Echo-specific k-space sampling with multi-echo sequences

Abstract

In a method, an imaging sequence is irradiated into an examination region in which an examination object is located. The imaging sequence includes an acquisition section. The acquisition section includes acquiring a plurality of echo signals, each of which samples a k-space region of a k-space. The plurality of echo signals comprises a plurality of first echo signals and a plurality of second echo signals. The plurality of first echo signals and the plurality of second echo signals are generated from different magnetization configurations. The k-space regions sampled by the plurality of first echo signals sample the k-space in a different order to the k-space regions sampled by the plurality of second echo signals.

Claims

1. A method for the acquisition of echo signals during a magnetic resonance examination of an examination object, the method comprising: irradiating an imaging sequence into an examination region in which the examination object is located, wherein the imaging sequence comprises an acquisition section, wherein the acquisition section comprises acquiring a plurality of echo signals, each echo signal of the plurality of echo signals sampling a k-space region of a k-space, wherein the plurality of echo signals comprises a plurality of first echo signals and a plurality of second echo signals, wherein the plurality of first echo signals and the plurality of second echo signals are generated from different magnetization configurations, wherein the k-space regions sampled by the plurality of first echo signals sample the k-space in a different order to the k-space regions sampled by the plurality of second echo signals, wherein the plurality of first echo signals are FID echo signals, and the plurality of second echo signals are stimulated echo signals, wherein the FID echo signals are acquired before the stimulated echo signals, or the stimulated echo signals are acquired before the FID echo signals, and wherein the order of the k-space regions sampled by the plurality of first echo signals is characterized in that spacing from a k-space center decreases during the course of acquisition of the plurality of first echo signals.

2. The method of claim 1, wherein the acquisition section further comprises irradiating a plurality of gradient pulses in a phase encoding direction, which is capable of setting the k-space regions to be sampled by the echo signals.

3. The method of claim 1, wherein each of the k-space regions is a k-space line.

4. The method of claim 1, wherein the order of the k-space regions sampled by the plurality of second echo signals is characterized in that spacing from a k-space center increases during the course of acquisition of the plurality of second echo signals.

5. The method of claim 1, wherein the acquisition section further comprises: irradiating a plurality of readout RF pulses, wherein each readout RF pulse of the plurality of readout RF pulses is operable to trigger at least one first echo signal of the plurality of first echo signals, at least one second echo signal of the plurality of second echo signals, or the at least one first echo signal and the at least one second echo signal.

6. The method of claim 1, wherein the imaging sequence further comprises a preparation section preceding the acquisition section, and wherein a phasing of the plurality of second echo signals is prepared by the preparation section.

7. The method of claim 5, further comprising: generating a main magnetic field during the imaging sequence; and deriving a B.sub.0 map that specifies an actual spatial distribution of a magnetic field strength of the main magnetic field, the B.sub.0 map comprising the at least one first echo signal and the at least one second echo signal.

8. The method of claim 7, wherein the imaging sequence further comprises a preparation section preceding the acquisition section, wherein the imaging sequence comprises: irradiating at least two preparation RF pulses during the preparation section, wherein there is a time span between irradiating one of the at least two preparation RF pulses and irradiating another one of the at least two preparation RF pulses; irradiating a plurality of readout RF pulses during the acquisition section; acquiring one first echo signal of the plurality of first echo signals in each case after one readout RF pulse of the plurality of readout RF pulses, wherein there is a time span between irradiating the respective one readout RF pulse of the plurality of readout RF pulses and acquiring the respective one first echo signal; acquiring one second echo signal of the plurality of second echo signals in each case after one readout RF pulse of the plurality of readout RF pulses, wherein there is a time span between irradiating the respective one readout RF pulse of the plurality of readout RF pulses and acquiring the respective one second echo signal, wherein the time span between irradiating the one preparation RF pulse and irradiating the another one preparation RF pulse is chosen such that between an instant at which the at least one first echo signal is acquired, and an instant at which the at least one second echo signal is acquired, a signal component of echo signals of the plurality of echo signals from protons bound in water has a same phase difference to a signal component of echo signals of the plurality of echo signals from protons bound in fat.

