Echo Planar Slice Multiplexing

Abstract

Method for separating measurement data of an examination object, which data was acquired in collapsed form simultaneously for slices using an EPI SMS technique, into measurement data of individual slices, first and second sets of reference measurement data for separating the measurement data are acquired for each of the slices using a GRE acquisition technique, wherein the reference measurement data in the first set is acquired during switching of readout gradients of a first polarity, and the reference measurement data in the second set is acquired during switching of readout gradients of a second polarity. Based on the two sets of reference measurement data, corresponding separate first calibration data is determined from the reference measurement data acquired using a GRE acquisition technique while switching readout gradients of a first polarity, and second calibration data is determined from the reference measurement data acquired while switching readout gradients of a second polarity.

Claims

1. A method for separating measurement data of an object under examination acquired in collapsed form simultaneously for at least two slices, using an echo planar (EPI) simultaneous multi-slice (SMS) technique, into measurement data of individual slices, the method comprising: loading measurement data to be separated which was generated by generating a train of at least two echo signals from at least two different slices of the object under examination after one radio frequency (RF) excitation pulse, and by acquiring the echo signals while switching readout gradients of alternating polarity for consecutive echo signals, and by capturing as the measurement data the echo signals acquired simultaneously for the at least two slices; for each of the at least two slices, acquiring using a gradient echo (GRE) acquisition technique a first set of reference measurement data for separating the measurement data, wherein reference measurement data in the first set of reference measurement data is acquired after a plurality of excitations having different phase encoding and in each case during switching of identically shaped readout gradients of a first polarity; for each of the at least two slices, acquiring using a GRE acquisition technique a second set of reference measurement data for separating the measurement data, wherein reference measurement data in the second set of reference measurement data is acquired after a plurality of excitations having different phase encoding and in each case during switching of identically shaped readout gradients of a second polarity, which differs from the first polarity; determining first calibration data based on acquired first sets of reference measurement data; determining second calibration data based on acquired second sets of reference measurement data; applying the first calibration data in order to separate into first measurement data of individual slices the measurement data captured in collapsed form that was acquired using readout gradients of the first polarity; applying the second calibration data in order to separate into second measurement data of individual slices the measurement data captured in collapsed form that was acquired using readout gradients of the second polarity; and storing and/or processing further the first measurement data of individual slices and the second measurement data of individual slices, in each case for at least one individual slice of the at least two slices for which measurement data was acquired simultaneously in collapsed form.

2. The method as claimed in claim 1, wherein reference measurement data in the first set of reference measurement data and in the second set of reference measurement data is acquired with the same phase encoding after joint excitation during two immediately consecutive readout gradients of the first polarity and the second polarity.

3. The method as claimed in claim 2, wherein reference measurement data in the first set of reference measurement data and/or in the second set of reference measurement data is acquired at least twice with an identical polarity with a same phase encoding after a joint excitation, with a result that at least two sets of reference measurement data of a same type are acquired.

4. The method as claimed in claim 3, wherein at least two sets of reference measurement data of the same type are combined into a combined set of reference measurement data.

5. The method as claimed in claim 4, wherein the at least two sets of reference measurement data of the same type are combined such that at least one combined set of reference measurement data has a virtual echo time that effectively equals an echo time of a set of reference measurement data that was acquired using readout gradients of a different polarity than the reference measurement data in the combined sets of reference measurement data.

6. The method as claimed in claim 3, wherein, if at least two sets of reference measurement data of the same type have been acquired, the set of reference measurement data of the at least two sets of reference measurement data of the same type that was acquired first in time is discarded.

7. The method as claimed in claim 1, wherein the readout gradients used in the acquisition of the first set of reference measurement data and of the second set of reference measurement data are selected such that they are as similar as possible to the readout gradients that were used in the capture of the measurement data in at least one parameter from a group of parameters consisting of size of amplitude, rise slew rate, duration, fall slew rate, readout bandwidth, resolution in a readout direction, positioning relative to the readout gradient of an acquisition window that is used, and symmetry of the echo.

8. The method as claimed in claim 1, wherein preprocessing steps that are applied to the measurement data to be separated before the first and/or second calibration data is applied to the measurement data to be separated, are applied analogously also before the determining of the first and second calibration data to the first set of reference measurement data and/or to the second set of reference measurement data.

9. The method as claimed in claim 1, wherein the measurement data to be separated was acquired in-plane incompletely in accordance with a parallel acquisition technique, and the first set of reference measurement data and the second set of reference measurement data are used as reference measurement data as part of the parallel acquisition technique to complete the data that was not acquired.

