JPH0811117B2 - Pulse sequence method in MRI - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a pulse sequence method in MRI for simultaneously obtaining magnetic resonance signals from two tomographic planes of a subject arranged in a static magnetic field.

(Prior Art) In a conventionally used MRI, that is, a magnetic resonance imaging apparatus, an MR (radio frequency wave) pulse is applied to an object placed at a predetermined position in a static magnetic field to perform MR.
A signal is obtained and the signal is subjected to, for example, Fourier transform to obtain a tomographic image of the subject.

In such a magnetic resonance imaging apparatus, adjacent tomographic planes of a specific part of the subject are sequentially excited with time.
Obtaining MR signal. This is also called multi-slice. That is, the object P shown in FIG.
In order to obtain images on the specific tomographic planes adjacent to each other indicated by S, T, and U, the RF pulses a, b, d, e, g, and h shown in FIG. The operation of receiving the signals c, f, and i is repeated. Here, assuming that this repetition time is T _R , image information of a plurality of tomographic images is sequentially obtained within this time T _R.

That is, in the conventional magnetic resonance imaging apparatus as described above, a plurality of tomographic planes are sequentially excited with RF pulses over time. That is, in order to perform multi-slicing, the number N of tomographic planes obtained within the repetition time T _R satisfies the relational expression N ≦ T _R / T _O (1). Here, the time required to generate the MR signal and receive this signal is T _O. Then, according to the above formula (1), when the repetition time T _R becomes a small value compared with T _{O in} order to obtain an emphasized image or to shorten the scan time, images of a plurality of tomographic planes are obtained. There is a problem that information cannot be obtained. Further, when the MR signals from these tomographic planes are reconstructed into an image, the required time increases in proportion to the number of the tomographic planes, and therefore the time until the image is displayed becomes long. There was a problem of doing.

(Problems to be Solved by the Invention) As described in detail above, in the conventional pulse sequence method in MRI, the time T _O and the repetition time T _R required to generate an MR signal and receive this signal are Compare, T _R
When is a small value, there is a problem that it is not possible to obtain a plurality of tomographic plane images. Furthermore, when the MR signals from these tomographic planes are reconstructed into an image, the required time increases in proportion to the number of the tomographic planes, so it takes a long time to display the image information. There was a problem of becoming.

The present invention, while a simple sequence repeats even when the time T _R becomes smaller, the pulse sequence method in the MRI that can shorten the time required for reconstitution with obtaining a tomographic image of a sufficient number For the purpose of providing.

[Structure of the Invention] (Means for Solving the Problems) The structure of the present invention for solving the above problems applies an RF pulse containing two frequency components to a subject placed in a static magnetic field, After applying this RF pulse, after applying a gradient magnetic field that makes the phases of spins of two excited tomographic planes specified by the frequency different, then collecting a MR signal while applying a reading gradient magnetic field. It is what

(Operation) According to the pulse sequence method in the MRI having the above-mentioned configuration, by applying RF pulses containing two frequencies to the subject, different tomographic planes of the subject are simultaneously excited, and the excited tomographic planes are respectively excited. By applying a gradient magnetic field that makes the phases of the planes different between spins, slice position information is added, and MR signals are simultaneously acquired from each tomographic plane of the subject by one RF pulse.

(Example) Hereinafter, a magnetic resonance imaging apparatus for implementing the pulse sequence method in MRI of the present invention will be described with reference to the drawings.

P is the subject In FIG. 3, 1 is the static magnetic field generating unit for applying a static magnetic field H ₀ to the subject P, 2 are RF pulse transmitting unit for providing an RF pulse to the subject P, 3 is the static magnetic field H ₀ It is a gradient magnetic field generator that generates a superimposed gradient magnetic field. The gradient magnetic field generator 3 generates a slicing gradient magnetic field G _Z , a phase coding gradient magnetic field G _Y, and a reading gradient magnetic field G _X , respectively.

