JPH11221200A - Mr imaging device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To dislocate the center of magnetic field specified by an inclined magnetic field to any eccentric position. SOLUTION: A pair of inclining coils 12 and 13 to form an inclined magnetic field is supplied with a current from two inclination power supplies 21 and 22 which are controllable independently, and the currents supplied are changed to any values by a controller 23, and thereby the center of the magnetic field (position where the intensity of the synthetic inclined magnetic field becomes zero) synthesized by the two coils 12 and 13 is shifted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an MR imaging apparatus for performing imaging by utilizing an NMR (nuclear magnetic resonance) phenomenon, and more particularly to an improvement in a gradient magnetic field generation system.

[0002]

2. Description of the Related Art In an MR imaging apparatus, a subject is arranged in a static magnetic field generated by a main magnet, a high-frequency signal is irradiated from the outside to the subject, and nuclear spin of protons in the tissue is applied to nuclear magnetic resonance. A phenomenon is caused and the nuclear magnetic resonance signal is received. At this time, a gradient magnetic field is applied to add position information to the resonance signal. The gradient magnetic field has a magnetic field intensity that changes (inclines) in a predetermined direction. By superimposing the gradient magnetic field on a static magnetic field, the resonance frequency is changed according to the position. Is added.

That is, in a conventional MR imaging apparatus, as shown in FIG. 5, a main magnet 11 for forming a strong static magnetic field in a space where a subject 10 is placed, and a gradient magnetic field superimposed on the static magnetic field are generated. Gradient coil 1
2 and 13 are provided. A transmitting / receiving coil 14 is mounted on the subject 10, and a high frequency power is supplied to the coil 14 by a transmitter 15 and a high frequency power amplifier 16,
Thereby, the subject 10 is irradiated with the high-frequency signal.

An echo signal generated by the subject 10 is sent to a receiver 18 via a transmitting / receiving coil 14 and a preamplifier 17, and the obtained data is sent to a sequence controller 24. The sequence controller 24 operates in cooperation with the host computer 25 to control the transmitter 15 to determine the frequency and waveform of the high-frequency signal, and to send a control signal to the gradient magnetic field controller 23 to transmit the gradient coils 12, 1, and 1.
The amplitude, waveform, and timing of the current flowing through 3 are determined. The collected data is processed in the host computer 25 to reconstruct an image.

The gradient coils 12 and 13 generate magnetic fields in directions opposite to each other, so that the coils 12 and
A synthetic magnetic field whose magnetic field strength is inclined is created in the space between the positions where 13 is placed. Conventionally, one gradient power supply (power amplifier) 26 is used, and a current of the opposite polarity is caused to flow through each of the gradient coils 12 and 13. Therefore, when the amplitude of the current is changed by the gradient magnetic field controller 23, the gradient of the gradient magnetic field is changed to a straight line 3 in FIG.
5, 36 can be changed. Note that the gradient magnetic field needs to be formed in each of the three-dimensional directions. For this reason, although only one pair of gradient coils 12 and 13 is shown in the figure, actually, three pairs of gradient coils (and their currents are transmitted). Power supply).

[0006]

However, in the conventional MR imaging apparatus, the position defined by the gradient magnetic field cannot be moved, so that there is a problem that the apparatus is unprotected against artifacts.

That is, in the conventional MR imaging apparatus, as described above, currents of opposite polarities are supplied from one gradient power supply 26 so that magnetic fields of opposite polarities are generated from the pair of gradient coils 12 and 13. Because
Even if the slope is changed by changing the current value (amplitude value),
The position where the gradient magnetic field becomes 0 (magnetic field center position)
There is no change as the center of each of the positions 2 and 13.