9. The method of claim 8, wherein $\mathrm{TS}=N*\frac{1}{{\delta }_{\mathrm{WF}}*\left(\gamma \text{/}2\pi \right)*B_{0,\mathrm{desired}}}+{\mathrm{TE}}_{\mathrm{STE}}-{\mathrm{TE}}_{\mathrm{FID}}$ wherein TS is the time span between irradiating the one of the at least two preparation RF pulses and irradiating the another one of the at least two preparation RF pulses, TE.sub.STE is the time span between irradiating the respective one readout RF pulse of the plurality of readout RF pulses and acquiring the respective one second echo signal, TE.sub.FID is the time span between irradiating the respective one readout RF pulse of the plurality of readout RF pulses and acquiring the respective one first echo signal, N is a whole number>0, δ.sub.WF specifies a chemical shift of water and fat, and γ specifies a gyromagnetic ratio of protons bound in water.

10. The method of claim 7, wherein the main magnetic field has a desired magnetic field strength of less than 2 T.

11. A magnetic resonance device operable to acquire echo signals during a magnetic resonance examination of an examination object, the magnetic resonance device comprising: a coil operable to irradiate an imaging sequence into an examination region in which the examination object is located, wherein the imaging sequence comprises an acquisition section, wherein the acquisition section comprises acquisition of a plurality of echo signals, each echo signal of the plurality of echo signals sampling a k-space region of a k-space, wherein the plurality of echo signals comprises a plurality of first echo signals and a plurality of second echo signals, wherein the plurality of first echo signals and the plurality of second echo signals are generated from different magnetization configurations, and wherein the k-space regions sampled by the plurality of first echo signals sample the k-space in a different order to the k-space regions sampled by the plurality of second echo signals, wherein the plurality of first echo signals are FID echo signals, and the plurality of second echo signals are stimulated echo signals, wherein the FID echo signals are acquired before the stimulated echo signals, or the stimulated echo signals are acquired before the FID echo signals, and wherein the order of the k-space regions sampled by the plurality of first echo signals is characterized in that spacing from a k-space center decreases during the course of acquisition of the plurality of first echo signals.

12. In a non-transitory computer readable storage medium that stores instructions executable by a system control unit of a magnetic resonance device to acquire echo signals during a magnetic resonance examination of an examination object, the instructions comprising: irradiating an imaging sequence into an examination region in which the examination object is located, wherein the imaging sequence comprises an acquisition section, wherein the acquisition section comprises acquiring a plurality of echo signals, each echo signal of the plurality of echo signals sampling a k-space region of a k-space, wherein the plurality of echo signals comprises a plurality of first echo signals and a plurality of second echo signals, wherein the plurality of first echo signals and the plurality of second echo signals are generated from different magnetization configurations, wherein the k-space regions sampled by the plurality of first echo signals sample the k-space in a different order to the k-space regions sampled by the plurality of second echo signals, wherein the plurality of first echo signals are FID echo signals, and the plurality of second echo signals are stimulated echo signals, wherein the FID echo signals are acquired before the stimulated echo signals, or the stimulated echo signals are acquired before the FID echo signals, and wherein the order of the k-space regions sampled by the plurality of first echo signals is characterized in that spacing from a k-space center decreases during the course of acquisition of the plurality of first echo signals.

13. The non-transitory computer readable storage medium of claim 12, wherein the acquisition section further comprises irradiating a plurality of gradient pulses in a phase encoding direction, which is capable of setting the k-space regions to be sampled by the echo signals.

14. The non-transitory computer readable storage medium of claim 12, wherein each of the k-space regions is a k-space line.

15. The non-transitory computer readable storage medium of claim 12, wherein the order of the k-space regions sampled by the plurality of second echo signals is characterized in that spacing from a k-space center increases during the course of acquisition of the plurality of second echo signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages, features, and details of the present embodiments may be found in the exemplary embodiments described below and with reference to the drawings. Mutually corresponding parts are provided with same reference characters in all figures.

