WO2003098267A1

WO2003098267A1 - Chemical species suppression in magnetic resonance imaging

Info

Publication number: WO2003098267A1
Application number: PCT/US2003/015241
Authority: WO
Inventors: Christopher A. Flask; Jonathan S. Lewin; Hisamoto Moriguchi; Claudia Hillebrand; Jeffrey L. Duerk; Brian M. Dale
Original assignee: Case Western Reserve University
Priority date: 2002-05-17
Filing date: 2003-05-15
Publication date: 2003-11-27
Also published as: AU2003234577A1

Abstract

A new improved method for chemical species suppression MRI (10) including the steps of: (12) generating at least one pulse sequence using a first TE for a first data set having a first trajectory, and a second TE for a second data set having a second trajectory, (14) acquiring the first data set and the second data set without chemical species suppression wherein a substantial majority of the second trajectory covers k-space that is not covered by the first trajectory, (16) combining the first and second data sets to suppress the second chemical species, present in both data sets, and (18) generating an MRI image that shows the unsuppressed chemical species while suppressing the chemical species of the preceding step.

Description

CHEMICAL SPECIES SUPPRESSION IN MAGNETIC RESONANCE IMAGING

BACKGROUND OF THE INVENTION

The present invention relates to a chemical species suppression in Magnetic Resonance Imaging (MRI). More particularly, the invention relates to a method for species suppression using two or more trajectories having different echo times (TEs) and covering substantially mutually exclusive portions of k-space.

A number of suppression techniques have been implemented to differentiate chemical species in MRI imaging. Suppression techniques, known as fat suppression, are often used to in MRI to differentiate fat from water in an imaged object. The majority of fat suppression techniques require specialized Radio Frequency RF excitation schema to selectively excite water protons or saturate fat protons. Binomial and CHESS are known excitation schema which are extremely effective at 1.5T. However, these techniques have encountered limitations clinically. For example, at higher field strengths imposed Specific Absorption Rate (SAR) constraints limit the range of spectrally selective excitations for fat suppression. At lower field strengths, Tl and T2 relaxation causes problems, and the increased duration of the RF excitations needed for the low field strengths can cause undesirable extension to the overall imaging time.

Known multi-point Dixon fat suppression methods take advantage of the relative difference in precession frequency of fat and water to create water and fat images from at least two full acquisitions. Higher order Dixon methods can be used to correct for field inhomogeneities and/or susceptibility artifacts in the fat suppressed images. Dixon fat suppression is obtained without specialized RF excitation pulses, thereby eliminating the SAR constraints of binomial and CHESS excitations. However, the multi-point Dixon methods significantly extend the overall imaging time by requiring multiple acquisitions of the some of the same portions of k-space. Spiral imaging is a widely used non-uniform data acquisition technique in MRI. Currently, spatially spectrally selective excitation RF pulses (spsp) are the most commonly used methods for fat suppression in spiral imaging. However, these excitation pulses may result in a significant increase in total acquisition time; the time for application of spsp pulses is near the same order time duration as the gradient waveforms used to collect data during coverage of k-space.

Longer acquisition times limit the use of these techniques for rapid or real-time MR applications such as cardiac imaging. Therefore, it is desirable to provide chemical species suppression utilizing short acquisition times for rapid MR imaging applications. It is also desirable to provide more complete k-space sampling than conventional multi-Point Dixon acquisitions of the same duration to decrease blurring and streak artifacts associated with angular undersampling or that result in other ways from the k-space trajectory.

SUMMARYOF THE INVENTION According to the present invention, a new and improved method for chemical species suppression MRI is provided.

