METHOD FOR EXCITING NUCLEAR SPINS

Abstract

Nuclear spins are excited in a region of interest in an object under examination by a radio-frequency pulse. During at least one phase of the radio-frequency pulse, excitation fields are transmitted while magnetic field gradients are simultaneously applied so that the magnetization of the nuclear spins moves on a trajectory through a transmission k-space. In a first phase of the at least one phase of the radio-frequency pulse, the trajectory moves at a radial distance around the center of the transmission k-space. The radial distance corresponds to the radius of a sphere superimposed with at least one radial harmonic.

Claims

1. A method for exciting nuclear spins in a region of interest in an object under examination by a radio-frequency pulse, the method comprising: during at least one phase of the radio-frequency pulse, excitation fields are transmitted while magnetic field gradients are simultaneously applied so that the magnetization of the nuclear spins moves on a trajectory through a transmission k-space, wherein, in a first phase of the at least one phase of the radio-frequency pulse, the trajectory moves at a radial distance around a center of the transmission k-space, wherein the radial distance corresponds to the radius of a sphere superimposed with at least one radial harmonic.

2. The method as claimed in claim 1, wherein the amplitude of the at least one radial harmonic is up to 30% of the radius of the sphere.

3. The method as claimed in claim 1, wherein the radial harmonics contain a fundamental with a half wavelength corresponding to the length of the first phase and at least one further harmonic with a shorter wavelength.

4. The method as claimed in claim 1, wherein, in a second phase of the at least one phase of the radio-frequency pulse, the trajectory moves on a continuous curve toward the center of the transmission k-space.

5. The method as claims in claim 4 wherein the continuous curve comprises a differentiable curve.

6. The method as claimed in claim 1, wherein, during a third phase of the at least one phase of the radio-frequency pulse, the trajectory remains in the center of the transmission k-space.

7. The method as claimed in claim 1, wherein different azimuth and polar angles are scanned, during the first phase and a second phase of the at least one phase of the radio-frequency pulse.

8. The method as claimed in claim 1, wherein a plurality of radio-frequency coils are actuated in parallel during transmission of the excitation fields.

9. The method as claimed in claim 1, wherein parameters of the trajectory are optimized on a basis of field distribution maps of the region of interest.

10. The method as claimed in claim 9, wherein the optimization is for uniformity of the excitation of the nuclear spins over the region of interest.

11. The method as claimed in claim 9, wherein the parameters of the trajectory comprise the radius of the sphere, a length of the first phase, a second phase, and/or a third phase of the at least one phase, and an amplitude of the at least one radial harmonic, and wherein the field distribution maps comprise a B.sub.0 map and/or at least one B.sub.1 map.

12. The method as claimed in claim 2, wherein the radial harmonics contain a fundamental with a half wavelength corresponding to the length of the first phase and at least one further harmonic with a shorter wavelength.

13. The method as claimed in claim 12, wherein, in a second phase of the at least one phase of the radio-frequency pulse, the trajectory moves on a continuous curve toward the center of the transmission k-space.

14. The method as claimed in claim 13, wherein, during a third phase of the at least one phase of the radio-frequency pulse, the trajectory remains in the center of the transmission k-space.

15. The method as claimed in claim 14, wherein different azimuth and polar angles are scanned, during the first phase and the second phase of the at least one phase of the radio-frequency pulse.

16. The method as claimed in claim 15, wherein a plurality of radio-frequency coils are actuated in parallel during transmission of the excitation fields.

17. The method as claimed in claim 16, wherein parameters of the trajectory are optimized on a basis of field distribution maps of the region of interest.

