Method for position dependent change in the magnetization in an object in a magnetic resonance experiment
A method for position dependent change in the magnetization in an object, according to a requirement in a magnetic resonance measurement, wherein radio-frequency pulses are irradiated in conjunction with supplementary magnetic fields that vary in space and over time and are superposed on the static and homogeneous basic field of a magnetic resonance measurement apparatus along a z-direction, is characterized in that non-linear supplementary magnetic fields are used, whose spatial gradient of the z-component is not constant at least at one instant of the irradiation, and that the radio-frequency pulses to be irradiated are calculated in advance, wherein progressions over time of the field strengths of the supplementary magnetic fields in the region of the object that are calculated and/or measured position-dependently are included in this calculation. This enables change in the magnetization with an at least locally spatially higher resolution and/or shorter irradiation duration of the RF pulses and supplementary magnetic fields than is feasible with linear supplementary magnetic fields produced by conventional gradient systems. In particular, this is possible under the technical and physiological conditions that currently constrain the performance of the known methods using linear supplementary fields.
1. A method for position dependent change of magnetization in an object, according to a requirement, in a measurement performed in a magnetic resonance apparatus, the method comprising the steps of:
a) applying a static and homogeneous basic field oriented along a z-direction;
b) superposing non-linear supplementary magnetic fields, varying in space and over time, on the static and homogeneous basic field, wherein dependences over time of field strengths of the supplementary magnetic fields in a region of the object are calculated and/or measured in a position dependent manner;
c) calculating radio-frequency pulses using the time and position dependences of the supplementary magnetic fields determined in step b); and
d) irradiating the radio-frequency pulses calculated in step c) in conjunction with the supplementary magnetic fields of step b), wherein the supplementary magnetic fields have a z-component spatial gradient which is not constant during at least at one instant of the irradiation.
2. The method of claim 1 , wherein, if at least one group of equivalent positions occurs within the object during irradiation with a single radio-frequency transmission antenna whose transmission profile covers an entire object such that changes in the magnetization at the equivalent positions cannot be performed mutually independently, at least two transmission antennas are used having different spatial transmission profiles, wherein a change in the magnetization at at least one of the equivalent positions can be performed independently of a change in magnetization at other positions of the group.
3. The method of claim 1 , wherein said supplementary magnetic fields effect a change in the magnetization with spatially varying spatial resolution.
4. The method of claim 1 , wherein a setting of a position dependent change in the magnetization is chosen such that the magnetization undergoes a selectively non-vanishing change in only one or more regions within the object.
5. The method of claim 4 , wherein at least one region is chosen such that it is adapted to anatomical, morphological, or functional properties of the object under examination and/or to a mapping characteristic of the magnetic resonance apparatus.
6. The method of claim 4 , wherein the regions are chosen such that MR signals originating from a totality of all regions can be uniquely spatially encoded with a gradient system used in data acquisition.
7. The method of claim 1 , wherein a phase of transverse magnetization is changed according to a defined spatial distribution.
8. The method of claim 7 , wherein, in the magnetic resonance measurement, a change in spatial distribution of transverse magnetization phases is performed repeatedly in at least two phase encoding periods according to a spatial encoding scheme, with data acquisition being performed after each of these phase encoding periods, wherein a totality of the data thereby acquired is reconstructed having been resolved spatially according to the spatial encoding scheme and stored and/or displayed as an image.