JP5392803B2 - Nuclear magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a multi-nuclide nuclear magnetic resonance imaging apparatus that simultaneously resonates (NMR) a plurality of different nuclear magnetic resonance nuclides and presents the two-dimensional and three-dimensional spatial distributions and temporal changes thereof as images and numerical data. Is.

  Nuclear magnetic resonance technology is used to analyze internal structures and physical properties (non-destructive testing) of various materials except metals and dielectrics of magnetic materials, visualization of fluids, chemical structures and pharmacological effects in pharmacy and chemistry, zoology, biological It can be used for analysis of biological tissue structure and function in science, medicine, physiology, etc., measurement of biological reaction, disease inspection / diagnosis (non-invasive inspection), measurement during treatment (see Patent Document 1) .

MRI (Magnetic Resonance Imaging) based on the principle of nuclear magnetic resonance (NMR), and to realize this, the MRI apparatus includes a magnet for generating a static magnetic field and a temporal / space around the object to be imaged. A gradient magnetic field generating coil for a magnetic field whose magnetic field intensity can be varied, and a spatially uniform magnetic field Bo is formed by a static magnetic field generating magnet, and the magnetic field strength B ₁ is locally generated by the gradient magnetic field generating coil. Control. The MRI apparatus applies an RF (radio frequency) wave to an imaging target for nuclear magnetic resonance in a static magnetic field Bo, and in that state, changes the phase of spins in an atomic nucleus that undergoes nuclear magnetic resonance with a gradient magnetic field to change the signal ( An echo signal or FID (free induction decay) signal is obtained. When this signal is reconstructed by Fourier transform or the like, an image (MRI image) is obtained. In order to encode the spin phase and distribute it in the k-space, a gradient magnetic field is applied in the x, y, and z directions to spatially change the magnetic field strength.

In the MRI apparatus, the SE (spin echo) method, IR (inversion) have been conventionally used to measure the relaxation time that changes depending on the binding state between the nuclides and the spatial displacement of the nuclides such as molecular diffusion and perfusion. Recovery) method, diffusion-weighted imaging method, phase contrast method, time-of-flight method, and these high-speed and ultra-high-speed imaging methods have been proposed, and measurement in a two-dimensional plane or a three-dimensional space is possible.
However, all of the MRI methods proposed so far and all of the MRI apparatuses for realizing the MRI method resonate one nuclide and measure its spatiotemporal distribution. For example, measurement of different nuclides is desired for analysis of substances having chemically complicated bonds.

JP 2006-87499 A

It is an object of the present invention to enable nuclear magnetic resonance of a plurality of different nuclides simultaneously.
By simultaneous resonance of multiple nuclides, the molecular structure can be grasped from the density and chemical bonding state of the nuclides, and the geometrical structure and metabolic changes of cells and living tissues based on them can be elucidated in detail. In non-biological measurement, it can be used for analysis of internal structure analysis, fluid analysis, effects of drugs and chemical substances, etc. of non-magnetic substances.

  The nuclear magnetic resonance imaging apparatus of the present invention includes an excitation and detection coil arranged around an imaging target, and applies an RF wave having a frequency equal to the resonance frequency to the excitation and detection coil in the form of a pulse to cause the nuclear to the imaging target. Magnetic resonance is induced and output as a detection signal or an echo signal by the excitation and detection coils. In order to target different types of nuclides, the nuclear magnetic resonance imaging apparatus matches an RF pulse generator that generates RF pulses having a plurality of types of frequencies corresponding to the resonance frequencies of the types of nuclides. And a receiver having a reception band.

  The receiver is connected to an image reconstruction device that reconstructs an image and an image display device that displays images of different nuclides in a superimposed manner. This image reconstruction device corrects the image size determined depending on the gyromagnetic ratio of the nuclide to be measured.

  According to the present invention, simultaneous measurement and imaging of a plurality of different nuclides are possible. It is desirable to measure for different nuclides for the analysis of chemically complex bonds. According to the present case, a plurality of different nuclides are simultaneously resonated and measured, and the spatial distribution can be drawn as an image, so that the chemical structure of a substance including a living body can be analyzed with higher accuracy.

