JPS59230148A - Nmr imaging device - Google Patents

Abstract

PURPOSE:To stabilize a static magnetic field and to obtain a distinct image by detecting a static magnetic field value in a prescribed period, and controlling a static magnetic field or a resonance frequency in accordance with a variation portion. CONSTITUTION:A static magnetic field value by an electrostatic field aircore electromagnet is detected by a probe 30 in a thermal balance waiting time of a body to be inspected 5 of a stop period in which X-Z inclined magnetic field coils are not excited by exciting power sources 7-9, processed by an RF receiver 26, a detector 27, etc., and a feedback signal corresponding to its variation portion is outputted from a feedback amplifier 28. By this output, an electromagnet 1 is brought to feedback control through a magnet power source 6, and a static magnetic field is stabilized. Accordingly, a projected output to the body to be inspected is not varied, an out-of-focus, etc. are not generated, and a distinct image is formed. In this regard, even when a resonance frequency is brought to feedback control, the same result is obtained.

Description

[Detailed Description of the Invention] [Field of Application] The present invention relates to a nuclear magnetic resonance apparatus, and in particular, the present invention relates to a nuclear magnetic resonance apparatus, and in particular, to obtaining medically effective diagnostic information from a subject, that is, a cross-section of a living body, using a nuclear magnetic resonance phenomenon. The present invention relates to an NMR imaging device that obtains images.

[Background of the invention]

Nuclear magnetic resonance (hereinafter referred to as NMFt) is used for structural analysis of organic compounds and research on condensed material physics, and has become one of the important analytical instruments. 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. In an NMR imaging apparatus, which is an inspection apparatus using NMR, it is common to add a second magnetic field having a spatially different intensity to the static magnetic field H6. There are several methods for applying this second magnetic field, depending on how the NMR signal is processed. Here, XIC
An overview of the NMR imaging apparatus (or sometimes referred to as NMR-CT) that reproduces images using the same method as T. In addition to a uniform magnetic field Ho, a static magnetic field with a spatial gradient G is applied to the subject. Let the direction of the magnetic field HO be the Z axis, and assume that the gradient G
is in the X direction, and if the strength of the static magnetic field at X=Q is H, then the static magnetic field H applied to the subject is given by H""Ho 10-X. The resonance frequency at this time is ω = γH - γH
a + rQ-x = ω0 + γQ -x (1) Therefore, ω0 = γHo, and γ becomes a linear function of X like the unique gyromagnetic ratio of the nuclear spin. When a resonance spectrum is measured for this object, the signal at frequency ω is only from the nuclear spin population in the corresponding plane where X=constant, as shown in FIG. Therefore, the measured spectrum P(ω) is calculated using the nuclear spin density function ρ(x, y, z) as P(ωI=ffρ(x, )', Z) dyd!
- According to equation (2) or (1), P(ω.+γG-x)=ffρ(x, y, z)dydz
−(3). Now, if we newly set the left side as f (x), f < X) = ffp (x, y, z l dydz
-(4) can be written. 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 a method of selectively exciting a resonance phenomenon is combined, it is possible to detect only a signal at a specific position on the Z axis, as shown in FIG.

Rotate the object around the Z axis or change 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. The above is a description of the NMR imaging apparatus that obtains tomographic images.

Here, we will discuss Ho and G in some detail. If Ho is an ideally uniform magnetic field, the NMR signal of the specimen to which no gradient G is applied will exhibit a resonance spectrum that is naturally determined by nuclear spins. Actually, HO itself has non-uniform components. This value depends on the structure of the magnet, but it is around 100p, and the resonance spectrum reflects the non-uniformity of the static magnetic field Ho even without adding the gradient G.
It becomes broad and has a spread of 100 IF.

If the nonuniformity of the static magnetic field Ha does not overlap spatially, it is possible to determine the nuclear spin density of each part of the object without the gradient G, and a tomographic image can be obtained without using the back projection method described earlier. . However, since the static magnetic field HO is non-uniformly distributed on concentric circles, it is necessary to add a gradient G to obtain a resonance spectrum corresponding to the spatial position. The value of the 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 applied at several times the non-uniformity of the static magnetic field Ho (several 10011 pH).