In accordance with a first aspect of the invention, the invention includes acquiring a first data set corresponding to a first echo time (TE) and having a first trajectory, acquiring a second data set corresponding to a second TE and having a second trajectory, wherein a substantial majority of the second trajectory covers k- space not covered by the first trajectory, and combining the first and second data sets to suppress the second chemical species. h accordance with a second aspect of the invention the method includes generating an MRI image including the first chemical species and suppressing the second chemical species. h accordance with another aspect of the invention the method includes generating at least one pulse sequence using a first TE for the first data set and a second TE for the second data set. Other features, benefits and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may take form in certain components and structures, preferred embodiments of which will be illustrated in the accompanying drawings wherein: Fig. 1 illustrates steps of the invention; Fig. 2 illustrates alternating radial k-space trajectories in accordance with the invention; and

Fig. 3 illustrates interleaved reverse spiral k-space trajectories in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1 a method for chemical species suppression in MRI is shown generally at 10. The method includes generating a pulse sequence at 12 using a first echo time (TE) for a first data set, the first data set having a first k-space trajectory, and using a second TE for a second data set, the second data set having a second k-space trajectory. The generating step at 12 can include generating one pulse sequence using different TEs, or more than one pulse sequence using different TEs.

The method 10 also includes acquiring the first data set and the second data set at 14, wherein a substantial majority of the second trajectory covers k-space not covered by the first trajectory as will be described in farther detail below. The first and second data sets can be acquired in a single acquisition, or in separate acquisitions.

The second TE is set in the generating step at 12 above such that the magnetization of the second chemical species in the second data set is out of phase from its orientation in the first data set. The method 10 also includes combining the first and second data sets to suppress one of the chemical species at 16. The method 10 can also include generating an MR image at 18 including one of the chemical species and suppressing the other of the chemical species. Any suitable known reconstruction method can be used to generate the MR image at 18.

Any suitable pulse sequence with a first TE corresponding to a first data set having a first trajectory and a second TE corresponding to a second data set having second trajectory can be used, wherein a substantial majority (typically more than 99%) of all of the second trajectory covers k-space not covered by the first trajectory. For the examples given below, the first chemical species is water and the second chemical species is fat. Although alternatively, the first chemical species can be fat and the second chemical species can be water. Alternatively, the first and second chemical species can be any other suitable chemically shifted chemical species.

Referring now to Fig. 2, by way of example which should not be considered limiting, the pulse sequence generated at 12 can be for a steady-state radial sequence for generating radial k-space projections shown generally at 20. The pulse sequence uses alternating TEs, the first TE corresponding to a first data set for producing a first trajectory including even radial k-space projections shown using solid lines at 22. The second TE corresponds to a second data set for producing a second trajectory including odd radial k-space projections shown using dashed lines at 24.

The pulse sequence is chosen so that a substantial majority, typically more than 99%, and more preferably more than 99.9%, of the second trajectory 24 covers k-space not covered by the first trajectory 22. The first and second trajectories 22, 24 can cover a small amount of the same k-space which, for example, can be portions of k- space near the origin. However, very little of the two trajectories 22, 24 cover the same k-space, and thus the two trajectories can be considered as substantially mutually exclusive. The first and second trajectories 22 and 24 typically cover less than 5% of the same k-space, preferably less than 1% of the same k-space, and often less than 0.1%) of the same k-space.

The pulse sequence used was a radial FLASH (Fast Low Angle SHot) sequence that allowed alternate TE settings between even and odd lines in radial k-space (α=20°, TR = 20ms). The TE values were established to acquire the fat and water signals out- of-phase with each other in one data set, using a TE = 6.6ms, and to acquire the fat and water signals in-phase with each other in the other data set using a TE = 8.8ms at 1.5T. A Siemens Sonata MR scanner was used although any suitable MR scanner using any suitable magnetic field strength can be used. Alternatively, any other pulse sequence(s) for generating suitable radial trajectories can be used including those using different TEs, TRs, and tip angles.

Since the fat magnetization detected in one data set was out-of-phase with the fat magnetization detected in the other data set, combining the first and second data sets in step 16 results in suppressing the fat signals. The same k-space data can also be used to suppress the water signals with no additional acquisitions by multiplying either the even or the odd k-space lines by -1.

The method 10 can also include generating an MR image at 18 including the first chemical species and suppressing the second chemical species using the combined data sets. A known fast look-up table image reconstruction method was used to generate the image at step 18 although any suitable method can be used.

The fat suppression obtained from the alternating TE sequence was evaluated by comparing the alternating radial TE sequence with a known radial 2-Point Dixon method using a similar FLASH sequence. Each projection in the radial 2-Point Dixon trajectory was acquired twice along the same k-space trajectory for both the in-phase and out-of-phase echo times. The in-phase and out-of-phase radial 2-Point Dixon data sets were summed prior to image reconstruction to produce fat-suppressed images.