(2) FIG. 1 shows one embodiment of a magnetic resonance device in a schematic representation;

(3) FIG. 2 shows one embodiment of a method in a block representation;

(4) FIG. 3 shows exemplary magnetization components that are available after suitable preparation for generation of a stimulated echo signal and an FID echo signal, in a simplified representation;

(5) FIG. 4 shows an exemplary linear order for sampling a k-space;

(6) FIG. 5 shows an exemplary centric order for sampling a k-space;

(7) FIG. 6 shows an exemplary anticentric order for sampling a k-space,

(8) FIG. 7 shows an exemplary sequence diagram for acquisition of stimulated echo signals and FID echo signals with optimized k-space sampling; and

(9) FIG. 8 shows exemplary measurement results with and without optimized k-space sampling.

DETAILED DESCRIPTION

(10) FIG. 1 schematically represents one embodiment of a magnetic resonance device 10. The magnetic resonance device 10 includes a magnet unit 11 that has a main magnet 12 for generating a strong and, for example, time-constant main magnetic field 13 with a desired magnetic field strength B.sub.0,desired. The desired magnetic field strength B.sub.0,desired is, for example, less than 2 T. In addition, the magnetic resonance device 10 includes an examination region 14 for a scan of a patient 15. In the present exemplary embodiment, the examination region 14 is cylindrical in design and surrounded in a circumferential direction by the magnet unit 11. In principle however, a different design of the examination region 14 may be provided. The patient 15 may be pushed by a patient supporting device 16 of the magnetic resonance device 10 into the examination region 14. For this purpose, the patient supporting device 16 has a patient couch 17 configured so the patient couch 17 may move inside the examination region 14.

(11) The magnet unit 11 also has a gradient coil unit 18 for generation of magnetic field gradients by irradiating gradient pulses that are used, inter alia, for spatial encoding during imaging. The gradient coil unit 18 includes, for example, three gradient coils for each one spatial direction. The gradient coil unit 18 is controlled by a gradient control unit 19 of the magnetic resonance device 10. The magnet unit 11 also includes a radio-frequency antenna unit 20 that, in the present exemplary embodiment, is configured as a body coil permanently integrated in the magnetic resonance device 10. The radio-frequency antenna unit 20 is controlled by a radio-frequency antenna control unit 21 of the magnetic resonance device 10 and irradiates radio-frequency pulses into an examination space, which is substantially formed by an examination region 14 of the magnetic resonance device 10. As a result, an excitation of atomic nuclei is established in the main magnetic field 13 generated by the main magnet 12. Magnetic resonance signals (e.g., echo signals) are generated by relaxation of the excited atomic nuclei. The radio-frequency antenna unit 20 is configured to receive the echo signals.

(12) The magnetic resonance device 10 has a system control unit 22 for control of the main magnet 12 of the gradient control unit 19 and for control of the radio-frequency antenna control unit 21. The system control unit 22 centrally controls the magnetic resonance device 10, such as performance of a predetermined imaging gradient echo sequence. In addition, the system control unit 22 includes an evaluation unit (not shown) for an evaluation of the echo signals that are acquired during the magnetic resonance examination. For example, the evaluation unit is configured to generate a B.sub.0 map based on the echo signals. The B.sub.0 map specifies an actual spatial distribution of the magnetic field strength of the main magnetic field 13. Further, the magnetic resonance device 10 includes a user interface 23 that is connected to the system control unit 22. Control information, such as imaging parameters, and reconstructed magnetic resonance mappings may be displayed on a display unit 24 (e.g., on at least one monitor) of the user interface 23 for a medical operator. Further, the user interface 23 has an input unit 25, by which information and/or parameters may be input by the medical operator during a measurement process.

(13) FIG. 2 schematically represents a method for the acquisition of echo signals during a magnetic resonance examination of an examination object. In S1, a preparation section of an imaging sequence is irradiated into the examination region in which the examination object is located. A longitudinal magnetization, for example, is prepared by the preparation section.

