U.S. Philips Corporation

Cornelis L Ham

Johannes A Overweg

Wilhelmus R Mens

Antoon F Mehlkopf

The gradient coil system in a conventional MRI apparatus is optimized in respect of the shielding effect by the shielding coil of the system. Consequently, the system generally is not optimized in respect of the Lorentz forces occurring in the system, resulting in noise of a level such that it is annoying to the users. In order to avoid such noise, the system can be force-optimized. Because the shielding effect is partly lost in that case, however, eddy currents and hence disturbing noise would occur again. However, if the eddy currents are made to occur in acoustically insulated eddy current conductors which also have a large time constant for the decay of the eddy currents, the adverse effects of these eddy currents are adequately counteracted and the disturbing noise is reduced to an adequate extent. Particularly, the (cold) radiation shields of a cryogenic magnet system, arranged in vacuum, can be used as eddy current conductors. The vacuum then constitutes the acoustic insulation.

Magnetic resonance apparatus provided with force-optimized gradient coils

Magnetic resonance imaging includes a system of gradient coils (3) for generating gradient fields in a measuring space (35), a power supply source (7) for the gradient coils, and a communication system for transferring acoustic information from at least a first region (39) in which the level of gradient noise generated by the gradient coils (3) is comparatively high to at least a second region (41). The communication system includes a reference signal generating device for generating a reference signal which is dependent on the gradient noise, a microphone (43) which is arranged in the first region (39) so as to pick up a mixture of sound information and gradient noise, and a sound reproduction device (65, 67), at least a part of which is situated in the second region (41). The communication system also includes a noise suppression device, formed by a filter device (61) for converting the reference signal into a signal which corresponds substantially to the gradient noise at the area of the microphone (43), and a summing device (63) for adding the output signal of the filter device to the output signal of the microphone in phase opposition, the output of the summing device being connected to the sound reproduction device. Between the microphone (43) and the summing device (63) a signal delay device (53) is inserted which delays the microphone signal for a predetermined period of time. The sound reproduction device (65, 67) is provided with a device (69) for attenuating sound which does not originate from the sound reproduction device.

Magnetic resonance imaging apparatus comprising a communication system

A magnetic resonance apparatus includes a magnet system (1) for generating a steady magnetic field in a measuring space (35), a gradient coil system (3) for generating gradient fields in the measuring space, and a power supply source (7) for the gradient coils (3) which comprises at least one gradient signal generator (9) and a number of gradient amplifiers (11), each of which is connected between an output of the gradient signal generator and at least one of the gradient coils. In order to cancel annoying noise caused by the gradient coils (3), the apparatus also includes a noise cancellation device which includes the following elements: a) a first filter device (39) having a number of inputs which corresponds to the number of gradient amplifiers (11), each of the inputs being connected to an output of the gradient signal generator (9), and also includes a corresponding number of outputs, the first filter device having transfer functions, all of which correspond approximately to the transfer function of one of the chains extending from the gradient signal generator (9), via one of the gradient amplifiers (11), the gradient coil (coils) (3) connected to this gradient amplifier, and air, to a region (40, 42) which is referred to as a region of silence and in which the sound of the gradient coils (3) is to be cancelled; b) an adder circuit (51), having inputs which are connected to outputs of the first filter device (39) and an output (63) which is connected to an input of a sound reproducing device which comprises at least one sound reproducer (57) arranged in or near the region of silence (40); and c) a signal delay device (41) for inducing a predetermined time difference between the signal transfer from the gradient signal generator (9) to the gradient coils (3) on the one hand and the signal transfer from the gradient signal generator to the first filter device (39) on the other hand.

Magnetic resonance apparatus with noise cancellation

In a magnetic resonance apparatus, a coil system is added to a gradient coil system, as a result of which a measuring field range is adapted to the geometry of an object to be examined. For example, by addition of a Bo coil system to be activated simultaneously with the z-gradient system, an asymmetry is realized in the z-gradient field, as a result of which return of disturbing resonance signals from an object part located outside the gradient field operating range is avoided. Further, by addition of additional arc conductors, the z-gradient linearity range is displaced in the z-direction.

Magnetic resonance apparatus

In a gradient coil system for a magnetic resonance apparatus arc conductors of notably X coils and Y coils are stacked axially or radially so as to form rigid stacks. The arc conductors are interconnected by means of electrical conductors and the stacks are also interconnected, so that a rigid, self-supporting coil system is obtained. Because the coil system is composed of rigid elements, is comparatively open and does not require a coil former, the production of noise in a magnetic resonance apparatus comprising such a coil system is substantially reduced and field disturbances due to coil deformations are also reduced.

Magnetic resonance apparatus comprising a low-noise gradient coil

In a magnetic resonance apparatus a gradient coil system in which 'return' arc conductors, which contribute substantially less to the gradient field formation because they are situated further outwards radially and/or axially from the center of the system, are positioned to compensate for stray fields of 'effective' are conductors, which are more centrally situated and which contribute more substantially to the gradient field formation. Due to both radial and axial displacement of the return arc conductors relative to the effective arc conductors, the measuring space of the apparatus can be conically shaped, so that a higher patient accessibility is achieved or a smaller diameter can be imparted to a central portion.

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Eindhoven, Netherlands

Cornelis L Ham