CN112858974A - Shimming method for local region, local coil and magnetic resonance imaging device - Google Patents
Shimming method for local region, local coil and magnetic resonance imaging device Download PDFInfo
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- CN112858974A CN112858974A CN201911189312.0A CN201911189312A CN112858974A CN 112858974 A CN112858974 A CN 112858974A CN 201911189312 A CN201911189312 A CN 201911189312A CN 112858974 A CN112858974 A CN 112858974A
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
- G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
- G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
- G01R33/387—Compensation of inhomogeneities
Abstract
The embodiment of the invention discloses a shimming method of a local region, a local coil and a magnetic resonance imaging device. The shimming method of the local region comprises the following steps: a silicon steel sheet part is arranged inside the local coil; when a main magnetic field is applied, the silicon steel sheet component induces a local magnetic field in the main magnetic field, and the local magnetic field can perform shimming compensation on the nonuniformity of the main magnetic field in the local coil area. The technical scheme in the embodiment of the invention can improve the magnetic field uniformity of the local area and further ensure the magnetic resonance imaging quality.
Description
Technical Field
The invention relates to the field of magnetic resonance imaging, in particular to a shimming method of a local region, a local coil and a magnetic resonance imaging device.
Background
Magnetic Resonance Imaging (MRI) apparatuses acquire molecular structures and structural information of the inside of a human body by using a nuclear Magnetic Resonance phenomenon based on a Magnetic field generated by a magnet. In accordance with the principles of magnetic resonance imaging, to obtain high quality MR images, the magnet in an MRI apparatus is required to provide a magnetic field of very high homogeneity in a particular space, i.e. the examination region. Among these, the homogeneity of the main magnetic field (B0 field) is an important factor affecting the imaging quality, especially for fat suppression. In the fat compression sequence, there are two kinds of radio frequency pulses, one is a pulse for imaging of normal water signals, and the other is a fat compression pulse for suppressing fat signals. The two resonance frequencies of water and fat are very close, and if the B0 field is not uniform, the resonance frequencies of water and fat are blurred, and the fat signal is difficult to accurately suppress by the fat pressing pulse.
Although the uniformity of the B0 field is easily achieved at the center of the magnet, the uniformity of the magnetic field at the off-center position of the magnet is difficult to ensure. For example, the knee joint is an important scanning site in MRI examination, and in MRI systems, in order to obtain good image quality, a dedicated knee coil is developed to receive MRI signals of the knee joint of a patient, particularly for an injured knee joint, the patella is an important region of interest, and the requirement for good image quality is high. However, the fat inhibition of the patella is not good, and the B0 field at the patella is not uniform.
To address this problem, it is often necessary to place a shim device, such as a shim shell (shim shell), in the magnet of the MRI apparatus to compensate for B0 field inhomogeneity.
Disclosure of Invention
In view of this, in the embodiments of the present invention, on the one hand, a shimming method for a local region is provided, and on the other hand, a local coil and a magnetic resonance imaging apparatus are provided to improve magnetic field uniformity of the local region, so as to further ensure magnetic resonance imaging quality.
The shimming method for the local region provided by the embodiment of the invention comprises the following steps: a silicon steel sheet part is arranged inside the local coil; when a main magnetic field is applied, the silicon steel sheet component induces a local magnetic field in the main magnetic field, and the local magnetic field can perform shimming compensation on the nonuniformity of the main magnetic field in the local coil area.
In one embodiment, the size and configuration of the silicon steel sheet components are determined as follows: acquiring original imaging data of a plurality of scanning objects based on a local coil, acquiring field intensity distribution of a corresponding scanning part based on the original imaging data, and determining magnetic field variation of a main magnetic field in the local coil according to the field intensity distribution; and determining the size and the structure of the silicon steel sheet capable of generating the reverse magnetic field variation according to the magnetic field variation.
In one embodiment, the local coil comprises: knee, shoulder or neck coils.
In one embodiment, when the local coil is a knee coil, the silicon steel sheet member includes: and the strip-shaped silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee.
In one embodiment, the at least one elongated silicon steel sheet is: a plurality of strip-shaped silicon steel sheets with the same or different lengths corresponding to the shape of the knee.
