ECHO PLANAR IMAGING METHOD CAPABLE OF REDUCING IMAGE DISTORTION

Abstract

An echo planar imaging method capable of reducing image distortion is provided. Two radio frequency (RF) pulses and are used to respectively flip over a longitudinal magnetization into a transverse plane at different moments, so as to obtain observable transverse magnetization; the transverse magnetization obtained by flipping over the longitudinal magnetization by means of and are respectively rephased in two different echo trains by means of gradient pulses. Navigator echo signals are respectively collected before the two gradient echo trains, and the amplitude difference between two gradient echo train signals is corrected on the basis of the amplitude difference between the two navigator echo signals. Correcting the signals collected by the two gradient echo trains, so as to obtain two pieces of image data, and the two pieces of image data are averaged to obtain a final image.

Claims

1. An echo planar imaging method capable of reducing image distortion, based on a first radio frequency (RF) pulse with a flip angle of , a second RF pulse with a flip angle of , a spin echo module, as well as a first gradient echo train and a second gradient echo train that are used for collecting echo-planar signals, and specifically comprising the following steps: step 1: exciting a first-part magnetization into a transverse plane by using an excitation level of the first RF pulse and keeping a second-part magnetization in a longitudinal plane; step 2: after a delay, exciting the second-part magnetization into the transverse plane by using an excitation level of the second RF pulse; step 3: applying the spin echo module; step 4: rephasing the first-part magnetization by using gradient pulses, and detecting a signal of the transverse first-part magnetization in the first gradient echo train to obtain a first signal; step 5: rephasing the second-part magnetization by using gradient pulses, and detecting a signal of the transverse second-part magnetization in the second gradient echo train to obtain a second signal; and step 6: alternately storing the first signal and the second signal along a phase encoding direction to reconstruct k-space, thereby obtaining a final image.

2. The echo planar imaging method capable of reducing image distortion according to claim 1, wherein the flip angle of the first RF pulse is 47, and the flip angle of the second RF pulse is 122.

3. The echo planar imaging method capable of reducing image distortion according to claim 1, wherein in step 2, the delay is equal to a time interval between centers of the first gradient echo train and the second gradient echo train.

4. The echo planar imaging method capable of reducing image distortion according to claim 1, wherein in step 3, the spin echo module containing a 180 RF pulse is applied to obtain T.sub.2-weighted images with reduced distortion.

5. The echo planar imaging method capable of reducing image distortion according to claim 4, wherein in step 3, diffusion-weighted gradients are applied at both sides of the 180 RF pulse to obtain diffusion-weighted images with reduced distortion.

6. (canceled)

7. The echo planar imaging method capable of reducing image distortion according to claim 1, wherein in step 6, first and second navigator echo signals are respectively collected before the first gradient echo train and the second gradient echo train, and an amplitude difference between signals of the first gradient echo train and the second gradient echo train is corrected by using an amplitude difference between the two first and second navigator echo signals.

8. The echo planar imaging method capable of reducing image distortion according to claim 1, wherein in step 6, signals of the first gradient echo train and the second gradient echo train are reconstructed using a MUSSELS method, to obtain two sets of image data, and the two sets of image data are averaged to obtain the final image.

9. The echo planar imaging method capable of reducing image distortion according to claim 1, wherein in step 4 and step 5, phase encoding gradients with different polarities are used for the first gradient echo train and the second gradient echo train; two images are reconstructed from the signals of the first gradient echo train and the second gradient echo train, and magnetic field distribution is estimated based on the two images, thereby correcting image distortion.

10. The echo planar imaging method capable of reducing image distortion according to claim 1, a third RF pulse and a third gradient echo train are further applied; signals are collected by using the first gradient echo train, the second gradient echo train, and the third gradient echo train; in step 4 and step 5, a momentum of 3/(.sub.H*FoV.sub.PE) is selected for phase encoding gradients, wherein .sub.H is a gyromagnetic ratio of hydrogen nuclei, and FoV.sub.PE is a size of an imaging field of view in the phase encoding direction.

