METHOD FOR ASCERTAINING FLIP ANGLES, MAGNETIC RESONANCE APPARATUS, AND COMPUTER PROGRAM PRODUCT

Abstract

A computer-implemented method for ascertaining flip angles of a magnetic resonance sequence with variable flip angles, a magnetic resonance apparatus, and a computer program product are disclosed. In this case, the magnetic resonance sequence includes at least one echo train with an excitation pulse and a plurality of refocusing pulses. In each case, an adapted flip angle is ascertained for at least one part of the plurality of refocusing pulses.

Claims

1. A computer-implemented method for ascertaining flip angles of a magnetic resonance sequence with variable flip angles, wherein the magnetic resonance sequence comprises at least one echo train with an excitation pulse and a plurality of refocusing pulses, the method comprising: ascertaining in each case an adapted flip angle for at least one part of the plurality of refocusing pulses.

2. The method of claim 1, wherein the flip angle of the refocusing pulses of the at least one part of the plurality of refocusing pulses is adapted such that at least one specified boundary condition is met.

3. The method of claim 1, wherein the ascertaining of the respective adapted flip angle comprises: providing in each case an initial flip angle for each refocusing pulse of the at least one part of the plurality of refocusing pulses; providing a magnitude integral of the excitation pulse; providing a respective magnitude integral of the plurality of refocusing pulses; and ascertaining the respective adapted flip angle for at least one part of the plurality of refocusing pulses based on the magnitude integral of the excitation pulse, the respective magnitude integral of the plurality of refocusing pulses, and the respective initial flip angle for each refocusing pulse of the at least one part of the plurality of refocusing pulses.

4. The method of claim 3, wherein the ascertaining of the respective adapted flip angle further comprises providing an initial reduction factor for reducing the excitation pulse and the plurality of refocusing pulses, and wherein the ascertaining of the respective adapted flip angle for at least one part of the plurality of refocusing pulses is further carried out based on the initial reduction factor.

5. The method of claim 3, wherein the ascertaining of the respective adapted flip angle for at least one part of the plurality of refocusing pulses comprises ascertaining a global reduction factor, and wherein the global reduction factor is applied to each refocusing pulse of the at least one part of the plurality of refocusing pulses.

6. The method of claim 3, wherein the ascertaining of the respective adapted flip angle for at least one part of the plurality of refocusing pulses comprises ascertaining an individual reduction factor for each refocusing pulse of the at least one part of the plurality of refocusing pulses.

7. The method of claim 6, wherein the ascertaining of the individual reduction factors takes into account a weighting of the magnitude integral of the respective refocusing pulse with respect to a mean magnitude integral.

8. The method of claim 7, wherein the weighting of the magnitude integral comprises linear weighting, quadratic weighting, or a combination thereof.

9. The method of claim 3, wherein the ascertaining of the respective adapted flip angle for at least one part of the plurality of refocusing pulses is carried out based on a maximum magnitude integral available in an entire echo train and/or a predetermined contrast behavior.

10. The method of claim 3, further comprising: ascertaining an adapted flip angle for a refocusing pulse preceding the at least one part of the plurality of refocusing pulses as a function of a refocusing pulse of the at least one part of the plurality of refocusing pulses.

11. The method of claim 1, wherein the ascertaining of the respective adapted flip angle for at least one part of the plurality of refocusing pulses comprises ascertaining a global reduction factor, and wherein the global reduction factor is applied to each refocusing pulse of the at least one part of the plurality of refocusing pulses.

12. The method of claim 1, wherein the ascertaining of the respective adapted flip angle for at least one part of the plurality of refocusing pulses comprises ascertaining an individual reduction factor for each refocusing pulse of the at least one part of the plurality of refocusing pulses.

13. The method of claim 12, wherein the ascertaining of the individual reduction factors takes into account a weighting of an magnitude integral of the respective refocusing pulse with respect to a mean magnitude integral.

14. The method of claim 13, wherein the weighting of the magnitude integral comprises linear weighting, quadratic weighting, or a combination thereof.

