JPH0322775B2 - - Google Patents

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention uses the magnetic resonance (hereinafter referred to as "MR") phenomenon to determine the spin density and relaxation time (relaxation time) of a specific atomic nucleus present in a specimen. ) Obtain images that reflect constants, etc. For example, in a diagnostic MR device
This relates to an MR imaging device.

[Technical Background of the Invention] For example, in a diagnostic MR apparatus, a tomographic image (for example, a spin density distribution image of a specific atomic nucleus on a tomographic plane) at a certain position of a subject is obtained as follows.

As shown in FIG.
Applying a very uniform static magnetic field H ₀ along the axial direction,
Furthermore, a linear magnetic field gradient Gz in the Z-axis direction is added to the static magnetic field H ₀ by a pair of gradient magnetic field coils 1A and 1B. A specific atomic nucleus resonates with the static magnetic field H ₀ at an angular frequency ω ₀ expressed by the following equation.

ω ₀ =γH ₀ ...(1) In this equation (1), γ is the gyromagnetic ratio and is unique to the type of atomic nucleus. So further,
A rotating magnetic field H ₁ with an angular frequency ω ₀ that causes only specific atomic nuclei to resonate is applied to the subject P via a pair of transmitting coils 2A and 2B. In this way, MR can be applied selectively to only the illustrated XY plane portion (a planar portion perpendicular to the Z axis, but actually has a certain thickness) that is selectively selected in the Z-axis direction by the magnetic field gradient Gz. A phenomenon occurs. This MR phenomenon is observed through a pair of receiving coils 3A and 3B, and the obtained
By Fourier transforming the MR signal, a single spectrum for the rotational angular frequency of a specific nuclear spin can be obtained. In order to obtain a tomographic image using a computer tomography (CT) method, projection images in multiple directions within the XY plane, which is a slice portion, are required. Therefore, after exciting the sliced portion to cause the _{MR phenomenon, a linear magnetic field gradient G XY having} a linear gradient in a specific direction on the When it acts,
The isomagnetic field lines in the sliced portion of the object P are parallel straight lines perpendicular to the inclination direction of the magnetic field gradient G _XY , and the rotational angular frequency of a specific nuclear spin on each of these isomagnetic field lines is expressed by the above equation (1). Therefore, by Fourier transforming the MR signal observed in such a state, the magnetic field gradient G of the slice part is projected in _{the XY} direction, that is, the projection information on the axis parallel to the above-mentioned isomagnetic field lines (one-dimensional image ). In this way, _the magnetic field gradient G _XY is rotated in the X-Y plane (the rotation of the magnetic field gradient _G
This is done by creating a magnetic field gradient G _XY as a composite magnetic field of G _Y and changing the composite ratio of the magnetic field gradients G _X and G _Y. ), X
- Projection information in each direction within the Y plane is obtained, and a tomographic image can be obtained by image reconstruction processing based on this information.

Note that there are various patterns for the sequence of MR excitation, MR signal collection, and accompanying magnetic field gradient application described above, and the above is only one basic example.

By the way, in this type of diagnostic MR device, it is difficult to avoid drifts in the uniform static magnetic field H ₀ generation area and other parts, and it is difficult to maintain a predetermined resonance condition for a long time. There is a tendency to deviate from the resonance condition as time progresses. If the resulting deviation Δω of the resonance frequency ω ₀ deviates by, for example, the order of several kHz in the case of a conventional device, resonance will no longer occur and MR excitation will no longer be possible. In addition, in a similar case, the resonant frequency shift Δω is several times
If the deviation is on the order of 10 Hz to several 100 Hz, excitation is possible, but the resulting image will be blurred or artifacts will appear. Therefore, in this type of device, the above deviation Δω is expressed as a number
Must be kept below Hz.

Generally, the following two methods can be considered as methods for matching the above resonance conditions.

(a) Adjust the static magnetic field H ₀ as variable.

(b) Adjust the resonance frequency ω ₀ as a variable.

The method (b) above has the following problems.

(b) Since the frequency band is wide, it is easy to pick up noise due to disturbances, and there is a risk of deterioration of the S/N (signal-to-noise ratio).