  Further, according to the present invention, it can be applied to all MRI imaging methods. Conventionally, various MRI (nuclear magnetic resonance imaging) methods such as relaxation time, diffusion coefficient, and temporal / spatial distribution of measured quantities such as chemical shift have been proposed and utilized. By adding (transmission / reception system) and software (correction due to frequency separation and magnetic rotation ratio), a conventional MRI apparatus can be diverted, and a conventional MRI imaging method can be utilized.

Device configuration Nuclide resonance imaging system (broadband consultation system) Nuclide resonance imaging system (narrow band consultation system) Observed signal Image reconstruction Image size correction Image size correction factor Image size correction and compositing Measurement target Magnetic field strength and frequency distribution (difference between hydrogen and Zenon) Enlargement of images (hydrogen and xenon) Signals observed in 2 Tesla MRI system (computer simulation)

Hereinafter, the present invention will be described based on examples. FIG. 1 shows the configuration of a nuclear magnetic resonance imaging apparatus for realizing the present invention. 1 shows a hardware configuration of an MRI apparatus for realizing the present invention. The nuclear magnetic resonance imaging apparatus is based on the principle of NMR (nuclear magnetic resonance). Outside the excitation / detection coil, a static magnetic field generating magnet (nuclear magnetic resonance (NMR)) for generating a temporally and spatially constant magnetic field on both sides of the imaging object (or measurement object, measurement substance) In principle, a magnet for generating a spatially constant magnetic field Bo, and a shim coil for locally adjusting the magnetic field strength to improve the spatial uniformity of Bo), A gradient magnetic field generating coil for spatially varying the magnetic field strength is provided. The static magnetic field magnet is connected to a static magnetic field magnet / shim coil power source and a control device for controlling the strength of the magnetic field. The gradient magnetic field generating coil is connected to a gradient magnetic field generating coil power source and a control device for controlling the strength of the magnetic field. The excitation / detection coil is connected to an image processing device including a receiver that receives a signal corresponding to nuclear magnetic resonance, an image reconstruction device that reconstructs an image, and an image display device. In addition, the excitation / detection coil has a frequency equal to the resonance frequency in order to resonate the nuclear magnetism (spin derived from the nucleus of the nuclear magnetic resonance nuclide) of the nucleus of the object to be imaged (or measurement object or measurement substance). An RF pulse generator (electromagnetic wave (radio wave) generator) that applies RF (radio frequency) waves in a pulsed manner is connected. The RF pulse generator and receiver are matched to the resonance frequency of the nuclide to be measured. The illustrated RF pulse generator generates RF pulses having the corresponding N types of frequencies in order to target different N types of nuclides. The receiver has a reception band that matches the resonance frequency of N different nuclides.

  When the RF pulse of the RF pulse generator is applied to the excitation / detection coil, the imaging target induces nuclear magnetic resonance, and the detection coil (FID [free induction decay] signal or echo signal) is detected by the detection coil arranged around the imaging target. ) Is output. This signal is collected by a receiver connected to the excitation / detection coil. Thereafter, the image is reconstructed by the image reconstruction device.

  In the nuclear magnetic resonance phenomenon, when the magnet and the static magnetic field strength used are Bo, the resonance frequency ω is given by ω = γ · Bo, where the magnetic rotation ratio γ inherent to the nuclide to be resonated is a proportional coefficient. For this reason, when the same MRI apparatus is used, that is, when Bo is fixed, the resonance frequency ω is defined based on the magnetic rotation ratio. In order to resonate a plurality of nuclides simultaneously, it is necessary to apply RF pulses having different resonance frequencies for each nuclide. Moreover, the receiving system of the observed signal must receive the corresponding frequency band.

When the gradient magnetic field g is applied to the magnet of the MRI apparatus only in the x direction, the resonance frequency ω _{1 of the} nuclide at a distance of ± x from the center of the magnet is given by the following equation.
± ω ₁ = ω ₀₁ ± ω ₁₁ = γ ₁ (Bo ± g * x) = γ ₁ * Bo ± γ ₁ * g * x
Here, the gyromagnetic ratio of the resonating nuclide is γ ₁ .

On the other hand, for other nuclides:
± ω ₂ = ω ₀₂ ± ω ₁₂ = γ ₂ (Bo ± g * x) = γ ₂ * Bo ± γ ₂ * g * x
Here, for the sake of simplicity, it is assumed that the gradient magnetic field is applied in the x direction, but the same applies when the gradient magnetic field is applied in the y and z directions. The frequency range (spectrum width) in which the signal is distributed is determined by the gyromagnetic ratio γ specific to the nuclide when the gradient magnetic field g is the same in both.

