Imaging systems
First Claim
1. A nuclear magnetic resonance apparatus, for examining a substantially planar slice of a body, the apparatus including means for applying magnetic fields to cause resonance preferentially in the plane of said slice, means for applying a pulsed magnetic field having a gradient across the plane of the slice to produce phase dispersion in said resonance, means for sensing resonance signals induced during said pulsed field for a plurality of different directions thereof, means for demodulating the resonance signals and means for further processing the demodulated signals to provide a representation of said slice, wherein the resonance signals are demodulated at a frequency which, in the presence of the said magnetic fields and pulsed magnetic field, is the resonance frequency for a position in the plane lying outside said slice.
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Accused Products
Abstract
In known NMR imaging apparatuses it has been the practice to arrange the GR field gradient, which rotates in the plane of the examined slice, with a zero crossing substantially in the center of the slice and to demodulate at the Larmor frequency at the zero crossing. It is now proposed to demodulate the detected resonance signals at a frequency which, in the presence of the field gradient, is for a part of the plane lying outside the body slice. This may be by arranging the zero-crossing to be outside the slice. In that case it is preferable that the zero-crossing of GR lies on a straight line (27) maintained tangential to a circle (28) lying outside the body (23) about an axis (24) through the slice. Control may then be by precalculated coil currents or by precalculating fields at probe positions and adjusting the coil currents to give the expected fields.
Alternatively the zero-crossing may be maintained at the center and demodulation may be at a different frequency. It is then desirable also to measure the resonance frequency in the absence of the field gradient across the slice. The resonance signals are then effectively further demodulated at that measured frequency. The effective demodulation is preferably achieved by complex multiplication of the two signals.
54 Citations
24 Claims
- 1. A nuclear magnetic resonance apparatus, for examining a substantially planar slice of a body, the apparatus including means for applying magnetic fields to cause resonance preferentially in the plane of said slice, means for applying a pulsed magnetic field having a gradient across the plane of the slice to produce phase dispersion in said resonance, means for sensing resonance signals induced during said pulsed field for a plurality of different directions thereof, means for demodulating the resonance signals and means for further processing the demodulated signals to provide a representation of said slice, wherein the resonance signals are demodulated at a frequency which, in the presence of the said magnetic fields and pulsed magnetic field, is the resonance frequency for a position in the plane lying outside said slice.
- 7. A nuclear magnetic resonance apparatus, for examining a substantially planar slice of a body, the apparatus including means for applying magnetic fields to cause resonance preferentially in the plane of said slice, means for applying a pulsed magnetic field having a gradient across the plane of the slice to produce phase dispersion in said resonance and repeating the pulsed magnetic field for a plurality of different directions of said gradient in sequence, and means for sensing a resonance signal induced during said pulsed field for each of said directions thereof wherein the means for applying the pulsed field is arranged so that said pulsed field is only of one polarity within said slice.
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8. A nuclear magnetic resonance apparatus, for examining a substantially planar slice of a body, the apparatus including means for applying magnetic fields to cause resonance preferentially in the plane of said slice, means for applying a pulsed magnetic field having a gradient across the plane of the slice in each of a plurality of different directions at different times, to produce phase dispersion in said resonance, and means for sensing a resonance signal induced during said pulsed field for each of said plurality of different directions thereof, wherein the means for applying the pulsed field is arranged so that the pulse field has a zero value along a straight line which is in the place of said slice and which, for said different directions of said gradient, is tangential to a circle surrounding the body.
- 11. A nuclear magnetic resonance apparatus for examining a substantially planar slice of a body, the apparatus including means for applying magnetic fields to cause resonance preferentially in the plane of said slice, means for applying a pulsed magnetic field having a gradient across the plane of the slice to produce phase dispersion in said resonance and repeating the pulsed magnetic field for a plurality of different directions of said gradient at different times, the means for applying the pulsed field being arranged so that the pulsed field is only of one polarity within the slice, means for sensing a resonance signal induced during said pulsed field for each of said directions, probe means to measure the magnetic fields at different positions in the region of the slice and means for determining the differences between the measured fields and for controlling the means for applying the pulsed magnetic field to reduce said differences.
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13. A method of examining a substantially planar slice of a body by nuclear magnetic resonance, the method including:
- applying magnetic fields to cause resonance preferentially in the plane of the slice;
applying a pulsed magnetic field having a gradient across the plane of the slice to produce phase dispersion in said resonance;
sensing the resonance signals induced during said pulsed field for a plurality of different directions thereof;
demodulating the resonance signals and further processing the demodulated signals to provide a representation of said slice;
wherein the resonance signals are demodulated at a frequency which, in the presence of the said magnetic fields and pulsed magnetic field, is the resonance frequency for a position in the plane lying outside said slice. - View Dependent Claims (14, 15, 16, 17, 18, 19)
- applying magnetic fields to cause resonance preferentially in the plane of the slice;
- 20. A method of examining a substantially planar slice of a body by nuclear magnetic resonance, the method including applying magnetic fields to cause resonance preferentially in the plane of the slice, applying a pulsed magnetic field having a gradient across the plane of the slice to cause phase dispersion in said resonance, sensing the resonance signals produced thereby and repeating the procedure for a plurality of different directions of said gradient in sequence, wherein the pulsed field is arranged to have a zero value which lies in said plane but outside said slice.
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21. A method of examining a substantially planar slice of a body by nuclear magnetic resonance, the method including applying magnetic fields to cause resonance preferentially in the plane of the slice, applying a pulsed magnetic field, having a gradient across the plane of said slice and a zero value which is in said plane but outside said slice, to cause phase dispersion in said resonance, sensing the resonance signals, produced thereby and repeating the procedure for each of a plurality of different directions of said gradient, wherein the zero-value of said pulsed field is arranged to be along a straight line which is in the plane of the slice and which, for said different directions of said gradient, is tangential to a circle surrounding the body.
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24. A method of examining a substantially planar slice of a body by nuclear magnetic resonance, the method including:
- applying magnetic fields to cause resonance preferentially in the plane of the slice;
applying a pulsed magnetic field having a gradient across the plane of the slice with a zero crossing within the slice to produce phase dispersion in said resonance;
sensing the resonance signals induced during said pulsed field;
demodulating the resonance signals at a frequency which, in the presence of said magnetic fields and pulsed magnetic field, is the resonance frequency for a position in the plane lying outside said slice;
repeating the sensing of the resonance signals in the presence of the first mentioned magnetic fields but not the pulsed gradient field;
effectively demodulating said first obtained resonance signals at the frequency of said second sensed signals;
repeating the preceding steps for a plurality of different directions of said gradient and further processing the demodulated signal for said different directions to provide a representation of said slice.
- applying magnetic fields to cause resonance preferentially in the plane of the slice;
Specification