(b) To change the frequency band related to transmission and reception,
The circuit constants related to transmission and reception must be changed.

Therefore, in this case, method (a) above, that is, a method of making the static magnetic field H ₀ variable, does not have the problems of (a) and (b) above and is considered to be preferable. Consider maintaining stable resonance conditions.

The conventional method of variably adjusting the static magnetic field H ₀ to correct for deviations in the resonance conditions is to
The probe for detecting H ₀ deviation is
A method of correcting the static magnetic field H 0 using a probe that is provided separately from that for MR signal detection and using reference data set for correction by this probe, or a method of correcting the static magnetic field H ₀ using a magnetic sensor that is also provided _separately. There are methods to detect the deviation of the static magnetic field H ₀ and correct the static magnetic field H
).

[Problems with the background art] However, both of the above methods require the addition of special external equipment, and there are disadvantages such as the time required for correction being longer than necessary due to signal processing, etc. Ta. In contrast, by using the subject to be imaged, the inventors were able to accurately correct the deviation in the resonance conditions without adding any special external equipment, and to capture MR images while always satisfying the resonance conditions. As a result, the spatial resolution is always high.
We previously proposed a technology that made it possible to obtain MR images (Japanese Patent Application Laid-Open No. 59-200947). That is, by using only the gradient magnetic field in the direction perpendicular to the imaging target surface as the gradient magnetic field superimposed on the static magnetic field, MR
By detecting the echo signal and detecting the peak of the projection signal obtained by Fourier transforming it,
By detecting the frequency shift Δω and calculating the static magnetic field conversion part ΔH ₀ from ΔH ₀ =Δω/γ (2), and feeding back this static magnetic field fluctuation part ΔH ₀ to the static magnetic field power supply, the static magnetic field H ₀ (hereinafter, such control will be referred to as "magnetic field lock").

However, in this method, even if a frequency deviation of the static magnetic field H ₀ occurs, it cannot be corrected by adjusting the static magnetic field power supply during the image data collection process, especially during the long image data collection process. Sometimes it was a problem.

[Object of the Invention] The present invention aims to achieve spatial uniformity of the static magnetic field and temporal uniformity of the static magnetic field in the local area, and to reduce the frequency of the static magnetic field H ₀ even during long-term image data collection processing. MR images can be captured while always satisfying the resonance conditions, with no deviations, and without adding any special external equipment. The purpose is to provide an MR imaging device that can obtain MR images.

[Summary of the Invention] The present invention involves placing a subject in a static magnetic field generated by an electromagnetic coil type static magnetic field generator, and applying a gradient magnetic field and an excitation rotating magnetic field to the static magnetic field for signal collection according to a predetermined pulse sequence. superimpose the MR phenomenon on the intended tomographic plane of the object, and based on the induced MR signal, at least one of the spin density distribution and relaxation time constant distribution of a certain atomic nucleus on the tomographic plane of the object is determined. An MR imaging apparatus that obtains reflected image information includes means for obtaining and holding a resonance frequency of a region different from the imaging region in advance, and a first resonant frequency for collecting MR signals of the imaging region.
means for operating a second pulse sequence for collecting a pulse sequence of 1 and a MR signal of a region different from the imaging region; and a means for repeatedly executing the first pulse sequence to collect a group of MR signals of the imaging region. and a means for executing the second pulse sequence between repeated executions of the first pulse sequence, and an MR signal of a region different from the imaging region resulting from the execution of the second pulse sequence obtained during the operation of this means. and means for detecting a shift in resonance frequency for the region based on MR signals of a region different from the imaging region held in advance; A means for adjusting the static magnetic field generated by the static magnetic field generator by controlling the amount of electricity of the generator; It is characterized by achieving temporal uniformity in the local area.

[Embodiments of the invention] FIG. 2 shows the configuration of an embodiment of the present invention.

