Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2002-07-22
2004-12-14
Arana, Louis (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06831461
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to magnetic resonance (MR) tomography as used in medicine for examining patients. The present invention relates in particular to an MR tomography apparatus of the type wherein vibrations of apparatus components which negatively influence many aspects of the overall system are reduced.
2. Description of the Prior Art
MR tomography is based on the physical phenomenon of nuclear spin resonance and has been used successfully as an imaging method for over 15 years in medicine and in biophysics. In this method of examination, the object is exposed to a strong, constant magnetic field. This aligns the nuclear spins of the atoms in the object, which were previously oriented irregularly. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation. In MR tomography, this oscillation generates the actual measurement signal which is picked up by means of suitable receiving coils. By the magnetic fields which are not spatially constant, generated by gradient coils, it is possible to spatially code the measurement signals from the object in all three spatial directions. The method permits a free choice of the layer to be imaged, so that it is possible to obtain tomographic images of the human body in all directions. MR tomography in medical diagnostics is distinguished first and foremost as a “non-invasive” method of examination by a versatile contrast capability. MR tomography currently uses sequences with high gradient performance which permit an excellent image quality with measuring times of the order of magnitude of seconds and minutes.
Continuous technical development of the components of MR systems, and the introduction of high-speed imaging sequences have opened ever more fields of use for MR tomography in medicine. Real time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples.
The basic design of the examination portion of such an MR apparatus is illustrated in FIG.
4
. This has a basic field magnet
1
(for example an axial superconducting air-coil magnet with active stray field shielding) which generates a homogeneous magnetic basic field its interior volume. The superconducting magnet
1
has superconducting coils which are contained in a vessel of liquid helium. The basic field magnet
1
is surrounded by a two-shell vessel arrangement which is made from stainless steel, as a rule. The inner vessel, which contains the liquid helium, and serves in part also as a winding body for the magnet coils, is suspended at the outer vessel, which is at room temperature, via fiber-glass-reinforced plastic rods which are poor conductors of heat. A vacuum prevails between inner and outer vessels.
The cylindrical gradient coil arrangement
2
in the interior of the basic field magnet
1
is inserted concentrically into the interior of a support tube by means of support elements
7
. The support tube is delimited externally by an outer shell
8
, and internally by an inner shell
9
. The function of the layer
10
will be explained later.
The gradient coil arrangement
2
has three such windings which, respectively generate gradient fields, proportional to the current impressed in each case, and which are spatially perpendicular to one another. As illustrated in
FIG. 5
, the gradient coil
2
has an x-coil
3
, a y-coil
4
and a z-coil
5
, which are respectively wound around the coil core
6
and thus generate respective gradient fields in the directions of the Cartesian coordinates x, y and z. Each of these coils is provided with a dedicated power supply unit in order to generate independent current pulses with accurate amplitudes and timing in accordance with the sequence programmed in the pulse sequence controller. The required currents are approximately 250 A. Since the gradient switching time is to be as short as possible, current rise rates of the order of magnitude of 250 kA/s are necessary. In an exceptionally strong magnetic field such as is generated by the basic field magnet
1
(typically between 0.22 and 1.5 tesla), such switching operations are associated with strong mechanical vibrations because of the Lorentz forces that occur. All system components (housing, covers, vessels of the basic field magnet and magnet housing, respectively, RF body coil etc.) are excited to produce forced vibrations.
Since the gradient coil is generally surrounded by conductive structures (for example magnet vessel made from stainless steel), the pulsed fields start in eddy currents in such structures, which exert forces on these structures due to interaction with the basic magnetic field, and likewise excite these structures to vibrations.
There is one further source of vibration which chiefly excites the magnet vessel to vibrations. This is the so-called cold head
15
, which ensures that the temperature of the basic field magnet
1
is maintained. The cold head
15
is driven by a compressor and exerts mechanical forces on the housing of the basic field magnet
1
.
These vibrations of the various MR components act negatively in many ways on the MR system:
1. Decidedly stronger airborne noise is produced, which constitutes an annoyance to the patient, the operating staff and other persons in the vicinity of the MR system.
2. Vibrations of the gradient coil and of the basic field magnet, and their transmission to the RF resonator and the patient couch in the interior of the basic field magnet and/or the gradient coil, are expressed in inadequate clinical image quality which can even lead to misdiagnosis (for example in the case of functional imaging, fMRI).
3. High costs arise also due to the need for a vibration-damping system setup—similar to that of an optical table—in order to prevent transmission of the vibrations to the ground, or vice versa.
In known systems, the transmission of vibrational energy between the gradient coil and tomograph is countered by the use of mechanical and/or electromechanical vibration dampers.
German Offenlegungsschrift 197 22 481 discloses an MR tomography apparatus which has a magnet body surrounded by a magnet housing, and this magnet housing surrounds and delimits an interior. A gradient coil system is located in this interior. A damping structure is provided on an inner side, delimiting the interior, of the magnet housing for absorbing acoustic vibrations which are caused by switching the currents generating the gradient fields. At least one damping cushion is formed of liquid, gaseous or flowable materials.
German Offenlegungsschrift 197 34 138 likewise discloses the use of passively acting elastomeric damping elements (for example rubber bearings), as well as the encapsulation of the gradient coil in a separate vacuum housing.
Normally use is made as well of the following further passive measures in order to reduce the vibrations:
general encapsulation of the source of vibration
use of thicker and heavier materials
damping layers (for example tar) applied from “outside”.
In particular, the noise production path over the interior of the MR apparatus, i.e., production of noise by vibration of the gradient coil and transmission of the noise to the support tube located in the gradient coil (FIG.
4
), which emits the noise inwardly to the patient and the interior, is blocked in the system disclosed in U.S. Pat. No. 4,954,781 by a damping viscoelastic layer
10
(
FIG. 4
) in the double-ply inner layer
9
of the support tube.
Furthermore, it is known to achieve this by inserting sound-absorbing so-called acoustic foams into the region between support tube and gradient coil.
German Offenlegungsschrift 196 43 116 discloses the integration of magnetostrictive materials, in particular into the gradient coil, permitting active, controlled opposing vibration-countering during operation, thereby reducing the vibration amplitude of the gradient coil.
German Patentschrift DE 44 32 747 likewise teaches the use of active damping, but by means of integrat
Arz Winfried
Boemmel Franz
Dietz Peter
Weber Matthias
Arana Louis
Schiff & Hardin LLP
Siemens Aktiengesellschaft
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