SPECTRAL SATURATION IN MAGNETIC RESONANCE IMAGING

Description

[0001] The present invention relates to a method for generating a saturation pulse for spectral saturation in magnetic resonance imaging, wherein the method comprises specifying or determining a first resonance frequency of an MR spectrum of a first substance, specifying or determining a first saturation frequency in a predetermined range around a second resonance frequency of the MR spectrum of a second substance and generating a saturation pulse which does not lead to saturation of the first substance at the first resonance frequency, wherein the saturation pulse has at least three spectral peaks for saturating the second substance in a predetermined range of the first saturation frequency (in the present document, a spectral peak is also referred to as a saturation peak if it is intended for saturation).Furthermore, the present invention relates to a method for imaging in magnetic resonance tomography, a computer program product, a computer-readable medium and a corresponding magnetic resonance system.

[0002] Magnetic resonance imaging (MRI) is an imaging technique primarily used in medical diagnostics. MRI requires a strong, homogeneous static magnetic field, also known as a B0 field. MRI observes the relaxation times of nuclear spins in response to a radiofrequency (RF) excitation pulse, particularly the so-called longitudinal relaxation time T1 and the transverse relaxation time T2. The measurement of the relaxation times of the hydrogen nucleus, i.e., a proton, plays the most important role due to its abundance in the human body.

[0003] A challenge in MRI is to create good contrast between adipose tissue and aqueous tissue, and in particular, to minimize the interference of fat signals in aqueous tissue. Separation of the signals from fat and water molecules is possible due to the different chemical shifts of both signals. Chemical shift is a shift in the resonance frequency of a nucleus depending on its electrical and chemical environment.

[0004] One method for suppressing fat signals due to the different chemical shifts of hydrogen nuclei in adipose tissue and in aqueous tissue is known as spectral fat saturation. In a magnetic resonance (MR) spectrum, the resonances of hydrogen nuclei in fat and water molecules appear as separate signals in the form of a fat and water peak. A spectrally selective RF excitation pulse is emitted that excites only the hydrogen nuclei in the adipose tissue, converting the longitudinal magnetization in the fat into a transverse magnetization. This is immediately dephased by a magnetic field gradient, so that the adipose tissue can no longer be visualized by the immediately following imaging sequence.

[0005] However, spectral fat saturation is susceptible to inhomogeneities in the B0 field. These determine the position and width of the fat peak in the MR spectrum. This dependence means that the quality of fat signal suppression can be spatially inhomogeneous. One way to adjust the homogeneity of the B0 field is called "shimming." Therefore, most MRI systems incorporate so-called shim coils, which are capable of correcting even more complex spatial magnetic field patterns.

[0006] Multichannel applications are known from the publication "Broadband Slab Selection with B1+ Mitigation at 7T via Parallel Spectral-Spatial Excitation" (Setsompop et al., MRM 2009). There, pTX layer or slab excitations (parallel transmission mode) are performed across a wide range of excitation frequencies.

[0007] Automated spectral fat saturation is known from the publication DE 10 2012 214 660 B4. The method is based on the acquisition of an MR spectrum using an RF coil and an MR device, with a static magnetic field B 0 prevailing in the acquisition volume of the MRI device. This method is also based on an automatic analysis of this MR spectrum by searching for two resonance signals in the form of a fat peak and a water peak, and for a minimum between the two peaks. This automatic analysis is used, depending on a predefined criterion, to automatically decide whether to perform a series of steps that include adjusting the homogeneity of the static magnetic field B 0 using a shimming coil of the MRI device and calculating an RF pulse for fat saturation based on the result of the above search.

[0008] The document LEE JAE-SEUNG ET AL: "Uniform saturation of a strongly coupled spin system by two-frequency irradiation", THE JOURNAL OF CHEMICAL PHYSICS, Vol. 134, No. 23, June 21, 2011 (2011-06-21), page 234504, XP055894208, US ISSN: 0021-9606, DOI: 10.1063 / 1.3600758, describes a theory for two-frequency saturation. The parameters that influence saturation are discussed and experimentally investigated using a model liquid crystal system. It has been shown that two-frequency irradiation can be extremely efficient if the excitation frequencies used are on opposite sides of the characteristic frequency of the spin system. The frequency spacing of the excitation frequencies in a two-frequency excitation system can be varied over a wide frequency range.

[0009] Document US 2008 / 238421 A1 discloses a magnetic resonance imaging scanner comprising a unit for setting imaging parameters and an image data acquisition unit. The unit for setting the imaging parameters sets an imaging parameter by applying first and second suppression pulses, which differ at least in type, center frequency, or frequency band. The first and second suppression pulses frequency-selectively suppress at least one of the group of fat and silicone. The image data acquisition unit acquires image data according to the set imaging parameters.

