JPS62231645A - Homogenous magnetic field forming apparatus for nmr, nmr spectroscopic apparatus and method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

TECHNICAL FIELD The present invention relates to a magnetic field forming device used for the generation of nuclear magnetic resonance in NMR spectroscopy, in particular by providing the device with a sufficiently short aspect ratio. For example, when performing spectroscopy on the human head, the inner diameter thereof can be made slightly larger than the human head.

The present invention also relates to NMR spectroscopy using such a magnetic field forming device and a method of using the NMR spectrometer.

Prior art and its problems Nuclear magnetic resonance (NMR) spectroscopy has traditionally been used in the fields of physical chemistry or analytical chemistry, but in recent years, proton images of human tissues or organs have also been used in the medical field. images) without causing damage to the human body.

Also, for example, when a nuclear magnetic substance such as water has a magnetic field strength H
The angular frequency of precession of the nucleus W when in a homogeneous and static external magnetic field of o. is known to be calculated using the following formula.

W6 = TH.

However, T is a specific constant that indicates the magnetic properties of the nucleus to be observed, that is, the rotational magnetic ratio (gyro-magnetic ratio).
ic ratio).

In NMR spectroscopy, research has mainly been carried out in two types of magnetic fields: a static and homogeneous magnetic field and a periodically changing high-frequency electromagnetic field.As a result, if the above equation is satisfied, It was discovered that nuclear magnetic resonance can be induced. In other words, the presence of a unique atomic nucleus in a target substance is detected from the occurrence of a certain type of nuclear magnetic resonance. A static magnetic field is created by a suitable coil supplied with a steady current, but to obtain a strong magnetic field,
Preferably, the coil is a superconducting coil. Further, the high frequency electromagnetic field is formed by an auxiliary coil or a radio frequency (RF) coil to which a high frequency current is supplied. In addition,
Nuclear magnetic resonance is detected by other coils or detection coils provided around the target substance, while the auxiliary coils are driven in a time-division manner.

NMR spectroscopy and NMR imaging differ in their requirements in two respects: magnetic field strength and magnetic field homogeneity.

That is, whereas NMR spectroscopy requires a strong magnetic field and homogeneity to perform spectral analysis with suitable resolution, NMR imaging requires a relatively weak magnetic field and a moderate amount of heat within a relatively large volume. Requires homogeneity. In addition,
The above requirements are incompatible with each other when considering the efficiency of the NMR imaging method.

In addition, in NMR spectroscopy, a static and homogeneous magnetic field is used together with a periodically changing high-frequency electromagnetic field. In different environments, it resonates at slightly different frequencies. Such a phenomenon is
This is as shown in FIG. 1, and is called a chemical shift.

This FIG. 1 shows a spectrum of phosphorus in a certain region of the brain, and in the figure, the presence of seven types of phosphorus compounds is expressed at a frequency difference of about 30 ppm. The peak curve corresponding to each phosphorus compound is its half-width (HMLW)
) is about 1 to 2 ppm, i ppm
The magnetic field must be more homogeneous than the magnetic field, and as the field becomes stronger, the distance between each peak increases and the spectral resolution expands. Here, the above-mentioned unique spectrum is an average signal value of 60 detection data. Note that, as is well known, the signal-to-noise ratio (S/N ratio) of NMR increases by n times 2 (n is the number of detections). In addition, in NMR spectroscopy used for biological tissues, it is considered appropriate that the homogeneity is 0.1 to 0.2 ppm over a range larger than the volume of the target substance, and NMR sensitivity is low and In order to suitably analyze phosphorus compounds that have low concentrations in the body, a magnetic field strength of at least 2 T (Tesla) is required, and the amount of water used is several moles (the amount of water used in NMR imaging is several hundred). moles).

Also, in NMR spectroscopy, seven peaks are formed for phosphorus compounds, whereas in NMR imaging, a single peak is preferably formed, and in a fatty environment. The peak is shifted several ppm from the peak position indicating mobile protons in tissue water. It would therefore be desirable to have a relatively low homogeneity of 10-100 ppm in order to remove fat-induced artifact noise, but the broad spectrum that results from this low homogeneity does not allow for spatial resolution. It becomes difficult to perform spectral analysis to obtain .