11. An echo planar imaging method capable of reducing image distortion, based on a first radio frequency (RF) pulse with a flip angle of , a second RF pulse with a flip angle of , a spin echo module, as well as a first gradient echo train and a second gradient echo train that are used for collecting echo-planar signals, and specifically comprising the following steps: step 1: exciting a first-part magnetization into a transverse plane by using an excitation level of the first RF pulse and keeping a second-part magnetization in a longitudinal plane; step 2: after a delay, exciting the second-part magnetization into the transverse plane by using an excitation level of the second RF pulse; step 3: rephasing the first-part magnetization by using gradient pulses, and detecting a signal of the transverse first-part magnetization in the first gradient echo train to obtain a first signal; step 4: rephasing the second-part magnetization by using gradient pulses, and detecting a signal of the transverse second-part magnetization in the second gradient echo train to obtain a second signal; and step 5: alternately storing the first signal and the second signal along a phase encoding direction to reconstruct k-space, thereby obtaining a final image.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a schematic diagram of a single-shot EPI sequence in the prior art;

[0029] FIG. 2 is a schematic diagram of a k-space filling trajectory in the prior art;

[0030] FIG. 3 is a schematic diagram of a pulse sequence according to an embodiment of the present disclosure;

[0031] FIG. 4 is a schematic diagram of a k-space filling trajectory according to an embodiment of the present disclosure;

[0032] FIG. 5 shows a slice-9 brain image according to an embodiment of the present disclosure;

[0033] FIG. 6 shows a slice-9 brain image using the prior art;

[0034] FIG. 7 shows a slice-15 brain image according to an embodiment of the present disclosure; and

[0035] FIG. 8 shows a slice-15 brain image using the prior art.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] A number of preferred examples of this application will be introduced below with reference to the accompanying drawings of the specification, such that the technical content can be clearly and easily understood. This application can be embodied through examples of many different forms, and the protection scope of this application is not limited to the examples mentioned herein.

[0037] The general design idea of the present disclosure is as follows: [0038] 1. Two RF pulses and are used to respectively flip over a longitudinal magnetization into a transverse plane at different moments, so as to obtain observable transverse magnetization. [0039] 2. The transverse magnetization obtained by flipping over the longitudinal magnetization by means of and are respectively rephased in two different echo trains by means of gradient pulses. [0040] 3. Navigator echo signals are respectively collected before the two gradient echo trains, and an amplitude difference between two gradient echo train signals is corrected on the basis of an amplitude difference between the two navigator echo signals. [0041] 4. A MUSSELS method is used to reconstruct signals collected by the two gradient echo trains, so as to obtain two sets of image data, and the two sets of image data are averaged to obtain a final image.

[0042] A momentum of a phase encoding blip gradient can be set to 2/(.sub.H*FoV.sub.PE) or 4/(.sub.H*FoV.sub.PE). .sub.H is a gyromagnetic ratio of hydrogen nuclei, and FoV.sub.PE is a size of an imaging field of view in the phase encoding direction. This further reduces image distortion, thereby obtaining images with higher resolution.

[0043] Phase encoding blip gradients with different polarities are used for the first gradient echo train and the second gradient echo train. Two images are reconstructed from the signals of the first gradient echo train and the second gradient echo train, and magnetic field distribution is estimated based on the two images, thereby correcting image distortion.

[0044] Three RF pulses and three gradient echo trains are used to collect signals, thereby obtaining the image through reconstruction.

[0045] Specifically, a spin echo module containing a 180 RF pulse is applied, to obtain T.sub.2-weighted images with reduced distortion. Diffusion-weighted gradients are applied at both sides of the 180 RF pulse to obtain diffusion-weighted images with reduced distortion. The spin echo module is not applied, to obtain T.sub.2*-weighted images with reduced distortion.

[0046] Specifically, this embodiment provides an echo planar imaging method capable of reducing image distortion. The method is based on a first RF pulse with a flip angle of , a second RF pulse with a flip angle of , a spin echo module, as well as a first gradient echo train and a second gradient echo train for collecting echo-planar signals. The flip angle of the first RF pulse is 47, and the flip angle of the second RF pulse is 122. The method specifically includes the following steps: [0047] Step 1: Excite a first-part magnetization into a transverse plane by using an excitation level of the first RF pulse and keep a second-part magnetization in a longitudinal plane. [0048] Step 2: After a delay, excite the second-part magnetization into the transverse plane by using an excitation level of the second RF pulse, where the delay is equal to a time interval between centers of the first gradient echo train and the second gradient echo train. [0049] Step 3: Apply the spin echo module, where the spin echo module containing a 180 RF pulse is applied, to obtain T.sub.2-weighted images with reduced distortion; diffusion-weighted gradients are applied at both sides of the 180 RF pulse to obtain diffusion-weighted images with reduced distortion; and the spin echo module is not applied, to obtain T.sub.2*-weighted images with reduced distortion. [0050] Step 4: Rephase the first-part magnetization by using gradient pulses, and detect a signal of the transverse first-part magnetization in the first gradient echo train to obtain a first signal. [0051] Step 5: Rephase the second-part magnetization by using gradient pulses, and detect a signal of the transverse second-part magnetization in the second gradient echo train to obtain a second signal.

