JPH01284239A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To obtain an image having high quality without disturbing the uniformity of the static magnetic field by providing the shim coils for correcting the nonuniformity of the static magnetic field and cancellation coils for offsetting the voltage induced on the shim coils by the coupling of the shim coils and the inclined magnetic field coils. CONSTITUTION:The inclined magnetic field coil side cancellation coils 2x-2z are installed onto the inclined magnetic field coils 2X-2Z on the X, Y and Z axes, and shim coil side cancellation coils 12x, 12y and 12z-14x, 14y and 14z are installed onto three shim coils 12-14, and when the mutual inductance between the inclined magnetic field coils on the X, Y and Z axes and three shim coils is M, the mutual inductance between the inclined magnetic field coil side cancellation coils and the shim coil side cancellation coils is determined so that the polarity becomes -M. The first shim coil 12 and the shim coil side cancellation coils 12x, 13x and 14x are connected in series, and connected with the first shim electric power source 15, and also the second and third shim coils 13 and 14 are similarly connected with the second and third shim electric power sources 16 and 17.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention is directed to magnetic resonance (MR)
In particular, regarding magnetic resonance imaging devices that utilize the resonance phenomenon to obtain morphological information such as slice images of a subject (living body) and functional information such as spectroscopy,
The present invention relates to a magnetic resonance imaging apparatus capable of generating a uniform static magnetic field.

(Prior art) Magnetic resonance phenomenon is a phenomenon in which an atomic nucleus with non-zero spin and magnetic moment placed in a static magnetic field resonantly absorbs and emits only electromagnetic waves of a specific frequency. The angular frequency ω shown in Eq. (ωo−2πshi0.shi0
; Larmor frequency).

ω0−γH0 Here, γ is the gyromagnetic ratio specific to the type of atomic nucleus,
Further, Ho is the static magnetic field strength.

An apparatus that performs biological diagnosis using the above-mentioned principle processes the electromagnetic waves of the same frequency as the above induced after the above-mentioned resonance absorption, and calculates the nuclear density, longitudinal relaxation time TI, transverse relaxation time T2. Diagnostic information that reflects information such as flow and chemical shift, such as slice images of a subject, can be obtained non-invasively.

Collecting diagnostic information by magnetic resonance can excite all parts of a subject placed in a static magnetic field and collect signals, but there are limitations in the equipment configuration and clinical requirements for imaging images. Therefore, in an actual device, a specific part is excited and its signal is collected.

In this case, the specific region to be imaged is generally a sliced region with a certain thickness;
Magnetic resonance signals (MR multiplied signals) of echo signals and FID signals from this slice site are collected by performing a data encoding process many times, and these data groups are subjected to image reconstruction processing using, for example, a two-dimensional Fourier transform method. By doing so, an image of the specific slice region is generated.

FIG. 6 is a diagram showing the overall configuration of this type of magnetic resonance imaging apparatus.

As shown in FIG. 6, a magnet assembly MA capable of accommodating a subject P therein includes a static magnetic field coil 1 using a normal conduction or superconductivity method;
A gradient magnetic field generating coil 2 for x, y, and z axes generates a gradient magnetic field for providing positional information of an induced site of a magnetic resonance signal, and a magnetic resonance signal (MR multiplication) that transmits a rotating high-frequency magnetic field and It has a transmitting/receiving system for detecting signals, for example, a probe 3 consisting of a transmitting coil and a receiving coil, and if it is a superconducting type, it includes a refrigerant supply control system and mainly controls the energization of the static magnetic field power source. The static magnetic field control system 4, the transmitter 5 that controls the transmission of RF pulses, the receiver 6 that controls the receiver of induced MR signals, and the excitation control of the gradient magnetic field generating coils 2 of the x, y, and z axes. X-axis, Y-axis, and Z-axis gradient magnetic field power supplies 7, 8.9, a sequencer 10 capable of implementing a pulse sequence for data collection, and a computer system that controls these and performs signal processing of detection signals and display thereof. 11.

