CN116152367A - Spin echo magnetic resonance imaging method and device based on diffusion weighting - Google Patents
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Abstract
The invention discloses a spin echo magnetic resonance imaging method and a device based on dispersion weighting, comprising the following steps: the dispersion gradient is applied to different gradients of the spin echo signals, so that phase differences of the FID signals of the gradients are obtained, phases of the corresponding gradients are corrected through the phase differences, preprocessing of the spin echo signals is completed, the FID signals of the acquisition nodes and the magnetic resonance imaging signals form echo chains of the spin echo signals, signal differences between the subsequent echo chains and the first echo chain are eliminated, k-space data of the spin echo magnetic resonance imaging are obtained, the magnetic resonance imaging is obtained through the k-space data, and artifacts of images are effectively reduced.
Description
Technical Field
The invention relates to the technical field of magnetic resonance imaging, in particular to a spin echo magnetic resonance imaging method and device based on diffusion weighting.
Background
Diffusion weighted imaging is a new magnetic resonance imaging technique, and hydrogen protons show different signal amplitudes in the gradient direction after applying a diffusion gradient by utilizing the motion diffusion characteristic of water molecules. Since the diffusion coefficients of different tissues are different, the different tissues can be distinguished by the signal intensity.
The fast spin echo in the diffusion weighted imaging has the characteristics of high scanning speed, low requirement on shimming conditions and small motion artifact, and can effectively reduce the deformation and artifact of plane echo images when being used in the diffusion weighted imaging. The fast spin echo has some defects, and because the system is difficult to reach a theoretical ideal state, a large number of phase-focusing pulses and gradients cause the difference of signal phases in an echo chain, and the signals show non-negligible artifacts in images.
Disclosure of Invention
In view of the above technical problems, the present invention provides a spin echo magnetic resonance imaging method based on diffusion weighting, including:
respectively applying the dispersion gradient to a first preset node pulse and a plurality of second preset node pulses of different gradients of the spin echo signal;
acquiring FID signals of a layer selection gradient, a readout gradient and a phase gradient, which are respectively applied to the spin echo signals, of the dispersion gradient, and acquiring phase differences of the FID signals;
correcting the phase of the corresponding gradient by using the phase difference, and obtaining a corrected spin echo signal to finish preprocessing of the spin echo signal;
sequentially acquiring corrected spin echo signals, applying a dispersion gradient to a preset acquisition node of the spin echo signals, and obtaining FID signals of the acquisition node and magnetic resonance imaging signals after the acquisition node;
and forming an echo chain of a spin echo signal by the FID signal and the magnetic resonance imaging signal, comparing the FID signal of the subsequent echo chain with the FID signal of the reference signal by taking the first echo chain as the reference signal, eliminating the signal difference between the subsequent echo chain and the first echo chain, obtaining k-space data of spin echo magnetic resonance imaging, and obtaining magnetic resonance imaging by the k-space data.
Further, after applying the diffusion gradient to the first preset node pulse and the plurality of second preset node pulses, respectively, the spin echo signal is different from the spin echo signal, the method includes:
the diffusion gradient is applied to the spin echo signal X, Y, Z after 90 ° of the first preset node pulse and 180 ° of the second preset node pulses, respectively.
Further, acquiring FID signals of the diffusion gradient applied to a slice-selecting gradient, a readout gradient, and a phase gradient of the spin echo signal, respectively, and acquiring a phase difference of the FID signals, includes:
after the diffusion gradient is applied to the selected layer gradient of the spin echo signal, acquiring a FID signal, denoted as S1;
after the diffusion gradient is applied to the readout gradient of the spin echo signal, a FID signal is acquired, denoted S2;
after the dispersion gradient is applied to the phase gradient of the spin echo signal, a FID signal is acquired, denoted S3;
when S1 is used as a reference signal, the correction phase phi of the layer direction s ＝0，
S 2 ’＝S2*conj(S1)；
S 3 ’＝S3*conj(S1)；
Wherein conj represents a complex conjugate signal;
will S 2 ' and S 3 ' calculating the phase difference phi between the two components and S1 through a point-by-point phase difference method respectively R ,φ P 。
Further, correcting the phase of the corresponding gradient by using the phase difference, and obtaining a corrected spin echo signal, so as to complete the preprocessing of the spin echo signal, including:
the phase difference is carried into the spin echo signal, and the phase signal of the corresponding axis is corrected to obtain a corrected spin echo signal S (S，R，P) ＝S (S，R，P) *exp(i*φ (S，R，P) )。
Further, sequentially acquiring the corrected spin echo signals, applying a diffusion gradient to a preset acquisition node of the spin echo signals, and obtaining FID signals and correction signals of the acquisition node, including:
setting the read gradient of the acquired corrected spin echo signal after the second 180 DEG pulse; the FID signal of the first 180-degree pulse of the preset acquisition node is acquired, and the correction signal after the first 180-degree pulse of the acquisition node is acquired.
