JPS61254839A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To always obtain an MR (magnetic resonance) image with high spatial resolution, by detecting frequency deviation of a static magnetic field H0 at positions other than the position for collecting picture data and synthesizing frequencies by adjusting a power source for static magnetic field even when picture data collection is processed. CONSTITUTION:Air core coil systems 4A and 4B are excited by means of a static magnetic field power source 13 and a uniform magnetic field H0 is impressed upon an object P to be inspected. Then an inclined magnetic field having a magnetic field gradient GZ in the direction vertical to the cross section is superposed by means of a coil 5 under this condition and, simultaneously, a selected excitation pulse SEP is impressed upon the object P to be inspected from a transmitter 9. After the magnetic field gradient GZ and pulse SEP are impressed, an inclined magnetic field having a magnetic field gradient GXY in the direction along the cross section is superposed upon the static magnetic field H0. Thereafter, an MR echo signal from the object P to be inspected is received by means of a transmitting-receiving high-frequency coil system 7. The signal is processed by an adder 11 and an arithmetic mean is obtained. This data is Fourier-transformed by means of a fast Fourier transformer 12 and projecting data are obtained. This data are found for each direction by changing the inclining direction of the magnetic field gradient GXY.

Description

Detailed Description of the Invention [Technical Field of the Invention] The present invention uses magnetic resonance (hereinafter referred to as rMRJ) phenomenon to determine the spin density,
The present invention also relates to an MR imaging apparatus, such as a diagnostic MRtii system, which obtains images that reflect relaxation time constants and the like.

[Technical background of the invention II] 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 specific position of a subject is obtained as follows.

As shown in FIG. 1, a very uniform static magnetic field Ha along the illustrated Z-axis direction is applied to the subject P, and a pair of gradient magnetic field coils 1A and IB are applied to the static magnetic field Ha in two axial directions. Add a linear magnetic field gradient Gz. A specific atomic nucleus resonates with the static magnetic field Ha at an angular frequency ω0 expressed by the following equation.

ωa−γHa (1) In equation (1), γ is the gyromagnetic ratio and is specific to the type of atomic nucleus. Therefore, a rotating magnetic field H1 with an angular frequency of 0 that makes only specific atomic nuclei resonate is applied to a pair of transmitting coils 2A.
, 2B on the subject P. In this way, the MR can be selectively performed only on the X-Y plane portion shown in the figure, which is selected in the Z-axis direction by the magnetic field gradient Gz (although it is a planar portion perpendicular to the Z-axis, in reality it has a certain thickness). A phenomenon occurs. This MR phenomenon is caused by a pair of receiving coils 3A and 3B.
By Fourier transforming the obtained MR multiplied 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 computerized tomography (CT) method, projection images in multiple directions within the X-Y plane, which is a slice portion, are required.

Therefore, after exciting the sliced portion to cause the MR phenomenon, a linear magnetic field gradient GXY having a linear gradient in a specific direction on the X-Y plane is applied to the magnetic field HO (by a coil, etc. not shown). Then, 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 Gxy, and the rotational angular frequency of a specific nuclear spin on each of these isomagnetic field lines is given by the above equation (1). Therefore, by Fourier transforming the MR multiplied signal observed in such a state, projection information in the magnetic field gradient GXY direction of the sliced portion, that is, projection information (one-dimensional image) on the axis parallel to the above-mentioned isomagnetic field lines is obtained. . In this way, the magnetic field gradient GXY is rotated within the X-Y plane (the rotation of the magnetic field gradient Gxy is performed using, for example, two pairs of gradient magnetic field coils, and the magnetic field gradient GXY is rotated as a composite magnetic field of the magnetic field gradients Gx and Gy in each of the X and Y directions. magnetic field gradient G
This is done by creating xy and changing the composite ratio of the magnetic field gradients Gx and Gy. ), projection information in each direction within the XY plane can be obtained in the same manner as described above, 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 multiplied signal collection, and application of magnetic field gradients in connection with these, and the above is just a basic example.