Here, as a data collection procedure, the transmitter 5 is driven, and the RF pulse of the rotating magnetic field (
90@pulse, 180@pulse) are applied, and the gradient magnetic field power supplies 7, 8.9 are driven to generate a gradient magnetic field Gx from the gradient magnetic field generating coil 2.

Gy and Gz are added for 1-phase encoding for slicing and for reading, and the signal from a specific region is collected by the receiving coil of probe 3. This sequence is repeated a predetermined number of times to obtain a data group, and an image is generated from this data group.

Here, we will discuss the conditions for obtaining high image quality in magnetic resonance imaging. That is, if the static magnetic field is non-uniform, the MR multiplication signal phase at each point in space is disturbed due to it, causing distortion etc. in the image. For this reason, it is important to create a spatially highly uniform static magnetic field in the imaging area. However, it is extremely difficult to produce a uniform static magnetic field using only static magnetic field coils. Furthermore, in general, the uniformity of the static magnetic field changes depending on whether or not there is a magnetic body around the magnetic resonance imaging apparatus and how the magnetic body is arranged.

Therefore, even if it were possible to create a uniform static magnetic field using only static magnetic field coils under a specific environment, it would be extremely difficult to deal with a variety of surrounding environments. In order to eliminate such problems, multiple sets of correction coils (hereinafter referred to as "shim coils") for uniformity of the static magnetic field are placed in the static magnetic field generation space, and these are connected to a power source (hereinafter referred to as "shim coil"). Generally, a method is employed to correct the non-uniformity of the static magnetic field by supplying appropriate currents from respective shim power sources (referred to as ``shim power supplies''). In this case, the magnetic field generated by the shim coil requires good uniformity of the static magnetic field and good stability over time, so a constant current type power source is used as the shim power source. be done.

In the above, since the shim coil is arranged in the static magnetic field generation space, it is magnetically coupled to the gradient magnetic field coil. In other words, it means that there is a mutual inductance M. Therefore, when the law of electromagnetic induction acts and a gradient magnetic field is generated by a gradient magnetic field coil, magnetic flux interlinks with the shim coil, and a voltage is induced with a polarity that attempts to cancel it. This induced voltage V1 is approximately expressed by the following equation (1) using a current 1 (t) flowing through the gradient magnetic field coil.

Vl--M・di(t)/dt (1) As is clear from the above equation (1), the magnitude of this voltage V1 and the length of time are determined by the magnitude of mutual inductance M and the current r ( Although it depends on the magnitude of the time change amount of t), in general,
For the shim power supply, this is recognized as a very large change in load conditions. For example, the mutual inductance M
For example, when the current I (t) is 10 μH and a current of 10 OA flows with a rise time of 1 m5 ec,
The voltage v1 according to the above formula is -1V.

(Problem to be solved by the invention) In this way, the shim coil provided for static magnetic field uniformity and the gradient magnetic field coil are magnetically coupled, so the gradient magnetic field is generated by the gradient magnetic field coil. On many occasions, the shim power supply was temporarily unable to operate at a constant current, causing a transient current, or coupling current, to flow through the shim coil. When such a coupling current flows, the uniformity of the static magnetic field, which has been ensured by the shim coil and shim power supply, is temporarily disturbed, resulting in negative effects such as image distortion. There was a problem.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a magnetic resonance imaging apparatus that can obtain high-quality images without disturbing the uniformity of the static magnetic field.

[Structure of the Invention] (Means for Solving the Problems) The present invention has a structure in which the following means are taken to solve the above problems and achieve the objects.

That is, the present invention causes a magnetic resonance phenomenon to occur in a specific part of the subject by applying a gradient magnetic field by a gradient magnetic field coil and an excitation magnetic field by a transmitting/receiving system to the subject placed in a static magnetic field by a static magnetic field magnet. The magnetic resonance imaging apparatus includes a shim coil for correcting non-uniformity of the static magnetic field; The present invention is characterized by comprising a canceling coil for canceling the voltage induced in the shim coil by coupling with the gradient magnetic field coil.