(14) In S2, an acquisition section of the imaging sequence is irradiated into the examination region. The acquisition section includes acquiring a plurality of echo signals, each of which samples a k-space region of a k-space. The plurality of echo signals include a plurality of first echo signals and a plurality of second echo signals. The plurality of first echo signals and the plurality of second echo signals are generated from two different magnetization configurations, with one of these magnetization configurations having been prepared in the preparation section. In this case, the k-space regions sampled by the plurality of first echo signals sample the k-space in a different order to the k-space regions sampled by the plurality of second echo signals.

(15) In S3, a map (e.g., a B.sub.0 map) is derived from the plurality of first echo signals and the plurality of second echo signals.

(16) The further figures are intended to explain why an echo-specific order may be advantageous when sampling the k-space.

(17) This may take place based on an imaging sequence, as is represented in FIG. 7. This sequence allows, for example, measurement of a B.sub.0 map. The first echo signals are, for example, FID echo signals, and the second echo signals are, for example, stimulated echo signals. The sequence includes a preparation section PS and an acquisition section AS. RF pulses are represented on the axis RF, readout windows are represented on the axis ADC, and gradient pulses are represented on the axes G.sub.R, G.sub.P and G.sub.S. The axis G.sub.R corresponds to a readout direction (e.g., frequency encoding direction), the axis G.sub.P corresponds to a phase encoding direction, and the axis G.sub.S corresponds to a slice selection direction.

(18) In the preparation section PS, a longitudinal magnetization or a phasing of a subsequent echo signal is prepared. For this, two preparation RF pulses PHF1 and PHF2 are irradiated into the examination region 14 (see axis RF). A phase accumulation takes place for this purpose in the time span between irradiating the preparation RF pulses PHF1 and PHF2. Any generated transverse magnetization may be dephased after the second preparation RF pulse PHF2 with a spoiler gradient pulse represented here on the axes G.sub.P and G.sub.S.

(19) The subsequent acquisition section AS includes a gradient echo train. Only four elements AS1, AS2, AS3, AS4 of the gradient echo train are represented here, but the gradient echo train may include many more elements. For example, the number of elements corresponds to the number of k-space lines acquired for each echo signal. Each element includes a readout RF pulse that is capable of triggering at least two echo signals. For example, at least two echoes are refocused in each element of the gradient echo train: at least one stimulated echo (e.g., generated from the longitudinal magnetization prepared in the preparation section PS) and at least one “normal” gradient echo, which may also be regarded as free induction decay (FID) and phasing of which was not prepared in the preparation section PS. Each of the elements includes a readout RF pulse (e.g., AF1 in the element AS1) and two acquisition windows (e.g., FID1 and STE1 in the element AS1) for the acquisition of one echo signal respectively (see axis ADC). One FID echo signal, respectively, is acquired during the acquisition windows FID1, FID2, FID3 and FID4. One stimulated echo signal, respectively, is acquired during the acquisition windows STE1, STE2, STE3 and STE4.

(20) In order to set the instants at which the FID echo signal and the stimulated echo signal occurs, first, a gradient pulse G.sub.prep is irradiated into the examination region 14 between the two preparation RF pulses PHF1 and PHF2 during the preparation sections PS. Further, a dephasing gradient pulse G.sub.deph is irradiated into the examination region 14 in each element of the acquisition section AS after irradiating the readout RF pulse, (e.g., AHF1 for AS1) and before the acquisition of the FID signal (e.g., FID1 for AS1) and of the stimulated echo signal (e.g., STE1 for AS1). In addition, a rephasing gradient pulse G.sub.reph is irradiated into the examination region 14 during acquisition of the FID signal and of the stimulated echo signal during the acquisition section AS.

(21) In general, the respective echo signal occurs if the rephasing gradient moment is the same as a preceding dephasing gradient moment. The instants of the FID echo signal and of the stimulated echo signal may be set accordingly by the timing and the form (e.g., the sign and amounts) of the gradient pulses G.sub.prep, G.sub.deph and G.sub.reph.

(22) The unit including preparation section PS and acquisition section AS may be repeated for the measurement of different slices, and an implementation for 3D measurement is likewise possible.