The invention also provides a spin echo magnetic resonance imaging device based on dispersion weighting, which comprises:
a diffusion gradient applying unit for applying diffusion gradients to the first preset node pulses of different gradients of the spin echo signal and after the plurality of second preset node pulses, respectively;
a phase difference acquisition unit configured to acquire FID signals of a slice selection gradient, a readout gradient, and a phase gradient, to which the diffusion gradient is applied, respectively, to a spin echo signal, and acquire a phase difference of the FID signals;
the preprocessing unit is used for correcting the phase of the corresponding gradient by using the phase difference, obtaining a corrected spin echo signal and finishing preprocessing of the spin echo signal;
the signal acquisition unit is used for sequentially acquiring the corrected spin echo signals, applying a dispersion gradient to a preset acquisition node of the spin echo signals, and acquiring FID signals of the acquisition node and magnetic resonance imaging signals after the acquisition node;
and the magnetic resonance imaging unit is used for forming an echo chain of a spin echo signal by the FID signal and the magnetic resonance imaging signal, comparing the FID signal of the subsequent echo chain with the FID signal of the reference signal by taking the first echo chain as the reference signal, eliminating the signal difference between the subsequent echo chain and the first echo chain, obtaining k-space data of spin echo magnetic resonance imaging, and obtaining magnetic resonance imaging by the k-space data.
Further, the dispersion gradient applying unit includes:
a diffusion gradient applying subunit for applying a diffusion gradient to the spin echo signal X, Y, Z after the first preset node pulse of three axes by 90 ° and the plurality of second preset node pulses by 180 °, respectively.
Further, the phase difference acquisition unit includes:
a first acquisition subunit for acquiring an FID signal after the diffusion gradient is applied to the slice-select gradient of the spin echo signal, denoted S1;
a second acquisition subunit for acquiring a FID signal, denoted S2, after the diffusion gradient is applied to the readout gradient of the spin echo signal;
a third acquisition subunit for acquiring a FID signal after the dispersion gradient is applied to the phase gradient of the spin echo signal, denoted S3;
phase difference calculating subunit, when S1 is used as reference signal, correcting phase phi of layer direction s ＝0，
S 2 ’＝S2*conj(S1)；
S 3 ’＝S3*conj(S1)；
Wherein conj represents a complex conjugate signal;
will S 2 ' and S 3 ' calculating the phase difference phi between the two components and S1 through a point-by-point phase difference method respectively R ,φ P 。
Further, the preprocessing unit includes:
a modifying subunit for bringing the phase difference into the spin echo signal, and modifying according to the phase signal of the corresponding axis to obtain a modified spin echo signal S (S，R，P) ＝S (S，R，P) *exp(i*φ (S，R，P) )。
Further, the magnetic resonance imaging unit comprises:
a signal acquisition subunit for setting a read gradient of the acquired modified spin echo signal after the second 180 ° pulse; the FID signal of the first 180-degree pulse of the preset acquisition node is acquired, and the correction signal after the first 180-degree pulse of the acquisition node is acquired.