10. The method as claimed in claim 1, wherein the measurement data to be separated is processed using a dual-polarity GRAPPA (DPG) algorithm, and the first and second sets of reference measurement data are also used as reference measurement data as part of the DPG algorithm.

11. The method as claimed in claim 1, wherein a phase correction method is applied to the first set of reference measurement data and/or to the second set of reference measurement data in order to align a phase evolution of the reference measurement data in the sets of reference measurement data to those of the measurement data to be separated.

12. The method as claimed in claim 1, wherein the measurement data to be separated is acquired using an acquisition technique from a group of acquisition techniques consisting of multi-shot EPI, spin echo EPI, dual-spin-echo EPI, stimulated echo EPI, gradient echo EPI, and GRASE (gradient and spin echo) EPI.

13. The method as claimed in claim 1, wherein the measurement data was generated using a saturation technique and/or a spectrally selective excitation technique, with a result that the acquired echo signals originate in a spectrally selective manner from a spin species defined by the saturation technique and/or spectrally selective excitation technique, and a saturation technique and/or a spectrally selective excitation technique is likewise used in the acquisition of the first and second sets of reference measurement data, with a result that the acquired reference measurement data in the first and second sets of reference measurement data also originates in a spectrally selective manner from the same spin species as the echo signals from which the measurement data was generated.

14. A magnetic resonance system, comprising: a magnet unit; a gradient unit; a radiofrequency unit; and a control device having a radiofrequency transmit/receive controller and a reference measurement data unit, wherein the control device is designed to perform on the magnetic resonance system a method as claimed in claim 1.

15. A non-transitory computer-readable storage medium comprising commands which, when executed by a control device of a magnetic resonance system, cause the magnetic resonance system to perform the method as claimed in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Further advantages and details of the present disclosure are presented in the exemplary aspects described below and with reference to the drawings, where the examples given have no limiting effect on the disclosure, in which drawings:

[0048] FIG. 1 is a schematic flow diagram of a method according to the disclosure;

[0049] FIG. 2 shows a segment of a pulse sequence scheme for acquiring measurement data for imaging by means of an EPI SMS technique;

[0050] FIGS. 3-5 show segments of pulse sequence schemes for acquiring reference measurement data;

[0051] FIG. 6 shows a schematic representation of a magnetic resonance system according to the disclosure.

DETAILED DESCRIPTION

[0052] FIG. 1 is a schematic flow diagram of a method according to the disclosure for separating measurement data MD.sub.sms of an object under examination, which data was acquired in collapsed form simultaneously for at least two slices by means of an echo planar (EPI) simultaneous multi-slice (SMS) technique, into measurement data of individual slices MD.sup.+.sub.es, MD.sup.?.sub.es.

[0053] In the method, measurement data MD.sub.sms to be separated is loaded, which measurement data was generated by generating a train of at least two echo signals from at least two different slices of an object under examination after one RF excitation pulse, and acquiring the echo signals while switching readout gradients A of alternating polarity for consecutive echo signals. The echo signals simultaneously acquired for the at least two slices were captured as measurement data MD.sub.sms, for instance in a measurement dataset (block 301). Acquiring echo signals from at least two slices can be understood here to mean analogously also acquiring echo signals from at least two slabs (with three-dimensional spatial encoding).

[0054] FIG. 2 shows a possible pulse sequence scheme for acquiring such measurement data for imaging by means of an EPI SMS technique. The top line RF shows RF pulses to be applied and generated echo signals, the second line Gs shows gradients to be switched in the slice-selection direction, the third line Gr shows gradients to be switched in the readout direction, and the bottom line Gp shows gradients to be switched in the phase-encoding direction.

[0055] After an RF excitation pulse MB, which, in particular, can be a multiband RF excitation pulse and which excites the at least two slices (i.e., SMS factor ?2), a train of echo signals is generated from the at least two excited slices by a readout gradient A of alternating polarity (as is usual in EPI techniques), which echo signals are acquired and captured as measurement data MD.sup.+.sub.sms, MD.sup.?.sub.sms. As a result of the alternating polarity of the readout gradients A, in the example shown, measurement data MD.sup.+.sub.sms is captured with a positive polarity of the readout gradient, and measurement data MD.sup.?.sub.sms with a negative polarity of the readout gradient. The problems resulting from this have already been described above in the text.

