JP2560300B2 - MRI equipment - Google Patents

Description

Detailed Description of the Invention [Industrial applications]

The present invention relates to an MRI apparatus that performs imaging by using an NMR phenomenon (nuclear magnetic resonance phenomenon), and particularly relates to a technique of performing spectrum measurement using this MRI apparatus.

[Prior art]

An MRI device with a static magnetic field strength of 1 terrace or more can obtain a tomographic image of a living body and a spectrum such as ³¹ P at the same time. This ³¹ P spectrum shows the activity status of the living body and can obtain extremely useful information. In this case, since the signal level is very low, it is usual to collect signals using a surface coil, but it is desired to limit the data collection site. Therefore, conventionally, a method of obtaining only the signal from the body excluding the signal from the body surface has been proposed (Journal of Mag
netic Resonance, 65, p522-525, 1985). This is the sixth
As shown in the figure, the surface coil 62 and the gradient magnetic field coil 63 are arranged parallel to the body surface of the subject 61. Both the surface coil 62 and the gradient magnetic field coil 63 are formed in a loop shape, and FIG. 6 is a lateral view of a portion where they are applied to the body surface. In such a coil configuration, in the sequence as shown in FIG.
An RF pulse is generated from the surface coil 62, a weak gradient magnetic field pulse is generated from the gradient magnetic field coil 63, and an NMR signal is received by the surface coil 62 to collect data. First, in the first half of this sequence, a gradient magnetic field pulse is applied together with a 90 ° pulse, thereby exciting all spins in the body of the subject 61. Here, since the gradient magnetic field pulse is weak, it is not used even for spins deep inside the body. Therefore, when a 180 ° pulse is applied next, the spin inside the body produces an echo signal. That is, the spin on the body surface is disturbed by the gradient magnetic field pulse,
Since the phases are separated, no echo signal is generated in this portion, and the signal from the body surface can be suppressed.

[Problems to be Solved by the Invention]

However, such a conventional method has the following problems. First, as can be seen from FIG. 7, since it takes a long time from the first application of the 90 ° pulse to the data acquisition, signal attenuation occurs. This is especially fatal for weak signals such as ³¹ P. Next, since the strength of the RF pulse given from the surface coil is attenuated in the direction perpendicular to the surface coil plane, it is difficult to give an accurate 90 ° or 180 ° pulse to all parts of the body. The present invention provides an MRI apparatus capable of obtaining a spectral signal such as ³¹ P having a high S / N ratio, which does not include a signal from a specific portion, by using a gradient magnetic field coil and a surface coil which are normally provided. The purpose is to do.

[Means for solving problems]

The MRI apparatus according to the present invention includes a means for repeatedly exciting an RF pulse a plurality of times while applying a gradient magnetic field for slice plane selection to selectively excite that portion to bring the magnetization of spins to 90 ° and a means to bring the spin magnetization to 90 °. Means for repeatedly applying a gradient magnetic field pulse for perturbing the phase of the generated spin to the slice plane selection multiple times during each application of the RF pulse during the application of the slice plane selection gradient magnetic field; After the application of the gradient magnetic field for use in the above-mentioned disturbed state, it has means for applying a 90 ° pulse and surface coil means for receiving the generated NMR signal.

[Action]

In principle, the magnetization disappears if a gradient magnetic field for phase disturbance is applied after exciting the spin magnetization by 90 ° by applying an RF pulse. The magnetization does not disappear with only one subsequent gradient magnetic field pulse for phase disturbance. This is because some RF pulse applications do not fall to 90 °. Therefore, the RF pulse is repeatedly applied a plurality of times, and each time, the gradient magnetic field pulse for phase disturbance is applied immediately after that.
Then, the magnetization, which was not extinguished by only one excitation / disturbance, is extinguished by the next RF pulse application / disturbance, and the magnetization can be completely extinguished by repeating this multiple times. The RF pulse application and the disturbance magnetic field gradient pulse application are performed a plurality of times while applying the slice plane selection gradient magnetic field. Then, the magnetization can be completely extinguished only in the selected slice portion. Then, after applying the slice plane selecting gradient magnetic field and the disturbing gradient magnetic field, a 90 ° pulse is applied when the phase is disturbed and the magnetization disappears in the specific slice portion as described above. This completely suppresses the signal from the particular slice mentioned above, leaving only the NMR from other parts.
A signal can be generated. Therefore, it is possible to obtain a spectrum signal from a deep portion inside the body, excluding the body surface portion.