To better quantify the effects of azimuthal undersampling, point-spread functions (PSF) were measured for the radial alternating TE and radial 2-Point Dixon sequences. It is known that more complete k-space coverage results improved effective resolution, in less streak artifacts and also in an increased diameter of the primary ring-lobe in the PSF. The radial alternating TE sequence provided improved k-space coverage for the on-resonance spins. It was found that the PSF from the radial alternating TE sequence provided a primary ring-lobe with a larger diameter than the PSF from the radial 2-Point Dixon sequence thereby demonstrating an improved effective resolution.

To generate the measured PSFs, a 3ml syringe (8mm ID) was filled with saline and positioned with its long axis aligned with the main magnetic field. It was placed near isocenter in the magnet to act as an approximation to a point signal source for axially acquired images. The sequences were all executed with a 300mm FOV to improve the visualization of the PSFs. On-resonance (i.e., fat-suppressed) PSFs from the radial alternating TE and radial 2 point Dixon sequences for both 128 and 256 projections were generated and compared. The on-resonance radial alternating TE PSFs were obtained by directly gridding and transforming the radial k-space data from the radial alternating TE acquisition of the saline syringe. The on-resonance radial 2-Point Dixon PSFs were generated by summing the in-phase and out-of-phase data sets prior to gridding and image reconstruction.

As expected, the diameter of the primary ring-lobe in the on-resonance radial alternating TE PSF was found to be twice that of the on-resonance radial 2 point Dixon PSF with the same total number of acquired projections. These results demonstrate that the radial alternating TE sequence provided twice the k-space coverage of the radial 2-Point Dixon sequence for unsuppressed, on-resonance water spins. Thus, the image generated in accordance with the invention has twice the effective resolution of an image created with two data sets, similar in size to the first and second data sets, generated from two TEs and having trajectories covering the same k-space trajectory.

Off-resonance (i.e., water-suppressed) PSFs from the radial alternating TE sequence with 128 and 256 projections were also generated to examine the effects of incomplete nulling of the fat signal at higher spatial frequencies and TE variation on the level of blurring and streak artifacts. The off-resonance PSFs were obtained by performing a fat reconstruction (water suppression) from the same saline phantom data sets used to create the on-resonance radial alternating TE PSFs. The off-resonance PSFs were reconstructed by multiplying the in-phase projections by -1 prior to gridding. The off-resonance PSFs have primary ring-lobe diameters that are equivalent to the on-resonance radial 2 point Dixon PSFs and smaller than the on-resonance radial alternating TE PSFs.

Phantom images from the radial 2-Point Dixon and radial alternating TE sequences with 128 total projections (equal scan times) were generated. The streak artifacts from the radial alternating TE sequence were found to be less pronounced and better distributed than the radial 2-Point Dixon images. The CNRs of the phantom images from these two sequences were equivalent at approximately 50, with 256 projections (128 in-phase and 128 out-of-phase). The radial alternating TE sequence provided a marginally better CNR (36 vs. 33) with 128 projections. The radial alternating TE sequence generated in accordance with the invention provided effective fat suppression with improved k-space coverage resulting in reduced artifacts in the reconstructed images as compared to the radial 2-Point Dixon sequence for equal scan times. The reduction in artifacts suggests an opportunity to improve the temporal resolution of the radial 2-Point Dixon method by reducing the total number of views required to obtain the same image quality. The main advantage of improving the temporal resolution of the Dixon methods is to provide a fast and efficient method for fat suppression without the SAR constraints and/or acquisition time increases of CHESS pulses and inversion recovery sequences. The invention can also be extended to other trajectories including but not limited to spiral trajectories, rosette trajectories, and rectilinear echo planar imaging (EPI) to provide suppression of a chemical species in real-time sequences.

Referring now to Fig. 3, by way of a second example which should not be considered limiting, the invention can also use interleaved reversed spiral imaging generating spiral trajectories shown generally at 30 using alternate TEs for fat suppression. In the pulse sequence generated at 12 alternating TEs were used for even reversed spiral interleaves, shown as solid lines at 32, and odd reversed spiral interleaves, shown as dashed lines at 34.