In one embodiment, when the local coil is a knee coil, the silicon steel sheet member includes: and the circular silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee.
The local coil provided in the embodiment of the present invention includes: a local coil body; the silicon steel sheet component can induce a local magnetic field in the main magnetic field, and the local magnetic field can perform shimming compensation on the nonuniformity of the main magnetic field in the local coil region; wherein the size and configuration of the silicon steel sheet component is determined by the amount of change in the magnetic field of the main magnetic field within the local coil.
In one embodiment, the size and structure of the silicon steel sheet are determined as follows: acquiring original imaging data of a plurality of scanning objects based on a local coil, acquiring field intensity distribution of a corresponding scanning part based on the original imaging data, and determining magnetic field variation of a main magnetic field in the local coil according to the field intensity distribution; and determining the size and the structure of the silicon steel sheet capable of generating the reverse magnetic field variation according to the magnetic field variation.
In one embodiment, the local coil comprises: knee, shoulder or neck coils.
In one embodiment, when the local coil is a knee coil, the silicon steel sheet member includes: and the strip-shaped silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee.
In one embodiment, the at least one elongated silicon steel sheet is: a plurality of strip-shaped silicon steel sheets with the same or different lengths corresponding to the shape of the knee.
In one embodiment, when the local coil is a knee coil, the silicon steel sheet member includes: and the circular silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee.
The magnetic resonance imaging apparatus provided in an embodiment of the present invention includes: the local coil as set forth in any of the above embodiments.
According to the scheme, the silicon steel sheet is added at the proper position on the local coil, so that the distortion of a magnetic field giant variable region, such as a B0 field near a patella, can be compensated, and the fat inhibition problem of the local coil, such as a knee coil, can be effectively solved. This approach is less costly and less complex than other approaches. In addition, due to the adoption of passive connection, the silicon steel sheet can be conveniently integrated into the existing MR local coil. The method not only effectively solves the problem of fat inhibition in certain local areas, but also can lead the research and development of local coils such as knee coils.
When the silicon steel sheet is specifically realized, the structural forms of the silicon steel sheet can be various, and the realization is more flexible. For example, for a knee coil, it may be in the form of a strip or a disc.
Drawings
The foregoing and other features and advantages of the invention will become more apparent to those skilled in the art to which the invention relates upon consideration of the following detailed description of a preferred embodiment of the invention with reference to the accompanying drawings, in which:
FIG. 1 is a graph of the field strength distribution of the knee scanning section in one example of the present invention.
FIG. 2 is a graph of phantom scan results of B0 field variations of a sheet of silicon steel in an example of the present invention.
Fig. 3 is an image of a water phantom imaged by a knee coil added with silicon steel sheets according to an example of the present invention.
Fig. 4A to 5B are comparison diagrams of magnetic resonance images of a sequence of silicon steel sheets with and without the addition of silicon steel sheets, using the fat-suppressed pulses and the fat-not-suppressed pulses in one example of the present invention. In which FIGS. 4A and 4B employ a sequence of suppressed fat pulses,
fig. 5A and 5B adopt a sequence of no fat-suppressing pulses, fig. 4A and 5A are images obtained by imaging a volunteer with a knee coil to which no silicon steel sheet is added, and fig. 4B and 5B are images obtained by imaging a volunteer with a knee coil to which a silicon steel sheet is added.
Fig. 6 is an exemplary flowchart of a local region shimming method in an embodiment of the present invention.
Fig. 7 is an exemplary block diagram of a local coil in an embodiment of the invention.
Wherein the reference numbers are as follows:
| S601～S602 | step (ii) of |
| 710 | |
| 720 | Silicon |
| 721～725 | Silicon steel sheet |
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail by referring to the following examples.
In embodiments of the present invention, it is considered that although shimming devices are placed in the magnet of an MRI apparatus, current shimming devices mainly compensate for a wide range of field inhomogeneities. For the marginal region or some local regions (such as knee, shoulder, neck, etc.), the shimming effect of the shimming device in this region is not ideal because the B0 field changes rapidly. Although in some applications it is contemplated to provide a second coil in the local coil for shimming the region in which the local coil is located, or to place shimming compensation means in the fringe region for shimming fringe inhomogeneities of the field over a large range. To date, however, no simple and satisfactory solution has been found for shimming such local regions of the knee joint.
