SPECTRAL SATURATION IN MAGNETIC RESONANCE TOMOGRAPHY

Abstract

In order to improve fat saturation in magnetic resonance technology (MRT) methods, a method for spectral saturation that includes specifying or ascertaining a first resonance frequency of a first substance and a first saturation frequency for a second substance is provided. A saturation pulse that causes no saturation of the first substance at the first resonance frequency is generated. The saturation pulse has a first spectral peak for saturation of the second substance at the first saturation frequency and a second spectral peak at a second saturation frequency. This allows a widening of a spectral saturation bandwidth of a dynamic saturation.

Claims

1. A method for generating a saturation pulse for spectral saturation in magnetic resonance tomography, the method comprising: specifying or ascertaining a first resonance frequency of a magnetic resonance (MR) spectrum of a first substance; specifying or ascertaining a first saturation frequency in a predefined range around a second resonance frequency of the MR spectrum of a second substance; and generating a saturation pulse that causes no saturation of the first substance at the first resonance frequency, wherein the saturation pulse has a first spectral peak for saturation of the second substance at the first saturation frequency, and wherein the saturation pulse has a second spectral peak for saturation of the second substance at a second saturation frequency in the predefined range, the second saturation frequency differing from the first saturation frequency.

2. The method of claim 1, wherein the saturation pulse has, in addition to the first spectral peak and the second spectral peak, at least one further spectral peak for saturation of the second substance.

3. The method of claim 1, wherein the first substance is water, and the second substance is fat.

4. The method of claim 1, wherein each spectral peak of the saturation pulse corresponds to a peak of an MR spectrum of the second substance.

5. The method of claim 1, wherein before generating the saturation pulse, the method further comprises: ascertaining a spatially resolved B.sub.0 field map of a magnetic static field of the magnetic resonance tomography; and effecting a spectral shift of the saturation pulse at respective locations of the B.sub.0 field map as a function of the spatially resolved B.sub.0 field map.

6. The method of claim 1, further comprising: determining or automatically ascertaining a parameter value of the magnetic resonance tomography; and automatically ascertaining a spectral location of the second spectral peak as a function of the parameter value.

7. The method of claim 6, wherein the parameter value relates to a fluctuation in a field strength of a magnetic static field or a region of an object that is to be examined by the magnetic resonance tomography.

8. The method of claim 1, wherein a volume that is to be examined by the magnetic resonance tomography is clustered into two regions in relation to the first substance and the second substance, and wherein the saturation pulse has the first spectral peak and the second spectral peak for one of the two regions and only a single spectral peak for the other of the two regions.

9. The method of claim 8, wherein the single spectral peak differs from the first spectral peak and the second spectral peak with respect to a resonance frequency.

10. A method for imaging in magnetic resonance tomography, the method comprising: generating a magnetic static field; generating an excitation pulse having a saturation pulse for spectral saturation in the magnetic resonance tomography, generating the saturation pulse comprising: specifying or ascertaining a first resonance frequency of a magnetic resonance (MR) spectrum of a first substance; specifying or ascertaining a first saturation frequency in a predefined range around a second resonance frequency of the MR spectrum of a second substance; and generating the saturation pulse that causes no saturation of the first substance at the first resonance frequency, wherein the saturation pulse has a first spectral peak for saturation of the second substance at the first saturation frequency, and wherein the saturation pulse has a second spectral peak for saturation of the second substance at a second saturation frequency in the predefined range, the second saturation frequency differing from the first saturation frequency; capturing a magnetic resonance signal as a response to the magnetic static field and the excitation pulse; and generating an image from the magnetic resonance signal.

11. The method of claim 10, further comprising capturing an image for each slice of a plurality of adjacent slices of an object that is to be examined, wherein a same saturation pulse is used in each case for the plurality of adjacent slices, even though different static field distributions are present in the plurality of adjacent slices.

12. The method of claim 10, further comprising automatically deciding that a saturation pulse will be used for the plurality of adjacent slices when a variation of the static field in the plurality of adjacent slices is less than a specified amount or less than an amount that is proportional to a spectral width of the saturation pulse.

