Improved determination of magnetic resonance data with spectral selection

Abstract

Method for recording measurement data of an object under investigation (U), which comprises spins of at least two different spin species, using a magnetic resonance system (1), comprising the steps: - Irradiation of a composite RF pulse, for example a binomial pulse, comprising at least two subpulses with a predetermined phase offset between successive subpulses, - Switching of bipolar layer selection gradients so that successive subpulses of the composite RF pulse are encoded with differently polarized layer selection gradients, - Recording magnetic resonance signals triggered by the composite RF pulse as measurement data, - Storing and / or further processing the recorded measurement data, wherein the subpulses are irradiated with a frequency detuned by a detuning shift relative to a resonance frequency of a spin species to be represented, such that the detuning shift (Δf) results in a linear evolution of the phase over the time course of the composite RF pulse, wherein the detuning of the frequency of the subpulses by the detuning shift is only generated during the respective duration of a respective emission of the subpulses and / or the frequency of the subpulses is additionally detuned by a layer selection shift.

Description

[0001] The invention relates to an improved determination of magnetic resonance data with spectral selection.

[0002] Magnetic resonance imaging (MRI) is a well-known technique used to generate images of the interior of an object. In simplified terms, the object is positioned in a magnetic resonance imaging (MRI) scanner within a relatively strong, static, homogeneous background magnetic field, also known as the B0 field, with field strengths ranging from 0.2 Tesla to 7 Tesla and higher. This causes the object's nuclear spins to align with the background magnetic field. To trigger measurable nuclear spin resonances, high-frequency excitation pulses (RF pulses) are applied to the object. The resulting nuclear spin resonances are measured as so-called k-space data using specially designed coils, and MR images or spectroscopic data are then determined based on these measurements. The alternating magnetic field generated by the excitation pulses applied via at least one transmitting coil is also referred to as the B1 field.To spatially encode the measurement data, rapidly switched magnetic gradient fields, or gradients for short, are superimposed on the underlying magnetic field. A scheme used that describes a temporal sequence of RF pulses to be applied and gradients to be switched is called a pulse sequence (or simply sequence). The recorded measurement data are digitized and stored as complex numerical values ​​in a k-space matrix. A corresponding MR image can be reconstructed from the k-space matrix containing these values, for example, using a multidimensional Fourier transform.

[0003] Protons in different (chemical) environments are also referred to as having different spin species. Different environments shield protons to varying degrees from the B0 field, resulting in different magnetic fields at the nucleus and thus different resonance frequencies. This is also known as a chemical shift between the different spin species. This can lead to different phase angles in the acquired signals if no specific countermeasures are implemented. The most prominent representatives of different spin species in (animal or human) patients as study objects are fat on the one hand and water on the other, although applications for other spin species, such as silicon, are also possible. The resonance frequency of protons bound to lipid molecules, which are assigned to the spin species "fat," is approximately 3.3–3.5 ppm.(parts per million) lower than the resonance frequency of water-bound protons assigned to the spin species "water".

[0004] This fact can be used to suppress the fat signal with frequency-selective RF saturation or RF inversion pulses, which are inserted prior to a subsequent signal acquisition, e.g., using a multi-echo technique. A disadvantage of such fat suppression techniques with frequency-selective RF saturation or RF inversion pulses is that the required suppression pulses add time to the sequence, increase the applied specific absorption rate (SAR), and always affect the entire imaging volume, meaning they cannot be optimized for a specific slice.

[0005] The targeted excitation or suppression of signal contributions from spin species with a specific chemical shift is highly relevant in MRI. For example, for certain clinical questions, the exclusive visualization of the water signal may be of interest – but cases are also conceivable in which only fat-bound protons (or those from other chemical compounds, such as silicone) are to be imaged.

[0006] Furthermore, there are imaging techniques where a spectrum of spin species with different chemical shifts would lead to undesirable image artifacts, and therefore spectral selection is desired. This includes, for example, echoplanar imaging (EPI), a multi-echo acquisition technique in which, due to the inherently low pixel bandwidth along the phase-encoding direction, spin species with different precession frequencies are spatially shifted in the image, with shifts of several millimeters. Sub-images of the individual spin species overlap due to the shift and interfere with each other, significantly complicating diagnosis for medical personnel based on the resulting MR images. This is clinically relevant, for example, in diffusion-weighted imaging, where EPI techniques are commonly used.