[0010] Document US 10598746 B2 describes a SPAIR pulse for reducing artifacts when acquiring magnetic resonance data using a magnetic resonance device. The SPAIR pulse, which acts on spins in a first, predetermined frequency range, and a saturation pulse, which acts on spins in a second predetermined frequency range, are emitted. A gradient field for spatial encoding is applied simultaneously, so that the saturation pulse acts on an edge region adjacent to the volume segment. The edge region delimits an ellipsoidal usable volume of the magnetic resonance device in which the strength of the B0 field varies by less than 30 ppm. Spoiler gradient fields are applied to destroy transverse magnetization before a radiofrequency excitation pulse, adapted to the SPAIR pulse, is emitted. Magnetic resonance data are acquired after the SPAIR pulse, the saturation pulse, and the excitation pulse.The second frequency range is adjusted to the first frequency range.

[0011] The object of the present invention is to reduce the effort required to calculate a saturation pulse. According to the invention, this object is achieved by a method, a magnetic resonance system, a computer program product, and a computer-readable medium according to the independent patent claims. Advantageous developments of the invention are set forth in the dependent claims.

[0012] Accordingly, the present invention provides a method for generating a saturation pulse for spectral saturation in magnetic resonance imaging. Spectral saturation is intended to suppress the signal of a specific substance from an MRI signal. The method can thus be part of a magnetic resonance application, e.g., MR imaging. In particular, a substance is intended to be suppressed that, for example, interferes with the imaging or otherwise in the MRI signal. In a specific example, fatty tissue is to be suppressed, which is why the method for spectral fat saturation can then be used.

[0013] In a (first) step of the method, a first resonance frequency of an MR spectrum of a first substance is specified or determined. This first substance therefore has a first resonance frequency in the MR spectrum. Such a resonance frequency can be located, in particular, at the location of a spectral peak. This resonance frequency can be known in advance and thus specified. Alternatively, the resonance frequency can also be determined manually or automatically. In either case, it is known where the first substance produces a maximum in the MR spectrum.

[0014] A similar procedure applies in a further step, in which a first saturation frequency is specified or determined in a predetermined range around a second resonance frequency of the MR spectrum of a second substance. The second substance causes a second spectral peak or a second maximum in the MR spectrum. This maximum is located at the location of the second resonance frequency. A first saturation frequency, with which saturation of the second substance is to be achieved, is now determined or specified (manually or automatically). The predetermined range in which the first saturation frequency lies results either from a previous measurement or, for example, .from experience or the like. For example, typical inhomogeneities of the B 0 field lead to known shifts of a fat peak. In this case, the first saturation frequency could be at the resonance frequency of the shifted fat peak. The range can be very small and, in limiting cases, even zero. Furthermore, the range can also be defined relative to the first resonance frequency (e.g., -3.0 to -3.5 ppm). The range can be defined solely by its outer limits (e.g., -3.0 ppm and -3.5 ppm).

[0015] For saturation, it is necessary to generate a saturation pulse within the scope of the method according to the invention. This generation of the saturation pulse involves not only the mathematical determination of the pulse parameters, but also the physical emission of this saturation pulse. This saturation pulse is specified in such a way that it does not lead to saturation of the first substance at the first resonance frequency. Since the MR spectrum of even a single compound is not a line spectrum but a distribution spectrum, it has a value other than zero at every frequency. However, these values ​​are generally negligible at a certain distance from the resonance frequency. For example, fat also has a small spectral component at the resonance frequency of water, but this is generally negligible. In practice, it is assumed that the saturation pulse does not lead to saturation of the first substance at the first resonance frequency.This means that the first substance provides virtually no MR signal immediately after the saturation pulse.

[0016] Furthermore, the saturation pulse is designed in such a way that it has a first spectral peak for saturating the second substance at the first saturation frequency. While the first substance cannot be saturated by the saturation pulse, the second substance is saturated when excited by the saturation pulse, at least under certain conditions (e.g., the B0 field). Preferably, the second substance is saturated to a degree of 80 to 100%. 100% is referred to as complete saturation. Such complete saturation is generally always desirable, but can usually not be achieved. Thus, the saturation pulse has a maximum at the first saturation frequency and the lowest possible value at the first resonance frequency.

