WO2011080693A1

WO2011080693A1 - Water -fat separation in mri using partial echoes at different echo times

Info

Publication number: WO2011080693A1
Application number: PCT/IB2010/056083
Authority: WO
Inventors: Miha Fuderer; Holger Eggers
Original assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips Electronics NV
Current assignee: Philips Intellectual Property and Standards GmbH; Koninklijke Philips NV
Priority date: 2009-12-30
Filing date: 2010-12-27
Publication date: 2011-07-07
Anticipated expiration: 2012-06-30

Abstract

The invention relates to a method of imaging at least two chemical species using magnetic resonance imaging with signal separation for the at least two chemical species, the method comprising: acquiring dual partial echoes (204, 210, 406, 408, 502, 506, 10, 612) at different echo times, wherein the partial echoes at the respective echo times are located in opposite regions of k- space, processing the partial echoes for reconstruction of a first and second image data set, the first and second image data set comprising separate image data of the first and second chemical species.

Description

WATER -FAT SEPARATION IN MRI USING PARTIAL ECHOES AT DIFFERENT ECHO TIMES

FIELD OF THE INVENTION

The present invention relates to a method of imaging at least two chemical species using magnetic resonance imaging (MRI) with signal separation for the at least two chemical species, a computer program product and a magnetic resonance imaging apparatus for imaging at least two chemical species with signal separation for the at least two chemical species.

Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.

According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the coordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view, the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis (also referred to as longitudinal axis), so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°).

After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant Tl (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the

magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.

In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The signal data obtained via the receiving coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to an MR image by means of Fourier transformation.

In MRI, it often is desired to obtain information about the relative contribution of two dominant chemical species, usually water and fat, to the overall signal, either to suppress the contribution of one of them or to separately or jointly analyze the contribution of both of them. These contributions can be calculated if information from two or more corresponding echoes, acquired at different echo times, is combined. This approach to water fat separation is commonly referred to as "Dixon water/fat imaging" or "Dixon imaging".

Dixon imaging usually relies on the acquisition of at least two echoes to separate water and fat signal. In its simplest form, water and fat images are generated by either addition or subtraction of the "in-phase" and "out-of-phase" data sets. This separation is possible because of the precessional frequency difference of hydrogen in fat and water.

One drawback of Dixon imaging is that the acquisition of at least two echoes results in a substantial prolongation of the overall scan time.

BACKGROUND OF THE INVENTION

US 2006/0250132 Al does disclose a homodyne reconstruction of water and fat images based on iterative decomposition of MRI signals. Homodyne image reconstruction is combined here with an iterative decomposition of signals for at least two species such as water and fat in MR signals obtained from a partial k-space signal acquisition in order to maximize the resolution of calculated water and fat images.

However, the problem here remains that the total data acquisition time is unacceptable long since nevertheless low resolution image information has to be acquired at multiple echo times by means of the Dixon imaging approach described above.

SUMMARY OF THE INVENTION

From the foregoing it is readily appreciated that there is a need for an improved MR imaging method. It is consequently an object of the invention to enable MR imaging in a fast and reliable manner in order to determine the relative contribution of two dominant chemical species to an acquired overall signal.

In accordance with the invention, a method of imaging at least two chemical species using magnetic resonance imaging with signal separation for the at least two chemical species is disclosed. The method comprises acquiring dual partial echoes at different echo times, wherein the partial echoes of the respective echo times are located in opposite regions of k-space. In a further step, processing of the partial echoes for

reconstruction of a first and second image data set is performed, wherein the first and second image data set comprise separate image data of the first and second chemical species. It has to be mentioned that US 2006/0250132 Al considers the case of three (or more) echo times and multi-acquisition (instead of multi-echo) sequences only, whereas in the present invention for example two echo times and multi-echo sequences, where all echoes are partial, are desired.