  FIG. 2 shows the internal configuration of the device. FIG. 2 shows a hardware configuration for resonating two different nuclides at the same time, in which nuclides A and B are simultaneously resonated and imaged. When different N types of nuclides are targeted, RF pulses having the corresponding N types of frequencies are generated. FIG. 2 shows a case where a broadband receiving system is used, and signals from a plurality of nuclides are received by one receiving system. Here, in the case where the phase is detected and received, in order to receive a phase component (sin component, cos component) different by 90 degrees, an equal two-channel receiving system is prepared. If two or more nuclides are targeted, the RF pulse and the reception system are transmitted and received in the corresponding frequency band, and the signals after reception and image reconstruction are individually performed.

  Similarly, FIG. 3 shows a narrow-band receiving system that receives each corresponding frequency. When different N types of nuclides are targeted, RF pulses having the corresponding N types of frequencies are generated, and amplifiers of the corresponding N types of frequency bands are used. By narrowing the band, only necessary signal components can be received, and unnecessary noise can be avoided.

  When different nuclides are resonated simultaneously, the frequency band (resonance frequency, spectral band) of the observed signal is different due to the difference in the gyromagnetic ratio for each nuclide that resonates. For this reason, when resonating images of different nuclides simultaneously, it is necessary to take one of the following methods.

When an AD converter is used for each nuclide that resonates and sampling can be performed at different frequency intervals, N points are sampled at intervals shown by the following equation. Sampling frequency S ₂ nuclides having the sampling frequency S ₁ and gyromagnetic ratio gamma ₂ nuclear species with gyromagnetic ratio gamma ₁ are each given by the following equation.
S ₁ = 2 * γ ₁ * g * x / N
S ₂ = 2 * γ ₂ * g * x / N
Therefore, the sampling frequency ratio SR is γ ₂ / γ ₁ . Here, N is an integer, but in general, a power of 2 is frequently used.

FIG. 4 shows the observed signal. FIG. 4 shows a case where two different nuclides are targeted, and nuclides A and B resonate at different frequencies due to the difference in the gyromagnetic ratio. In MRI, a gradient magnetic field g is applied for imaging, and due to this, the frequency is dispersed within a range of ± ω ₁₁ or ± ω ₁₂ . The receiving system detects these signals. Signal detection can be performed by receiving signals from a plurality of nuclides (broadband receiving system) or receiving each nuclide (narrowband receiving system).

  FIG. 5 shows image reconstruction after signal collection. FIG. 5 shows the case of two nuclides. The signals observed at different frequencies are frequency separated and image reconstruction is performed individually. Image reconstruction uses Fourier transform or the like. When a gradient magnetic field is applied to any two axes of x, y, and z, a two-dimensional image is reconstructed, and when applied to three axes, three-dimensional imaging is enabled. Since the signal reception needs to detect the phase of the signal, the phase is detected by 90 ° and separated into a sin component and a cos component. Of course, when the phase information is not required, only one of the components can be used.

FIG. 6 is a diagram for explaining the size correction of the reconstructed image. FIG. 7 is a diagram showing correction coefficients for the image size. As described above, the resonance frequency is different due to the difference in the magnetorotation ratio. Since the observation frequency matches this, if the sampling interval is the same in different nuclides, it is necessary to correct the size after image reconstruction. For example, in the case of two nuclides, the image of another nuclide B having a gyromagnetic ratio γ ₂ is enlarged by a coefficient (γ ₁ / γ ₂ ) with respect to the gyromagnetic ratio γ ₁ of the nuclide A. The image after enlargement is illustrated on the left side of FIG. 6, and the image before enlargement is illustrated on the right side.

Since the image size is defined by the magnetic rotation ratio γ, it is defined by the following equation.
M = ω ₁₁ / ω ₁₂ = γ ₁ * g * x / γ ₂ * g * x = γ ₁ / γ ₂
Therefore, when displaying both nuclides superimposed, size correction is performed according to the following equation.
A = 1 / M = γ ₂ / γ ₁
For example, when hydrogen 1H and xenon 129Xe are subjected to double resonance, the magnetorotational ratios of both are 42.58 MHz / T (hydrogen) and 11.78 MHz / T (zenon).
A = 1 / M = 42.58 / 11.78 = 3.61
Therefore, it is necessary to enlarge the Zenon image by 3.61 times.

