JPH0686764A - Mri system - Google Patents

Abstract

PURPOSE:To simplify the correcting work of a primary offset of a static magnetic field by calculating a correcting coil current for generating a correcting magnetic field with regard to one kind of gradient magnetic field coil, and obtaining a correcting coil current related to other kind of gradient magnetic field coil, based on a ratio of its correcting coil current and the maximum rated output. CONSTITUTION:A first correcting coil current for generating a correcting magnetic field of a primary offset by a gradient magnetic field coil 5a is contained by actual measurement, and thereafter, based on a ratio of the maximum rated output of the gradient magnetic field coil 5a and a gradient magnetic field coil 5b and a first correcting coil current, a second correcting coil current for generating the correcting magnetic field by the gradient magnetic field coil 5b is calculated. Each correcting coil current thereof is stored in a data file in a computer 2. Subsequently, the correcting oil current corresponding to the gradient magnetic field coil to be used is fetched, and based thereon, the gradient magnetic field is generated by the coil 5a or the coil 5b. As a result, a correcting work of a primary offset of a static magnetic field can be simplified.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI apparatus,
More specifically, the present invention relates to an MRI apparatus that properly uses a plurality of types of gradient magnetic field coils according to observation and corrects the primary offset of a static magnetic field by the gradient magnetic field coils used.

[0002]

2. Description of the Related Art In an MRI apparatus, a primary offset of a static magnetic field that cannot be removed only by shimming a magnet is corrected by a correction magnetic field generated by a gradient magnetic field coil. The correction by the gradient magnetic field coil will be described below.

First, a predetermined gradient magnetic field is generated by a gradient magnetic field coil and a phantom is observed. Then, the primary offset of the static magnetic field is extracted from the observation result. Next, the correction magnetic fields Hxof, Hyof, and Hzof of each axis for correcting the primary offset of the static magnetic field obtained by the actual measurement are determined. FIG. 6 illustrates the primary offset of the static magnetic field obtained by actual measurement. Next, in order to generate the correction magnetic fields Hxof, Hyof, and Hzof, x coils and y that form a gradient magnetic field coil.
Correction coil currents xof, yo supplied to the coil and z coil
f and zof are calculated based on the following equation.

[0004]

[Equation 1]

The square matrix on the right side of the above equation (1) represents the magnetic field generation characteristics of the gradient magnetic field coil, which is called the magnetic field generation matrix.

The gradient magnetic field of each axis by the gradient magnetic field coil is X
When out, Yout, Zout and the coil currents supplied to the x-coil, y-coil, and z-coil to generate the target gradient magnetic field are xin, yin, and zin, the following equations hold.

[0007]

[Equation 2]

The above equation (2) is simply expressed as follows. Π = Ψ ・ Φ + Ψ ・ Θ (3) In other words, the gradient magnetic field coil is supplied with a coil current obtained by adding the coil current Φ and the correction coil current Θ to the target gradient magnetic field Ψ ・ Φ with the correction magnetic field Ψ ・ Θ. Generates a superposed gradient magnetic field Π. Ψ is the magnetic field generation matrix. The primary magnetic field is corrected in the combined magnetic field in which the gradient magnetic field Π is superimposed on the static magnetic field, and has good homogeneity as shown in FIG. 7.

[0009]

In the conventional MRI apparatus, the correction coil current is set in the gradient magnetic field power source in an analog manner. However, MR equipped with multiple types of gradient magnetic field coils, which are used properly according to the observation
In the I-apparatus, each time the type of gradient magnetic field coil used changes, the above-mentioned primary offset correction work (measurement, calculation,
It is necessary to perform the setting) to set the correction coil current in the gradient magnetic field power source in an analog manner, which is time-consuming and troublesome. Therefore, an object of the present invention is to provide an MRI apparatus capable of easily performing a correction work of a primary offset of a static magnetic field in a short time when a plurality of types of gradient magnetic field coils are selectively used.

[0010]

SUMMARY OF THE INVENTION An MRI apparatus of the present invention is an MRI apparatus that uses a plurality of types of gradient magnetic field coils depending on the observation and corrects the primary offset of the static magnetic field by using the gradient magnetic field coils. And a first correction coil current for generating a correction magnetic field for correcting the measured primary offset of the static magnetic field by one kind of gradient magnetic field coil. The first correction coil current calculation means, the ratio of the rated maximum output of the one type of gradient magnetic field coil to the other type of gradient magnetic field coil, and the other type of gradient based on the first correction coil current. Second correction coil current calculating means for calculating a second correction coil current for generating the correction magnetic field in the magnetic field coil, and the first correction coil Flow and the second correction coil current are stored in the storage means, and the correction coil current corresponding to the type of the gradient magnetic field coil to be used is read out from the storage means and the gradient magnetic field coil drive is controlled based on the read out. A structural feature is that the control means is provided.

