JP2734061B2 - MR imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to an MR imaging apparatus that obtains in-vivo image information using nuclear magnetic resonance (NMR).

[Prior art]

In the field echo method, as shown in FIG. 4, a gradient magnetic field (Gz) for selecting a slice plane is added at the same time as applying a high frequency signal for excitation, and then a gradient magnetic field (Gy) for phase encoding is applied. Then, an NMR signal generated while applying a gradient magnetic field for frequency encoding (Gx) is sampled to collect data. This is repeated while changing the magnitude of the gradient magnetic field for phase encoding.
Repeat with TR. A high-speed imaging method for collecting data at high speed by shortening the repetition time TR of the field echo method is known. When an image is reconstructed from the data thus obtained, as shown in FIG. A linear artifact 52 in the frequency modulation direction (here, the X direction) is generated in the portion. This artifact 52 is caused by a reduction in the repetition time TR to generate a steady state signal (ADRIAN P. CRAWLEY et al., “Elimination of
Transverse Coherences in FLASH MRI ", MAGNETIC RESO
NANCE IN MEDICINE, 8, P248-260, 1988). Therefore, conventionally, as shown in FIG. 6, it has been proposed to add an extra gradient magnetic field Gz 'by randomly changing the magnitude of the gradient magnetic field Gz at the end of the repetition time TR (JENS FRAHM et al.). ., “Transverse Coherences in Rap
id FLASH NMR Imaging ", Journal of Magnetic Resonanc
e, 72, 307-314, 1987). This is based on the fact that the phase of the NMR signal generated each time is disturbed by Gz ', and the generation of a steady state signal is suppressed.

[Problems to be solved by the invention]

However, when the phase is disturbed by using the gradient magnetic field Gz as described above, there are the following problems. First, as shown in FIG. 7, the intensity of the gradient magnetic field Gz in the Z direction is inclined in the Z direction so that it does not change much at the center, so that the effect of disturbing the phase depends on where the slice plane is set. become. Therefore, when a slice plane is set near the center in the Z direction, 5 mse
It is necessary to generate a gradient magnetic field Gz of several tens of Gauss / m in the application time of c. In this case, a large current of several tens of amps must be applied to the gradient coil. As a result, a very large eddy current is induced in the magnet bobbin and the like, resulting in a problem that the image quality is deteriorated. The present invention can suppress the occurrence of a steady-state signal and remove artifacts in a high-speed imaging method using a field echo method with a reduced repetition time without causing a problem of image quality deterioration due to eddy current.
An object of the present invention is to provide an MR imaging apparatus.

[Means for Solving the Problems]

In order to achieve the above object, according to the present invention, an MR imaging apparatus for acquiring data by a field echo method with a reduced repetition time is provided with a coil for uniformly changing a static magnetic field regardless of location, and for each repetition time And generating a randomly changing offset magnetic field.

[Action]

If a magnetic field is generated by using a coil that uniformly changes the static magnetic field regardless of the location, the same magnetic field change can be generated regardless of the location. Therefore, if an offset magnetic field that changes randomly at each repetition time is generated from this coil, the effect of similarly disturbing the phase can be obtained no matter where the slice plane is set. Therefore, a very small magnetic field may be generated from this coil even at the maximum amplitude, and the current flowing through this coil may be smaller. As a result, the image quality does not deteriorate due to the eddy current. In a typical MR imaging device, in order to obtain the uniformity of the static magnetic field by the main magnet, a shim coil (so-called Gz0 shim coil) that uniformly changes the static magnetic field regardless of the location, and the gradient of the intensity of the static magnetic field by X, Y, Various shim coil groups such as shim coils that change in each direction of Z are provided. Thus, in such a normal MR imaging apparatus, the Gz0 shim coil is a coil that generates a random magnetic field for suppressing the generation of the steady state signal by disturbing the phase of the NMR signal generated at each repetition time TR. Can also be used.