The invention provides a spin echo magnetic resonance imaging method and a device based on diffusion weighting, which are characterized in that diffusion gradients are applied to different gradients of spin echo signals, so that the phase difference of FID signals of each gradient is obtained, the phase of the corresponding gradient is corrected through the phase difference, the preprocessing of the spin echo signals is completed, the FID signals of a plurality of acquisition nodes and the magnetic resonance imaging signals form an echo chain of the spin echo signals, the signal difference between the subsequent echo chain and the first echo chain is eliminated, k-space data of the spin echo magnetic resonance imaging is obtained, the magnetic resonance imaging is obtained through the k-space data, and the artifact of the image is effectively reduced.
Drawings
FIG. 1 is a flow chart of a spin echo magnetic resonance imaging method based on diffusion weighting provided by the invention;
FIG. 2 is a timing diagram of a basic diffusion-weighted spin echo sequence in accordance with the present invention;
FIG. 3 is a reference signal position diagram in accordance with the present invention;
FIG. 4 is a graph of error correction between spin echoes according to the present invention;
fig. 5 is a comparison of the magnetic resonance image processing before and after the present invention;
fig. 6 is a schematic structural diagram of a spin echo magnetic resonance imaging device based on diffusion weighting according to the present invention.
Detailed Description
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. The present invention may be embodied in many other forms than those herein described, and those skilled in the art will readily appreciate that the present invention may be similarly embodied without departing from the spirit or essential characteristics thereof, and therefore the present invention is not limited to the specific embodiments disclosed below.
The invention provides an imaging method aiming at a spin echo series of diffusion weighted imaging, which can effectively reduce the artifact of an image, and a flow chart of the method is shown in figure 1 and comprises the following steps:
step S101, applying the diffusion gradient to the first preset node pulses and the second preset node pulses of different gradients of the spin echo signal respectively.
Basic diffusion weighted spin echo sequence as shown in fig. 2, a diffusion gradient is applied to the spin echo signal X, Y, Z after a first preset nodal pulse of 90 ° and a plurality of second preset nodal pulses of 180 °, respectively.
The other parts are consistent with the conventional rapid spin echo sequence, and signals are acquired in an echo chain mode.
Step S102, acquiring FID signals of a slice selection gradient, a readout gradient and a phase gradient, which are respectively applied to the spin echo signals, and acquiring phase differences of the FID signals.
As the influence of factors such as eddy current remanence and the like caused by different axes of the dispersion gradient can influence the consistency of echo signals, the pre-scanning is performed before formal acquisition.
After the diffusion gradient is applied to the selected layer gradient of the spin echo signal, a FID signal is acquired, denoted as S1, at the position shown by the arrow in FIG. 3;
likewise, the same is true. After the diffusion gradient is applied to the readout gradient of the spin echo signal, a FID signal is acquired, denoted S2;
likewise, after the dispersion gradient is applied to the phase gradient of the spin echo signal, a FID signal is acquired, denoted as S3;
when S1 is used as a reference signal, the correction phase phi of the layer direction s ＝0，
S 2 ’＝S2*conj(S1)；
S 3 ’＝S3*conj(S1)；
Wherein conj represents a complex conjugate signal;
will S 2 ' and S 3 ' calculating the phase difference phi between the two components and S1 through a point-by-point phase difference method respectively R ,φ P 。
And step S103, correcting the phase of the corresponding gradient by using the phase difference, and obtaining a corrected spin echo signal to finish preprocessing of the spin echo signal.
The phase difference is carried into the spin echo signal, and the phase signal of the corresponding axis is corrected to obtain a corrected spin echo signal S (S，R，P) ＝S (S，R，P) *exp(i*φ (S，R，P) )。
Step S104, sequentially acquiring corrected spin echo signals, applying a diffusion gradient to a preset acquisition node of the spin echo signals, and obtaining FID signals of the acquisition node and magnetic resonance imaging signals after the acquisition node.
Besides errors caused by imperfect spin echo, dispersive gradients are also particularly sensitive to motion artifacts and the like, and eventually can also lead to different degrees of artifacts and errors. To eliminate the effect, we modify the sequence timing diagram of FIG. 2 by setting the read gradient of the acquired modified spin echo signal after the second 180 pulse; the FID signal of the first 180 deg. pulse of the preset acquisition node is acquired, and the correction signal after the first 180 deg. pulse of the acquisition node is acquired, as shown in fig. 4, specifically,
(1) the equilibrium gradient (arrow 2) of the read gradient is shifted from after the 90 pulse to before the second 180 phase-focusing pulse (position shown).