[0056] FIG. 2 shows, by way of example, a pulse sequence scheme for imaging by means of single-shot gradient echo EPI. This shall not be interpreted restrictively; the method described herein is suitable for any EPI acquisition techniques, in particular also for multi-shot EPI acquisition techniques, spin echo EPI acquisition techniques, dual-spin-echo EPI acquisition techniques, stimulated echo EPI acquisition techniques, gradient echo EPI acquisition techniques, or GRASE (gradient and spin echo) EPI acquisition techniques.

[0057] For each of the at least two slices, a first set of reference measurement data RD.sup.+ for separating the measurement data MD.sub.sms (block 303) is acquired by means of a GRE acquisition technique, wherein the reference measurement data RD.sup.+ in the first set of reference measurement data RD.sup.+ is acquired after a plurality of excitations having different phase encoding and in each case during switching of identically shaped readout gradients, which have a first polarity. The first polarity can be positive or negative here.

[0058] For each of the at least two slices, a second set of reference measurement data RD.sup.? for separating the measurement data MD.sub.sms (block 303) is acquired by means of a GRE acquisition technique, wherein the reference measurement data RD.sup.? in the second set of reference measurement data RD.sup.? is acquired after a plurality of excitations having different phase encoding and in each case during switching of identically shaped readout gradients, which have a second polarity, which differs from the first polarity.

[0059] Examples of possible pulse sequence schemes of GRE acquisition techniques for acquiring reference measurement data RD.sup.+, RD.sup.? for a slice are described below in FIGS. 3 to 5, wherein in each figure again, the top line RF shows RF pulses to be applied and generated echo signals, the second line Gs shows gradients to be switched in the slice-selection direction, the third line Gr shows gradients to be switched in the readout direction, and the bottom line Gp shows gradients to be switched in the phase-encoding direction.

[0060] First calibration data KD.sup.+ is determined on the basis of acquired first sets of reference measurement data RD.sup.+ (block 305), and second calibration data KD.sup.? is determined on the basis of acquired second sets of reference measurement data RD.sup.? (block 305). The first calibration data and second calibration data are determined in a manner known per se in the context of SMS techniques but, as described, are determined in each case on the basis of the respective acquired sets of reference measurement data.

[0061] In order to separate the measurement data MD.sup.+.sub.sms captured in collapsed form and acquired using readout gradients of the first polarity into first measurement data of individual slices MD.sup.+.sub.es, the determined first calibration data KD.sup.+ is applied to the measurement data MD.sup.+.sub.sms to be separated that was captured in collapsed form and acquired using readout gradients of the first polarity (block 309).

[0062] In order to separate the measurement data MD.sup.?.sub.sms captured in collapsed form and acquired using readout gradients of the second polarity into second measurement data of individual slices MD.sup.?.sub.es, the determined second calibration data KD.sup.? is applied to the measurement data MD.sup.?.sub.sms to be separated that was captured in collapsed form and acquired using readout gradients of the second polarity (block 309).

[0063] For this purpose, the measurement data MD.sub.sms to be separated can be divided into measurement data MD.sup.+.sub.sms captured in collapsed form and acquired using readout gradients of the first polarity and into measurement data MD.sup.?.sub.sms captured in collapsed form and acquired using readout gradients of the second polarity (block 307).

[0064] It is conceivable that the measurement data MD.sup.+? was generated using a saturation technique, for instance, a fat saturation technique, and/or a spectrally selective excitation technique, for example, a water excitation technique, with the result that the acquired echo signals originate in a spectrally selective manner from a spin species, in particular from water spins, defined by the saturation technique and/or spectrally selective excitation technique. In particular, when the measurement data MD.sup.+? is generated in this spectrally selective manner, it is also possible to use a saturation technique and/or a spectrally selective excitation technique in the acquisition of the first and second sets of reference measurement data RD.sup.+, RD.sup.?, with the result that the acquired reference measurement data in the first and second sets of reference measurement data RD.sup.+, RD.sup.? also originates in a spectrally selective manner from the same spin species as the echo signals from which the measurement data MD.sup.+? was generated. It is not necessary here that the same saturation technique or spectrally selective excitation technique is used in each case. However, rather it is still possible to achieve a further improvement in the consistency of the reference measurement data in the first and second sets of reference measurement data RD.sup.+, RD.sup.? with the measurement data MD.sup.+? acquired for the imaging just by defining the same spin species albeit in different ways in the acquisitions of the reference measurement data and the echo signals of the measurement data MD.sup.+?.