Additionally, each phosphorus compound has a proton observable signal and other signals that cannot be observed by nuclear magnetic resonance of phosphorus, such as lactate or ADP (adenosine diphosphate or ammonium dihydrogen phosphate), respectively. Therefore, in recent years, each resonance part (peak part) on the spectrum has come to be considered to be the nuclear magnetic resonance of protons in the living body.

Therefore, a highly homogeneous magnetic field generating device is required to obtain a high resolution that allows discrimination of phosphorus and the protons mentioned above.

Further, magnets for NMR spectroscopy have evolved from those for spectroscopic analysis of target substances in liquids or solutions, and have now become magnets for NMR imaging.

This spectroscopic magnet is a magnetic field or its gradient end (
A thick-walled cylindrical coil mechanism for generating a strong magnetic field with homogeneity of the 6th to 20th orders (gradient)" (Journal of Applied Physics, 1967, Vol. 38, No. 6, written by M.W. and Garrett). It is based on the so-called 6th order solenoid.

The magnetic field H formed by a geometrically symmetrical cylindrical coil
(r, θ) is explained by Tiller's expansion formula around the origin of cylindrical coordinates.

・ ・ ・(1) However, H(r, θ) is the magnetic field at the point with radius r, angle θ), Pfi(cos θ) is the n-th order Legendre polynomial, and β7 is defined by the shape and current of the coil. It is a number.

The magnetic field H2 on the central axis of the cylindrical coil is as shown in the following equation because the odd terms are zero based on the n-th order magnetic field gradient and the symmetry of the coil around the origin.

6! 8! Conventionally, in order to cancel the magnetic field gradient H2 of the second-order term and the magnetic field gradient H4 of the fourth-order term, a long solenoid with an inherently low magnetic field gradient is used with an internal or external nozzle.
tches ), thereby removing the remaining sixth-order term Hb, which is an extremely small value. Such long solenoids were chosen to simplify the manufacturing of the device and to allow easier and more accurate positioning of the coils coaxially with each other than would be possible with shorter sized coils.

As mentioned above, an NMR imaging magnet needs to have a suitable homogeneity within a relatively large volume, and its shape should be an approximation of a perfect sphere or a nearly perfect sphere. It is necessary. The shape of this magnet includes 4.6 or 8 coaxial coil pairs, as shown in the description by Garrett,
Theoretically, the coils located on the same axis cancel out the magnetic field gradient of the higher-order terms in equation (2) up to the 20th-order term. However, within a relatively large volume with a diameter close to the diameter of the magnet hole, the magnetic field gradient of higher-order terms comes to have a large influence. For example, if the homogeneity is doubled, 2
The 12th order term increases by 1Z times or 4096 times.

In addition, when applying NMR spectroscopy to a human head using conventional equipment, the entire torso or at least both shoulders of the human body is inserted into the magnet hole, and the head is placed in the center of the magnet where the homogeneity is highest. The aspect ratio of the magnet had to be large because the magnet had to be able to reach the target area. Therefore, N
There has been a problem in that the cost of the MR spectrometer changes approximately in proportion to the square of the diameter of the magnet.

OBJECT OF THE INVENTION The present invention has been made with the aim of solving the problems in conventional devices as described above.

Another object of the present invention is to provide a magnetic field forming device that has sufficient magnetic field strength and homogeneity and has a small aspect ratio when performing NMR spectroscopy on a human body.

Another object of the present invention is to have a diameter slightly larger than that of an average human brain, and to sufficiently shorten the length (in the axial direction) of the head of the subject. An object of the present invention is to provide an NMR spectroscopic device which is inserted into an opening so that NMR spectroscopy can be applied to the brain.

Another object of the present invention is to provide a magnetic field forming device capable of forming a magnetic field having an intensity of at least 2 Tesla and a homogeneity of at least 0.2 ppm.

