WO2004099809A1

WO2004099809A1 - Method of magnetic resonance imaging

Info

Publication number: WO2004099809A1
Application number: PCT/EP2004/004989
Authority: WO
Inventors: Ke Cheng Liu
Original assignee: Siemens Aktiengesellschaft
Priority date: 2003-05-09
Filing date: 2004-05-10
Publication date: 2004-11-18
Also published as: CN1548007A; CN100366218C

Abstract

This invention relates to a method of magnetic resonance ima-ging, includes following steps: sending a radio frequency ex-citation impulse to selection slice and putting a transverse magnetic field on a slice of three-dimensioned volume above; generating a first phase-encoding gradient impulse and a se-cond phase-encoding gradient impulse which sample k-space respectively; receiving a echo signal and a read out gra-dient, repeatly receiving the echo signals to form an echo train; modulating the phases of the echo signals in k-space so that the echo signals are essentially arranged in linear fashion according to phase discrepancy; then the image is re-constructed with Fourier transform. This invention can reduce artifacts of magnetic resonance imaging and improve the image quality.

Description

Method of Magnetic Resonance Imaging

(I) Technical Field

The present invention relates to a method of magnetic resonance imaging and particularly, to a method of magnetic resonance imaging capable of reducing artifacts and improving the quality of image.

(II) Background Art

US 6,037,771 disclosed a method of magnetic resonance ima- ging, when a three dimensional volume of interest, such as a human tissue, is subjected to a static magnetic field B0 of a magnetic resonance system, the spin magnetic moment will attempt to align with the magnetic field B0. If an external magnetic field Bl is applied to the tissue, the spin moments will precess at the Larmor frequency. When the three dimensional volume is further applied with a radio frequency excitation magnetic field Bl, the magnetic moment Mz originally arranged along B0 may be rotated into the X-Y plane to become a transverse magnetic moment Mt . After the excitation magnetic field Bl is terminated, the excited spin emits a signal which is acquired and processed to form image. After termination of said excitation magnetic field and before the acquisition of the signals, magnetic field gradients (Gx, Gy, Gz) are applied on said three dimensional volume, i.e., a read out gra- dient Gx, a second phase-encoding gradient Gy and a first phase-encoding gradient Gz . A sequence of scanning cycles are preformed in the area to be imaged, in such cycles, said gradients are varied with a particular locating method, to determine the spatial position of the signal . An echo signal is constructed by said result of scanning together with the signal emitted by said excited spins, the echo signals are acquired and digitized, and applied a k-space Fourier transform by a computer, to reconstruct the image. The k-space is also called a frequency space, the axes of the original data matrix are called Kx and Ky, respectively, said matrix is divided into four quadrants, and the plane formed by said two axes is call a k-space.

There are two methods of acquiring the echo signal in the prior art. One is to acquire an echo signal from the three dimensional volume subjected to a static magnetic field BO for each excitation, and a next echo signal is acquired for the next excitation, the time interval between the two excitation impulses is called repetition time (TR) , this procedure is repeated until sufficient echo signal are collected, then Fourier transform is applied on those signals in the k- space, so as to reconstruct an image. The defect of such method is that a period of magnetic moment recovery time shall be given to said three dimensional volume of interest after each excitation before the application of the next excitation, thus the speed of imaging is relatively slow.