13. In a non-transitory computer-readable storage medium that stores instructions executable by one or more processors to generate a saturation pulse for spectral saturation in magnetic resonance tomography, the instructions comprising: specifying or ascertaining a first resonance frequency of a magnetic resonance (MR) spectrum of a first substance; specifying or ascertaining a first saturation frequency in a predefined range around a second resonance frequency of the MR spectrum of a second substance; and generating a saturation pulse that causes no saturation of the first substance at the first resonance frequency, wherein the saturation pulse has a first spectral peak for saturation of the second substance at the first saturation frequency, and wherein the saturation pulse has a second spectral peak for saturation of the second substance at a second saturation frequency in the predefined range, the second saturation frequency differing from the first saturation frequency.

14. The non-transitory computer-readable storage medium of claim 13, wherein the saturation pulse has, in addition to the first spectral peak and the second spectral peak, at least one further spectral peak for saturation of the second substance.

15. The non-transitory computer-readable storage medium of claim 13, wherein the first substance is water, and the second substance is fat.

16. The non-transitory computer-readable storage medium of claim 13, wherein each spectral peak of the saturation pulse corresponds to a peak of an MR spectrum of the second substance.

17. The non-transitory computer-readable storage medium of claim 13, wherein before generating the saturation pulse, the instructions further comprise: ascertaining a spatially resolved B.sub.0 field map of a magnetic static field of the magnetic resonance tomography; and effecting a spectral shift of the saturation pulse at respective locations of the B.sub.0 field map as a function of the spatially resolved B.sub.0 field map.

18. The non-transitory computer-readable storage medium of claim 13, wherein the instructions further comprise: determining or automatically ascertaining a parameter value of the magnetic resonance tomography; and automatically ascertaining a spectral location of the second spectral peak as a function of the parameter value.

19. A magnetic resonance system comprising: a data processing facility configured to: specify or ascertain a first resonance frequency of a magnetic resonance (MR) spectrum of a first substance; and specify or ascertain a first saturation frequency in a predefined range around a second resonance frequency of the MR spectrum of a second substance; and a radio frequency (RF) coil configured to generate a saturation pulse that causes no saturation of the first substance at the first resonance frequency, wherein the saturation pulse has a first spectral peak for saturation of the second substance at the first saturation frequency, and wherein the saturation pulse has a second spectral peak for saturation of the second substance at a second saturation frequency in the predefined range, the second saturation frequency differing from the first saturation frequency.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG. 1 shows a magnetic resonance (MR) spectrum with spectral peaks for the purpose of saturation;

[0033] FIG. 2 shows a schematic flow diagram of an exemplary embodiment of a method;

[0034] FIG. 3 shows a magnetic resonance tomography (MRT) image of a back with standard fat saturation;

[0035] FIG. 4 shows a measured B.sub.0 field map;

[0036] FIG. 5 shows an MRT image of a back with fat saturation according to an embodiment;

[0037] FIG. 6 shows an MRT image of a cranium with standard fat saturation;

[0038] FIG. 7 shows an MRT image of a cranium with fat saturation according to an embodiment; and

[0039] FIG. 8 shows the basic structure of an MRT system.

DETAILED DESCRIPTION

[0040] The exemplary embodiments described in detail below represent embodiment variants. Identical or similar elements in the figures are designated by same reference signs. The figures are also schematic illustrations of various embodiment variants. The elements represented in the figures are not necessarily illustrated to scale. Rather, the elements are depicted such that function and purpose of the elements may be understood by a person skilled in the art. The connections illustrated in the figures between functional units or other elements may also be implemented as indirect connections, where a connection may be wireless or wire-based. Functional units may be implemented as hardware, software, or a combination of hardware and software.

[0041] The present embodiments have, as a starting point, a method for spectral saturation of matter or substances such as fat or water. Variations of the magnetic static field (e.g., B.sub.0 field) in the image region, caused, for example, by the system (e.g., magnet, eddy currents) or the patient (e.g., anatomy, for example in the neck), are taken into consideration so that the correct target frequency is saturated at each location point. The target frequency is shifted relative to B.sub.0 if necessary. The method is based first on the determination of the B.sub.0 field map (see FIG. 4) using measurement and/or simulation; the method is based second on the dynamic excitation pulses that are calculated, for example, in a PTX framework (e.g., parallel transmission technology).