[0008] For this reason, in the case of 3D imaging (three-dimensional imaging), artifacts due to noise are formed on the central slice of the region of interest, which causes a problem that image diagnostic performance is impaired. In general, 3D imaging is highly sensitive and extremely sensitive to noise (drift). Drift noise has the property of being imaged on the center slice of the magnetic field, so that artifacts occur in the center slice of the region of interest. In addition, most of the application applications of 3D imaging are angiography. In this angiography, only a weak signal is obtained, and MIP (maximum intensity projection) processing is performed. It is imaged. Furthermore, the 3D imaging itself requires a long imaging time, which makes it more susceptible to drift noise that is not affected by normal imaging.

In view of the above, the present invention has been made so that the center of the magnetic field defined by the gradient magnetic field can be arbitrarily shifted, so that it is possible to easily deal with artifacts due to noise. The purpose is to provide.

[0010]

According to the present invention, there is provided a means for generating a static magnetic field, and a means for irradiating a subject placed in the static magnetic field with a high-frequency signal. An MR imaging apparatus comprising: a means for generating a gradient magnetic field for adding positional information; a means for receiving echo signals emitted from a subject to collect data; and a means for processing the data and reconstructing an image. Wherein the gradient magnetic field generating means is characterized by comprising a pair of coils and a pair of independently controllable current supply devices for supplying a current flowing through each of the coils.

A pair of coils for generating a gradient magnetic field form a magnetic field of opposite polarity, whereby the magnetic field strength changes between the two coils in a space sandwiched between the coils. A (tilted) magnetic field is synthesized. The currents flowing through these coils are respectively supplied from a pair of current supply devices, and their current values can be controlled independently. Therefore, if the current supplied to one coil (for example, the positive side coil) is increased and the current supplied to the other coil (the negative side coil) is reduced, the position where the combined magnetic field becomes zero is two coils. From the center to the negative coil. Since the center of the magnetic field defined by the gradient magnetic field can be arbitrarily shifted in this way, in the case of 3D imaging in which an artifact that forms an image at the center of the magnetic field occurs, shifting the center of the magnetic field reduces the slice where the artifact occurs ,
The diagnostic ability can be improved by following the edge of the region of interest so as not to be affected by the edge.

[0012]

Next, embodiments of the present invention will be described in detail with reference to the drawings. As shown in FIG. 1, an MR imaging apparatus according to an embodiment of the present invention generates a main magnet 11 for forming a strong static magnetic field in a space where a subject 10 is placed, and generates a gradient magnetic field superimposed on the static magnetic field. And gradient coils 12 and 13 for causing the coils to move. Current is supplied to each of the gradient coils 12 and 13 from two gradient power supplies (power amplifiers) 21 and 22 to form magnetic fields of opposite polarities. Therefore, it is possible to form a composite magnetic field having a tilted magnetic field strength in the space between the gradient coils 12 and 13.

This will be described in more detail. The horizontal axis in FIG. 2 indicates the direction in which the gradient coils 12 and 13 are arranged, and the vertical axis indicates the intensity of the gradient magnetic field. One coil generates a positive magnetic field (this superimposes an increasing magnetic field on the static magnetic field, and if it is used alone, it becomes like a dotted line 31), and the other coil generates a negative magnetic field (this is a static magnetic field). A magnetic field is superimposed with a decreasing magnetic field.
2), the intensity of the magnetic field synthesized in the middle thereof changes in the coil arrangement direction as indicated by a straight line 33. Therefore, if the currents flowing through both coils 12 and 13 are made unequal, the position where the combined magnetic field becomes 0 shifts from the center of both coils 12 and 13. For example, if the positive magnetic field is made larger than the negative magnetic field, a composite magnetic field such as the straight line 33 in FIG. 2 can be formed, and thus the magnetic field center 34 can be shifted in the direction of the coil generating the negative magnetic field. .

Although FIG. 1 shows only a pair of gradient coils 12 and 13 for forming a gradient magnetic field in one direction, a pair of gradient coils for forming a gradient magnetic field in other two axes perpendicular to the pair is shown. The same applies to the gradient coils, and a current is supplied independently from each pair of gradient power supplies. This allows
The center of the magnetic field can be shifted in any three-dimensional direction.