The other one is the so-called fast echo acquisition method, as shown in Fig. 1, a number of echo signals are acquired continuously after each of the excitations applied on the three dimensional volume in the static magnetic field B0, ho- wever, only four of those echo signals (1, 2, 3 and 4) are illustrated in the figure 1. The intensities of the signals are gradually attenuating. Although those echoes are consistent in phase theoretically, however, practically, phase discrepancies exist between those echoes due to the imperfec- tion of the magnetic resonance system, and the phase discrepancies between the echo signals are increasing, as shown in Fig.2. In addition, the motion of the substance to be imaged during the signal acquisition procedure may also cause said phase discrepancy. Figs. 3 and 4 show a typical known sy - metric arrangement for amplitude modulation and phase modulation of said echo signal in k-space. The first echo signal is placed at the centre of the k-space during amplitude modula- tion, and the three subsequent echoes are placed symmetrically on both sides of said first echo, respectively, the amplitudes of the echo signals 1, 2, 3 and 4 are indicated with 11, 21, 31 and 41, respectively. The first echo signal is placed at the centre of the k-space in phase modulation, the three subsequent echoes are placed symmetrically on both sides of said first echo, respectively, the phase differences of the echo signals 1, 2, 3 and 4 are indicated with 12, 22, 32 and 42, respectively. Those signals are then processed using one or several known techniques including the Fourier transform in k-space, to reconstruct the image. Although this approach has the advantage of fast imaging, however, artifacts exist in the magnetic resonance image which cause the worse quality of the image, because the magnetic resonance imaging system is impossible to be perfect due to the existence of eddy current in the pole plates and the gradient coils. The so called artifact is a criterion for the quality of the image, which indicates the signal intensity of the portion of the magnetic resonance image which do not cor- respond to the spatial distribution of the tissue in the image plane.

The hardware improvement of the magnetic resonance imaging system has been suggested in attempts to enhance the quality of image, for example, to reduce the generation of eddy current by the use of masked gradient coils. Such improvement is effective, however, the cost of manufacturing is increased as well. Another method of improving the quality of the image is to decrease the performance of the MR system, of example, by limiting the max. performance of the gradient coils. However, the disadvantage thereof is that the performance of the device can not be exploited sufficiently. (III) Content of the invention

It is an object of the present invention to provide a method of magnetic resonance imaging which is capable of reducing artifacts and increasing the resolution of the image.

It is another object of the present invention to provide a method of reducing artifacts in magnetic resonance image, the artifacts in an image can be reduced under the fast echo col- lection mode without the need to alter the hardware of the magnetic resonance imaging system or to reduce the performance of the system.

The above mentioned objects of the present invention are a- chieved by the technical solution of claim 1 in which a method of magnetic resonance imaging, which acquires resonance data from a interesting three-dimensioned volume to form image, includes steps of: (1) sending a radio frequency excitation impulse to a selection slice and putting a transverse magnetic field on the slice of the three-dimensioned volume above; (2) generating a first phase-encoding gradient impulse which samples k-space along a first axis traversing the above slice; generating a second phase-encoding gradient impulse which samples k-space along a second axis in the above slice plane; (4) receiving an echo signal and a read out gradient which samples k-space along a third axis vertical to the above second axis in the above slice plane; repeating the steps (2), (3) and (4) to collect the echo signals above- mentioned to form an echo train; the present invention is characterized by: (6) modulating the phases of the respective echo signals in k-space so that the echo signals are arranged essentially in linear fashion according to phase discrepancy; and (7) reconstructing the image with Fourier transform by using the echo signal modulated in step (6) .

According to an aspect of the present invention, in the above step (6) , arranging the above echo signals in ascending order from small to large according to phase discrepancy, such arrangement also appears in stepped form, approximately in linear arrangement, without the use of the previously mentioned symmetric arrangement. Furthermore, the amplitudes of the above echo signals are modulated in k-space, in which the arrangement order of the echo signals is the same as their order of phase discrepancy.

According to another aspect of the present invention, in the above modulation step (6) , the above echo signals can also be arranged in descending order from large to small according to phase discrepancy. This arrangement also appears in stepped form, approximately in linear arrangement. Similarly, the amplitudes of the above signals are modulated in k-space, in which the arrangement order of the echo signals is the same as their order of phase discrepancy.