In Fig. 2, 4A and 4B are air-core magnetic field coil systems which are electromagnetic coil type static magnetic field generators for applying a uniform static magnetic field _H0 to the subject P, and 5 is a direction perpendicular to the tomographic plane (the tomographic plane If is the X-Y plane, then Z
A gradient magnetic field coil system for generating a gradient magnetic field having a magnetic field gradient Gz in the axial direction), 6 has a magnetic field gradient G _XY or -G _XY in the fault plane direction (each direction on the fault plane X-Y plane) This is a gradient magnetic field coil system for generating gradient magnetic fields. 7 is a high-frequency coil system for transmitting and receiving, and 8 is a (quadrature) two-phase detection to obtain an MR echo signal by detecting the in-phase component (real part) and 90° phase component (imaginary part) of the MR echo received signal with respect to the reference signal. 9 is a transmitter that generates selective excitation pulses (90° pulses) and non-selective excitation pulses (180° pulses) consisting of high-frequency pulses with an angular frequency ω, and transmits them via a high-frequency coil system 7; 11 is an A/D (analog-digital) converter that converts the MR echo detection output of the phase detection device 8 into a digital value; 11 is an adder that averages the MR echo data output of the A/D converter 10 a predetermined number of times; This is a fast Fourier transformer that performs fast Fourier transform on the MR echo data averaged by the adder 11.
Reference numeral 13 denotes a static magnetic field power supply that excites the air-core magnetic field coil systems 4A and 4B to generate a static magnetic field _H0 , and 14 a static magnetic field that controls the static magnetic field power supply 13 using information obtained by the fast Fourier transformer 12 during magnetic field lock operation. It is a controller. 15 is an image reconstruction device that performs image reconstruction processing based on the output of the fast Fourier transformer 12 (other than during magnetic field lock operation); 16 is a display device that displays the image obtained by the image reconstruction device 15; and 17 is a timing control system that manages the operation timing of each part in the above configuration.

Next, the operation in such a configuration will be explained.

First, the case of normal MR imaging will be described with reference to the timing chart shown in FIG.

In this case, first the air-core magnetic field coil systems 4A and 4B
is excited by the static magnetic field power supply 13 to apply a uniform static magnetic field H ₀ to the subject P. Next, in this state, a gradient magnetic field having a magnetic field gradient Gz in the direction (Z direction) perpendicular to the desired tomographic plane (X-Y plane) is superimposed on the uniform static magnetic field H ₀ for a predetermined time using the gradient magnetic field coil 5. and at the same time selective excitation pulse (90° pulse)
SEP is applied from the transmitter 9 to the subject P within the magnetic field via the high-frequency transmitting/receiving coil system 7. After completing the application of these magnetic field gradients Gz and selective excitation pulse SEP, the magnetic field gradient is applied in the direction along the above fault plane (predetermined direction on the X-Y plane) (with the static magnetic field _H0 still applied).
A gradient magnetic field _{having G} This magnetic field gradient G _XY
After the application of , the magnetic field gradient _Gz and the selective excitation pulse (180° pulse) SEP are transmitted from the transmitter 9 via the transmitting/receiving high frequency coil system 7 and the gradient magnetic field coil system 5 (with the static magnetic field H 0 still applied). The magnetic field gradient Gz is applied to the specimen P, and the magnetic field gradient Gz is continued to be superimposed on the static magnetic field _H0 for a predetermined period of time. After applying this magnetic field gradient Gz,
While superimposing a gradient magnetic field having _the above-mentioned magnetic field gradient _G
Receive echo signals. The phase detection device 8 performs phase detection on the MR echo signal received by the high frequency coil system 7, and the detected waveform of this MR echo signal is converted into a digital value by the A/D converter 10 and input to the adder 11. Thereafter, the above-described operation is repeated several times, and the adder 11 averages the MR echo data. Here, the purpose of taking the additive average is to improve the S/N. The thus averaged MR echo data is Fourier-transformed by a fast Fourier transformer 12 to obtain projection data as shown in FIG. This projection data is obtained in each direction by changing the inclination direction of the magnetic field gradient G _XY , and based on this data, the image reconstruction device 15 performs reconstruction processing to form an image, which is displayed on the display device 16 .

Next, the case of magnetic field lock will be explained.

In the case of magnetic field lock, as shown in Fig. 5, the gradient magnetic field coil system 6 is changed from the sequence for normal imaging.
Obtain projection data using a sequence that excludes the application of the magnetic field gradient G _XY .