【Example】

Next, an embodiment of the present invention will be described with reference to the drawings. In the MR imaging apparatus according to one embodiment of the present invention, a pulse sequence as shown in FIG. 1 is performed. This pulse sequence is basically
This is based on a high-speed imaging method based on a field echo method with a reduced repetition time TR. That is, the gradient magnetic field for selecting a slice plane is applied at the same time as applying the excitation high frequency signal.
Gz is added, then a gradient magnetic field Gy for phase encoding is added, and data is collected by sampling an NMR signal generated while adding a gradient magnetic field Gx for frequency encoding. This is repeated with a short repetition time TR while changing the magnitude of the phase encoding gradient magnetic field. According to the present invention, a Gz0 magnetic field is applied at the end of each repetition time TR.
The intensity is generated such that the intensity changes randomly at each repetition time. The Gz0 magnetic field changes the static magnetic field uniformly regardless of the location. FIG. 2 shows a system configuration of an MR imaging apparatus that performs the above-described pulse sequence. The subject 11 is laid on a bed 12 and sent into a static magnetic field formed by a main magnet 21. Inside the main magnet 21, three orthogonal axes (X,
A gradient magnetic field coil 22 for generating three gradient magnetic fields whose magnetic field strengths incline in the (Y, Z) directions, and an RF coil 23 are arranged. A shim coil 24 for correcting the uniformity of the static magnetic field is also provided. The gradient magnetic field coil 22 is connected to a gradient magnetic field power supply 31 and supplied with a current for generating a gradient magnetic field, and the shim coil 24 is connected to a correction magnetic field power supply 33 and supplied with a current for generating a correction magnetic field. The RF coil 23 is connected to the RF transmitting / receiving device 32, irradiates the subject 11 with an RF signal to excite it, and receives an NMR signal generated in the subject 11. The transmission and reception of these RF signals and the generation of the gradient magnetic field and the correction magnetic field are controlled by the measurement control device 34, and the data obtained from the received NMR signal is sent to the image reconstruction device 35, where the image such as a two-dimensional Fourier transform is obtained. Upon receiving the reconstruction process, the reconstructed image is sent to the image display device 36 and displayed.
The computer 37 performs overall control. The magnetic field Gz0 for uniformly changing the static magnetic field regardless of the location is, as shown in FIG. 3, two circular coils arranged inside the main magnet 21 and having the same radius r and the same distance r. In this embodiment, the Gz0 shim coil 25 of the shim coil 24 is used to generate the current. That is, the correction magnetic field power supply 33 is controlled by the measurement control device 34,
The correction magnetic field power supply 33 supplies the Gz0 shim coil 25 with the repetition time TR at a timing near the end of the repetition time TR.
A pulse-like current that changes randomly every time,
The Gz0 magnetic field as shown in the figure is generated. The random magnetic field to disturb the phase of the NMR signal for each repetition time TR, and the static magnetic field is changed uniformly
Since the Gz0 magnetic field is used, the intensity at the maximum amplitude of the Gz0 magnetic field may change the phase of the NMR signal to about 2π. Therefore, when the application time is set to 5 msec, it may be as small as about 0.047 Guas / m. Therefore, the current flowing through the Gz0 shim coil 25 may be as small as about several amperes, and the eddy current induced on the bobbin of the magnet may be so small as to be negligible, so that the image quality does not deteriorate.

【The invention's effect】

According to the MR imaging apparatus of the present invention, in a high-speed imaging method using a field echo method with a shortened repetition time, a random state that disturbs the phase of an NMR signal generated every time to suppress the occurrence of a steady state signal and remove artifacts. Since a strong magnetic field is generated by using a coil that uniformly changes the static magnetic field, a very small magnetic field is required, and a small current is required to flow through the coil, and the image quality does not deteriorate due to eddy current. .

[Brief description of the drawings]

FIG. 1 is a time chart showing a pulse sequence according to one embodiment of the present invention, FIG. 2 is a block diagram showing a system configuration of the embodiment, FIG. 3 is a schematic diagram showing a Gz0 shim coil, and FIG. FIG. 5 is a time chart showing a pulse sequence of the echo method, FIG. 5 is a diagram showing an artifact generated in a high-speed imaging method by the field echo method with a shortened repetition time, FIG. 6 is a time chart showing a conventional pulse sequence for artifact removal, and FIG. FIG. 7 is a diagram showing the distribution of the gradient magnetic field Gz intensity in the Z direction. 11 ... subject, 12 ... bed, 21 ... main magnet, 22
…… Gradient magnetic field coil, 23 …… RF coil, 24 …… Shim coil, 25 …… Gz0 shim coil, 31 …… Gradient magnetic field power supply, 32…
… RF transmitter / receiver, 33… correction magnetic field power supply, 34… measurement control device, 35… image reconstruction device, 36… image display device, 37
...... Computer, 51 ... Reconstructed image, 52 ... Artifact.

Claims (1)

(57) [Claims] An MR imaging apparatus for acquiring data by a field echo method with a shortened repetition time is provided with a coil for uniformly changing a static magnetic field regardless of a location, and an offset magnetic field that changes randomly at each repetition time is provided. An MR imaging apparatus characterized in that the image is generated.