Method for adapting a magnetic resonance sequence, magnetic resonance device and computer program product

Abstract

The invention relates to a method for adapting a magnetic resonance sequence, a magnetic resonance device, and a computer program. According to the method, an (original) magnetic resonance sequence is provided. The magnetic resonance sequence includes at least one gradient pulse. Furthermore, at least one piece of diagnostic information is provided. The at least one gradient pulse of the magnetic resonance sequence is adapted based on the at least one piece of diagnostic information. In this adaptation, the gradient strength of the at least one gradient pulse of the magnetic resonance sequence is adjusted, in particular reduced, in an adaptation section such that the gradient strength before and after the adaptation section is at least sectionally higher than in the adaptation section.

Description

[0001] The invention relates to methods for adapting a magnetic resonance sequence, a magnetic resonance apparatus and a computer program product.

[0002] In medical technology, imaging using magnetic resonance (MR), also known as magnetic resonance tomography (MRI), is characterized by high soft tissue contrast. A subject, particularly a patient, is positioned in the examination zone of a magnetic resonance scanner. A main magnetic field, also known as the B0 field, is generated in the examination zone using a main magnet. During a magnetic resonance measurement, radio-frequency (RF) pulses are applied using a radio-frequency antenna unit and gradient pulses are applied using a gradient coil unit, according to a magnetic resonance sequence. The generated pulses excite and trigger spatially encoded magnetic resonance signals in the patient. The triggered magnetic resonance signals are received by a receiver coil arrangement of the magnetic resonance scanner and used to reconstruct magnetic resonance images.

[0003] The publication SA Seada et al.: Minimum TR radifrequency pulse design for rapid gradient echo sequences. In Magnetic Resonance in Medicine, 2021; 86: 182-196. ISSN 1522-2594 discloses a method for reducing a pulse duration using a VERSE (variable-rate selective excitation) technique. The publication US 2019 / 0064293 A1 discloses determining a gradient waveform based on a VERSE factor.

[0004] To obtain magnetic resonance images of the highest possible quality, the magnetic resonance sequence should be adapted as well as possible to the examination problem and / or the examination conditions. The object of the present invention can be considered to be to perform such an adaptation of the magnetic resonance sequence.

[0005] The problem is solved by the features of the independent claims. Advantageous embodiments are described in the dependent claims. Regardless of the grammatical gender of a particular term, it encompasses persons with male, female, or other gender identities.

[0006] A method for adapting a magnetic resonance sequence for examining an examination subject using a magnetic resonance device is proposed. An (original) magnetic resonance sequence is provided. The magnetic resonance sequence provides at least one gradient pulse. Furthermore, at least one piece of examination information is provided. The at least one gradient pulse of the magnetic resonance sequence is adapted based on the at least one piece of examination information. A gradient strength of the at least one gradient pulse of the magnetic resonance sequence is adapted, in particular reduced, in an adaptation section such that the gradient strength before and after the adaptation section is at least partially higher than in the adaptation section.Preferably, after adaptation, the at least one gradient pulse has a gradient strength that is higher at the beginning of the gradient pulse and / or at the end of the gradient pulse than in the adaptation section in between. The adaptation section is preferably located centrally within the gradient pulse. Advantageously, a magnetic resonance examination of the examination subject is performed according to the adapted magnetic resonance sequence.

[0007] A magnetic resonance sequence is provided, for example, using a first provisioning unit. A magnetic resonance sequence is provided, for example, using a second provisioning unit, which may also be identical to the first provisioning unit.

[0008] The gradient strength of the at least one gradient pulse of the magnetic resonance sequence is adjusted based on the at least one piece of examination information, for example, using an adjustment unit. This can, for example, be part of a system control unit of the magnetic resonance device. The adjustment unit preferably comprises a computing unit and / or a memory unit. The computing unit can comprise one or more processors and / or one or more memory modules.

[0009] The magnetic resonance sequence typically comprises several modules, each of which includes at least one gradient pulse. In addition, the magnetic resonance sequence may also include additional modules, such as RF pulses and / or readout modules.

[0010] Advantageously, a magnetic field gradient is generated in an examination region of the magnetic resonance device by applying a gradient pulse during the execution of the magnetic resonance sequence. A magnetic field gradient is typically a change in the magnetic field in a specific direction, in particular a linear increase or decrease. The magnetic gradient fields are typically generated using gradient coils of the magnetic resonance device. They can impart spatial coding to the magnetic resonance signals and determine, for example, the spatial resolution in the magnetic resonance image.