That is, the air-core magnetic field coils 4A, 4 are attached to the subject P.
A uniform static magnetic field H ₀ is applied at B, and in this state, a gradient magnetic field with a gradient Gz in the direction perpendicular to the desired fault plane is superimposed for a predetermined time by the gradient magnetic field coil system 5, and at the same time a selective excitation pulse is applied from the high frequency coil system 7. SEP (90° pulse) is applied to the subject P within the magnetic field. Next, the magnetic field gradient Gz and the selective excitation pulse SEP (180° pulse) are applied to _the subject P from the high frequency coil system 7 and the gradient magnetic field coil system 5 (without applying the magnetic field gradient G continues to be superimposed on the static magnetic field H ₀ for a predetermined time. Thereafter, the high frequency coilel system 7 receives the MR echo signal from the subject P. Then, as in the case of normal imaging, the phase of the received MR echo signal is detected by the phase detection device 8, and this detected waveform is converted into a digital value by the A/D converter 10 and input to the adder 11. Thereafter, the above-described operation is repeated several times, and the adder 11 averages the MR echo data. The thus averaged MR echo data is Fourier-transformed by a fast Fourier transformer 12 to obtain projection data as shown in FIG. This projection data is input to the static magnetic field controller 14, which was not used during the normal imaging described above.

This static magnetic field controller 14 detects the maximum peak of the input projection data, calculates the frequency shift Δω (see Figure 6), and calculates the static magnetic field variation ΔH ₀ from the above equation (2), The static magnetic field power supply 13 is controlled in accordance with this, and is specifically configured as shown in FIG. 7, for example.

In FIG. 7, 14-1 is a Δω detector which calculates the frequency deviation Δω from the value on the frequency axis where input projection data takes the maximum value, and 14-2 is a Δω detector.
The multiplier 14-3 calculates the static magnetic field variation ΔH by applying the above equation (2) to the frequency deviation Δω obtained by the detector 14-1 and multiplying it by 1/γ. D/A (digital-analog) that converts the static magnetic field variation ΔH into an analog value as the static magnetic field correction amount
It is a converter. Then, the output of the D/A converter 14-3 is supplied to the static magnetic field power supply ΔH to correct the static magnetic field H ₀ and eliminate the frequency deviation Δω.

Here, in the present invention, preprocessing is performed before image data collection. Here, preprocessing is processing for obtaining reference data used for static magnetic field adjustment, and
For example, the sequence for magnetic field lock shown in FIG. 5 is executed once for side A (image data collection side) in the figure to collect projection data, and then the sequence for side B (image data collection side) shown in FIG. Regarding the data collection aspect), the sequence for magnetic field lock shown in FIG. 5 is executed once, for example, to collect projection data. Then, the frequency of the A plane is set as the resonance frequency in image data collection, and the frequency of the B plane is held for use in adjusting the static magnetic field.

Next, regarding image data collection, the sequence shown in FIG. 3 is executed multiple times in order to collect a group of image data for side A. During execution of the sequence shown in FIG. 3, the sequence shown in FIG. The frequency deviation Δω from the frequency is determined, and the same processing as the above means is performed to calculate ΔH, the static magnetic field H ₀ is corrected, and the frequency of the A plane is adjusted to the resonance frequency.

Here, the method of obtaining a resonance signal from the B side is to excite the A side, obtain the resonance signal of the A side, and then use the recovery time of the A side to excite the B side at an excitation frequency different from the excitation frequency of the A side. By selectively excitation, a B-plane resonance signal can be obtained.

In this way, since the configuration was designed to achieve spatial uniformity of the static magnetic field and temporal uniformity of the static magnetic field in the local area, the static magnetic field H ₀ due to drift etc.
etc. fluctuates, causing a shift in angular frequency Δω,
By operating the magnetic field lock described above, the static magnetic field H ₀ can be appropriately corrected in a short time, and there is no need to additionally use a special external device for correction, and image data can be collected for a long time. No frequency shift occurs during processing.

It should be noted that the present invention is not limited to the embodiments described above and shown in the drawings, but can be implemented with various modifications without changing the gist thereof.