[0007] As an alternative to the aforementioned (simple) spectral saturation or inversion, so-called composite RF pulses can be used for spectral selection of the imaged spin species (for two-dimensional or three-dimensional imaging techniques). These consist of at least two consecutive (concatenated) RF pulses, also called subpulses of the composite RF pulse. The term "composite" already indicates that the subpulses are not emitted in isolation, but are combined in such a way that the resulting signal is optimally influenced for a specific imaging goal, e.g., frequency selectivity. The composite RF pulse thus emulates the effect of a simple RF pulse, but has a built-in compensation mechanism that may, if necessary,which takes only slightly longer than a conventional RF pulse, but eliminates the otherwise necessary application of additional pulses to suppress unwanted signals, while simultaneously reducing the applied SAR and enabling layer-specific optimizations.

[0008] When compound RF pulses are used for frequency selection to excite the spin species water, this is also referred to as "water excitation." For such water excitation, compound RF pulses known as binomial pulses are particularly common, in which the relative flip angles of the subpulses are coefficients of a binomial sequence (e.g., 1-1, 1-2-1, 1-3-3-1, etc.). A desired spectral selectivity can be achieved by adjusting the time interval ΔT between the subpulses, as described, for example, in the article by P.J. Hore, "A new method for water suppression in the proton NMR spectra of aqueous solutions," J. Magn. Reson. 54: pp. 539-542, 1983.

[0009] Known spatially and spectrally selective water excitations include, for example, water excitation with unipolar layer selection gradients. Fig. Figure 1 shows an example schematic representation of a section of a pulse sequence scheme for a unipolar 1-3-3-1 binomial pulse as a composite RF pulse RF1. The top row, RF, represents the RF pulses to be applied, and the bottom row, G, represents the pulses to be applied. SThe switching layer selection gradients (without ramps) are shown. The excitation scheme consists of four successive layer-selective subpulses RF* with an amplitude ratio of 1:3:3:1, such that, for example, if a flip angle of 90° is to be generated with the composite RF pulse, the subpulses RF* each have flip angles of 11.25°, 33.75°, 33.75°, and 11.25°. All four subpulses RF* are applied with the same phase and synchronously with the spin species to be excited (i.e., exactly at the resonance frequency of the spin species to be excited, e.g., water). The middle row PhN shows an example of how to generate the subpulses with the same phase by applying a constant phase using a numerically controlled oscillator (NCO).

[0010] During a Fig. In the unipolar layer selection shown in Figure 1, a layer selection gradient with the same polarity (here, for example, positive) is applied during each subpulse RF*. Between the subpulses RF*, a gradient with inverted polarity in the layer selection direction is applied for rephasing. However, this results in only a short maximum pulse duration T for each subpulse RF*. RF , which is significantly smaller than the time interval ΔT of the subpulses RF*. A consequence of the short pulse duration T available for the subpulses RF*. RF One limitation is the reduced quality of the achievable layer profile, since the subpulses RF* can only be applied with a small bandwidth-time product (BWT). Another limitation is the available pulse duration T. RFthe minimum achievable layer thickness (and thus the possible clinical applications) is a factor, since at least the central half-wave must be applied for a duration appropriate to the layer thickness.

[0011] One advantage of using such unipolar layer selection in conjunction with binomial pulses is that the spatial layer profiles of the spins manipulated by the respective subpulses RF* are identical for all subpulses RF*, so that the desired binomial amplitude ratio of the subpulses RF* (here e.g. 1-3-3-1) is maintained for every spatial position within the layer profiles.

[0012] Furthermore, spatially and spectrally selective water excitations with bipolar layer selection gradients are already known. Fig. Figure 2 shows an exemplary schematic representation of a section of a pulse sequence scheme of a variant of a 1-3-3-1 binomial pulse as a composite RF pulse RF1 with bipolar layer selection. Analogous to Fig. 1 are in Fig. 2 RF pulses to be emitted in the top row RF and in the bottom row G S The layer selection gradients to be switched (without ramps) are shown. The excitation scheme here consists (as in Fig. 1) from four successive layer-selective subpulses RF* with an amplitude ratio of 1:3:3:1. All four subpulses RF* are irradiated with the same phase and synchronously with the spin species to be excited (i.e., exactly at the resonance frequency of the spin species to be excited, e.g., water). The middle row PhN shows an example of a possible generation of the subpulses with the same phase by applying a constant phase using a numerically controlled oscillator (NCO).

