JPS6244231A - Diagnostic magnetic resonance imaging apparatus - Google Patents

Abstract

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

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention utilizes magnetic resonance (hereinafter referred to as MR), which is a resonance phenomenon for electromagnetic waves of atomic nuclei, to detect specific The present invention relates to a diagnostic magnetic resonance imaging apparatus that performs imaging by non-invasively measuring MR multiplied signals, which are information from atomic nuclei, outside a subject.

[Technical background of the invention]

CT in which a computer performs image reconstruction processing based on X-ray absorption data or magnetic resonance signals (MR multiplied signals, etc.) obtained from various projection directions for a desired tomographic plane of the subject to obtain an image of the tomographic plane. When using a device to obtain a tomographic image of a subject's abdomen, for example, movements in the position of organs due to the subject's breathing etc. are the main cause of blurring the obtained image (out of focus).Especially when using diagnostic resonance equipment (hereinafter referred to as M
RI), projection data (referred to as RI) is
Since it takes several minutes to collect data (collected data in each projection direction for the photographed cross section), when photographing the abdomen, chest, etc., the organs are significantly blurred due to the effects of breathing, etc.

Therefore, by combining MRI with a respiratory synchronization device, for example, projection data of the part of the subject to be imaged is collected in synchronization with the respiratory level of the respiration curve, and an image is generated based on the projection data from each projection direction. If reconstruction is performed, each projection data will be data collected in a state where there is little movement of the subject's organs, so a clear reconstructed image without blur will be obtained.

By the way, some conventional breathing apparatuses utilize an impedance method in which a weak high-frequency current is passed through a subject and a breathing curve is extracted from the biological impedance that changes due to breathing.

However, in this method, the RF signal applied for data collection in MRI is induced and mixed into the respiration curve as noise, so it cannot be said to be a method suitable for the purpose. Furthermore, one of the important factors imparted to images in MRI is reconstruction based on data with different excitation repetition times T and I (that is, different signal intensities) for information collection. However, this appears as image non-uniformity, which is a major hindrance to advanced medical diagnosis.

Here, the MR phenomenon will be explained. In other words, many atomic nuclei each have their own unique rotation, or spin, and therefore have angular momentum. And since the nucleus has a load, its spin corresponds to a current flowing around its axis, generating a small magnetic field. Therefore, unless the spin is zero, the nucleus will have a magnetic moment (magnetic dipole).

Normally, the magnetic dipole of a nucleus with spin points in an arbitrary direction, but when placed in a magnetic field, the magnetic dipole orients in the direction of the magnetic field lines. In a nucleus with a spin of 1/2, such as the proton ('H), which is the nucleus of hydrogen, there are only two possible orientations for the dipole: parallel to the magnetic field or antiparallel (opposite direction).

The two orientations have slightly different energies, and this is called Zeeman splitting of energy levels.

The magnetic behavior of a group of atomic nuclei as a whole can be known by defining the macroscopic magnetization vector M. M represents the net value of all the magnetic moments of the nucleus of interest in the sample to be measured. If no external magnetic field is applied, the macroscopic magnetization is naturally zero, but when a magnetic field is applied to the sample, the nuclear dipoles become oriented, so macroscopic magnetization parallel to the magnetic field is generated. This direction is customarily defined as the Z axis.

The rotating nucleus behaves like a small "top" like a gyroscope.

When the axis of a rotating gyroscope is tilted from the vertical, it begins a so-called "precession" movement, which is macroscopically equivalent to a group of nuclei rotating in a magnetic field. Since the magnetization vector M is
When this is tilted from the Z-axis direction, M begins to precess around the Z-axis in the same way.

The way to tilt this M is to apply a very small magnetic field that rotates in the X, 7 plane perpendicular to the static magnetic field applied in the Z-axis direction.In reality, the rotating magnetic field is generated from a high-frequency power source. The method is to add it to the sample through a coil.

However, the frequency of the applied high-frequency magnetic field must match the precession frequency of the nuclei in the sample. That is, it causes a so-called resonance phenomenon, which is why it is called magnetic resonance.

