JPS60151545A - Nmr imaging device - Google Patents

Abstract

PURPOSE:To correct deterioration in magnetic field uniformity due to magnetic field disturbance by providing annular ferromagnetic bodies in a cylinder at right angles of the in-cylinder static magnetic field direction of a magnet. CONSTITUTION:A current is flowed through four electromagnets 10-40 which constitute a normal conductive electromagnet to produce a magnetic field in the internal space of the electromagnet coils. A pipe 80 is provided in the internal space of the coils 20 and 30, and annular ferromagnetic bodies 50 and 60 are arranged therein symmetrically at specific distance to correct terms Z<4> and Z<2> of magnetic field nonuniformity. Namely, the magnetic field nonuniformity due to magnetic field disturbance is corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an NMR imaging apparatus, and particularly to an NMFL imaging apparatus capable of improving the uniformity of a magnetic field.

[Background of the invention]

In general, nuclear magnetic resonance (hereinafter referred to as NMR) is an analysis method often used for structural analysis of organic compounds and research on condensed material physics. Recently, many attempts have been made to image the nuclear spin density of a biological cross section using this NMR technique, and it has become possible to obtain NMR images that can be compared with X-ray CT. With this NM imaging device,
Several methods are available, depending on the method of applying a second magnetic field having spatially different strengths to the static magnetic field I (o) and the method of processing the NMR signal. A reproducing NMR imaging device will be outlined.

First, in addition to a uniform magnetic field H, a static magnetic field that waits for a spatial gradient G is applied to the subject. If we assume that the direction of the magnetic field H6 is the z-axis and the gradient G is in the X direction, and the strength of the W magnetic field at X-0 is Ho, the static magnetic field H applied to the subject is It is given by H0+G −X.The resonance frequency ω at this time is ω2γH−
γHo+γG-X = ωθ10γG-X ・・・・・・・・・(1) However, ω
0=γH0 γ: As shown by the specific gyromagnetic ratio of nuclear spin, it is a linear function of X. When a resonance spectrum is measured for this object, the signals at frequency ω are only those from the nuclear spin population in the corresponding plane where X=constant, as shown in FIG.

Therefore, the measured spectrum P(ω) can be expressed as P(ω) using the nuclear spin tightness function ρ(X+Y+'')
= f / p (x, y, z)dydz ・−・
++... (2) Or, according to the above formula (1), P ((13O+rG-X>-ffp(x, y, z
)dy'z......(3) Now, if we set the left side as f(l, then f(x
)=ffp (x, y, z)dydz・・・・・・
...(4) becomes. In this case, the measured resonance spectrum is a line integral or projection of the nuclear spin density in the direction perpendicular to the X-axis. If we combine the method of selectively exciting the resonance phenomenon, the second
As shown in the figure, only the signal at a specific position on the z-axis can be detected. Rotate the object around the z-axis or
The magnetic field gradient vector G can be rotated to obtain projections from each direction.

To approximately restore the two-dimensional distribution on the display screen of the device from projections from each direction, as shown in Figure 3, an amount proportional to the intensity of each projection is returned onto the screen along the direction of projection. This is a method of adding together in all directions. This image reconstruction method is called a back projection method.

Here, to explain the relationship between the static magnetic field Ho and the gradient G, if the static magnetic field Ho is an ideally uniform magnetic field, the gradient G
The NMR signal of the analyte without the addition of neutrons will exhibit a resonance spectrum that is naturally determined by nuclear spins. but,
In reality, the static magnetic field Ho itself has a non-uniform component,
This value depends on the structure of the magnet, but 1001P
Before and after, the resonance spectrum reflects the non-uniformity of the static magnetic field H0 even without adding the gradient G, and has an expanse of 100 degrees. If this non-uniformity of the static magnetic field H6 does not overlap spatially, it will be possible to calculate the nuclear spin density in each part of the object without the gradient G, and a tomographic image can be obtained without using the back projection method described earlier. It will be done. However, the static magnetic field Ho
Since non-uniformity is distributed on concentric circles, a resonance spectrum corresponding to the spatial position must be obtained by adding a gradient G. The value of this gradient G is the minimum value necessary to avoid spatial overlap due to non-uniformity of the static magnetic field HO. In reality, it is several times the non-uniformity of the static magnetic field Ha (several 100p
) is applied. In other words, the value of the gradient G is the static magnetic field H, which is a value less than 0.1% of the static magnetic field HO, and the gradient G.
The frequency ω of the resonance spectrum of the NMR, 11-singing apparatus using two magnetic fields also greatly depends on the static magnetic field HO.