(Function) According to such a configuration, non-uniformity of the static magnetic field can be corrected by passing an appropriate current through the shim coil, and when the gradient magnetic field coil is operated, the magnetic field between the gradient magnetic field coil and the shim coil can be corrected. The induced voltage caused by this coupling can be canceled out by the voltage induced in the cancellation coil, and therefore, a high-quality image can be obtained without disturbing the uniformity of the static magnetic field.

(Embodiment) An embodiment of the magnetic resonance imaging apparatus according to the present invention will be described below with respect to the relationship between the gradient magnetic field coil and shim coil, which are the main parts, and the cancellation coil, which is a new configuration.

The configuration of FIG. 1 includes gradient magnetic field power supplies 7.8.
Gradient field coil 2X connected to 9.

This is a conceptual configuration example applied when 2Y, 2Z, and three shim coils 12, 13, and 14 are coupled to each other, and X.

The gradient magnetic field coils 2X, 2Y, and 2Z of the Y and z axes each have gradient magnetic field coil side canceling coils 2x.

2y and 2z are provided, and three shim coils 12.13°14
Shim coil side cancellation coil 12x.

12y, 12z, 13x, 13y, 13z.

14x, 14y, and 14z are provided.

Here, the gradient magnetic field coils 2X for the x, y, and z axes.

2Y, 2Z, and three simcois, +I/12, 13°1
The mutual inductance with 4 is M (M xi, M y
l.

Mzl, Mx2. My2. Mz2. Mx3.
My3. Mz3), the gradient magnetic field coil side canceling coils 2x, 2y, 2z and the shim coil side canceling coils 12x, 12y, 12z, 13x, 13y.

The mutual inductance with 13z, 14x, 14y, and 14z is -M(-Mxl, -
Myl, -Mzl, -Mx2. -My2゜-
Mz2. - Mx3. -My3. - Mz3).

The first shim coil 12 and its shim coil side cancellation coils 12x, 12y, 12z are connected in series and connected to the first shim power supply 15, and the second shim coil 13 and its shim coil side cancellation coils 13x, 13y,
13z is connected in series to the second shim power supply 16, and the third shim coil 14 and its shim coil side cancellation coils 14x, 14y, 14z are connected in series to the second shim power source 16.
It is connected to the shim power supply 17. Here, Figure 2 is the first
It is a specific example wiring diagram of a figure.

Further, FIG. 3 shows an example in which the X-axis gradient magnetic field coil 2X and three shim coils 12, 13.14 each cause coupling with each other, and the gradient magnetic field coil side cancellation coil 2X is 2xl.

The mutual inductance between the X-axis gradient magnetic field coil 2X and the three shim coils 12, 13°14 is divided into 2X2 and 2x3, and the mutual inductance M xl, M x2. M x 3, gradient magnetic field coil side cancellation coil 2 x 112 x 2.
2x3 and shim coil side cancellation coil 12x, 13x
, 14x and the mutual inductance −M xl,
-M x2. - Mx3 is supported.

FIG. 4 is a modification of FIG. 3, and corresponds to the relationship between the X-axis gradient magnetic field coil 2X and three shim coils 12.degree. 13.14 in FIG.

Figure 5 shows the X, Y, and Z axis gradient magnetic field coils 2X.

This is a configuration example applied when 2Y, 2Z, and one shim coil 12 are coupled to each other, and the gradient magnetic field coils 2X, 2Y, 2
Z gradient magnetic field coil side cancellation coils 2x, 2y, 2
z and shim coil side cancellation coils 12x, 12y, 1
2z, the gradient magnetic field coils 2X, 2 of the x, y, and z axes are
Mutual inductance M xi, M yl, M zl due to Y, 2Z and one shim coil 12, mutual inductance -Mxl due to gradient magnetic field coil side cancellation coils 2x, 2y, 2z and shim coil side cancellation coils 12x, 12y, 12z , -Myl, -M
zl is supported.

Here, gradient magnetic field coil side cancellation coil 2 x +
The self-inductance and resistance of 2 y and 2 Z are
It is sufficiently smaller than that of the gradient magnetic field coils 2X, 2Y, and 2Z. Also, the shim coil side cancellation coil (12x, 12y).