[0013] During a Fig. In the bipolar layer selection shown in Figure 2, a layer selection gradient with a different polarity (here, for example, positive-negative-positive-negative) is alternately switched during the successive subpulses RF'. This results in a maximum pulse duration T for each emission of the subpulses RF'. RFavailable, which can now take the entire time interval ΔT of the subpulses RF', which, compared to the unipolar variant, increases the quality of an achievable layer profile and also allows excitation of thinner layers.

[0014] However, the application of layer selection gradients with different polarities, in conjunction with the different precession frequencies for the non-resonant spin species to be suppressed (e.g., fat) due to chemical shifts, generates a spatial shift in the layer profiles of the positively polarized subpulses RF' (e.g., the first and third) relative to the layer profiles of the negatively polarized subpulses RF' (e.g., the second and fourth). Particularly in the boundary regions of the desired layer to be manipulated with the binomial pulse, this leads to a deviation from the amplitude ratio of the binomial pulse required for spectral selection. Consequently, the desired spectral selection is not complete, and a residual magnetization of the unwanted spin species (e.g., fat) remains, which is also measured as a signal.Such contamination with signal from unwanted spin species negatively affects the achievable image quality.

[0015] The spatially selective composite RF pulses described in the prior art, e.g. binomial pulses, therefore have limitations: in some methods the spectral selectivity is limited, while other methods have limitations regarding the quality of layer profiles or the minimum achievable layer thicknesses.

[0016] Examples of spectrally selective RF pulses can be found in the articles by Thomasson et al., “Phase-modulated binomial RF pulses for fast spectrally-selective musculoskeletal imaging”, Magn. Reson. Med. 35: pp. 563-568, 1996; by Grissom et al., “Maximum linear-phase spectral-spatial radiofrequency pulses for fat-suppressed proton resonance frequency-shift MR Thermometry”, Magn. Reson. Med. 62: pp. 1242-1250, 2009; and by Zur et al., “Design of improved spectral-spatial pulses for routine clinical use”, Magn. Reson. Med. 43: pp. 410-420, 2000.

[0017] The invention is based on the objective of avoiding the aforementioned disadvantages and enabling a high degree of selection of a spin species while simultaneously achieving good quality and low thickness of the resulting layer profiles.

[0018] The problem is solved by a method for recording measurement data of an object of investigation comprising spins of at least two different spin species, using a magnetic resonance system according to claim 1, a magnetic resonance system according to claim 13, a computer program according to claim 14, and an electronically readable data carrier according to claim 15.

[0019] A method according to the invention for recording measurement data of an object under investigation, which comprises spins of at least two different spin species, using a magnetic resonance system, comprises the steps: - Irradiation of a composite RF pulse, for example a binomial pulse, comprising at least two subpulses with a predetermined phase offset between successive subpulses, - Switching of bipolar layer selection gradients so that successive subpulses of the composite RF pulse are encoded with differently polarized layer selection gradients, - Recording magnetic resonance signals triggered by the composite RF pulse as measurement data, - Storing and / or further processing the recorded measurement data, wherein the subpulses are irradiated with a frequency detuned by a detuning shift relative to a resonance frequency of a spin species to be represented, such that the detuning shift results in a linear evolution of the phase over the time course of the composite RF pulse, wherein the detuning of the frequency of the subpulses by the detuning shift is only generated during the respective duration of a respective emission of the subpulses and / or the frequency of the subpulses is additionally detuned by a layer selection shift.

[0020] By detuning the frequency at which subpulses of the composite RF pulse are irradiated according to the invention, the layers manipulated by the subpulses can be adapted in their layer profiles in such a way that both good spectral selectivity and a high quality of the spatial selectivity achieved, in particular with regard to the achievable layer profiles and their minimum thickness, are achieved.

[0021] A magnetic resonance system according to the invention comprises a magnet unit, a gradient unit, a radio frequency unit and a control device designed for carrying out a method according to the invention, including a detuning unit.

[0022] A computer program according to the invention implements a method according to the invention on a control device when it is executed on the control device. For example, the computer program includes instructions that, when the program is executed by a control device, e.g., a control device of a magnetic resonance system, cause this control device to execute a method according to the invention. The control device can be in the form of a computer.

[0023] The computer program can also be in the form of a computer program product that can be directly loaded into a memory of a control device, with program code means to execute a method according to the invention when the computer program product is executed in a computing unit of the computing system.

[0024] A computer-readable storage medium according to the invention comprises instructions which, when executed by a control device, e.g. a control device of a magnetic resonance system, cause it to execute a method according to the invention.