Another object of the present invention is to reduce the cost of an NMR spectrometer by using the magnetic field forming device.

Another object of the invention is that the dimensions are reduced;
Moreover, it is an object of the present invention to provide an NMR spectrometer that increases internal homogeneity.

In addition, the above-mentioned objects and other objects of the present invention are:
As the main coil, we use Helmholtz coils, which are a pair of annular coils that have the same number of turns and the same diameter, are coaxially connected, and are connected in series. This is achieved by using a correction coil of 5 IMi wound to eliminate .

EXAMPLE Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 2 is a schematic cross-sectional view showing an NMR spectrometer that is an embodiment of the present invention. 10 well-known cryostats
is a superconducting magnetic coil 18, which will be described later, which functions as a main magnet.
a and 18b. This low temperature holding device
O is constructed by disposing a liquid helium container 12 within a liquid nitrogen container 14 within a vacuum container 16.

Inside the low temperature holding device 10, superconducting magnetic coils 18a, 1
8b, a set of correction coils 22, 24a, 2, which will be described later.
4b, 26a and 26b, and a set of superconducting rectangular correction coils 21 are provided.

This correction coil 21 is described in US Pat. No. 3,622, for example.
．． No. 869 (Golay), U.S. Patent No. 4.500,86
No. 0.

and U.S. Patent No. 4,506,247 (Vermilly)
This is provided to achieve the well-known effect of eliminating manufacturing tolerances and the effects of the environment of use, as described in . Furthermore, a butterfly-shaped RF coil 30 that functions as a high-frequency probe is provided within the large diameter hole 29 formed in the cryostat 10 within the diameter of the hole 29 . The NMR spectrometer shown in the figure is used to perform NMR spectroscopy on a human brain, and the subject's head 28 is located within the hole 29 of the cryostat 10, facing and adjacent to the RF coil 30. It will be enshrined there. R
The F coil 30 detects resonance and transmits a high frequency signal (not shown).
The received signal is electrically supplied to a suitable type of computer 31 via a receiving circuit and an A/D converter.

The embodiment of FIG. 3 shows in detail the coil arrangement used in the magnetic field forming device. In the figure, a pair of superconducting magnetic coils 18a, 18b are separated by a gap 20, and their length dimensions are set so that H2 of the second-order term of superconducting magnetic coils 18a, 18b is canceled. .

These superconducting magnetic coils 18a, 18b and the gap (81×2 space in FIG. 3) 20 are generally known as Helmholtz coils, have the same number of turns and the same diameter, and are coaxially arranged. It is a pair of annular coils connected in series. Conceptually, coil 1
The gap 20 between 8a and 18b is a reversely wound coil in the direction of reducing the magnetic field H6 of the first-order term, the magnetic field gradient H2 of the second-order term, the magnetic field gradient H4 of the fourth-order term, the magnetic field gradient H6 of the sixth-order term, etc. in the equation (2). It is believed that there is. When the magnetic field gradient H2 is canceled, the magnetic field gradient H4 in the opposite direction due to the gap 20
is larger than H4 of coils 18a and 18b, leaving a non-negligible magnetic field gradient. Note that the composite magnetic field gradient of the magnetic field gradient H6 caused by the superconducting magnetic coils 18a and 18b and the opposite direction H4 caused by the gap 20 is small, so that it does not impair the homogeneity region in NMR spectroscopy.

As mentioned above, the residual magnetic field gradient H4 in the opposite direction due to the gap 20 is quite large, so the required 0,
The volume (space) in which magnetic field homogeneity of about 1 ppm is formed is small, for example, about 2 cm in diameter. Therefore, based on the law that the strength of the fourth-order term magnetic field gradient H4 increases in inverse proportion to the fifth power of the radius, the residual magnetic field gradient H4 is eliminated by the correction coil. therefore,
By providing five coaxial correction coils, the following items 1 to 2 can be obtained.