Since in all measurements only partial echoes are sampled, this allows to either reduce the repetition time for a given phase difference between water and fat protons or to increase the phase difference between water and fat protons at a given repetition time. As a consequence, either the data acquisition process can be accelerated or the determination of the relative contribution of the two dominant chemical species to a given overall signal can be made more reliable or the SNR in the separated images can be optimized.

In accordance with an embodiment of the invention, the processing of the partial echoes comprises a Dixon reconstruction technique.

In accordance with a further embodiment of the invention, the two chemical species are water and fat. In accordance with a further embodiment of the invention, the dual partial echoes are acquired by means of a balanced fast field echo pulse sequence (BFFE sequence). This has the advantage, that the repetition rate for data acquisition can be further increased. In this case, the second partial echo immediately follows the first partial echo such that these two echoes are embedded between two refocusing pulses. As a consequence, less refocusing pulses are required in a given time frame in order to achieve a reliable separation of image data of water and fat based on the acquired echo data.

In accordance with a further embodiment of the invention, the processing of the partial echoes comprises removing the effect of main field inhomogeneity of the main magnetic field used for imaging the chemical species. This ensures that with respect to the acquired data the main field inhomogeneity excluding chemical shift is only varying smoothly over space. As a consequence, state of the art partial echo reconstruction can be performed which relies on the assumption that all phase variations over space are smooth.

In accordance with a further embodiment of the invention, the echo times are chosen such that a phase shift due to chemical shift is large compared to the phase errors induced by the partial echo reconstruction. As a consequence, the phase shift induced by the partial echo reconstruction will be at most π/2 while the phase shift due to the chemical shift is between π/2 and π such that it will dominate over any phase errors induced by the partial echo reconstruction process. As a consequence, a more precise determination of desired phase shifts due to a chemical shift, depending on the water/fat fraction in the voxel (three dimensional pixel) and the difference in echo times can be performed. Further, said desired phase shift can be distinguished over undesired phase shifts due to partial echo

reconstruction, i.e. e.g. the convolution of the complex image with the Fourier transform of the weighting function applied in k-space. Consequently, possible phase shifts due to chemical shift are constrained and a separation of both effects mentioned above becomes feasible ensuring a good separation of water signal and fat signal.

In accordance with a further embodiment of the invention, the echo times are chosen such that their differences in phase shift due to chemical shift is either 0 or π, independent of the water/fat fraction. This also ensures in a similar manner, as described above, that the possible differences between phase shifts due to the chemical shifts are constrained and a separation of both effects, namely desired and undesired phase shifts, becomes feasible and a clear separation of water and fat signal is obtained.

In accordance with a further embodiment of the invention, a central k-space area is covered by each partial echo. This allows deriving low resolution estimates of water signal, fat signal, and field strength from two such echoes without relying on a partial echo reconstruction. This information can then be used to remove the influence of the main field inhomogeneity from the data and additionally to optionally constrain subsequently derived high resolution estimates.

For example, on the lowest resolution level, the estimate on the field strength may be initialized using a priori information, for instance from results for adjacent slices or for models of the field distortion. Otherwise, the estimate from the previous iteration on the low resolution level may be interpolated to the current resolution level. An appropriate starting point may be selected, usually based on criteria such as the signal strength, the estimate of the field strength at the spatial position, and the fit error. The water signal, the fat signal and the field strength are calculated along a predefined path covering the field of view, for instance with a least squares estimation. Dependent on the obtained results and the signal- to-noise ratio (SNR), the estimates are either marked as reliable or unreliable. The calculation is preferably performed in several passes, wherein in this way pixels for which the estimates are initially considered as unreliable may benefit from the availability of more reliable information in their vicinity in later passes. These steps may be repeated for two or more resolution levels.

Constraining the deviation of the field strength from the estimate found on a coarser resolution level prevents to a large extend the local swapping of water and fat signal on finer resolution levels. The allowed maximum deviation may be made dependent on the local signal strength and the spatial position, for instance to account for larger variations in the field strength at tissue air interfaces and towards the edges of the homogeneity volume.