[0065] The first measurement data MD.sup.+.sub.es of individual slices and the second measurement data MD.sup.?.sub.es of individual slices are stored in each case for at least one of the at least two slices for which measurement data was acquired simultaneously in collapsed form and/or processed further, for instance, into image data BD (block 311).

[0066] FIG. 3 shows a segment of a pulse sequence scheme for acquiring reference measurement data by means of a GRE acquisition technique. In this case, after each RF excitation pulse RF1 an echo signal can be generated from a slice of the object under examination, which is acquired with each switching of a readout gradient A+ as reference measurement data RD.sup.+ along a k-space row, wherein the generated echo signals are encoded spatially with different phase encodings by switching different gradients in the phase-encoding direction PE1, PE2, PE3, PE4, with the result that the reference measurement data RD.sup.+ is acquired along k-space rows that are shifted with respect to each other in the phase-encoding direction. The number of excitations, each by means of an RF excitation pulse RF1, and the different phase encodings PE1, PE2, PE3, PE4 can be selected such that a desired set of reference measurement data RD.sup.+ is acquired in total.

[0067] The performed measurements are repeated using readout gradients A? in a readout direction (shown dotted) of different polarity from the polarity of the readout gradients A+, but otherwise having identical gradients and RF excitations pulses, thereby acquiring a set of reference measurement data RD.sup.? (not shown).

[0068] Thus, in a first measurement for acquiring a first set of reference measurement data, only reference measurement data RD.sup.+ having a first, for instance, positive, polarity of the readout gradients used can be acquired, and in a second (separate) measurement for acquiring a second set of reference measurement data only reference measurement data RD.sup.? having a second, for instance negative, polarity of the readout gradients used can be acquired. It can thereby be achieved that both sets of reference measurement data (and all the acquired reference measurement data RD.sup.+ and RD.sup.?) have an identical echo time TE. Both measurements mentioned here can be acquired consecutively or else in an interleaved manner.

[0069] FIG. 4 shows a segment of a pulse sequence scheme for acquiring reference measurement data by means of a dual-echo GRE acquisition technique, in which two echo signals are generated by alternating readout gradients after an RF excitation pulse RF1. In this case, an echo signal generated first after each RF excitation pulse RF1 can be acquired with each switching of a first readout gradient A+ of a first polarity as reference measurement data RD.sup.+ in a first direction along a k-space row and a second echo signal generated each time can be acquired with each switching of a second readout gradient A? of a second polarity, which differs from the first polarity, as reference measurement data RD.sup.? in a second direction along the same k-space row. The generated first and second echo signals can again, in this case, be spatially encoded with different phase encodings by switching different gradients in the phase-encoding direction PE1, PE2, PE3, with the result that the reference measurement data RD.sup.+ and RD.sup.? is acquired along k-space rows that are shifted with respect to each other in the phase-encoding direction. The number of excitations, each by an RF excitation pulse RF1, and the different phase encodings PE1, PE2, PE3 can again be selected such that a desired first set of reference measurement data RD.sup.+ and a desired second set of reference measurement data RD.sup.? is acquired in total.

[0070] Reference measurement data RD.sup.+, RD.sup.? in the first set of reference measurement data RD.sup.+ and in the second set of reference measurement data RD.sup.? can thus be acquired with the same phase encoding after joint excitation during two immediately consecutive readout gradients of the first polarity and the second polarity.

[0071] An acquisition of both sets of reference measurement data in this way by means of a dual-echo GRE acquisition technique can be performed in a shorter time than an acquisition of both sets of reference measurement data in accordance with a pulse sequence scheme shown in FIG. 3 because the acquisition of the reference measurement data RD.sup.? in the second set of reference measurement data RD.sup.? is performed without separate excitation immediately after acquisition of reference measurement data RD.sup.+ in the first set of reference measurement data RD.sup.+ by adding a further readout gradient A? after each excitation by an RF excitation pulse RF1. Reference measurement data RD.sup.+ in the first set of reference measurement data RD.sup.+ and reference measurement data RD.sup.? in the second set of reference measurement data RD.sup.? that are acquired in this way have different echo times TE, however.