By the way, in this type of diagnostic MR apparatus, it is difficult to avoid drifts in the uniform static magnetic field HO generation part and other parts, and it is difficult to maintain a predetermined resonance condition for a long period of time. There is a tendency for the resonance condition to deviate from the resonance condition. If the resulting deviation Δω of the resonance frequency 0 is on 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. Moreover, in the same case, the resonant frequency shift Δω is several tens of H.
If the deviation is on the order of 2 to several 100 H2, excitation is possible, but the resulting image may be blurred or have artifacts. Therefore, in this type of device, the above-mentioned deviation Δω must be kept below -12.

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

(a) Adjust the static magnetic field HO as variable.

(b) Adjusting the resonance frequency ωa 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) In order to change the frequency band related to transmission and reception, it is necessary to change the circuit constants related to transmission and reception.

Therefore, in this case, the method (a) above, that is, the method of making the static magnetic field Ho variable, is the method (b) above.

Since there is no problem (b) and it is considered to be preferable, we will consider stably maintaining the above-mentioned resonance conditions using this method.

The conventional method of variably adjusting the static magnetic field HO to correct deviations in the resonance conditions is to provide a probe for detecting deviations in the static magnetic field (IO) separately from the original one for detecting MR signals; There is a method of correcting the static magnetic field HO using the probe based on reference data set for correction by this probe, or a method of detecting the deviation of the static magnetic field HO using a separately provided magnetic sensor and correcting the static magnetic field HO. Both of these processes were executed only before performing MR signal data collection processing (hereinafter referred to as "image data collection processing") for obtaining images.

[Problems in 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 photographed, the inventors have been able to correct the deviation in resonance conditions with high precision without adding any special external equipment, and the Wi We proposed a technology that makes it possible to obtain MR images with high spatial resolution at all times.

In other words, MR
By detecting the echo signal and detecting the peak of the projection signal obtained by Fourier transforming it, the frequency deviation Δω is detected, and the static magnetic field variation ΔHO is calculated from γ. Adjusting the static magnetic field HO and matching the frequency by feeding back to the magnetic field power supply (hereinafter, such control is referred to as "11 field lock")
).

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

[Object of the Invention] The present invention is capable of correcting deviations in resonance conditions with high precision without causing any deviation in the frequency of the static magnetic field HO even during long-term image data collection processing, and without adding any special external equipment. It is an object of the present invention to provide an MR imaging apparatus that can perform m-images of MR images while always satisfying resonance conditions, and as a result can always obtain MR images with high spatial resolution.

[Summary of the Invention] The present invention is characterized by applying a magnetic field locking method using multi-slicing technology, and adjusting the frequency of the static magnetic field HO in a region other than the region where image data is collected. It is characterized by detecting deviation and adjusting the static magnetic field power source to match the frequency even during image data collection processing.

[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 for applying a uniform static magnetic field HO to the subject P, and 5 is a direction perpendicular to the tomographic plane (if the tomographic plane is the X-Y plane, the Z-axis 6 is a gradient magnetic field coil system for generating a gradient magnetic field having a magnetic field gradient Gz in the direction); 6 is a gradient coil system having a magnetic field gradient G X Y l or -GXY in the fault plane direction (each direction on the fault plane This is a gradient magnetic field coil system that generates a magnetic field. 7 is a high-frequency coil system for transmitting and receiving, and 8 is an in-phase component (real part) and a 90° phase component (with respect to the reference signal of the MR echo reception signal).
imaginary part) to obtain an MR echo signal (quadrature) two-phase detection type phase detection device; reference numeral 9 indicates a selective excitation pulse (90° pulse) consisting of a high-frequency pulse with an angular frequency ω and a non-selective excitation pulse (180° pulse); 10 is a phase detection device 8
A/D that converts the MR echo detection output of
(analog-digital) converter; 11 is an adder that averages and corrects the MR echo data output of the A/D converter 10 a predetermined number of times; 12 is a high-speed Fourier transform of the MR echo data averaged by the adder 11; It is a Fourier transformer. 13 is a static magnetic field power source that excites the air-core magnetic field coil systems 4A and 4B to generate the static magnetic field HO; 14 is a static magnetic field w that controls the static magnetic field power source 13 based on information obtained by the fast Fourier transformer 12 during magnetic field locking operation. It is a controller. 15 is the fast Fourier transformer 12 (other than during magnetic field lock operation)
Image reconstruction device that performs image reconstruction processing based on output, 1
6 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 explained with reference to the timing chart shown in FIG.