[0052] Optionally, in step 4 and step 5, a momentum of 2/(.sub.H*FoV.sub.PE) or 4/(.sub.H*FoV.sub.PE) can be selected for phase encoding gradients, where .sub.H is a gyromagnetic ratio of hydrogen nuclei, and FoV.sub.PE is a size of an imaging field of view in the phase encoding direction. In some embodiments, three RF pulses and three gradient echo trains are used to collect signals. In this case, the momentum of the phase encoding gradients can be 3/(.sub.H*FoV.sub.PE).

[0053] Step 6: Alternately store the first signal and the second signal along a phase encoding direction to reconstruct k-space, thereby obtaining a final image. Specifically, navigator echo signals are respectively collected before the first gradient echo train and the second gradient echo train, and an amplitude difference between signals of the first gradient echo train and the second gradient echo train is corrected by using an amplitude difference between the two navigator echo signals, where a navigator echo is a gradient echo without phase encoding. The signals collected by the first gradient echo train and the second gradient echo train are reconstructed using a MUSSELS method, to obtain two sets of image data, and the two sets of image data are averaged to obtain the final image.

[0054] FIG. 3 shows an echo planar imaging sequence based on multi-RF excitation provided in this embodiment. The echo planar imaging sequence includes a first RF pulse with a flip angle of , a second RF pulse with a flip angle of , a spin echo module containing a 180 RF pulse, and two gradient echo trains (a first gradient echo train and a second gradient echo train) for collecting echo-planar signals. RF corresponds to RF pulses, G.sub.RO corresponds to gradient pulses applied in a readout direction, G.sub.PE corresponds to gradient pulses applied in a phase encoding direction, and G.sub.ss corresponds to gradient pulses applied in a slice selection direction. FIG. 4 shows a k-space filling trajectory corresponding to the echo planar imaging sequence based on multi-RF excitation provided in this embodiment, where k.sub.x corresponds to k-space of the readout direction, k.sub.y corresponds to k-space of the phase encoding direction, and the signals collected by the two gradient echo trains are alternately combined and filled into k-space in a manner depicted in FIG. 4.

[0055] A comparative measurement of a healthy volunteer's head scan was performed on a 3T magnetic resonance imaging system equipped with a 32-channel head receiving coil array using both the conventional single echo planar imaging sequence and the echo planar imaging sequence according to the embodiments of the present disclosure. Two sequences were employed in the test: the echo planar imaging sequence based on the prior art and the echo planar imaging sequence according to this embodiment. In the comparative experiment of the two echo planar imaging sequences, the imaging field of view was 240240 mm.sup.2, the slice thickness was 5 mm, TR was 4000 ms, TE was 92 ms, echo spacing was 0.57 ms, in-plane resolution was 2.52.5 mm.sup.2, and the number of slices collected was 19. For the echo planar imaging sequence based on the prior art, a RF pulse with a flip angle of 90 was used to excite the signal, then a spin echo module containing a 180 RF pulse was applied, and a single gradient echo train, with a length of 96, was used to collect signals. For the echo planar imaging sequence according to this embodiment, an RF pulse with a flip angle of 47 and an RF pulse with a flip angle of 122 were used to excite the signal, then a spin echo module containing a 180 RF pulse was applied, and two gradient echo trains, each with a length of 48, were used to collect signals. The amplitude of the phase encoding gradient was twice that of the echo planar sequence in the prior art. For the echo planar imaging sequence according to this embodiment, the MUSSELS method was used to reconstruct the signals collected by the two gradient echo trains, resulting in two sets of image data. The two sets of image data were averaged to obtain the final image.

[0056] FIG. 5 to FIG. 8 display the results of the volunteer experiments using the two sequences described above. FIG. 5 shows a slice-9 image using the echo planar imaging sequence according to the present embodiment; FIG. 7 shows a slice-15 image using the echo planar imaging sequence according to the present embodiment; FIG. 6 shows a slice-9 image using the echo planar imaging sequence based on the prior art; and FIG. 8 shows a slice-15 image using the prior art using the echo planar imaging sequence based on the prior art. Compared with FIG. 6, FIG. 5 exhibits reduced geometric distortion in the frontal area (indicated by the arrow). In comparison with FIG. 8, FIG. 7 notably shows reduced geometric distortion in the eye area (indicated by arrow b) and significantly improved signal pile-up (indicated by arrow a).

[0057] Preferred specific examples of this application are described in detail above. It should be understood that, a person of ordinary skill in the art can make various modifications and variations according to the concept of this application without creative efforts. Therefore, all technical solutions that can be obtained by a person skilled in the art based on the prior art through logical analysis, deduction, or limited experiments according to the concept of this application should fall within the protection scope defined by the claims.