FIG. 8 shows image size correction and image composition after reconstruction. The image size is determined depending on the gyromagnetic ratio of the nuclide to be measured. In order to make the γ ₂ nuclide coincide with γ ₁ , the image size is multiplied by a coefficient of γ ₁ / γ ₂ . After the size correction, if the images are superimposed as they are, images of different nuclides are displayed simultaneously.

  FIG. 9 shows the results of nuclear magnetic resonance imaging of hydrogen (1H) and xenon (129Xe). The measurement object was an aqueous copper sulfate solution as hydrogen, and a rare gas as Zenon. As shown in the figure, the former was enclosed in an acrylic container and the latter was enclosed in a crow tube to form a double structure. There is a gap between the periphery and each container.

  FIG. 10 shows a resonance frequency when a gradient magnetic field g of 100 mT / m is applied using an MRI apparatus having a 2 Tesla static magnetic field strength Bo and a frequency distribution resulting from the application of the gradient magnetic field in the case of hydrogen. The number of sampling points N was 256. Since the resonance frequency of hydrogen 1H is 85.160 MHz, the frequency of the collected data of the image when the gradient magnetic field g is 100 mT / m and the imaging region FOV is 100 mm with a 2 Tesla MRI apparatus is distributed in ± 2.129 MHz. If the number of data sampling is 256, the sampling interval is ± 16.633 KHz / point. On the other hand, 129Xe has a resonance frequency of 23.560 MHz due to the difference in the magnetic rotation ratio, and when the gradient magnetic field g is 100 mT / m and the imaging region FOV is 100 mm, the collected data is distributed in ± 0.589 MHz. Therefore, in order to match the number of data samplings to 256 points, the sampling interval is set to ± 4.501 KHz / point.

  On the other hand, in the case of a receiving system in which a sampling frequency cannot be set for each nuclide to be measured, the same sampling frequency must be set. In this case, the reconstructed image is enlarged at a ratio calculated according to the magnetic rotation ratio.

  FIG. 11 shows the case of hydrogen 1H and Zenon 129Xe. The magnetorotation ratio of hydrogen is 42.58 MHz / T, and Zenon is 11.78 MHz / T. Zoom in twice (or reduce hydrogen to 3.61 times).

  FIG. 12 shows signals (computer simulation results) observed under the conditions of FIG. Both signals are mixed from the detection coil. The image is reconstructed by frequency separation.

  Nuclear magnetic resonance imaging technology analyzes the internal structure and physical properties of various materials except metals and magnetic materials (nondestructive inspection), fluid visualization, chemical structure and pharmacological effects in pharmacy and chemistry, zoology, biology It can be used for analysis of tissue structure and function of a living body in medicine, physiology, biological reaction, measurement of disease inspection / diagnosis (non-invasive inspection), contrast medium, and the like.

Claims (5)

An excitation and detection coil arranged around the imaging target is provided, and an RF wave having a frequency equal to the resonance frequency is applied in pulses to the excitation and detection coil to induce nuclear magnetic resonance in the imaging target, and the excitation And a nuclear magnetic resonance imaging apparatus that outputs a detection signal or an echo signal by a detection coil,
An RF pulse generator for generating RF pulses having different types of frequencies in order to target different types of nuclides;
A receiver having a reception band matching the resonance frequency of the plurality of types of nuclides,
Wherein at the receiver, nuclear magnetic resonance imaging apparatus characterized by sampling at different sampling frequencies for each nuclear species.   2. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein an image reconstruction device that reconstructs an image and an image display device that superimposes and displays images of different nuclides are connected to the receiver. 2. The nuclear magnetic resonance imaging apparatus according to claim 1 , wherein the sampling frequency of the receiver AD converter is varied depending on the observed resonance frequency and spectrum width due to the difference in the gyromagnetic ratio for each nuclide.   2. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the receiver includes a single wide-band receiving system that receives signals from the plurality of nuclides, and individually performs signal separation and image reconstruction after reception. .   The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the receiver includes a narrow-band receiving system that receives signals from the plurality of nuclides for each corresponding frequency.

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