Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
2000-11-15
2002-12-31
Lateef, Marvin M. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
C324S307000, C324S309000, C600S407000
Reexamination Certificate
active
06501977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for operating a magnetic resonance tomography device with a gradient system having at least one gradient coil arrangement for generating a gradient field in a spatial direction and which contains an energy supply device that is connected to the gradient coil arrangement; the gradient coil arrangement having at least one first sub-coil and one second sub-coil, and wherein the energy supply device is fashioned such that the currents in the sub-coils can be adjusted independently of one another.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for acquiring images of the inside of the body of a living examination subject. For this purpose, rapidly switched magnetic gradient fields, which have a high amplitude and which are generated by a gradient system, are superimposed on a static basic magnetic field.
The gradient system includes gradient coils, gradient amplifiers and a gradient control. One of the gradient coils, for a specific spatial direction, generates a gradient field having a gradient, which, at least within an imaging volume of the magnetic resonance tomography device, is approximately of the same magnitude in a location-independent manner at any arbitrary point in time. Since the gradient field is a chronologically variable magnetic field, the aforementioned is still valid for any point in time but the magnitude is variable from one point in time to another point in time. Normally, the direction of the gradient is strictly prescribed by the gradient coil design.
The currents are adjusted in the gradient coil for generating the gradient field. The amplitudes of the required currents amount to several 100 A. The current rising and falling rates (“slew rate”) amount to several 100 kA/s. The gradient coil is connected to a gradient amplifier for the current supply. Since the gradient coil represents an inductive load, high initial voltages of the gradient amplifier are necessary for generating the aforementioned currents.
In the case of magnetic resonance image pickups in living examination subjects, unwanted stimulations in the examination subject can be triggered due the switching of the gradient fields. The gradient fields thereby have an effect on the examination subject and are characterized by a chronologically changing magnetic flux density, which generates eddy and inductance currents in the examination subject.
Methods are known for predicting these stimulations. One of these methods for monitoring the stimulation is based on the dB/dt model, for example. In this method, the values of the chronological change of the magnetic flux density (dB/dt-values) of gradient fields are controlled and monitored, these values occurring during magnetic resonance tomography. The maximally allowable dB/dt values derive from the result of a stimulation study with the corresponding gradient coil, or from the limiting values that are strictly prescribed by the facility operating the tomography apparatus, for example. Further details are provided by J. Abart et al. “Peripheral Nerve Stimulation by Time-Varying Magnetic fields”, J. Computer Assisted Tomography (1997) 21 (4), pages 532 to 538.
The initiation of stimulations essentially depends on the type of pulse sequence employed in the imaging. Such sequences are broadly differentiated between conventional sequences and the fast sequences. Normally, conventional sequences require a high linearity of the gradient fields within a specific linearity volume, for example a linearity of approximately 5% in a spherical linearity volume having a diameter of approximately 40 to 50 cm given moderate gradient intensities of 10 to 20 mT/m and switching times of approximately 1 ms. High gradient intensities, e.g. 20 to 40 mT/m, are extremely rapidly switched for the fast sequences (switching times circa 100 to 500 &mgr;s). The time-varying magnetic flux density of the gradient fields induces electric currents in the examination subject, and these electric currents can initiate stimulations of the examination subject. As a result of faster time variations, i.e., faster switching times and higher values of the magnetic flux density of gradient fields, the induced currents become larger and the likelihood of stimulations increases. Values that are the largest in terms of magnitude are reached at the edges and outside of the linearity volumes; this is where the maximal field boost occurs. Given the requirements to be met by the gradient intensity and the switching time, the boost is reduced and therefore the risk of stimulation because a gradient coil having a smaller linearity volume is utilized. Therefore, the linearity volume is reduced to a diameter of 20 cm, for example, in fast sequences. Normally, a gradient coil with the aforementioned properties for fast sequences is not suitable for conventional whole body applications, but it is suitable for magnetic resonance imaging techniques, such as the echo planar method and its hybrids.
Published German application OS195 40 746 describes a modular gradient coil system, which has two gradient coils for a spatial direction. One of the two coils or a series connection of both gradient coils is optionally connected to a gradient amplifier. For example, only one of the gradient coils is used for fast sequences and the series connection is used for conventional sequences. The gradient coil system thereby has a small linearity volume for fast sequences and allows the fast switching of gradient fields with large gradient intensities. Given the common operation of both coils, the gradient coil system has a larger linearity volume for conventional sequences with slowly switched gradient fields and with respect to smaller gradient intensities. A disadvantage of the aforementioned gradient coil system is that the size of the linearity volume and the quality of the linearity can be varied only in three steps at a maximum.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for operating a magnetic resonance tomography device of the type described above which improves the avoidance of producing stimulations of a living examination subject.
This object is achieved in accordance with the invention in a method for operating a magnetic resonance tomography device having a gradient system, which contains at least one gradient coil arrangement for generating a gradient field in a spatial direction and which contains an energy supply device that is connected to the gradient coil arrangement, wherein the gradient coil arrangement has at least one first sub-coil and one second sub-coil and wherein the energy supply device is fashioned such that currents can be adjusted independently of one another in the sub-coils, and wherein, for the continuous adjustment of at least one property of the gradient field, the current in at least one of the sub-coils is determined and adjusted by solving an optimization task containing a target function and at least one secondary condition, so that stimulations of a living examination subject are avoided.
For example, stimulations are prevented when an extreme value of the magnetic flux density of the gradient field remains below a fixable stimulation limiting value given a fixed slew rate of a sequence. The optimization task is solved by a variation calculation, for example. The target function contains coefficients of a spherical function development of a magnetic flux density of the gradient field, and the target function contains coefficients for each of the sub-coils. The coefficients for one of the sub-coils are multiplied with a factor that corresponds to a ratio of an adjustable current to a nominal current of the sub-coil. For example, a further secondary condition is that at least one of the coefficients multiplied with the factor is larger than a fixed limiting value. For example, it is possible to prescribe a minimally required gradient intensity with this version of the invention.
In addition to a current adjustm
Lateef Marvin M.
Qaderi Runa Shah
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