JP3169444B2 - MRI equipment - Google Patents

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 selectively uses a plurality of types of gradient magnetic field coils according to observation and corrects a primary offset of a static magnetic field using 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 which cannot be removed only by shimming of a magnet is corrected by a correction magnetic field using a gradient magnetic field coil. Hereinafter, the correction by the gradient magnetic field coil will be described.

[0003] 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, Hzof of each axis for correcting the primary offset of the static magnetic field obtained by the actual measurement are determined. FIG. 6 illustrates a primary offset of a static magnetic field obtained by actual measurement. Next, in order to generate the correction magnetic fields Hxof, Hyof, Hzof, x coils and y constituting a gradient magnetic field coil are used.
Correction coil current xof, yo supplied to coil and z coil
f and zof are calculated based on the following equations.

[0004]

(Equation 1)

[0005] The square matrix on the right side of the above equation (1) represents the magnetic field generation characteristics of the gradient magnetic field coil, and is called a magnetic field generation matrix.

The gradient magnetic field of each axis by the gradient magnetic field coil is represented by X
out, Yout, and Zout, and when the coil currents supplied to the x, y, and z coils for generating the target gradient magnetic field are xin, yin, and zin, the following equation is established.

[0007]

(Equation 2)

The above equation (2) is simply expressed as follows. Π = Ψ · Φ + Ψ · Θ (3) That is, a coil current obtained by adding the coil current Φ and the correction coil current Θ is supplied to the gradient magnetic field coil, and the correction magnetic field Ψ · Θ is added to the target gradient magnetic field Ψ · Φ. Generate a superimposed gradient magnetic field Π. Ψ is the above magnetic field generation matrix. In the composite magnetic field in which the gradient magnetic field Π is superimposed on the static magnetic field, the primary offset is corrected, and as shown in FIG. 7, excellent uniformity is obtained.

[0009]

In a conventional MRI apparatus, a correction coil current is set in a gradient magnetic field power supply in an analog manner. However, an MR that has a plurality of types of gradient magnetic field coils and uses them properly according to observations
In the I device, each time the type of the gradient magnetic field coil used changes, the above-described work of correcting the primary offset (actual measurement, calculation,
Setting) to set the correction coil current in the gradient magnetic field power supply in an analog manner, which takes time and effort. SUMMARY OF THE INVENTION It is an object of the present invention to provide an MRI apparatus that can easily and quickly correct a primary offset of a static magnetic field when a plurality of types of gradient magnetic field coils are properly used.

[0010]

An MRI apparatus according to the present invention uses a plurality of types of gradient magnetic field coils in accordance with observation and corrects a primary offset of a static magnetic field by using a gradient magnetic field coil. And a first correction coil current for generating a correction magnetic field for correcting the primary offset of the actually measured static magnetic field with one kind of gradient coil. First correction coil current calculation means, and the other type of gradient magnetic field coil based on a ratio of a rated maximum output of the one type of gradient magnetic field coil to another 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 for generating the correction magnetic field with a magnetic field coil, and the first correction coil Storage means for storing the current and the second correction coil current, and a gradient coil drive for reading the correction coil current corresponding to the type of the gradient coil to be used from the storage means and controlling the driving of the gradient coil based on the readout. And a control means.

[0011]

In the MRI apparatus of the present invention, the primary offset amount of the static magnetic field is actually 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. A first correction coil current is calculated. By the way, when a gradient magnetic field of a predetermined magnitude is separately generated by different types of gradient magnetic field coils a and b, the ratio of the coil current supplied to each gradient magnetic field coil is determined by the rated maximum output of each gradient magnetic field coil. It almost matches the ratio. Therefore, if one coil current is known, the other coil current can be calculated from the ratio of the rated maximum output. Therefore, the second correction coil current calculating means calculates another correction coil current based on the first correction coil current and a ratio between the rated maximum output of one type of gradient magnetic field coil and the rated maximum output of another type of gradient magnetic field coil. A second correction coil current for generating the correction magnetic field by 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 includes:
A correction coil current corresponding to the type of the gradient magnetic field coil to be used is taken out, and the driving of the gradient magnetic field coil is controlled based on the current.

