WO2011065357A1 - Mri装置用磁場調整方法 - Google Patents
Mri装置用磁場調整方法 Download PDFInfo
- Publication number
- WO2011065357A1 WO2011065357A1 PCT/JP2010/070881 JP2010070881W WO2011065357A1 WO 2011065357 A1 WO2011065357 A1 WO 2011065357A1 JP 2010070881 W JP2010070881 W JP 2010070881W WO 2011065357 A1 WO2011065357 A1 WO 2011065357A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- magnetic field
- magnetic
- distribution
- adjustment method
- mri apparatus
- Prior art date
Links
Images
Classifications
-
- 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
- G01R33/3873—Compensation of inhomogeneities using ferromagnetic bodies ; Passive shimming
-
- 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
- G01R33/3875—Compensation of inhomogeneities using correction coil assemblies, e.g. active shimming
Definitions
- the present invention relates to a superconducting magnet apparatus, and to a nuclear magnetic resonance tomography apparatus (Magnetic Resonance Imaging).
- the accuracy required for the magnetic field intensity generated by the magnet system has a problem of a fluctuation of about one millionth of the magnetic field intensity. It is accuracy.
- the magnetic field (1) is constant in time, and in the area where tomography of the human body is spatially required, the magnetic field strength is required to be highly accurate and uniform.
- the high accuracy is, for example, an imaging space FOV (Field (of View) having a diameter of 40 cm, and accuracy of the order of 1 / million is required such as ⁇ 1.5 ppm.
- FOV Field (of View) having a diameter of 40 cm, and accuracy of the order of 1 / million is required such as ⁇ 1.5 ppm.
- the magnetic field distribution that requires extremely high precision uniformity requires the magnetic field to be accurately adjusted after the magnet is manufactured and excited.
- an error magnetic field due to a manufacturing error is 1000 times larger than an allowable error magnetic field required for a uniform magnetic field.
- Magnetic field adjustment (shimming) required at the time of installation after manufacture is to reduce the error magnetic field from several hundred ppm to several ppm, and an extremely high-precision magnetic field adjustment device and its method are required.
- FIG. 2 is a diagram showing an example of a conventional magnetic field adjustment method, which uses a spherical function (Japanese Patent Laid-Open No. 2001-87245).
- Spherical harmonics form a base orthogonally on the spherical surface, but there is mutual interference on the magnetic field adjustment mechanism and on the aspherical magnetic field evaluation surface, generating a magnetic field with a precise spherical harmonic distribution. If it tries to do so, fine adjustment on the magnetic field adjustment mechanism is required. For example, the uniform magnetic field distribution is the lowest order distribution of the spherical harmonics. However, it is impossible to actually accurately obtain this distribution unless the magnetic field adjustment mechanism completely surrounds the magnetic field adjustment region. There is no such magnetic field adjustment mechanism in the MRI targeted by the technology.
- the problem of the present invention is that the magnetic field adjustment device and its method include the solution of the problems described above, and confirms the progress of adjustment and the prospect of how much the final error magnetic field can be reduced during the adjustment work. It is another object of the present invention to provide a method and an apparatus that can reliably complete adjustment. Provided are a method including a function that can be easily and automatically corrected in the case of an erroneous operation in order to complete the adjustment quickly, and a device for displaying a guideline for a magnetic field adjustment operation including the method.
- Non-Patent Document 1 As a method for obtaining a current distribution on an arbitrary surface such as a curved surface or a flat surface with respect to a target magnetic field, there is a method using a current potential described in a paper (Non-Patent Document 1). This calculation method is named DUCAS in the paper. The magnetic field adjustment is performed by applying this method, in particular, the current potential and singular value decomposition concept used in this method.
- the magnetic field distribution input as the error magnetic field to be corrected is obtained by the difference between the target magnetic field determined by the plasma confinement theory and the magnetic field distribution calculated by the assumed current potential, that is, by numerical calculation.
- the difference between the target magnetic field and the measurement magnetic field is used as an error magnetic field, and multiple measurement magnetic fields are handled to grasp the error magnetic field distribution.
- Non-Patent Document 1 the distribution of the current potential T is obtained, and the current density vector j is the vector product of the current potential T and the normal of the surface, and the current is given by ( ⁇ T) ⁇ n.
- the magnetic moment distribution or the iron piece density distribution is used.
- an MRI apparatus that generates a highly accurate magnetic field can be manufactured at low cost. Moreover, even if it is not MRI, it can apply to the magnetic field adjustment method of the magnet which requires a highly accurate magnetic field.
- the magnetic field adjustment flowchart which is one suitable Example of this invention is shown.
- the shimming flow figure of a conventional method is shown. It is a figure which shows the view of conversion of the current potential required for the magnetic field correction
- FIG. The figure of the calculation model which applies this invention to the magnetic field adjustment mechanism of FIG. 5 is shown.
- mold MRI made symmetrical of one Example of this invention.
- a diagram of a calculation system for simulating a shim tray of a horizontal magnetic field type MRI apparatus, and a surface for evaluating a current potential for obtaining the amount of iron for shimming are arranged in a ring shape.
- the amount of iron pieces for fine adjustment of the magnetic field is changed depending on the position.
- the magnetic moment in the area divided by the grid is converted into an iron amount, and iron pieces are arranged at the same position on the shim tray shown above in the amount of the calculation result.
- the figure which shows the view which calculates the magnetic moment in a mesh Area integration of the magnetic moments of the nodes in the area corresponding to the area inside the shim tray.
- the iron piece quantity is arranged in the grid by the calculated quantity, but it is divided into several prepared quantities.
- FIG. 3 shows the equivalence of the current potential, the small coil 3 (current loop 4c), the permanent magnet piece 4p, and the magnetized iron piece.
- FIG. 4 As the iron piece 4, a bolt-like shape and a plate-like shape are shown. Even if it is not an iron piece but a magnet, it is equivalent via the magnetization current 2 on the surface. In this case, the direction of the magnetization current 2 is determined by the magnetization of the permanent magnet 4p regardless of the surrounding magnetic field.
- an iron piece may be a ferromagnetic material such as Co or an alloy thereof.
- it may be replaced with an iron piece as long as it is a ferromagnetic substance.
- it is simply referred to as an iron piece.
- FIG. 3A shows the current 21 generated by the finite element 12 and the contact 11 and the current potential T at the time of calculation
- FIG. 3B shows the generation of the magnetic moment by the current 1 flowing through the small coil 3
- FIG. The magnetic moment by the magnetizing current 2 by the magnetized iron piece 4 is shown.
- a bolt-like shape is shown on the upper side, and a plate-like shape is shown on the lower side.
- the current potential value T used to express the current distribution in DUCAS has a unit of [A] as a unit, which is the density [1 / m 2 ] of the magnetic moment [Am 2 ], A] can also be considered.
- the sufficiently magnetized iron piece 4 has a magnetic moment proportional to the volume because the magnetic moment is proportional to the product of the area surrounded by the magnetizing current and the length in the direction of the magnetic field. That is, at the time of adjusting the magnetic field, the current potential T is an amount proportional to the density of the iron piece 4 [weight g / m 2 or volume cc / cm 2 per unit area].
- the eigen distribution function and the singular value obtained by the singular value decomposition used in DUCAS are used instead of the spherical harmonic function of the conventional method.
- This provides a device that uses DUCAS to perform a support calculation for adjusting a magnetic field for a magnetic field generator and displays the arrangement of iron pieces or magnetic moments arranged for the adjustment. The operator can adjust the target magnetic field distribution by proceeding with the adjustment according to the display.
- an arbitrary magnetic field distribution can be set as the target magnetic field, but the following discussion will mainly be made on the assumption that the target magnetic field is a uniform magnetic field. However, whether the target magnetic field has a distribution does not affect the following discussion. It is simply to facilitate understanding of the discussion.
- FIG. 4 is a diagram showing a calculation system of the present embodiment. It consists of a current potential evaluation surface 13 and a set 14 of magnetic field measurement evaluation points.
- the current potential evaluation surface 13 may have a plurality of surfaces, but here, each of them will be discussed as one.
- Magnetic field evaluation points do not necessarily constitute a surface, but are shown here as points on the surface.
- the measurement point j has three-dimensional magnetic field components B xj , B yj , and B zj, but one point of measurement indicates the measurement magnetic field component by the measurement position and the unit vector p defined at that position. Even a single point in space may be three data in the present invention.
- a plurality a is the error magnetic field measurement data the difference between the measured value and the target magnetic field, as a whole becomes a column vector representing the B e.
- Error magnetic field B e is the difference between the magnetic field strength B tg adjusted when adjusting the measurement magnetic field B m and uniform magnetic field.
- FIG. 1 The general system for applying singular value decomposition is shown in FIG.
- the subscript j is a numerical value in the order in which the eigendistribution is numbered in the order of the singular values.
