JP3212644B2 - MRI equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an MRI apparatus for imaging a subject using a magnetic resonance phenomenon, and more particularly, to an MRI apparatus for generating a real-time M-mode image.

[0002]

2. Description of the Related Art When a nucleus is placed in a static magnetic field, the nucleus precesses at an angular velocity proportional to a constant that varies depending on the strength of the magnetic field and the type of the nucleus. When a high-frequency rotating magnetic field having the above frequency is applied to an axis perpendicular to the static magnetic field, magnetic resonance occurs, and a group of specific nuclei having the constant causes transition between levels by a high-frequency magnetic field satisfying the resonance condition, and energy Transit to the higher level. Nuclei excited to a higher level after resonance return to a lower level and emit energy. The MRI is a device for observing a nuclear magnetic resonance phenomenon caused by a specific atomic nucleus and capturing a tomographic image of a subject.

[0003] In MRI, a pulse sequence for applying a high-frequency magnetic field and a gradient magnetic field used for the Fourier transform method is a seventh sequence.
Shown in the figure. In period 1, spins in a slice plane perpendicular to the z direction centered on z = 0 are selectively excited by the 90 ° pulse and the slice gradient. The rephase gradient in the period 2 is for restoring the phase of the spin disturbed by the slice gradient. The dephase gradient in the same period 2 is for giving a phase difference according to the place to the spin so that the center of the SE signal coincides with the time center of the data read period 4. In period 2, a warp gradient is applied to shift the phase of the spin in proportion to the position in the y direction. The warp gradient is applied with its intensity changed every period. Thereafter, a 180 ° pulse is applied to align the magnetic moments, and the SE signal appearing thereafter is observed. In period 4, a read gradient is applied to the x-axis. As a result, the phase difference given by the dephase gradient is canceled at the time center of the read gradient in period 4 and S
The E signal appears. This sequence is called a view,
After the pulse repetition period TR, a 90 ° pulse ′ is applied again to start the next view.

[0004]

In the imaging in the above case, a high-speed scanning method for shortening the pulse repetition period is used in order to reduce the influence of movement in a portion to be imaged.

As the method, for example, FAST (Four
ier Acquired Steady-state Technique), FISP (Fa
st Imaging with Steady Precession), CE-FAST
(Contrast Enhansed FAST). Each of these scan sequences uses an RF pulse having one flip angle, the FAST sequence uses the FID signal immediately after the RF pulse, and the CE-FAST sequence uses the R pulse.
The echo signal immediately before F (the echo by the previous RF pulse appears immediately before the next RF pulse) is observed. That is, immediately after the RF pulse, a linear gradient magnetic field for shifting the phase of the spin in the direction of the read magnetic field (dephase) is applied, a linear gradient magnetic field in the opposite direction is immediately applied, and the spin is refocused to produce an echo signal (gradient). e
cho). In this gradient echo, measurement can be performed at high speed as much as the 180 ° pulse is not used.

However, in the MRI described above, if a normal sequence is performed, one scan is performed in several tens of milliseconds to ten seconds, so that it takes several seconds to several tens of minutes to obtain one image. For this reason, real-time properties are lacking, and it is difficult to image a moving organ.

On the other hand, in an ultrasonic diagnostic apparatus, there is a diagnostic method called an M mode in which a probe is fixed and a temporal change in a distance to a moving object (echo source) is displayed on a CRT. In this case, although it is a one-dimensional image, a temporal change can be displayed in real time. However, the measurement method is completely different between ultrasonic diagnosis and MR, and real-time measurement could not be performed in MR.

The present invention has been made in order to solve the above-mentioned problems, and an object of the present invention is to provide an MRI apparatus capable of generating a real-time image.

[0009]

According to a first method of the present invention for solving the above-mentioned problems, a spin in a subject placed in a static magnetic field is brought into a steady state, and the slice direction, warp direction, In an MRI in which a gradient magnetic field and an RF pulse are applied in accordance with a predetermined sequence in each direction of the read direction and an MR signal from a slice plane is detected to obtain a tomographic image of a subject, a 90 ° pulse and a slice gradient in the slice direction are obtained. And then apply a spoiler in the slice direction and the warp direction to excite the first plane including the desired portion, apply a 180 ° pulse and a slice gradient in the warp direction, and then apply the slice gradient and the warp direction. Apply rephase and spoiler gradient to intersect the first plane,
A second plane including a desired portion is excited, and a readout gradient is applied in a read direction to detect a spin echo signal from the desired portion.

