Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-01-31
2002-09-24
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000, C324S309000
Reexamination Certificate
active
06456076
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to magnetic resonance imaging coils and, more particularly, to z-gradient shielding coils.
2. Background of the Invention
Magnetic resonance imaging (MRI) systems are currently employed in forming images of the internal human anatomy. In such systems, a patient is placed in a magnetic field and is subjected to radio-frequency electromagnetic pulses. The magnetic resonance of the atomic nuclei of the patient are detected with a radio frequency receiver to provide information from which an image of that portion of the patient containing these nuclei may be formed. The magnetic field includes a main magnetic field and three additional fields with linear spatial gradients in the x, y, and z directions
The main magnetic field is a very strong magnetic field, which may be created by a super-conducting coil, a resistive coil, or a permanent magnet. Normally, the z-axis is parallel to the axis of the main magnetic field for systems in which the magnet has cylindrical geometry, such as for whole body imaging. The linear gradient magnetic fields are typically created by resistive coils and are referred to as gradient coils. The resistive coils create a magnetic field within the coil with a linear spatial gradient, also referred to as a magnetic gradient. Typically, there is one gradient coil for each of the x, y and z-axes, which create x, y, and z magnetic gradients, respectively. Two different types of gradient coils are typically used to produce the magnetic gradients for MRI, one which creates a magnetic gradient along the z (or longitudinal) axis of the coil, and two others which create magnetic gradients along either the x or y (transverse) axes.
In operation, for imaging purposes, it is necessary to rapidly pulse electrical current through the three gradient coils. When this is done, a problem commonly encountered is the induction of eddy currents in various metallic parts of the MRI system. The MRI system typically contains a metallic cylinder called a bore tube. The inside of the bore tube is an image volume; however, most imaging occurs only in the central portion ofthe bore tube. The current in the gradient coils induce eddy currents in the bore tube of the MRI system that, in turn, induce a magnetic field within the image volume, referred to as the error field. The magnetic field created by the eddy currents is undesirable in the image volume. In many medically useful imaging procedures, it is highly desirable to reduce or eliminate these eddy currents.
Typically, eddy currents are reduced by surrounding each gradient coil, also referred to as the inner coil, with another similar coil, an outer coil or a shielding coil, to cancel the magnetic and induced electric fields in the region outside of the outer coil. A set of a gradient coil and its associated shielding coil is referred to as a shielded gradient coil set. Ideally, the shielding coil is designed to exactly cancel the electric and magnetic field outside of the coil set. If no field exists outside of the shielded gradient coil set, then no eddy currents can be induced in the metallic parts of the MRI system, and therefore, no error field will be produced in the image volume.
Not all eddy currents affect the imaging volume equally, in particular, the induced eddy currents and magnetic fields can be analyzed in terms of the azimuthal harmonic number, m. The azimuthal harmonic number m means that the field or gradient varies in azimuthal angle like cosine(m&phgr;), sin(m&phgr;), or a linear combination of the two, where &phgr; is the azimuthal angle as shown in FIG.
5
B. That is, the field goes through exactly m full cycles as the angle varies from 0 to 360 degrees. The worst effects are seen from eddy currents with m=0. These harmonics also have the longest lifetime, which can be as long as several seconds. In general, the lower the m number, the worse the effects on an MRI system.
Existing attempts to reduce the eddy current effects have only been partially successful, especially for the z-gradient coil set. One common technique for making z-gradient coils is using circular parallel loops of wire, all of which lie in planes that are perpendicular to the z-axis. The loops are interconnected by straight wires that lie on the outer cylindrical surface of a support structure and are parallel with the z-axis. This design has the advantage that it creates no x or y magnetic gradient. This is important because it is undesirable to use a z-gradient coil that creates x or y magnetic gradients. However, the z-gradient that is created is not exactly homogenous, but varies with the radius from the z-axis.
The problem with this conventional shielding is that it is impossible to exactly cancel the field outside of the z-gradient coil set. A continuous surface current distribution would be required on the surface of the shielding coil to exactly cancel the field outside of the gradient coil set. Conventional shielding simulates a continuous surface current distribution by winding several discrete circular loops around a support structure. However, these discrete circular loops cannot exactly simulate a continuous surface distribution, and therefore, never exactly cancel the field outside of the gradient coil set.
While is not possible to exactly cancel the entire field outside of the gradient coil set, it would be desirable to cancel the specific harmonics that are most troublesome to the MRI system. Therefore, a shielding coil for a z-gradient coil that exactly cancels the magnetic fields of low azimuthal harmonic number, m, outside of the z-gradient coil set would be very desirable.
FIG. 1
illustrates an exemplary prior art MRJ system
10
as disclosed in U.S. Pat. No. 4,733,189. As shown in
FIG. 1
, the MRI system
10
includes a main magnetic component
20
, gradient coils
30
, shielding coils
40
, and a detection component
50
.
The main magnetic component
20
can be a permanent magnet, a resistive electromagnet, or a superconducting system as shown, in which a solenoidal electromagnet
22
is encased within a cryogenic vessel
26
. Bore tube
28
supports the solenoidal electromagnet
22
. Image volume
24
is located centrally to the main magnetic component
20
.
Gradient coils
30
include an x-gradient coil
32
, a y-gradient coil
34
and a z-gradient coil
36
, disposed to create gradient fields orthogonal to each other. X and y gradient-producing coils are preferably implemented by saddle-shaped coil elements disposed about the main magnetic field axis and rotated ninety degrees from each other in orientation. As shown, the z-gradient coil
36
is implemented by a parallel loop gradient coil coaxial with the main magnetic field axis.
Detection component
50
includes a radio frequency (RF) coil
52
and an RF interrogator
56
and receiver
58
. The interrogator
56
produces a pulse of radio frequency excitation and the energy emitted as the atoms return to an aligned state is captured via coil
52
and used to obtain an image signal. In use, a patient or other object is positioned within the image volume
24
of the system
10
.
Shield component
40
includes an x-shielding coil
42
, a y-shielding coil
44
, and a z-shielding coil
46
disposed to counteract the eddy currents induced by the gradient-producing coils
32
,
34
and
36
, respectively. The x and y shielding coils,
42
and
44
may be implemented by saddle-shaped coils cut from flat copper sheets and rolled into the appropriate saddle shapes. As shown, the z-shielding coil
46
is implemented by a parallel loop shielding coil coaxial with the main magnetic field axis.
FIG. 2
illustrates an exemplary prior art parallel loop gradient coil of a type which may be used as a z-gradient coil
36
in FIG.
1
. As shown in
FIG. 2
, parallel loop gradient coil
80
includes loops
81
interconnected by straight wires
82
that lie on the outer cylindrical surface of a support structure
84
and parallel with the z-axis. The loops
81
and straight wires
82
Lefkowitz Edward
Shrivastav Brij B.
The Trustees of the University of Pennsylvania
Woodcock & Washburn LLP
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