Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2003-04-29
2004-05-11
Arana, Louis M. (Department: 2859)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06734672
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed in general to magnetic resonance tomography as employed in medicine for examining patients. The present invention is more specifically directed to a method for the automatic determination of system-specific acoustic or mechanical resonances in an individual manner for each magnetic resonance tomography apparatus. Given knowledge of such resonances, they can be avoided during data acquisition from an examination subject in the MR apparatus by limitations of the system parameters.
2. Description of the Prior Art
MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully utilized for more than 15 years as an imaging method in medicine and in biophysics. In this examination modality, the subject is disposed in a strong, constant magnetic field. As a result, the nuclear spins of the atoms in the subject, which were previously irregularly oriented, are aligned. Radiofrequency waves can then excite these “ordered” nuclear spins to a precessional movement. This precession generates the actual measured signal in MRT that is picked up with suitable reception coils. The measured subject can be spatially encoded in all three spatial directions by utilizing non-homogeneous magnetic fields generated respectively by gradient coils.
In one method for generating MRT images, a slice, for example in the z-direction of a Cartesian coordinate system, is first selectively excited. The encoding of the location information in the slice ensues combined phase and frequency encoding with two orthogonal gradient fields that, in the example of a slice excited in the z-direction, are generated in the x-direction and the y-direction by the aforementioned gradient coils. The imaging sequence is repeated N times for different values of the phase encoding gradient, for example Gx, and the magnetic resonance signal is digitized and stored N times in each sequence execution in the presence of the readout gradient Gy. A number matrix (matrix in k-space) with N×N data points is obtained in this way. An MR image of the observed slice having a resolution of N×N pixels can be directly reconstructed from this dataset by means of a two-dimensional Fourier transformation.
The method allows a free selection of the slice to be imaged, so that tomograms of the human body can be acquired in all directions. As a “non-invasive” examination method, MRT is distinguished first and foremost as a tomographic method in medical diagnostics by a versatile contrast capability. Due to the excellent presentation of the soft tissue, MRT has developed into a method that is often superior to X-ray computed tomography (CT). MRT is currently based on the application of spin echo sequences and gradient echo sequences that enable an excellent image quality with measuring times in the range of seconds through minutes.
Constant technical improvements of the components of MRT devices and the introduction of fast imaging sequences have created an increasing number of medical applications for MRT. Real-time imaging for supporting minimally invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are examples.
The basic structure of one of the components of such an MRT apparatus is shown in FIG.
9
. This component includes superconducting basic field magnet
1
(for example, an axial superconducting magnet with active stray field shielding) that generates a homogeneous basic magnetic field in an interior space. The inside of the superconducting basic field magnet
1
is composed of coils situated in liquid helium. The basic field magnet is surrounded by a double shell cryostat (not shown) that is usually composed of stainless steel. The inner shell that contains the liquid helium and also partly serves as a winding body for the magnet coils is suspended at the outer shell via poorly thermally conductive rods, the outer shell being at room temperature. A vacuum exists between the inner and outer shells. The inner and outer shells are referred to as magnet vessel.
The cylindrical gradient coil
2
is concentrically introduced into the inside of a carrying tube in the interior of the basic field magnet
1
by means of carrier elements
7
. The carrying tube is outwardly limited by an outer shell
8
and inwardly limited by an inner shell
9
.
The gradient coil
2
has partial windings that respectively generates gradient fields that are proportional to the impressed current and are spatially perpendicular to one another. As shown in
FIG. 10
, the gradient coil
2
has an x-coil
3
, a y-coil
4
and a z-coil
5
that are respectively wound around the coil core
6
and thus generate respective gradient fields in the direction of the Cartesian coordinates x, y and z. Each of these gradient coils
3
,
4
and
5
is equipped with its own power supply in order to generate independent current pulses with correct amplitude and at the proper time in conformity with the sequence programmed in the pulse sequence controller.
The radio-frequency resonator (RF coil or antenna; not shown in
FIGS. 4 and 5
) is situated inside the gradient coil
2
. It converts the RF pulses supplied from a power transmitter into an electromagnetic alternating field and subsequently converts the alternating field emitted by the precessing nuclear moment into a voltage supplied to the reception branch.
Since the gradient switching times should be as short as possible, gradient rise rates on the order of magnitude of a few 10 mT/m are necessary. In an extremely strong magnetic field like that generated by the basic field magnet
1
(typically between 0.2 and 1.5 Tesla), strong Lorentz forces occur given such switching events. All system components (housing, covers, shell of the basic field magnet, RF body coil, etc.) that are mechanically coupled to the gradient system move (vibrate) due to the influence of these forces.
Since the gradient coil is almost always surrounded by conductive structures (for example, magnet vessel of stainless steel), the pulsed fields create eddy currents in them that, due to interaction with the basic magnetic field, exert forces on these structures and likewise cause them to move. It is standard in many imaging methods to employ periodically repeated gradient pulses, thereby causing a forced mechanical oscillation of the system to occur. If excitation occurs with periods/frequencies that correspond exactly to one of the natural resonant frequencies that every mechanical system has, resonant amplification of the oscillation (resonance step-up) occurs and the motion amplitudes increase noticeably.
These resonant oscillations of the various MRT apparatus components have a negative influence on the MRT system in many respects:
1. Strong air-borne sound (noise) is generated that represents a nuisance to the patient, the operating personnel and other persons close to the MRT apparatus.
2. The vibrations of the gradient coil as well as of the basic field magnet and their transmission to the RF resonator and the patient bed in the interior of the basic field magnet and the gradient coil contribute to an inadequate clinical image quality that can even lead to misdiagnoses (for example, in functional imaging, fMRI).
3. When the oscillations of the outer shell are transmitted via the poorly thermally conducting rods to the inner shell, or when the superconductor itself is excited to oscillate, then an increased helium evaporation occurs in the inside of the shell, so that an correspondingly greater amount of liquid helium must be replenished, leading to higher costs.
4. High costs also arise due to the necessity of installing a vibration-damping system (similar to an optical table) in order to suppress transmission of the oscillations to the floor.
The excitation of these mechanical or acoustic resonances is dependent on the parameters that define the imaging sequences and including the switching of the gradient pulses. Parameters that excite the gradient-induced mechanical oscillations are, for
Arana Louis M.
Schiff & Hardin LLP
Siemens Aktiengesellschaft
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