[0017] Furthermore, it is provided that the saturation pulse has a second spectral peak for saturating the second substance at a second saturation frequency that is different from the first saturation frequency in the predetermined range. The saturation pulse therefore has at least two spectral peaks for saturating the second substance (e.g. for two different conditions with regard to the homogeneity of the B 0 field) as well as a spectral component at the position of the first resonance frequency that does not lead to saturation of the first substance. It should therefore be possible to specifically bring about saturation of the second substance under different conditions using two spectral peaks. The second spectral peak should therefore be able to ensure that the second substance is saturated even if, for example, the MR resonance frequency of the second substance shifts due to a deviation in the B 0 field. This has the consequence that the second substance can be saturated more reliably.For example, if fat saturation is desired, the second fat saturation peak can be used to increase the proportion of fat saturation, making fatty tissue appear darker in an MRI image and not obscuring other tissue sections that are not, or only to a lesser extent, based on fatty tissue. This second spectral peak can thus be used to more specifically deactivate an MRI peak of the second substance.

[0018] Furthermore, as already mentioned above, the saturation pulse contains at least one additional spectral peak in addition to the first and second spectral peaks to saturate the second substance. In this case, the saturation pulse therefore contains at least three spectral peaks to achieve saturation of the second substance. This allows multiple spectral components of the second substance to be specifically deactivated in the MR procedure. If necessary, the width of the individual spectral peaks can also be modified.

[0019] This results in the particular advantage that fluctuations in the B0 map for one layer of an object under investigation do not have too great an impact on the imaging when the second substance is saturated (e.g. fat saturation). A second, third, fourth, etc. spectral peak in the saturation pulse for the saturation of the second substance increases the probability that the second substance will also be saturated to a high degree (preferably 100%). At the same time, however, this also results in the advantage that fluctuations in the B0 field from layer to layer are less noticeable due to the multiple spectral peaks for the saturation of the second substance. This in turn means that a saturation pulse can be used not only for a single layer, but also for one or more neighboring layers if necessary.

[0020] In a preferred embodiment, the first substance is water and the second substance is fat. The method according to the invention is intended to create the possibility of achieving high fat saturation, whereas structures based on water compounds are not saturated. This ensures that, particularly in MR imaging, structures of an organism based on water compounds are not obscured by fatty tissue. Since the fatty tissue in this case provides no or only a low MR signal, it appears dark, so that even tissue portions that only provide a weak signal can be visualized.

[0021] In a further embodiment, each spectral peak of the saturation pulse corresponds to a peak of an MR spectrum of the second substance. This is particularly useful, for example, when the second substance has multiple peaks in the MR spectrum. In this case, the second substance can provide multiple components in the MR signal that need to be individually saturated. Ultimately, this allows for more extensive saturation of the second substance.

[0022] In a particularly advantageous embodiment of the method, a spatially resolved B0 field map of a constant magnetic field of the magnetic resonance imaging is determined before the saturation pulse is generated, and depending on the spatially resolved B0 field map, a spectral shift of the saturation pulse is effected at respective locations of the B0 field map. The B0 field map is preferably determined as a function of B0 deviations induced by a patient. Furthermore, the B0 field map can be determined as a function of dynamic effects caused by gradient fields at the time of the excitation pulse. The B0 field map contains data on spatial variations of the B0 field in the acquisition area of ​​the magnetic resonance imaging scanner. In contrast to the B1 field, a B0 field is considered to be a magnetic field that exhibits temporal variations only at frequencies that are significantly below the Larmor frequency, for example by a factor of 10, 50, or greater.The B0 field map can be stored in the controller's memory, for example, if it was already determined during production by measuring with a field camera or by calculating a field distribution for the magnetic resonance imaging scanner. However, it is also conceivable that the controller can be configured at the beginning of a sequence by a magnetic resonance measurement or by simulation, e.g. . taking into account the position and other characteristics of the patient or settings of the magnetic resonance imaging system such as shim currents through shim coils, these are currently determined.

[0023] According to an advantageous development, a parameter value of the magnetic resonance imaging can be fixed or determined automatically, and depending on the parameter value, a spectral position of the first and / or second spectral peak is automatically determined. If, for example, the height of the B 0 field is exactly known at a certain point in a slice, the spectral position of a peak can be determined more precisely so that the saturation pulse can be adjusted accordingly. However, the parameter value of the magnetic resonance imaging can also refer to a specific region of the object to be examined (e.g., the human body). For example, a specific body region can be defined using the parameter value. For example, the spectral peak for fat in the head region lies at a different resonance frequency than in the neck region.If the parameter value defines the corresponding region of the body or object, the position of the spectral peak or saturation peak can also be adjusted more precisely. However, the parameter value can also refer to other environmental variables that influence the MR spectrum and thus the saturation peaks.