(IV) Description of figures

The embodiments of the present invention will be explained in detail with reference to the accompanying drawings to facilitate the understanding of the present invention. In which:

Fig. 1, shows the case of fast acquiring several echoes after an excitation in the prior art, the intensity of each of the echo signals is substantially successively decreasing; Fig. 2 shows the phase difference value of each of the collected echo signals shown in Fig. 1, the phase differences of the subsequent echo signals being successively increasing; Fig. 3 shows a method of modulating the echo signals in k- space according to the prior art, a typical symmetric arrangement being formed after the modulation;

Fig. 4 shows a symmetric arrangement formed after the phase modulation of the echo signals according to the method of Fig. 3;

Fig. 5 shows a sequence of echo signals re-arranged in k- space according to an embodiment of the method of the present invention, each of the echo signals being substantially linearly arranged according to the order of their phase differences from small to large;

Fig. 6 shows the echo signals of the embodiment in Fig. 5 ha- ving also amplitude modulated correspondingly; and

Fig. 7 shows an image A obtained in the symmetric arrangement mode of the echoes of the prior art , and an image B obtained after linearly arranging the echoes according to their phase differences of the present invention, the artifact in image B is significantly reduced, and the quality of image is enhanced.

(V) Embodiments

In a method of magnetic resonance imaging according to an embodiment of the present invention, a three dimensional volume of interest is placed in a static magnetic field B0 of a magnetic resonance imaging system, and a radio frequency excitation magnetic field is further applied to the volume. Af- ter the radio frequency excitation magnetic field is terminated, the excited spin emits magnetic resonance signal to produce phase encoding gradients in three mutually vertical directions, respectively. The magnetic resonance signals are encoded, those echo signal are then acquired and Fourier transform is performed on said echo signal in k-space, to thereby obtain an image. The specific steps are: (1) sending a radio frequency excitation impulse to a selection slice to apply a transverse magnetic field on the slice of said three dimensional volume; (2) generating a first phase-encoding gradient impulse which samples k-space along a first axis transverse to said slice; (3) generating a second phase- encoding gradient impulse which samples k-space along a second in said slice plane; (4) acquiring an echo signal and then acquiring a read out gradient which samples k-space along a third axis perpendicular to said second axis in said slice plane; (5) repeating said steps (2), (3) and (4) to repeatedly acquire sequence of said echo signals to form an echo train. The above mentioned technique pertains to the well known technique of multiple echo acquisition, which is not to be explained here for simplicity.

As shown in Figs . 5 and 6 , the improvement of the method of the present invention is that a sequence of echo signals are rearranged in k-space. In the invention, the respective echo signals are modulated in k-space, such that said respective echo signals are arranged essentially in linear fashion ac- cording to phase difference values from small to large. Referring to Fig. 2 ad 4 first for better understanding, in an echo train, since each of the echo signals is acquired at different times and under different conditions (e.g., the influence of residue magnetic field) by the use of multiple echo acquisition technique, each of the echoes has a certain phase discrepancy, the phase discrepancy 12 of the first echo signal 1 is the smallest one, the phase discrepancy 22 of the second echo signal 2 is slightly larger, the phase discrepancy 32 of the third echo signal 3 is much larger, and the pha- se discrepancy 42 of the fourth echo signal 4 is the largest, it will be a rule that the later the echo signal is acquired the larger the phase discrepancy will be. It is understood that more than the four echoes can be acquired by the use of multiple echo acquisition technique after each radio frequen- cy excitation. The sequence of the echo signals in k-space can be altered only by altering the software in accordance with the invention. In the embodiment shown in Fig. 5, the echo signals are rearranged according to sequence of their phase discrepancy, from small to large, such that the sequen- ce of the phase discrepancy of the echoes is approximately linearly arranged. From left to right, the echo signal 1 is arranged in the leftmost for its smallest phase discrepancy 12, the second echo signal 2 is arranged in the second position to the left for its slightly larger phase discrepancy 22, the third echo signal 3 is arranged in the third position to the left for its much larger phase discrepancy 32, and the fourth echo signal 4 is positioned in the rightmost end for its largest phase discrepancy 42. In this way, the phase shift of each of the echo signals is arranged in a stepped increasing manner, which is approximately in linear relationship as shown by the dotted line in the figure 5.