[0072] FIG. 5 shows a segment of a pulse sequence scheme for acquiring reference measurement data RD.sup.+, RD.sup.? by means of a multiple-echo GRE acquisition technique, in which a plurality of echo signals, for instance, three, four, five, or more, are generated by alternating readout gradients A+, A? after an RF excitation pulse RF1. In this case, an echo signal generated first after each RF excitation pulse RF1, and each additional odd-numbered generated echo signal, can be acquired with each switching of a first readout gradient A+ of a first polarity as reference measurement data RD.sup.+ in a first direction along a k-space row. A second echo signal is generated each time, and each additional even-numbered generated echo signal can be acquired with each switching of a second readout gradient A? of a second polarity, which differs from the first polarity, as reference measurement data RD.sup.? in a second direction along the same k-space row. The generated first and second echo signals can again, in this case, be spatially encoded with different phase encodings by switching different gradients in the phase-encoding direction PE1, PE2, with the result that the reference measurement data RD.sup.+ and RD.sup.? is acquired along k-space rows that are shifted with respect to each other in the phase-encoding direction. The number of excitations, each by means of an RF excitation pulse RF1, and the different phase encodings PE1, PE2 can again be selected such that a desired first set of reference measurement data RD.sup.+ and a desired second set of reference measurement data RD.sup.? is acquired in total.

[0073] Reference measurement data RD.sup.+, RD.sup.? in the first set of reference measurement data RD.sup.+ and in the second set of reference measurement data RD.sup.? can thus be acquired with the same phase encoding after joint excitation during at least three immediately consecutive readout gradients of the first polarity and the second polarity, and acquired multiple times at least for one of the two sets of reference measurement data RD.sup.+, RD.sup.?.

[0074] It is hence conceivable that reference measurement data RD.sup.+, RD.sup.? in the first set of reference measurement data RD.sup.+ and/or in the second set of reference measurement data RD.sup.? is acquired at least twice with an identical polarity with the same phase encoding after a joint excitation, with the result that at least two sets of reference measurement data RD.sup.+, RD.sup.? of the same type (same polarity and same spatial encoding, e.g. the same k-space row) are acquired.

[0075] Acquiring the two sets of reference measurement data in this way by means of a multiple-echo GRE acquisition technique requires (as in the example of the dual-echo GRE acquisition technique described with reference to FIG. 4) fewer excitations to acquire the two sets of reference measurement data than an acquisition of the two sets of reference measurement data in a manner described with reference to FIG. 3. Furthermore, acquiring the two sets of reference measurement data in this way by means of a multiple-echo GRE acquisition technique allows additional freedoms:

[0076] If the two sets of reference measurement data are acquired using a three-echo GRE acquisition technique, for example, in which three echo signals are generated, and each time the echo signal that is generated third is acquired with a readout gradient of the same polarity as the echo signal generated first, then the two sets of reference measurement data of the same type, which were acquired by acquiring reference measurement data during the echo signals generated first and third, can be combined into a combined set of reference measurement data so that a better consistency of the reference measurement data can be achieved. Said combining of sets of reference measurement data of the same type can be performed in particular as part of the determining of associated calibration data (block 305 or 305), or before the calibration data is determined.

[0077] The two sets of reference measurement data of the same type can be combined, in particular by averaging (e.g., in a similar way to that described in the above-cited U.S. Pat. No. 6,043,651 for navigator data), in such a way that the combined set of reference measurement data has a virtual echo time TE that effectively equals the echo time TE of the reference measurement data acquired during the second echo signal.

[0078] Thus, at least two sets of reference measurement data of the same type can be combined in such a way that at least one combined set of reference measurement data has a virtual echo time that effectively equals an echo time of a set of reference measurement data that was acquired using readout gradients of a different polarity than the reference measurement data in the combined sets of reference measurement data. This can achieve an even better consistency of the reference measurement data.

[0079] Alternatively, the echo signal always generated first in a three-echo GRE acquisition technique can be ignored, and only the echo signals generated second and third in each case are acquired as reference measurement data RD.sup.+, RD.sup.? (rather like in a dual-echo GRE acquisition). In this manner it can be achieved that the echo signals used to acquire the sets of reference measurement data RD.sup.+, RD.sup.? are more similar to most of the echo signals in the echo train that were acquired as measurement data MD.sub.sms. This is particularly the case if transient processes generate effects during the acquisition of the measurement data MD.sub.sms to be separated, which cause changes in the acquired measurement data, especially changes in the signal phases, across the acquired EPI echo train. Such a situation can be caused, for example, by resonance effects of the gradient unit of the magnetic resonance system being used. In addition, the gradients switched before a first echo signal of a three-echo GRE acquisition technique (e.g., for slice selection, phase encoding, and/or prephasing of the readout gradients) can generate interference or eddy currents. Therefore, first echo signals are affected differently or more severely than other echo signals generated later after the joint excitation, whereby first echo signals could be deemed particularly unrepresentative of the majority of the echo signals in the EPI echo train that was used to acquire the measurement data MD.sub.sms to be separated.