[0025] The computer-readable storage medium can be designed as an electronically readable data carrier which includes electronically readable control information stored on it, which includes at least one computer program according to the invention and is designed in such a way that, when the data carrier is used in a control unit of a magnetic resonance system, it carries out a method according to the invention.

[0026] The advantages and explanations given regarding the procedure also apply analogously to the magnetic resonance system, the computer program product and the electronically readable data carrier.

[0027] Further advantages and details of the present invention will become apparent from the exemplary embodiments described below and from the drawings. The examples shown do not constitute a limitation of the invention. They show: Fig. 1 An exemplary, schematic representation of a section of a pulse sequence scheme for a unipolar 1-3-3-1 binomial pulse as a composite RF pulse from the prior art, Fig. 2 an exemplary, schematic representation of a section of a pulse sequence scheme for a bipolar 1-3-3-1 binomial pulse as a composite RF pulse from the prior art, Fig. 3 a schematic flow diagram of a method according to the invention for recording measurement data of a test object comprising spins of at least two different spin species with improved simultaneous spatial and spectral selection, Fig. 4-5 exemplary, schematic representations of a section of pulse sequence schemes for possible composite RF pulses according to the invention, Fig. 6 a schematically illustrated magnetic resonance system according to the invention.

[0028] Fig. Figure 3 is a schematic flowchart of a method according to the invention for recording measurement data of a test object comprising spins of at least two different spin species Sp1 and Sp2, using a magnetic resonance system. The at least two different spin species can be spin species from the group consisting of water, fat, and silicone.

[0029] At least one composite RF pulse, comprising at least two subpulses with a predetermined phase offset between successive subpulses, is injected into the object under investigation (Block 101). The composite RF pulse can be a binomial pulse. However, other types of composite RF pulses are also conceivable. The phase offset between successive subpulses of the at least two subpulses can, for example, be chosen such that the signal contribution of the spin species to be visualized is as large as possible compared to the signal contributions of other spin species of the at least two spin species. This can be achieved, for example, by minimizing the signal contribution of a spin species to be suppressed among the at least two spin species by means of the selected phase offset. A phase offset of 180° or 180° is used for this purpose.The π between successive subpulses can also be achieved by alternating the (complex) amplitudes of the successive subpulses.

[0030] Fig. Figure 4 is an exemplary, schematic representation of a section of a pulse sequence scheme for possible composite RF pulses RF2.1, RF2.2 according to the invention, which can each be used individually or in combination in a sequence.

[0031] Analogous to the Fig. 1 and Fig. The top row contains RF pulses to be emitted, and the bottom row contains G. S The switching layer selection gradients (without ramps) are shown. For better comparability, the excitation scheme here again consists of four consecutive layer-selective subpulses RF'' with an amplitude ratio of 1:3:3:1 (the underlines indicate an inverted phase (phase offset of 180°) between the subpulses RF'' in this example).

[0032] During the transmission of a composite RF pulse, bipolar layer selection gradients are switched, so that successive subpulses of a composite RF pulse are encoded with differently polarized layer selection gradients (Block 103). If the composite RF pulse is a binomial pulse, then the switched layer selection gradients make it a bipolar binomial pulse.

[0033] In Fig. 4 is an example analogous to Fig. 2 a bipolar layer selection is shown in which a layer selection gradient with different polarity (here e.g. positive-negative-positive-negative) is switched alternately during the successive subpulses RF''.

[0034] The subpulses of an irradiated composite RF pulse are irradiated with a carrier frequency that is detuned by a detuning shift Δf relative to a resonance frequency of a spin species to be represented, such that the detuning shift Δf results in a linear evolution of the phase over the time course of the composite RF pulse.

[0035] In Fig. Figure 4 in the middle row PhV shows an example of a possible linear evolution of the phase as it is generated by the detuning shift according to the invention.

[0036] Magnetic resonance signals induced in the object under investigation by irradiated composite RF pulses are recorded as measurement data MD in a recording window A (Block 105).

[0037] A time interval ΔT between subpulses of the composite RF pulse can be determined as a function of a chemical shift of two of the at least two different spin species.

[0038] As described above, the spectral selectivity can be determined via the time interval ΔT between the subpulses of a composite RF pulse. For binomial pulses, which are composite RF pulses consisting of subpulses without a phase offset between them, signal contributions from a chemically shifted spin species by ΔCS can be suppressed, for example, by a time interval of ΔT = π / (γ ΔCS B0) between the subpulses, where y is the gyromagnetic ratio and B0 is the strength of the background magnetic field. For example, for B0 = 3T, γ / 2π = 42.575 MHz / T, and ΔCS = 3.3 ppm, a time interval ΔT ≈ 1190 µs is obtained.