(23) The stimulated echo has a different history or prepared magnetization, and therewith a different signal behavior, to the FID echo. The longitudinal magnetization prepared in the preparation section PS relaxes during the course of the echo train, so the transverse magnetization generated by the readout RF pulses AHF1, AHF2, AHF3, AHF4 likewise decreases during the course of the echo train. As is represented in FIG. 3, the magnetization component, which is available for the formation of the stimulated echoes STE, consequently decreases during the echo train. By contrast, the fraction of the unprepared longitudinal magnetization, from which the FID echo FID is formed, increases during the course of the echo train, and this may accordingly result in an increase in the transverse magnetization generated by the readout RF pulses AHF1, AHF2, AHF3, AHF4. As a result, it may be the case that data sets or images reconstructed from the different echoes, STE compared to FID, therefore, exhibit different artifacts.

(24) Conventional sequences often use a linear reordering in which the k-space is run through linearly to a certain extent, beginning with the outer k-space lines of the one half of the k-space, then central lines and then outer lines of the other half of the k-space. This is shown by way of example in FIG. 4 for eight k-space lines.

(25) One possibility for the optimization of the sequence may be to apply a centric reordering to both echoes (e.g., the k-space regions sampled by the echo signals the k-space are brought into an order), so the central k-space lines are measured first of all. This is represented by way of example in FIG. 5 for eight k-space lines. For one possible calculation of a B.sub.1 map, which is calculated from the magnitude ratios of the two data sets, this may be advantageous in order to minimize any distortions of the B.sub.1 values to be calculated caused by T.sub.1 effects. This may also be advantageous for the calculation of a B.sub.0 map that is calculated from phase differences of the two data sets in order to maximize the signal-to-noise ratio of the stimulated echo signal. The central k-space lines of the stimulated echoes STE are then acquired while the magnetization available for this is at a maximum.

(26) This is shown in FIG. 3, which in a simplified manner represents magnetization components that are available for the generation of the stimulated echo STE and of the FID echo FID, as a function of the time t following the preparation section PS. An image that is reconstructed from the stimulated echo STE and magnetization of which decreases during the course of the echo train then conventionally has blurring in the phase encoding direction, while the one image, which is reconstructed from the FID echo FID, may have an edge overshoot and ghosting of the edges in the phase encoding direction; see the upper illustration in FIG. 8. Both effects may contribute to maps ascertained from the acquired FID echo signals and stimulated echo signals, such as B.sub.1 maps or B.sub.0 maps, being distorted to a great extent by artifacts. A further problem of centric reordering emerges: even if the signal of the central k-space lines of the stimulated echoes STE is maximized as a result, the central k-space lines of the FID echo FID are measured while the magnetization thereof is at a minimum. The has an adverse effect on the signal-to noise ratio of the image reconstructed from the FID echo signal.

(27) Conventional approaches to the minimization of the above-described problems envisage an optimization of the sequence over time. Due to the minimization of the length of the echo train the effects of the T.sub.1 relaxation over this echo train, and therewith the opposed artifacts (e.g., blurring of the stimulated echoes and edge overshoot in the FID echo signal) are also minimized.

(28) The present embodiments may provide the possibility of effectively reducing the above-described artifacts of the two echoes STE and FID and of maximizing the echo signals of the two echoes.

(29) For example, provision is made in this case for using different reorderings for the individual types of echo in a multi-echo method (e.g., different orders of the acquisitions of the k-space regions, such as k-space lines, for the different types of echo). It is conventional for the same reordering to be applied to both echoes. Examples of different reorderings are shown in a simplified manner in FIGS. 4-6 for eight k-space lines. A linear reordering is shown in FIG. 4. The k-space lines are, for example, measured in turn (e.g., in ascending or descending order). A centric reordering is shown in FIG. 5. In FIG. 5, the central k-space line is measured first, then the k-space lines located further out are successively measured. The order of the k-space regions sampled by the echo signals is characterized in that spacing from the k-space center increases during the course of acquisition of the echo signals, therefore. Anticentric reordering is shown in FIG. 6. In FIG. 6, the outer k-space lines are measured first, and then, k-space lines located further in are successively measured until, at the end, the central k-space line is measured. The order of the k-space regions sampled by the echo signals is characterized in that spacing of the k-space regions from the k-space center decreases during the course of acquisition of the echo signals.