Propagating the field strength from one slice to adjacent slices considerably reduces the likelihood of the swapping of water and fat signal in whole slices. Alternatively, a multi path strategy or a path covering the field of view may be extended to 3D.

In another aspect, the invention relates to a magnetic resonance imaging apparatus for imaging at least two chemical species with signal separation for the at least two chemical species, wherein the apparatus comprises a magnetic resonance imaging scanner for acquiring magnetic resonance image data, a controller adapted for controlling a scanner operation of acquiring dual partial echoes at different echo times, wherein the partial echoes of the respective echo times are located in opposite regions of k-space. The apparatus further comprises a data reconstruction system adapted for processing the partial echoes for reconstruction of a first and second image data set, wherein the first and second image data set comprises separate image data of the first and second chemical species. In accordance with an embodiment of the invention, the data reconstruction system may be further adapted for processing of the partial echoes by means of a Dixon reconstruction technique.

In general, in accordance with an embodiment of the invention, the apparatus may be adapted in order to carry out any of the method steps described above.

The method of the invention can be advantageously carried out in most MR devices in clinical use at present. To this end, it is merely necessary to utilize a computer program by which the MR device is controlled such that it performs the above explained method steps of the invention. The computer program may be present either on a data carrier or be present in a data network so as to be downloaded for installation in the control unit of the MR device. Therefore, the invention also relates to a computer program product comprising computer executable instructions to perform the method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The enclosed drawings disclose preferred embodiments of the present invention. It should be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention. In the drawings:

Fig. 1 shows an MR device for carrying out the method of the invention;

Figs. 2a,2b illustrate a pulse sequence diagram for Dixon imaging;

Figs. 3a,3b show a pulse sequence diagram employing a combination of a fast spin echo sequence with partial echo imaging;

Fig. 4 illustrates a pulse sequence diagram in which a fast spin echo sequence with interleaving of echo times by RF pulses is used;

Fig. 5 illustrates a pulse sequence diagram in which a GRASE like fast spin echo sequence with three echoes is used in combination with partial echo imaging;

Fig. 6 illustrates a pulse sequence diagram in which a balanced fast field echo in combination with dual partial echoes is used;

Figs. 7a,7b show a pulse sequence diagram employing a combination of a fast spin echo sequence with partial echo imaging and slightly asymmetric echo time formation.

DETAILED DESCRIPTION OF EMBODIMENTS

With reference to Figure 1, a MR device 1 is shown. The device comprises superconducting or resistive main magnet coils 2 such that a substantially uniform, temporally constant main magnetic field is created along a z-axis through an examination volume.

A magnetic resonance generation and manipulation system applies a series of RF pulses and switched magnetic field gradients to invert or excite nuclear magnetic spins, induce magnetic resonance, refocus magnetic resonance, manipulate magnetic resonance, spatially and otherwise encode the magnetic resonance, saturate spins, and the like to perform MR imaging.

Most specifically, a gradient pulse amplifier 3 applies current pulses to selected ones of whole-body gradient coils 4, 5 and 6 along x, y and z-axes of the

examination volume. A RF frequency transmitter 7 transmits RF pulses or pulse packets, via a send-/receive switch 8, to a whole-body volume RF coil 9 to transmit RF pulses into the examination volume. A typical imaging sequence is composed of a packet of RF pulse segments of short duration which taken together with each other and any applied magnetic field gradients achieve a selected manipulation of nuclear magnetic resonance. The RF pulses are used to saturate, excite resonance, invert magnetization, refocus resonance, or manipulate resonance and select a portion of a body 10 positioned in the examination volume. The MR signals are also picked up by the whole-body volume RF coil 9.

For generation of MR images of limited regions of the body 10, a set of local array RF coils 11, 12, 13 are placed contiguous to the region selected for imaging. The array coils 11, 12, 13 can be used to receive MR signals induced by body-coil RF transmissions.