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

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the embodiment shown in the drawings. It should be noted that the present invention is not limited by this. FIG. 1 is a block diagram of an MRI apparatus 1 according to one embodiment of the present invention. The computer 2 controls the entire operation based on an instruction from the console 13. The sequence controller 3 activates the gradient magnetic field drive circuit 4 based on the stored sequence to generate a gradient magnetic field with the gradient magnetic field coil 5a or the gradient magnetic field coil 5b of the magnet assembly 5. Also, the gate modulation circuit 7
And modulates the RF pulse generated by the RF oscillation circuit 6 into a predetermined waveform, and applies the RF pulse to the transmission coil of the magnet assembly 5 from the RF power amplifier 8.

The NMR signal obtained by the receiving coil of the magnet assembly 5 is input to a phase detector 10 via a preamplifier 9 and further input to a computer 2 via an AD converter 11. The computer 2 reconstructs an image based on the NMR signal data obtained from the AD converter 11 and displays the image on the display device 12. In the 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 flowchart 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, a primary offset of the static magnetic field is extracted from the measurement result. FIG. 3 illustrates a primary offset of the extracted static magnetic field. Then, the correction magnetic fields Haxof, 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
First correction coil currents xaof, yao to be supplied to the xa, ya, and za coils constituting the gradient coil 5a in order to generate f, Hayof, and Hazof.
f and zaof are calculated based on the following equations.

[0018]

(Equation 3)

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

FIG. 4 shows an example of the configuration of the data file F. In this data file F, the gradient magnetic field coils 5a,
5b are associated with storage areas Da and Db, and each storage area has a correction coil current Θ (xof, y
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). The correction coil current Θa of the gradient magnetic field coil 5a is determined in step S4.
Is stored in. The correction coil current Θb of the gradient coil 5b is stored in step S6 described below. The rated maximum output MG, primary adjustment coefficient K and zero-order adjustment coefficient E are
Each predetermined value is stored by the user. Note that the primary 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), which are determined by the user in consideration of, for example, the individuality of the gradient magnetic field coil and changes with time.

In step S5, the rated maximum output MGa of the area Da in the data file F and the correction coil current Θa
And the rated maximum output MGb of the area Db and the primary adjustment coefficient K
b, using the zero-order adjustment coefficient Eb, Θb = Kb · [{MGa / MGb} · Θa] + Eb (6) xb coil and y constituting the gradient magnetic field coil 5b
A 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 data file F. Then, the process ends.

The computer 2 reads the correction coil current ら れ corresponding to the gradient magnetic field coil to be used from the data file F based on the scan instruction from the console 13, and drives the gradient magnetic field drive via the sequence controller 3. Input to the circuit 4. That is, in the scan using the gradient magnetic field coil 5a, 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 according to the procedure stored in the computer 2 and the sequence controller 3.

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

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 calculation of the first correction coil current 算出 a. Is also good.

[0026]

According to the MRI apparatus of the present invention, when a correction coil current for generating a 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. In order to obtain the correction coil current for other types of gradient coils, time and labor can be saved.

[Brief description of the drawings]

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

FIG. 2 is a flowchart for correcting a primary offset by the apparatus of FIG. 1;

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

FIG. 4 is a conceptual diagram of a database of the apparatus in FIG. 1;

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

FIG. 6 is an illustration 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]

 DESCRIPTION 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

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) A61B 5/055

Claims (1)

(57) [Claims] 1. An MRI apparatus which selectively uses a plurality of types of gradient magnetic field coils according to observation and corrects a primary offset of a static magnetic field by using a gradient magnetic field coil, wherein a primary offset of a static magnetic field is actually measured. Quantity actual measurement means, first correction coil current calculation means for calculating a first correction coil current for generating a correction magnetic field for correcting the primary offset of the actually measured static magnetic field with one kind of gradient magnetic field coil, Generating the correction magnetic field by the other type of gradient magnetic field coil 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 and the first correction coil current. A second correction coil current calculation means for calculating a second correction coil current; and storing the first correction coil current and the second correction coil current. MRI characterized by comprising storage means, and gradient magnetic field coil drive control means for reading a correction coil current corresponding to the type of gradient magnetic field coil to be used from said storage means and controlling the driving of the gradient magnetic field coil based on the current. apparatus.