[0011] A gradient pulse is typically characterized by a time-dependent gradient intensity. Typically, the gradient intensity increases sharply at the beginning of the gradient pulse and decreases again at the end.

[0012] The magnetic resonance sequence preferably provides for the application of at least one RF pulse during the at least one gradient pulse. The magnetic resonance sequence is preferably a VERSE sequence. "VERSE" stands for "Variable-Rate Selective Excitation." This type of magnetic resonance sequence is described, for example, in the publication Hargreaves, BA, Cunningham, CH, Nishimura, DG, & Conolly, SM (2004). Variable-rate selective excitation for rapid MRI sequences. Magnetic Resonance in Medicine, 52(3), 590-597.

[0013] In most types of magnetic resonance sequences, the gradient strength is constant throughout an RF pulse. This is especially different for VERSE pulses, where the gradient strength is typically lower in the center of the pulse than at the beginning and end of the pulse. This can be implemented particularly advantageously if free or arbitrary gradient shapes are permitted during the pulse and the pulse and gradient shape can be optimized based on constraints, for example, to achieve a short duration and / or low SAR exposure.

[0014] It can be helpful to significantly reduce the gradient strength in the middle of the pulse compared to pulses without VERSE. However, this makes the pulses significantly more susceptible to inhomogeneities in the B0 field in the object under investigation, for example, in the vicinity of air-water interfaces or metal implants.

[0015] Therefore, it is often unclear or difficult to determine whether or to what extent the gradient strength should be reduced in the middle of the pulse. Furthermore, with a very small gradient strength, the inhomogeneities of the B0 field that occur outside the center of the magnetic resonance scanner can dominate the field change induced by the gradient pulse. The resulting disruption of the linearity of the gradient field can lead to artifacts that can be advantageously reduced by the proposed adaptation of the at least one gradient pulse of the magnetic resonance sequence based on the at least one piece of examination information.

[0016] Preferably, adapting the gradient strength of the at least one gradient pulse of the magnetic resonance sequence comprises determining a maximum gradient strength reduction factor and / or a minimum gradient strength based on the at least one piece of examination information and adapting the gradient strength of the at least one gradient pulse of the magnetic resonance sequence based on the maximum gradient strength reduction factor and / or a minimum gradient strength.

[0017] For example, when adjusting the gradient strength of the at least one gradient pulse, at least in the adjustment section, the gradient strength is reduced by the maximum gradient strength reduction factor. Preferably, when adjusting the gradient strength of the at least one gradient pulse, at least in the adjustment section, the gradient strength is divided by the maximum gradient strength reduction factor. The resulting minimum gradient strength preferably represents a lower limit up to which the gradient strength of the at least one gradient pulse is adjusted in the adjustment section.

[0018] For example, the maximum gradient strength reduction factor MGSRF and the original gradient strength A G,P is adjusted in the center of the gradient pulse according to the following equation to the resulting gradient strength A G,min reduced: A G,min = A G,P / MGSRF. But it is also conceivable that A G,min represents only a lower limit for the reduction and sets the gradient strength to a value between A G,P and A G,min is reduced.

[0019] Advantageously, the determined minimum gradient strength represents a lower (absolute) limit up to which the gradient strength of the at least one gradient pulse is adjusted in the adjustment section.

[0020] Preferably, the at least one piece of examination information comprises information about a B0 magnetic field distribution in an area to be measured. For this purpose, a measurement to acquire a B0 map can be performed, for example, before adjusting the gradient strength. The area to be measured can, for example, be part of the examination area in which the object under examination is placed.

[0021] The area to be measured is, for example, the area defined by the field of view (FOV) of a magnetic resonance measurement, e.g., the FOV of a measured B0 map. This information can preferably be measured directly.

[0022] Information about a B0 magnetic field distribution can, for example, come from a B0 map and / or shim information. Shim information is often advantageously available for magnetic resonance imaging and contains information about phase responses. From this, an at least relative B0 map can be determined, on which the deviations from the fundamental frequency can be identified. In addition, other calculations or estimates based on the patient's anatomy can also be used.