[0080] If at least two sets of reference measurement data of the same type have been acquired, it is also possible to discard the set of reference measurement data of the at least two sets of reference measurement data of the same type that was acquired first in time. This can achieve similar advantages to the case described above in which echo signals generated first after an excitation are ignored.

[0081] Adding another one or more echo signals by additional alternating readout gradients after a joint excitation pulse RF1 results in four-echo or five-echo GRE acquisition techniques (etc.).

[0082] It is also possible for such multiple-echo GRE acquisition techniques, similar to the options described with reference to the three-echo GRE acquisition, to acquire or take into account reference measurement data either from just individual echo signals (e.g., the last two in each case) after a joint excitation and/or if at least two sets of reference measurement data of the same type have been acquired, to combine these into a combined set of reference measurement data.

[0083] For example, in the case of a four-echo GRE acquisition technique, a first echo signal after each excitation could be ignored, and sets of reference measurement data of the same type acquired during a second and fourth echo signal, respectively, could be averaged to form a combined set of reference measurement data. Reference measurement data in such a combined set of reference measurement data can (as described above) have a virtual echo time TE that effectively equals an echo time TE of the reference measurement data of a second set of reference measurement data that was acquired during the third echo signal after the joint excitation. First calibration data can then be determined on the basis of the reference measurement data in the combined set of reference measurement data for a first polarity, and second calibration data can then be determined on the basis of the reference measurement data (having effectively identical echo time TE) in the second set of reference measurement data for a second polarity.

[0084] By ignoring first echo signals, the acquired reference measurement data is more representative of the echo signals in an EPI echo train that was used to acquire the measurement data MD.sub.sms to be separated.

[0085] In the simplest case, properties, apart from the specified polarity, of readout gradients used for acquiring the reference measurement data for the two sets of reference measurement data (in particular properties such as duration of constant amplitude, amplitude, ramp times or rise and fall slew rates) can be selected independently of the alternating readout gradients of the SMS EPI technique that was used to acquire the measurement data MD.sub.sms to be separated. This gives maximum flexibility in the parameterization of the reference measurement data, and this data can be acquired, for example, with a reduced resolution and hence very quickly in a short measurement time.

[0086] In general, in order to improve the consistency between the measurement data MD.sub.sms to be separated and the acquired reference measurement data, readout gradients used in the acquisition of the first set of reference measurement data and of the second set of reference measurement data can be selected such that they are as similar as possible, in particular identical, to the readout gradients that were used in the capture of the measurement data MD.sub.sms to be separated in at least one of the parameters from the group comprising size of amplitude, rise slew rate, duration of constant amplitude, fall slew rate, readout bandwidth, resolution in the readout direction, positioning relative to the readout gradient of an acquisition window that is used (e.g. for what is known as ramp sampling, in which acquisition of the (reference) measurement data is started already during the rising edge of the readout gradient), symmetry of the echo (e.g. in connection with partial Fourier techniques in the readout direction).

[0087] The more similar the properties of the readout gradients used in the acquisition of the reference measurement data are chosen to be to the properties of the readout gradients of the SMS EPI technique that was used to acquire the measurement data MD.sub.sms to be separated or the more of these properties that are selected to be as similar as possible, the more the achievable consistency can be improved between the acquired reference measurement data and the measurement data MD.sub.sms to be separated. For maximum consistency, the parameters and, hence, properties of the readout gradients for the acquisitions of the reference measurement data can be chosen to be completely identical to those of the SMS EPI technique that was used to acquire the measurement data MD.sub.sms to be separated.

[0088] When identical properties of the readout gradients of the reference measurement data to those of the measurement data MD.sub.sms to be separated are used, preprocessing steps such as correction methods, for instance, that are applied to the measurement data MD.sub.sms to be separated can be applied in an identical manner to the reference measurement data RD.sup.+, RD.sup.?. Examples of preprocessing steps that are eligible here are corrections of a variable sampling density in the case of ramp sampling and/or corrections of asymmetries between positive and negative readout gradients (for instance, based on navigator data), as already mentioned above.

