METHOD AND APPARATUS FOR ACQUIRING MAGNETIC RESONANCE DATA

Abstract

In a method and apparatus for acquiring magnetic resonance data from a slice package composed of multiple measured slices as a target volume by executing a measuring sequence, prior to each scan of one of the measured slices, the measuring sequence includes a preparation pulse associated with the measured slice for signal suppression of a type of saturation molecule. This said preparation pulse acts on the entire target volume, and a pulse parameter of the preparation pulse is chosen for a measured slice group, composed of at least one measured slice, as a function of resonance information on the contiguous partial volume covered by the measured slice group. The pulse frequency and/or the pulse bandwidth are chosen as pulse parameters as a function of resonance information describing at least the resonance frequencies of the type of saturation molecule and a type of target molecule, the magnetic resonance data of which is to be acquired, in the contiguous partial volume of the target volume that covers the measured slice group.

Claims

1. A method for acquiring magnetic resonance data from a slice stack comprising: operating a magnetic resonance data acquisition scanner to acquire magnetic resonance data from a slice stack, comprising a plurality of measurement slices, as a target volume by executing a measurement sequence, with each acquisition of magnetic resonance data from each measurement slice constituting a scan; operating the magnetic resonance data acquisition scanner to radiate, prior to each scan of each individual measurement slice, a preparation pulse in said measurement sequence that is associated with the respective individual measurement slice, said preparation pulse suppressing a magnetic resonance signal from a type of saturation molecule, said preparation pulse acting on an entirety of said target volume; operating said magnetic resonance data acquisition scanner to select at least one of a pulse frequency and a pulse bandwidth as at least one pulse parameter of said preparation pulse for a measurement slice group, comprising at least one of said measurement slices, dependent on resonance information of a contiguous partial volume covered by said measurement slice group, said resonance information describing at least resonance frequencies of said type of said saturation molecule and a type of target molecule from which said magnetic resonance data are to be acquired in said contiguous partial volume of said target volume; and providing said magnetic resonance data to a processor and making the magnetic resonance data available from the processor in electronic form, as a data file.

2. A method as claimed in claim 1 comprising selecting said pulse bandwidth as said at least one pulse parameter, and selecting said pulse bandwidth dependent on a chemical shift in said partial volume described by said resonance information.

3. A method as claimed in claim 1 comprising selecting said resonance information from the group consisting of a resonance frequency spectrum of said type of said saturation molecule, a resonance frequency spectrum of said type of target molecule, a peak frequency of the resonance frequency spectrum of the type of saturation molecule, a peak frequency of the resonance frequency spectrum of said type of target molecule, a peak width of said resonance frequency spectrum of said type of saturation molecule, and a peak width of said resonance frequency spectrum of said type of target molecule.

4. A method as claimed in claim 1 comprising, in a processor, determining said resonance frequency infoimation from B0 field maps for said type of target molecule and said type of saturation molecule determined for respective target volumes in an acquisition position.

5. A method as claimed in claim 1 comprising, in a processor, determining said resonance frequency information dependent on an anatomical model, of at least said target volume, that describes an influence on magnetic fields.

6. A method as claimed in claim 1 comprising selecting a time order in which to scan said measurement slice groups so that, when target molecules of a further measurement slice group are excited outside of a scan of said further measurement slice group, the excitation of said further measurement slice group has decayed before a start of a scan of said further measurement slice group, or magnetic resonance data from said further measurement slice group have already been acquired.

7. A method as claimed in claim 6 comprising in said time order, increasing the resonance frequencies of the type of saturation molecule associated with the measurement slice group at a start of said time order with a minimum value, from said minimum value, if said resonance frequency of said type of saturation molecule is lower than the resonance frequency of the target molecule, and reducing said resonance frequency at the start of said time order with a maximum value, from said maximum value, if the resonance frequency of the type of saturation molecule is higher than the resonance frequency of the type of target molecule, with each of said increase and said decrease being monotonic or as a general tendency across the measurement slice group in said sequence.