[0024] In another advantageous development of the method according to the invention, it is provided that a space to be examined by means of magnetic resonance imaging is clustered into two regions with respect to the first and second substance, and the saturation pulse has the first and second spectral peaks for one of the regions and only a single spectral peak for the other of the regions. For example, a body can be divided into regions that have a predominantly fat content or a predominantly water content. This makes it possible to define a water region and a fat region. For the water region with only a low fat content, it may be sufficient for the saturation pulse to have only a single spectral peak. Such a saturation pulse can be calculated with reduced effort. In the fat region, however, where the fat content predominates, it may be useful to equip the saturation pulse with two or more spectral peaks for fat saturation.Accordingly, calculating the saturation pulse for the fat range becomes somewhat more complex. However, since multiple spectral peaks for fat saturation do not need to be used in both ranges, overall computing time for calculating the saturation pulse across all ranges can be saved.

[0025] The first and second spectral peaks in one region (e.g., the fat region) may have a different resonance frequency than the single spectral peak for the other region (e.g., the water region). For example, the spectral peak for fat saturation in the water region may be at -3.4 ppm, while the two spectral peaks for fat saturation in the fat region may be at -3.0 ppm and -3.6 ppm. Thus, with the two spectral peaks, fat saturation can be achieved over a broader spectral range. This is particularly important for the fat region, as it would otherwise be dominated by fat signals in the MR image.

[0026] The present invention also includes a method for imaging in magnetic resonance imaging. This method comprises generating a constant magnetic field, namely the B 0 field. Furthermore, an excitation pulse is generated which has a saturation pulse generated according to the method described above. In the present case, the excitation pulse is therefore viewed as an overall pulse containing several alternating components. On the one hand, high-frequency components of the excitation pulse are used to obtain corresponding MR signals from the corresponding relaxations of the spin precisions, and on the other hand, high-frequency saturation components of the excitation pulse, namely the saturation pulse, are used to scatter the MR signals of a second substance (e.g., fat) so that these regions of the object to be examined do not obscure the desired structures.Finally, for imaging, a magnetic resonance signal is acquired in response to the DC magnetic field and the excitation pulse (including the saturation pulse), and an image is generated from the magnetic resonance signal. Due to the saturation of the second substance (e.g., fat), signals from the second substance do not obscure the MR signals from other substances.

[0027] In a special embodiment of the imaging method, one image is acquired for each of several adjacent slices of an object to be examined, and the same saturation pulse is used for each of the adjacent slices, even if the slices have different DC field distributions. A single saturation pulse is therefore used for several slices, even if these slices have different DC field distributions. Not only can the DC field distributions vary regionally, but the mean values ​​of the DC field in the individual slices can also be different. However, due to the multiple saturation peaks in the saturation pulse, sufficient saturation of the second substance can be achieved despite the differences in the DC field in all slices. This eliminates the need to calculate a separate saturation pulse for each slice.

[0028] In a further development, it is automatically decided whether a saturation pulse is to be used for the multiple adjacent slices if the deviation of the DC field in the multiple adjacent slices is smaller than a predetermined value or a value proportional to the spectral width of the saturation pulse. It can therefore be automatically determined whether the saturation pulse can be used for multiple slices and, if applicable, for how many slices. This decision can be made dependent on the regional change in the DC field within a slice and / or on the change in the DC field from slice to slice. Appropriate threshold values ​​can be used for the decision.

[0029] Furthermore, a computer program product is provided which can be called up in an internal memory of a computer and comprises a computer program for carrying out the steps of an above-mentioned method. A computer program is therefore additionally provided with program means which can be loaded directly into a memory unit of a control unit of the MR system in order to carry out the steps of the method described above or described below when the program means are executed in the control unit. Likewise, a computer-readable medium, i.e. an electronically readable data carrier, is provided on which the computer program product of the above type is stored in an executable manner. Electronically readable control information is therefore stored on the data carrier. This control information is designed such that it carries out the method described above or below when the data carrier is used in a control unit of the MR system.

[0030] The above-mentioned object is also achieved according to the invention by a magnetic resonance system comprising a data processing device for specifying or determining a first resonance frequency of an MR spectrum of a first substance, and for specifying or determining a first saturation frequency in a predetermined range around a second resonance frequency of the MR spectrum of a second substance, an RF coil for generating a saturation pulse that leads to substantially no saturation of the first substance at the first resonance frequency, wherein the saturation pulse has a first spectral peak for saturating the second substance at the first saturation frequency, wherein the saturation pulse has a second spectral peak for saturating the second substance at a second saturation frequency different from the first saturation frequency in the predetermined range, wherein the data processing device is designed to determine the saturation pulse such that the saturation pulse, in addition to the first and second spectral peaks, has at least one another spectral peak to saturate the second substance.