【Example】

FIG. 1 shows an embodiment of the present invention. In this figure, a computer 1 generates a Gx power source 2 connected to a Gx coil for generating a gradient magnetic field Gx in the X direction and a gradient magnetic field Gy in the Y direction. The Gy power source 3 connected to the Gy coil 6 and the Gz power source 4 connected to the Gz coil 7 that generates the gradient magnetic field Gz in the Z direction are controlled. On the other hand, the RF oscillator 8 oscillates a high frequency having a frequency proportional to the static magnetic field strength, and this high frequency signal is amplitude-modulated by the modulator 9 under the instruction of the computer 1. Is sent to the surface coil 11 after being amplified by the RF power amplifier 10 whose amplification degree is controlled by the computer 1, and the subject (not shown) is excited. The NMR signal generated from the subject is received by the surface coil 11, sent to the detector 13 via the amplifier 12 and detected, and the detected output is sampled by the A / D converter 14 and converted into a digital signal, and the computer Taken in 1. These Gx coil 5, Gy coil 6, and Gz coil 7 are originally used when a tomographic image is taken, generate a gradient magnetic field for selecting a slice plane, and perform frequency coding in one direction on the slice plane. It has a function of generating a gradient magnetic field for generating a gradient magnetic field for performing phase coding in another direction on the surface. In this embodiment, the spectrum is measured by utilizing the gradient magnetic fields generated by the coils 5, 6, and 7 as follows. As shown in FIG. 2, surface coils 11 to 90
When a pulse is applied, a gradient magnetic field for selecting a slice (Gz in this embodiment) is applied at the same time to selectively excite spins only in the selected slice plane, that is, only the body surface portion. As a result, the magnetization of this portion is tilted by 90 ° as shown in FIG. 3A, but the gradient magnetic field Gz is continuously given and the other two gradient magnetic fields Gx and Gy are added later. The phase is disturbed by the pulse, the spin directions are scattered as shown in FIG. 3B, and the magnetization is erased. Therefore, as shown in FIG. 2, if a 90 ° pulse is given immediately after this, spins in parts other than the body surface part, that is, parts in the body are excited, and only the magnetization of this part is tilted 90 °,
A signal is generated only in this body part, which is received by the surface coil 11 and data is collected. By the way, this is the ideal case, and in fact
Generally, it is difficult to give an accurate 90 ° pulse using a surface coil. Therefore, it is the actual situation that the body surface portion that is selectively excited does not fall to 90 ° as shown in FIG. 4A. In such a case, the magnetization of the constituents is not disturbed by the gradient magnetic field for phase perturbation, but the longitudinal component is the fourth component.
As shown in Fig. B, the signal from the body surface is mixed in a little when the 90 ° pulse for reading is given. Therefore, as shown in FIG. 5, the body surface portion is selectively excited by exciting with a 90 ° pulse while applying a gradient magnetic field Gz for slice plane selection, and a gradient magnetic field for phase disturbance is applied sequentially. Then, even if the longitudinal magnetization remains as shown in FIG. 4B due to the first excitation / disturbance, the remaining longitudinal magnetization is excited the second time and tilted by the same angle as the first magnetization, and the phase As the transverse component is erased by the disturbance, the magnetization of the body surface portion decreases at the same rate, and the magnetization completely disappears after several repetitions. In this way, if the 90 ° pulse is applied after the magnetization of the body surface portion is completely erased, the NMR signal is generated only from the body portion excluding the body surface portion, and the body surface portion signal can be removed. . In the above-mentioned embodiment, the RF coil for excitation is transmitted from the surface coil 11, and the surface coil 11
Although the NMR signal is received, another antenna coil such as a large saddle type head coil or body coil is used for transmission, and a surface coil is used for reception in the case of the transmission / reception separation method described above. Figure or 5th
The sequence in the figure can be adopted.

【The invention's effect】

According to the MRI apparatus of this invention, since the RF pulse application and the disturbance magnetic field gradient pulse application are performed plural times while applying the slice plane selecting gradient magnetic field, the magnetization is completely performed in the selected slice portion. It can be extinguished, and using a normal surface coil and a normal gradient magnetic field coil, it is possible to effectively obtain a spectrum signal with a limited region. Moreover, since the signal can be sampled immediately after the excitation RF pulse is given, data with a high S / N ratio can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIGS. 2 and 5 are time charts showing a pulse sequence of the embodiment, and FIGS. 3A and 3B and FIGS. 4A and 4B are spin magnetization. FIG. 6 is a schematic view for explaining FIG. 6, FIG. 6 is a side view of a conventional example, FIG.
The figure is a time chart showing a pulse sequence of a conventional example. 1 ... Computer, 2 ... Gx power supply, 3 ... Gy power supply, 4
…… Gz power supply, 5 …… Gx coil, 6 …… Gy coil, 7 ……
Gz coil, 8 ... RF oscillator, 9 ... Modulator, 10 ... RF power amplifier, 11 ... Surface coil, 12 ... Amplifier, 13
...... Detector, 14 …… A / D converter.

Claims (1)

(57) [Claims] 1. An RF pulse is repeatedly applied a plurality of times while applying a gradient magnetic field for slice plane selection to selectively excite that portion to bring the magnetization of spins to 90 °, and 90 °. A means for repeatedly applying a gradient magnetic field pulse for disturbing the phase of spins a plurality of times for each application of an RF pulse during the application of the gradient magnetic field for slice plane selection, and a gradient magnetic field for the slice plane selection and disturbance An MRI apparatus comprising: means for applying a 90 ° pulse in the above-mentioned disturbed state after applying a gradient magnetic field; and surface coil means for receiving a generated NMR signal.