8. A method as claimed in claim 1 comprising, selecting said at least one pulse parameters for said measurement slice group, using resonance information of at least one further measurement slice group, adjacent to the measurement slice group.

9. A method as claimed in claim 8 comprising adding said resonance information from said further measurement slice group with a lower weight to the resonance information of said measurement slice group.

10. A method as claimed in claim 8 comprising using said resonance information from said further measurement slice group in an optimization algorithm, with a weighted penalty term, to deteimine said pulse parameters.

11. A method as claimed in claim 1 wherein said type of saturation molecule and said type of target molecule are selected from the group consisting of fat molecules and water molecules.

12. A magnetic resonance apparatus comprising: a magnetic resonance data acquisition scanner; a control computer configured to operate said a magnetic resonance data acquisition scanner to acquire magnetic resonance data from a slice stack, comprising a plurality of measurement slices, as a target volume by executing a measurement sequence, with each acquisition of magnetic resonance data from each measurement slice constituting a scan; said control computer being configured to operate said magnetic resonance data acquisition scanner to radiate, prior to each scan of each individual measurement slice, a preparation pulse in said measurement sequence that is associated with the respective individual measurement slice, said preparation pulse suppressing a magnetic resonance signal from a type of saturation molecule, said preparation pulse acting on an entirety of said target volume; said control computer being configured to operate said magnetic resonance data acquisition scanner to select at least one of a pulse frequency and a pulse bandwidth as at least one pulse parameter of said preparation pulse for a measurement slice group, comprising at least one of said measurement slices, dependent on resonance information of a contiguous partial volume covered by said measurement slice group, said resonance information describing at least resonance frequencies of said type of said saturation molecule and a type of target molecule from which said magnetic resonance data are to be acquired in said contiguous partial volume of said target volume; and said control computer being configured to provide said magnetic resonance data to a processor and make the magnetic resonance data available from the processor in electronic form, as a data file.

13. A non-transitory, computer-readable data storage medium encoded with programming instructions, said storage medium being loaded into a control computer of a magnetic resonance apparatus, that comprises a magnetic resonance data acquisition scanner, said programming instructions causing said control computer to: operate the magnetic resonance data acquisition scanner to acquire magnetic resonance data from a slice stack, comprising a plurality of measurement slices, as a target volume by executing a measurement sequence, with each acquisition of magnetic resonance data from each measurement slice constituting a scan; operate the magnetic resonance data acquisition scanner to radiate, prior to each scan of each individual measurement slice, a preparation pulse in said measurement sequence that is associated with the respective individual measurement slice, said preparation pulse suppressing a magnetic resonance signal from a type of saturation molecule, said preparation pulse acting on an entirety of said target volume; operate said magnetic resonance data acquisition scanner to select at least one of a pulse frequency and a pulse bandwidth as at least one pulse parameter of said preparation pulse for a measurement slice group, comprising at least one of said measurement slices, dependent on resonance information of a contiguous partial volume covered by said measurement slice group, said resonance information describing at least resonance frequencies of said type of said saturation molecule and a type of target molecule from which said magnetic resonance data are to be acquired in said contiguous partial volume of said target volume; and provide said magnetic resonance data to a processor and making the magnetic resonance data available from the processor in electronic form, as a data file.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is a flowchart of an exemplary embodiment of the inventive method.

[0027] FIG. 2 shows theoretical, ideal resonance frequency spectra for fat and water.

[0028] FIG. 3 shows real resonance frequency spectra for fat and water in a measured slice group affected by field inhomogeneities.

[0029] FIG. 4 shows a characteristic of peak frequencies across different measured slices to be acquired.

[0030] FIG. 5 shows peak frequency characteristics in the case of a changed acquisition sequence.