The resultant MR signals are picked up by the whole body volume RF coil 9 and/or by the array RF coils 11, 12, 13 and demodulated by a receiver 14 preferably including a preamplifier (not shown). The receiver 14 is connected to the RF coils 9, 11, 12 and 13 via send-/receive switch 8.

A host computer 15 controls the gradient pulse amplifier 3 and the transmitter

7 to generate any of a plurality of imaging sequences, such as echo planar imaging (EPI), echo volume imaging, gradient and spin echo imaging, fast spin echo imaging, and the like. For the selected sequence, the receiver 14 receives a single or a plurality of MR data lines in rapid succession following each RF excitation pulse. A data acquisition system 16 performs analogue-to-digital conversion of the received signals and converts each MR data line to a digital format suitable for further processing. In modern MR devices the data acquisition system 16 is a separate computer which is specialized in acquisition of raw image data.

Ultimately, the digital raw image data is reconstructed into an image representation by a reconstruction processor 17 which applies a Fourier transform or other appropriate reconstruction algorithms. The MR image may represent a planar slice through the patient, an array of parallel planar slices, a three-dimensional volume, or the like. The image is then stored in an image memory where it may be accessed for converting slices, projections, or other portions of the image representation into appropriate format for visualization, for example via a video monitor 18 which provides a man-readable display of the resultant MR image.

The host computer 15 and the reconstruction processor 17 comprise a programming by which they are enabled to execute the above-described MR imaging method of the invention.

Figure 2 illustrates a pulse sequence diagram of a state of Dixon imaging technique. For the sake of clarity, the phase encoding and slice selection gradients are not shown in this diagram of a fast spin echo sequence. Only shown are excitation and refocusing pulses, as well as frequency encoding gradients and acquired signals.

The sequence starts in Figure 2a with an excitation pulse 100, followed by a frequency encoding gradient of area b. Subsequently, a refocusing pulse 102 is applied which leads to the formation of an echo 110. A frequency encoding gradient 106 is centered on the centre 112 of the echo 110 and has an area which equals 2b.

This may be repeated with one or more subsequent identical refocusing pulses, interleaved by respective frequency encoding gradients centered on the echo 110 having an area 2b.

Further as illustrated in Figure 2b, a second fast spin echo sequence is applied starting again with an excitation pulse 100 and followed by a frequency encoding gradient 118 of area b. The frequency encoding gradient 118 is followed by a refocusing pulse 120 in such a manner that this time the measurement samples the echo at a slightly different point in time than in the previous measurement illustrated in Figure 2a. In general, one set of measurements (e.g. the top one) samples the echo at a slightly different point in time than the RF echo time to obtain a controlled dephasing between water and fat protons. Another set of measurements (e.g. the bottom row) samples the echo time at another point in time. The difference between these two points in time is denoted by T_WF- In Dixon imaging, the multiple images acquired at different echo times are jointly processed to produce water and fat selective images.

The pulse sequence shown in Figure 2a,b has the disadvantage that in case a large T_WF is required in order to improve the visualization of the difference between water and fat protons, the interval between the refocusing pulses has to be increased. This also increases the repetition time, which is often undesirable.

This problem is solved in Figure 3a,b. Here, also a fast spin echo sequence is shown which consists of two measurements as illustrated in Figure 3 a and Figure 3b. To avoid a prolongation of the interval between refocusing pulses in case a large T_WF is required, both measurements sample only partial echoes, wherein the echoes are partial on opposite sides of k-space. Thus, T_WF is increased for a given time interval between the refocusing pulses. Alternatively, for a given T_WF the time interval between subsequent refocusing pulses may be decreased which allows shortening the repetition time when carrying out the pulse sequence.