From a quantum mechanical point of view, the tilting of the macroscopic magnetization formed by the collection of nuclei from the equilibrium position is the same as a transition from a low energy unit to a high energy unit, and the frequency of the high-frequency magnetic field is equal to the transition between two energy units. A transition occurs only when the difference in magnetic energy between

Angular frequency ω to resonate. is also called the Lamor frequency,
It has a simple mathematical relationship with the strength of an externally applied static magnetic field. That is, the frequency ω. is equal to the magnetic field strength Ho multiplied by the gyromagnetic ratio γ.

Here, the gyromagnetic ratio T is a constant specific to each nucleus whose spin is not zero. For example, the resonance frequency of a hydrogen nucleus (proton) in a magnetic field of 1 Tesla (10,000 Gauss) is 42.57 MH
The resonant frequency of phosphorus 31 (“P”) is 17.24 MHz in a 1 Tesla magnetic field. Both of these resonant frequencies are in the radio frequency band of the electromagnetic spectrum, and are more powerful than X-rays and visible light. Since it is much lower, it does not have the power to cut molecules in living organisms.Therefore, by appropriately selecting the frequency and tuning to a specific nuclide, the response signal can be isolated and observed.

MRI utilizes this, and a linear magnetic field gradient is added to the sample in a static magnetic field applied in the Z-axis direction.
Then, a rotating magnetic field necessary to tilt the magnetization vector M is applied with a high-frequency pulse, the magnetization vector M is tilted, and after the application of this high-frequency pulse is finished, the electromotive force generated in the receiving coil is multiplied by MR, and this signal is obtained. Spatial information is obtained by Fourier transforming the image, and image reconstruction is performed from this information.

Specifically, in diagnostic MRI, in order to obtain a tomographic image of a specific position of the subject to be examined, a very uniform static magnetic field H0 is applied to the subject P along the Z direction as shown in FIG.
is applied, and a linear magnetic field gradient is added to the static magnetic field H0 by the pair of gradient magnetic field coils 21A and 21B. A specific atomic nucleus has the aforementioned Lamor frequency ω for the static magnetic field H0. Therefore, a rotating magnetic field at an angular frequency that causes only specific atomic nuclei to resonate is transmitted through a pair of transmitting coils 22A and 22B to the subject P within the illustrated X-Y plane, which is set using the linear magnetic field gradient. The MR phenomenon is caused only in a specific slice portion E (which is a planar portion but actually has a certain thickness) from which a tomographic image is obtained. The MR phenomenon is caused by free induction attenuation (
FID: Free Induction Decay
) signal (FID signal) is observed as an MR multiplied signal, and by Fourier transforming this signal, a single spectrum for a specific nuclear spin rotation frequency can be obtained. In order to obtain a tomographic image as a CT image, projection images of the slice portion S in multiple directions within the X-Y plane are required. Therefore, after exciting the slice portion S to cause an MR phenomenon, As shown in the figure, a linear magnetic field gradient GXY having a linear gradient in the X'-axis direction (coordinate system rotated by θ° from the X-axis) is applied to the magnetic field H0. As a result, the isomagnetic field line E in the slice portion S of the subject P becomes a straight line, and the rotational frequency of a specific nuclear spin on that line can be expressed by the above-mentioned Lamor frequency relational expression.

Here, for convenience of explanation, each of the isomagnetic field lines E (El to En)
It is considered that a signal RI, ~D, (a type of FID signal) is generated. The amplitudes of the signals DI-D, , are proportional to the nuclear spin densities on the isomagnetic field lines E, -En passing through the slice portion E, respectively. However, the FID signal that is actually observed is a composite FID signal that is the sum of all D1 to D7, so by Fourier transforming this composite FID signal FID, the projection information of the slice portion S onto the X' axis (
One-dimensional image) PD is obtained. By rotating this X' axis within the X-Y plane, projection information in each direction within the X-Y plane is obtained in the same manner as described above, and image reconstruction is performed based on this projection information, that is, projection data. A CT image can be obtained by performing the processing.