[0031] FIG. 6 is a block diagram of an inventive magnetic resonance apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] FIG. 1 is a flowchart of an exemplary embodiment of the present invention, as can be employed in the context of acquiring magnetic resonance data with fat saturation, if the patient's target volume to be acquired is divided into a plurality of slice packages containing measured slices and the measured slices are not acquired simultaneously, in particular individually one after the other. In general a preparation pulse (saturation pulse) is emitted prior to the acquisition of each measured slice, so as to selectively chemically excite the fat molecules as a type of saturation molecule and bring them to saturation such that, during the following excitation pulses, the measuring sequence for the measured slice ideally excites only the water molecules as a type of target molecule and delivers a signal. The preparation pulse and the excitation pulse consequently have a frequency shift that is dictated by the chemical shift between water and fat.

[0033] In a step S1 of the method according to FIG. 1, acquisition parameters are provided, which may have been entered by a user directly at a user interface of the magnetic resonance apparatus and/or are given by selecting a measuring protocol. The acquisition parameters describe the subsequent acquisition procedure initially in an abstract manner that is comprehensible for the user, by for example describing the target volume, the division thereof into measured slices, the measuring sequence to be used, the fat saturation technique to be used, etc., by virtue of the acquisition parameters. The acquisition parameters in step S1 are, for preparation of the actual acquisition procedure, converted into specific control parameters for the components of the magnetic resonance device, in particular a gradient coil arrangement, a radio frequency coil arrangement, of local coils, shim coils, etc. The inventive method relates to part of this procedure.

[0034] The exemplary embodiment described herein initially uses resonance information 1 in step S2 to determine specific pulse parameters of the preparation pulse, in this case at least the pulse frequency and the pulse bandwidth, wherein other pulse parameters can of course also be adjusted, for example the pulse amplitude, the actuation of individual antennas of the radio frequency coil arrangement via which the preparation pulse is emitted, etc. Pulse parameters adjusted in this case are determined for each individual measured slice or the preparation pulse associated therewith, i.e. the resonance information 1 is present in a manner specific to each measured slice and describes the resonance behavior both of the fats as a type of saturation molecule and also of the water molecules as a type of target molecule. In this case this is given in the form of resonance frequency spectra for the respective measured slice, consequently of resonance frequency distributions that indicate both for the water molecules and for the fat molecules how much signal response to a radio frequency pulse is likely at which frequencies. It has been shown that the relative position of the peak frequencies of fat and water, and consequently the chemical shift, can also change from measured slice to measured slice. In this case the peak frequency is the resonance frequency at which the maximum signal response of the respective type of molecule is expected in the resonance frequency spectrum. A peak width as well can be defined in the known manner, such that it is conceivable in exemplary embodiments to indicate merely the peak width and the peak frequencies as resonance information 1. However, the entire characteristic of the resonance frequency spectrum is preferably taken into consideration, as will be explained in greater detail in respect of FIG. 2 and FIG. 3.

[0035] FIG. 2 shows a resonance frequency spectrum 2 for water and a resonance frequency spectrum 3 for fat in an ideally homogeneous constant magnetic field in a measured slice. The resonance frequency spectra 2, 3 are here indicated via the chemical shift compared to the nominal Larmor frequency of the magnetic resonance device, wherein the positive shift extends leftward, i.e. the fat peak frequency 4 is less than the water peak frequency 5.

[0036] It is apparent that both resonance frequency spectra 2, 3 are clearly spaced apart and separated, such that a saturation range 6 that is to be affected by the preparation pulse can readily be defined.

[0037] FIG. 3 shows the effects of magnetic inhomogeneities on the resonance frequency spectra 2, which become wider and where appropriate may also change their position relative to one another. It is apparent that only one critical frequency range 7 occurs, in which the resonance frequency spectra 2, 3 overlap, and consequently both water molecules and fat molecules can be excited by a corresponding radio frequency pulse.

[0038] An optimization procedure performed in step S2 to determine optimum pulse parameters, and which in the corresponding measured slice is aimed at the saturation of the magnetization of as many fat molecules as possible with as little loss as possible of water signals as a result of unwanted water saturation, also consequently takes into consideration this critical frequency range 7 given by the overlap region of the resonance frequency spectra 2, 3. In this way the specific relationships within the individual measured slices can be taken into consideration and the saturation range 6 can be optimally determined on the basis of specific measured slices in a particularly effective manner.