Fig. 6 shows the amplitude modulation of these echo signals, it can be seen that the sequence of arrangement of each of the echoes is the same as that shown in Fig, 5. From left to right, the amplitude 11 of the echo signal 1 is arranged in the leftmost, the amplitude 21 of the second echo signal 2 is arranged in the second to the left, the amplitude 31 of the echo signal 3 is arranged in the third to the left, and the amplitude 41 of the fourth echo signal 4 is arranged at the right end. Then an image is reconstructed from those echo signals using Fourier transform.

In an echo train, the excessive phase discrepancy among the echo signals is an important factor that causes the occurrence of artifacts in a magnetic resonance image. It has been known that those phase discrepancies are caused by the imperfection of the system, eddy current or the like, therefore, the prior modifications were made, aiming at those problems. The inventor found that if the phase changing of each of the echo signals mentioned is approximately in a linear way, then said phase discrepancy would only cause the image to move for some pixels, but not cause any artifact or smearing, therefore without deteriorating the quality of the image. The present invention has been made based on this important discovery.

In another embodiment (not shown) of the present invention, each of the echo signals is rearranged in descending sequence according to the phase difference value thereof, from large to small. In such way, the phase shift of each of the echo signals is arranged in a stepped descending order, similarly, the phase of each of the echo signals can be arranged in approximately linear relationship according to phase discrepan- cy. In the same time, in the amplitude modulation of each of the echo signals, the sequence of arrangement of each of the echo signals is varied correspondingly. In this embodiment, other technical features are the same as that of the previous embodiment, which are not to be described here for simplification.

Fig. 7 shows an image A obtained in symmetric phase arrangement of the echoes and an image B obtained in linear arrange- ment of the phase discrepancy of the echoes according to the present invention. The significant effects of improvement on the quality of the magnetic resonance image can be seen clearly by comparison. The image A is obtained by the prior art and severe artifacts exist in the image. While the arti- facts in image B are significantly reduced, especially the edges near the boundary of the image are much clearer, the quality of the image is greatly improved.

According to the present invention, it is not necessary to alter the hardware design of the magnetic resonance system, the artifacts can be reduced and the quality of the image can still be improved only by modifying the software of the system, thus the cost of the technical solution of the present invention is relatively low, and an improved magnetic reso- nance system can be marketed in a short time.

Claims

Patent claims

1. A method of magnetic resonance imaging, which acquires magnetic resonance data from a interesting three- dimensioned volume to form image, includes steps of :

(1) sending a radio frequency excitation impulse to a selection slice and putting a transverse magnetic field on the slice of the three-dimensioned volume above;

(2) generating a first phase-encoding gradient impulse which samples k-space along a first axis traversing the above slice;

(3) generating a second phase-encoding gradient impulse which samples k-space along a second axis in the above slice plane; (4) receiving a echo signal and a read out gradient which samples k-space along a third axis vertical to the above second axis in the above slice plane; (5) repeating the step (2), (3), (4) to collect the echo signals above-mentioned to form an echo train; wherein characterized by:

(6) modulating the phases of the respective echo signals in k-space so that the echo signals are arranged essentially in linear fashion according to phase discrepancy; (7) reconstructing the image with Fourier transform by using the echo signal modulated in step (6) .

2. The method according to theclaim 1, wherein in the above step (6) , arranging the above echo signals in ascending order according to phase discrepancy.

3. The method according to the claim 2, wherein the above step (6) further includes modulating the amplitude of the above echo signals in k-space, in which the arrangement order of the echo signals is the same as their order of phase discrepancy.

4. The method according to the claim 1, wherein in the above step (6) , arranging the above echo signals in descending order according to phase discrepancy.

5. The method according to the claim 4, wherein the above step (6) further includes modulating the amplitude of the above echo signals in k-space, in which the arrangement order of the echo signals is the same as their order of phase discrepancy.