Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2000-05-24
2001-09-11
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S307000
Reexamination Certificate
active
06288547
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for determining the curve for a gradient current in a magnetic resonance tomography apparatus, as well as to a magnetic resonance tomography apparatus operating according to the method.
2. Description of the Prior Art
In MR tomography devices, pulse sequences are employed which are generated by gradient currents. Calculating the curve of the gradient current is very complicated. This is particularly true when the k-space is not scanned in straight sections, but instead a general k-space scanning strategy is used. An example of such a sequence is the spiral-imaging method disclosed in the U.S. Pat. No. 4,651,096, wherein the k-space path exhibits a spiral shape.
In order to attain maximum performance of the MR tomography device, effort is made to optimally utilize the gradient system with respect to the slew rate and gradient amplitude. The calculation of complicated differential equations is required for the determination of the curve of the gradient current with these prescriptions as boundary conditions. It would greatly exceed the computing capacity of the currently customary control computers to calculate these equations in realtime during the measurement. For MR devices as are offered, e.g. by Siemens under the name “Magnetom”®, it is therefore customary to calculate a table offline with support values for the curve of the gradient current and to store as the reference current curve for a class of pulse sequences (for example, for the aforementioned spiral-imaging method). Given a change of the measurement field, only the gradient amplitude is then scaled. Since it must be assured that the allowable slew rate is not exceeded, however, one cannot attain an optimal utilization of the gradient system. The reference current curve must be determined such that the scaling does not cause the limit values of the slew rate for any adjustable measurement field to be exceeded.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for determining the curve of the gradient current in an MR tomography device, or an MR tomography device that assures an improved utilization of the gradient system.
The above object is achieved in accordance with the principles of the present invention in a method and MR tomography apparatus wherein a reference current curve for a gradient current is established for a class of pulse sequences, and wherein a current curve for a gradient current is obtained from the reference current curve by scaling, for use with a particular pulse sequence, and wherein the scaling takes place, for at least a portion of the pulse sequence being currently used, both in the amplitude axis and in the time axis.
As a result of the scaling of the reference current curve in the amplitude and in the time axis, an improved utilization of the gradient system is attained since the slew rate is not automatically scaled along with the scaling of the gradient amplitude.
The inventive method can be used, e.g. for scaling on the basis of a desired measurement field or for adapting performance data of various MR tomography devices. The sequence that has been calculated once can be employed in between various systems using respectively different performance data.
Scaling of the amplitude and the time axis respectively can ensue by a factor n given a change of the measurement field by the factor 1
. Thus the scaling is divided uniformly on the amplitude axis and the time axis.
Given a pulse sequence with variable amplitude, one can advantageously provide time intervals within the pulse sequence in which an amplitude limitation of the curve of the gradient current is in effect and only a scaling in the time axis ensues. Thus, the desired k-space path can be approached without exceeding the allowable gradient amplitude.
REFERENCES:
patent: 4651096 (1987-03-01), Buonocore
patent: 5237277 (1993-08-01), Lenz
patent: 5856744 (1999-01-01), Block et al.
patent: 5914601 (1999-06-01), Goldfarb
Arana Louis
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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