JPH0572812B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Field of Industrial Application) The present invention is directed to nuclear magnetic resonance (NMR).
In particular, it involves a magnetic resonance imaging method that applies the phenomenon of magnetic resonance (magnetic resonance), in which the inclination angle of the spin-based magnetic moment can be adjusted to a predetermined value by changing the transmission pulse power used to generate an excitation rotating magnetic field according to the attributes of the subject. This invention relates to a high frequency output adjustment method for adjusting the output value.

(Prior Art) Nuclear magnetic resonance is a phenomenon in which atomic nuclei placed in a magnetic field resonate and absorb electromagnetic wave energy of a specific wavelength, and then emit this energy as electromagnetic waves. Devices that utilize this phenomenon to diagnose living organisms detect the electromagnetic waves (hereinafter referred to as MR signals) emitted from the above-mentioned atomic nuclei, especially protons, and
The detected MR signals are processed to determine nuclear (proton) density, longitudinal relaxation time T ₁ , transverse relaxation time T ₂ , flow,
Diagnostic information such as a tomographic image of the subject reflecting information such as chemical shift can be obtained.

By the way, in order to detect MR signals from protons using such a nuclear magnetic resonance phenomenon,
An excitation rotating magnetic field generated by a high-frequency pulse called a 90° pulse and a 180° pulse selectively excites the part to be measured of the subject, and the angle of inclination of the spin magnetic moment of protons in the part to be measured is adjusted to a predetermined angle. Must be a value. Here, a 90° pulse has the function of causing magnetic resonance absorption and rotating the magnetic moment of the spin of an atomic nucleus by 90° from a direction parallel to the direction of the static magnetic field in which the subject is placed until it rises perpendicularly. It is. The 180° pulse has the function of causing magnetic resonance absorption and rotating the magnetic moment of the spin of an atomic nucleus by 180° from a direction parallel to the direction of the static magnetic field in which the subject is placed.

For these 90° pulses and 180° pulses, it is necessary to set conditions such as the pulse size in advance before detecting the MR signal from the measurement site. However, when setting the conditions for this high-frequency pulse, since the straight capacitance Q, which is an electrical characteristic, changes depending on attributes such as the shape of the subject, it is necessary to use a 90° pulse or a 180° pulse at each slice position. The conditions for this will deviate, and the angle of inclination of the magnetic moment of the spin will differ from the set value. As a result, the obtained signals vary depending on the attributes of the subject. In particular, when the area to be measured is the thoracoabdominal area, the thorax and abdomen swing back and forth in the body due to respiratory movements, and the diaphragm moves up and down in the direction of the body height, making the slice position relative to the area to be measured. 90° pulse or
The conditions for the 180° pulse deviate, and furthermore, a phase change of the spin occurs during data collection, resulting in
There is a problem that the intensity of the MR signal changes.

(Problems to be Solved by the Invention) As mentioned above, when obtaining tomographic images of the chest and abdomen using nuclear magnetic resonance phenomena, MR
Setting the conditions for the high frequency output of the 90° pulse or 180° pulse, which is done prior to detecting the signal, will result in a shift in the relative slice position, resulting in a shift in the conditions for the 90° pulse or 180° pulse. There is a problem in that the angle of inclination of the magnetic moment of the spin differs from the set value, making it impossible to obtain an accurate signal.

SUMMARY OF THE INVENTION An object of the present invention is to provide a high frequency output adjustment method that allows accurate setting of high frequency output conditions for 90° pulses or 180° pulses and obtains accurate signals.

[Structure of the Invention] (Means for Solving the Problems) The present invention provides a transmission pulse that generates an excitation rotating magnetic field in a predetermined region in order to generate an inclination angle in a spin-based magnetic moment in a predetermined region in a static magnetic field. , a step of detecting a magnetic resonance signal induced from the predetermined region by the generation of the tilting angle of the spin-based magnetic moment using a high-frequency coil, In the high frequency output adjustment method comprising the step of adjusting the magnitude of the transmission pulse so that the magnitude of the transmission pulse is adjusted to a value of
The slice thickness of the predetermined region where the magnetic resonance signal is detected is set such that the sensitivity of the high frequency coil is substantially uniform;
This high-frequency output adjustment method is characterized in that the high-frequency output is set to be almost the same as the maximum region.

(Function) Adjustment of the size of the transmitted pulse is performed while detecting the magnetic resonance signal induced in the static magnetic field using a high-frequency coil.

In the present invention, when detecting a magnetic resonance signal, the region where the magnetic resonance signal is detected, that is, the slice thickness, is set to be almost the same as the region where the sensitivity of the high frequency coil is almost uniform and maximum. The method is thicker.

Until now, even when adjusting the size of the transmitted pulse, the same slice thickness was used as for imaging, so the maximum thickness was only a few millimeters.

In the case of the present invention, since the slice thickness is increased, the region where magnetic resonance signals are detected becomes thicker accordingly.