[0023] When adjusting the gradient strength of at least one gradient pulse, the information about the B0 magnetic field distribution can advantageously be used to calculate the extent to which any deviations will influence the magnetic resonance image to be generated and the maximum gradient strength reduction factor and / or gradient strength that would be acceptable. Furthermore, for example, in the case of shim information, the maximum gradient strength reduction factor and / or minimum gradient strength can be determined based on empirical values.

[0024] Preferably, the at least one piece of examination information comprises information about the examination of an examination subject, in particular about a region of the examination subject to be examined. For example, the examination information comprises which body part is to be examined, such as the patient's head or abdomen.

[0025] If the magnetic resonance imaging examination to be performed is performed in an area where significant B0 effects are not typically observed (e.g., in the lumbar spine) and / or the clinical question is not sensitive to potential B0 effects, a high maximum gradient strength reduction factor and / or a small minimum gradient strength can be selected. However, if regions are measured where B0 deviations are large (e.g., in the neck, abdomen, or orbit), the maximum gradient strength reduction factor should be selected to be small or the minimum gradient strength to be large when adjusting at least one gradient pulse.

[0026] Preferably, the at least one piece of examination information comprises information about the examination object, such as a body size of the examination object and / or the presence of implants in the examination object.

[0027] If a magnetic resonance examination is performed, for example, in the vicinity of an implant, the maximum gradient strength reduction factor should be selected to be rather small or the minimum gradient strength to be large when adjusting at least one gradient pulse.

[0028] Preferably, the at least one piece of examination information comprises information about the B0 homogeneity of the magnetic resonance device and / or about a coil configuration to be used for the examination.

[0029] Preferably, the information about the B0 homogeneity of the magnetic resonance device includes information about the B0 field distribution in a region that extends beyond the field of view. In particular, this region encompasses the entire area covered by the radio-frequency antenna unit, in particular by the transmit and / or receive coils, of the magnetic resonance device. This information cannot usually be measured directly, but can have an effect on the required gradient strength.

[0030] If, for example, parts of the patient's body are located in areas outside the center of the magnetic resonance device during the magnetic resonance examination, where stronger B0 inhomogeneities occur, the maximum gradient strength reduction factor should be selected to be small or the minimum gradient strength to be large when adjusting at least one gradient pulse.

[0031] Furthermore, a magnetic resonance apparatus is proposed that is configured to carry out a method described above. The advantages of the proposed magnetic resonance apparatus essentially correspond to the advantages of the proposed method for adapting a magnetic resonance sequence, which were described in detail above. Features, advantages, or alternative embodiments mentioned herein can also be applied to the other claimed subject matter, and vice versa.

[0032] Furthermore, a computer program product is proposed which comprises a program and can be loaded directly into a memory of a programmable computing unit of a magnetic resonance apparatus and has program means, e.g., libraries and auxiliary functions, for executing a proposed method for adapting a magnetic resonance sequence for examining an examination subject using a magnetic resonance apparatus when the computer program product is executed in the computing unit of the magnetic resonance apparatus. The computer program product can comprise software with source code that still needs to be compiled and linked or that only needs to be interpreted, or executable software code that only needs to be loaded into the computing unit for execution. The computer program product enables the inventive method to be executed quickly, identically repeatably, and robustly.The computer program product is configured such that it can execute the method steps according to the invention using the computing unit. The computing unit has the necessary prerequisites, such as a suitable RAM, a suitable graphics card, or a suitable logic unit, so that the respective method steps can be executed efficiently.

[0033] The computer program product is stored, for example, on a computer-readable medium or on a network or server, from where it can be loaded into the processor of a local computing unit, which can be directly connected to the magnetic resonance device or formed as part of the magnetic resonance device. Furthermore, control information of the computer program product can be stored on an electronically readable data carrier. The control information of the electronically readable data carrier can be configured such that, when the data carrier is used in a computing unit of a magnetic resonance device, it carries out a method according to the invention. Examples of electronically readable data carriers are a DVD, a magnetic tape, or a USB stick on which electronically readable control information, in particular software, is stored.If this control information is read from the data carrier and stored in a processing unit of the magnetic resonance device, the embodiments of the methods described above can be carried out. Thus, the invention can also be based on said computer-readable medium and / or said electronically readable data carrier.

[0034] Further advantages, features, and details of the invention will become apparent from the exemplary embodiments described below and from the drawings. Corresponding parts are provided with the same reference numerals in all figures.

[0035] They show: Fig. 1 a magnetic resonance device in a schematic representation, Fig. 2 a flowchart of a method for adapting a magnetic resonance sequence, Fig. 3 idealized gradient pulses before and after an adjustment.

[0036] In Fig. 1 schematically illustrates a magnetic resonance apparatus 10. The magnetic resonance apparatus 10 comprises a magnet unit 11 having a main magnet 12 for generating a strong and, in particular, temporally constant main magnetic field 13. Furthermore, the magnetic resonance apparatus 10 comprises an examination region 14 for accommodating a patient 15, who is the examination subject here. The examination region 14 in the present exemplary embodiment is cylindrical and is surrounded in a circumferential direction by the magnet unit 11. In principle, however, a different design of the examination region 14 is conceivable at any time. The patient 15 can be pushed into the examination region 14 by means of a patient support device 16 of the magnetic resonance apparatus 10. For this purpose, the patient support device 16 comprises a patient table 17 designed to be movable within the examination region 14.

[0037] The magnet unit 11 further comprises a gradient coil unit 18 for generating magnetic field gradients used for spatial encoding during imaging. The gradient coil unit 18 is controlled by a gradient control unit 19 of the magnetic resonance device 10. The magnet unit 11 further comprises a radio-frequency antenna unit 20, which in the present exemplary embodiment is designed as a body coil permanently integrated into the magnetic resonance device 10. The radio-frequency antenna unit 20 is controlled by a radio-frequency antenna control unit 21 of the magnetic resonance device 10 and radiates radio-frequency magnetic resonance sequences into an examination space, which is essentially formed by an examination region 14 of the magnetic resonance device 10. As a result, the main magnetic field 13 generated by the main magnet 12 excites atomic nuclei.Magnetic resonance signals are generated by the relaxation of the excited atomic nuclei. The radio-frequency antenna unit 20 is designed to receive the magnetic resonance signals.

[0038] 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 centrally controls the magnetic resonance apparatus 10, such as performing a magnetic resonance sequence, in particular a VERSE sequence. Such a magnetic resonance sequence typically comprises several modules, such as RF pulses generated by the radio-frequency antenna unit 20 during the magnetic resonance sequence, and gradient pulses generated by the gradient coil unit 18.

[0039] In addition, the system control unit 22 includes an evaluation unit (not shown in detail) for evaluating the magnetic resonance signals acquired during the magnetic resonance examination. Furthermore, the magnetic resonance device 10 includes a user interface 23 connected to the system control unit 22. Control information such as imaging parameters and reconstructed magnetic resonance images can be displayed on a display unit 24, for example, on at least one monitor, of the user interface 23 for medical personnel. Furthermore, the user interface 23 has an input unit 25, by means of which information, such as examination information, and / or parameters, can be entered by the medical personnel during a measurement procedure.

[0040] In Fig. 2 shows a possible sequence of a method for adapting a magnetic resonance sequence for examining the patient 15 by means of the magnetic resonance device 10.

[0041] In S10, a magnetic resonance sequence comprising at least one gradient pulse is provided. This sequence can be selected, for example, by the operating personnel via the input unit 25 and provided to the system control unit 22.

[0042] In S20, at least one piece of investigation information is provided and evaluated. This at least one piece of investigation information is, for example, available prior information. This can consist of at least one of the following aspects: a. Information about the B0 distribution in the area to be measured. This can come from B0 maps, shim information, or similar sources. Shim information, in particular, is usually available for magnetic resonance imaging and contains information about the phase distribution. The B0 maps and / or shim information can be measured, in particular, in any preceding steps. b. Information about the planned examination and the area of ​​the patient’s body to be examined 15. c. Information about the patient 15, such as height, implants, etc. d. Information about the B0 homogeneity of the magnetic resonance device 10 used and the coil configuration used.

[0043] In S30, the magnetic resonance sequence is adapted. In this case, a gradient strength of the at least one gradient pulse of the magnetic resonance sequence is adapted in an adaptation section based on the at least one piece of examination information provided in S20. After the adaptation, the gradient strength is at least partially higher before and / or after the adaptation section than in the adaptation section.

[0044] This should be done with the help Fig. 3. Here, in the upper diagram, a possible course of an amplitude A RF an RF pulse P HF as a function of time t. The RF pulse P HF begins at time t RF,i and ends at time t RF,f . During the application of the RF pulse P HF A gradient pulse is also applied. The original, not yet adjusted, gradient pulse P Gi has a rectangular shape, ie at time t G,ithe gradient strength increases to a value A G,P , then remains until time t G,i constant and then drops again. (In particular, the pulse edges in Fig. 3 is only shown schematically; in fact, the pulses have certain rise and fall times.)

[0045] By adapting in S30 based on the at least one piece of examination information provided in S20, the gradient strength is determined in an adaptation section between the times t A,i and t A,f to a value A G,min reduced. The resulting gradient pulse P Ga therefore has a higher gradient strength in sections before and after the adaptation section, namely in the section between the times t G,i and t A,i before the adjustment section and in the section between T A,f and t G,f after the adjustment section.

[0046] The Pulse P HF and PGa schematically correspond to a typical excitation according to a VERSE magnetic resonance sequence. Preferably, during the gradient pulse P Ga the high-frequency pulse P HF applied.

[0047] Based on the information obtained in S20, the following aspects can be taken into account in S30, for example (the bullet points ad refer to the information mentioned above with the same bullet points used there): a. From the B0 information, it is possible to calculate the extent to which any deviations will affect the planned image and up to what factor these deviations would be acceptable. Furthermore, for example, in the case of shim information, a factor can be determined based on empirical values. b. If the examination is performed in an area where significant B0 effects are not usually present (e.g., the lumbar spine), or if the clinical question is not sensitive to possible B0 effects, a large factor can be selected. However, if the examination is performed in areas where B0 deviations are large, such as the neck, abdomen, or orbits, a small factor should be selected. c. If an examination is carried out near an implant, for example, the factor should also be chosen to be small. d. If parts of the patient's body 15 are located in areas outside the center of the examination area 14, in particular the field of view, where greater B0 inhomogeneities may occur, the factor should be selected to be small.

[0048] Finally, it should be noted once again that the methods described in detail above, as well as the magnetic resonance device illustrated, are merely exemplary embodiments that can be modified in a variety of ways by those skilled in the art without departing from the scope of the invention. Furthermore, the use of the indefinite articles "a" or "an" does not exclude the possibility that the respective features may be present in multiple instances. Likewise, the term "unit" does not exclude the possibility that the respective components consist of several interacting subcomponents, which may also be spatially distributed.

Claims

[1] Method for adapting a magnetic resonance sequence for examining an examination subject by means of a magnetic resonance device, the method comprising: - Providing a magnetic resonance sequence, wherein the magnetic resonance sequence comprises at least one gradient pulse, - Providing at least one piece of investigation information, - adapting, in particular reducing, a gradient strength of the at least one gradient pulse of the magnetic resonance sequence in an adaptation section based on the at least one piece of examination information, so that the gradient strength before and after the adaptation section is at least partially higher than in the adaptation section, wherein adjusting the gradient strength of the at least one gradient pulse of the magnetic resonance sequence comprises: - Determining a maximum gradient strength reduction factor and / or a minimum gradient strength based on the at least one piece of examination information, - Adjusting the gradient strength in the adjustment section based on the maximum gradient strength reduction factor and / or a minimum gradient strength. [2] The method of claim 1, wherein the method further comprises: Carrying out a magnetic resonance examination, in particular magnetic resonance measurement, using the adapted magnetic resonance sequence. [3] Method according to one of the preceding claims, wherein the at least one item of examination information comprises information about a B0 magnetic field distribution in a region to be measured. [4] Method according to one of the preceding claims, wherein the at least one item of examination information comprises information about the examination of an examination object, in particular about a region of the examination object to be examined. [5] Method according to one of the preceding claims, wherein the at least one item of examination information comprises information about the examination subject, in particular a body size and / or a presence of implants in the examination subject. [6] Method according to one of the preceding claims, wherein the at least one item of examination information comprises information about the B0 homogeneity of the magnetic resonance device and / or about a coil configuration to be used for the examination. [7] Method according to one of the preceding claims, wherein the magnetic resonance sequence provides for applying at least one radio frequency pulse during the at least one gradient pulse. [8] Method according to one of the preceding claims, wherein the magnetic resonance sequence is a VERSE sequence. [9] Magnetic resonance apparatus adapted to carry out a method according to any one of the preceding claims. [10] A computer program product comprising a program and being directly loadable into a memory of a programmable system control unit of a magnetic resonance device, with program means for carrying out a method according to any one of claims 1 to 8 when the program is executed in the system control unit of the medical imaging device.

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