Now, if the value of the static magnetic field H0 changes due to some influence, each projection will move left and right in accordance with the change in the static magnetic field Ho. For this reason, even if each projection is added onto the display screen using the back projection method, a reconstructed image will not be obtained, but the resulting image will be out of focus, which is insufficient as a medical diagnostic image.

As described above, in order to obtain high-quality images, the NMR imaging apparatus requires uniformity of the static magnetic field and linearity of the gradient magnetic field. That is, it is necessary to quantitatively measure the distortion of these magnetic fields and correct the distortion of images obtained by the NM imaging device due to the magnetic field.

Therefore, as a means for measuring magnetic field uniformity, conventionally, the "Magnetic Resonance Apparatus" of Japanese Patent Publication No. 47-28953, U.S. Pat.
No. 873909 ()yr□magneticAppar
atus Employing Computer M
ears for correctigits Qper
2tlng Pangmetens.

Also, U.S. Patent No. 3,443,209 Mign et
i C4F Meld ) (omogenity Co
As shown in ntrol Apparatus, the half-width of the signal is used to include a large sample within the field of view. In general, when the range in which a uniform magnetic field is used, that is, the range that requires magnetic field homogeneity is narrow, such as in high-resolution NMR equipment for analysis, a certain degree of magnetic field homogeneity can be obtained. This is an effective means for measuring the uniformity of the magnetic field when the degree of uniformity has already been obtained to some extent from the beginning.

This is how the uniformity of the magnetic field is measured.
This magnetic field is created by a normally conducting magnet. In other words, a normally conducting electromagnet (hereinafter referred to as RM) commonly used in NMR imaging equipment can be used to control the electromagnetic coils by supplying DC current to the four electromagnetic coils 1, 2° and 3.4°, as shown in Fig. 4. Generates a magnetic field in the internal space.

For example, the RM manufactured by Qxford Instruments (UK) has four electromagnetic coils 1, 2, 3.
．． By supplying a direct current (RI) to 4, a magnetic field is generated in the internal space of the electromagnetic coil. - For example, 0xf
Manufactured by ord■n8trumenta (UK), the M is made up of four electromagnetic coils and has an output of 22OA during i.
This generates a magnetic field of approximately 0.15 Tesla (T) in its internal space. The four electromagnetic coils 1, 2, 3, and 4 are designed to make the magnetic field generated in the internal space of the electromagnetic coil uniform, and these four electromagnetic coils 1, 2, and 3.4 are independent. As shown in FIG.

It is configured to be movable in the Y and Z directions. The magnetic field homogeneity is corrected by adjusting the positions of these four electromagnetic coils. When this electromagnetic field uniformity is corrected, 50 I
A spherical space of P/30 cm and a spherical space of 100 p/40 cm (centered on point O, which is the center of the magnetic field) are obtained. On the other hand, 几M is generally housed in a building made of reinforced concrete.

In this case, since the RM is surrounded by iron such as reinforced concrete, that is, a ferromagnetic material, the magnetic flux generated by the KM is attracted by these, and the uniformity of the magnetic field in the RM internal space 7 is extremely deteriorated. In order to correct this, the electromagnet 1
, 2, 3. Adjust the positional relationship of 4. This electromagnet 1, 2
, 3. What if we express the inhomogeneity of the magnetic field as an orthogonal function by adjusting the positional relationship of 4?
Y.

