JPH03215246A - Shimming method by iron shim - Google Patents

Abstract

PURPOSE:To enhance operation speed by reducing the processing quantity of the installation spot by calculating the mounting position of an iron piece correcting the non-uniformity of a magnetic field before the delivery from a factory by measuring the same in such a state that an iron piece having fixed thickness is placed at that position and calculating the thickness of the iron piece corresponding to each function system on the installation spot. CONSTITUTION:The distribution of the magnetic fields at respective measuring points of an iron piece having specific thickness is measured in a factory to form a data file and (k) of a function system phik is set to 1 and the value of the magnetic field at the measuring points in the function system phik are substituted for the non-uniformity (b) of the magnetic field as the value of the magnetic field of a calculation formula from the data file. Next, the thickness of the iron piece at each position realizing each function system phik is calculated using a linear algorithm and a shim group is set and, when (k) is not equal to the max. value, k=k+1 is formed to return to the origin. After a static magnetic field coil is arranged on the installation spot, the distribution b1 of the non-uniformity of the magnetic field is measured and, when the magnetic field is not uniform, the coefficient Ck corresponding to phik when the measured magnetic field b1 is made approximate in the function system #phik is calculated by a linear programming algorithm and the shim group of the function system phik having the corresponding thickness is arranged. When the magnetic field is uniform, a measuring procedure is completed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to shimming of the main magnet of an MHI (magnetic resonance imaging apparatus) using iron shims, and particularly to a shimming method using iron shims that can be quickly implemented at an installation site.

(Prior art) When an atomic nucleus is placed in a static magnetic field, it precesses at an angular velocity proportional to a constant that varies depending on the strength of the magnetic field and the type of nucleus. Magnetic resonance occurs when a high-frequency rotating magnetic field of the above-mentioned frequency is applied to an axis perpendicular to this static magnetic field, and a group of specific atomic nuclei having the above-mentioned constant undergoes a transition between levels due to the high-frequency magnetic field that satisfies the resonance condition, resulting in energy Transition to the higher level. After resonance, the atomic nucleus excited to a higher level returns to a lower level and radiates energy.

MHI is nuclear magnetic resonance (NM
This is a device that observes a phenomenon (referred to as R) and captures a tomographic image of a subject.

In this MHI, the magnetic field created by the main magnet for creating the static magnetic field is required to have a uniformity of 20 pp or less at least in the range where the object is to be observed. To achieve this requirement, shims are used to correct the non-uniformity of the magnetic field, and there are two methods: a current shim method using multiple shim coils, and an iron shim method that corrects multiple iron pieces within the magnetic field. The method using shim coils usually requires 16 to 18 shim coils, each of which is connected to a power source to supply current, which requires relatively large equipment and increases the cost of the magnet. Iron shims have the advantage that the scale of the equipment does not change because iron shims are simply placed, so shimming using iron shims is now being performed sequentially. In shimming using this iron shim, it is necessary to select the installation location of the iron shim, the thickness of the iron shim, etc., and perform optimal shimming.

Conventional shimming methods include the following methods.

1. Spherical function expansion method The magnetic field in the static magnetic field coil can be expanded into each term of a spherical function. Among these, the lower-order terms are the main non-uniform components, so the magnetic field component corresponding to each term is generated. Make a shim coil. As the function system φ of this shim coil, the one shown in FIG. 6 is used. The function system shown in Figure 6 has a function φ1 as a curve for correcting the measured magnetic field curve. , φk indicate the functions of the product curves of the two axes and the X axis, respectively. Since this method requires a small amount of calculation, it can be processed at a practical speed using a computer with normal processing power.

2. In shimming, in which pieces of iron are subdivided and placed on the coil surface, the magnetic field created at each point in the image area by a piece of iron with a magnetic vector placed in the subdivision area is calculated in advance and corrected from the uniformity measurement data. Find the amount of magnetic vector at each point required for this using the least squares method.

3. Method using linear programming Similar to the least squares method described above, the magnetic field created at each point in the image area by each iron piece placed in the subdivided area is stored in advance as a data file, and the optimal one for correction is calculated from the uniformity measurement data. Find the thickness of the iron piece using linear programming.

(Problem to be solved by the invention) However, the above method has the following problems.

1. Spherical function expansion method When using iron shims in place of the shim coils of the function system determined as described above, it is difficult to determine the arrangement of the iron pieces, and magnetic field components other than the desired components are generated, resulting in calculation predictions. In order to correct the uniformity, it is necessary to repeat the so-called trial and error process of repeating measurements many times.

2. Least Squares Method This method differs from current shims in that there are no negative thicknesses, so special measures are required for solutions with negative thicknesses.

In addition, the iron piece is divided into 300 to 400 positions,
It is necessary to solve the corresponding number of simultaneous equations,
It has the following drawbacks.