[0089] Thus preprocessing steps, in particular correction methods, that are applied to the measurement data MD.sub.sms to be separated before the first and/or second calibration data is applied to the corresponding captured measurement data (e.g. in block 307) can be applied analogously also before the determining of the first and second calibration data KD.sup.+, KD.sup.? (block 305 and block 305 respectively) to the first set of reference measurement data RD.sup.+ and/or to the second set of reference measurement data RD.sup.?.

[0090] A phase correction method can be applied to the first set of reference measurement data RD.sup.+ and/or to the second set of reference measurement data RD.sup.? in order to align the phase evolution of the reference measurement data RD.sup.+, RD.sup.? in the sets of reference measurement data RD.sup.+, RD.sup.? to those of the measurement data MD.sub.sms to be separated. Such phase correction methods can be used to reduce variations in the phase evolution of the reference measurement data RD.sup.+, RD.sup.? (acquired using a gradient echo measurement and short echo time) and in the phase evolution of the measurement data MD.sub.sms to be separated that was acquired by means of the SMS EPI technique for imaging (acquired using an EPI measurement and longer echo time) when the spatial distribution of the main magnetic field B0 is known. DE 10 2016 200889 B4 describes such a phase correction method, for example. A further improvement in the consistency of reference measurement data and measurement data MD.sub.sms to be separated associated with such a phase correction method can lead to even better quality of the results of the separation into measurement data of individual slices.

[0091] It is conceivable that the captured measurement data MD.sub.sms to be separated is acquired in-plane incompletely in accordance with a parallel acquisition technique such as GRAPPA, and the first set of reference measurement data RD.sup.+ and the second set of reference measurement data RD.sup.? are used additionally as reference measurement data as part of the parallel acquisition technique to complete the data that was not acquired because acquired reference measurement data in the first and second sets of reference measurement data RD.sup.+, RD.sup.? is inherently suitable for such an application as part of parallel acquisition techniques. Acquiring further reference measurement data specifically for the parallel acquisition technique can be omitted.

[0092] The captured measurement data MD.sub.sms to be separated can also be processed by means of a dual-polarity GRAPPA (DPG) algorithm, as described, for example, in the article by Hoge et al., Dual-polarity GRAPPA for simultaneous reconstruction and ghost correction of echo planar imaging data, Magn. Reason. Med. 76: pages 32-44, 2016. In this case, the first and second sets of reference measurement data can also be used as reference measurement data as part of the DPG algorithm. Acquiring further reference measurement data specifically for the DPG algorithm can be omitted.

[0093] Hence using the method described herein, in particular by determining polarity-specific calibration data, it is possible to significantly increase a consistency of measurement data MD.sub.sms to be separated with reference measurement data acquired to determine the calibration data (convolution kernels) despite different acquisition techniques in the acquisition of the reference measurement data and the measurement data MD.sub.sms to be separated (GRE vs. EPI), with the result that it is possible to separate (unfold) the signal contributions of the different slices with higher quality (lower noise and/or reduced unfolding artifacts).

[0094] Imaging methods in which the acquisition time for the measurement data MD.sub.sms to be separated is very short benefit, in particular from acquiring reference measurement data according to a method described herein in first and second sets of reference measurement data. The shorter this acquisition time, the more important is the time for the acquisition of the reference measurement data (in relative terms). For example, on typical clinical magnetic resonance systems, diffusion-weighted imaging of the entire head by means of single-shot EPI, for instance, (e.g. with an SMS factor of 2 and an (in-plane) GRAPPA factor of 2) within 4 repetitions (1? without diffusion weighting, 3? with diffusion weighting along orthogonal directions) with a repetition time TR of 3 seconds can be performed in a total of 4?3=12 seconds. If applicable, a further dummy repetition and, thus, a further repetition time TR for establishing a state of equilibrium of the magnetization can be required, resulting in a total of 5?3=15 seconds for the time length of the entire measurement. For a previously known acquisition of reference measurement data by means of an EPI acquisition technique for separating the measurement data acquired in collapsed form for (in this example) two slices, 3-6 seconds are needed in the known prior art. For a previously known acquisition of further reference measurement data by means of a GRE or an EPI acquisition technique for completing unacquired measurement data within a slice, a further 3-6 seconds are needed in the known prior art. In total, an acquisition of all the necessary reference measurement data thus takes 6-12 seconds in the prior art, and thus almost as long as the acquisition of the measurement data for the imaging. If GRE reference measurement data known from the prior art and acquired using just one polarity of the readout gradients is used both for separating the slices acquired in collapsed form and for completing the unacquired measurement data within a slice, the time needed for acquiring the reference measurement data can be reduced to 3-6 seconds, but an achievable quality for the separation of the slices acquired in collapsed form (slice unfolding) suffers under such a choice of reference measurement data.