In this case, the air-core magnetic field coil systems 4A and 4B are first excited by the static magnetic field power supply 13, and a uniform static magnetic field HO is applied to the subject P.
Apply. Next, in this state, the required tomographic plane (X-Y
A gradient magnetic field having a magnetic field gradient G2 in the direction (two directions) perpendicular to the plane) is superimposed on the uniform static magnetic field HO for a predetermined time by the gradient magnetic field coil 5, and at the same time a selective excitation pulse (90°
A pulse) SEP is applied from the transmitter 9 to the subject P within the magnetic field via the transmitting/receiving high frequency coil system 7. After finishing the application of these magnetic field gradient Gz and selective excitation pulse SEP, (
A gradient magnetic field having a magnetic field gradient GXY in the direction along the above-mentioned tomographic plane (a predetermined direction on the X-Y plane) is superimposed on the static magnetic field HO for a predetermined time by the gradient magnetic field coil system 6, while the static magnetic field (-1o remains applied). After the application of the magnetic field gradient GXY is completed, the magnetic field gradient Gz and the selective excitation pulse (180@pulse) SEP are sent from the transmitter 9 to the high-frequency coil system 7 and the gradient magnetic field coil system 5 (with the static magnetic field HO still applied). The magnetic field gradient Gz is applied to the subject P through the static magnetic field HO.
continues to be superimposed on for a predetermined period of time. After the application of the magnetic field gradient Gz is completed, the gradient magnetic field having the magnetic field gradient GXY is superimposed on the static magnetic field HO from the gradient magnetic field coil system 6 for a predetermined period of time, and the MR echo signal from the subject P is received by the transmitting/receiving high frequency coil system 7. do.

The phase of the MR echo signal received by the high frequency coil system 7 is detected by the phase detection device 8, and the detected waveform of this MR echo signal is converted into a digital value by the A/D converter 10, and the detected waveform is converted into a digital value by the adder 1.
Enter 1. Thereafter, the above-described operation is repeated several times, and the adder 11 averages the MR echo data. Here, the purpose of taking the 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 for each direction by changing the gradient direction of the magnetic field gradient Gxy, and based on this, the image reconstruction device 115
performs reconstruction processing and converts it into an image, which is then displayed on the display device 16.
Displayed by

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

In the case of magnetic field lock, projection data is obtained by a sequence obtained by excluding the application of the magnetic field gradient GXY by the gradient magnetic field coil system 6 from the sequence in the case of normal image transfer, as shown in FIG.

That is, a uniform static magnetic field Ho is applied to the subject P by the air-core magnetic field coils 4A and 4B, and in this state, a gradient magnetic field with a gradient Gz in the direction perpendicular to the desired tomographic plane is superimposed for a predetermined time by the gradient magnetic field coil system 5. At the same time, a selective excitation pulse SEP (90° pulse) is applied from the high-frequency coil system 7 to the subject P within the magnetic field. Next, the magnetic field gradient Gz and 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 Gxy), and the gradient magnetic field of the gradient Gz is further applied. It continues to be superimposed on the static magnetic field HO for a predetermined time. Thereafter, the MR echo signal from the subject P is received by the high frequency coil system 7. 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 MR echo data thus averaged 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, determines the frequency shift Δω (see Figure 6), determines the static magnetic field variation ΔHO from the above (2 formula), and It controls the static magnetic field power supply 13, and is specifically configured as shown in FIG. 7, for example.

In FIG. 7, 14-1 is a Δω detector that calculates the frequency deviation Δω from the value on the frequency axis where the input projection data takes the maximum value, and 14-2 is the Δω detector that calculates the frequency deviation Δω obtained by the Δω detector 14-1. (A multiplier that applies the '2J formula and multiplies by 1/Y to calculate the static magnetic field variation ΔH. 14-3 is an analog value that uses the static magnetic field variation ΔH calculated by the multiplier 14-2 as the static magnetic field correction amount. This is a D/A (digital-to-analog) converter for converting into a D/A converter 14-.
The output of No. 3 is supplied to the static magnetic field power supply ΔH to correct the static magnetic field ()-1o and eliminate the frequency deviation Δω.