[0031] The advantages and refinements outlined in connection with the above-described inventive method also apply analogously to the magnetic resonance system. The magnetic resonance system has corresponding means, such as the data processing device and the RF coil, which are designed to carry out the above-mentioned method steps.

[0032] The features set out above and the features described below may be used not only in the corresponding explicitly set out combinations, but also in other combinations, unless explicitly stated otherwise.

[0033] The invention will now be explained in more detail with reference to the accompanying drawings, in which: FIG 1 shows an MR spectrum with spectral peaks for saturation; FIG 2 shows a schematic flow diagram of some steps of a method according to the invention; FIG 3 shows an MRI image of a back with standard fat saturation; FIG 4 shows a measured B0 field map; FIG 5 shows an MRI image of a back with fat saturation according to the invention; FIG 6 shows an MRI image of a skull with standard fat saturation; FIG 7 shows an MRI image of a skull with fat saturation according to the invention; and FIG 8 shows the basic structure of an MRI system.

[0034] The exemplary embodiments described in more detail below represent preferred embodiments of the present invention. In the figures, identical reference numerals designate identical or similar elements. Furthermore, the figures are schematic representations of various embodiments of the invention. The elements shown in the figures are not necessarily shown to scale. Rather, they are shown in such a way that their function and purpose are understandable to those skilled in the art. The connections shown in the figures between functional units or other elements can also be implemented as indirect connections, wherein a connection can be wireless or wired. Functional units can be implemented as hardware, software, or a combination of hardware and software.

[0035] The invention is based on a method for the spectral saturation of materials or substances such as fat or water, whereby deviations of the constant magnetic field (B 0 field) in the image area - induced, for example, by the system (magnet, eddy currents) or the patient (anatomy, e.g., in the neck) - are taken into account in such a way that the correct target frequency, possibly shifted with respect to B 0, is saturated at each location. The method is based on the determination of the B 0 field map (see FIG 4 ) by measurement and / or simulation and on the other hand on the dynamic excitation pulses, which are calculated, for example, in a so-called PTX framework (parallel transmission technology).

[0036] The problem of conventional fat saturation can be FIGS 3 and 4 be recognized. FIG 3shows an MRI image of a patient's back. Good fat saturation can be achieved in the central region of the spine, as evidenced by the dark areas that reveal lighter structures. However, in the upper region of the image (see arrow), fat saturation is insufficient. The fatty tissue produces MR signals, resulting in gray areas that obscure spinal structures. This is caused by changes in the B0 field or inhomogeneities that arise from the system or from the patient.

[0037] FIG 4shows a B0 field map revealing the inhomogeneities. Particularly in the upper left area of ​​the B0 field map, significant deviations from the mean magnetic field strength of the B0 field are evident. These lead to a shift in the fat peak in the MR spectrum, which is why only low fat saturation can be achieved in the corresponding image area. Thus, the goal to date has always been to generate a B0 field map that is as error-free as possible as a basis for calculating the excitation pulse or saturation pulse.

[0038] According to the invention, an alternative approach is proposed which, on the one hand, reduces the influence of deviations in the B 0 field map and, on the other hand, allows advantageous reuse of a pulse (short for excitation pulse including saturation pulse) in adjacent slices.

[0039] So far, only two spectral boundary conditions are specified for the pulse calculation, namely: 1. For water protons, no saturation (0%) may occur at the Larmor frequency (0 Hz deviation), 2. for fat-bound protons, full saturation (100%) should occur at -3.4 ppm compared to the resonance frequency of the water protons (in short: of the water).

[0040] FIG 1shows an MR spectrum 1 of water and fat. Water produces a spectral peak 2 at approximately 4.9 ppm on the scale shown, while spectral peak 3 of fat is at approximately 1.5 ppm. Both peaks 2 and 3 are therefore separated by approximately 3.4 ppm. These values ​​refer to the same values ​​of the B0 field in both the water and fat regions. However, if the values ​​of the constant magnetic field B0 differ between the water and fat regions, peaks 2 and 3 can shift relative to each other. Therefore, in a homogeneous magnetic field, full fat saturation can be achieved with a saturation pulse at -3.4 ppm relative to the resonant frequency of the water protons. However, in an inhomogeneous magnetic field, when the resonant frequency of fat peak 3 is no longer 3.4 ppm away from the resonant frequency of water peak 2, full saturation cannot be achieved with a saturation peak at -3.4 ppm.However, the goal should be to achieve at least 80 to 100% saturation of the fat (generally the second substance). For this reason, the spectral width of the saturation pulse is increased.