A 12-interleave reversed spiral sequence was generated on a Siemens 1.5 T Magnetom Sonata (Siemens, Erlangen, Germany), though any suitable number of interleaves may be used. The first TE = 11.0ms was used for the first data set corresponding to even spiral interleaves 32, and the second TE = 13.2ms was used for the second data set corresponding to odd spiral interleaves 34, with TR = 18.0ms. The sequence flip angle, slice thickness and field of view (FA/TH/FOV) were 10°, 10mm, and 210mm, respectively. Separate pulse sequences can be used for the interleaves or a single shot sequence can be used. Further, any other pulse sequence(s) for generating suitable spiral sequences can be used, including those using different TEs, TRs, etc.

The TEs used generated fat signals in one data set which were out-of-phase with the fat signals in the other data set, similar to the first embodiment described above. The TE used allowed the fat magnetization to rotate 180° out of phase between successive interleaves. As a result, the fat signal cancels in the lower frequency region of k-space in the combining step 16 in a similar manner as the first embodiment described above.

The reversed spiral sequence was designed to oversample the central k-space region by a factor of 2. The radius of the oversampled region was 50% of k_max, though any suitable amount of oversampling my be used. With oversampling, a higher density of data points are generated in a region. However, a substantial majority of the second trajectory still covers k-space not covered by the first trajectory. The first and second interleaves 32 and 34 typically cover less than 5% of the same k-space, preferably less than 1% of the same k-space, and often less than 0.1% of the same k-space. The results obtained by this embodiment of the invention were compared with a 12-interleave reversed spiral sequence using a constant a TE (TE/TR=8.5/17.0ms, 10° tip angle) with and without a 1-2-1 spatial-spectral excitation pulse scheme. Each of the sequences was used to scan a portion of the right thigh from a healthy volunteer. All the images were reconstructed using a known Block Uniform Resampling (BURS) regridding algorithm with minor modifications.

The images generated in step 18 demonstrated that the alternating reversed spiral TE sequence provides fat suppression equivalent to the reversed spiral sequence with a spatial-spectral excitation pulse. Although some artifacts were visible, the invention requires only 2.2ms delay for even (or alternatively odd) interleaves in a 1.5 T MR system. In a 12 interleave spiral sequence, this results in a 13.2(=2.2x6) msec increase in scan time for fat suppression through TE alternation, versus about 60 msec additional scan time for a 5 msec spatial spectral pulse (5 msec x 12 interleaves). Thus, the chemical species suppression technique using alternating TEs in interleaved reversed spiral imaging is a faster suppression technique than known reversed spiral imaging techniques reducing the acquisition time of the reversed spiral sequences through the elimination of long spatial-spectral excitation RF pulses.

The invention can also be used for forward spiral interleaves. The same alternating TE technique was implemented in a forward spiral sequence with (TE(odd)/TE(even)/TR=2.0/4.2/18.0ms, 10° tip angle). The reversed spiral imaging used in accordance with the invention demonstrated some advantages over forward spiral imaging used in accordance with the invention including better resolution and more T2* weighted contrast. The invention has been described with reference to preferred embodiments.

Obviously, modifications and alterations will occur to others upon reading and understanding the preceding specification. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

What is claimed is:

1. A method of chemical species suppression for Magnetic Resonance imaging (MRI) of a subject having at least two chemically shifted chemical species with different resonant frequencies comprising: acquiring a first data set corresponding to a first echo time (TE) and having a first trajectory; acquiring a second data set corresponding to a second TE and having a second trajectory, wherein a substantial majority of the second trajectory covers k-space not covered by the first trajectory; and combining the first and second data sets to suppress the second chemical species.

2. The method defined in claim 1 generating an MRI image including the first chemical species and suppressing the second chemical species.

3. The method defined in claim 1 further including generating at least one pulse sequence using a first TE for the first data set and a second TE for the second data set.

4. The method defined in claim 3 wherein the second TE is set such that the magnetization of the second chemical species is out of phase from its orientation in the first data set.

5. The method defined in claim 3 wherein the first and second TE values are set such that the magnetization of the first and second chemical species is out-of- phase with each other in one data set and in-phase with each other in the other data set, and the magnetization of one of the chemical species has the same phase in both data sets.