Here, in the present invention, as a preprocessing before the image data collection process, the magnetic field shown in FIG. Collect projection data in the lock sequence and match the frequency of side A to the resonance frequency. At this time, the frequencies of 8 planes are held.

Next, on side A, image data collection processing is in progress (sequence is 3rd
Figure), the sequence for detecting the frequency of side B is Figure 5),
Frequency deviation Δω from the frequency of surface B obtained in preprocessing
is calculated, ΔH is calculated by performing the same processing as the above means, and the static magnetic field HO is corrected to match the frequency of the A plane to the resonance frequency.

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

In this way, when the static magnetic field HO etc. fluctuates due to drift etc. and an angular frequency deviation Δω occurs, by performing the magnetic field locking operation described above, the static magnetic field HO can be appropriately corrected in a short time, and the correction is Therefore, there is no need to additionally use a special external device, and no frequency shift occurs even during long-term image data collection processing.

Note 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. 2, part or all of the adder 11, fast Fourier transformer 12, static magnetic field controller 14, and timing control system 17 may be replaced with electronic computers, and these functions may be implemented using computer software. It can also be realized through processing. 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 magnetic field locking 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 Δ1-1o in the control section] means that the static magnetic field power supply 1
This means that ΔHO is given as a correction amount to the part that generates the output to 3, and the static magnetic field power supply 13 is controlled accordingly. Moreover, the value of this ΔHo is O in the initialized state.
is set to . This FIG. 9 is substantially the same as the flowchart showing the operation of the relevant portion in FIG. 2.

[Effects of the Invention] According to the present invention, without adding any special external device,
Since the static magnetic field power supply can be corrected during the image data collection process using the object to be imaged, deviations in resonance conditions can be corrected with high precision, and MR imaging can be performed while always satisfying the resonance conditions. As a result, the spatial It is possible to provide an MR imaging device that can obtain MR images with high resolution.

[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 during normal imaging in the same embodiment. Fig. 4 is a timing chart showing an example of projection data obtained during the same shooting, Fig. 5 is a timing chart showing a time schedule during magnetic field lock in the same example, and Fig. 6 is a timing chart showing an example of projection data obtained during the same magnetic field lock. FIG. 7 is a block diagram showing in detail an example of a specific configuration of the main parts of the embodiment; FIG. 8 is a diagram for explaining the main parts of the embodiment; FIG. 9 is a flowchart showing 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. Applicant's agent Patent attorney Takehiko Suzue Figure 1 A Figure 2 Figure 31! ! 11 41st I Q Li2 5th gm Figure 6 Figure 7 Figure 8R A side Bffil

Claims (1)

[Claims] A static magnetic field generator that generates a uniform static magnetic field; a gradient magnetic field generator that generates a gradient magnetic field in a predetermined direction by superimposing the uniform static magnetic field; a selective excitation pulse signal consisting of a high-frequency pulse at a magnetic resonance frequency; A transmitter that generates a non-selective excitation pulse signal; and a transmitter that applies a selective excitation pulse and a non-selective excitation pulse to a subject within the static magnetic field based on the signal generated by the transmitter, and a magnetic resonance signal generated in the subject. It is equipped with a transmitter/receiver that detects the magnetic resonance of a specific atomic nucleus in a subject, and a control processor that controls the operation of each of the above components and performs signal processing based on the signal received by the transmitter/receiver to form an image. In a magnetic resonance imaging apparatus that obtains image information reflecting the density distribution, relaxation time constant, or information related to the density distribution of the atomic nucleus from the induced resonance signal, multi-slicing technology is applied to the above-mentioned object for image data collection. means for collecting projection data in a sequence of magnetic field locking by setting a first surface and a second surface for data collection for magnetic field locking;
means for detecting the frequency of the second surface during image data collection processing on the first surface; and means for correcting the static magnetic field from the static magnetic field generator based on the detection output from the means. A magnetic field resonance imaging device characterized by comprising: