Method and apparatus for SAR reduction using B0 specific RF excitation
A method for generation of a radio-frequency (RF) pulse for excitation of nuclear spins in a predetermined layer of a specimen for magnetic resonance imaging and a magnetic resonance imaging device for performing the method are provided. The method includes determining a variation of a magnetic field in a measuring volume, and defining a spectral frequency distribution of the RF pulse. The RF pulse with the spectral frequency distribution is configured to excite nuclear spins in the specimen. The nuclear spins are polarized by the magnetic field at a predetermined flip angle in the measuring volume under a boundary condition of a substantially minimum energy content. The method also includes generating the RF pulse with the defined spectral frequency distribution.
1. A method for generation of a radio-frequency (RF) pulse using a magnetic resonance imaging device for excitation of nuclear spins in a predetermined measuring volume for magnetic resonance imaging, wherein the magnetic resonance imaging device comprises a supply unit and a magnet unit, and wherein the supply unit comprises a controller and an RF pulse generating unit, the method comprising:
determining, with the controller, a spatial variation of a magnetic field strength of a magnetic field in the predetermined measuring volume;
defining, with the controller, a spectral frequency distribution of the RF pulse under a boundary condition of a substantially minimum energy content of the RF pulse, wherein the spectral frequency distribution only encompasses a previously determined frequency range of the spatial variation of the magnetic field strength of the magnetic field in the predetermined measuring volume, and wherein the RF pulse with the spectral frequency distribution is configured to excite nuclear spins in a specimen in the magnetic field in the predetermined measuring volume at a predetermined flip angle with a predetermined variance of the flip angle;
generating, with the RF pulse generating unit, the RF pulse with the defined spectral frequency distribution; and
imaging, with the magnetic resonance imaging device, the specimen using the generated RF pulse.
2. The method of claim 1 , wherein the substantially minimum energy content is at the most twice a minimum possible energy content.
3. The method of claim 1 , wherein determining the spatial variation of the magnetic field comprises measuring the magnetic field.
4. The method of claim 3 , wherein determining the spatial variation of the magnetic field further comprises using an analysis of a spectral distribution of a resonance line of a non-spatially resolved reference measurement in the specimen.
5. The method of claim 1 , wherein defining the spectral frequency distribution of the RF pulse comprises using a definition of a duration of the RF pulse.
6. The method of claim 1 , wherein the excitation of the nuclear spins is performed by the RF pulse in the magnetic field, and the magnetic field has a gradient field as a component,
wherein defining the spectral frequency distribution of the RF pulse comprises defining a temporal course of an amplitude of the RF pulse as a function of a temporal course of an amplitude of the gradient field.
7. The method of claim 1 , wherein the RF pulse comprises a plurality of partial pulses, and
wherein defining the spectral frequency distribution of the RF pulse comprises defining a number of the plurality of partial pulses.
8. A magnetic resonance imaging device comprising:
a magnet unit operable to generate a magnetic field that has a variation of the magnetic field in a predetermined measuring volume; and
a supply unit comprising a control unit and a radio-frequency (RF) pulse generating unit,
wherein the control unit is configured to:
determine a spatial variation of a magnetic field strength of the magnetic field in the predetermined measuring volume;
determine a spectral frequency distribution for an RF pulse under a boundary condition of a substantially minimum energy content of the RF pulse, wherein the spectral frequency distribution only encompasses a previously determined frequency range of the spatial variation of the magnetic field strength of the magnetic field in the predetermined measuring volume, and wherein the RF pulse is suitable to excite nuclear spins in a specimen in the magnetic field in the predetermined measuring volume at a predetermined flip angle with a predetermined variance of the flip angle,
wherein the RF pulse generating unit is configured to generate the RF pulse with the spectral frequency distribution, and
wherein the magnetic resonance imaging device is configured to image the specimen using the generated RF pulse.
9. The magnetic resonance imaging device of claim 8 , wherein the substantially minimum energy content is at the most twice a minimum possible energy content.
10. The magnetic resonance imaging device of claim 8 , wherein the control unit is further configured to determine the spatial variation of the magnetic field in the predetermined measuring volume using an analysis of a spectral distribution of a resonance line of a non-spatially resolved reference measurement in the predetermined measuring volume.
11. The magnetic resonance imaging device of claim 8 , wherein the control unit is further configured to adjust the spectral frequency distribution of the RF pulse by a duration of the RF pulse.
12. The magnetic resonance imaging device of claim 8 , wherein the excitation of the nuclear spins is performed by the RF pulse in the magnetic field, and the magnetic field has a gradient field as a component,
wherein the magnetic resonance imaging device is configured to excite the nuclear spins by the RF pulse in the magnetic field, and the magnetic field has a gradient field as a component,
wherein the control unit is configured to adjust the spectral frequency distribution of the RF pulse by specifying a temporal course of an amplitude of the RF pulse as a function of a temporal course of an amplitude of the gradient field.
13. The magnetic resonance imaging device of claim 8 , wherein the RF pulse includes a plurality of partial pulses, and the control unit is configured to adjust the spectral frequency distribution of the RF pulse by a number of the plurality of partial pulses.
14. The magnetic resonance imaging device of claim 8 , wherein the control unit is further configured to determine the variation of the magnetic field in the predetermined measuring volume using an analysis of a spectral distribution of a resonance line of a non-spatially resolved reference measurement in the predetermined measuring volume.
15. The magnetic resonance imaging device of claim 8 , wherein the control unit is further configured to adjust the spectral frequency distribution of the RF pulse by a duration of the RF pulse.
16. The magnetic resonance imaging device of claim 8 , wherein the excitation of the nuclear spins is performed by the RF pulse in the magnetic field, and the magnetic field has a gradient field as a component,
wherein the magnetic resonance imaging device is configured to excite the nuclear spins by the RF pulse in the magnetic field, and the magnetic field has a gradient field as a component,
wherein the control unit is configured to adjust the spectral frequency distribution of the RF pulse by specifying a temporal course of an amplitude of the RF pulse as a function of a temporal course of an amplitude of the gradient field.