is the applied magnetic field. For a hydrogen nucleus, &ggr;=42.57 MHz/T.
When the nuclear spins are aligned in the static magnetic field B
0
, it is possible to “flip” the spins by applying an alternating magnetic field B
1
. In order to do this, the alternating magnetic field must be at 90° to the static magnetic field and it must alternate at substantially the Larmor frequency. When such an alternating field B
1
is applied the spins will tend to align themselves parallel to B
1
, and when the alternating field is removed the spins will relax back into the state in which they are aligned with the static magnetic field B
0
. The alignment of the spins with the alternating field decreases the magnetisation in the longitudinal direction (parallel to B
0
) and increases the magnetisation in the transverse plane (that is, the plane perpendicular to B
0
), and the subsequent relaxation of the spins when the alternating field is removed produces the reverse effects. These changes in the magnetisation are detected in the MRI process, and are processed to provide a visible display of the nuclei.
FIG. 1
at
11
shows a typical MRI system in block diagram form. The magnet
12
provides the static magnetic field B
0
. In principle, the magnet
12
could be a superconductive magnet, an electromagnet or a permanent magnet. However, a super-conducting magnet is commonly used, since readily provide a large, homogeneous static magnetic field. The magnet
12
contains a bore
13
enabling the entry of a patient into the static magnetic field. A patient shown at
14
is inserted into the bore
13
using bed arrangement
16
so as to be within the static magnetic field.
Radio frequency (R. F.) pulses generated by transmitter
22
and applied through multiplexer
23
and radio frequency coil apparatus
24
act to tip the aligned spins so as to have a projection, for example, in the X, Z plane; the X, Y plane or the Y, Z plane. The X, Y and Z nomenclature refers to the imaginary orthogonal axes shown at 221 used in describing MRI systems; where the Z axis is an axis coaxial with the axis of the bore hole. The Y axis is the vertical axis extending from the centre of the magnetic field and the X axis is the corresponding horizontal axis orthogonal to the other axes.
The spins, when realigning after the radio frequency pulse is removed generate, free induction decay (F.I.D.) signals which are received by the radio frequency coil apparatus
24
and transmitted through the multiplexer to the receiving circuit
26
. From the receiving circuit the received signals go through the controller
25
to an image processor
27
. The image processor works in conjunction with a display memory
28
to provide the image displayed on display monitor
29
. It should be noted that the radio frequency coil apparatus
24
can comprise separate coils for transmitting and receiving or the same coil apparatus
24
could be used for both transmitting and receiving the R. F. pulses.
In order to spatially resolve the MRI signal, encoding signals within the static magnetic field are provided by gradient coils (not shown in FIG.
1
). There are typically three sets of gradient coils. X gradient coils alter the strength of the static magnetic field along the X axis, Y gradient coils alter the strength of the static magnetic field along the X axis, and Z gradient coils alter the strength of the static magnetic field along the Z axis. The strength of the static magnetic field along other axes, such as XZ axis for example, can be changed using two or three of the gradient coils in combination.
The X, Y and Z gradient coils are driven by X gradient driver
17
, Y gradient driver
18
and Z gradient driver
19
, respectively. It is possible to modify the static magnetic field B
0
using the gradient coils so that only nuclei within a small volume element of the of the patient have a Larmor frequency equal to the frequency of the R. F. field B
1
. This means that the F.I.D. signal comes only from nuclei within that volume element of the patient. In practice the gradient coils are supplied with time-varying electrical currents from a power supply, such as a power amplifier, so that the volume element in which the nuclei have a Larmor frequency equal to the frequency of the applied R.F. field is scanned over the patient so as to build up a 2-D or 3-D image of the patient.
A typical prior art set of gradient coils is disclosed in, for example, “Foundations of Medical Imaging” by Z.H. Cho et al (published by Wiley International), and is shown schematically in FIG.
2
. The X gradient coils are shown in FIG.
2
(
a
). FIGS.
2
(
b
) and
2
(
c
) show the Y gradient coils and the Z gradient coils respectively.
It will be noted that the X gradient coils and the Y gradient coils shown in FIGS.
2
(
a
) and
2
(
b
) are in the form of saddle coils. In each case, two saddle coils are placed either side of the Z=0 plane.
In the prior art, the gradient coils are constructed over a tubular base, the Z gradient coils are placed over the X gradient coils, and finally the Y gradient coils are placed over the Z gradient coils (although the order in which the gradient coils are provided on the former is not limited to this particular order).
As shown in FIG.
2
(
c
), the Z gradient coils are axial coils of solenoidal form. These axial coils are wound into pre-formed slots in an insulating former. The former is prepared by machining grooves in a fiberglass tube. The tube is then split into four sections, which are glued to the tope of the X gradient coils. The Z gradient coils are then wound in grooves
41
in the preferably fiberglass former using copper wire or copper bar
42
. FIG.
3
(
a
) is a side view of such a prior art former
40
showing the grooves
41
, and FIG.
3
(
b
) is an end view of the former of FIG.
3
(
a
).
To reduce the noise created by the Z gradient coils when they are energized, grooves
41
in former
40
are lined with a rubber sheet
43
, as shown in
FIG. 4
, which is an enlarged partial sectional view of the former of FIG.
3
(
a
). Rubber sheet
43
acts as a shock absorber and damps the vibration of the Z gradient coils thereby reducing the maximum acceleration of the coils and the noise generated by the coils in use (the vibrations are caused by the coils moving as a result of the magnetic forces acting on the coils).
It can therefore be seen that the prior art method of constructing the Z gradient coils is expensive. In particular, the construction of finely machined thin-walled glass-fiber tubes further cut into four sections is very costly. Moreover, two winding steps are required after the former has been glued in position. Firstly the grooves are lined with the rubber sheet
43
and then, secondly, the copper wire or bar is wound into the rubber-lined grooves.
SUMMARY OF THE INVENTION
A first aspect of some preferred embodiments of the present invention provides a method of manufacturing a coil for a magnet comprising: a curved former; and disposing an electrical conductor around th
Armstrong Alan George Andrew Marcel
Bird John Maurice
Palkovich Alex
Della Penna Michael A.
Donovan Lincoln
Horton Carl B.
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