■ Minimum value of Ho ■ Zero Hz ■ H4 equal and opposite to the main magnetic field ■ Minimum value of H8 Correction coil 22 for correcting the magnetic field gradient H4 of the 4th order term
．． 24a, 24b, 26a, 26b are arranged on the inner circumferential side of the superconducting magnetic coils 18a, 18b, and
Like the coils 18a and 18b, they are superconducting coils, and are housed in the cryostat 10 as shown in FIG. 2, or as shown in FIG. It is located outside. Further, a correction coil 22. 24 a, 24 b, 26
a, 26 b can be operated in conjunction with the main magnet or independently. In addition, since the magnetic force of a coil usually becomes larger as its radius dimension becomes smaller, the correction coils 22.24a, 24b,
It is desirable that the diameters of 26a and 26b be set as small as possible in consideration of the average diameter of the head or end of the human body being tested.

In this embodiment, the Helmholtz coil 18a.

18b and correction coils 22.24a, 24b.

Geometry and current for each of 26a and 26b.

and magnetic field related data are shown in Table 1 below. Note that the symbols and symbols used in Table 1 correspond to those in FIG. 3. In FIG. 3, 34 is the central axis of each magnetic coil, and 36 is a plane that passes through the center of the gap 20 and is perpendicular to the central axis 34, or a bisector line that bisects the gap 20. be. Each coil 18a.

18b, 22.24 and 26a, 26b are arranged in symmetrical shapes or positions with respect to plane 36. In Table 1, A1 and A2 indicate the inner radius and outer radius of each coil, respectively, and B and B2
represent one end of each coil that is the shortest distance from the plane 36 and the other end that is the longest distance from the plane 36, respectively. Although these symbols A+, A2, Bl, and B2 are also present in each coil, in order to simplify the explanation, they are attached only to the coil 18a in FIG. 3. Further, the central axis 34 and the line 36 indicating the plane in FIG. 3 are the Y-axis and the Y-axis of Cartesian (parallel) coordinates, respectively. Further, negative numbers in Table 1 indicate that the coils are wound in the opposite direction.

The NMR spectrometer of this example uses the depth pulse method (dep
th pulsing) and water suppression techniques (wate
In the adult brain cortex, phosphorus metabolites and sensitive proton resonances are separated, as shown in Figure 1. Therefore, the overall status of brain energy metabolism can be identified, and the measurement results can be effectively used for treatment of the living body.

Note that the present invention can be suitably implemented in other embodiments as well.

For example, by driving a superconducting coil made of a superconducting alloy (Nb-Ti, obu-titanium) with an appropriate current, the magnetic field at the center of the coil can generate about 2.5 Tesla, and the maximum magnetic field can generate about 4.8 Tesla. Obtainable. Note that if a coil of the same shape is made of a superconducting alloy (Nb, Sn, oboustin) or a superconducting alloy (V, Ga) (vanadium-gallium), the central magnetic field will be 5.
It can have an intensity of 0 tesla.

In addition, in the above embodiment, the length of the magnet and the shape and size of the hole etc. are based on the average diameter dimension (9,5>') and average length dimension (10!n) of the head and neck of the human body. determined according to. The length dimension of the cryostat 10 is such that each coil forms a predetermined magnetic field strength and homogeneity so that the subject's brain can be accessed without inserting the subject's shoulder into the hole 29 of the cryostat 10. On the premise that NMR spectroscopy can be performed on the coils, the length can be shortened to the extent that each coil can be accommodated. Here, since the price of an NMR spectrometer is approximately proportional to the square of its diameter, the NMR
A remarkable effect is obtained in that the spectroscopic device becomes inexpensive.

Furthermore, if suitable dimensions are selected for the superconducting coil, the length of the cryostat 10 can be shortened to some extent. On the other hand, as a magnetic field forming device, resistive
ive) magnets, water-cooled magnets, or permanent magnets, with the advantage that such magnets can be made somewhat smaller.