Therefore, in the embodiment of the invention, a passive compensation method is adopted in the local coil, namely a non-coil implementation scheme without wiring and power connection is considered, and after a series of creative work such as thinking, experiments and the like, the silicon steel sheet is proposed to be adopted to realize local shimming treatment. Silicon steel sheet is a kind of magnet, but it exhibits a significant magnetic property only in an environment where a magnetic field exists, and it does not need to be connected to any device. Taking the knee coil as an example, a common shim magnetic field for compensating different knees is induced in the main magnetic field by being placed at the top inside the knee coil near the knee.
In order to know how much magnetic field the silicon steel sheet needs to generate locally, large data of the B0 field near the patella are needed, and therefore the large data can be collected by scanning some volunteers. During scanning, a field map sequence can be selected to obtain original imaging data, and then Matlab is used to process the original imaging data, so that the field intensity distribution of the scanning part shown in FIG. 1 can be obtained. In the above process, historical original imaging data of some patients can be obtained, and then Matlab is used to process the historical original imaging data to obtain the field intensity distribution of the corresponding scanning part.
It can be seen that the B0 field changes more strongly from the center of the knee towards the patella. The approximate field difference between the patella edge and the knee center is approximately 230 Hz. Taking 1.5T system as an example, when the echo time TE is 64ms63.6 × 10 for total hertz of the main magnetic field6In Hertz, the B0 field variation from the center of the knee to the patella can be calculated as shown in equation (1):
as can be seen from the calculation of equation (1), when TE is 64ms, the B0 field variation from the center of the knee to the patella is about 3.6ppm (parts per million). According to the experimental results, the optimum results of about 3.6ppm B0 change were obtained by performing phantom scanning with silicon steel sheets of different sizes placed at the top of the knee coil. Meanwhile, in order to avoid the influence of the signal-to-noise ratio of the knee coil, the volume of the silicon steel sheet cannot be too large.
After analysis and comparison, the finally determined silicon steel sheet can obtain a phantom scanning result graph of B0 field variation as shown in FIG. 2. It can be seen that the B0 field has changed after the addition of the silicon steel sheet. There were approximately 15 black bands, each equal to 0.25ppm, so 15 black bands equal to 3.75ppm, which is very close to the ideal value of 3.6 ppm.
In order to verify the shimming effect, in the embodiment of the present invention, magnetic resonance imaging is performed by using a phantom (phantom) first. In a specific implementation, the die body can be a water die or an oil die. As shown in fig. 3, fig. 3 is an image of a water phantom imaged by a knee coil added with silicon steel sheets according to an example of the present invention. It can be seen that the water model imaging has clear boundary and uniform signal-to-noise ratio.
In addition, in the embodiment of the present invention, since the silicon steel sheet is placed on the coil capable of generating the reverse magnetic field, that is, is in direct contact with the coil, another problem needs to be considered, that is, the silicon steel sheet does not affect the image quality of the coil. Therefore, it is necessary to compare the magnetic resonance image with the silicon steel sheet and the magnetic resonance image without the silicon steel sheet, and the sequence used has a sequence of the fat-suppressing pulse and the fat-suppressing pulse, as shown in fig. 4A to 5B, where fig. 4A and 4B adopt the sequence of the fat-suppressing pulse, fig. 5A and 5B adopt the sequence of the fat-suppressing pulse, fig. 4A and 5A are images obtained by imaging the volunteer with the knee coil without the silicon steel sheet, and fig. 4B and 5B are images obtained by imaging the volunteer with the knee coil with the silicon steel sheet. It can be seen that in the patellar region, compared to the coil image without the silicon steel sheet in fig. 4A, the coil with the silicon steel sheet added thereto has no fat saturation problem. And it can be concluded from the results of fig. 4B and 5B that the silicon steel sheet has almost no influence on the quality of the coil.
Fig. 6 is an exemplary flowchart of a local region shimming method in an embodiment of the present invention. As shown in fig. 5, the method may include the steps of:
step S601, a silicon steel sheet member is placed inside the local coil. Wherein the size and configuration of the silicon steel sheet component may be determined based on the amount of change in the magnetic field of the main magnetic field within the local coil. The method specifically comprises the following steps: acquiring original imaging data of a plurality of scanning objects based on a local coil, acquiring field intensity distribution of a corresponding scanning part based on the original imaging data (for example, Matlab can be used for processing the original imaging data to acquire field intensity distribution of the corresponding scanning part), and determining magnetic field variation of a main magnetic field in the local coil according to the field intensity distribution; and determining the size and the structure of the silicon steel sheet capable of generating the reverse magnetic field variation according to the magnetic field variation.
In concrete implementation, the silicon steel sheet component does not need to be connected with any device and does not relate to the problem of routing, so long as the position of the silicon steel sheet component in the local coil can be fixed. In particular, there are several ways to implement the installation. For example, the silicon steel sheet member may be fixed on the inner case surface of the partial coil by glue or other means. The local coil may be a knee coil or a shoulder coil or a neck coil, etc. For example, if the local coil is a knee coil, the silicon steel sheet component may be located within the knee coil at a position corresponding directly above the knee.
In particular, the silicon steel sheet component can have various structural forms. For example, still exemplified as a knee coil, the silicon steel sheet member may include: and the strip-shaped silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee. For example, the length of the silicon steel strip may be one strip corresponding to the position right above the knee, or a plurality of strips of silicon steel strips with the same or different lengths corresponding to the shape of the knee. For example, it may include: the first silicon steel sheet of rectangular shape and be located two rectangular shape second silicon steel sheet of first silicon steel sheet both sides directly over corresponding knee. Wherein the second silicon steel sheet is shorter than the first silicon steel sheet. Or comprises the following steps: two strip-shaped silicon steel sheets with the same length are symmetrically arranged above the knee. In addition, the silicon steel sheet member may be a circular silicon steel sheet located inside the local coil above the corresponding knee. Of course, the size, number, position and the like of the silicon steel sheets can be realized in other manners, and the size, number, position and the like can be selected according to actual needs in practical application.
Step S602, when a main magnetic field is applied, the silicon steel sheet component induces a local magnetic field in the main magnetic field, and the local magnetic field can perform shimming treatment on the nonuniformity of the main magnetic field in the local coil area.
Fig. 7 is an exemplary block diagram of a local coil in an embodiment of the invention. The local coil may be a knee coil, a shoulder coil, a neck coil, or the like. As shown in fig. 7, the local coil in the embodiment of the present invention may include: a local coil body 710; and a silicon steel sheet member 720 positioned in the local coil body 710 (in the present embodiment, a knee coil is taken as an example).
The silicon steel sheet part 720 can induce a local magnetic field in the main magnetic field, and the local magnetic field can compensate the inhomogeneity of the main magnetic field in the local coil area. Wherein the size and configuration of the silicon steel sheet component is determined by the amount of change in the magnetic field of the main magnetic field within the local coil. The method specifically comprises the following steps: acquiring original imaging data of a plurality of scanning objects based on a local coil, processing the original imaging data by using Matlab to obtain field intensity distribution of a corresponding scanning part, and determining the magnetic field variation of a main magnetic field in the local coil according to the field intensity distribution; and determining the size and the structure of the silicon steel sheet capable of generating the reverse magnetic field variation according to the magnetic field variation.
In particular, the silicon steel sheet component can have various structural forms. It can have different configurations for different local coils. For example, when the local coil is a knee coil, the silicon steel sheet member 720 may include: and at least one strip-shaped silicon steel sheet positioned inside the local coil 710 and corresponding to the upper part of the knee. For example, the length of the silicon steel strip may be one strip corresponding to the position right above the knee, or a plurality of strips with the same or different lengths corresponding to the shape of the knee, such as 721-725 shown in the figure. For example, it may include: the first silicon steel sheet of rectangular shape and be located two rectangular shape second silicon steel sheet of first silicon steel sheet both sides directly over corresponding knee. Wherein the second silicon steel sheet is shorter than the first silicon steel sheet. Or comprises the following steps: two strip-shaped silicon steel sheets with the same length are symmetrically arranged above the knee. Of course, the size, number, position and the like of the silicon steel sheets can be realized in other manners, and the size, number, position and the like can be selected according to actual needs in practical application.
In addition, the silicon steel sheet member may be a circular silicon steel sheet located inside the local coil above the corresponding knee.
An embodiment of the present invention further provides a magnetic resonance imaging apparatus, which may include the local coil according to any one of the above embodiments.
From the above analysis, it can be seen that the silicon steel sheet added at the proper position on the local coil can compensate the distortion of the B0 field in the magnetic field giant variable region, such as around the patella, and can effectively solve the fat inhibition problem of the local coil, such as the knee coil. This approach is less costly and less complex than other approaches. In addition, due to the adoption of passive connection, the silicon steel sheet can be conveniently integrated into the existing MR local coil. The method not only effectively solves the problem of fat inhibition in certain local areas, but also can lead the research and development of local coils such as knee coils.
When the silicon steel sheet is specifically realized, the structural forms of the silicon steel sheet can be various, and the realization is more flexible. For example, for a knee coil, it may be in the form of a strip or a disc.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.
Claims (13)
1. A method of shimming a local region, comprising:
placing a silicon steel sheet member inside the local coil (S601);
upon application of the main magnetic field, the silicon steel sheet assembly induces a local magnetic field within the main magnetic field that compensates for inhomogeneity of the main magnetic field in the local coil region (S602).
2. The method of shimming local regions according to claim 1, wherein the size and configuration of the silicon steel sheet components are determined as follows: acquiring original imaging data of a plurality of scanning objects based on a local coil, acquiring field intensity distribution of a corresponding scanning part based on the original imaging data, and determining magnetic field variation of a main magnetic field in the local coil according to the field intensity distribution; and determining the size and the structure of the silicon steel sheet capable of generating the reverse magnetic field variation according to the magnetic field variation.
3. The method of shimming a local region according to claim 1, wherein the local coil comprises: knee, shoulder or neck coils.
4. The local region shimming method according to claim 3, wherein when the local coil is a knee coil, the silicon steel sheet member includes: and the strip-shaped silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee.
5. The local area shimming method according to claim 4, wherein the at least one strip-shaped silicon steel sheet is: a plurality of strip-shaped silicon steel sheets with the same or different lengths corresponding to the shape of the knee.
6. The local region shimming method according to claim 3, wherein when the local coil is a knee coil, the silicon steel sheet member includes: and the circular silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee.
7. A local coil, comprising:
a local coil body (710); and
the silicon steel sheet component (720) is positioned in the local coil main body (710), the silicon steel sheet component (720) can induce a local magnetic field in the main magnetic field, and the local magnetic field can perform shimming compensation on the nonuniformity of the main magnetic field in the local coil area; wherein the size and structure of the silicon steel sheet member (720) is determined according to the amount of change in the magnetic field of the main magnetic field in the local coil.
8. The local coil as set forth in claim 7, wherein the silicon steel sheet member (720) is sized and configured as follows: acquiring original imaging data of a plurality of scanning objects based on a local coil, acquiring field intensity distribution of a corresponding scanning part based on the original imaging data, and determining magnetic field variation of a main magnetic field in the local coil according to the field intensity distribution; and determining the size and the structure of the silicon steel sheet capable of generating the reverse magnetic field variation according to the magnetic field variation.
9. The local coil of claim 8, wherein the local coil comprises: knee, shoulder or neck coils.
10. The local coil as set forth in claim 9, wherein the silicon steel sheet member (720) includes, when the local coil is a knee coil: and the strip-shaped silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee.
11. The local coil as claimed in claim 10, wherein said at least one elongated silicon steel sheet is: a plurality of strip-shaped silicon steel sheets (721-725) having the same or different lengths corresponding to the shape of the knee.
12. The local coil as set forth in claim 9, wherein the silicon steel sheet member (720) includes, when the local coil is a knee coil: and the circular silicon steel sheet is positioned in the local coil and corresponds to the upper part of the knee.
13. A magnetic resonance imaging apparatus, characterized by comprising: the local coil of any one of claims 7 to 12.
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