In this way, MRI uses a magnetic field to cause resonance, and images are reconstructed based on the weak MR multiplied signals obtained at that time.Since a homogeneous and stable magnetic field is required, the magnetic field is Disturbing respiratory synchronization devices cannot be used.

[Purpose of the invention]

The present invention has been made in view of the above circumstances, and detects the state of respiration using a respiration sensor made of a non-magnetic material. It is an object of the present invention to provide a diagnostic magnetic resonance imaging apparatus that can display images of tomographic sections that are easily affected by respiratory motion as clear images without blurring the images.

[Summary of the invention]

In order to achieve the above object, the present invention obtains projection information in multiple directions of the spin density of a specific atomic nucleus based on magnetic resonance signals induced by a magnetic resonance phenomenon and taken out by a receiving coil, and makes a diagnosis by converting this projection information into an image. In a magnetic resonance imaging apparatus for use in magnetic resonance imaging, a body movement detecting means made of a non-magnetic material, a level setting means for setting a level in advance on a curve according to the body movement, and a level setting means for detecting the projection information when the level is within the set level by the level setting means. The present invention is equipped with an excitation generation control means that generates an excitation start signal for collecting information, collects information only in a range with little body movement, and performs image reconstruction to obtain a tomographic image with less blur. be able to.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

In FIGS. 1 and 2, 1 is a body movement detecting means made of a non-magnetic material, such as a breathing sensor, and includes an air bag 2, a tube 3 for transmitting pressure changes inside the air bag 2, and this tube. 3.Receive pressure changes and electrocardiogram signals,
A selection type A/D conversion circuit (hereinafter referred to as A/DMPX) 4 that performs A/D conversion as well as selection is provided to directly detect body movement and remove harmful noise sources from the MR multiplied signal. 5
is a synchronization control unit connected to the body movement detection means 1, which includes a level setting means 6 that sets a level in advance to a curve according to the body movement, and information of projection information when the level is within the level set by the level setting means 6. and excitation generation control means for generating an excitation start signal for acquisition. 7 is an MRI connected to the synchronization control unit 5.
This is an MRI system controller used by J. 8 is A/D
This is an arithmetic control circuit that receives signals and A/B marker signals from the MPX 4 and the level setting means 6, and transmits a comparator signal, a mono-multi signal, and a heartbeat/respiration synchronization signal for controlling the MRI system controller 7. 9 is an arithmetic control circuit 8
Display device 1 converts the electrocardiogram signal and respiratory signal from
0 is an input D/A conversion circuit, 11 is a mono multi signal that sends a mono multi signal to select and set the number of times of information collection, 12
For example, 13 is a DLY that performs image tube freezing of an ultrasonic diagnostic device (not shown), and 13 is an MR
This is an AND circuit that sends data to the I system controller.

The operation of the present invention configured as described above will be explained with reference to FIG. 3. The level setting means displays the respiration curve in FIG. 3(a) and the electrocardiogram waveform in FIG. The comparator level is set while looking at it, and the excitation generation control means in the arithmetic control circuit 8 obtains the comparator signal shown in FIG. 3(C) and transmits it to the MRI system controller 7.

Figure 3 (d) (e) only when the comparator signal operates
MR multiplied signal collection is performed as shown in . In this case, the third
In the figure, the repetition time TR is set to 5 as the MR multiplication signal acquisition method.
00m5. Furthermore, the delayed signal from the DLY 12 causes the image of the ultrasound diagnostic apparatus, that is, the ultrasound image obtained by emitting an ultrasound beam to a subject and receiving ultrasound echoes reflected from a predetermined region, to be stopped at a desired point. Diagnosis can be made by comparing with magnetic resonance images. Further, by adjusting the monomulti 11, the number of times information is collected can be freely selected for diagnosis.

Information collection methods include the gate priority method, in which information is collected continuously while the comparator signal is operating, as shown in Figure 3(e), and the MR double signal precision method, as shown in Figure 3(d). In consideration, it is possible to use a number-of-times priority method in which the comparator signal is only excited for collecting the MR multiplied signal at the start of operation, and information is collected for the same repetition time T11.