In Figure 3a, the first measurement starts with an excitation pulse 100, followed by a frequency encoding gradient 200 of area a. This gradient is followed by a refocusing pulse 102 and a frequency encoding gradient 202, wherein the gradient 202 is dimensioned in such a manner that a partial echo 204 is formed. The frequency encoding gradient 202 consists of an inverted part with area b-a and a positive part of area b+a. After application of the frequency encoding gradient, a further refocusing pulse 104 may be applied, followed by a further frequency encoding gradient of similar type. This may be repeated one or several times. For the sake of completeness it has to be mentioned that the reason for using a combination of an inverted and positive frequency encoding gradient is that the application of a fast spin echo sequence requires keeping the frequency encoding gradient area between two refocusing pulses constant.

The second measurement shown in Figure 3b also starts with an excitation pulse 100, followed by an frequency encoding gradient 206 of area a. Subsequently, a refocusing pulse 120 is applied which is followed by a frequency encoding gradient 208. While in Figure 3 a the frequency encoding gradient 202 was arranged for an echo formation at a time later than the RF echo time, in Figure 3b the gradient 208 is arranged to permit an echo formation at a time earlier than the RF echo time to obtain a controlled dephasing between water and fat protons. As a consequence, a partial echo 210 which builds up in sequence 3b is pushed to an earlier formation time compared to the echo 204 in Figure 3 a.

It has to be noted, that in general with respect to all embodiments the two echo formation times should not be symmetric with respect to the RF echo time, since this makes the separation difficult. Therefore, as shown with respect to Figure 7a,b (which is basically identical to Figure 3), a slightly asymmetric setup is also desired. As further shown in Figure 3b, the frequency encoding gradient may be followed by one or more further refocusing pulses 126, 128, as well as further frequency encoding gradients 210.

Figure 4 illustrates a pulse sequence diagram in which a fast spin echo sequence with interleaving of echo times by RF pulses is used. The sequence starts with an excitation pulse 100, followed by a frequency encoding gradient 400 of area a. This is followed by a refocusing pulse 102 and subsequently a frequency encoding gradient 402. This frequency encoding gradient 402 is similar to the frequency encoding gradient 202 which was described with respect to Figure 3 a. Instead of using two sets of different measurements as discussed with respect to Figures 3a and 3b, in Figure 4 only one set of measurement is used. Therefore, the frequency encoding gradient 402 which leads to the formation of the partial echo 406 is followed by a refocusing pulse 104 and subsequently by a further frequency encoding gradient 404. This frequency encoding gradient 404 is similar to the frequency encoding gradient 208 as discussed with respect to Figure 3b. As a

consequence, a partial echo 408 forms.

Therefore, in one measurement two different echoes appearing at different points in time than the RF echo time are obtained and can be combined in order to obtain information with respect to the relative contribution of the two dominant chemical species like water and fat.

Figure 5 illustrates a GRASE like fast spin echo sequence. GRASE is the abbreviation of "Gradient and Spin Echo" and stands for a hybrid sequence with a combination of a gradient and spin echo sequences.

The GRASE like fast spin echo sequence in Figure 5 comprises three echoes 502, 504 and 506, wherein the echoes 502 and 506 are partial echoes on opposite sides of k- space.

The sequence starts with an excitation pulse 100, followed by a frequency encoding gradient 508 of area a. This is followed by a refocusing pulse 102, a frequency encoding gradient 500 of area a+b which leads to the formation of a partial echo 502, an inverted frequency encoding gradient of area 2b which leads to the formation of the inverted echo 504, as well as a frequency encoding gradient 500 of length b+a which leads to the formation of the partial echo 506.

The principle of dual partial echoes may equally be applied advantageously to balanced fast field echo (BFFE) sequences, as shown in the embodiment of Figure 6. The sequence starts with a pulse a, denoted by the reference numeral 600. A subsequent frequency encoding gradient 606 leads to the formation of two consecutive partial echoes 610 and 612, which are both partial on opposite sides of k-space. Subsequently, a further pulse -a is applied, as denoted by the reference numeral 604. This pulse 604 is again followed by the frequency encoding gradient 606 which similarly as before leads to the formation of two partial echoes 610 and 612 which are formed at different points in time than the RF echo time. The sequence may then be continued by again a further pulse 600.