In a second method of the present invention for solving the above-mentioned problems, a spin in a subject arranged in a static magnetic field is brought into a steady state, and the spin in the subject is sliced, warped, and read. A gradient magnetic field and an RF pulse are applied according to a predetermined sequence, and M
In MRI that obtains a tomographic image of a subject by detecting an R signal, an RF pulse having a constant flip angle and a slice gradient are applied in a slice direction, and then a spoiler is applied in a slice direction and a warp direction, and rephase is performed in a readout direction. After applying the readout gradient, rephase and spoiler gradients are applied in the slice direction and warp direction, and dephase is applied in the read direction to excite a plane including a desired portion with an RF pulse having a constant flip angle. After applying an RF pulse having a fixed flip angle and a slice gradient in the slice direction, applying a spoiler in the slice direction and the warp direction, applying a rephase in the readout direction, applying the readout gradient, and then applying the slice gradient and the warp. Apply rephase and spoiler gradients in the direction A phase is applied to sequentially excite a plurality of planes intersecting the plane at a desired site by an RF pulse having a constant flip angle, and detect a gradient echo from a predetermined site where the plurality of excited planes intersect. It is characterized by the following.

[0011]

In the first method, a 90 ° pulse excites spins in a first plane including a desired portion, and causes
The 0 ° pulse crosses the first plane and excites spins in a second plane containing the desired site. As a result, only the spin echo at a predetermined portion where the first plane and the second plane intersect is observed.

In the second method, a plane including a desired portion is excited by an RF pulse having a fixed flip angle by an RF pulse having a fixed flip angle, and a plurality of planes intersecting the plane at the desired portion are fixed. By sequentially exciting with the RF pulse having the flip angle, a gradient echo signal from a predetermined portion where a plurality of excited planes intersect is detected.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a pulse sequence diagram showing a signal waveform in one embodiment of the method of the present invention. Here, an embodiment to which the spin echo method is applied will be described.

In this figure, (1) is an RF pulse,
(2) is a slice gradient on the slice axis Gz for determining a plane for selectively exciting spin, (3) is a slice gradient on the Gy axis, and (4) is a read gradient on a read axis for reading. Is the read gradient of

FIG. 2 is an explanatory view showing a slice plane for explaining the embodiment. First 90 ° pulse (Fig. 1 (1)
(A)) and a slice gradient (FIGS. 1 (2) and (b)) selectively excite spins in a slice plane perpendicular to the z direction including the intersecting columns pq. Thereby, the slice plane A (thickness = d) including the intersecting columns pq to be imaged is excited.

Next, a 180 ° pulse (FIG. 1 (1)
(F)) and the slice gradient (FIGS. 1 (3) (g)) selectively excite spins in a slice plane perpendicular to the y-axis direction including the intersecting columns pq. Thereby, the slice plane B (thickness = d) including the intersecting columns pq to be imaged is excited.

Here, immediately after the 90 ° pulse and the 180 ° pulse, the spoiler gradient pulse (FIGS. 1 (c), (d),
(E), (h), (i): However, the pulse immediately after the 180 ° pulse includes rephasing), and the
Parts other than q can be dephased, and the generation of echo signals from them can be suppressed. Therefore, by observing the spin echo while applying the readout gradient magnetic field (FIG. 1 (4) (j)) after the 180 ° pulse, the intersecting column pq where the slice plane A and the slice plane B intersect can be formed. Can be imaged. By repeating the above operation, a real-time M-mode image can be generated.

The above is the case of a single echo.
By obtaining a multi-echo by continuously applying a 180 ° pulse, many one-line images can be obtained in a short time (for example, several tens of milliseconds). In this case, since the signal strength is weakened for each echo, it is necessary to correct it.

As described in detail above, by applying the spin echo and observing the echo excited on the slice plane orthogonal to the desired axis (intersection) to be observed in real time, It becomes possible to observe a real-time M-mode image in MRI.

Although the ultrasonic M mode can basically be observed only in the sound ray direction at the same time, according to the real time M mode image of MRI according to the present embodiment, a line having an arbitrary direction can be observed. Image can be obtained.

Next, an embodiment to which the gradient echo method is applied will be described with reference to FIGS. The sequence in this case is based on CE-FAST as shown in FIG. Slice plane (thickness = d) S0 including the intersecting column pq to be imaged with an RF pulse having a flip angle α degrees
To excite. Then, the slice plane S0, which is obtained by rotating the slice plane S0 by θ0 °, is excited by the next α ° RF pulse. Hereinafter, in the same manner, the slice planes intersect one another pq
The RF pulse application and the slice plane excitation are repeated while rotating about. The n-th slice plane S
The slice plane Sn rotated by θn-1 with n-1 as the axis of the intersecting column pq is excited by an RF pulse of α °.

In this manner, since the RF pulse is continuously applied only to the intersection pq, a gradient echo can be generated only in the intersection. On the other hand, portions other than the intersecting columns pq in each slice plane do not receive the application of the RF pulse twice or more consecutively, so that no echo is generated. Therefore, by adjusting the area of the spoiler after the RF pulse and applying the readout gradient magnetic field, the intersecting column pq can be imaged.