(30) Even further reorderings may be provided, however. For example, randomized arrangements and different interleavings of the acquisition order of the two phase encoding directions in the case of 3D measurements may be provided.

(31) By way of example, the following imaging sequence is provided: a sequence with a preparation section PS and an acquisition section AS is used, with the acquisition section AS including an echo train with a plurality of elements, where at least two echo signals in each element are used for acquisition. A B.sub.0 map may be reconstructed from the at least two echo signals in that the phase differences of the at least two echo signals are used for the calculation thereof. At least two different reorderings are used for the at least two echo signals. For example, centric reordering is used for at least one of the echo signals; anticentric reordering is used for at least one other echo signal.

(32) For example, an imaging sequence according to FIG. 7 may be provided. Here, two echo signals are used per element of the echo train AS1, AS2, AS3, AS4 or per readout RF pulse AHF1, AHF2, AHF3, AHF4 (e.g., the FID echo signal with an anticentric reordering and the stimulated echo signal with a centric reordering). A B.sub.0 map may be reconstructed from the phase differences of the data sets reconstructed from the two echo signals. The method provided is not limited to this special case.

(33) The different reordering of the individual echoes may be generated by additional gradient pulses in the phase encoding direction between the acquisition windows of the echoes (e.g., by the gradient pulse GP.sub.1 between the acquisition windows FID1 and STE1 in the element AS1). The same applies to the gradient pulses GP.sub.2, GP.sub.3, and GP.sub.4 in the following elements AS2, AS3 and AS4. The first echo signal (e.g., in the example shown, the FID echo signal), therefore, is anticentrically ordered here, while the second echo signal (e.g., in the example shown, the stimulated echo), therefore, is centrically ordered.

(34) The different reorderings for the stimulated echo and the FID echo provide that the artifacts of the two types of echo become more similar: if the central k-space lines of the two echoes are measured when the maximum magnetization is available (see FIG. 3: STE at the start, FID at the end), blurring will appear in the phase encoding direction in the images of the two echoes. In the data calculated from the two echo signals (e.g., a B0 map), the similarity of the artifacts may be advantageous since then, for example, edge artifacts are no longer intensified to the usual extent. In addition, the combination of a centric reordering with an anticentric reordering results in the possible advantage that the signal-to-noise ratio of both the stimulated echo signal and of the FID echo signal are maximized simultaneously. With a centric acquisition for the two echoes, this maximization may only be attained for the stimulated echo STE. As a result, the flip angle of the preparation RF pulses PHF1, PHF2 may be increased without the FID echo signal suffering too greatly as a consequence.

(35) A comparison of measured data of a phantom is shown in FIG. 8. In the top illustrations, the data was acquired with a centric reordering for the stimulated echo signal and the FID signal. In the bottom illustrations, the data was acquired with a centric reordering for the stimulated echo signal and an anticentric reordering for the FID signal. Here, an image reconstructed from the FID echo signal is represented on the left, an image reconstructed from the stimulated echo signal is represented in the center, and a B.sub.0 map derived therefrom is represented on the right.

(36) The image reconstructed from the FID echo signal, for example, exhibits considerably fewer artifacts and an increased signal-to-noise ratio if an anticentric reordering is applied. This also manifests itself in the improved signal-to-noise ratio and the reduced artifacts in the B.sub.0 map. While with the same reordering considerable edge ghosting artifacts may clearly still be seen (illustration top right) in the vertical direction (e.g., the phase encoding direction applied here), this is barely still the case (illustration bottom right) with opposed reordering.

(37) The methods described in detail above and the represented magnetic resonance device are merely exemplary embodiments that may be modified in a wide variety of ways by a person skilled in the art without departing from the scope of the invention. Further, use of the indefinite article “a” or “an” does not preclude the relevant features from also being present multiple times. Similarly, the term “unit” does not preclude the relevant components from including a plurality of cooperating sub-components that may optionally also be spatially distributed. Further, the term “echo” and “echo signal” may usually be used synonymously.

(38) 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.

(39) 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.