[0042] The problems of conventional fat saturation are shown in FIGS. 3 and 4. FIG. 3 shows a magnetic resonance tomography (MRT) image of the back of a patient. Good fat saturation may be achieved in a central region of the spine, as shown by the dark regions that allow lighter structures to be identified. However, in the upper region of the image (see arrow), the fat saturation is inadequate. The fatty tissue returns MR signals, thereby producing gray regions that conceal structures of the spine. This is caused by changes in the B.sub.0 field or inhomogeneities that are system-related or produced by the patient concerned.

[0043] FIG. 4 shows a B.sub.0 field map from which the inhomogeneities are evident. For example, clear variations from the average magnetic field strength of the B.sub.0 field are apparent in the upper left-hand region of the B.sub.0 field map. These result in a shift of the fat peak in the MR spectrum, and therefore, only low fat saturation may be achieved in the corresponding image region. Until now, the objective has therefore always been to generate a largely error-free B.sub.0 field map as a basis for calculating the excitation pulse or saturation pulse.

[0044] The present embodiments provide an alternative approach, by which both the influence of variations in the B.sub.0 field map may be reduced and further use of a pulse (e.g., abbreviation for excitation pulse including saturation pulse) may be achieved in adjacent slices.

[0045] Until now, only two spectral boundary conditions were specified for pulse calculation: For water protons, no saturation (0%) is to occur at the Larmor frequency (0 Hz variation); and for fat-bound protons, total saturation (100%) may occur at −3.4 ppm relative to the resonance frequency of the water protons (e.g., abbreviated to water).

[0046] Regarding this, FIG. 1 shows an MR spectrum 1 of water and fat. Water returns a spectral peak 2 at approximately 4.9 ppm on the illustrated scale, while the spectral peak 3 of fat lies at approximately 1.5 ppm. Both peaks 2 and 3 therefore have a separation of approximately 3.4 ppm. These values relate to identical values of the B.sub.0 field in both the water region and the fat region. If, however, the values of the magnetic static field B.sub.0 differ in the water region and in the fat region, the two peaks 2 and 3 may shift relative to each other. Therefore, in the case of a homogeneous magnetic field, a total fat saturation may be achieved with a saturation pulse at −3.4 ppm relative to the resonance frequency of the water protons, but in the case of an inhomogeneous magnetic field, if the resonance frequency of the fat peak 3 no longer has a separation of 3.4 ppm from the resonance frequency of the water peak 2, it is not possible to achieve full saturation with a saturation peak at −3.4 ppm. The aim may, however, be to achieve at least 80 to 100% saturation of the fat (e.g., generally the second substance). For this reason, the spectral width of the saturation pulse is increased.

[0047] In FIG. 1, for the case of the homogeneous magnetic field B.sub.0, a fat saturation pulse 4 is shown with a spectral peak at −3.4 ppm relative to water. The saturation peak 4 is marked in simply as a spectral line, but is intended nonetheless to represent a spectral peak. In order to guard against a shift of the fat spectrum, the saturation pulse is widened by adding a further spectral peak 5 (e.g., at −3.0 ppm (as a further saturation frequency)). The further spectral peak 5 or the further saturation frequency is therefore situated, for example, in a predefined range of 0.4 ppm around the resonance frequency of the fat peak 3. If the spectral peak 3 of fat then moves closer to the spectral peak 2 of water due to an inhomogeneity of the B.sub.0 field, it is possible, if applicable by the second saturation peak 5 again, to achieve a higher saturation than may be achieved by the first saturation peak 4. Therefore, for example, the following condition is added for the purpose of calculating the excitation pulse or saturation pulse: For fat-bound protons, total saturation (100%) may occur at −3.0 ppm.

[0048] However, in order to cover the case in which the spectral peak 3 of fat drifts further from the spectral peak 2 of the water, the saturation pulse may alternatively or additionally be provided with a third saturation peak 6. This is situated, for example, at −3.6 ppm (e.g., as a second saturation frequency) relative to the spectral peak 2 of the water. Accordingly, the following additional condition may be introduced for the purpose of calculating the excitation pulse or saturation pulse: For fat-bound protons, total saturation (100%) may occur at −3.6 ppm.

[0049] In this way, for the purpose of fat saturation (e.g., generally saturation of the second substance), one or a plurality of saturation peaks may be inserted into the saturation pulse at a desired separation from the spectral peak of water (e.g., generally first substance). Specifically, additional saturation peaks at corresponding saturation frequencies may therefore also be introduced between or beyond the two saturation peaks 4, 5.