- One basis vector corresponding to each current potential distribution and magnetic field distribution corresponds to one number.
- the two basis vectors and one singular value related to this one number are collectively called one eigenmode.
- the order number j is the order of the eigenmode.
- the low-order eigenmode having a large singular value can generate a large magnetic field as can be understood from the fact that the magnetic field intensity per unit current potential distribution v j is ⁇ j u j .
- the magnetic field distribution is small.
- This property will be described later, it plays an important role in this magnetic field adjustment method.
- the distribution of eigenvectors obtained by singular value decomposition is used as the distribution function of equation (1).
- the relationship between current potential and iron piece density is described below.
- the iron piece can be replaced with a magnetic moment in consideration of the surface magnetizing current.
- M magnetization (T). If the iron piece is in a saturated state, M is about 2.1T. Therefore, j m is about 1.7 ⁇ 10 6 A / m. Therefore the volume of the iron a cubic meter has a magnetic moment of about 1.7 ⁇ 10 6 Am 2 (170Acm 2 / 1cc). Since this value depends on the type of magnet, particularly the magnetic field strength, it needs to be examined individually. However, in the case of a magnet having a magnetic field strength exceeding about 1T, it can be considered that the iron piece is magnetized in a state close to saturation. At this time, the magnetic moment of iron is proportional to the volume.
- d j ⁇ v j C j / ( ⁇ j j m ) (10)
- the current density vector j is a vector product of the current potential T and the normal of the surface, and a current given by ((T) ⁇ n is arranged.
- the above is the basic correction method.
- This is correction of the component by the eigen distribution (u j ) of one error magnetic field.
- the correction of the present invention is characterized in that the distribution function (v j , u j ) of the magnetic field adjustment means for the magnetic field distribution is the basis of each and corresponds one-to-one, and one eigen distribution component is corrected. For this purpose, the distribution function of only one adjusting means is adjusted.
- an eigen distribution having a large singular value is used for adjustment in order to generate a uniform magnetic field with as small an amount as possible.
- a component having a small component intensity of the intrinsic magnetic field distribution included in the measurement magnetic field can be ignored. If the component intensity calculated by the inner product [Expression (7)] is sufficiently smaller than the error magnetic field allowed at the target uniform magnetic field level, there is no need to correct it. Even an eigen component having a large singular value does not need to be used for shimming and is not selected if the component intensity is small.
- the eigendistribution function judged by the operator as being particularly necessary for correction is individually selected and corrected with the strength obtained from the inner product or the artificially determined strength.
- the correction for artificially reducing the peak is corrected by selecting an appropriate eigen distribution function and the magnitude.
- the uniformity (reached uniformity) after correcting the current potential component of the selected eigendistribution function is obtained to determine whether the selection of the eigendistribution function is appropriate. If the arrival uniformity is not sufficient, the selection of the eigendistribution function is reviewed.
- the homogeneity indicates the ratio of the maximum and minimum magnetic field strengths among multiple measurement points in the magnetic field evaluation region, in other words, the difference between the maximum peak and the minimum peak of the error magnetic field to the average magnetic field.
- the iron pieces to be arranged for the adjustment are required to manage the quantity with an accuracy smaller than 1/1000, but it is not easy to manage this accuracy in actual work. Therefore, according to the present invention, the amount of magnetic field is controlled with an accuracy of about 1/10 or less in one adjustment, and the error magnetic field is reduced each time the number of times is adjusted, and the relative ratio with the final magnetic field accuracy is reduced. A sufficient final magnetic field accuracy can be obtained even with a quantity control of / 10 or less.
- the correction amount D corresponding to the selected eigen distribution is the sum of the correction amounts based on the respective eigen distribution functions.
- the sum ⁇ is executed for the selected eigendistribution function. It can be easily calculated and predicted how the magnetic field distribution in the imaging region will be after this correction is executed.
- One method is a method of obtaining from a magnetic field distribution function of an eigen distribution function.
- the following formula. B shim B e - ⁇ C j u j (12)
- the other is a method of reconfiguration by the reconfigured current potential.
- the integration region will be discussed with an example.
- the discretized display shows the actual calculation contents because it is divided and calculated by the mesh.
- the previously described sum ⁇ is performed at node i in the integration region, and Sij is the area of element j belonging to the i-th node. Since it is a triangular element, 1/3 is considered to contribute to the i-th node.
- the sum ⁇ written behind is executed for the triangular element j to which the i-th node belongs.
- Si ⁇ Sij / 3.0 (16) Discuss as.
- the current potential has been discussed as a variable, but considering the area Si involved in the node from the beginning, TiSi ⁇ Mi (17) , The discussion will deal with the magnitude Mi of the magnetic moment, but the discussion up to now is simply the same as the argument using the singular value decomposition, only by converting the area by the magnification of the size. .
- the integral of Equation 16 is simply the sum of the magnetic moments belonging to that region.
- the position where the magnetic moment is arranged is not limited to the surface as in the current potential.
- the schematic structure of the magnetic field adjustment mechanism used in actual shimming is a structure having a flat surface or a cylindrical surface, and will be discussed below with a current potential on a curved surface.
- the selection of the eigenmode in item (1) is selected to correct the error magnetic field especially for the low-order eigendistribution function.
- a low-order distribution function is selected as long as the magnetic field can be corrected with a relatively small amount of iron pieces. Even if it is only on the lower order side, tens to hundreds of eigendistribution functions are usually selected.
- By correcting the magnetic field according to the iron piece (current potential) arrangement of the eigendistribution function it is possible to correct the eigendistribution that has not been selected so as not to have a large influence and a new error magnetic field. This is particularly advantageous in that it does not disturb the higher order (numbered large numbered eigendistribution) that was not selected. That is, when the magnetic field adjustment is performed, the high-order eigen distribution not selected is not disturbed, and the work is not complicated.
- the low-order eigendistribution function selected in the singular value decomposition can be corrected with a small amount of iron pieces, but a large amount of iron is required to change the higher order.
- the reason why the higher order part is not disturbed is that, besides the fact that the distribution is orthogonal, a higher amount of iron pieces is required. That is, in the correction of the low-order distribution function in which a small amount of iron pieces is arranged, even if the arrangement is disturbed due to an error, the intensity of the high-order component does not change. In this sense as well, correction is performed by selecting from the low-order eigendistribution.
- the magnetic field that can be corrected is large in proportion to the singular value in the low order, the magnetic field adjustment, that is, shimming can be performed efficiently with a small amount of iron pieces.
- Item (2) does not correct eigen distribution function components that do not require correction. However, even if selected and included in the correction amount, the correction amount is small, so that the higher-order components are not disturbed as described above, so that no problem occurs.
- Item (3) adjusts the selection to adjust the iron piece arrangement amount and magnetic field distribution.
- the magnetic field is corrected using only the iron piece, it may be difficult to perform adjustment by the amount of the negative iron piece that removes the iron piece.
- the high-order distribution generates only a small magnetic field even if iron pieces are arranged. That is, a high-order component iron piece is arranged to make room for removing the iron piece during low-order correction.
- the uniformity is defined in the range from the positive peak to the negative peak value, there is a case where only the peak portion is concentrated and the display of the uniformity is deteriorated. In this case, an appropriate correction component is artificially added. This makes it easier to reach the target uniformity.
- the negative iron piece amount is a case where the magnetic moment necessary for the magnetic field adjustment obtained by Expression (15) shows a magnetic moment in the direction opposite to the direction of iron piece magnetization by the surrounding magnetic field.
- this correction can be performed by using a current loop or a permanent magnet.
- the procedure and the structure are complicated, the above procedure is used if possible.
- Item (5) is selection of how much the magnetic field strength to be made uniform is.
- the eigen distribution function is selected while changing the target magnetic field, the arrival uniformity and the amount of iron pieces are checked, and the target magnetic field with good uniformity and easy arrangement of the iron pieces is selected.
- the easy placement of the iron pieces does not simply mean that the amount is small, the necessary relatively low-order distribution function can be corrected sufficiently, and there is no region where the iron pieces cannot be arranged with the calculated negative iron amount Arrangement.
- Item (6) completes the magnetic field adjustment by repeating the work from the measurement to the iron piece arrangement.
- the eigendistribution function changes.
- the eigen distribution is initially selected up to the higher order, the magnetic field adjustment is performed, and the quantity is adjusted to be large, and the upper limit of the order is gradually lowered.
- the adjustment accuracy which is about 1/10 of the quantity, is also improved.
- a target magnetic field uniformity can be obtained with a small correction amount while predicting the adjusted magnetic field. It is necessary to perform shimming work repeatedly. In the middle of repetitive work, especially when high-order distribution shimming is performed, the error magnetic field component corresponding to the low-order distribution function may increase and the uniformity may appear to deteriorate.