Reference numeral 4 is a signal collecting unit for receiving the MR signal from the subject P, and 5 is for reconstructing the MR image of the subject P by taking in the MR signal received by the signal collecting unit 4 and executing a two-dimensional Fourier transform. This is an image creating unit. An image display unit 6 visualizes the MR image reconstructed by the image creating unit 5.
Reference numeral 7 denotes a system controller that controls the operation of the apparatus of this embodiment. Specifically, each operation of the static magnetic field generator 1, the excitation pulse transmitter 2, the gradient magnetic field generator 3, and the signal collector 4 is preset. The pulse sequence is controlled according to the pulse sequence.

Next, an embodiment of the RF pulse transmitter 2 will be described with reference to FIG.

As shown in the figure, the RF pulse transmission unit 2 is composed of a high frequency generation circuit 2A, an envelope generation circuit 2B, and a modulation circuit 2C which modulates a high frequency and an envelope to generate an RF pulse.

As shown in the block diagram of FIG. 2, the high frequency generating circuit 2A is provided with local oscillators A ₁ and A ₂ which respectively generate different frequency components. The frequencies Δω ₁ and Δω ₂ respectively corresponding to the magnetic field strength of the gradient magnetic field and the slice position can be set to arbitrary values by the connected system controller 7. In the figure, A ₀ is a pulse generator that generates an RF wave of a fundamental frequency ω ₀ , and this RF wave is modulated to be a combination of frequencies ω ₀ + Δω ₁ and ω ₀ + Δω ₂ and then an envelope is provided in the subsequent stage. The subject P is modulated by being modulated by the circuit 2B and the modulation circuit 2C.

As described above, in the present invention, since the RF pulse includes two frequency components, it is possible to excite two tomographic planes at the same time. Therefore, since it is not necessary to transmit the RF pulse for each tomographic plane to be excited as in the conventional case, image information of the tomographic plane can be obtained in a short time.

The tomographic plane obtained by the RF pulse is given a phase difference by the gradient magnetic field Gz generated by the gradient magnetic field generation unit 3. More specifically, the above
Since the RF pulse has two carrier frequency components ω ₁ and ω ₂ , it is necessary to change the phases of the images from the two tomographic planes specified by the RF pulse in order to perform Fourier transform in the image creating section 5. . Here, when z is the distance in the slice direction, the gradient g of the gradient magnetic field Gz is expressed by g = ∂Gz / ∂z (2), and the slicing gradient magnetic field Gz shown in FIG. By doing so, the “deviation” δφ of the spin phase at δz with respect to the phase at Z = 0 can be obtained by δφ = 2πg · δz · δt (3). Therefore, when δφ = π / 2, the application time δt may be set to δt = 1 / 4g · δz (4).

By the way, even when the phase difference is not given as described above,
If there is a phase "shift" of the excited spins,
Correction can be made by the method disclosed in Japanese Patent Application No. 60-147770. Here, the phase shift between the slice planes is δφ ₁ and δφ ₂ , respectively, and the original reconstructed images are f ₁ (x, y), f ₂ (x,
y), the reconstructed image f (x, y) obtained is Is represented by Even if the phase difference is not positively applied as described above, if there is a phase shift in each carrier, the phase shift is corrected by the above method.

As described above in detail, for example, when obtaining image information of two tomographic planes within the repetition time T _R , if δφ _K = (k / 2) π, then f (x, y) = F ₁ (x, y) + if ₂ (x, y) (6) By extracting the real part and the imaginary part of f (x, y) by this equation, the respective values of f ₁ and f ₂ can be obtained. In this way, one RF pulse can be applied to two tomographic planes at the same time.
MR images can be obtained.

For example, considering the case where the method of the present invention is applied to the conventional multi-slice case, T _R = 210 (ms), T _O = 7
When set to 0 (ms), 3 in the case of conventional multi-slice
Although only images from the tomographic planes at the locations can be obtained, according to the present invention, images at 6 locations can be obtained at the same time.

 The effects of the present invention detailed above will be described.