For example, in the configuration shown in FIG.
1. Fast Fourier transformer 12, static magnetic field controller 14
It is also possible to replace part or all of the timing control system 17 with an electronic computer and realize these functions through software processing by the electronic computer. Further, in this case, the electronic computer constituting the image reconstruction device 15 may be shared, and in this case, the hardware configuration can be exactly the same as that required for normal imaging.

FIG. 9 shows a control flowchart of the magnetic field lock when the adder 11, Fourier transformer 12, static magnetic field controller 14, and timing control system 17 are replaced with electronic computers. In this case, "writing ΔH ₀ in the control section" means giving ΔH ₀ as a correction amount to the part that generates the output to the static magnetic field power supply 13, and the static magnetic field power supply 13 is controlled accordingly. Also, the value of this ΔH ₀ is 0 in the initialized state.
is set to . This figure 9 is actually the second
The operation of the relevant part in the figure is no different from that shown in the flowchart.

[Effects of the Invention] According to the present invention, since the structure is configured to achieve spatial uniformity of the static magnetic field and temporal uniformity of the static magnetic field in the local area, imaging can be performed without adding any special external device. Since the static magnetic field power supply can be corrected during the image data acquisition process using a subject to be examined, deviations in resonance conditions can be corrected with high precision, and MR images can be performed while always satisfying the resonance conditions, resulting in consistently high spatial resolution. MR capable of obtaining high MR images of
An imaging device can be provided.

[Brief explanation of drawings]

Fig. 1 is a schematic perspective view for explaining the principle of an MR imaging device, Fig. 2 is a block diagram showing the configuration of an embodiment of the present invention, and Fig. 3 is a time schedule for normal imaging in the same embodiment. 4 is a diagram showing an example of projection data obtained during the same imaging. FIG. 5 is a timing chart showing the time schedule during magnetic field lock in the same example. FIG. 6 is a diagram showing an example of the projection data obtained during the same imaging. FIG. 7 is a block diagram showing in detail an example of a specific configuration of the main parts in the same embodiment, and FIG. 8 explains the main parts of the same embodiment. Figure for, No. 9
The figure is a flowchart showing the main processing in another embodiment of the present invention. 4A, 4B...Air core magnetic field coil system, 5,6...
Gradient magnetic field coil system, 7... High frequency coil system, 8...
...Phase detection device, 9...Receiver, 10...A/D
Converter, 11... Adder 12... Fast Fourier transformer, 13... Static magnetic field power supply, 14... Static magnetic field controller, 15... Image reconstruction device, 16... Display device, 17... Timing control System, 14-1
...Δω detector, 14-2... Multiplier, 14-3
...D/A converter.

Claims (1)

[Claims] 1. A subject is placed in a static magnetic field generated by an electromagnetic coil type static magnetic field generator, and a gradient magnetic field and an excitation rotating magnetic field are superimposed on the static magnetic field for signal collection according to a predetermined pulse sequence. , causing a magnetic resonance phenomenon in a predetermined tomographic plane of the subject, and based on the induced magnetic resonance signal, at least one of the spin density distribution and relaxation time constant distribution of a certain atomic nucleus in the tomographic plane of the subject is determined; A magnetic resonance imaging apparatus that obtains reflected image information includes: means for obtaining and holding a resonance frequency of a region different from the imaging region in advance; a first pulse sequence for collecting magnetic resonance signals of the imaging region; means for operating a second pulse sequence for collecting magnetic resonance signals in a region different from the imaging region; and repeatedly executing the first pulse sequence to collect a group of magnetic resonance signals in the imaging region; means for executing the second pulse sequence between repeated executions of the first pulse sequence; and magnetic resonance of a region different from the imaging region obtained during the operation of the means and accompanied by the execution of the second pulse sequence. means for detecting a shift in resonance frequency for the region based on a signal and a pre-held magnetic resonance signal of a region different from the imaging region; and the electromagnetic coil method based on the shift in resonance frequency detected by the means. A magnetic resonance imaging apparatus comprising: means for adjusting the static magnetic field generated by the static magnetic field generator by controlling the amount of electricity of the static magnetic field generator.