In other words, the value of the gradient G is the static magnetic field I, which is a value less than 0.1 of the static magnetic field Ho. It is heavily dependent on Io. If this static magnetic field HOO value changes due to some influence, each projection will move left and right in accordance with the change in the static magnetic field H. Even if each projection is added onto the display screen using the back projection method described above, the restored image will either not form an image or will be an out-of-focus image, making it insufficient as a medical diagnostic image. No.

[Purpose of the invention]

An object of the present invention is to provide an N M FL imaging device that can stabilize the magnetic field Ho generated by the magnetic field device.

[Summary of the Invention] The present invention detects a static magnetic field H8 when no gradient magnetic field is applied during projection data collection, detects a change ΔHa in the detected value, and detects a change ΔHa when the change ΔHa becomes zero. The idea is to stabilize the magnetic field Ho generated by the magnetic field device by controlling the magnetic field device that applies a static magnetic field or the resonance frequency ω0 so that

[Embodiments of the invention]

Examples of the present invention will be described below.

FIG. 4 shows an embodiment of the invention.

In the figure, the magnetic field device that generates the static magnetic field H8 is constituted by an air-core electromagnet 1 consisting of four coils so as to improve the uniformity. Inside this air core electromagnet 1, there are X, Y,
A bobbin 2 is provided in which three sets of coils are wound to generate gradient magnetic fields in each Z direction. Inside this bobbin 2, an irradiation coil 3 for exciting resonance and four receiving coils for detecting the qsi signal are arranged. Within this receiving coil 4, the subject 5 is installed at the center of the air-core electromagnet 1, which has the best homogeneity of static magnetic field (0).The air-core electromagnet 1 has a magnet t
An L source 6 is connected.

Furthermore, excitation power supplies 7, 8.9 are connected to the X gradient magnetic field coil, the X gradient magnetic field coil, and the Z gradient magnetic field coil wound on the bobbin 2, respectively. This excitation power supply 7, 8.9
are connected to the computer 11 via an interface 12, respectively.

On the other hand, a high frequency transmitter 14 and an RF transmitter 22 are connected to the frequency synthesizer 13. This high frequency transmitter f'14 is connected to F'14 via a modulator 15.
An LF power amplifier 16 is connected. The irradiation coil 3 is connected to this RF power amplifier 16.

Furthermore, an RF transmitter amplifier 23 and a wave detector 27 are connected to the RF transmitter 22 . A matching circuit 24 is connected to the RF power amplifier 23, and a static magnetic field H and a globe 30 for detection are connected to the matching circuit 24. An RF receiver 2G is connected to the matching circuit 24 via a preamplifier 25, and a feedback amplifier 28 is connected to the RF receiver 26 via a detector 27. This feedback amplifier 28 is connected to the magnet power supply 6.

In addition, the receiving coil 4 is connected to an RF
A receiver 18 is connected, and an audio amplifier 20 is connected to the RF receiver 18 via a detector 19. The output of the audio amplifier 20 is converted into a digital signal by an A/D converter 21 and input to the computer 11 via the interface 12. A tisgerei 29 is connected to the ink face 12, and an operation console 10 is connected to the computer 11.

Since it is constructed in this way, the air core electromagnet 1 is excited by the magnet power supply 6 to 1500 Gauss, and the gradient magnetic field coil wound around the bobbin 2 has a power of 1cn+6 for each of X, Y, and Z. , 3 Gauss and 1 inch by an excitation power supply 7,8°9.

When the inspection technician who is the operator starts the inspection by operating the console 10, the computer 11 instructs the sequence controller in the ink face 12 to perform a predetermined series of measurements in accordance with the changes in the switches on the console 10. Apply. The series of 41 ['' in total includes the collection of projection data from each direction divided into 128 equal parts and the display of a tomographic image after calculation processing, and takes about 2 minutes.