[0011]

In the MRI apparatus of the present invention, the primary offset amount of the static magnetic field is measured by the static magnetic field primary offset amount measuring means, and the correction magnetic field is generated by the one kind of gradient magnetic field coil by the first correction coil current calculating means. The first correction coil current for the calculation is calculated. By the way, when the gradient magnetic fields of different types are separately generated in the gradient magnetic field coils a and b, the ratio of the coil currents supplied to the respective gradient magnetic field coils is determined by the maximum rated output of each gradient magnetic field coil. Almost matches the ratio. Therefore, if one coil current is known, the other coil current can be calculated from the rated maximum output ratio. Therefore, the second correction coil current calculation means determines whether the first correction coil current of another type is based on the ratio of the rated maximum output of one type of gradient magnetic field coil to another type of gradient magnetic field coil. A second correction coil current for generating the correction magnetic field in the gradient magnetic field coil is calculated.

The storage means stores the first correction coil current and the second correction coil current. The drive control means is
A correction coil current corresponding to the type of gradient magnetic field coil to be used is taken out, and the drive of the gradient magnetic field coil is controlled based on this.

Therefore, when a plurality of types of gradient magnetic field coils are used, it is not necessary to perform the primary offset correction work (measurement, calculation, setting) for each type of gradient magnetic field coil to be used. You can save time.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail based on the embodiments shown in the drawings. The present invention is not limited to this. FIG. 1 is a block diagram of an MRI apparatus 1 according to an embodiment of the present invention. The computer 2 controls the overall operation based on the instruction from the console 13. The sequence controller 3 operates the gradient magnetic field driving circuit 4 based on the stored sequence, and causes the gradient magnetic field coil 5a or the gradient magnetic field coil 5b of the magnet assembly 5 to generate a gradient magnetic field. In addition, the gate modulation circuit 7
Is controlled to modulate the RF pulse generated by the RF oscillation circuit 6 into a predetermined waveform, and the RF pulse is applied from the RF power amplifier 8 to the transmission coil of the magnet assembly 5.

The NMR signal obtained by the receiving coil of the magnet assembly 5 is input to the phase detector 10 via the preamplifier 9 and further to the computer 2 via the AD converter 11. The computer 2 reconstructs an image based on the data of the NMR signal obtained from the AD converter 11, and displays it on the display device 12. In this MRI apparatus 1, the primary offset of the static magnetic field is corrected by the procedure stored in the computer 2 and the sequence controller 3.

When the user gives an instruction to correct the primary offset of the static magnetic field using the console 13, the computer 2 executes the processing of the flow chart shown in FIG. In step S1, a predetermined gradient magnetic field is generated by the gradient magnetic field coil 5a,
The gradient magnetic field is superimposed on the static magnetic field and the phantom is measured. In step S2, the primary offset of the static magnetic field is extracted from the measurement result. FIG. 3 illustrates the primary offset of the extracted static magnetic field. Then, the correction magnetic fields Haxof and Hayo of each axis for correcting the primary offset of the static magnetic field.
f, Hazof are determined.

In step S3, the correction magnetic field Haxo is generated.
First correction coil currents xaof, yao to be supplied to the xa coil, ya coil, and za coil forming the gradient magnetic field coil 5a in order to generate f, Hayof, and Hazof.
f and zaof are calculated based on the following equation.

[0018]

[Equation 3]

In step S4, the first correction coil currents xaof, yaof, zaof are associated with the gradient magnetic field coil 5a, and the data file F in the computer 2 (see FIG. 4).
To store.

FIG. 4 illustrates the structure of the data file F. In this data file F, the gradient magnetic field coils 5a,
Storage areas Da and Db are assigned in association with 5b, and the correction coil current Θ (xof, y is assigned to each storage area.
of, zof), rated maximum output MG (MGx, MGy, MG
z) (gauss / cm), primary adjustment coefficient K (K
x, Ky, Kz), zero-order adjustment coefficient E (E
(x, Ey, Ez) are provided. The correction coil current Θa of the gradient magnetic field coil 5a is determined by the above step S4.
Stored at. The correction coil current Θb of the gradient magnetic field coil 5b is stored in step S6 described later. Rated maximum output MG, primary adjustment coefficient K and zero-order adjustment coefficient E are
Each predetermined value is stored by the user. The first-order adjustment coefficient K and the zero-order adjustment coefficient E are ideally K
= 1.0 (Kx = 1.0, Ky = 1.0, Kz = 1.0), E = 0.0 (E
x = 0.0, Ey = 0.0, Ez = 0.0), but it is determined by the user in consideration of, for example, the individuality of the gradient magnetic field coil and changes with time.