15. The method of claim 1, wherein the ascertaining of the respective adapted flip angle for at least one part of the plurality of refocusing pulses is carried out based on a maximum magnitude integral available in an entire echo train and/or a predetermined contrast behavior.

16. The method of claim 1, further comprising: ascertaining an adapted flip angle for a refocusing pulse preceding the at least one part of the plurality of refocusing pulses as a function of a refocusing pulse of the at least one part of the plurality of refocusing pulses.

17. The method of claim 1, wherein the magnetic resonance sequence is a Turbo Spin Echo (TSE) sequence.

18. The method of claim 11, wherein the TSE sequence is a Half-Fourier Acquisition Single-shot Turbo spin Echo imaging (HASTE) sequence, or a Sampling Perfection with Application optimized Contrasts using different flip angle Evolution (SPACE) sequence.

19. A magnetic resonance apparatus comprising: a control unit configured to: receive a magnetic resonance sequence having variable flip angles, wherein the magnetic resonance sequence comprises at least one echo train with an excitation pulse and a plurality of refocusing pulses; and ascertain in each case an adapted flip angle for at least one part of the plurality of refocusing pulses.

20. A computer program product that comprises a program and is configured to be loaded directly into a memory of a programmable system control unit of a magnetic resonance apparatus, wherein the program, when executed by the system control unit of the magnetic resonance apparatus, is configured to: receive a magnetic resonance sequence having variable flip angles, wherein the magnetic resonance sequence comprises at least one echo train with an excitation pulse and a plurality of refocusing pulses; and ascertain in each case an adapted flip angle for at least one part of the plurality of refocusing pulses.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0055] FIG. 1 depicts a diagrammatic view of an example of a magnetic resonance apparatus.

[0056] FIG. 2 depicts a diagram of an example of a method for ascertaining flip angles of a magnetic resonance sequence with variable flip angles.

[0057] FIG. 3 depicts an example of a flip angle curve.

DETAILED DESCRIPTION

[0058] FIG. 1 is a diagrammatic view of a magnetic resonance apparatus 10. The magnetic resonance apparatus 10 includes a magnet unit 11 having a main magnet 12 for generating a strong and, in particular, temporally constant main magnet field 13. In addition, the magnetic resonance apparatus 10 includes a patient receiving area 14 for receiving a patient 15. The patient receiving area 14 in the present exemplary embodiment is cylindrical in design and is surrounded in a circumferential direction by the magnet unit 11 in a cylindrical shape. In principle, however, an embodiment of the patient receiving area 14 deviating therefrom is conceivable at any time. The patient 15 may be pushed into the patient receiving area 14 by a patient positioning apparatus 16 of the magnetic resonance apparatus 10. The patient positioning apparatus 16 has a patient table 17 for this purpose which is designed to be movable inside the patient receiving area 14.

[0059] The magnet unit 11 also has a gradient coil unit 18 for generating magnetic field gradients that are used for spatial encoding during imaging. The gradient coil unit 18 is controlled by a gradient control unit 19 of the magnetic resonance apparatus 10.

[0060] The magnet unit 11 further includes a radio-frequency antenna unit 20, which in the present exemplary embodiment is designed as a body coil permanently integrated into the magnetic resonance apparatus 10. The radio-frequency antenna unit 20 is controlled by a radio-frequency antenna control unit 21 of the magnetic resonance apparatus 10 and radiates radio-frequency magnetic resonance sequences, (e.g., a TSE sequence such as a HASTE or a SPACE sequence), into an examination room formed by a patient receiving area 14 of the magnetic resonance apparatus 10. The radio-frequency antenna unit 20 is in particular configured to emit excitation pulses and/or refocusing pulses. As a result, it is possible to excite atomic nuclei in the main magnetic field 13 generated by the main magnet 12. Magnetic resonance signals are generated through relaxation of the excited atomic nuclei. The radio-frequency antenna unit 20 is configured to receive the magnetic resonance signals. For transmitting RF pulses, in particular excitation pulses and/or refocusing pulses, the magnetic resonance apparatus 10 includes in particular a radio-frequency amplifier, by which signals from HP pulses may be amplified. The amplified signals may be transmitted to the radio-frequency antenna unit 20.