[0041] In FIG 1For the case of a homogeneous magnetic field B 0 , a fat saturation pulse 4 with a spectral peak at -3.4 ppm compared to water is shown. Saturation peak 4 is merely drawn as a spectral line, but it is intended to represent a spectral peak. To prevent a shift in the fat spectrum, the saturation pulse is broadened by adding a further spectral peak 5, for example, at -3.0 ppm (as an additional saturation frequency). The additional spectral peak 5 or the additional saturation frequency is therefore located, for example, in a predetermined range of 0.4 ppm around the resonance frequency of fat peak 3. If spectral peak 3 of fat then moves closer to spectral peak 2 of water due to an inhomogeneity of the B 0 field, the second saturation peak 5 may again achieve a higher saturation than the first saturation peak 4.For example, the following condition is added for the calculation of the excitation or saturation pulse: 3. for fat-bound protons, preferably a complete saturation (100%) at -3.0 ppm.

[0042] However, to cover the case where, for example, spectral peak 3 of fat migrates further away from spectral peak 2 of water, the saturation pulse will be additionally equipped with a third saturation peak 6. This is located, for example, at -3.6 ppm (as the second saturation frequency) compared to spectral peak 2 of water. Accordingly, the following additional condition could be introduced for the calculation of the excitation pulse or saturation pulse: 4. For fat-bound protons, full saturation (100%) is preferably at -3.6 ppm.

[0043] In this way, one or more saturation peaks can be inserted into the saturation pulse at any distance from the spectral peak of water (generally: the first substance) for fat saturation (generally: saturation of the second substance). Specifically, additional saturation peaks can also be inserted between or outside the two saturation peaks 4, 5 at corresponding saturation frequencies.

[0044] Fat to be saturated has peaks in the MR spectrum at -3.3 ppm, -2.5 ppm, +0.7 ppm, -3.7 ppm, -1.8 ppm, and -0.4 ppm relative to water. Covering all peaks is not possible with conventional fat saturation methods. Therefore, the present inventive method can be used to design a spectral fat pulse that can cover one, several, or all peaks of this spectrum.

[0045] This has the following effects: If there are erroneous deviations in the B0 field map used for pulse calculation that are smaller than the "width" of the saturation pulse (in the example above, from -3.0 ppm to -3.6 ppm), a more robust fat saturation can be achieved despite the deviations. If, in multi-slice acquisitions, different B0 field maps are present in the slices, with different distributions between the slices, it may still be possible to use a single saturation pulse or excitation pulse for multiple slices. This is possible if the deviation of the B0 field at a position does not differ more than the "width" of the saturation pulse. The proposed method also includes an algorithm that can determine, based on the measured B0 field maps across multiple slices, whether a new pulse calculation is required for a slice. The pulse calculation takes longer due to the added boundary conditions.As a first approximation, it can be assumed that the computation time increases linearly with the number of spectral boundary conditions. However, this increase can be compensated, especially in multi-slice measurements, by requiring fewer pulses or only a single pulse to be computed.

[0046] The method can be designed so that the spectral width of the entire saturation pulse can either be specified by the user or automatically determined based on other environmental variables such as B 0 field strength, body region, etc. For example, a second saturation peak 5 is automatically used alongside the first saturation peak 4 if the B 0 field exhibits a certain inhomogeneity. If the inhomogeneity is greater, it is automatically determined that the saturation pulse is equipped with a third saturation peak 6. The use of the saturation peaks and their order can, of course, be chosen arbitrarily.

[0047] Optionally, a static B0 field map of the MR scanner is determined, at least for the volume to be scanned. The B0 field map can be stored, for example, in a memory of the MRI scanner's control system and retrieved from there by the control system. Retrieval from an external storage device or via a network is also conceivable.

[0048] The B0 field map can, for example, already be provided by simulation during design or by measurement with a field camera in the manufacturing process.

[0049] Additionally or alternatively, the controller can measure a B0 field map prior to the measurement using a preferably fast sequence, which shows the B0 changes induced by the patient, at least in the examination volume. It would also be possible for the controller itself to provide the B0 field map through simulation, possibly with simplified assumptions.

