JPH0577417B2 - - 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, relaxation time constant, etc. of a certain atomic nucleus existing in a specimen. The present invention relates to an MR imaging apparatus that obtains an image in which the magnetic field is reflected, and particularly relates to an MR imaging apparatus that corrects non-uniformity of a static magnetic field with high accuracy and obtains an MR image with high spatial resolution.

[Technical background of the invention]

For example, in a diagnostic MR imaging apparatus, a tomographic image at a certain position of a subject, that is, a spin density distribution image of a specific atomic nucleus on a tomographic plane is obtained in the following manner.

As shown in FIG.
Applying a very uniform static magnetic field H ₀ along the axial direction,
Furthermore, by a pair of gradient magnetic field coils 1A and 1B,
Linear magnetic field gradient Gz about the Z-axis direction in the static magnetic field H ₀
Add. 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, which is specific to the type of atomic nucleus. Therefore, a rotating magnetic field H ₁ having an angular frequency ω ₀ that makes only specific atomic nuclei resonate is applied to the subject P via the 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 by Fourier transforming the obtained MR signal, a single spectrum for the rotation angle local wave number of a specific nuclear spin can be obtained.

In order to obtain a tomographic image by computer tomography, projection images in multiple directions within the X-Y plane, which is a slice portion, are required. Therefore, after exciting the sliced part and causing the MR phenomenon, the magnetic field is
_When a linear magnetic field gradient _G This becomes a parallel straight line perpendicular to _{the XY} tilt direction, 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, we can obtain the projection information of the magnetic field gradient G of the slice part in _{the XY} direction, that is, the projection information of the one-dimensional image on the axis parallel to the above-mentioned isomagnetic field lines. get. 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, and changing the composite ratio of _the magnetic field gradients _G projection information 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 above-mentioned MR excitation, MR signal collection, and accompanying magnetic field gradient application sequence, and the above is just a basic example.

By the way, in this type of MR imaging device, it is difficult to avoid the drift of the uniform static magnetic field _H0 in the generating part and other parts, and it is difficult to maintain the uniformity of the static magnetic field over a long period of time. The static magnetic field tends to become non-uniform over time. If the static magnetic field is not completely uniform, the nuclei in different parts of the sample will feel slightly different magnetic fields, and as can be seen from equation (1), they will resonate at slightly different angular frequencies. For this,
Inhomogeneities in the static magnetic field cause distortions in the resulting image with a spread in angular frequencies. Therefore, in this type of device, it is necessary to maintain the uniformity of the static magnetic field with high precision.

[Purpose of the invention]

The present invention was made based on the above circumstances, and
The purpose of this is to use the subject to be imaged without adding any special external equipment, to compensate for the non-uniformity of the static magnetic field with high precision, and to capture MR images while always satisfying uniform conditions. The object of the present invention is to provide an MR imaging device that can perform this and, as a result, always obtain MR images with high spatial resolution.

[Summary of the invention]

To achieve this purpose, the MR according to the present invention
An imaging device obtains a uniform static magnetic field by superimposing an offset on a gradient magnetic field to compensate for non-uniformity of the static magnetic field. The MR echo signal is detected using only the gradient magnetic field in the direction perpendicular to the imaging target surface as a gradient magnetic field superimposed on the static magnetic field, and the peak value of the echo signal is attenuated to 1/e (e is a natural number). Detect time T2. Here, it is generally known that the echo signal decay time T2 is longest in a uniform static magnetic field. Therefore, FUC (Field
Uniformity Control).

[Embodiments of the invention]

The MR imaging apparatus according to the present invention will be explained below according to an embodiment shown in FIG.

In FIG. 2, 4A and 4B are air-core magnetic field coil systems for applying a uniform static magnetic field H ₀ to the subject P;
5 is a gradient magnetic field coil system for generating a gradient magnetic field having a magnetic field gradient Gz in the direction perpendicular to the fault plane, that is, in the Z-axis direction if the fault plane is the This is a gradient magnetic field coil system for generating a gradient magnetic field having a magnetic field gradient G _XY in each direction on the −Y plane.

7 is a high-frequency coil system for transmitting and receiving, and 8 is an in-phase component (real part) of the MR echo reception signal with respect to the reference signal.
and a 90° phase component (imaginary part) to obtain an MR echo signal. 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 (90° pulse). A transmitter that generates a selective excitation pulse (180° pulse) and transmits it via the high frequency coil system 7; 10 is a phase detector 8;
An A/D (analog-digital) converter that converts the MR echo detection output into a digital value; 11 an adder that averages the MR echo data output of the A/D converter 10 a predetermined number of times; 12, the adder 11 averages This is a fast Fourier transformer that performs fast Fourier transform on the added MR echo data.

13 is a static magnetic field power source that excites the air-core magnetic field coil systems 4A and 4B for generating the static magnetic field H ₀ ; 14 is a gradient magnetic field that controls the gradient magnetic field coil systems 5 and 6 based on the MR echo data obtained by the adder 11; 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 an image reconstruction device 15;
17 is a display device for displaying the image obtained in 1, and a timing control system 17 for managing the operation timing of each part in the above configuration.