The other structure is the same as that of FIG. 5 described above. That is, the transmitting and receiving coil 1 attached to the subject 10
4, a transmitter 15, a high-frequency power amplifier 16 is provided,
The subject 10 is irradiated with a high-frequency signal. Subject 1
The echo signal generated at 0 is transmitted to the receiver 18 via the transmission / reception coil 14 and the preamplifier 17, and the obtained data is transmitted to the sequence controller 24, and further processed by the host computer 25 to reconstruct an image. The same is true for The sequence controller 24 operates based on the sequence information from the host computer 25, controls the transmitter 15 to determine the frequency and waveform of the high-frequency signal, and sends a control signal to the gradient magnetic field controller 23 to transmit the gradient coils 12 and 13. The amplitude, waveform and timing of the current flowing through the coil are determined.
5 is different from FIG. 5 in that the amplitudes of the currents flowing through 2 and 13 are independently determined.

Under the control of the host computer 25 and the sequence controller 24, the pulse sequence of 3D imaging as shown in FIG. Here, 90 °
A spin echo sequence that generates an echo signal using a pulse and a 180 ° pulse is represented by Gz as a gradient magnetic field for selective excitation, Gy as a gradient magnetic field for phase encoding, and Gx as a gradient magnetic field for reading (and frequency encoding).
Is repeated using.

For example, as shown in FIG. 4, a 90 ° pulse having a wide frequency band and a 180 °
By applying a gradient magnetic field Gz in the Z direction at the same time as the ° pulse, a large volume (shaded portion) including many slices is selectively excited at a time. And 90 °
A Gz pulse and a Gy pulse are applied between the pulse and the 180 ° pulse, and the pulse sequence is repeated many times while changing their amplitudes, whereby the position information in the Y direction is encoded into a phase and selectively excited. Phase encoding of the Z-direction position information (slice position information) in the volume.

In this case, if the gains of the gradient power supplies 21 and 22 are controlled independently so that the magnetic field center in the Z direction shifts (eccentric), the magnetic field center slice can be positioned at the end of the region of interest. As a result, artifacts due to drift noise can be caused in the slice located at the end of the selectively excited volume, and it is possible to prevent the image diagnosis of the region of interest from being disturbed.

The above description relates to the embodiment of the present invention, and the present invention is of course not limited to this description. For example, the overall configuration of the MR imaging apparatus can be another configuration. Further, it is only an example that the direction in which the region is selected is the Z direction, the direction in which the frequency is encoded is the X direction, and the direction in which the phase is encoded is the Y direction. Various imaging pulse sequences other than those shown in FIG. 3 can be employed.

[0020]

As described above, according to the MR imaging apparatus of the present invention, the center of the magnetic field can be decentered arbitrarily, thereby obtaining advantages such as easy handling of image artifacts. it can.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a graph showing a distribution of a gradient magnetic field intensity in one direction in the embodiment.

FIG. 3 is a time chart showing an example of a pulse sequence performed in the embodiment.

FIG. 4 is a schematic diagram showing a positional relationship among a subject, an excitation volume, and a slice.

FIG. 5 is a block diagram showing a conventional example.

FIG. 6 is a graph showing the distribution of the gradient magnetic field intensity in one direction in the conventional example.

[Explanation of symbols]

Reference Signs List 10 subject 11 main magnet 12, 13 gradient coil 14 transmitting and receiving coil 15 transmitter 16 high-frequency power amplifier 17 preamplifier 18 receiver 21, 22, 26 gradient power supply 23 gradient magnetic field controller 24 sequence controller 25 host computer

Claims (1)

[Claims] A means for generating a static magnetic field; a means for irradiating a subject placed in the static magnetic field with a high-frequency signal;
Means for generating a gradient magnetic field for adding position information,
In an MR imaging apparatus comprising: means for receiving and receiving data of an echo signal emitted from a subject; and means for processing the data and reconstructing an image, the gradient magnetic field generating means includes a pair of coils; Characterized by comprising a pair of independently controllable current supply devices for supplying a current flowing to each of the MR imaging devices.