Comparative data when using both are shown in Figure 4 (a>
, (b) and will be explained.

The MR signal ■ is expressed by the following formula, and has a repetition time T.

It has been shown that when T is changed, the signal magnitude changes even in tissues with the same T.

■″(1e-TI/TI) However, T1 is the longitudinal relaxation time.

Figure 4 (a) and (b) both show the same T when T and I change.
, the level of organization is changing. For example, looking at the results in FIG.
=1000 signal strength ratio is 0.710.45 = 1.
56, which is approximately 1.6 times. Therefore, normally the T value of normal tissue depends on the magnetic field strength H0, but is 200~lO
It is considered to be OOms, and it is understood that it is important to align Tll.

It goes without saying that the present invention is not limited to the embodiments described above, and includes various modifications within the scope of the gist of the invention. For example, in order to eliminate pain to the subject as the body movement detection means 1, a non-magnetic thermistor may be used, or a Hg
, A device that detects changes in impedance of a non-magnetic conductive material such as ZnC1 due to body movement may be used.

Furthermore, as the excitation generation control means 7, one provided with means for selecting the number of times of information collection may be used.

〔Effect of the invention〕

The present invention can display tomographic images that are easily affected by respiratory motion, such as tomographic images and sagittal and coronal cross-sectional (longitudinal and transverse) images of the subject's abdomen and chest, as the best images that are less affected by body movements. .

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a diagnostic magnetic resonance imaging apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram showing body movement detection means, level setting means, and excitation generation control means, and FIG. ) to (e) are graphs showing the information collection state related to the comparator signal, Fig. 4(a) (b)
5 is a diagram showing the basic configuration of an example of diagnostic MHI, and FIG. 6 is a diagram showing the principle of obtaining information by the magnetic resonance phenomenon. DESCRIPTION OF SYMBOLS 1... Body movement detection means, 6... Level setting means, 7.
...Excitation generation control means. Agent Patent Attorney Ken Nori Chika Kanshu Norio Ogo Figure 4 (b)

Claims (8)

[Claims] (1) In a diagnostic magnetic resonance imaging device that obtains projection information in multiple directions of the spin density of a specific atomic nucleus based on magnetic resonance signals induced by a magnetic resonance phenomenon and extracted by a receiving coil, and converts this projection information into an image. , a body movement detecting means made of a non-magnetic material, a level setting means for setting a level in advance on a curve according to the body movement, and an excitation for collecting the projection information when the level is within the set level by the level setting means. 1. A diagnostic magnetic resonance imaging apparatus comprising: excitation generation control means for generating a start signal. (2) The diagnostic magnetic resonance imaging apparatus according to claim 1, wherein the body movement detection means uses a thermistor made of a non-magnetic material. (3) The body motion detection means is characterized by comprising an airbag, a tube that transmits pressure changes within the airbag, and a conversion means that converts the pressure changes transmitted by the tube into an electrical signal. A diagnostic magnetic resonance imaging apparatus according to claim 1. (4) Claims characterized in that the body movement detection means includes a non-magnetic conductive material wrapped around the outside of the body, and uses a device that detects changes in impedance of the non-magnetic conductive material due to body movement. 2. The diagnostic magnetic resonance imaging apparatus according to item 1. (5) The diagnostic magnetic resonance imaging apparatus according to claim 1 or 4, wherein the nonmagnetic conductive substance is Hg or ZnCl. (6) The diagnostic magnetic resonance imaging apparatus according to claim 1, wherein the excitation generation control means includes an acquisition frequency selection device for selecting the information acquisition frequency. (7) Diagnostic magnetic resonance imaging according to claim 1, wherein the excitation generation control means is set to only excite the first signal and not collect information within a predetermined period. Device. (8) The diagnostic magnetic resonance imaging apparatus according to claim 1, wherein the level setting means is capable of setting one or more levels.