[0050] Fat, which is 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. It is not possible to cover all peaks using conventional fat saturation methods. For this reason, the present method allows the creation of a spectral fat pulse that is able to cover a plurality of peaks or all peaks in this spectrum.

[0051] This has the following effects. If erroneous variations are present in the B.sub.0 field map that is used for the pulse calculation, the variations being nonetheless smaller than the “width” of the saturation pulse (e.g., from −3.0 ppm to −3.6 ppm in the example above), greater resilience with respect to fat saturation may be achieved in spite of the variations. For recordings involving a plurality of slices, if different B.sub.0 field maps are present in the slices and their distribution differs between the slices, it is now possible to use a single saturation pulse or excitation pulse for a plurality of slices. This is possible if the variation of the B.sub.0 field at a position does not differ further than the “width” of the saturation pulse. The method also includes an algorithm in this case, that may determine, based on the measured B.sub.0 field maps over a plurality of slices, whether a new pulse calculation is required for a slice. The pulse calculation lasts longer due to the additional boundary conditions. At first approximation, it may be assumed that the computing time increases linearly with the number of spectral boundary conditions. This longer time may nonetheless be balanced against the fact that overall fewer pulses or only one pulse is to be calculated (e.g., in the case of multi-slice measurements).

[0052] The method may be embodied such that the spectral width of the overall saturation pulse may either be specified by the user or automatically determined based on further environment variables such as B.sub.0 field strength, body region, etc. For example, a second saturation peak 5 is automatically used in addition to the first saturation peak 4 if the B.sub.0 field has a certain inhomogeneity. If the inhomogeneity is greater, it may be automatically determined that the saturation pulse has a third saturation peak 6. The use of the saturation peaks and their sequence may be selected as desired.

[0053] A static B.sub.0 field map of the MR scanner is optionally ascertained at least for the examination volume to be scanned. The B.sub.0 field map may be stored for example in a memory of the control unit for the magnetic resonance tomography system and retrieved from there by the control unit. However, retrieval from an external memory or via a network may also be provided.

[0054] The B.sub.0 field map may already be available as a result of for example simulation at the time of configuration or measurement with a field camera during the production process.

[0055] Additionally or alternatively, before the measurement, the control unit may measure a B.sub.0 field map using, for example, a rapid sequence in order to show the B.sub.0 changes caused by the patient, at least in the examination volume. The control unit itself may also provide the B.sub.0 field map using simulation, using simplified assumptions if necessary.

[0056] As a function of the spatially resolved B.sub.0 field map, it is now possible to effect a spectral shift of the saturation pulse at respective locations of the B.sub.0 field map. This has the effect that the locally variable (e.g., dynamic) saturation pulse compensates for the inhomogeneities of the B.sub.0 field.

[0057] In a further embodiment variant, the object to be examined or the B.sub.0 field map may also be spatially clustered into fat regions (e.g., voxels >50% fat) and water regions (e.g., voxels >50% water). The spectral expansions described above may then be restricted to the fat regions. This provides that an exemplary third condition and an exemplary fourth condition may be a combination of a spectral concurrence of total saturation at −3.4 ppm for water regions and total saturation at −3.0 ppm and −3.6 ppm for fat regions. The available spatial-spectral degrees of freedom are thereby concentrated primarily on the problematic zones, instead of being used for spatially global suppression in each case. A fat mapping or water mapping may be derived from an existing Dixon measurement or, for example, from a B.sub.0 mapping sequence.

[0058] FIG. 2 schematically shows the execution of a method in an embodiment variant. For this, provision is initially made in an optional act S1 for clustering the examination region or the B.sub.0 field map 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 (subsequently designated as fat), provision is made in act S2 for specifying or ascertaining a first resonance frequency of the MR spectrum of the water (subsequently designating the first substance).

[0059] In a further act S3, provision is made for specifying or ascertaining a first saturation frequency in a predefined range around a second resonance frequency of the MR spectrum of fat (this subsequently designating the second substance). In an optional act S4 following thereupon, a second saturation frequency in the predefined range is specified or ascertained (e.g., by measurement) for the purpose of saturating the fat.

[0060] Depending on the clustering in act S1, the method may jump to act S5 instead of jumping to act S2. In act S5, only a single saturation frequency or a single saturation peak is specified or ascertained. For example, in a water region of the object to be examined, it is sufficient to use a single fat peak for the purpose of fat saturation.