[0061] The magnetic resonance apparatus 10 has a system control unit 22 for controlling the main magnet 12, the gradient control unit 19, and the radio-frequency antenna control unit 21. The system control unit 22 controls the magnetic resonance apparatus 10 centrally, for example, the performance of a predetermined magnetic resonance imaging sequence. The system control unit 22 may be configured to carry out a computer-implemented method for ascertaining flip angles of a magnetic resonance sequence with variable flip angles. For this purpose, the system control unit 22 includes, for example, a computing unit and/or a memory unit.

[0062] In addition, the system control unit 22 includes an evaluation unit (not shown in more detail) for evaluating the magnetic resonance signals which are detected during the magnetic resonance examination. Furthermore, the magnetic resonance apparatus 10 includes a user interface 23 connected to the system control unit 22. Control information such as imaging parameters, as well as reconstructed magnetic resonance images may be displayed on a display unit 24, (e.g., on at least one monitor), of the user interface 23 for a medical operator. Furthermore, the user interface 23 has an input unit 25, by which information and/or parameters may be entered by the medical operator during a measurement process.

[0063] FIG. 2 shows a method for operating the magnetic resonance apparatus 10, in particular for ascertaining flip angles of a magnetic resonance sequence with variable flip angles, wherein the magnetic resonance sequence includes at least one echo train with an excitation pulse and a plurality of refocusing pulses.

[0064] In S10, a respective initial flip angle is provided for each refocusing pulse of at least one part of the plurality of refocusing pulses. In S20, a magnitude integral of the excitation pulse is provided. In S30, a magnitude integral of the plurality of refocusing pulses is provided in each case. In S40, an initial reduction factor for reducing the excitation pulse and the plurality of refocusing pulses is provided. S10, S20, S30, and S40 may be carried out simultaneously and/or in any order.

[0065] In S50, in each case, an adapted flip angle is ascertained for at least one part of the plurality of refocusing pulses based on the magnitude integral of the excitation pulse, of the respective magnitude integral of the plurality of refocusing pulses, of an initial reduction factor for reducing the excitation pulse and the plurality of refocusing pulses and of the respective initial flip angle for each refocusing pulse of the at least one part of the plurality of refocusing pulses.

[0066] In S60, a magnetic resonance measurement is carried out according to a magnetic resonance sequence with the adapted flip angles.

[0067] Possible aspects of the method shown in FIG. 2 are described hereinafter.

[0068] In the case of magnetic resonance sequences with variable flip angles, (e.g., in a TSE echo train, HASTE echo train, or SPACE echo train), each refocusing pulse may be assigned its own flip angle. This may be used in hyperecho, T2var, T1var, and/or Pdvar technology.

[0069] An example of a flip angle curve of an echo train according to hyperecho technology is shown by way of example in FIG. 3.

[0070] The voltage of the RF pulses is plotted on the vertical axis. An initial excitation pulse (x=1) is followed by a plurality of refocusing pulses (x=2, 3, . . . , n). With the same pulse length and pulse shape, the maximum voltage of an RF pulse also reflects its flip angle. That is the case for all the refocusing pulses in this example.

[0071] The initial 90? excitation pulse with a flip angle a.sub.1 is followed by a first refocusing pulse, the flip angle a.sub.2 of which is in turn dependent on the start or plateau flip angle a of the hyperecho train according to:

[00008]$a_2=90?+a/2$

[0072] This is followed by a plurality of refocusing pulses, starting, for example, with a flip angle of 165? and a downward trend in further sales.

[0073] These two RF pulses may not be changed (e.g., initially), as a result of which a reduction in the flip angles should only have an effect on the subsequent RF pulses. The flip angle of the first refocusing pulse a.sub.2 may be adapted retrospectively after ascertaining a.

[0074] As a<a, ultimately the first refocusing pulse will also become smaller.