(b) A special calculation precision called double precision is required.

(Example) The amount of calculation is enormous, and a computer with high processing speed is required.

3. Method using linear programming Because the number of shims is large, processing speed becomes a major issue and a computer with high processing speed is required.

The present invention has been made in view of the above points, and its purpose is to reduce the number of repeated measurements, reduce the amount of calculation, and increase the calculation speed when arranging iron shims at the installation site. The objective is to realize a shimming method using shims.

(Means for Solving the Problems) The present invention solves the above-mentioned problems, and each piece of iron has a magnetic field distribution data file generated in the magnetic field measurement area obtained in advance. A step in which a shim group corresponding to the function φk is determined by a linear programming algorithm before shipment from the factory, a step in which the magnetic field is measured and its homogeneity is examined, and a linear programming algorithm is used from the data obtained in the magnetic field measurement step. The method is characterized by comprising a step of determining the coefficient of each term of the magnetic field function φk, and a step of arranging a shim group of each function system having a thickness corresponding to the coefficient of each term, which is performed at the installation site. It is something.

(Function) Before shipping from the factory, the installation positions of the iron pieces that correct magnetic field inhomogeneity are determined by placing iron pieces of a certain thickness at each iron piece installation position and measuring them, and at the installation site, each function system determined in the above step is used. By determining the thickness of the iron piece to be used, the amount of processing at the installation site can be reduced and calculation speed can be improved.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Fig. 1 is a flowchart of the procedure for performing magnetic field correction using iron shims to correct magnetic field non-uniformity due to the environment of the MRI installation location according to an embodiment of the present invention, and Fig. 2 is a flowchart of the procedure for implementing magnetic field correction performed in a factory. It is. Before explaining the implementation procedure using a flowchart, an example of a static magnetic field coil and an iron shim attached thereto will be explained with reference to FIG. In the figures, (a) is a front view, (open) is a side view, and (c) is a view of a tray for attaching iron pieces. In the figure, reference numeral 1 denotes a static magnetic field coil for applying a static magnetic field to the subject, and the coil is wound along the illustrated circle. 2 is a plastic tray for attaching the iron piece 3 by a method such as screwing; 4 is the tray 2;
This is the iron piece attachment position for attaching the iron piece 3 on top. The iron piece 3 is indicated by hatching.

In this static magnetic field coil 1, the tray 2 has 30
There are 12 attached at ° intervals. There are 23 iron piece mounting positions 4 on the tray 2 to which iron pieces 3 are attached, so that in the end, 276 iron pieces can be attached.

In order to set the uniformity of the magnetic field obtained by the static magnetic field coil 1 to a desired value, an algorithm is used to calculate where the iron piece 3 should be attached to the iron piece attachment position 4 and how thick the iron piece should be attached. demand.

Next, the non-uniformity of the magnetic field is determined in order to determine the installation position of the iron piece 3, the thickness of the iron piece 3 to be attached, etc. The measurement method is usually to use an NMR probe to measure the desired shimming area (for example, a diameter of 40 buttons). , a sphere with a diameter of 35(1), etc.).Measure the magnetic field at each point. An example of this measurement point is shown in FIG. In the figure, (a) is a diagram showing the shimming area 5 in which the magnetic field is made uniform by shimming in the static magnetic field coil 1, and (b) is a diagram showing the shimming area 5 divided into 13 planes. , (c) Figure is (mouth) Figure 1
FIG. 3 is a diagram of one plane divided into three planes; The shimming area 5 has a diameter of 35 cm, for example, as shown in FIG.
ball (35 cm d.s.v.) or 40(1)
d. s. set by the sphere of v. This shimming area 5 is cut and divided into 13 planes perpendicular to the cylindrical axis of the static magnetic field coil 1 as shown in the figure (c), and one plane is divided into each plane as shown in the figure (c). Divide into 24 points at 15° intervals, obtain 13×24-312 points from the spherical shimming area 5 as measurement points 6, and measure at each measurement point 6.

Next, the linear programming algorithm used in this embodiment will be explained. FIG. 5 is a graph showing the distribution of the non-uniformity b of the magnetic field obtained by measuring the magnetic field of the static magnetic field coil at the 312 points mentioned above at a certain point in time. In the figure, the horizontal axis represents the position of the measurement point, and the vertical axis represents the non-uniformity of the magnetic field.

ETOL is a set target value of non-uniformity, for example 10 ppm
This is the value set to . This figure shows an attempt to transfer the value of the magnetic field inhomogeneity at each measurement point j to b,' to fit it within the width of the ETOL.

Therefore, the value of the magnetic field measured at each point j (magnetic field inhomogeneity) b,
′ should be in the following range.