[0039] However, other time intervals ΔT are also suitable for this purpose. In particular, the same spectral selectivity can be achieved with a phase difference of π + N * 2π, so that a time interval ΔT between subpulses of the composite RF pulse can be determined as the quotient of the sum of the number Pi with a natural multiple N, where N can also be zero, but in particular with N at least one, twice the number Pi (dividend), and the product of the gyromagnetic ratio with a chemical shift of two of the different spin species Sp1, Sp2, and a strength of a main magnetic field of the magnetic resonance system (divisor): ΔT=(π+N​*2π) / (γΔCS B0) min N=0,1,2,3,…, preferably with N=1,2,3,….

[0040] With larger multiples N, the time interval ΔT of the subpulses becomes larger, thus providing more time for the subpulses to be played out, which can, for example, lower the SAR applied by the composite RF pulse and further improve a layer profile.

[0041] For binomial pulses, which are composite RF pulses consisting of subpulses with a predetermined phase offset between successive subpulses, signal contributions from a spin species chemically shifted by ΔCS of a spin species to be suppressed are obtained under the same condition for the time interval ΔT = π / (γ ΔCS B0) between the subpulses. This means that without the detuning according to the invention, spin species excited "on-resonant" (i.e., at their resonance frequency) by the composite RF pulse would not generate a signal, while a signal from a spin species "off-resonant" due to the chemical shift would be retained. However, the detuning shift described here shifts the "on-resonant" condition back to the spin species to be suppressed.

[0042] The advantage of this approach is that, due to the constant frequency shift by the detuning shift Δf, the layer profiles of the spin species to be suppressed now coincide with the positive and negative polarity of layer-coded subpulses, so that, regardless of the spatial position within the layer manipulated by the composite RF pulse, the desired amplitude ratio, e.g., as in Fig. 4 1-3-3-1, remains unchanged, and the signal of the spin species to be suppressed, e.g., fat, is indeed suppressed. Although the detuning shift Δf now shifts the layer profiles of the spin species to be visualized, e.g., water, for subpulses with positive layer selection gradients compared to those with negative layer selection gradients, which can lead to a slight reduction in the signal of the spin species being visualized, this is generally not significant. For example, with water excitation and fat suppression, a slight loss of water signal (i.e., a slightly lower signal-to-noise ratio, SNR) is significantly less critical for image quality and diagnosis than even slight contamination of the image with residual fat signal, which would produce noticeable image artifacts.

[0043] Comparisons of layer profiles and signal amplitudes of spin species to be represented for unipolar binomial pulses, carried out using Bloch simulations (see...). Fig. 1), bipolar binomial pulses (cf. Fig. 2) and composite RF pulses according to the invention (see Fig. 4) The inventive method yields a quality of layer profiles comparable to that of bipolar binomial pulses, with a significant improvement in spectral selectivity comparable to that of unipolar binomial pulses, while only very slightly reducing the obtained signal.

[0044] The detuning shift Δf of the frequency of the subpulses can be chosen such that, over the time course of the composite RF pulse, it detunes the frequency of the subpulses to a resonance frequency of one of the at least two spin species to be suppressed. This can be achieved, for example, by choosing the detuning shift Δf as a function of the chemical shift of the spin species to be suppressed relative to one of the at least two spin species Sp1, Sp2 to be represented, according to the following formula: Δf=(γΔCS B0) / π, where γ is the gyromagnetic ratio, ΔCS is the chemical shift, B0 is the main magnetic field of the magnetic resonance system used, and π is the pi number.

[0045] The detuning of the frequency of the subpulses by the detuning shift Δf can occur at least during the time of emission of the subpulses (pulse duration T). RFThe process involves switching an NCO with a frequency constantly shifted by the detuning factor relative to the resonance frequency of a spin species to be represented over the time course of the composite RF pulse. In this way, the frequency is detuned by means of an NCO. Additionally or alternatively, the frequency detuning can include a known frequency and / or phase modulation of the irradiated subpulses, so that the frequency is detuned via frequency and phase modulation.

[0046] The detuning of the frequency of the subpulses by the detuning shift can occur at least during the time of emission of the subpulses, i.e., simultaneously with the emission of the subpulses for the respective pulse duration T. RF take place.

[0047] The detuning can be permanently effective throughout the entire compound RF pulse RF2.1.