12z), (13x, 13y, 13z), (14x
, 14y, 14z), the self-inductance and resistance are 3
It is sufficiently smaller than that of the two shim coils 12, 13, and 14.

With the above configuration, for example, in FIG. 1 or 2, when the voltage Vl of equation (1) is induced in the shim coils 12, 13, and 14 as a whole, the shim coil side canceling coil 12x.

12y, 12z, 13x, 13y, 13z.

14x, 14y, and 14z also have an overall voltage of ■2 (lV2
1-IVI l induces atte-V2- V l ), and vl is canceled by vl. Therefore, equivalent voltages are not applied to the shim power supplies 15, 16, and 17. As a result, the gradient magnetic field coils 2X, 2
Even if a gradient magnetic field is generated at y, 2Z, the shim power supply 1
Since the load conditions for the shim coils 12, 13, and 17 do not change, the shim power source 15, 16, and 17 can always perform constant current operation, and no coupling current flows in the shim coils 12, 13, and 14. In this way, since no coupling current flows, the uniformity of the static magnetic field is not temporarily disturbed, and image quality does not deteriorate.

Note that although the above example describes an example where there are three shim coils, the number is not specified;
Furthermore, examples of combinations of canceling coils are not specified. The invention can be modified in various ways without departing from the gist of the invention.

[Effects of the Invention] As described above, the present invention includes a shim coil for correcting the non-uniformity of a static magnetic field, and a shim coil for canceling the voltage induced in the shim coil by the coupling between the shim coil and the gradient magnetic field coil. By equipping the shim coil with a cancellation coil, it is possible to correct the non-uniformity of the static magnetic field by passing an appropriate current through the shim coil, and when operating the gradient magnetic field coil, the magnetic field between the gradient magnetic field coil and the shim coil can be adjusted. The induced voltage due to the coupling can be canceled by the voltage induced in the canceling coil, so that a high-quality image can be obtained without disturbing the uniformity of the static magnetic field.

Therefore, according to the present invention, it is possible to provide a magnetic resonance imaging apparatus that makes it possible to obtain high-quality images without disturbing the uniformity of the static magnetic field.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a conceptual configuration example of a gradient magnetic field coil, a shim coil, and a cancellation coil, which are the main parts of a magnetic resonance imaging apparatus according to the present invention, and FIG. 2 is a diagram showing a specific example of FIG. 1. , FIG. 3 to FIG. 5 are views showing other embodiments, respectively, and FIG. 6 is a view showing the entire magnetic resonance imaging apparatus. MA... Magnet assembly, 1... Static magnetic field coil, 2... Gradient magnetic field generating coil for x, y, and z axes (2
X, 2Y, 2Z), 2x, 2y, 2z-... Gradient magnetic field coil side cancellation coil, 3... Probe, 4...
・Static magnetic field control system, 5... Transmitter, 6... Receiver, 7
...X-axis gradient magnetic field power supply, 8...Y-axis gradient magnetic field power supply,
9...Z-axis gradient magnetic field power supply, 10...Sequencer, 1
1...computer system, 12,13゜14...
・Shim coil, 12 x, 12 y, 12 z
. 13x, 13y, 13z, 14x, 14y. 14...Shim coil side cancellation coil, 15°16
．． 17...Sim power supply. Applicant's agent Patent attorney Takehiko Suzue Figure 1 Figure 2 Figure 6

Claims (1)

[Claims] A magnetic resonance phenomenon is caused in a specific part of the subject by applying a gradient magnetic field by a gradient magnetic field coil and an excitation magnetic field by a transmitting/receiving system to a subject placed in a static magnetic field by a static magnetic field magnet, and The magnetic resonance imaging apparatus collects magnetic resonance signals induced from the magnetic resonance imaging apparatus to obtain diagnostic information of the site, and the magnetic resonance imaging apparatus includes a shim coil for correcting non-uniformity of the static magnetic field, and a combination of the shim coil and the gradient magnetic field coil. A magnetic resonance imaging apparatus comprising a canceling coil for canceling a voltage induced in the shim coil by coupling.