The frequency synthesizer 13 is set at 6.38 MHz, at which hydrogen nuclei resonate at 1500 Gauss. Its frequency stability is per hour. ! is less than lXl0-'-\russo, and the variation within a series of measurement times is of the order of being negligible. The 6.38 MHz radio wave is divided by the high frequency transmitter 14 into an output for irradiation and an output for detection. This irradiation output is amplitude-modulated into a Gaussian wave by a modulator 15 so as to have a specific frequency band,
The power is amplified to 500 watts at the peak by the RF power amplifier 16 and applied to the irradiation coil 3. By simultaneously applying the gradient magnetic field G-Z, hydrogen nuclear spins within the frequency band of radio waves in the subject 5 are excited to a resonance state. Since the Z axis is aligned with the body axis direction of the subject, a surface to be imaged with z=constant is determined. When a radio wave with double the amplitude is applied to the irradiation coil 3 after 5 milliseconds, it becomes an NM multiplied signal echo signal and is induced in the receiving coil 4 after another 5 milliseconds. NMR number is Griamp 17,
The RF receiver 18, detector 19, and audio amplifier 20 generate a signal of approximately ±5 volts. Sampling is N
Starting from the maximum amplitude point of the MRM signal, the A/D converter 21 stores 256 points of discrete data in the memory circuit in the ink face 12 as one projection take. The time required for sampling is determined by the value of the simultaneously applied gradient magnetic field G which determines the projection direction. Since the values of G and A are ±0.3 Gauss/cnt, the measurement space is 40
The change in the static magnetic field at an is from -6 Gauss to +6 Gauss, and the band of the resonance signal of the hydrogen nuclear spin is ±26 kilohertz. To sample a 26 kHz signal, A/D conversion is performed at 20 microsecond intervals, which is 256 kHz.
At the point, the sampling time is 5 milliseconds. Even if the multi-echo 1'1ill method is used, in which radio waves with twice the amplitude are irradiated multiple times in order to improve the S/N ratio of the resonance signal, collecting one projection data requires A The time required to complete the /D conversion is 0.1 seconds or less. On the other hand, in order to collect the next projected cheater, it is necessary to wait for about 1 second, which is determined by the physical quantity of the longitudinal relaxation time T1 of the hydrogen nuclear spin of the object. During this time, no gradient magnetic field is applied, and the nuclear spin co-II* condition is determined only by the magnetic field generated by the air-core electromagnet l. When this waiting time is reached, the RF transmitter 22 uses the signal from the sequence controller in the ink face 12 as a trigger, and 6.
It outputs a radio wave modulated into a 38 megabyte rectangular pulse. A rectangular pulse in pulse width will contain a frequency component of 1/τ centered at 6.38 MHz. The radio waves were amplified to 310 watts on an RFt power amplifier,
The resonance energy is applied through the matching box 24 to the glove 30 for error signal detection.

The resonance signal of the hydrogen nucleus of the sample in the globe is observed after the rectangular pulse of radio waves, and is again transferred to the matching box 24.
After that, it is amplified by a preamplifier 25 and an RF receiver 26,
The detector 27 converts it into a DC component.

During this period, when the value of the static magnetic field changes somewhat and becomes 10+ΔH, the resonant frequency ω becomes ω−γ(Ha+ΔH1−γ−1−1−1.0+γ・Δ14=
ωθ+γ・ΔH. If we set γ・ΔH=Δω0, the above formula becomes ω=ωθ+Δ
becomes ω. The detector 27 uses a reference signal of 6.38 MHz (
Since ω0 ) is used in the output, the component of Δω can be outputted to the feedback amplifier 28 as error No. 16. 1”1 power of feedback amplifier 28 is Δω
The output jJ of the magnet power supply 6 is controlled by '1111 so that the value becomes zero. During the one-second waiting period, the error signal is detected, and when the next projection data measurement begins, the sequence controller in the interface 12 executes the sequence again and works to maintain the output voltage of the feedback amplifier 28. . From the first projection to the measurement of the 128th projection, the inspection is performed while maintaining the magnetic field generated by the air-core electromagnet 10 at an intensity of 1500 Gauss.

FIG. 5 shows a time chart of this embodiment. That is, in FIG. 5, section A is a period in which the spins of the subject 5 are excited, and a high frequency magnetic field is applied as shown in FIG. 5 (4). In sections A and B, a Z gradient magnetic field is applied as shown in FIG. 5 σ3), and the imaging Ur plane of the subject 5 is determined.