[0075] First, an initial reduction factor c is ascertained, for example, by the system control unit 22. However, this reduction factor c may relate to the entire echo train, including the first two RF pulses that may not be adapted, however.

[0076] In a first act, a corrected reduction factor c is determined using the magnitude integral M.sub.x of an RF pulse at the point x in the echo train:

[00009]$c.Math.\underset{x=1}{\overset{n}{.Math.}}{M}_x=M_1+M_2+c^{}.Math.\underset{x=3}{\overset{n}{.Math.}}{M}_x$$c^{}=\frac{c.Math.{.Math.}_{x=1}^n{M}_x-M_1-M_2}{{.Math.}_{x=3}^n{M}_x}$

[0077] In the case of a restore pulse at the end of the echo train, the flip angle of the last refocusing pulse x=n is also coupled to the previous RF pulse with 90?+a/2. This may be taken into account when calculating c.

[0078] The flip angle curve is now reduced on the basis of c. This may take place in two ways in particular, referred to hereinafter as constant reduction or area-proportional reduction.

[0079] According to constant reduction, all RF pulses x=3 . . . n are reduced by the factor c, e.g.:

[00010]${a^{}}_{x=3...n}=c^{}.Math.a_{x=3...n}$

[0080] In this case, c represents a constant factor, in particular a global reduction factor, for all initial flip angles a.sub.x, x=3 . . . n.

[0081] According to area-proportional reduction, in each case an individual reduction factor c.sub.x is determined individually for each RF pulse x=3 . . . n based on the area of the RF pulse:

[00011]${c^{}}_{x=3...n}=c^{}\frac{M_{x=3...n}}{.Math.M.Math.}$

[0082] In this case, <M> is a mean magnitude integral which is calculated, for example, according to:

[00012]$.Math.M.Math.=\frac{{.Math.}_{x=3}^n{M}_x}{n-2}$

[0083] Alternatively, other distribution functions may also be used, for example, with a quadratic weighted area:

[00013]${c^{}}_{x=3...n}=c^{}\frac{M_{x=3...n}^2}{{.Math.M.Math.}^2}$

[0084] According to area-proportional reduction, the reduced flip angles then result in:

[00014]${a^{}}_{x=3...n}=c^{}.Math.a_{x=3...n}$

[0085] Area-proportional reduction is advantageous as the flip angle at the rear end of an echo train is already quite small when using hyperecho and thus reacts more sensitively to further reduction (for example, flow sensitivity) and, in addition, the effect of reduction for larger flip angles is greater due to the larger area.

[0086] The flip angles are also adapted in such a way that at least one specified boundary condition is met. For example, such a boundary condition is that CBM limits are not exceeded. A CBM limit may be understood to mean a Limit of a Charge Balance Model (CBM). A CBM may be used in particular to avoid measurement interruptions due to overloading of the radio-frequency amplifier. For example, such a model is used to check whether a performance limit of the radio-frequency amplifier is exceeded at a certain time before a magnetic resonance measurement is carried out on the basis of runtime information of the RF pulses. If there is a conflict between the power required and the power available, it is advantageous to find and implement a solution.

[0087] In a further embodiment, a new flip angle curve is calculated with the maximum magnitude integral available in the entire echo train and a desired contrast behavior as boundary conditions by formulation as an optimization problem. For this purpose, for example, an Extended Phase Graph Simulation may be used.

[0088] Finally, it is pointed out once again that the methods described in detail above, as well as the magnetic resonance apparatus shown, are only exemplary embodiments that may be modified in a wide variety of ways by a person skilled in the art without departing from the scope of the disclosure. Furthermore, the use of the indefinite article a or an does not rule out the possibility that the features in question may also be present more than once. Likewise, the term unit does not rule out the possibility that the components concerned include a plurality of interacting part components that may also be spatially distributed.

[0089] 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 disclosure. Thus, whereas the dependent claims appended below depend on only a single independent or dependent claim, it is to be understood that these dependent claims may, 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.

[0090] While the present disclosure has been described above by reference to various embodiments, it may be understood that many changes and modifications may be made to the described embodiments. It is therefore 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.