Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Reexamination Certificate
2000-03-08
2001-05-22
Arana, Louis (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
C324S307000
Reexamination Certificate
active
06236204
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance tomography device, that contains a basic field magnet, a gradient coil system and a control system, that among other things, controls the currents in the gradient coils on the basis of gradient pulses in a pulse sequence, wherein significant noise occurs due to the exciting of resonant oscillations of the gradient coil system.
2. Description of the Prior Art
Magnetic resonance tomography is a known modality for the acquisition of images of the interior of the body of the living patient. In magnetic resonance tomography devices, dynamic magnetic fields with linear gradients in all three spatial directions are superimposed on the static basic magnetic field. In this situation the time-variable currents in the gradient coils achieve amplitude values of up to several 100 A and the current flowing therein executes frequent and rapid changes in direction, with increase and decrease rates of several 100 kA/s. These currents interact with the strong basic magnetic field, of an order of magnitude of 1 Tesla, an produce significant Lorentz forces, that lead to oscillations of the gradient coil system and thereby generate noise.
A trend in magnetic resonance tomography is toward shortening of measurement times and improvement of imaging properties, achieved with increasingly faster pulse sequences. These require enhanced gradient coil performance. Higher gradient coil currents lead to higher Lorentz forces and thereby to an increase in noise. Efforts to reduce noise led to a change in the mechanical design of the magnetic resonance tomography device. For example, the rigidity of the gradient coil system has been increased, the gradient coils have been acoustically damages or isolated, and the fastening points of the gradient coil system have been placed at oscillation nodes. The faster pulse sequences control more frequent and rapid changes in the gradient coil currents, i.e. the dominant components of these currents in the frequency spectrum shift to higher frequencies. Although, for example, a doubling of the rigidity of the gradient coil system increases the resonant frequencies only by a factor of approximately 1.4, the probability increases that the fast pulse sequences will excite the resonances of the gradient coil system in spite of increased rigidity. This then leads to significant noise.
SUMMARY OF THE INVENTION
In addition to the previously described, purely mechanical design measures, an object of the invention is to reduce the noise that arises in the implementation of a pulse sequence.
The object is inventively achieved in a method for operating a magnetic resonance tomography device wherein portions of a gradient pulse sequence actively damp noise-producing resonances of the gradient coil system excited by other portions of the sequence.
A particular advantage of this active damping is that this type of noise reduction without mechanical design measures is effective for any fast pulse sequences. It is particularly advantageous to apply the active damping to pulse sequences where the form of its gradient pulse sequences, for example, for obtaining a desired imaging property, cannot be configured such that the resonances are not excited or are only weakly excited from the onset.
The noise behavior of a gradient coil system is advantageously described per gradient coil by means of a transfer function, that results from the gradient coil current as an input quantity and from the noise as an output quantity. In the case of magnetic resonance tomography devices with cylindrical patient opening, for example, the gradient coils are usually a component of the tube-like arrangement, referred to as the gradient tube. The transfer functions of the gradient tubes exhibit noticeable resonances. The dominant resonance (mode) of a gradient tube lies typically in the range of 750 Hz, has a resonant width of few Hz and slight damping. During the implementation of a gradient pulse sequence, if the spectrum thereof has a component at the same frequency as the dominant resonant frequency, an especially loud noise occurs.
Even in the event that a pulse sequence excites no resonances of a gradient coil system, a base noise level still occurs, but this is a multiple lower than if resonances are excited. The goal of the active damping is to limit the noise to a specific base noise level. The excitation of a resonant mode is a time dependent process; i.e. there is a specific excitation time, or a number of gradient pulses necessary in order to stimulate the resonance so that the maximum noise occurs. Due to this fact, it suffices to employ measures to counter an excited resonant mode in relatively long time intervals of approximately 10 to 100 ms after excitation begins. Determining the number of gradient pulses after which the active measures are to be implemented depends on the selected limit value for the maximum allowable noise.
In an embodiment, a gradient pulse sequence is interrupted after the completion of a specific pattern of gradient pulses and before this pattern is repeated for odd-numbered, positive multiples of half of the reciprocal value of the excited dominant resonant frequency of the gradient coil system. The aforementioned interruption is a variation of a pulse sequence that is very simple and surveyable in terms of its imaging properties and causes no additional electrical load on the gradient coil.
In an embodiment a gradient pulse sequence is continued with the pattern mirrored on the time axis after completion of a specific pattern of gradient pulses. The aforementioned configuration of a gradient pulse sequence causes no lengthening of the measurement time and represents no electrical additional load for the gradient coil.
In a further embodiment, additional gradient pulses that counteract the excited noise-producing resonant modes of the gradient coil system are inserted in a gradient pulse sequence after the completion of a set pattern of gradient pulses. Given a corresponding form, height and duration, a quick damping with shorter time constants than those associated with the excitation of the resonant modes is achieved with the aforementioned additional gradient pulses.
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Arana Louis
Schiff & Hardin & Waite
Siemens Aktiengesellschaft
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