[0050] Depending on the spatially resolved B0 field map, a spectral shift of the saturation pulse can now be achieved at specific locations on the B0 field map. This has the effect that the spatially variable (dynamic) saturation pulse compensates for the inhomogeneities of the B0 field.

[0051] In a further embodiment, the object to be examined or the B0 field map can additionally be spatially clustered into fat regions (e.g., voxels > 50% fat) and water regions (voxels > 50% water). The spectral extensions described above can then be restricted to the fat regions. This means that an exemplary third and fourth condition could be a combination of a spectral composition of full saturation at -3.4 ppm for water regions and full saturation at -3.0 ppm and -3.6 ppm for fat regions. This concentrates the available spatial-spectral degrees of freedom primarily on the problematic zones instead of using them up for a respective spatially global suppression. A fat / water mapping can be taken from a preceding Dixon measurement or, preferably, from a B0 mapping sequence.

[0052] FIG 2shows a partial schematic of the sequence of a method according to the invention in one embodiment. For this purpose, in an optional step S1, the examination region or the B0 field map is first clustered into a region of the first substance (e.g., water) and a region of the second substance (e.g., fat). For the regions of the second substance (hereinafter referred to as fat), in a step S2, a first resonance frequency of the MR spectrum of the water (hereinafter referred to as the first substance) is specified or determined. In a further step S3, a first saturation frequency is specified or determined in a predetermined range around a second resonance frequency of the MR spectrum of fat (hereinafter referred to as the second substance). Furthermore, in a step S4, a second saturation frequency in the predetermined range is specified or determined (e.g., by measurement) for saturating the fat.

[0053] Depending on the clustering in step S1, the method can skip to step S5 instead of jumping to step S2. In step S5, only a single saturation frequency or a single saturation peak is specified or determined. For example, in a water area of ​​the object under investigation, it is sufficient to use only a single fat peak for fat saturation.

[0054] In a step S6, which follows step S4 or S5, a saturation pulse, which can be part of an excitation pulse, is generated. This saturation pulse does not lead to saturation of the water at the first resonance frequency of the water. In addition, the saturation pulse has a first spectral peak (first saturation peak at a first saturation frequency) for saturating the fat at the first saturation frequency, e.g., a resonance frequency of the fat. Finally, the saturation pulse also has a second spectral peak (second saturation peak at a second saturation frequency) for saturating the fat at the second saturation frequency, which is different from the first saturation frequency, in the predetermined range. Under certain circumstances, one or more parameters P of the MRI system or from the environment (e.g., the object to be examined) are also taken into account to generate the saturation pulse in step S6.In a subsequent step S7, an excitation pulse for the MR analysis is generated. This step S7 can be performed together with step S6.

[0055] Subsequently, in step S8, a magnetic resonance signal is acquired in response to the excitation pulse or saturation pulse. Finally, in step S9, an image is generated from the magnetic resonance signal. However, only steps S2, S3, and S6 are of primary importance for the present invention.

[0056] The FIGS 3 to 5 demonstrate the advantage of the method according to the invention using an MRI image of the spine. As in connection with the FIGS 3 and 4 As already explained above, certain structural areas in the area of ​​the arrow of FIG 3 by fatty tissue, as only a conventional fat saturation method was used. In the example of FIG 5In contrast, the method according to the invention uses an additional spectral peak for fat saturation. In this case, the structural areas of the spine can be FIG 5 The arrow shown can be better recognized because a high fat saturation was achieved.

[0057] Similar results can be obtained from the skull images of FIGS 6 and 7 be confirmed. When recording FIG 6 In turn, conventional fat saturation was achieved. FIG 7 However, the fat saturation according to the invention was achieved with at least one additional spectral peak. As the eye sockets marked by arrows show, fat content in the tissue behind the eye obscures the optic nerve. FIG 7 The optic nerve, however, is clearly visible because it is not covered by fatty tissue in the eye socket.

[0058] FIG 8shows an embodiment of a magnetic resonance system for improved spectral fat saturation in cross-section, comprising an MRI device 11, a control unit or data processing device 19 and an input unit 21 as well as an output unit 22. A computer-readable medium 20 (e.g. DVD, USB stick or the like) can be processed by the data processing device 19. In particular, a computer program is stored on the computer-readable medium 20, with which the steps of the FIG 2 can be triggered or controlled by the method shown. The data processing device 19 is accordingly designed to control or carry out these method steps.

[0059] In the following example, the MRI device 11 comprises a cryostat 12 containing a magnet made of superconducting material. Typically, such a cryostat 12 is filled with liquid helium to cool the magnet below the transition temperature and convert it into the superconducting state. A superconducting magnet is required to generate a high static magnetic field B 0 17 up to a strength of several Tesla in a large recording volume 13. The cryostat 12 and the magnet are typically essentially designed as a hollow cylinder, in whose hollow interior the static magnetic field B 0 17 can be generated. Furthermore, the MRI device 11 has RF coils 18 that surround the recording volume 13.

Claims

1. Method for generating a saturation pulse for spectral saturation in magnetic resonance tomography, wherein the method has steps as follows: - specifying or ascertaining (S2) a first resonance frequency of an MR spectrum of a first substance, - specifying or ascertaining (S3) a first saturation frequency in a predefined range around a second resonance frequency of the MR spectrum of a second substance, and - generating (S6) a saturation pulse which causes no saturation of the first substance at the first resonance frequency, wherein - the saturation pulse has a first spectral peak (4) for saturation of the second substance at the first saturation frequency, wherein - the saturation pulse has a second spectral peak (5) for saturation of the second substance at a second saturation frequency, which second saturation frequency differs from the first saturation frequency, in the predefined range, characterised in that the saturation pulse has, in addition to the first and second spectral peak, at least one further spectral peak (6) for saturation of the second substance.

2. Method according to claim 1, wherein the first substance is water and the second substance is fat.

3. Method according to one of the preceding claims, wherein each spectral peak (4, 5, 6) of the saturation pulse corresponds to a peak of an MR spectrum of the second substance.

4. Method according to one of the preceding claims, wherein before the generation of the saturation pulse, a spatially resolved B0 field map of a magnetic static field or B0 field in the image region of the magnetic resonance tomography is ascertained and, as a function of the spatially resolved B0 field map, a spectral shift of the saturation pulse is effected at respective locations of the B0 field map.

5. Method according to one of the preceding claims, wherein a parameter value (P) of the magnetic resonance tomography is determined or automatically ascertained and, as a function of the parameter value (P), a spectral location of the second spectral peak (5) is automatically ascertained.

6. Method according to claim 5, wherein the parameter value relates to a fluctuation in the field strength of a magnetic static field or a region of an object (15) that is to be examined by means of the magnetic resonance tomography.

7. Method according to one of the preceding claims, wherein a volume that is to be examined by means of the magnetic resonance tomography is clustered (S1) into two regions in relation to the first and second substance, and the saturation pulse has the first and the second spectral peak (4, 5) for one of the regions and only a single spectral peak for the other region.

8. Method according to claim 7, wherein the single spectral peak differs from the first and the second spectral peak (4, 5) in respect of its resonance frequency.

9. Method for imaging in magnetic resonance tomography, wherein the method has steps as follows: - generating a magnetic static field or B0 field in the image region of the magnetic resonance tomography, - generating (S7) a saturation pulse according to the method according claim 1 for saturation of the second substance and generation of an excitation pulse for excitation of the first substance, - capturing (S8) a magnetic resonance signal as a response to the magnetic static field and the excitation pulse, and - generating (S9) an image from the magnetic resonance signal.

10. Method according to claim 9, wherein an image is captured for each of a plurality of adjacent slices of an object (15) that is to be examined, and the same saturation pulse is used in each case for the plurality of adjacent slices, even though different static field distributions are present in the slices.

11. Method according to claim 9 or 10, wherein it is automatically decided that a saturation pulse will be used for the plurality of adjacent slices if a variation of the static field in the plurality of adjacent slices is less than a specified amount or less than an amount which is proportional to the spectral width of the saturation pulse.

12. Computer program product, comprising commands which, when the program is executed by a processor of a magnetic resonance system, cause the magnetic resonance system to carry out the method steps according to one of the claims 1 to 11.

13. Computer-readable medium (20) on which the computer program product according to claim 12 is executably stored.

14. Magnetic resonance system having - a data processing facility (19) for specifying or ascertaining (S2) a first resonance frequency of an MR spectrum of a first substance, and for specifying or ascertaining (S3) a first saturation frequency in a predefined range around a second resonance frequency of the MR spectrum of a second substance, - an RF coil (18) for generating a saturation pulse which causes no saturation of the first substance at the first resonance frequency, wherein - the saturation pulse has a first spectral peak (4) for saturation of the second substance at the first saturation frequency, - the saturation pulse has a second spectral peak (5) for saturation of the second substance at a second saturation frequency, which differs from the first saturation frequency, in the predefined range, characterised in that the data processing facility (19) is configured to determine the saturation pulse such that, in addition to the first and second spectral peak, the saturation pulse has at least one further spectral peak (6) for saturation of the second substance.