Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
1999-10-08
2001-03-27
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S318000, C324S322000
Reexamination Certificate
active
06208140
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for operating a magnetic resonance tomography device and to a magnetic resonance tomography apparatus for implementing the method.
2. Description of the Prior Art
The requirements for a gradient coil are mainly dependent on the pulse sequence that is applied for MR-imaging. In the following, “conventional” pulse sequences are differentiated from “fast” pulse sequences. As used herein, “conventional” pulse sequences can be characterized by only one nuclear magnetic resonance signal being selected per excitation. Examples of conventional pulse sequences are traditional spin echo methods or gradient echo methods for example, but also the faster methods such as FLASH (described in U.S. Pat. No. 4,707,658) and FISP (described in the U.S. Pat. No. 4,769,603) are examples thereof. As used herein, “fast” MR-imaging techniques are those wherein a large number of nuclear magnetic resonance signals are selected subsequent to an excitation. The EPI method (described in the U.S. Pat. No. 4,165,479), in particular, is such a sequence, as well as the turbo spin echo method, the GRASE method and the HASTE methods.
A high image quality and a large measuring volume are particularly important given conventional pulse sequences. The following requirements with respect to the gradient coils thereby result: Large linearity volume (≈5% linearity in the linearity volume of 40-50 cm), moderate gradient intensities (10-20 mT/m) and moderate switching times (≈1 ms).
Great value is particularly placed on the speed in fast pulse sequences, but compromises must be made regarding other parameters. High gradients (20-40 mT/m) must be switched or activated in a very fast manner; this is a specific requirement with respect to the gradient system (switching times approximately 100-500 &mgr;s). Due to the necessary high amplitude change rates of the magnet fields, currents are induced in a patient to be examined, which currents can lead to peripheral muscle stimulations. The stimulation is particularly determined by the maximum field boost. With the given requirements with respect to the gradient intensity and the switching time, the field boost and thus the stimulation risk can only be reduced by reducing the linearity volume of the gradient coil.
To that end, it is known from the German OS 195 40 746 to use a modular gradient coil system. A central modular coil alone is utilized for fast pulse sequences. This central modular coil exhibits only a relatively small linearity volume. Since the efficiency of a coil is approximately proportional to the volume, high gradient intensities and short rise times can be realized with such a coil. Although the measuring volume is limited, fast pulse sequences can be realized, the field boost remains limited. Given operation of the MR-tomography device with conventional sequences, correcting coils are connected to the central part of the coil, these correcting coils increasing the linearity volume; however, this is at the expense of the gradient performance.
Only prior to the measurement, the user has the choice to either optimize the gradient coil system for maximal gradient intensity/gradient rise speed or to optimize it for a maximal linearity volume given this modular gradient coil system.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for the operation of an MR-tomography device, and an MR-tomography device, that can be utilized more universally.
The above object is achieved in accordance with the principles of the present invention in a magnetic resonance tomography apparatus, and a method for operating such a magnetic resonance tomography apparatus, wherein a gradient coil system of the apparatus can be switched to different field qualities during a pulse sequence forming a single scanning/data acquisition sequence having measuring phases including spin preparation, slice excitation, k-space filling and readout, with a field quality being employed in one of these measuring phases which is different from the field qualities employed in the other measuring phases.
The switchability of the gradient coil system in different measuring phases during the measuring process allows the field quality that is optimal for the respective measuring phase to be used. This is based on the knowledge that, given a number of pulse sequences, the competing requirements of gradient intensity/gradient rise rate and linearity volume are different during the sequence process. Given the “fast” pulse sequences defined above, the gradient intensity and the gradient rise times are also critical mainly in the readout phase. In the portion of the sequence wherein spin preparation and the slice excitation take place, the good linearity is more important. This can be taken into consideration by the switchability of the field quality regarding these parameters during the sequence process.
Here, the switchability of the field quality is not limited to the factors of linearity and gradient rise time/gradient intensity, but must be viewed more broadly. A field quality, for example, which is optimized to an off-center linearity volume can be effective in an embodiment. Such an optimization is advantageous for measurements in the shoulder area for example, since the measurement must hereby ensue outside of the magnet center.
Given saturation pulses with which the spin magnetization is saturated in a volume outside of the measuring volume, the gradient linearity is typically more important than the gradient intensity/gradient rise rate, so that the field quality can be correspondingly optimized.
REFERENCES:
patent: 4707658 (1987-11-01), Frahm et al.
patent: 4769603 (1988-09-01), Oppelt et al.
patent: 5512858 (1996-04-01), Pausch et al.
patent: 5736858 (1998-04-01), Katznelson et al.
patent: 2 314 934 (1998-01-01), None
Gebhardt Matthias
Schmitt Franz
Oda Christine
Shrivastan Brij B.
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