- FIG. 5 shows a system for shimming (magnetic field adjustment) the magnetic field generated by the magnet of the MRI apparatus. This figure assumes an open type machine in which the direction of the magnetic field (lines of magnetic force) is oriented in the vertical direction.
- the conceptual shape of the open MRI magnet is also shown in FIG.
- coils 62a including a vacuum vessel 62c for securing a heat insulating vacuum, a radiation shield 62d, a cryogenic vessel 62e, and a magnetic shielding coil 62b.
- the examinee lies on the examinee's bed 61 and performs nuclear magnetic resonance tomography.
- FIG. 1 shows a shimming flow for adjusting the magnetic field distribution in this embodiment. It is an Example applied to the magnetic field adjustment of the imaging area
- the surface of the magnetic field adjustment mechanism (the shim tray 5) is above and below the imaging region with the strength of the magnetic field component perpendicular to the ground, and the iron piece 4 is disposed on the surface.
- FIG. 6 shows an example of mesh generation when the present embodiment is applied to shimming of an MRI apparatus.
- the spherical surface is a set 14 of magnetic field measurement evaluation points, and hundreds of magnetic field measurement points are arranged.
- the upper and lower disk surfaces of the sphere are the calculation model of the surface on which the iron piece 4 is arranged when performing shimming, that is, the current potential evaluation surface 13.
- a finite element calculation system composed of triangular elements having contacts on this surface is constructed.
- Step 1 includes a singular value decomposition calculation step 32S for calculating the preliminary calculation portion 1B in the broken line prior to the shimming operation, and includes a calculation mesh generation step 31S, and an eigen distribution function and a singular value as a result of the singular value decomposition.
- Step 33S is included.
- This portion is a preliminary calculation portion 1B including a singular value decomposition of the response matrix A from the contact corresponding to several thousand current potential values to the magnetic field measurement points of about several hundred points in the imaging region. Need time. For this reason, the eigendistribution function for shimming is calculated with a calculation system adapted to the magnet system, thereby shortening the calculation time during the shimming operation.
- the data calculated in advance is stored in the storage area of the computer in the storing step 33S, and is read out when necessary (single value decomposition result reading step 16S).
- the computer there are several or more eigendistribution functions that are the basis vectors of the magnetic field distribution, the same number of basis vectors that are the distribution functions of the current surface, and the same number of singular values that are conversion information of the magnitudes of both. Are stored in combination.
- the magnetic field adjustment starts 11S. Work is performed according to the flow of FIG. After the magnetic field measurement step 12S and the magnetic field distribution data storage step 13S and the magnetic field data read step 14S, it is determined whether the uniformity is good in the magnetic field uniformity determination step 15S. If the uniformity is sufficient, shimming is not necessary, and the magnetic field adjustment end step 40S is performed. This may be the case when the device used with sufficient uniformity is de-energized after degaussing at the time of maintenance, etc., but the new magnet has a uniformity of several hundred to 1000 ppm due to manufacturing errors, and the magnetic field It is determined that adjustment (shimming) is necessary.
- step 18 S calculations of the equations (1) to (14) such as the intensity C j of each eigenmode, the correction current potential ⁇ T, the correction iron piece arrangement and the correction magnetic field distribution, and the calculation of reachable uniformity are selected.
- the intensity C j of each eigenmode the intensity of each eigenmode
- the correction current potential ⁇ T the correction current potential
- the correction iron piece arrangement and the correction magnetic field distribution the calculation of reachable uniformity
- Step 18 The calculation result of S is displayed to determine the validity of the eigenmode selection.
- FIG. 7 is a diagram in which the strength of the eigen distribution of the magnetic field included in the error magnetic field obtained by the equation shown in equation (7) is shown on the vertical axis, and the order of the eigen mode is shown on the horizontal axis, which is called a spectrum. To do.
- the vertical axis shows the logarithmic scale.
- FIG. 7 also shows the eigenmode selection range and reachable uniformity.
- FIG. 8 shows a display example of the iron piece arrangement amount for shimming work together with current potential contour lines.
- the calculation contents shown in this example are the system shown in FIGS. 5 and 6, but the target is a uniform magnetic field where the magnetic field evaluation point is on a surface having a diameter of 40 cm and the error magnetic field of this surface is 20 ppm or less.
- the eigen distribution function to be corrected is selected.
- x corresponds to each eigenmode, but the one surrounded by a circle is the selected eigenmode 15. Those not circled are non-selected eigenmodes. This selection is made by the method already described.
- the reachable uniformity can be calculated and predicted by subtracting the error magnetic field component from the measurement error magnetic field.
- the reachable uniformity 17 is shown in a portion surrounded by an ellipse at the top.
- FIG. 7A is before shimming and FIG. 7B is after shimming. It is 726 ppm in this example before shimming, and it can be seen that the error magnetic field component in the low-order mode is large in the spectrum diagram.
- A is an eigenmode selected as an error magnetic field component having an order of 80 or less and a strength almost equal to or greater than the lower limit of measurement accuracy. In this example, if the selected eigenmode is corrected, it is predicted to be 15.25 ppm.
- a line 22 indicating the upper limit of the eigenmode selection and a line 23 indicating the lower limit of the eigenmode selection are displayed on the spectrum diagram of FIG. 7 to select the eigenmode.
- the selection of the eigendistribution function is reconsidered. Adjusting the number of unique distribution functions, namely to adjust the lower limit of the specific distribution function selection number on the lower limit or eigenmode intensity C j of. There are also options such as adjusting the correction ratio of the individually selected eigendistribution function.
- the other display in step 19S is used for checking whether or not shimming is possible in the iron piece arrangement amount instruction diagram of FIG.
- the circle in the figure shows the shim tray 5 shown in FIG. Although there are two upper and lower sheets, this is the lower shim tray.
- the cells 7 in the figure are sections arranged on the shim tray 5, and each is assigned an address. In FIG. 7, addresses are designated by A, B, C,... In the left-right direction and 1, 2, 3,.
- the numerical value in the mesh 7 indicates the iron volume 18 arranged in the mesh.
- the unit is 0.1 cc.
- the grid has a structure in which iron pieces of about 5 cc can be sufficiently arranged, and the amount of display is sufficiently small and can be arranged. Since the amount of iron pieces to be handled is gradually reduced during the repeated adjustment, it is displayed in units as small as 1/10, 1/100, 1/1000 of the original.
- FIG. 8 shows a contour line 19 of the current potential on the current potential evaluation surface 13 in which the shim tray 5 is also modeled, in addition to the iron piece amount 10 of the mesh 7 and the mesh.
- the current potential contour line 19 is considered as a coil shape, the error magnetic field can be corrected by the coil having this shape. This is described in the previously published paper mentioned above.
- the contour display in the present invention has another advantage.
- the distribution function obtained by singular value decomposition requires an arrangement of iron pieces or magnetic moment with a spread on the surface. However, the most arrangement (or removal) is required in the vicinity of the peak 8 of the contour line and the valley 9 of the contour line. Using these two properties, the arrangement position of the iron piece for adjusting the magnetic field is flexibly considered.
- the amount of iron pieces of the current potential contour line 19 closed with the same sign around the contour peak is added and placed near (removed from) the peak 8 position of the contour line. Also, if the vicinity cannot be placed due to, for example, a shim tray fixing support, the same amount of placement may be placed (removed) in other parts within the closed contour region. .
- FIG. 9 shows a conversion concept from the contact potential value to the magnetic moment and the iron piece amount in the magnetic field adjustment calculation of this embodiment.
- equation (9) it was stated that the volume of iron and the magnetic moment are proportional.
- the current potential can be interpreted as indicating the magnetic moment per unit area. Therefore, in order to obtain the amount of iron pieces in a certain area, the current potential T is integrated in the area to obtain the magnetic moment necessary for the area, and converted to the iron amount as already described.
- FIG. 9 schematically shows the relationship between the mesh 7 shown in FIG. 8 and the contacts. A point indicated by ⁇ is a contact point.
- the product of the contact point and the area corresponding to the contact point is added to obtain the magnetic moment in the mesh 7 as in the equation in the figure.
- the area corresponding to the contact may be 1/3 of the element to which the contact belongs (in the case of ⁇ element).
- the size of the mesh 7 is required to be fine enough to have the resolution of the iron amount distribution shown in FIG.
- the contour line distribution is confirmed near the upper limit of the order of the eigenmode necessary for obtaining the uniformity, and is assumed to be smaller than the size of the peak or valley portion. On the other hand, it takes time and effort to make a fine mesh. In FIG. 8, it is approximately the same as the minimum size of the peak 8 of the contour line and the valley 9 of the contour line. Because of the same degree, there are some areas where fine contour lines do not have sufficient resolution with only the mesh 7. In this case, the iron piece is arranged by adjusting the position with reference to the peak and valley positions of the contour lines.