[0039] A further circumstance, which arises from the fact that the preparation pulse acts on the entire target volume, will be explained for further measures to improve the quality of the magnetic resonance data to be acquired.

[0040] To this end, FIG. 4 shows the characteristics 8, 9 of peak frequencies 4, 5 across measured slices M1 to M16 to be acquired, wherein the characteristic 8 relates to the fat molecules, and the characteristic 9 relates to the water molecules. Not only can it be seen from these characteristics 8, 9, as has already been indicated, that the chemical shift can also change for spatially adjacent measured slices when the peak frequencies 4, 5 are considered, but it also becomes clear that in regions of major change there is a risk that, as a result of a preparation pulse of a measured slice, for example the measured slice M1, water molecules can inadvertently be excited and saturated in a further subsequent measured slice, for example the measured slice M2, in the acquisition sequence, such that their signal may be lost. It is possible, in another exemplary embodiment, to seek to prevent this, by taking into consideration penalty terms from the resonance information for further subsequent measured slices in the acquisition sequence in the context of the optimization procedure in step S2. However, this is less preferred, since then damage may occur in respect of the fat saturation in the measured slice to be currently acquired.

[0041] Consequently an alternative and preferred variant, also used in the exemplary embodiment illustrated in FIG. 1, provides for the acquisition sequence of the measured slices M1, . . . , M16 to be adjusted adroitly, in order to prevent this, wherein once again the resonance information 1 is taken into consideration.

[0042] According to FIG. 1 this also occurs in step S3, wherein the acquisition sequence of the individual measured slices M1 to M16 is chosen such that in the acquisition sequence the fat saturation frequency, in other words the fat peak frequency 4, develops such that it increases monotonically from a minimum value to a maximum value. This is illustrated in greater detail in FIG. 5 following on from the example in FIG. 4.

[0043] In order to achieve the monotonically increasing behavior, the acquisition sequence M6, M5, M3, M2, M1, M4, M7, M8 . . . M16 is consequently chosen. The corresponding, resulting peak frequency characteristics 8′, 9′ across the cited acquisition sequence are shown in FIG. 5 and for both peak frequencies 4, 5 exhibit monotonically increasing behavior. This effectively prevents unwanted water saturation occurring in measured slices that are not acquired until later in the acquisition sequence.

[0044] In step S4 (see FIG. 1 once again) the magnetic resonance data is then acquired using the measured-slice-specific pulse parameters determined in step S2 and the acquisition sequence of measured slices M1-M16 determined in step S3. Although the present case has been described with respect to individual measured slices, for which in each case the acquisition position and the preparation pulse are adjusted, it is of course also possible for multiple measured slices to be combined in one measured slice, which is appropriate, for example, if breath-holding techniques are being used. Then, for example, the measured slices that are acquired during a single breath-holding procedure can always be combined to form a measured slice group.

[0045] FIG. 6 shows a schematic diagram of an inventive magnetic resonance apparatus 10 which, as is known in principle, has a scanner 11 that contains the basic field magnet for generating the constant magnetic field. Surrounding the cylindrical patient aperture 12, into which a patient can be introduced by a patient couch (not shown), are a radio frequency coil arrangement and a gradient coil arrangement (not shown). The preparation pulses and the excitation pulses are emitted via the radio frequency coil arrangement. The local coils designed to transmit can also of course be used for this purpose, as is known in principle. The operation of the magnetic resonance apparatus 10 is controlled by a controller 13, which is also designed to perform the inventive method. To this end, the controller 13 can have not only a sequence control processor, known in principle, for controlling the components for acquiring magnetic resonance data, but also a pulse parameter adjustment processor and an acquisition sequence adjustment processor.

[0046] The methods described herein can be the controller executed by the controller 13, when an electronically readable data carrier (not shown) with electronically readable control information (code) stored thereon is loaded into the controller 13 of the magnetic resonance apparatus 10. The program code cause the controller 13 to carry out the method as described.

[0047] Although modifications and changes may be suggested by those skilled in the art, it is the intention of the Applicant to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of the Applicant's contribution to the art.