Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-04-24
2002-11-12
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06479997
ABSTRACT:
This application claims Paris Convention priority of DE 199 20 085 filed May 3, 1999 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
An electric arrangement, with a gradient coil for generating magnetic gradient fields, which rapidly change with time, in the working volume of a magnetic resonance apparatus, with a current path guided in winding on the surface of a geometric body, the geometry of which results in the desired spatial dependence of the magnetic field of said gradient coil in the working volume, and with N current power supplies which can be controlled by an analog or digital control signal to each generate a controlled time-dependent electric current which is proportional to the control signal.
An electric arrangement of this type is disclosed e.g. in U.S. Pat. No. 4,928,063.
An essential component of nuclear magnetic resonance (NMR) apparatus, generally used for tomography, but sometimes also for spectroscopy, is a system of normally three gradient coils consisting of several partial coils which are independently supplied with currents of different strengths. These gradient coils superimpose spatially constant magnetic field gradients of adjustable strength on a homogeneous magnetic field B
0
z, extending along a z-axis, of the main field magnet of the NMR apparatus, wherein the direction of one of these gradients (dBz/dz) extends generally parallel to the direction of the homogeneous main field B
0
z (z gradient=axial gradient), and the two other gradients (dBz/dx, dBz/dy) are mutually orthogonal and transverse to the direction of the main field (x and y gradients=transverse gradients). The strength of the gradient field is designated with g (g=dBz/dr, r=x or y or z). The volume within which the magnetic fields of these gradient coils have an approximately linear dependence can be used for NMR methods requiring spatial resolution (imaging, spatially selective spectroscopy) if this volume is not further constrained by inhomogeneities in the main field.
A further essential component of a magnetic resonance apparatus is the current power supply for the gradient coils. This unit supplies an electric current I, which can be generated within wide limits, into a gradient coil with a precisely determined time dependence. The time dependence of the electric current is generally specified by an analog or digital control signal which serves as command variable for a control loop, wherein the electric current is controlled in correspondence with the time dependence of this command variable. Current power supplies which are particularly advantageous for these applications and which have particularly low electrical power losses are constructed as cycled switching power supplies and are described e.g. in U.S. Pat. No. 5,113,145. The gradient coil, constituting a substantially inductive load, is thereby connected via a transistor, acting as a switch S, to an electric voltage source having constant voltage U and is charged quickly. When the switch S is open, the coil decharges slowly via a recovery diode D connected in parallel to the coil. The current in the coil therefore remains largely constant for short switch opening times. Current regulation is thereby principally effected by adjusting the cycling times with which the switch is closed or opened.
The properties of a magnetic resonance apparatus improve with increasing maximum gradient strength gmax generated with the three gradient coils and with increasing maximum change in the gradient strength dg per unit time dt,(dg/dt)max. Both properties are suitably combined as product in a dynamic characteristic variable kdyn=gmax*(dg/dt)max.
The gradient coils may e.g. be x-, y- and z-coils disposed on cylindrical surfaces for conventional tomography magnets or be gradient coils for gradient accelerated NMR spectroscopy. In addition, flat gradient plates for pole shoe magnets have also been used in NMR tomography. U.S. Pat. No. 5,666,054 and U.S. Pat. No. 5,563,567, the complete disclosure of which are hereby incorporated by reference, extensively describe the spatial construction of other possible geometrical arrangements of gradient coils.
U.S. Pat. No. 5,323,135 describes a particularly advantageous gradient coil system, wherein the inductance L can be optimized for given boundary conditions and additional technically relevant parameters of the magnet coil arrangement, such as e.g. current density distributions, shielding effect etc. can also be optimized, independently of one another. The shielding effect describes the degree to which high quality gradient coils generate minimum or theoretically vanishing stray magnetic fields in the vicinity of the main field magnet. This is advantageous since the gradient fields, varying rapidly with time, which are generated by these gradient coils, do not induce eddy currents in the metal structure of the main field magnet, the additional magnetic fields of which would be superimposed on the magnetic field of the gradient coil and distort same in a disturbing manner.
A common feature of all gradient coils mentioned herein, is that they are constructed from an even number of at least two partial coils, arranged symmetrically with respect to one another to generate the normally desired spatially symmetric magnetic field strength dependence. In principle, these partial coils can be generated using two symmetry operations: mirrored reflection on a plane by rotation about an axis through 180 degrees. The transverse gradient coils disclosed e.g. in U.S. Pat. No. 5,323,135 comprise four such partial coils disposed on the surface of a circular cylinder. The transverse gradient coils disclosed in U.S. Pat. No. 5,343,148 comprise two partial coils disposed on the surface of a circular cylinder. Axial gradient coils and coils for generating a homogeneous magnetic field consist in general of two partial coils arranged on the surface of a circular cylinder. The transverse gradient coils disclosed in U.S. Pat. No. 5,959,454 consist of two partial coils arranged in a common plane.
In many cases, a gradient coil is constructed from one single conductor path extending along the current path guided in windings on the surface of a geometric body, which is connected to one single current power supply. For the dynamic performance data of such an arrangement, the following holds:
gmax=g
0
*Imax
(dg/dt)max=(g
0
/L)*Umax
kdyn=(g
0
*g
0
)*Imax*Umax,
wherein g
0
is the gradient strength per current magnitude unit (normalized gradient strength), L the inductance of the coil, Imax the maximum current magnitude of the current power supply, and Umax the maximum electric voltage of the current power supply.
For given performance data for the current power supply, Imax and Umax, the e.g. maximum gradient strength gmax can be adjusted within large limits by constructing the gradient coil with more or less windings, since the normalized gradient strength is proportional to the number of windings W of the current path. Since the inductance L is inversely proportional to the square of the number of windings W, an increase in the maximum gradient strength gmax is linked with a corresponding reduction in the maximum change with time of the gradient strength (dg/dt)max.
The dynamic characteristic variable kdyn thereby remains unchanged. The dynamic characteristic variable kdyn can also be maximized by maximizing the characteristic parameter of the gradient coil (g
0
*g
0
/L) through adjustment of the detailed dependence of the geometry of the current path. Methods therefor are e.g. described in the above mentioned U.S. Pat. No. 5,323,135. A further increase in the dynamic characteristic variable kdyn is only possible by increasing the performance values Imax and Umax of the current power supply. These performance values also depend on technical limits. Typical performance values of modern cycled switching power supplies are about Imax=600 A and Umax=400 V.
To further improve the dynamic characteristic varia
Schmidt Hartmut
Westphal Michael
Bruker Biospin GmbH
Lefkowitz Edward
Vargas Dixomara
Vincent Paul
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