(2) Instead of acquiring a conventional image signal after the first 180 deg. phase-focusing pulse, an FID signal is acquired (arrow 1).
(3) The acquisition of a conventional image signal begins after the second 180 deg. phase-focusing pulse.
And step 105, forming an echo chain of a spin echo signal by the FID signal and the magnetic resonance imaging signal, comparing the FID signal of the subsequent echo chain with the FID signal of the reference signal by taking the first echo chain as the reference signal, eliminating the signal difference between the subsequent echo chain and the first echo chain, obtaining k-space data of spin echo magnetic resonance imaging, and obtaining magnetic resonance imaging by the k-space data.
According to the step of collecting signals in the previous step, the FID signals and the magnetic resonance imaging signals form echo chains of spin echo signals, and after the first 180 DEG phase-gathering pulse in each echo chain, a corrected signal is obtained, and the signal records the phase difference of each echo chain collection time caused by the factors such as the movement of volunteers.
During reconstruction, according to the calculation step of preprocessing the spin echo signals, the information of the first echo chain is used as a reference, the follow-up FID signals are compared with the reference, the difference between the echo chains can be removed, the echo signals with good consistency are obtained, and complete K space data are formed.
Fig. 5 shows the results before and after the spin echo processing, it can be seen that fig. 5 (a) has an unclear structure of ventricle and brain tissue due to a large error between K-space data, even the lower half of the image is obviously lost, obvious signal loss occurs, specific tissue information is difficult to distinguish, and the contrast difference between the ventricle and brain tissue is obviously improved after the processing in fig. 5 (b).
Based on the same inventive concept, the present invention also provides a spin echo magnetic resonance imaging apparatus 600 based on diffusion weighting, as shown in fig. 6, comprising:
a diffusion gradient applying unit 610 for applying diffusion gradients to the first preset node pulses of different gradients of the spin echo signal and after the plurality of second preset node pulses, respectively;
a phase difference acquisition unit 620 for acquiring FID signals of which the diffusion gradient is applied to a slice selection gradient, a readout gradient, and a phase gradient of the spin echo signal, respectively, and acquiring a phase difference of the FID signals;
a preprocessing unit 630, configured to correct the phase of the corresponding gradient using the phase difference, and obtain a corrected spin echo signal, so as to complete preprocessing of the spin echo signal;
a signal acquisition unit 640, configured to sequentially acquire the corrected spin echo signals, apply a diffusion gradient to a preset acquisition node of the spin echo signals, and obtain FID signals of the acquisition node and magnetic resonance imaging signals after the acquisition node;
the magnetic resonance imaging unit 650 is configured to form an echo chain of spin echo signals from the FID signal and the magnetic resonance imaging signal, compare the FID signal of the subsequent echo chain with the FID signal of the reference signal, eliminate the signal difference between the subsequent echo chain and the first echo chain, obtain k-space data of spin echo magnetic resonance imaging, and obtain magnetic resonance imaging from the k-space data.
Further, the dispersion gradient applying unit includes:
a diffusion gradient applying subunit for applying a diffusion gradient to the spin echo signal X, Y, Z after the first preset node pulse of three axes by 90 ° and the plurality of second preset node pulses by 180 °, respectively.
Further, the phase difference acquisition unit includes:
a first acquisition subunit for acquiring an FID signal after the diffusion gradient is applied to the slice-select gradient of the spin echo signal, denoted S1;
a second acquisition subunit for acquiring a FID signal, denoted S2, after the diffusion gradient is applied to the readout gradient of the spin echo signal;
a third acquisition subunit for acquiring a FID signal after the dispersion gradient is applied to the phase gradient of the spin echo signal, denoted S3;
phase difference calculating subunit, when S1 is used as reference signal, correcting phase phi of layer direction s ＝0，
S 2 ’＝S2*conj(S1)；
S 3 ’＝S3*conj(S1)；
Wherein conj represents a complex conjugate signal;
will S 2 ' and S 3 ' calculating the phase difference phi between the two components and S1 through a point-by-point phase difference method respectively R ,φ P 。
Further, the preprocessing unit includes:
a modifying subunit for bringing the phase difference into the spin echo signal, and modifying according to the phase signal of the corresponding axis to obtain a modified spin echo signal S (S，R，P) ＝S (S，R，P) *exp(i*φ (S，R，P) )。
Further, the magnetic resonance imaging unit comprises:
a signal acquisition subunit for setting a read gradient of the acquired modified spin echo signal after the second 180 ° pulse; the FID signal of the first 180-degree pulse of the preset acquisition node is acquired, and the correction signal after the first 180-degree pulse of the acquisition node is acquired.
The invention provides a spin echo magnetic resonance imaging method and a device based on diffusion weighting, which are characterized in that diffusion gradients are applied to different gradients of spin echo signals, so that the phase difference of FID signals of each gradient is obtained, the phase of the corresponding gradient is corrected through the phase difference, the preprocessing of the spin echo signals is completed, the FID signals of a plurality of acquisition nodes and the magnetic resonance imaging signals form an echo chain of the spin echo signals, the signal difference between the subsequent echo chain and the first echo chain is eliminated, k-space data of the spin echo magnetic resonance imaging is obtained, the magnetic resonance imaging is obtained through the k-space data, and the artifact of the image is effectively reduced.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made to the specific embodiments of the present invention without departing from the spirit and scope of the present invention, and all modifications and equivalents are intended to be included in the scope of the claims of the present invention.
Claims (10)
1. A spin echo magnetic resonance imaging method based on diffusion weighting, comprising:
respectively applying the dispersion gradient to a first preset node pulse and a plurality of second preset node pulses of different gradients of the spin echo signal;
acquiring FID signals of a layer selection gradient, a readout gradient and a phase gradient, which are respectively applied to the spin echo signals, of the dispersion gradient, and acquiring phase differences of the FID signals;
correcting the phase of the corresponding gradient by using the phase difference, and obtaining a corrected spin echo signal to finish preprocessing of the spin echo signal;
sequentially acquiring corrected spin echo signals, applying a dispersion gradient to a preset acquisition node of the spin echo signals, and obtaining FID signals of the acquisition node and magnetic resonance imaging signals after the acquisition node;
and forming an echo chain of a spin echo signal by the FID signal and the magnetic resonance imaging signal, comparing the FID signal of the subsequent echo chain with the FID signal of the reference signal by taking the first echo chain as the reference signal, eliminating the signal difference between the subsequent echo chain and the first echo chain, obtaining k-space data of spin echo magnetic resonance imaging, and obtaining magnetic resonance imaging by the k-space data.
2. The method of claim 1, wherein applying a diffusion gradient to the first preset node pulse and the plurality of second preset node pulses, respectively, of different axes of the spin echo signal, comprises:
the diffusion gradient is applied to the spin echo signal X, Y, Z after 90 ° of the first preset node pulse and 180 ° of the second preset node pulses, respectively.
3. The method of claim 1, wherein acquiring FID signals for which the diffusion gradient is applied to a slice gradient, a readout gradient, and a phase gradient, respectively, of a spin echo signal, and acquiring a phase difference of the FID signals comprises:
after the diffusion gradient is applied to the selected layer gradient of the spin echo signal, acquiring a FID signal, denoted as S1;
after the diffusion gradient is applied to the readout gradient of the spin echo signal, a FID signal is acquired, denoted S2;
after the dispersion gradient is applied to the phase gradient of the spin echo signal, a FID signal is acquired, denoted S3;
when S1 is used as a reference signal, the correction phase phi of the layer direction s ＝0，
S 2 ’＝S2*conj(S1)；
S 3 ’＝S3*conj(S1)；
Wherein conj represents a complex conjugate signal;
will S 2 ' and S 3 ' calculating the phase difference phi between the two components and S1 through a point-by-point phase difference method respectively R ,φ P 。
4. A method according to claim 1 or 3, wherein using the phase difference to correct the phase of the corresponding gradient and obtain a corrected spin echo signal, the preprocessing of the spin echo signal is done, comprising:
the phase difference is carried into the spin echo signal, and the phase difference is corrected according to the phase signal of the corresponding axis, so as to obtain a corrected spin echo signal S # S，R，P )＝S( S，R，P )*exp(i*φ( S，R，P ))。
5. The method of claim 1, wherein sequentially acquiring the modified spin echo signals, applying a dispersion gradient to a preset acquisition node of the spin echo signals, obtaining FID signals and correction signals for the acquisition node, comprises:
setting the read gradient of the acquired corrected spin echo signal after the second 180 DEG pulse; the FID signal of the first 180-degree pulse of the preset acquisition node is acquired, and the correction signal after the first 180-degree pulse of the acquisition node is acquired.
6. A diffusion weighting based spin echo magnetic resonance imaging apparatus, comprising:
a diffusion gradient applying unit for applying diffusion gradients to the first preset node pulses of different gradients of the spin echo signal and after the plurality of second preset node pulses, respectively;
a phase difference acquisition unit configured to acquire FID signals of a slice selection gradient, a readout gradient, and a phase gradient, to which the diffusion gradient is applied, respectively, to a spin echo signal, and acquire a phase difference of the FID signals;
the preprocessing unit is used for correcting the phase of the corresponding gradient by using the phase difference, obtaining a corrected spin echo signal and finishing preprocessing of the spin echo signal;
the signal acquisition unit is used for sequentially acquiring the corrected spin echo signals, applying a dispersion gradient to a preset acquisition node of the spin echo signals, and acquiring FID signals of the acquisition node and magnetic resonance imaging signals after the acquisition node;
and the magnetic resonance imaging unit is used for forming an echo chain of a spin echo signal by the FID signal and the magnetic resonance imaging signal, comparing the FID signal of the subsequent echo chain with the FID signal of the reference signal by taking the first echo chain as the reference signal, eliminating the signal difference between the subsequent echo chain and the first echo chain, obtaining k-space data of spin echo magnetic resonance imaging, and obtaining magnetic resonance imaging by the k-space data.
7. The apparatus according to claim 6, wherein the dispersion gradient applying unit includes:
a diffusion gradient applying subunit for applying a diffusion gradient to the spin echo signal X, Y, Z after the first preset node pulse of three axes by 90 ° and the plurality of second preset node pulses by 180 °, respectively.
8. The apparatus according to claim 6, wherein the phase difference acquisition unit includes:
a first acquisition subunit for acquiring an FID signal after the diffusion gradient is applied to the slice-select gradient of the spin echo signal, denoted S1;
a second acquisition subunit for acquiring a FID signal, denoted S2, after the diffusion gradient is applied to the readout gradient of the spin echo signal;
a third acquisition subunit for acquiring a FID signal after the dispersion gradient is applied to the phase gradient of the spin echo signal, denoted S3;
phase difference calculating subunit, when S1 is used as reference signal, correcting phase phi of layer direction s ＝0，
S 2 ’＝S2*conj(S1)；
S 3 ’＝S3*conj(S1)；
Wherein conj represents a complex conjugate signal;
will S 2 ' and S 3 ' calculating the phase difference phi between the two components and S1 through a point-by-point phase difference method respectively R ,φ P 。
9. The apparatus according to claim 6 or 8, characterized by a preprocessing unit comprising:
a modifying subunit for carrying the phase difference into spin echo signals, and modifying according to the phase signals of the corresponding axes to obtain modified spin echo signals S # S，R，P )＝S( S，R，P )*exp(i*φ( S，R，P ))。
10. The apparatus of claim 6, wherein the magnetic resonance imaging unit comprises:
a signal acquisition subunit for setting a read gradient of the acquired modified spin echo signal after the second 180 ° pulse; the FID signal of the first 180-degree pulse of the preset acquisition node is acquired, and the correction signal after the first 180-degree pulse of the acquisition node is acquired.
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