Further, in section C, a gradient magnetic field of <X, Y as shown in FIG. 5(C) is applied, and an NMR signal as shown in FIG. 5(to) is measured from the receiving coil 4. Further, the period is a waiting time until the spin of the subject 5 reaches thermal equilibrium. In this section, high frequency power for locking is applied to the RFt force amplification amplifier as shown in FIG. NM for this lock
As for the R signal, a signal as shown in FIG. 5 [F] is obtained by the FLF receiver 26, and a signal as shown in FIG. 5 (CG) is obtained by the detector 27. The output of this detector 27 is integrated by a feedback amplifier 28 and output as a signal as shown in FIG. 50. The output of this feedback amplifier 28 corresponds to the aforementioned error signal Δω. Furthermore, section A
During the BC period, the output of the detector 27 becomes zero as shown in FIG. 50, so the output of the feedback amplifier 28 is held.

Therefore, according to this embodiment, since the static magnetic field H8 is controlled by the resonance frequency of the hydrogen nuclear spin, it is possible to prevent the drift of Ha due to temperature changes in the magnet power supply 6 and the coil of the air-core electromagnet 1.

Further, according to this embodiment, even if the magnetic body moves near the inspection device during inspection and cuts off the leakage magnetic flux of the air-core electromagnet 1, H
As long as the value of o is kept constant, there is no need to stop the test.

Furthermore, according to this embodiment, when the power is turned on, the static magnetic field H
, to an appropriate value, it is possible to omit the work of checking the value of H8 in advance of the laboratory technician.

In this embodiment, a case has been described in which the error signal is fed back to the magnet power supply 6, but the same effect can be expected even if the error signal is fed back to the frequency synthesizer 13.

Furthermore, the same effect can be expected even if a fluorine nucleus 19p, a phosphorus nucleus 31P, or the like different from the measurement nuclide of the subject is used as the sample in the glove for error signal detection.

In this case, the frequency synthesizer 13 oscillates at the resonance frequency of 6.38 MH2 for hydrogen nuclei, so the RF transmitter 22 oscillates at the resonance frequency of 6.00 MH2 for fluorine nuclei.
A frequency conversion function must be added to the resonance frequency of the phosphorus nucleus, which is 2.58MH2, but since it is different from the resonance frequency of the hydrogen nucleus of the measurement nuclide in the test object, the irradiation coil 3 for inspection,
There is no interference between the receiving coil 4 and the coil of the error detection glove 23, and no RF shield is required.

〔Effect of the invention〕

As explained above, according to the present invention, the magnetic field Ho generated by the magnetic field device can be stabilized.

[Brief explanation of the drawing]

1 and 2 are explanatory diagrams of NMR imaging, FIG. 3 is an explanatory diagram of back projection, FIG. 4 is a block diagram showing an embodiment of the present invention, and FIG. 5 is a time chart. DESCRIPTION OF SYMBOLS 1... Air core electromagnet, 2... Bobbin, 3... Irradiation coil, 4... Receiving coil, 5... Subject, 6...
Magnet power supply, 13... Frequency synthesizer, 24...
Matching circuit, 30...Glove. Agent Patent attorney Tatsuyuki Unuma $ 1 FREQL）ENCY Kaya 2 Prisoner FREσuENCγ Kaya 3 Figure 4

Claims (1)

[Claims] 1. A magnet formed in a cylindrical shape into which an object to be measured such as a human body can be inserted and removed at will, a first magnetic field device that generates a uniform static magnetic field H6 in a certain space formed by the magnet, and a spatial position. and a second magnetic field device that generates magnetic fields (gradient magnetic fields) with different strengths according to A detection means for detecting the static magnetic field HoO value while the magnetic field device is not operating, a calculation means for calculating a change ΔHo in the value of the static magnetic field detected by the detection means, and a calculation means for calculating the change ΔHo in the value of the static magnetic field detected by the detection means. An NMR imaging apparatus comprising: a control means for controlling the static magnetic field H6 or the upper magnetic field resonance frequency ω0 of the first magnetic field device so that the qH3 value ΔHa is zero.