Magnetic resonance imaging
An MRI apparatus employs a magnet for establishing a B 0 field, one or more imaging radiofrequency coils for creating a B 1 field, and one or more nulling radiofrequency coils. The RF energy emitted by the nulling radiofrequency coils may be configured such that it disrupts the nuclear magnetic resonance signals emitted by nuclei excited by the B 1 field. In addition, the nulling radiofrequency coils may be calibrated to be 180° out of phase such that the RF energy emitted by a pair of nulling radiofrequency coils is cancelled at a location between them.
1. A method of eliciting magnetic resonance signals from an object comprising:
subjecting the object to a magnetic field;
applying an excitation radiofrequency (“RF”) signal to the object so as to tilt spin axes of nuclei within the object relative to the magnetic field;
applying a spatially inhomogeneous RF variation signal to the object so that the variation signal affects spins within the object except in a zone within the object; and
receiving RF signals emitted by the object after application of said excitation and variation signals so that the received RF signals include signals emitted by nuclei within said zone.
2. The method of claim 1 wherein said variation signal is effective to substantially randomize the orientations of nuclear spin axes except within said zone.
3. The method of claim 2 wherein said variation signal includes a random or pseudorandom noise component.
4. The method of claim 1 wherein:
said variation signal is effective to shift the phase of rotation of nuclear spin axes except within said zone so that nuclei outside of said zone emit signals differing in phase from signals emitted by nuclei within zone; and
said receiving step includes selecting signals by phase so as to exclude signals emitted by nuclei outside of said zone.
5. The method of claim 1 wherein:
said variation signal is effective to shift the frequency of rotation of nuclear spin axes except within said zone so that nuclei outside of said zone emit signals differing in frequency from signals emitted by nuclei within said zone; and
said receiving step includes selecting signals by frequency so as to exclude signals emitted by nuclei outside of said zone.
6. The method of claim 1 wherein said variation signal is inhomogeneous in at least two orthogonal directions.
7. The method of claim 6 wherein said variation signal is inhomogeneous only in first and second orthogonal directions so that said zone is an elongated line extending in a third direction orthogonal to said first and second directions.
8. The method of claim 7 wherein said step of applying a magnetic field is conducted so as to apply a magnetic field gradient in a third direction orthogonal to said first and second directions.
9. The method of claim 8 further comprising varying the spatial inhomogeneity of said variation signal so as to shift said zone in at least one of said first and second directions.
10. The method of claim 9 wherein said step of applying an excitation signal is performed so that said excitation signal consists essentially of frequencies corresponding to the Larmor frequencies of nuclei in a slice extending in said first and second directions and having a thickness in said third direction, whereby the received signals are signals from nuclei in a voxel at the intersection of said elongated zone and said slice.
11. An apparatus for eliciting magnetic resonance signals from an object comprising:
a magnet assembly configured to subject an object to a magnetic field;
a first pair of radiofrequency coils configured to apply an excitation radiofrequency (“RF”) signal to the object so as to tilt spin axes of nuclei within the object relative to the magnetic field;
a second pair of radiofrequency coils configured to apply a spatially inhomogeneous RF variation signal to the object so that the variation signal affects spins within the object except in a zone within the object; and
a radiofrequency detector configured to detect RF signals emitted by the object after application of said excitation and variation signals so that the received RF signals include signals emitted by nuclei within said zone.
12. The apparatus of claim 11 wherein said variation signal is effective to substantially randomize the orientations of nuclear spin axes except within said zone.
13. The apparatus of claim 12 wherein said variation signal includes a random or pseudorandom noise component.
14. The apparatus of claim 11 wherein:
said variation signal is effective to shift the phase of rotation of nuclear spin axes except within said zone so that nuclei outside of said zone emit signals differing in phase from signals emitted by nuclei within zone; and
said radiofrequency detector is further configured to select signals by phase so as to exclude signals emitted by nuclei outside of said zone.
15. The apparatus of claim 11 wherein:
said variation signal is effective to shift the frequency of rotation of nuclear spin axes except within said zone so that nuclei outside of said zone emit signals differing in frequency from signals emitted by nuclei within said zone; and
said radiofrequency detector is further configured to detect signals by frequency so as to exclude signals emitted by nuclei outside of said zone.
16. The apparatus of claim 11 wherein said variation signal is inhomogeneous in at least two orthogonal directions.
17. The apparatus of claim 16 wherein said variation signal is inhomogeneous only in first and second orthogonal directions so that said zone is an elongated line extending in a third direction orthogonal to said first and second directions.
18. The apparatus of claim 17 wherein said magnet assembly is further configured to apply a magnetic field gradient in a third direction orthogonal to said first and second directions.
19. The apparatus of claim 18 wherein varying the spatial inhomogeneity of said variation signal is performed so as to shift said zone in at least one of said first and second directions.
20. The apparatus of claim 19 wherein the first radiofrequency coil is further configured to apply said excitation signal consisting essentially of frequencies corresponding to the Larmor frequencies of nuclei in a slice extending in said first and second directions and having a thickness in said third direction, whereby the detected signals are signals from nuclei in a voxel at the intersection of said elongated zone and said slice.