Item 21 etc. can be adjusted. But Z! Or, term 24 is 4 in a range of 30 cm (centered on the magnetic field center point O).
It reaches 00~10001F, which is connected to electromagnets 1, 2, and 3.
．． Electromagnets 1 and 2 to be adjusted according to the positional relationship of 4
, 3.4 position adjustment is required at least 10 crn, and 10
Since it is not possible to adjust by more than m due to the configuration, it is virtually impossible to adjust. There is a means of using current shims as a means of correcting the magnetic field uniformity.

That is, it is possible to obtain magnet uniformity by providing a group of current shim coils in the internal space of the electromagnet 1.2 and supplying DC current appropriately. However, the 400-100 oP
According to the method of using this current shim to perform the correction, it is necessary to supply a current of 20 to 50 A to the current shim coil. However, when a current of 20 to 50 A is passed through the shim coil, the shim coil generates heat. That is, for example, when a current of 5OA is passed through a coil resistance of 2Ω, the power supply voltage is 100V.
The electric power is approximately 5 KW, and the amount of heat generated in this case is approximately 2 Kcal/sec. Now, in order to maintain the temperature of the coil at 28C, if 12C tap water is used, 4.5 t/min of tap water and cooling system are required. Furthermore, a power supply device with a stability of 1/100 and a capacity of 5KW is required. If two terms, z2 and Z6, need to be adjusted, twice the amount of equipment and tap water described above will be required.

Therefore, it is practically impossible to obtain magnetic field uniformity using such a current shim coil. Therefore, in the past, it has been practically impossible to correct the deterioration of magnetic field uniformity due to magnetic field interference such as reinforcing bars.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide an NMR imaging apparatus capable of correcting deterioration of magnetic field uniformity due to magnetic field disturbance.

[Summary of the invention]

The present invention aims to correct the deterioration of magnetic field uniformity due to magnetic field disturbance by providing an annular ferromagnetic body perpendicular to the direction of the magnetic field in a quiet magnetic field inside the cylinder of the magnet.

[Embodiments of the invention]

Examples of the present invention will be described below.

FIG. 6 shows an embodiment of the invention.

In the figure, four electromagnetic coils 10,
By supplying direct current (+) to 20, 30, and 40, a magnetic field is generated in the internal space of the electromagnetic coil. The inner diameter of this electromagnetic coil 20.30 is approximately φ98.
0mm, the inner diameter of the electromagnetic coil 10°40 is approximately φ610non. A pipe 80 is provided in the internal space of this electromagnetic coil 20.30.
0 is NMR.

Glass epoxy resin (commonly known as GFRP) for coil groups that generate X, Y, and Z gradient magnetic fields essential for imaging
Use a made bobbin. Its inner diameter is W1680. An annular ferromagnetic material 50.60 is provided on the inner diameter of the pipe 80, which is made of a flexible rubber band (cross section: 4 x 4 wn") mixed with high-frequency soft ferrite. Two annular ferromagnetic bodies 50 and 60 are provided, and these two forming bodies are fixed to the inner wall of the pipe 80 with a distance of 240 rtrx from the center of the inner diameter of the pipe 80. The 400 value of the 2'' term can be corrected by the field 50.60. This correction effect can be easily obtained not only with the soft ferrite but also with other ferromagnetic materials. For example, the same effect can be obtained by winding and fixing S1 m piano wire (iron, 1 il) 10 turns around the inner diameter of the vibrator symmetrically at a distance of 240 cm from the center. This is because the correction of item 26 is effective when two annular ferromagnetic bodies 50 and 60 are installed. In this example, the annular ferromagnetic material 50.60 is centered symmetrically with 240
Only the Z'-term correction is performed with the ferromagnetic material 50.60 mm apart, but as is clear from FIG.
Since the term correction ability is reduced, the correction Z'→Z2+Z'→Z2 becomes possible. In Figure 7, the horizontal axis represents the center of the magnetic field direction l111Z of the magnetic field of M, the distance from the magnetic field center is scaled in units of (m), and the vertical axis represents the magnetic field strength (ΔHo) to be corrected, and only the terms Z2 and z4 are shown. It is shown. In addition, when the annular ferromagnetic bodies 50 and 60 are installed asymmetrically with respect to the center or only on one side, it is also possible to correct the zl and ZaO terms. Furthermore, if each of the annular ferromagnetic bodies 50 and 60 is provided with a movement adjustment mechanism in the x, y, and z directions, not only the above correction but also
It is also possible to correct terms including X, Y, and Z. In the above explanation, the number of annular ferromagnetic bodies is limited to one or two, but the above effects can be expected even when there are three or more. Furthermore, high-frequency ferrite is used because it has an eddy current prevention effect when a gradient magnetic field is applied, has good responsiveness during high-frequency switching, and does not cause distortion in the gradient magnetic field. Eddy currents can be prevented by applying insulation coating to the piano wire and opening the tip and end of the winding, or by dividing the piano wire winding at multiple points.