At step S5, the rated maximum output MGa of the area Da in the data file F and the correction coil current Θa are obtained.
And the rated maximum output MGb of the area Db and the primary adjustment coefficient K
b, and the zero-order adjustment coefficient Eb, Θb = Kb [{MGa / MGb} Θa] + Eb (6), which forms the gradient magnetic field coil 5b.
The second correction coil current Θb (xbof, ybof, zbof) to be supplied to the b coil and the zb coil is calculated.

In step S6, the calculated second correction coil current Θb is stored in a predetermined storage area in the data file F. Then, the process ends.

The computer 2 reads the correction coil current Θ associated with the gradient magnetic field coil to be used from the data file F based on the scanning instruction from the operator console 13, and drives the gradient magnetic field via the sequence controller 3. Input to circuit 4. That is, in the scan using the gradient magnetic field coil 5 a, the correction coil current Θa is read and input to the gradient magnetic field drive circuit 4. On the other hand, in the scan using the gradient magnetic field coil 5b, the correction coil current Θb is read and input to the gradient magnetic field drive circuit 4. The coil current Φ for generating the target gradient magnetic field is input to the gradient magnetic field drive circuit 4 by the procedure stored in the computer 2 and the sequence controller 3.

The gradient magnetic field drive circuit 4 supplies the gradient magnetic field coil 5a or 5b of the magnet assembly 5 with a coil current obtained by adding the correction coil current Θ to the coil current Φ.
The gradient magnetic field coil 5a or 5b is excited by the supplied coil current to generate a gradient magnetic field Π. When the gradient magnetic field coil 5a is used, Πa = Ψa · Φa + Ψa · Θa (7). When using the gradient magnetic field coil 5b, Πb = Ψb · Φb + Ψb · Θb (8). As a result, as shown in FIG. 5, the static magnetic field has good homogeneity.

In some cases (for example, when there is a margin in time), the second correction coil current Θb is calculated along steps S1 and S2 in the same manner as the first correction coil current Θa is calculated. Good.

[0026]

According to the MRI apparatus of the present invention, when the correction coil current for generating the correction magnetic field is calculated for one type of gradient magnetic field coil, it is based on the ratio between the correction coil current and the rated maximum output. Therefore, the correction coil currents for other types of gradient magnetic field coils are obtained, so that time and labor can be saved.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of an MRI apparatus of the present invention.

FIG. 2 is a flow chart for correction of a primary offset by the apparatus of FIG.

FIG. 3 is an exemplary diagram of a primary offset of a static magnetic field.

4 is a conceptual diagram of a database of the apparatus of FIG.

FIG. 5 is an exemplary diagram when a primary offset of a static magnetic field is compensated.

FIG. 6 is an exemplary diagram of a primary offset of a static magnetic field.

FIG. 7 is an exemplary diagram when a primary offset of a static magnetic field is compensated.

[Explanation of symbols]

 1 MRI apparatus 2 computer 3 sequence controller 4 gradient magnetic field drive circuit 5 magnet assembly 5a, 5b gradient magnetic field coil F data file

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location 9219-2J G01N 24/06 G 8203-2G G01R 33/22 N

Claims (1)

[Claims] 1. An MRI apparatus that uses a plurality of types of gradient magnetic field coils depending on the observation and corrects the primary offset of the static magnetic field by using the gradient magnetic field coils. An amount measurement means, a first correction coil current calculation means for calculating a first correction coil current for generating a correction magnetic field for correcting the measured primary offset of the static magnetic field with one kind of gradient magnetic field coil, and For generating the correction magnetic field in the other type of gradient magnetic field coil based on the ratio of the rated maximum output of the one type of gradient magnetic field coil to the other type of gradient magnetic field coil and the first correction coil current Second correction coil current calculation means for calculating a second correction coil current, and the first correction coil current and the second correction coil current are stored. The MRI is provided with a storage means and a gradient magnetic field coil drive control means for reading a correction coil current according to the type of the gradient magnetic field coil to be used from the storage means and controlling the drive of the gradient magnetic field coil based on the read. apparatus.