In this method, it is necessary to consider the magnetization state, gradient, and rotation angle θn in the intersecting columns pq. First, the magnetization state in the intersecting column pq, but the periodically moving magnetization is SSFP (stationary free precession) due to the movement.
It is preferable not to be in the state, but rather to be in the SSFP state. This is because in the SSFP state, higher-order echoes such as spin echoes and stimulated echoes reflecting the past state are superimposed, and the real-time meaning is reduced. Therefore, it is necessary to prevent the SSFP state from being caused by a random RF phase, a random spoiler, or the like. Then, the static magnetization is not in the SSFP state, but no particular problem occurs. As a result, the echo is 1
Some of the transverse magnetization excited by the previous RF is
It is only formed by F.

Next, regarding the gradient, since the slice plane is rotated every RF pulse, the gradient waveform must be converted according to the rotation angle and the angle of the intersecting column pq to be imaged. No. FIG. 3 shows only the most basic 1TR section before conversion. Since this waveform only needs to be converted according to each slice plane, the following description will be made using the gradient waveform of FIG.

Here, it is particularly necessary to consider the area of the spoiler, and any two adjacent TR (n−n).
(Represented by 1 and n) must be applied so as to always satisfy the following condition. S1n-1 + S2n-1 = S1n = Random By doing so, the SSFP state can be positively suppressed, and the occurrence of artifacts due to movement can be suppressed.

Finally, regarding the rotation angle θn of the slice plane, it is preferable that the slice plane once excited is never excited again, or is excited again after the magnetization is sufficiently restored to the thermal equilibrium state. For this purpose, θn is θn = 2π × f (n) / M where M is a prime sequence including f (n) = 0 greater than 1 and excluding M. Exciting the same plane as twice is impossible. Absent. For example, M = 17 is conceivable.

Next, specific positioning criteria are shown in FIGS.
This will be described with reference to FIG. As shown in FIG. 5, first, an image of a two-dimensional plane where a desired line exists is obtained (step in FIG. 6). Then, a line cursor is positioned on a desired line on the image to perform positioning (step in FIG. 6). Thereafter, a real-time image (real-time M-mode image) of the intersecting pillar is acquired by the above-described method (step in FIG. 6). Then, the real-time M-mode image is displayed beside the two-dimensional image while sweeping (step in FIG. 6). Thus, the original two-dimensional image and the corresponding M-mode image of the line cursor can be observed.

Thereafter, when it is desired to change the observation position of the M-mode image, the position of the intersection pq is changed by changing the position of the line cursor with a trackball or the like, and the real-time M-mode image is also immediately changed. Correspondence (Fig. 6 step ~
).

As described in detail above, the spin echo method or the gradient echo method is applied to observe an echo of a desired axis (intersection) to be observed in real time, thereby real-time M mode image in MRI. Can be observed.

Although the ultrasonic M mode can basically be observed only at the same time in the sound ray direction, according to the present embodiment, according to the observation of the MRI real-time M mode image, a line having an arbitrary direction can be observed. Image can be obtained. Since TR is short, it is easy to increase the time resolution.

[0031]

As described above in detail, according to the present invention, a real-time M-mode image of a desired axis can be generated in MRI by applying the conventional spin echo method and gradient echo method. it can.

[Brief description of the drawings]

FIG. 1 is a pulse sequence diagram of a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a state of a slice plane according to the first embodiment.

FIG. 3 is a pulse sequence diagram of a second embodiment of the present invention.

FIG. 4 is an explanatory diagram showing a state of a slice plane according to a second embodiment.

FIG. 5 is an explanatory diagram showing a display state of a two-dimensional image and an M-mode image.

FIG. 6 is an explanatory diagram showing a procedure for acquiring and displaying an M-mode image.

FIG. 7 is an explanatory diagram showing a conventional spin echo method.

[Explanation of symbols]

 a 90 ° pulse b slice gradient c spoiler d spoiler e dephase f 180 ° pulse g slice gradient h rephase spoiler i rephase spoiler j readout gradient

──────────────────────────────────────────────────続 き Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. An MRI apparatus for detecting an NMR signal from a predetermined slice in a subject, wherein the gradient echo method rotates the slice about a predetermined axis by using an RF pulse having a predetermined flip angle. Scanning means for performing a scan by means of X-rays and continuously detecting an NMR signal from a one-dimensional area as the axis; processing means for processing the NMR signal to continuously generate one-dimensional image data; An MRI apparatus comprising: display means for performing M-mode display by sequentially updating the one-dimensional image data on the image. 2. The MRI apparatus according to claim 1, wherein the scan uses CE-FAST.