[0095] Using a method according to the disclosure, an acquisition of the reference measurement data in both sets of reference measurement data is possible in 4-7 seconds, which represents an only insignificant increase compared with previously known acquisitions of reference measurement data by means of a GRE acquisition technique using only one polarity of the readout gradients used, while achieving a considerably higher quality for the slice unfolding.

[0096] The aspects of the disclosure succeed in combining the advantages of reference measurement data acquired by means of a GRE acquisition technique (e.g., short measurement times, low sensitivity to movement) with the advantages of reference measurement data acquired by means of an EPI acquisition technique (in particular improved consistency with the measurement data MD.sub.sms to be separated), and thus in achieving improved quality of the slice unfolding and hence of the reconstructible individual images after the slice unfolding.

[0097] In addition, the reference measurement data acquired according to the disclosure can also be used as part of parallel acquisition techniques for compensation for in-plane undersampling (e.g., with GRAPPA), are robust to movements of the object under examination, insensitive to inhomogeneities in the main magnetic field B0, and allow a high degree of flexibility in the choice of the parameters of the acquisition, in particular of the resolution.

[0098] FIG. 6 shows schematically a magnetic resonance system 1 according to the disclosure. This comprises a magnet unit 3 for generating the main magnetic field, a gradient unit 5 for generating the gradient fields, a radiofrequency unit 7 for emitting and receiving radiofrequency signals, and a control device 9 designed to implement a method according to the disclosure.

[0099] In FIG. 6, these sub-units of the magnetic resonance system 1 are not shown in detail. In particular, the radiofrequency unit 7 may consist of a plurality of sub-units, for instance, of a plurality of coils such as the coils 7.1 and 7.2 shown schematically or more coils, which may either be designed solely to transmit radiofrequency signals or solely to receive the induced radiofrequency signals, or be designed to do both.

[0100] In order to examine an object under examination U, for example, a patient or else a phantom, the object can be introduced into the magnetic resonance system 1 into the measurement volume thereof on a couch L. The schematically depicted slices S1 and S2 represent by way of example a plurality of slices of a target volume of the object under examination, which slices are to be acquired simultaneously by means of a slice multiplexing method, from which echo signals can be acquired in collapsed form and captured as measurement data.

[0101] The control device 9 is used to control the magnetic resonance system 1 and, in particular, can control the gradient unit 5 by means of a gradient controller 5 and can control the radiofrequency unit 7 by means of a radiofrequency transmit/receive controller 7. The radiofrequency unit 7 can here comprise a plurality of channels on which signals can be transmitted or received.

[0102] The radiofrequency unit 7, together with its radiofrequency transmit/receive controller 7 is responsible for generating and radiating (transmitting) an alternating radiofrequency field for manipulating the spins in a region to be manipulated (for instance in slices S to be measured) of the object under examination U. The center frequency of said alternating radiofrequency field, also referred to as the B1 field, as a rule is set so as to lie close to the resonant frequency of the spins to be manipulated. Off-resonance refers to deviations of the resonant frequency from the center frequency. In order to generate the B1 field, currents are applied to the RF coils, which currents are controlled in the radiofrequency unit 7 by means of the radiofrequency transmit/receive controller 7.

[0103] In addition, the control device 9 comprises a reference measurement data unit 15, which can be used to control an acquisition according to the disclosure of reference measurement data. The control device 9 is designed overall to perform a method according to the disclosure.

[0104] A computing unit 13 comprised of the control device 9 is designed to perform all the computing operations needed for the required measurements and determinations. Intermediate results and results required for this purpose or determined in this process can be saved in a memory unit S of the control device 9. The units shown need not necessarily be interpreted here as physically separate units but merely constitute a subdivision into logical units, which, however, can be implemented, e.g., in fewer physical units or even in just one physical unit.

[0105] Via an input/output device E/A of the magnetic resonance system 1, it is possible, for instance, for a user to direct control commands to the magnetic resonance system and/or to display results from the control device 9, for example, results such as image data.

[0106] A method described herein can also exist in the form of a computer program, which comprises commands that execute the described method in a control device 9. A computer-readable storage medium can likewise be provided that comprises commands which, on execution by a control device 9 of a magnetic resonance system 1, cause said device to execute the described method.

[0107] Independent of the grammatical term usage, individuals with male, female, or other gender identities are included within the term.