Further, in the above embodiment, N
Although MR spectroscopy was explained, NMR spectroscopy of the present invention
Spectroscopic devices are also applicable to the extremities of the human body, such as limbs. With conventional NMR spectrometers, when observing the part of the limb closest to the torso, it is not possible to insert the limb to a sufficient position within the magnet due to the size of the magnetic field forming device. However, according to the magnetic field forming device of the above-mentioned embodiment, since the diameter dimension and the aspect ratio are reduced, such conventional problems can be solved.

Further, in the above embodiment, the NMR spectrometer mainly uses NM
Although previously used to perform R spectroscopy, the use of this device in NMR imaging allows e.g.
It is also possible to obtain images of relatively small objects such as tumors. In such a case, an imaging coil 32 may be used instead of the RF coil 30 (both shown in FIG. 2).

In NMR imaging, artifacts from fat are removed by the computer, so that even the most extremely homogeneous magnetic field produced in the embodiment described does not interfere with imaging.

Note that the above-mentioned embodiment is merely one embodiment of the present invention, and various modifications may be made to the present invention without departing from the spirit thereof.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating chemical shift by showing an example of a rinse vector in a certain region of the brain in which phosphorus compounds are classified into seven categories with a difference of about 3°ppm in frequency, that is, in a magnetic field, The vertical axis is absorbed energy and the horizontal axis is RF frequency. FIG. 2 shows an embodiment of the present invention.
1 is a schematic cross-sectional view of an MR spectrometer. FIG. 3 is a schematic diagram corresponding to FIG. 2 of another embodiment of the present invention. 18a, 18b: Superconducting magnetic coil (Helmholtz coil)

Claims (7)

[Claims] (1) A magnetic field forming device having a small aspect ratio and capable of forming a high enough magnetic field and homogeneity to observe the inside of a human body in NMR spectroscopy, the device including a pair of Helmholtz coils; A main magnet in which a gap provided between the coils forms a magnetic field gradient larger than the magnetic field gradient of the Helmholtz coil to generate a fourth-order term magnetic field gradient in a negative direction; a set of correction coils for erasing the magnetic field gradient of the fourth-order term in the negative direction in the main magnet. (2) The set of correction coils are coaxially arranged with a radius determined to be as small as possible based on the radius of the part of the living body to be observed, and an intensity as large as possible. coils, and the pair of Helmholtz coils are
2. The homogeneous magnetic field forming device for NMR according to claim 1, which is coaxially disposed at a position as close as possible to the set of correction coils from the outside. (3) The set of correction coils are arranged coaxially inside the Helmholtz coil and are arranged symmetrically about a bisector that bisects the gap, and are similar to the Helmholtz coil. a central coil that is wound in the direction of the central coil, and a central coil that is located coaxially with each other and is arranged symmetrically about the bisector, has the same current as the central coil, and has a current similar to that of the central coil; A second coil and a third coil are wound in opposite directions, and the second coil and third coil are coaxial with each other, are arranged symmetrically about the bisector, and have the same current as the central coil. and a fourth coil and a fifth coil wound in the same direction as the central coil.
homogeneous magnetic field forming device. (4) The homogeneous magnetic field forming device for NMR according to claim 1, wherein the pair of Helmholtz coils are superconducting magnets. (5) a magnet, a high-frequency probe, a detection means for detecting induced nuclear magnetic resonance, the magnet, the high-frequency probe,
and a computer that drives each of the detection means, wherein the magnet is the magnetic field forming device according to claim 1. (6) A method for determining the resonance on the NMR spectrum of a predetermined atomic nucleus or proton in the living body by inserting the target living body into an NMR spectrometer and then driving the device, wherein the NMR spectrometer is The apparatus according to claim 5, wherein the observation point in the object is
The NMR spectrometer is located at a point where the central axes of the pair of Helmholtz coils intersect with a plane that is orthogonal to the central axis and bisects the gap between the pair of Helmholtz coils. Features of NMR spectroscopy. (7) The observation point is located inside the subject's brain, and the diameter of the opening of the NMR spectrometer is slightly larger than the diameter of the subject's head, and the shoulder of the subject is inserted. NM spectroscopy according to claim 6, characterized in that the NM spectroscopy is set so that no NM spectroscopy is obtained.