Next, the operation of this embodiment configured as described above 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 set to a uniform static magnetic field H ₀ by the gradient magnetic field coil 5. Time-superimposed and simultaneously selective excitation pulses (90° pulse)
SEP from the transmitter 9 to the high frequency coil system 7 for transmitting and receiving.
is applied to the subject P within the magnetic field.

After finishing the application of the magnetic field gradient Gz and the selective excitation pulse _SEP , a gradient magnetic field with a magnetic field gradient _G , is superimposed on the static magnetic field H ₀ for a predetermined time by the gradient magnetic field coil system 6. After the application of this magnetic field _{gradient G}
NSEP is applied to the subject P from the transmitter 9 via the transmitting/receiving high-frequency coil system 7, and a gradient magnetic field having the above magnetic field gradient Gz is superimposed on the static magnetic field _H0 from the gradient magnetic field coil system 5 for a predetermined time. This magnetic field gradient
After the application _{of Gz} is completed, the gradient magnetic field having the above-mentioned magnetic field gradient G Receive.
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.

After that, the above operation is repeated several times, and the adder 11
Take the average of the MR echo data. Here, the purpose of taking the additive average is to improve the S/N.

The MR echo data averaged in this way is
A fast Fourier transformer 12 performs a Fourier transform to obtain the fourth
Obtain projection data as shown in the figure. This projection data is obtained in each direction by changing the direction of inclination of the magnetic field gradient _G.

Next, the case of F.U.C will be explained.

In the case of FUC, as shown in FIG. 5, MR echo data is obtained by applying an offset to a sequence obtained by excluding the application of the magnetic field gradient G _XY by the gradient magnetic field coil system 6 from the sequence for normal imaging.

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 selective excitation is applied from the high frequency coil system 7. A pulse SEP is applied to the subject P within the magnetic field. Then, the magnetic field gradient G _XY
A non-selective excitation pulse NSEP is applied to the subject P from the high-frequency coil system 7 without applying any voltage, and a gradient magnetic field with a gradient Gz is applied to the static magnetic field H ₀ by the gradient magnetic field coil system 5.
for a predetermined period of time.

Thereafter, the high frequency coil system 7
Receive MR echo signals from. 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.

After that, the above operation is repeated several times, and the adder 11
Take the average of the MR echo data. The MR echo data thus averaged is input to the gradient magnetic field controller 14.

In this gradient magnetic field controller 14, the input MR
Find the time T2 value for decay from the peak value of the echo data to 1/e, find the optimal offset value of the gradient magnetic field voltage so that this decay time T2 is the longest, and set the offset value of the gradient magnetic field voltage to the normal MR image. The inhomogeneity of the static magnetic field is corrected and the distortion of the MR image is corrected by automatically adding it at the time of imaging.

As described above, according to this embodiment, the static magnetic field
If non-uniformity occurs in H ₀ , by performing the FUC 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. .

The present invention is not limited to the embodiments mentioned and described above, but can be implemented with various modifications without departing from the gist of the present invention.

〔Effect of the invention〕

According to the present invention, by using the subject to be imaged without adding any special external equipment,
It is an object of the present invention to provide an MR imaging device that can correct non-uniformity of a static magnetic field with high precision, perform MR imaging while always satisfying uniform conditions, and as a result can always obtain MR images with high spatial resolution. can.

[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, and FIG. 5 is a timing chart showing the time schedule during FUC in the same embodiment. 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...Transmitter, 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.

Claims (1)

[Scope of Claims] 1. Static magnetic field generating means for generating a static magnetic field; Gradient magnetic field generating means for generating a gradient magnetic field in a predetermined direction by superimposing the static magnetic field; and a pulse for generating a high-frequency pulse at a magnetic resonance frequency. a generating means, a receiving means for detecting the MR signal generated in the subject, and a control for correcting the uniformity of the static magnetic field by controlling the offset of the gradient magnetic field using the MR signal received by the receiving means. A magnetic resonance imaging apparatus comprising: means. 2. The control means determines the decay time of the MR signal, which is a quantity reflecting the uniformity of the static magnetic field, from the received MR signal, and controls the offset of the gradient magnetic field so that the decay time becomes the longest. A magnetic resonance imaging apparatus according to claim 1. 3. The control means determines the decay time of the MR signal, which is a quantity reflecting the uniformity of the static magnetic field, by determining the decay curve of the received MR signal and determining the time for decay from the peak to a predetermined ratio. A magnetic resonance imaging apparatus according to claim 2, characterized in that: 4. Claims characterized in that the control means controls the gradient magnetic field generation means so as to superimpose an offset voltage on the voltage applied to the gradient magnetic field generation means based on the longest decay time. The magnetic resonance imaging apparatus according to item 2 or 3.