- the size of the finite element is determined by the number of contacts in the grid. As already described, the accuracy of the iron piece arrangement amount may be about 1/10, and the uniformity is increased repeatedly. If the number of contacts is about 5 or more, even if a slight error occurs in the corresponding area, it is considered that the accuracy is sufficient. In the example of FIG. 8, one side has about 1500 contacts. As shown in FIG. 6, the upper and lower shim trays are considered as current evaluation surfaces, and the total of two sheets is considered to be about 3000 contacts or more.
- the iron piece arrangement work step 22S was performed.
- the basis for this prediction is that the reachable uniformity 17 is sufficiently better than the target value and the amount of iron pieces can be arranged.
- the process proceeds to a magnetic field adjustment determination step 21S for determining whether or not the target magnetic field adjustment is possible. If it is determined in the magnetic field adjustment possible determination step 21S that the magnetic field adjustment is possible (“YES”), “return to the eigenmode selection and target magnetic field determination step 17S again. However, even if various conditions are changed, the target magnetic field adjustment is performed. If it is determined that the magnetic field adjustment is not possible (“No”) in the magnetic field adjustment possible determination step 21S for determining whether it is possible, the magnet is defective, and the repair / adjustment step 41S is entered.
- Fig. 7 (b) shows the spectrum at the end of shimming. Shimming can be achieved up to a uniformity of 17 ppm. It can be said that the uniformity was predicted with good accuracy and with the same degree of uniformity compared with the initially estimated 15 ppm level. Until the spectrum uniformity shown in FIG. 7A is reached, the repetitive operation is performed as shown in the flow of FIG. The necessity of this repetition has already been described, but will be described later with an example.
- step 21S The possibility of adjusting the magnetic field is determined in step 21S, which will be described below.
- an appropriate correction amount shim iron piece amount.
- this is a case where the manufacturing accuracy of the magnet is insufficient, the magnetic field is poor, and a large amount of iron pieces is required to obtain the target uniformity, which is actually impossible.
- This evaluation enables detection known defects of the magnetic field without a magnetic field adjustment. If it is defective, appropriate correction is performed, but the problem location can be estimated from the correction amount distribution. Further, if it cannot be repaired, it is possible to determine that the product is defective, and the present invention also includes the advantage that determination can be made without using manual labor by repeatedly adjusting the magnetic field.
- the iron piece distribution calculation result necessary for correction is output by paper printing or enlarged display by project, and the shimming iron pieces are arranged according to the distribution.
- Do work There is an error in the amount and position of the iron piece to be placed in the shimming operation, and there is an error in the iron piece conversion from the current potential because the degree of iron piece magnetization depends on the material properties of the iron piece and the magnetic field distribution in the magnet. To do. For this reason, the reachable uniformity cannot be reached in one operation. Therefore, the work is repeated as shown in FIG. 1 to bring the magnetic field closer to uniform.
- a second embodiment will be described. Although it has already been described that it can be used for quality inspection after production, this method can be used for magnet design with the same judgment.
- the flow in this case is shown in FIG.
- the magnetic field adjustment is performed in calculation, and it is applied to the magnetomotive force arrangement design by confirming that the target magnetic field accuracy can be achieved.
- step 52S is performed to assume the magnetomotive force arrangement.
- a magnetic field calculation step 53S is performed based on this magnetomotive force arrangement.
- the singular value decomposition is executed based on the arrangement of the shimming trays from the magnetomotive force arrangement, and the result is stored.
- This preliminary calculation portion 1B is the same as that in the first embodiment.
- the portion 1B is executed based on the magnetomotive force arrangement assumption step 52S only when it is determined that the shim tray needs to be changed in the magnetomotive force arrangement improvement determination step 56S.
- This preliminary calculation unit 1B is the same as in FIG. In 56S, it is determined whether or not the existing singular value decomposition data can be used, and depending on the result, only the data set of the singular value decomposition result is read as 16S.
- step 56S If shimming is possible, whether or not to improve by correcting the magnetomotive force arrangement in step 56S will be examined with reference to the amount of iron pieces necessary for shimming and the structural design of the whole magnet. If reexamination of the magnetomotive force arrangement is not performed, the candidate magnetomotive force arrangement plan 57S is obtained. When correcting the magnetomotive force arrangement, the process returns to the assumption of the magnetomotive force arrangement again. In the case of reexamining the magnetomotive force arrangement, for example, the empirical magnetic field of the superconducting coil is excessive, or the support structure has a difficult electromagnetic force.
- the shimming of the present invention is virtually performed in calculation to obtain a magnetomotive force arrangement candidate.
- the entire design of the magnetomotive force amount, electromagnetic force, and stress is performed, and the feasibility of the magnet is further determined. If it is found that the feasibility is difficult, start by assuming the magnetomotive force arrangement again.
- FIG. 1 as a magnetic field adjusting means, a method using a magnetic moment of the magnetized iron piece 4 is described as iron piece arrangement work step 22S.
- the magnetized iron piece is equivalent to the current by the small coil 3. Therefore, it is also possible to arrange the small coils in the grid shape of FIG. 8 and adjust the current 1 according to the magnetic moment distribution calculated by this method as an alternative to the iron piece arrangement work step 22S.
- magnetic field adjustment requiring a negative amount may be required.
- the negative amount in this case is considered as follows.
- the negative part is not arranged.
- the requirement for a negative amount can usually be adjusted by a method of reducing the amount of iron pieces already arranged by magnetic field adjustment up to the higher order. .
- the iron amount is already zero in the mesh, it is removed from the vicinity. Neighboring is from a region of closed lines of contour lines. If there is still no iron piece to be taken, the specific natural mode is artificially removed from the size necessary for correction so that the negative amount is eliminated.
- the magnetic field adjustment calculation part 3B is made especially of software, so that when performing magnetic field adjustment, the stored data of the singular value decomposition result and the magnetic field adjustment support tool with good mobility. It becomes.
- the horizontal magnetic field type MRI magnet device 62 can also be used for magnetic field adjustment (shimming), It can be applied to quality control and magnetomotive force arrangement design.
- the shape of the magnet device 62 is different, there are also different points of view such as a calculation procedure and an iron piece 4 arrangement position. Therefore, it demonstrates as Example 3 below.
- the third embodiment is applied to the horizontal magnetic field type MRI magnet apparatus 62 shown in FIG.
- a region (shim tray 5) used for shimming is arranged in a cylindrical shape in a bore 62f (cylindrical hollow hole) penetrating the center of the magnet device in FIG.
- the shim tray 5 surface is a cylindrical region surrounding the medical examinee as shown in a cross section in FIG.
- Surrounding the cylindrical shim tray 5 is also surrounded by a cylindrical heat insulating vacuum vessel 62C, and has a radiation shield 62d and a cryogenic vessel 62e therein, and a coil group 62a is arranged in the cryogenic vessel 62e.
- a magnet device 62 including the above contents surrounds the shim tray 5.
- the imaging area 6 is the geometric center of the magnet, and is an area surrounded by a dotted line centering on the intersection of three orthogonal symmetry axes. Imaging is performed while the gradient magnetic field 19 is generated therein.
- the gradient coil is also disposed in the same region as the shim tray 5, but is omitted here.
- the shim tray 5 is often arranged inside the gradient coil assembly.
- the cross section of the shim tray 5 does not necessarily have a circular cross section. Since singular value decomposition is used, the response matrix from the magnetic material on the shim tray to the magnetic field in the imaging region can be applied to an arbitrary calculation system. For example, when this shim tray is disposed between the coil groups having the cross-sectional shape of the gradient magnetic field coil disclosed in Patent Document 3, a cylindrical shape having an elliptical cross section is formed. However, in this method, shimming can be performed by the method already described.
- the mesh of the shim tray 5 of this horizontal magnetic field type MRi magnet device is different from the mesh in the vertical magnetic field of FIG. The reason is that the surface which comprises the cylindrical shim tray 5 becomes parallel to a magnetic field.
- the shim tray surface is a surface perpendicular to the magnetic field.
- the direction of magnetization needs to be perpendicular to the surface, and the iron piece is magnetized by the influence of the surrounding magnetic field, and the direction of the magnetization is substantially the static magnetic field line direction 65 of the surrounding magnetic field. is there. Therefore, the direction of magnetization of the iron piece 4 is the in-plane direction of the shim tray surface.
- a large number of ring-shaped current potential evaluation surfaces 13 are arranged in the axial direction (the direction of the magnetic field lines of the horizontal magnetic field) as shown in FIG.
- a large number of nodes 11 (referred to as in-plane nodes) that do not become edges of the surface are secured in the circumferential direction.
- the in-plane node 11 is equivalent to assuming a circular current around the in-plane node, and a magnetic moment M is arranged.
- the state of the shim tray surface can be read by implementation to the horizontal magnetic field machine.
- the region (the shim tray 5) in which the iron pieces 4 that are a pair of planes are arranged as shown in FIG. 5 is a cylindrical position arranged in the magnet bore 62f as shown in FIG.
- the iron pieces 4 are arranged distributed in the circumferential direction and the axial direction. This arrangement is made according to the quantity of iron 10 determined by the calculation method common to the vertical magnetic field machine (open type machine) already described.
- the iron pieces 4 are distributed and arranged at the axial position (magnetic field position) and the circumferential position of the cylindrical cross section. In FIG. 14, this state is depicted by changing the thickness of the iron piece.
- the diagram showing the distribution of the iron piece arrangement and the current potential distribution for correction shown in FIG. 8 in the magnet device 62 for the vertical magnetic field type MRI includes the angular coordinates indicating the rotation direction and the angular direction address as shown in the lower part of FIG. It is a distribution map indicated by position coordinates and addresses occupying positions in the axial direction.
- the shim tray 5 is written in the upper part of FIG.
- the same address as the current potential evaluation surface 13 is also arranged on the shim tray 5 and the quantity of the address obtained by calculation is arranged.
- 24 divisions in the circulation direction and 14 divisions A to N in the axial direction are shown, but this address division method is corrected to achieve the required magnetic field accuracy (uniformity).
- the order to be performed (the number of eigenmodes) is examined, and the number of divisions that can reproduce the distribution of the order is required.
- the number is about 15 to 30 in both the circumferential direction and the axial direction.
- the grid 7 shown in the lower part of FIG. 15 actually displays a number indicating the amount of the iron piece 4 as shown in FIG. 8, but the display of the number indicating the amount of the iron piece 4 is omitted here.
- the plane on which the mesh is arranged and the plane for calculating the current potential for correction exist on the same plane. Therefore, the current potential evaluation surface 13 is different from the cylindrical surface of the shim tray 5. For this reason, the display of FIG. 15 projects the value of the intermediate node 11c of the current potential distribution calculated on each ring-shaped current evaluation surface 13 (FIG. 14) onto the cylindrical surface with a line passing through the geometric center of the magnet. To display. It should be considered that it is convenient for this display if all the intermediate nodes 11c are arranged on the same cylindrical surface.
- the calculation of the iron piece quantity 10 for displaying the arrangement amount of the iron pieces 4 will be described with reference to FIG.
- the iron piece arrangement amount is calculated by the area of the current potential.
- the current evaluation surface current potential evaluation surface 13
- integration is performed across several ring-shaped surfaces as shown in FIG. To do. Therefore, it is convenient to arrange a large number of ring-shaped current surfaces so that one or more ring-shaped current surfaces are disposed in a region corresponding to the mesh 7.
- Mf ⁇ Ti ⁇ Si (Am 2 ) (18)
- the amount of iron pieces 10 (volume) in the frame is adjusted by a combination of iron pieces 4 having different volumes.
- the sum is executed across the current potential evaluation surface at the position included in the cell 7.
- Ti is the current potential value (A) of the contact i in the frame
- Si is the element area on the surface of the current potential evaluation surface 13 attached to the contact. Since the contacts are attached to a plurality of elements, there is no problem with the triangular element shown here assuming that 1/3 of each element belongs to each contact 11.
- FIG. 17B In the state of the mesh 7 written in the upper part of FIG. 17 (FIG. 17B), the arrangement in the case of using the iron piece 4 or the bolt of the permanent magnet 4P is schematically written. Even in this case, a target amount is obtained by combining physical quantities corresponding to the magnitudes of several magnetic moments (Mf1 to Mf3, Mp1 to Mp3).
- the shim tray having a cylindrical surface in FIG. 14 actually has a sheath having a long structure parallel to the magnet axis, and assumes a structure in which an iron piece is disposed therein. In the winding direction, 24 sheaths are arranged in FIG.
- This sheath has a structure for fixing the iron piece and the like, and has 10 or more fixing points in a direction parallel to the magnet axis.
- eleven axial fixing points are drawn. If the number of the fixed parts in the axial direction and the number of fixed parts in the axial direction coincide with the calculation divisions of FIG. 15, the magnetic moment of the equation (18) is changed to the appropriate fixed part of the sheath. Deploy. Further, when the calculation category is large, it is distributed and arranged in a plurality of corresponding pods. When the calculation division is small, the calculation is added to the corresponding sheath fixed position.
- Examples 1 and 2 an example in which the iron piece 7 is arranged with an amount that generates a necessary magnetic moment has been shown. However, as already mentioned, a negative amount is required as the iron piece amount 10 to be arranged. It also occurs when the amount of iron that can be removed is insufficient or zero. If there is a case where sufficient uniformity does not occur even if it is handled as described above, the permanent magnet 4P or the current loop 4C is used instead of iron. There is no problem even if these are applied to a positive iron amount, but it is desirable to use an iron piece that can be shimmed at low cost if it can be dealt with by magnetization of iron. This is shown in the upper part of FIG.
- magnetization cannot be obtained from the magnetization curve.
- the surrounding magnetic field especially if the magnetic material is present in the surroundings
- the surrounding magnetic field is disturbed, and a case where the magnetic field is different from the original magnetic field may occur.
- the surrounding magnetic field is measured before and after placing a magnetic piece whose magnetization is actually unknown, and compared with the change in the magnetic field due to the placement when the magnetization is known. Or, it will be compared with the calculated magnetic field change.
- a detailed nonlinear magnetic field calculation is performed and used as a calculated value of magnetization 4 of the iron piece arranged from the calculation result.
- the final arrival uniformity is predicted, the quality of the magnet is confirmed, the error is automatically corrected, and the measurement and the corrected iron arrangement calculation and arrangement are repeated to ensure Magnetic field adjustment can be performed. It can also be used for magnetomotive force arrangement design of magnets that require high magnetic field accuracy.
- the present invention relates to a magnetic field distribution in a desired magnetic field intensity distribution in a magnet apparatus that generates a magnetic field by arranging a coil and a magnetic material such as iron, such as a nuclear magnetic resonance tomography apparatus (MRI) used for medical diagnosis.
- a nuclear magnetic resonance application apparatus such as MRI provides a method and apparatus for uniformizing with extremely high accuracy in a measurement region.
- the error magnetic field is corrected by arranging the iron pieces, and the error magnetic field distribution and the iron piece arrangement distribution are corrected to a uniform magnetic field distribution by a combination of the respective orthogonal bases in an operation called shimming for uniforming the magnetic field strength.
Landscapes
- Physics & Mathematics (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Magnetic Resonance Imaging Apparatus (AREA)
Abstract
Description
(1)時間的に定常で空間的にも一定な磁場で、通常0.1から数テスラ以上の強さである、撮像を行う空間(通常直径で30-40cmの球もしくは楕円体の空間)内で数ppm程度の変動範囲である。
(2)1秒程度以下の時定数で変化して、空間的に傾斜した磁場。
(3)核磁気共鳴に対応した周波数(数MHz以上)の高周波の電磁波によるもの。
(1)詳細な磁場計算を行うために多くの計算時間が必要。
(2)ここの鉄片や電流の設置や変化に高精度の磁場に対応した精度を要求。
(3)誤ったシミング作業を行った場合に、誤った箇所の特定が難しく回復に手間がかかる。
Berr(r)=ΣCmψm(r) …(1)
である。従来法ではルジャンドル多項式とか球面調和関数を用いている。本発明では、特異値分解による分布関数を用いる。加算する関数ψmとその係数Cmの決め方を具体的に説明していく。
Bej=Btg-Bmj …(2)
である。
B=AT …(3)
である。この式は、電流面上の接点の電流ポテンシャル値を要素に持つベクトルTから、磁場評価点の磁場の応答を示す式で、行列Aはm(磁場計測点数)行n(接点数)列である。
つまり磁場分布の基底である固有分布
u1,u2,u3 …(4)
と電流ポテンシャルの基底である固有分布
v1,v2,v3 …(5)
であり、ujとvjには
λjuj=A・vj …(6)
の関係があるが、ここでλjは特異値である。また添え字jは特異値の大きさの順に固有分布に番号を付けた順番の数値である。一つの番号に対して、電流ポテンシャル分布と磁場分布を示す基底ベクトルが各一個ずつ対応している。この一つの番号に関連した2つの基底ベクトルと一つの特異値をまとめて一つの固有モードと呼ぶことにする。また、順番の番号jは固有モードの次数である。
Cj=Be・uj …(7)
から
Dj=-Cj/λj …(8)
である。つまりj番目固有分布の誤差磁場はDjvjの電流ポテンシャル分布を与えることで、完全に補正できる。
jm=M/μ0 …(9)
が流れている。ここでMは磁化(T)である。鉄片が飽和状態にあればほぼMは2.1T程度である。従って、jmは約1.7×106A/mである。従って1立方メートルの体積の鉄は約1.7×106Am2(170Acm2/1cc)の磁気モーメントを持つ。この値は磁石の種類、特に磁場強度に依存するので、個々に検討する必要はある。しかし、磁石の磁場強度が1T程度を超える磁石では、鉄片はほぼ飽和に近い状態に磁化していると考えて差し支えない。この時には鉄の磁気モーメントは体積に比例する。
dj=-vjCj/(λjjm) …(10)
の成分djk(j番目固有分布関数に対応するk番目鉄片補正点の鉄片量)に相当する体積密度(m=m3/m2)で鉄片を配置する。また、電流で磁場補正を行う場合には電流密度ベクトルjが電流ポテンシャルTと面の法線のベクトル積で、(▽T)×nで与えられる電流を配置する。
(1)小さい電流ポテンシャル(つまり少ない鉄片量)で大きな磁場を補正できる固有モードから選択する。この指標が特異値λjである。特異値は本計算体系では固有分布毎の単位電流ポテンシャルあたりの磁場強度であるので、この特異値の小さい固有分布は選択しない。また言い換えると特異値は単位鉄片量あたりの磁場強度に比例する値であるとも言える。一般に、できるだけ小さな物量で均一磁場を生成したいために、特異値の大きな固有分布を、調整に用いる。
(2)計測磁場に含まれる固有磁場分布の成分強度が小さなものは無視できる。内積[式(7)]で計算する成分強度が、目標とする均一磁場レベルで許容される誤差磁場に比べて十分小さな強度であれば、あえて補正する必要はない。特異値が大きな固有分でも、成分強度が小さい場合にはシミングに用いる必要はなく、選択しない。
(3)特に補正を必要と作業者が判断する固有分布関数を個々に選択し、内積で求める強度または人為的に決めた強度で補正する。たとえば、誤差磁場分布のピークが重なり局所的に大きな誤差磁場が発生している場合には、人為的にピークを下げる補正を適切な固有分布関数の選択と大きさで補正する。
(4)選択した固有分布関数の電流ポテンシャル成分を補正した後の均一度(到達均一度)を求め固有分布関数の選択が妥当であるかどうかを判定する。到達均一度が十分でなければ固有分布関数の選択を再検討する。ここで均一度は、磁場評価領域の複数計測点のなかで、磁場強度の最大,最小の差、言い換えると、誤差磁場の最大ピークから最小ピークの差について、平均磁場に対する割合を示したもので、MRIでは通常1/百万(ppm)のオーダで議論する。
(5)目標磁場を変更すると、誤差磁場に含まれる各固有分布の強度と、残差として残る磁場の強度、つまり到達均一度も変化するので、固有分布選択には目標磁場も考慮が必要である。
(6)数回から数10回の回数を繰り返して調整を行う。これは、調整機構の精度が、目標とする磁場精度に比べて通常粗いために繰り返し操作して磁場精度を向上させる。たとえば、MRIの磁場調整(シミング)では、1マイクロTの精度で磁場調整を行う必要があるが、調整以前の誤差磁場は数mT程度のものである。これを一気に調整することにした場合、調整のために配置する鉄片は1/1000より細かい精度での物量の管理が要求されるが、実作業でこの精度を管理することは容易でない。そこで、本発明によると一回の調整では1/10程度以下の精度での物量管理で、回数を追う毎に誤差磁場を低下させ、最終的な磁場精度との相対比を「低下させ、1/10以下の物量管理でも十分な最終磁場精度を得る。
D=Σdj=Σ-vjCj/(λjjm) …(11)
Bshim=Be-ΣCjuj …(12)
ΔT=Σ-vjCj/λj …(13)
であり、補正後の誤差磁場分布は
Bshim=Be-AΔT …(14)
で与えられる。この2つの方法は同じ計算結果を与える。ここでも和Σは選択した固有分布関数に対して実行される。この計算法で、磁場調整後の到達均一度を予測して目標の精度で磁場調整が進行しているかどうかを判定する。
(a)製作完了時の品質管理
(b)コイルや磁性体配置の設計時に、起磁力配置の設計が妥当性,配置の再検討要否の検討に利用できる。必要な鉄量は式(9)に関連して議論したようにおおよそ170Acm2/1ccで換算可能である。また、補正に必要な磁気モーメントは、鉄片を配置する領域で
∫Tds=ΣTiΣSij/3.0(Am2) …(15)
の面積積分であるが、積分領域については実例で議論する。また離散化した表示はメッシュで分割して計算しているために実際の計算内容を示している。前に書いている和Σは積分領域内の節点iで行われ、Sijはi番目節点に属する要素jの面積である。三角要素であるために1/3がi番目節点に寄与すると考えている。後ろに書いている和Σは、i番目節点が属する三角要素jについて実行される。以下では簡単に
Si=ΣSij/3.0 …(16)
として議論する。
TiSi→Mi …(17)
と変換すれば、磁気モーメントの大きさMiを扱う議論になるが、これまでの議論は単に面積を大きさだけの倍率で変換するのみで、特異値分解を利用する議論としては全く同じとなる。この場合、式16の積分は単純にその領域に属する磁気モーメントの和となる。また、このように磁気モーメントMiを変数として式(1)から式(16)の計算を行う場合には、磁気モーメントを配置する位置は、電流ポテンシャルのように面上に限ることはない。しかし、実際のシミングで用いる磁場調整機構の概略構造は、平面もしくは筒状の面を有する構造であり、以下では曲面上の電流ポテンシャルで議論していく。
Mf=ΣTi×Si(Am2) …(18)
を発生できるように、枠内の鉄片物量10(体積)を、体積の異なる鉄片4の組み合わせで調整する。この式で、和は升目7に含まれる位置の電流ポテンシャル評価面を跨いで実行される。また、Tiは枠内の接点iの電流ポテンシャル値(A)で、Siはその接点に付属する電流ポテンシャル評価面13面上の要素面積である。接点は複数の要素に付属するので、ここで示す三角要素では各要素の1/3が個々の接点11に属すると考えて問題はない。鉄片物量10と磁気モーメントMの換算方法は既に図3で議論した170Acm2/1cc程度である。升目7に必要とされた磁気モーメントを鉄片の体積に換算して必要な体積を升目内に配置する。磁場が弱く、鉄片が磁気飽和してない場合には、磁化Mは飽和磁化とは異なり、この換算係数も異なるが、この場合には材料の磁化曲線(M-H曲線,M=磁化強度T,H=磁界の強さA/mもしくはT)を参考にしてMの大きさを決めておく。これらの考え方は垂直磁場(開放型)機用の磁場調整と同じ考え方である。
Mc=電流×Sl
である。この磁気モーメントが必要とされる磁気モーメントと同じになるように電流を電源10から、符号も考慮して調整する。
1B 予備計算部分
2 磁化電流
2B 磁場計測部分
3 小コイル
3B 磁場調整計算部分
4 鉄片
4C 電流ループ
4P 永久磁石
5 シムトレイ
6 磁場計測評価領域(撮像領域)
7 升目
8 等高線のピーク
9 等高線の谷
10 鉄片物量
11 接点
11S 磁場調整開始ステップ
12 有限要素
12S 磁場計測ステップ
13 電流ポテンシャル評価面
13S 計測磁場保存ステップ
14 磁場計測評価点の集合
14S 磁場データ読み出しステップ
15 選択した固有モード
15S 均一度判断ステップ
16 非選択の固有モード
16S 特異値分解結果読み出しステップ
17 到達可能均一度
17S 固有モード選択と目標磁場決定ステップ
18 升目に配置する鉄体積
18S 固有モード強度,補正電流ポテンシャル,鉄片量,補正磁場分布および到達可能
均一度計算ステップ
19 電流ポテンシャル等高線
19S スペクトル,到達可能均一度および鉄片配置量計算ステップ
20S シミング可否の判断ステップ
21 電流ポテンシャルによる電流
21S 品質良否判断ステップ
22 固有モード選択の次数上限を示す線
22S 鉄片配置作業ステップ
23 固有モード選択で強さ下限を示す線
31S 計算メッシュ生成ステップ
32S 特異値分解計算ステップ
33S 特異値分解結果保存ステップ
40S 磁場調整終了ステップ
41S 修理・調整ステップ
51S 起磁力配置検討開始ステップ
52S 起磁力配置仮定ステップ
53S 磁場計算ステップ
54S 磁場計算結果保存ステップ
55S シムトレイ変更要否判断ステップ
56S 起磁力配置改善判断ステップ
57S MRI磁石用起磁力配置候補案ステップ
60 被検診者
61 被検診者用ベッド
62 磁石装置
62a コイル
62b 能動磁気シールド用コイル
62c 真空容器
62d 輻射シールド
62e 極低温容器
62f 磁石中空穴(ボア)
63 連結柱
64 電源
65 静磁場磁力線方向
66 傾斜磁場ベクトル
Claims (14)
- 磁場発生装置に目標の磁場分布が与えられた領域があり、当該領域の磁場分布を前記目標の磁場分布に近づけるMRI装置用の磁場調整方法において、
調整手段として、電流ループ、受動的に磁化する鉄片などの磁性体、または外部磁場に
依存しない永久磁石を、その領域を含む筒状領域に配置できる磁場調整機構を持ち、
所定数の点において磁場計測を行い、前記目標磁場との差である誤差磁場を算出し、その誤差磁場を近似的に補正できる磁場調整機構領域の電流ポテンシャル分布を前記磁場調整機構領域内の磁場方向と交差する複数のリング状面上で求め、
前記電流ポテンシャル分布を磁気モーメントに換算し、1つまたは複数の計算点を含む区域で加算し、加算された磁気モーメントに相当するループ電流もしくは磁性体片を配置する磁場調整作業を特徴とするMRI装置用の磁場調整方法。 - 磁場発生装置に目標の磁場分布が与えられた領域があり、当該領域の磁場分布を前記目標の磁場分布に近づけるMRI装置用の磁場調整方法において、
調整手段として、電流ループ、受動的に磁化する鉄片などの磁性体、または外部磁場に
依存しない永久磁石を、その領域を含む筒状領域に配置できる磁場調整機構を持ち、
所定数の点において磁場計測を行い、前記目標磁場との差である誤差磁場を算出し、その誤差磁場を近似的に補正できる磁場調整機構領域に多数配置した磁気モーメント計算位置上で磁気モーメントの大きさを求め、
1つまたは複数の磁気モーメント計算点を含む区域で加算し、その磁気モーメントの大きさに相当するループ電流もしくは磁性体片を配置する磁場調整作業を特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第1項または第2項の磁場調整方法であって、
特異値分解により得た基底である固有分布関数の中か
ら分布関数を選択し、その組み合わせで近似的に誤差磁場を補正する電流ポテンシャルまたは磁気モーメントの分布を計算することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第3項の磁場調整方法であって、
近似的に磁場補正する電流ポテンシャルまたは磁気モーメントに基づいて、前記目標磁場が与えられた領域の磁場計測点の補正磁場量を計算し、
前記目標磁場から前記補正磁場量を差し引くことで残留誤差磁場の予測値を求め、
前記残留誤差磁場の予測値が許容残留誤差磁場の目標範囲内となる固有分布関数を選択することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第1項または第2項のMRI装置用の磁場調整方法であって、
磁気モーメントを鉄片密度に換算する、または電流ポテンシャルを磁気モーメントに比例する量として、鉄片量密度に換算し、
その換算した分布に従って鉄片を配置することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第1項または第2項のMRI装置用の磁場調整方法であって、
誤差磁場の補正に必要な電流ポテンシャル分布または磁気モーメントを求める固有分布関数の選択を、特異値の大きさの順に並べて番号付けした番号(次数)と、誤差磁場に含まれる固有分布の強さの相関図(スペクトル図)上で選択することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第1項または第2項のMRI装置用の磁場調整方法であって、
計算結果の電流ポテンシャル、または、磁気モーメントに相当する量について、密度分布を磁場調整機構に含まれる円筒面上に等高線を含む表示を行い、その表示に従って鉄片を作業者が配置することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第7項のMRI装置用の磁場調整方法であって、
等高線とともに鉄片を配置する磁場調整機構に含まれる面を多角形で分割し、分割した領域毎に、磁気モーメントの大きさもしくは鉄片量や永久磁石量を、面積積分値で、等高線と共に、もしくは等高線無しで表示することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第8項のMRI装置用の磁場調整方法であって、
等高線で示した山もしくは盆地部をまとめて積算し、その量を山もしくは盆地部内の一カ所もしくは複数箇所に分散して配置することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第1項または第2項のMRI装置用の磁場調整方法であって、
磁場計測から磁気モーメントの大きさもしくは鉄片量や永久磁石量の配置までの計算と作業を繰り返し実行することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第10項のMRI装置用の磁場調整方法であって、
繰り返し計算と作業で、誤差磁場の大きさと共に、特異値分解で得た磁場分布を表す基底である固有分布関数について、個々の強度の大きさを表示して、磁場調整の進展を把握することを特徴とするMRI装置用の磁場調整方法。 - 請求の範囲第4項の残留誤差磁場予測値の代表値、たとえば最小最大値の差を目標もしくは計測磁場の平均磁場強度で割った値を表示し、
請求の範囲第6項の相関図、または請求の範囲第7項乃至第9項のうちのいずれかの磁場補正作業で配置する磁性体,永久磁石もしくは電流ループの強さを表示することを特徴とするMRI装置用の磁場調整方法。 - 磁場発生用のコイルや磁性体の起磁力源を含む電磁石であって、
請求の範囲第1項または第2項のMRI装置用の磁場調整方法を用いて、目標の残留誤差磁場以下とする条件での、請求の範囲第7項または第8項の磁気モーメントの大きさもしくは鉄片量や永久磁石量の配置が実装上可能であることで、磁場調整が正常に実行できることを、磁場調整作業開始時に判断できることを特徴とするMRI装置用の磁石品質把握法。 - 磁場発生用のコイルや磁性体の起磁力源を含む電磁石の設計において、目標磁場分布が
与えられ、起磁力配置から磁場分布を計算し、前記請求の範囲第13項の磁場計測値の代替えに磁場計算値を入力とし、配置した起磁力源配置の妥当性確認し、妥当で無ければ、磁場調整が可能となるまで起磁力配置を変更して、磁場調整が可能な起磁力配置を求めることを特徴と磁石起磁力配置設計法。
Priority Applications (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US13/511,786 US20120268119A1 (en) | 2009-11-24 | 2010-11-24 | Magnetic field adjustment method for mri device |
CN201080053192.2A CN102665542B (zh) | 2009-11-24 | 2010-11-24 | 核磁共振断层摄像装置用磁场调整方法 |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2009-265849 | 2009-11-24 | ||
JP2009265849A JP5427565B2 (ja) | 2009-11-24 | 2009-11-24 | Mri装置用磁場調整 |
Publications (1)
Publication Number | Publication Date |
---|---|
WO2011065357A1 true WO2011065357A1 (ja) | 2011-06-03 |
Family
ID=44066462
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/JP2010/070881 WO2011065357A1 (ja) | 2009-11-24 | 2010-11-24 | Mri装置用磁場調整方法 |
Country Status (4)
Country | Link |
---|---|
US (1) | US20120268119A1 (ja) |
JP (1) | JP5427565B2 (ja) |
CN (1) | CN102665542B (ja) |
WO (1) | WO2011065357A1 (ja) |
Families Citing this family (17)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US8947089B2 (en) * | 2008-05-09 | 2015-02-03 | Hitachi, Ltd. | Magnetic field shimming adjustment: reducing magnetic distribution errors by obtaining current potential distributions of MRI apparatus |
JP5122029B1 (ja) | 2012-03-01 | 2013-01-16 | 三菱電機株式会社 | 超電導マグネットの調整方法 |
JP5802163B2 (ja) | 2012-03-29 | 2015-10-28 | 株式会社日立メディコ | 磁場均一度調整方法、磁石装置及び磁気共鳴撮像装置 |
US9883312B2 (en) | 2013-05-29 | 2018-01-30 | Qualcomm Incorporated | Transformed higher order ambisonics audio data |
WO2015005109A1 (ja) * | 2013-07-09 | 2015-01-15 | 株式会社 日立メディコ | 磁場調整支援装置、磁場調整支援方法、mri装置および磁石装置 |
US10156619B2 (en) * | 2014-03-06 | 2018-12-18 | Hitachi, Ltd. | Magnetic resonance imaging system, static magnetic field homogeneity adjusting system, magnetic field homogeneity adjusting method, and magnetic field homogeneity adjusting program |
JP6429312B2 (ja) * | 2014-08-06 | 2018-11-28 | 株式会社日立製作所 | 磁場調整装置、磁石装置および磁場調整方法 |
US10213133B2 (en) * | 2014-12-22 | 2019-02-26 | Biosense Webster (Israel) Ltd | Modeling of a magnetic field |
CN107205688B (zh) * | 2015-02-20 | 2020-04-28 | 株式会社日立制作所 | 磁场调整用磁矩配置计算方法以及磁场调整装置 |
WO2016132832A1 (ja) * | 2015-02-20 | 2016-08-25 | 株式会社日立製作所 | 磁場均一度調整方法、磁場均一度調整プログラムおよび磁場均一度調整装置 |
WO2016133204A1 (ja) * | 2015-02-20 | 2016-08-25 | 株式会社日立製作所 | 磁場調整支援システムおよび磁場調整方法 |
WO2016133205A1 (ja) * | 2015-02-20 | 2016-08-25 | 株式会社日立製作所 | 磁場調整方法 |
US10638950B2 (en) * | 2015-02-25 | 2020-05-05 | Hitachi, Ltd. | Magnetic resonance imaging apparatus, static magnetic field homogeneity adjustment method, program, and computer |
CN107847181B (zh) * | 2015-07-15 | 2020-12-22 | 圣纳普医疗(巴巴多斯)公司 | 用于偏移均匀磁场空间的有源线圈 |
JP6643110B2 (ja) * | 2016-01-27 | 2020-02-12 | 株式会社日立製作所 | 磁場調整装置、および磁気共鳴イメージング装置 |
US11986319B2 (en) | 2017-08-25 | 2024-05-21 | NEUROPHET Inc. | Patch guide method and program |
KR101950815B1 (ko) * | 2017-08-25 | 2019-02-21 | 뉴로핏 주식회사 | 패치 가이드 방법 및 프로그램 |
Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPH0838454A (ja) * | 1994-04-13 | 1996-02-13 | Oxford Magnet Technol Ltd | 磁気共鳴映像装置の磁石 |
WO2009136643A1 (ja) * | 2008-05-09 | 2009-11-12 | 株式会社日立製作所 | Mri装置用磁場調整 |
Family Cites Families (20)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US5428333A (en) * | 1993-01-22 | 1995-06-27 | New York University | Method and apparatus for compensation of field distortion in a magnetic structure |
US5642087A (en) * | 1994-10-25 | 1997-06-24 | Sandia Corporation | Generating highly uniform electromagnetic field characteristics |
US5717371A (en) * | 1994-10-25 | 1998-02-10 | Sandia Corporation | Generating highly uniform electromagnetic field characteristics |
US5532597A (en) * | 1994-11-04 | 1996-07-02 | Picker International, Inc. | Passive shimming technique for MRI magnets |
US6008647A (en) * | 1997-02-11 | 1999-12-28 | General Electric Company | Method for reducing Maxwell term artifacts in fast spin echo MR images |
US6002255A (en) * | 1997-11-20 | 1999-12-14 | Brigham & Women's Hospital | Planar open magnet MRI system having active target field shimming |
US6294972B1 (en) * | 2000-08-03 | 2001-09-25 | The Mcw Research Foundation, Inc. | Method for shimming a static magnetic field in a local MRI coil |
US6448772B1 (en) * | 2000-10-06 | 2002-09-10 | Sumitomo Special Metals Co., Ltd. | Magnetic field adjusting apparatus, magnetic field adjusting method and recording medium |
US20020179830A1 (en) * | 2000-11-01 | 2002-12-05 | Pearson Robert M. | Halbach Dipole magnet shim system |
US6566991B1 (en) * | 2001-04-24 | 2003-05-20 | Fonar Corporation | Apparatus and method of shimming a magnetic field |
ATE491958T1 (de) * | 2001-11-01 | 2011-01-15 | Peter Mansfield | Gradientenspulen für die bildgebende magnetische resonanz mit reduzierter nervenstimulation |
JP4040930B2 (ja) * | 2002-08-26 | 2008-01-30 | ジーイー・メディカル・システムズ・グローバル・テクノロジー・カンパニー・エルエルシー | 磁場均一化方法および装置 |
DE10330926B4 (de) * | 2003-07-08 | 2008-11-27 | Siemens Ag | Verfahren zur absoluten Korrektur von B0-Feld-Abweichungen in der Magnetresonanz-Tomographie-Bildgebung |
US20050154291A1 (en) * | 2003-09-19 | 2005-07-14 | Lei Zhao | Method of using a small MRI scanner |
JP4639948B2 (ja) * | 2005-05-17 | 2011-02-23 | 三菱電機株式会社 | 磁石装置及びそれを用いた磁気共鳴イメージング装置 |
JP5060384B2 (ja) * | 2008-05-09 | 2012-10-31 | 株式会社日立製作所 | 磁場均一度調整用ソフトウェア、磁場均一度調整方法、磁石装置及び磁気共鳴撮像装置 |
US20100019766A1 (en) * | 2008-07-28 | 2010-01-28 | Siemens Medical Solutions Usa, Inc. | System for Dynamically Compensating for Inhomogeneity in an MR Imaging Device Magnetic Field |
GB2468852A (en) * | 2009-03-23 | 2010-09-29 | Siemens Magnet Technology Ltd | Arrangements and Method for Shimming a Magnetic Field |
US8536870B2 (en) * | 2010-04-21 | 2013-09-17 | William F. B. Punchard | Shim insert for high-field MRI magnets |
EP2506026A1 (en) * | 2011-03-29 | 2012-10-03 | Universitätsklinikum Freiburg | Method of dynamically compensating for magnetic field heterogeneity in magnetic resonance imaging |
-
2009
- 2009-11-24 JP JP2009265849A patent/JP5427565B2/ja not_active Expired - Fee Related
-
2010
- 2010-11-24 US US13/511,786 patent/US20120268119A1/en not_active Abandoned
- 2010-11-24 CN CN201080053192.2A patent/CN102665542B/zh not_active Expired - Fee Related
- 2010-11-24 WO PCT/JP2010/070881 patent/WO2011065357A1/ja active Application Filing
Patent Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPH0838454A (ja) * | 1994-04-13 | 1996-02-13 | Oxford Magnet Technol Ltd | 磁気共鳴映像装置の磁石 |
WO2009136643A1 (ja) * | 2008-05-09 | 2009-11-12 | 株式会社日立製作所 | Mri装置用磁場調整 |
Also Published As
Publication number | Publication date |
---|---|
JP5427565B2 (ja) | 2014-02-26 |
US20120268119A1 (en) | 2012-10-25 |
CN102665542B (zh) | 2015-04-01 |
JP2011110065A (ja) | 2011-06-09 |
CN102665542A (zh) | 2012-09-12 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
JP5427565B2 (ja) | Mri装置用磁場調整 | |
JP4902787B2 (ja) | Mri装置用磁場調整 | |
JP5943355B2 (ja) | 静磁場均一度の調整方法、磁気共鳴イメージング用静磁場発生装置、磁場調整システム、プログラム | |
JP6018185B2 (ja) | Mri放射線治療装置の静磁場補正 | |
CN108139453A (zh) | 包含海尔贝克型圆柱环的核磁共振用磁性组件 | |
JP6001784B2 (ja) | 磁場調整支援装置、磁場調整支援方法、mri装置および磁石装置 | |
WO2009136642A1 (ja) | 磁場均一度調整用ソフトウェア、磁場均一度調整方法、磁石装置及び磁気共鳴撮像装置 | |
JP6639596B2 (ja) | 磁場調整方法 | |
US10156619B2 (en) | Magnetic resonance imaging system, static magnetic field homogeneity adjusting system, magnetic field homogeneity adjusting method, and magnetic field homogeneity adjusting program | |
Shan et al. | Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method | |
Wenzel et al. | B0-shimming methodology for affordable and compact low-field magnetic resonance imaging magnets | |
Tadic et al. | Design and optimization of a novel bored biplanar permanent-magnet assembly for hybrid magnetic resonance imaging systems | |
US9995805B2 (en) | Magnetic field homogeneity adjustment method, magnet device, and magnetic resonance imaging apparatus | |
WO2016132832A1 (ja) | 磁場均一度調整方法、磁場均一度調整プログラムおよび磁場均一度調整装置 | |
JP6807185B2 (ja) | 磁気共鳴イメージング装置のシミング方法、磁気共鳴イメージング装置及びシールドルーム | |
Ren et al. | Study on shimming method for open permanent magnet of MRI | |
JP6392141B2 (ja) | 磁場均一度調整方法、磁場均一度調整プログラムおよび磁場均一度調整装置 | |
US10408903B2 (en) | Shimming system and shimming method including a sensor unit having a pluraly of magnetic field sensors | |
Wen et al. | Shimming with permanent magnets for the x‐ray detector in a hybrid x‐ray/MR system | |
JPH01165106A (ja) | 磁界発生装置 | |
JPH03215246A (ja) | 鉄シムによるシミング方法 |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
WWE | Wipo information: entry into national phase |
Ref document number: 201080053192.2 Country of ref document: CN |
|
121 | Ep: the epo has been informed by wipo that ep was designated in this application |
Ref document number: 10833203 Country of ref document: EP Kind code of ref document: A1 |
|
NENP | Non-entry into the national phase |
Ref country code: DE |
|
WWE | Wipo information: entry into national phase |
Ref document number: 13511786 Country of ref document: US |
|
122 | Ep: pct application non-entry in european phase |
Ref document number: 10833203 Country of ref document: EP Kind code of ref document: A1 |