The NM imaging device is installed in hospitals, and most of these hospitals are constructed of reinforced concrete buildings. For this reason, the NMR imaging device will be installed in a reinforced concrete building. In this case, significant deterioration of magnetic field uniformity due to magnetic field interference caused by reinforcing bars and the like becomes a problem. Unless this is corrected, good quality NMR imaging images cannot be obtained. The most correctable term for magnetic field disturbance is Z2
゜It is item 24. In this embodiment, correction can be performed safely, stably, and easily, and the NMR imaging apparatus can be installed inside a reinforced concrete building.

〔Effect of the invention〕

As described above, according to the present invention, deterioration in magnetic field uniformity due to magnetic field disturbance can be corrected.

[Brief explanation of drawings]

Figures 1 and 2 are illustrations of NM imaging, Figure 3 is an illustration of back projection, Figure 4 is a diagram of a conventional air-core 4-split coil type normal conducting electromagnet, and Figure 5 is the direction of the magnetic field. , FIG. 6 is a diagram showing an embodiment of the present invention, and FIG. 7 is a diagram showing z", z'
FIG. 3 is a diagram showing magnetic field strength characteristics of terms. 10.20, 30.40... Electromagnetic coil, 50°6
0...Annular ferromagnetic material, 80...No (Eve. Agent Patent Attorney Tatsuyuki Unuma) Quality of the drawing (no change in content) Fig. 1 drawing 2) (no change in content) 2nd Map: JL comparison No. 3, No. 4, No. 5, No. Otsu, No. 1, No. 7, No. 2, Name of the invention NMR imaging device 3, Relationship with the case of the person making the amendment Name of patent applicant (510) Hitachi, Ltd. 4,
Agent 7, drawings subject to amendment. 8. Contents of amendment (1) Figures 1 and 2 will be corrected as shown in the attached sheet. that's all

Claims (1)

[Claims] 1. Inside the cylinder of a magnet - an object to be measured, such as a human body, is inserted into a magnetic field, and the object is irradiated with pulse-modulated high frequency waves to create a cross-section of the object using nuclear magnetic resonance. NMR for imaging
An NMR imaging apparatus characterized in that one or more annular ferromagnetic bodies are installed in the magnetic field in the cylinder so as to be orthogonal to the magnetic field direction of the static magnetic field. 2. In the invention as set forth in claim 1, the annular ferromagnetic material is a NM imaging device characterized in that it is mounted in a structure of several windings of iron wire. 3. Claim 2 3. The NMR imaging apparatus according to the invention described in 1., wherein the iron wire has an optical surface covered with an electrically insulating layer and a tip and a terminal end thereof being open. 4. The NMa imaging device according to claim 1, wherein the annular ferromagnetic material is made of soft 7-elite or hard ferrite. 5. In the invention described in claim 1, the annular ferromagnetic material is a flexible rubber band mixed with soft ferrite or hard ferrite.
MR imaging device.