6. The method defined in claim 1 wherein the first and second trajectories are radial trajectories.

7. The method defined in claim 6 wherein the first trajectory includes even lines in radial k-space and the second trajectory includes odd lines in radial k-space.

8. The method defined in claim 3 wherein the pulse sequence is a radial FLASH (Fast Low Angle SHot) sequence.

9. The method defined in claim 8 wherein the first chemical species is water and the second chemical species is fat, the first TE = 6.6ms and the second TE = 8.8ms at 1.5T α=20° and TR = 20ms.

10. The method defined in claim 1 wherein the second trajectory covers less than 1 % of the k-space of the first traj ectory.

11. The method defined in claim 10 wherein the second trajectory covers less than 0.1% of the k-space of the first trajectory.

12. The method defined in claim 2 wherein the image has twice the effective resolution of an image created with two data sets, similar in size to the first and second data sets, generated from two TEs having trajectories covering each other's k-space.

13. The method defined in claim 1 wherein the first and second trajectories are spiral trajectories.

14. The method defined in claim 13 wherein the first and second spiral trajectories are interleaved.

15. The method defined in claim 13 wherein the first and second trajectories are reverse spiral trajectories.

16. The method defined in claim 13 wherein the first and second trajectories are forward spiral trajectories.

17. The method defined in claim 13 further including generating a pulse sequence using a first TE for the first data set and a second TE for the second data set.

18. The method defined in claim 13 wherein the pulse sequence is an interleaved reversed spiral sequence.

19. The method defined in claim 18 wherein the first TE = 11.0ms and the second TE = 13.2ms at 1.5T with TR = 18ms.

20. The method defined in claim 18 wherein the reversed spiral sequence oversampled the central k-space region by a factor of 2.

21. The method defined in claim 20 wherein the radius of the oversampled region was 50% ofk_max.

22. The method defined in claim 14 wherein the pulse sequence is an interleaved forward spiral sequence.

23. The method defined in claim 22 wherein the first TE = 2.0ms and the second TE = 4.2ms at 1.5T with TR = 18ms.

24. The method defined in claim 1 wherein the first chemically sliifted species is water and second chemical species is fat.

25. The method defined in claim 1 wherein the first chemically shifted species is fat and second chemical species is water.

26. The method defined in claim 1 further including acquiring the first and second data sets in a single acquisition.

27. The method defined in claim 1 further including acquiring the first and second data sets in separate acquisitions.

28. The method defined in claim 27 further including multiplying half of the k-space lines by -1 to produce an image nulling water at DC k-space and summing fat.

29. A method of chemical species suppression for Magnetic Resonance imaging (MRI) of a subject having at least two chemically shifted chemical species with different resonant frequencies comprising: generating at least one pulse sequence using a first echo time (TE) for a first data set and a second TE for a second data set; acquiring the first data set having a first k-space trajectory; acquiring the second data set having a second k-space trajectory, wherein the first and second TE values are set to acquire the first and second chemical species signals out-of-phase with each other in one data set and in-phase with each other in the other data set, one of the chemical species has the same phase in both data sets, and a substantial majority of the second k-space trajectory covers k-space not covered by the first k-space trajectory; combining the first and second data sets to suppress the second chemical species; generating an MRI image including the first chemical species and suppressing the second chemical species.

30. A method of Magnetic Resonance (MR) imaging of one chemical species in a subject having at least two chemically shifted chemical species with different resonant frequencies comprising: acquiring a first data set corresponding to a first echo time (TE) and having a first trajectory; acquiring a second data set corresponding to a second TE and having a second trajectory, wherein a substantial majority of the second trajectory covers k-space not covered by the first trajectory; combining the first and second data sets to suppress the second chemical species; and generating an MRI image including the first chemical species and suppressing the second chemical species.

31. The method defined in claim 30 further including generating at least one pulse sequence using a first TE for the first data set and a second TE for the second data set.

32. The method defined in claim 31 wherein the first and second TE values are set such that the magnetization of the first and second chemical species is out-of- phase with each other in one data set and in-phase with each other in the other data set, and the magnetization of one of the chemical species has the same phase in both data sets.