First, the subject P is inserted and placed inside the apparatus, and a uniform static magnetic field H ₀
Add. Then, the pulse transmitter 2 generates an RF pulse based on the timing signal from the system controller 7 to generate a high-frequency pulse j on the mounted subject.
k is applied (see FIG. 4). Since the RF pulse in the present invention contains two frequency components as described above, it is possible to simultaneously excite two tomographic planes.

Then, after applying the RF pulse, a gradient magnetic field G _z that gives a phase difference to the spins in each slice plane is applied. This gradient magnetic field
The timing of applying G _z is such that it is applied after the 90-degree pulse is applied, as indicated by m in FIG. In the figure, a gradient magnetic field G _z that gives a phase difference after application of the 180-degree pulse k
An example in which is applied is shown. When the gradient magnetic field G _z is applied before the 180-degree pulse k is applied, the signs of the phases are simply reversed.

Specifically, for example, a portion shown by tomographic planes 1a and 1b at arbitrary two places set at a predetermined interval of the subject as shown in FIG. 6 is excited at the same time. The phase planes 1a and 1b to be excited have a phase difference that can be arbitrarily set depending on the application time of the gradient magnetic field G _{z. When} exciting two cross sections at the same time, the phase difference of each cross section is set. π / 2 is appropriate.

In this way, after the MR signals on the two tomographic planes 1a and 1b are received, the frequency component of the RF wave is changed by a predetermined value, and the MR signals on the tomographic planes 2a and 2b are received this time. This operation is repeated to collect MR images at each cross section. As described above, according to the present invention, not only can MR signals in each cross section be acquired in a shorter time than in conventional multi-slices, but also MR signals of two tomographic planes can be reconstructed at the same time. The image information can be displayed on the image display unit 6 in a short time.

The present invention is not limited to the above embodiments, and various modifications can be made within the scope of the present invention.

In the above embodiment, the local oscillators A ₁ and A ₂ of the high frequency generating circuit 2A are provided separately, but the present invention is not limited to this. For example, as shown in FIG. You may generate the synthetic wave which has. With such a configuration, there is also an effect that it can contribute to downsizing of the device according to the present invention.

Furthermore, in the above-described embodiment, the image of the tomographic plane is obtained based on the MR signal from the subject, but the invention is not limited to this. For example, the FID signal from the subject may be directly reconstructed. . Further, not only the above two-dimensional Fourier transform method but also a filtered back projection method may be used.

According to the present invention the method as described above in detail [Effect of the Invention], a simple sequence while, when the repetition time T _R becomes a value smaller, with obtaining a tomographic image of a sufficient number A pulse sequence method in MRI that can reduce the time required for reconstruction can be provided.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an RF pulse transmitter for carrying out the method of the present invention, FIG. 2 is a block diagram showing the configuration of the high-frequency generator circuit shown in FIG. 1, and FIG. 3 is a block diagram showing the configuration of a magnetic resonance imaging apparatus. FIG. 4 is an explanatory diagram of a pulse sequence of the magnetic resonance imaging apparatus, FIG. 5 is an explanatory diagram showing the arrangement of the tomographic planes obtained, and FIG. 6 is an explanatory diagram showing the mutual relationship of the tomographic planes from which MR signals are to be obtained. FIG. 7 is a block diagram showing a modification of the high-frequency generation circuit, FIG. 8 is an external perspective view showing the positions of the subject P and the tomographic plane, and FIG. 9 is a timing chart showing a multi-slice pulse sequence. is there. G _z …… Gradient magnetic field, Δω ₁ , Δω ₂ …… Modulated wave.

Claims (2)

[Claims] 1. An RF pulse containing two frequency components is applied to a subject placed in a static magnetic field, and after the application of the RF pulse, a phase between spins of two excited tomographic planes specified by the frequency. A pulse sequence method in MRI, wherein MR signals are acquired while applying a gradient magnetic field for reading after applying a gradient magnetic field that makes the difference. 2. The pulse sequence method in MRI according to claim 1, wherein each tomographic plane specified by the frequency has a phase difference of π / 2 between spins.