[0048] It is also conceivable that the detuning of the frequency of the subpulses by the detuning shift is only generated during the respective duration of each transmission of the subpulses, i.e., only during the times when subpulses are applied (i.e., with interruptions during pause periods, for example, during gradient ramps). With such interruptions, it is only necessary to ensure that the start phase of the successive subpulses is set appropriately so that it corresponds to the phase that would result from a continuous effect of the detuning shift. The right-hand composite RF pulse RF2.2 in Fig. The interrupted line shown in row PhV, indicating the phase evolution, illustrates that the detuning shift is only effective during the subpulses of the compound RF pulse RF2.2.

[0049] In addition to the detuning shift, the frequency of the subpulses can be detuned by a layer selection shift, as is known in the prior art, in order to manipulate layers S1 and S2 at different layer positions without having to switch other gradients. In this way, the described method can be used for acquiring a large number of layers, for example, for the complete imaging of an anatomy relevant for diagnosis.

[0050] The switched bipolar layer selection gradients can be layer selection gradients of a VERSE technique (VERSE: "variable-rate selective excitation"). VERSE employs time-varying layer selection gradients, for example, to reduce the applied SAR or the RF pulse duration. The dotted indentations of the layer selection gradients of the right composite RF pulse RF2.2 represent possible trajectories of VERSE layer selection gradients. The phase evolution remains linear and is not modulated depending on the variable amplitude of the layer gradients.

[0051] In addition, the frequency at which the subpulses are injected can be further optimized for a specific desired layer using a view-specific adjustment method. This allows, for example, setting an optimized center frequency and / or amplitude scaling for a desired layer (or block in three-dimensional imaging) and thus optimizing the image quality locally for the current layer. Particularly in water excitation—where, unlike chemically selective fat suppression, spectral selectivity acts only locally within the layer—such layer-specific adjustments can lead to significant improvements in image quality. The shifted center frequency determined by the adjustment can be added to the detuning shift required for the invention.Exemplary layer-specific adjustment methods that can be combined with a detuning shift described herein are known, for example, from documents DE 10 2014 219 778 B4 and (further in combination with a layer multiplexing technique for the simultaneous acquisition of signals from several layers) DE 10 2015 218 852 A1.

[0052] With a composite RF pulse described herein, which is applied together with a bipolar layer selection gradient, it is possible, as described, to determine the duration T. RFThe subpulses of the compound RF pulse are designed to be relatively long, up to a duration corresponding to a time interval ΔT between subpulses. This reduces the SAR applied by the compound RF pulse. The detuning shift Δf proposed here simultaneously improves the image quality of the spin species being visualized, enabling the acquisition of clinically relevant MR images. Particularly in slice multiplexing techniques, applied SAR values ​​are often undesirably high due to the simultaneous excitation of multiple slices. The compound RF pulses described herein can effectively counteract this.

[0053] Echoplanar diffusion imaging, in particular, where residual signals from non-resonant spin species can lead to pronounced image artifacts, benefits from a combination of the composite RF pulse described herein with layer-specific adjustments. Crucially, this involves, on the one hand, the effective suppression of unwanted spin species, which is further enhanced by layer-specific adjustments. On the other hand, only the longer duration of the subpulses achievable with the method described herein allows for the excitation of thin slices, and thus enables high-resolution imaging of clinically relevant details.

[0054] A composite RF pulse according to the invention can replace a standard (non-composite) RF pulse or a composite RF pulse known from the prior art in a wide variety of pulse sequence schemes. Depending on the desired application, the composite RF pulse can be used as an RF excitation pulse or as an RF refocusing pulse. In principle, its use as an RF inversion pulse or a so-called store and / or restore RF pulse, or (for all types of RF pulses) an embodiment as an asymmetric RF pulse, is also conceivable.

[0055] Accordingly, the recording technique used to acquire the MD measurement data can also be a recording technique that acquires measurement data in two-dimensional or three-dimensional space, in particular a layer multiplexing technique and / or a diffusion recording technique.

[0056] The magnetic resonance signals triggered by the composite RF pulse can be generated as gradient echo signals, spin echo signals, turbo spin echo signals, doubly refocused spin echo signals and / or stimulated echo signals.

[0057] It is conceivable that, following a composite RF pulse described herein, which is an RF excitation pulse (i.e., following a composite RF excitation pulse), at least one first RF refocusing pulse RF3.1, which can be a conventional RF refocusing pulse, and at least one second RF refocusing pulse RF3.2, which can also be a conventional RF refocusing pulse, are applied. In this way, a doubly refocused spin-echo signal can be obtained after a first RF refocusing pulse RF3.1 and a second RF refocusing pulse RF3.2 and recorded in a respective acquisition window A.In one embodiment of such a spin-echo method with at least two refocused pulses, a layer selection gradient with a first polarity can be switched during the application of a first RF refocusing pulse, and a layer selection gradient with a second polarity can be switched during the application of a second RF refocusing pulse, as shown schematically in . Fig. Figure 5 shows the terms for the same thing again analogous to the Fig. 1, Fig. 2 and Fig. 4 are selected. This means that the polarity of the layer selection gradients switched during the first RF refocusing pulse is reversed compared to the polarity of the layer selection gradients switched during the second RF refocusing pulse, thereby achieving, analogous to a so-called "gradient reversal" technique, even better suppression of a signal to be suppressed from a spin species to be suppressed.

[0058] In addition to the detuning shift, a phase of at least one subpulse of the composite RF pulse, preferably the respective phase of each subpulse of the composite RF pulse, can be manipulated in a known manner such that a signal phase generated by the composite RF pulse (the phase of the generated transverse magnetization) reaches a desired value. The desired signal phase can, for example, be one achieved with a prior art method that is to be improved by the detuning shift described herein, or it can be selected according to an RF spoiling method, such as the one described in the article by Zur et al., "Spoiling of Transverse Magnetization in Steady-State Sequences," Magn. Reson. Med. 21: pp. 251-263, 1991.

[0059] The recorded measurement data MD are stored and / or further processed (Block 107). For example, image data BD can be reconstructed from recorded measurement data MD.

[0060] Fig. Figure 6 schematically represents a magnetic resonance system 1 according to the invention. This comprises a magnet unit 3 for generating the basic magnetic field, a gradient unit 5 for generating the gradient fields, a radio frequency unit 7 for irradiating and receiving radio frequency signals, and a control device 9 designed for carrying out a method according to the invention.

[0061] In the Fig.Figure 6 shows only a rough schematic representation of these subunits of the magnetic resonance system 1. The high-frequency unit 7 can consist of several subunits and, for example, comprise several coils. In particular, the high-frequency unit 7 can comprise a body coil that is permanently integrated into the magnetic resonance system 1 and, in turn, can comprise, for example, two antenna elements 7.1 and 7.2. Furthermore, the high-frequency unit 7 can comprise one or more different local coils 7*, which can be designed either only for transmitting high-frequency signals or only for receiving the triggered high-frequency signals, or for both, and which themselves can comprise several antenna elements and associated coil channels.

[0062] To examine a test object U, for example a patient or a phantom, it can be placed on a table L in the magnetic resonance imaging (MRI) system 1 within its measurement volume. Layers S1 or S2 represent exemplary target volumes of the test object, from which echo signals can be recorded and acquired as measurement data.

[0063] The control unit 9 serves to control the magnetic resonance system 1 and can, in particular, control the gradient unit 5 by means of a gradient controller 5' and the radio frequency unit 7 by means of a radio frequency transmit / receive controller 7'. The radio frequency unit 7 can comprise several channels on which signals can be transmitted or received.

[0064] The high-frequency unit 7, together with its high-frequency transmit / receive control 7', is responsible for generating and transmitting a high-frequency alternating field to manipulate the spins in a region to be manipulated (for example, in layers S to be measured) of the object under investigation U. The center frequency of the high-frequency alternating field, also referred to as the B1 field, is generally set as close as possible to the resonance frequency of the spins to be manipulated. Deviations from the center frequency to the resonance frequency are referred to as off-resonance. To generate the B1 field, controlled currents are applied to the RF coils in the high-frequency unit 7 by means of the high-frequency transmit / receive control 7'.

[0065] Furthermore, the control unit 9 comprises a detuning unit 15 for detuning the frequencies of subpulses of compound RF pulses according to the invention. The control unit 9 is configured overall to carry out a method according to the invention.

[0066] A computing unit 13, encompassed by the control unit 9, is designed to perform all the necessary calculations for the required measurements and determinations. Intermediate results and final results required for this purpose, or determined in the process, can be stored in a storage unit S of the control unit 9. The units shown here are not necessarily to be understood as physically separate units, but merely represent a subdivision into conceptual units, which can also be realized, for example, in fewer or even just a single physical unit.

[0067] Via an input / output device (I / O) of the magnetic resonance system 1, control commands can be sent to the magnetic resonance system by a user, for example, and / or results from the control device 9, such as image data, can be displayed.

[0068] The method described herein may also be in the form of a computer program comprising instructions that execute the described method on a control unit 9. Likewise, a computer-readable storage medium may be present, comprising instructions that, when executed by a control unit 9 of a magnetic resonance system 1, cause it to execute the described method.

[0069] Regardless of the grammatical gender of a particular term, persons with male, female or other gender identities are included.

Claims

[1] Method for recording measurement data of an object of investigation (U) comprising spins of at least two different spin species, using a magnetic resonance system (1), comprising the steps: - Irradiation of a composite RF pulse, for example a binomial pulse, comprising at least two subpulses with a predetermined phase offset between successive subpulses, - Switching of bipolar layer selection gradients so that successive subpulses of the composite RF pulse are encoded with differently polarized layer selection gradients, - Recording magnetic resonance signals triggered by the composite RF pulse as measurement data, - Storing and / or further processing the recorded measurement data, wherein the subpulses are irradiated with a frequency detuned by a detuning shift relative to a resonance frequency of a spin species to be represented, such that the detuning shift (Δf) results in a linear evolution of the phase over the time course of the composite RF pulse, wherein the detuning of the frequency of the subpulses by the detuning shift is only generated during the respective duration of a respective emission of the subpulses and / or the frequency of the subpulses is additionally detuned by a layer selection shift. [2] Method according to claim 1, wherein a time interval (ΔT) between subpulses of the composite RF pulse is determined as a function of a chemical shift of two of the at least two different spin species. [3] Method according to one of the preceding claims, wherein the detuning shift is the frequency of the subpulses over the time course of the composite RF pulse to a resonance frequency of one of the spin species to be suppressed of the at least two spin species and / or the predetermined phase offset is a phase offset of 180°. [4] Method according to one of the preceding claims, wherein the detuning of the frequency of the subpulses by the detuning shift comprises, at least during times of emission of the subpulses, switching an NCO with a frequency shifted constantly by the detuning shift over the time course of the composite RF pulse and / or frequency and / or phase modulation of the emitted subpulses. [5] Method according to any of the preceding claims, wherein the switched bipolar layer selection gradients are layer selection gradients of a VERSE technique (VERSE: “variable-rate selective excitation”). [6] Method according to one of the preceding claims, wherein the frequency at which the subpulses are irradiated is further optimized for a respective desired layer by means of a layer-specific adjustment method. [7] Method according to any of the preceding claims, wherein the composite RF pulse is an RF excitation pulse or an RF refocusing pulse, an RF inversion pulse, a store and / or restore RF pulse, which is symmetrically or asymmetrically designed. [8] Method according to one of the preceding claims, wherein the measurement data are recorded using a recording technique that records measurement data in two-dimensional or three-dimensional space, in particular a layer multiplexing technique and / or a diffusion recording technique. [9] Method according to any of the preceding claims, wherein the magnetic resonance signals triggered by the composite RF pulse are generated as gradient echo signals, spin echo signals, turbo spin echo signals, doubly refocused spin echo signals and / or stimulated echo signals. [10] Method according to one of the preceding claims, wherein after a composite RF pulse irradiated as an RF excitation pulse, at least a first RF refocusing pulse and at least a second RF refocusing pulse are irradiated such that after a first RF refocusing pulse and a second RF refocusing pulse a doubly refocused spin echo signal is generated, wherein a polarity of layer selection gradients switched during a first RF refocusing pulse is inverse to a polarity of layer selection gradients switched during a second RF refocusing pulse. [11] Method according to one of the preceding claims, wherein a phase of at least one subpulse of the composite RF pulse is manipulated such that a signal phase generated by the composite RF pulse reaches a desired value. [12] Method according to any of the preceding claims, wherein the at least two different spin species are spin species from the group consisting of water, grease and silicone. [13] Magnetic resonance system (1) comprising a magnet unit (3), a gradient unit (5), a radio frequency unit (7) and a control unit (9) with a radio frequency transmit / receive control (7') and with a detuning unit (15), wherein the control unit (9) is configured to perform a method according to any one of claims 1 to 12 on the magnetic resonance system (1). [14] Computer program comprising commands which, when the program is executed by a control device (9) of a magnetic resonance system (1), cause it to execute the method according to any one of claims 1 to 12. [15] Computer-readable storage medium comprising instructions which, when executed by a control device (9) of a magnetic resonance system (1), cause it to execute the method according to any one of claims 1 to 12.

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