2 2 Here, b,' is the value of the magnetic field, expressed as ΣA1, .X1, measured at position j, generated by an iron piece of L length at position i, for example. This linear programming method attempts to minimize the number of shims used under the following constraint.

2 Next, the flowchart shown in FIG. 2 will be explained.

The flowchart in Figure 2 shows that the optimum solution for the thickness of the iron piece at each position to realize each function φk obtained by the spherical function expansion method is determined in advance by linear programming, and a shim group corresponding to each function φk is set. This shows the steps. In each function, the measured values at the positions of each shim can be added together according to the principle of superposition to obtain the distribution of the magnetic field of all shims. In this case, 1 at each of 276 positions
Distribution A of the magnetic field at each of the 312 measurement points of the iron piece of e + thickness,
．． 1 shall be measured in advance and kept as a data file.

Step 1 Set k of the function system φk of φ= (rI) to 1, and function φ
Calculate for 1.

Step 2 Measurement points j-1~ in the function system φk from the data file
The value of the magnetic field at the 312 points of 312 is substituted into b(r+) as the value of the magnetic field in the calculation formula.

To explain this example, if φ1 is a Z linear function, b
(Z j) -CZ j (2) where, j...Measurement points 1 to 312 Zj...Value of Z coordinate at measurement point j C...A constant.

Step 3 Using the previously described linear algorithm, calculate the iron piece thickness X1 at each position to realize each function system φk, and set the shim group Sk.

Step 4 Check whether k is equal to the maximum value, ie 16.

If they are equal, the process ends; if they are not equal, proceed to step 5.

Step 5 Add 1 to k to obtain k-k+1 and return to step 2. φ
When k is φ2, two-axis quadratic function system (22 in Figure 6)
A similar calculation is performed for shim group S, to obtain the shim group S. The above procedure is performed in a factory, and it takes time to measure each of 312 measurement points on 276 pieces of iron, but this is not a problem since it is a factory-level measurement.

Next, the procedure for determining the size of each term of the shim group Sk determined in flow 2 of FIG. 2 at the installation site will be explained with reference to the flowchart of FIG.

Step 1 After installing the static magnetic field coil at the installation location, the distribution b of the non-uniformity of the magnetic field is measured at measurement points j-1 to 312.

Step 2 Check the uniformity of the magnetic field using the measured values. If the distribution is uniform, this measurement procedure ends. If it is not uniform, proceed to step 3.

Step 3 Using the linear programming algorithm, set the measured magnetic field b, to φk
Find the coefficient C corresponding to φ1 when approximated by .

Step 4 Once Ck has been determined in step 3, a shim group of the function system φk having a thickness corresponding to each C is placed.

Step 5 The shim group obtained in Step 4 is placed and the non-uniformity of the magnetic field is measured at 312 measurement points.

Step 6 Check the uniformity of the magnetic field using the measured values. If the distribution is uniform, this measurement procedure ends. If it is not uniform, return to step 3.

As described above, according to this embodiment, the measurement at the installation site is performed using a function system expressing the nonuniform magnetic field b(r+) with about 16 terms at most, so that the shimming processing time can be extremely short.

Note that the present invention is not limited to the above embodiments.

In the embodiment, the number of shim positions has been described as 276, but this number may be further increased to improve accuracy, or this number may be decreased if lower accuracy is acceptable. Further, the number of function systems φk is not limited to 16, but may be increased.

Furthermore, the number of measurement points is not limited to 312, and may be further increased. Increasing the number of measurement points will further improve the uniformity of the magnetic field.

(Effects of the Invention) As described above in detail, according to the present invention, in shimming iron shims performed at the site where static magnetic field coils are installed, the number of measurements is small, the amount of calculation is also small, and the processing speed can be increased. The practical effect is great.

[Brief explanation of drawings]

Figure 1 is a flowchart of the iron shim setting procedure at the installation site of the present invention, Figure 2 is a flowchart of the iron shim installation position determination procedure at the factory stage, Figure 3 is an explanatory diagram of the iron shim installation position, and Figure 4 is the magnetic field. FIG. 5 is an explanatory diagram of the distribution of magnetic field non-uniformity at each measurement point; FIG. 6 is an explanatory diagram of the function system for magnetic field non-uniformity correction.

Claims (1)

[Claims] Using a magnetic field distribution data file generated in the magnetic field measurement region of each iron piece obtained in advance, a shim group corresponding to each function φ_k is created by approximating the magnetic field by the spherical function expansion method using a linear programming algorithm. a step of measuring the magnetic field and examining its uniformity; a step of calculating the coefficient of each term of the magnetic field function φ_k from the data obtained in the magnetic field measurement step using a linear programming algorithm; A shimming method using iron shims, comprising the steps of arranging shim groups of each function system having thicknesses corresponding to the coefficients of each term, and performing the shims at an installation site.