Again, by means of a combination of the partial echo data, the separation of water and fat is possible. It has to be noted again, that in order to obtain a precise estimation of the relative contribution of the different chemical species to the overall signal, two basic assumptions need to be satisfied, namely that the initial phase (the phase of the signal at excitation) is varying smoothly over space and that the main field inhomogeneity (excluding chemical shift) is varying smoothly over space. Since the central k-space area is fully covered by each echo, it is possible to derive low resolution estimates of water signal, fat signal and off resonance frequency from two such echoes without relying on a partial echo

reconstruction, wherein this information can be used to remove the influence of main magnetic field inhomogeneity from the data and in additional to optionally constrain subsequently derived high resolution estimates.

After removing the effect of the main field inhomogeneity from the data and after applying a partial echo reconstruction to each of the two echoes, the resulting complex images contain two kinds of phase information, namely desired phase shifts due to chemical shift, depending on the water/fat fraction in the voxel and the difference in echo times, and undesired phase shifts due to the partial echo reconstruction.

In order to simplify a separation of these two effects, and thus to simplify a separation of water and fat signals, two strategies to realize respective constraints are to assume that most voxels are containing water or fat only, wherein in this case the echo times are chosen such that phase shift due to the chemical shift is large compared to the phase errors induced by the partial echo reconstruction. The other strategy is to choose the echo times as in- and out-of-phase times such that the phase shift due to chemical shift is always either 0 or π, independent of the water/fat fraction.

Claims

CLAIMS:

1. A method of imaging at least two chemical species using magnetic resonance imaging with signal separation for the at least two chemical species, the method comprising:

acquiring dual partial echoes (204, 210, 406, 408, 502, 506, 10, 612) at different echo times, wherein the partial echoes at the respective echo times are located in opposite regions of k-space,

processing the partial echoes for reconstruction of a first and second image data set, the first and second image data set comprising separate image data of the first and second chemical species.

2. The method of claim 1, wherein the processing of the partial echoes (204, 210,

406, 408, 502, 506, 10, 612) comprises a Dixon reconstruction technique.

3. The method of claim 1, wherein the two chemical species are water and fat.

4. The method of claim 1, wherein the dual partial echoes (204, 210, 406, 408,

502, 506, 10, 612) are acquired by means of a balanced fast field echo pulse sequence or a Fast Spin Echo sequence.

5. The method of claim 1, wherein the processing of the partial echoes (204, 210, 406, 408, 502, 506, 10, 612) comprises removing the effect of inhomogeneity of the main magnetic field used for imaging the chemical species.

6. The method of claim 1, wherein the echo times are chosen such that a phase shift due to chemical shift is large compared to the phase errors induced by the partial echo reconstruction.

7. The method of claim 1, wherein the echo times are chosen such that their difference in phase shift due to chemical shift of one of the two chemical species is equal to π.

8. The method of claim 1, wherein a central k- space area is covered by each partial echo (204, 210, 406, 408, 502, 506, 10, 612).

9. A computer program product comprising computer executable instructions to perform any of the method steps as claimed in any of the previous claims 1 to 8.

10. A magnetic resonance imaging apparatus (1) for imaging at least two chemical species with signal separation for the at least two chemical species, the apparatus comprising: - a magnetic resonance imaging scanner for acquiring magnetic resonance image data,

a controller (15) adapted for controlling a scanner operation of acquiring dual partial echoes (204, 210, 406, 408, 502, 506, 10, 612) at different echo times, wherein the partial echoes at the respective echo times are located in opposite regions of k-space, - a data reconstruction system (17) adapted for processing the partial echoes for reconstruction of a first and second image data set, the first and second image data set comprising separate image data of the first and second chemical species.