[0061] In act S6, which follows act S4 or act S5, a saturation pulse that may be part of an excitation pulse is generated. This saturation pulse causes no saturation of the water at the first resonance frequency of the water. In addition, the saturation pulse has a first spectral peak (e.g., first saturation peak at a first saturation frequency) for saturation of 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 (e.g., second saturation peak at a second saturation frequency) for saturation of the fat at the second saturation frequency in the predefined range. The second saturation frequency is different than the first saturation frequency. If required, one or more parameters P of the MRT system or from the environment (e.g., of the object to be examined) are also taken into consideration for the purpose of generating the saturation pulse in act S6. In a next act S7, provision is made for generating an excitation pulse for the MR analysis. This act S7 may take place jointly with the act S6.

[0062] In act S8 following thereupon, provision is made for capturing a magnetic resonance signal as a response to the excitation pulse or saturation pulse. Finally, an image is generated from the magnetic resonance signal in act S9. For the present embodiments, however, only the acts S2, S3, and S6 are of primary significance.

[0063] FIGS. 3 to 5 show the advantage of the method of the present embodiments with reference to MRT imaging of the spine. As explained above in connection with FIGS. 3 and 4, certain structural regions in the region of the arrow in FIG. 3 are concealed by fatty tissue, since only a conventional fat saturation method was used there. In the example of FIG. 5, however, use was made of an additional spectral peak for the purpose of fat saturation in the method according to the present embodiments. In this case, the structural regions of the spine at the arrow marked in FIG. 5 may be identified more easily because a high fat saturation was achieved there.

[0064] This may be confirmed similarly by the cranial recordings in FIGS. 6 and 7. For the recording in FIG. 6, conventional fat saturation was used. For the recording in FIG. 7, however, the fat saturation of the present embodiments was used with at least one further spectral peak. As shown by the eye sockets indicated by arrows, fat content in the tissue behind the eye conceals the optic nerve. In FIG. 7, however, the optic nerve may easily be identified because the optic nerve is not concealed by fatty tissue in the eye sockets.

[0065] FIG. 8 shows a cross section of an exemplary embodiment of a magnetic resonance system for improved spectral fat saturation, including an MRT device 11, a control unit or data processing facility 19, an input unit 21, and an output unit 22. A computer-readable medium 20 (e.g., DVD, USB stick, or similar) may be processed by the data processing facility 19. For example, stored on the computer-readable medium 20 is a computer program by which the acts of the method illustrated in FIG. 2 may be initiated or controlled. The data processing facility 19 is configured accordingly in order to control or perform these method acts.

[0066] The MRT device 11 in the following example includes a cryostat 12 in which a magnet composed of a superconductive material is situated. Such a cryostat 12 is typically filled with liquid helium in order to cool the magnet to below the transition temperature and take the magnet into the superconductive state. A superconductive magnet is to be provided in order to generate a strong static magnetic field B.sub.0 17 (e.g., a plurality of tesla) in a large recording volume 13. The cryostat 12 and the magnet are typically configured essentially as a hollow cylinder. The static magnetic field B.sub.0 17 may be generated in a hollow interior of the hollow cylinder. Further to this, the MRT device 11 has RF coils 18 that surround the recording volume 13.

[0067] The patient 15 is moved into the recording volume 13 on a patient couch 14 for the purpose of examination by the MRT device 11. In order to record a tomographic image of the patient 15 using the MRT device 11, a local coil 16 may also be used. The RF coils and/or the local coil 16 are typically used for both transmitting and receiving.

[0068] These coils 16 and 18 are also used to transmit the saturation pulses with their saturation peaks or spectral peaks.

[0069] As shown by the exemplary embodiments above, the widening of the spectral saturation bandwidth of the present embodiments allows dynamic saturation because further boundary conditions may be specified for the pulse calculation, defining the saturation effect at desired frequencies and locations. Using this method, the resilience against inhomogeneities in the B.sub.0 field makes it possible to achieve improved stability of the method and faster calculation times for multi-slice measurements if applicable.

[0070] The elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present invention. Thus, whereas the dependent claims appended below depend from only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent. Such new combinations are to be understood as forming a part of the present specification.

[0071] While the present invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made to the described embodiments. It is therefore intended that the foregoing description be regarded as illustrative rather than limiting, and that it be understood that all equivalents and/or combinations of embodiments are intended to be included in this description.