18. A non-transitory computer readable storage medium storing instructions, which when executed by a computer, control a magnetic resonance device, the instructions comprising: actuation of the magnetic resonance device with an actuation sequence to capture magnetic resonance data from an object under examination, wherein the actuation sequence contains control signals for magnetic field gradients and transmit signals for one or more radio-frequency transmit antennas, during at least one phase of a radio-frequency pulse of the actuation sequence, transmission of excitation fields occurs while magnetic field gradients are simultaneously applied so that the magnetization of the nuclear spins moves on a trajectory through a transmission k-space, wherein, in a first phase of the at least one phase of the radio-frequency pulse, the trajectory moves at a radial distance around a center of the transmission k-space, wherein the radial distance corresponds to the radius of a sphere superimposed with at least one radial harmonic.

19. A magnetic resonance device comprising: a transmit coil; a gradient coil; and a computer configured to control transmission of a radio-frequency pulse from the transmit coil, wherein during at least one phase of the radio-frequency pulse, excitation fields are transmitted while magnetic field gradients of the gradient coil are simultaneously applied so that the magnetization of nuclear spins moves on a trajectory through a transmission k-space, wherein, in a first phase of the at least one phase of the radio-frequency pulse, the trajectory moves at a radial distance around a center of the transmission k-space, wherein the radial distance corresponds to the radius of a sphere superimposed with at least one radial harmonic.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The invention will now be described in more detail with reference to exemplary embodiments and with reference to the accompanying drawings. The drawings show:

[0043] FIG. 1 an example schematic representation of the Larmor wavelength at different magnetic field strengths;

[0044] FIG. 2 shows examples of B.sub.1 inhomogeneities in a human head at different B.sub.0 field strengths;

[0045] FIG. 3 illustrates a schematic representation of the different components that are measured or optimized for a pTx pulse, according to one embodiment;

[0046] FIG. 4 shows an example of the course of the trajectory and RF pulse shapes for the k.sub.t-points method;

[0047] FIG. 5 shows an example of the course of the trajectory and the RF pulse shapes for the spokes method;

[0048] FIG. 6 shows an example of the course of the trajectory and the RF pulse shapes for a SPINS trajectory;

[0049] FIG. 7 shows an example of the course of the trajectory and the RF pulse shapes for a SPENS trajectory;

[0050] FIG. 8 is an example graph of the radial trajectory component during the RF pulse;

[0051] FIG. 9 illustrates examples of the azimuth and polar components of the trajectory during the RF pulse;

[0052] FIG. 10 is a schematic representation of a magnetic resonance device according to one embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] FIG. 1 illustrates how greatly the Larmor wavelength is reduced when the B.sub.0 field strength increases from 1.5 tesla to 7.0 tesla: at 7.0 tesla, the Larmor wavelength is only approximately 12 cm, resulting in a B.sub.1 inhomogeneity in the tissue.

[0054] This is illustrated in FIG. 2 with reference to a simulation of an axial section through a human skull: at 3 tesla, the brain still appears uniformly illuminated. At 5 tesla, it is already clearly visible that a darker ring develops about a still bright core in which the tissue is less well illuminated, i.e., for example a smaller flip angle was realized than in the center. This darker ring becomes darker and narrower as the field strength B.sub.0 increases, until, at 12 tesla, there is clearly very poor illumination of the brain.

[0055] As described, this problem can be mitigated by using optimized pTx pulses. The components required for this are schematically illustrated in FIG. 3. In order to optimally adjust the pTx pulses to the field inhomogeneities present in each case, these methods, on the one hand, use a map 2 of the B.sub.0 field, i.e., knowledge of the B.sub.0 inhomogeneities in the object under examination. Furthermore, in order to be able to optimally actuate the parallel transmit antennas, their respective illumination field 3 are known, i.e., the so-called B.sub.1 maps. In the example depicted, an RF coil with 8 parallel transmit antennas is used, the individual B.sub.1 maps of which can be seen at 3. The B.sub.0 and B.sub.1 maps can be measured—both universally or averaged over a plurality of different subjects, or currently during an examination on a specific subject. This information can be used to optimize a trajectory 4 in the transmission k-space 5, such as, for example, the trajectory according to the embodiments. Advantageously, the trajectory is described generally by a plurality of parameters, and these parameters are then adapted to the current B.sub.0 and B.sub.1 maps in each case. Moreover, the pulse shapes 6, also called pulse profiles, can also be adapted, i.e., the voltages that are applied to the respective RF transmit antennas 3. Therefore, there are exactly as many pulse profiles 6 to be optimized as there are transmit antennas with a corresponding b.sub.1 map 3. mathematically, the relationships of a dynamic pTx pulse can be depicted as follows:

iγ|Σ.sub.c=1.sup.c=8S.sub.c(r).Math.∫.sub.0.sup.Te.sup.i(−Δω.sup.0.sup.(r)+rk(t))(T−t).Math.u.sub.c(t)dt|=α(r,T)  (6)

[0056] Here, S.sub.c(r) denotes the B.sub.1 field of the respective transmit antenna c, Δω.sub.0 is the deviation of the B.sub.0 field from the mean value B.sub.0, and u.sub.c denotes the voltage applied to the respective transmit antenna. α denotes the flip angle reached at the respective position r.

[0057] Various trajectory courses known from the prior art, (in each case on the left) and the associated RF pulse profiles or voltages applied to the plurality of transmit antennas (in each case on the right) are depicted in FIGS. 4 to 6 and compared with the trajectory according to the present approach in FIG. 7. |u(t)| in each case denotes the magnitude of the voltages applied to the RF transmit antennas; <(u(t)) depicted below describes the phase.

[0058] FIG. 4 depicts the k.sub.T trajectory in which in each case square-wave voltages u are applied to the different transmit antennas (indicated by different shades of gray in the righthand side of the figure).

[0059] In contrast, FIG. 5 depicts the spokes method according to EP 2 296 000 B.sub.1 in which the k-space is scanned in the form of “spokes”, i.e., in straight trajectory sections, as depicted on the lefthand side of FIG. 5.

[0060] FIG. 6 illustrates the SPINS trajectory, while FIG. 7 depicts the SPENS trajectory. In this three-dimensional representation of the k-space 5, the course of the trajectory 4 cannot be optimally identified and so reference is made to FIGS. 8 and 9 in this regard. On the righthand side of FIG. 7, it can be identified in the case of the RF pulses that initially different voltages are applied to all RF transmit antennas with relatively strong deflections. This characterizes the first phase up to time t.sub.1. This is followed by rather smoother courses of the voltages corresponding to the ramp to the k-space center in the second phase. Also, in the third phase after t.sub.2 until T, the voltages only have a steady change. In this phase, no more gradients are applied, instead the magnetization remains in the center of the k-space.

[0061] FIG. 8 illustrates the three phases of the RF pulse by the radial component k.sub.r of the k-space trajectory: in the first phase P1, which lasts from time 0 to t.sub.1, the trajectory 10 runs approximately at the level of the radius of the sphere ko, but superimposed by various harmonics. At the time t.sub.1, it starts to run toward 0 in the form of an inverted parabola 12, wherein this parabola 12 is mirrored at point 15 halfway through the second phase P2 and then arrives at the end of the second phase P2 at time t.sub.2. Here, the trajectory 14 remains during the third phase P3 until time T.

[0062] The azimuth and polar angles k.sub.θ or k.sub.φ are depicted in FIG. 9 and have a continuous and differentiable course over all three phases up to time T, which can be described by a quadratic formula. Obviously, another analytical description that is not quadratic is also conceivable.

[0063] Finally, FIG. 10 shows a magnetic resonance device 20 with which the method can be executed. The device has a B.sub.0 magnet under the housing that surrounds an examination region 22 into which an object under examination can be moved on a bench 24. The device can, for example, be controlled by a computer 26 with a screen 28 and hard disk 29 (memory or non-transitory computer readable storage medium) and further computing units. The computer program can be stored on the hard disk 29.

[0064] It is intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.

[0065] It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims can, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.