In this way, magnetic resonance signals are detected from a wide area, so even if the area to be examined is shifted due to respiratory movements, such as the chest and abdomen, the transmitted pulses, for example 90゜Pulse and
The size of the 180° pulse can be adjusted to obtain accurate tomographic images.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a nuclear magnetic resonance imaging apparatus used in an embodiment of the present invention. As shown in FIG. 1, this device 1 includes a gradient magnetic field generating coil 2 for generating a gradient magnetic field for obtaining positional information of a site where a magnetic resonance signal is induced, and a gradient magnetic field generating coil 2 for generating a gradient magnetic field for obtaining positional information of a site where a magnetic resonance signal is induced. It has a high frequency coil 3 which is a transmitting/receiving system for detecting magnetic resonance signals. This gradient magnetic field generating coil 2 has an X-axis gradient that generates a gradient magnetic field about these axes, where the axis in the height direction of the subject P is the Z-axis, and the axes orthogonal to this Z-axis are the X-axis and the Y-axis, respectively. It is composed of a magnetic field generating coil 2a, a Y-axis gradient magnetic field generating coil 2b, and a Z-axis gradient magnetic field generating coil 2c. Each gradient magnetic field generating coil 2a, 2b, 2c
are connected to power supplies for an X-axis gradient magnetic field, a Y-axis gradient magnetic field, and a Z-axis gradient magnetic field (not shown), respectively, to supply current for generating magnetic fields. In addition, as will be described later, the high-frequency coil 3 is placed above and below the subject P at a position corresponding to the position of the chest and abdomen of the subject P, and is also connected to a transmitter that supplies high-frequency pulses and a part to be measured. Connected to a receiver to receive the induced MR signal. Furthermore, this device 1 is capable of generating each gradient magnetic field and 90° pulse as well as
It also includes a sequencer that implements a pulse sequence of high-frequency pulses of 180° pulses, and a computer system that controls each power source, transmitter, receiver, and sequencer, and performs signal processing of detection signals. The signals processed by this computer system are displayed on a display or the like. This device 1 is
It also includes a static magnetic field coil (not shown) that generates a static magnetic field in the Z-axis direction with respect to the subject P, and a power source (not shown) that supplies current to the static magnetic field coil.

Next, regarding the high frequency output adjustment method of this example using the imaging apparatus having the above configuration,
explain. First, as shown in Figure 2, the following steps are required to set the high frequency output of 90° pulse and 180° pulse.
When obtaining MR signals, by adjusting the Z-axis gradient magnetic field, which is the magnetic field for slicing, in advance, the slice width S shown with diagonal lines in Fig. 1 can be adjusted so that the sensitivity of the high-frequency coil is almost uniform, and It is set to be almost the same as the maximum area, making it thicker than previous methods. Under these conditions, while applying the slicing gradient magnetic field shown in Fig. 3a,
As shown in Figure b, a 90° pulse and a 180° pulse are applied to the high-frequency coil, the induced MR signal is detected, and the magnitude of each pulse is adjusted so that the MR signal is maximized. The conditions for each pulse are set so that the angle of inclination of the system magnetic moment becomes a predetermined value.

When setting these conditions, for example, as a circuit system that can automatically set the conditions for each pulse,
The block diagram shown in FIG. 4 can be used.
That is, this circuit system is composed of a transmission pulse control means 10, a transmitting section 11, and a receiving section 12. The transmission pulse control means 10 is configured mainly of a central processing unit, for example, and is capable of changing the transmission pulse power used for generating the excitation rotating magnetic field by the high-frequency coil 3 according to the attributes of the subject. Accordingly, the angle of inclination of the spin-based magnetic moment is set to a predetermined value. for example,
While receiving the MR signal induced at the measurement site via a high-frequency coil, the transmitter pulse power value is determined to maximize the peak value of the received MR signal when the transmitter pulse power is varied within a predetermined range. Based on this value, the transmission pulse power when taking a tomographic image of the subject is automatically set.

Further, the transmitter 11 is connected to a high frequency coil 3 that emits each pulse. This transmitter 11 includes an oscillation means 13 that outputs a continuous wave, a modulation means 14 that modulates the continuous wave from this oscillation means 13, and a pulse wave amplitude and pulse width selected in this modulation means 14 for modulating the continuous wave. a power controller (APC) 1 that varies the transmission pulse power under the control of the setting selection means 15 and the transmission pulse control means 10;
6. First amplification means (radio frequency amplifier) 17 that amplifies the output of this APC 16, second amplification means (drive amplifier) 18, third amplification means (final amplifier) 19, and impedance matching that performs impedance matching. It is comprised of a means 20 and a tuning means 21 for tuning with the high frequency coil 3.

Further, the receiving section 12 is connected to the high frequency coil 3 that receives the MR signal induced by this pulse. When a common high-frequency coil 3 is used for both radiation of high-frequency pulses and reception of MR signals, it is necessary to separate the transmitting section 11 and the receiving section 12 using an isolator or the like. Furthermore, in the high frequency output adjustment method of this embodiment, the slice thickness is set so as to be approximately the same as the region where the sensitivity of the high frequency coil is substantially uniform and maximum, and is made thicker than during imaging. Therefore, when the magnitude of the transmitted pulse is adjusted, a stronger MR signal is detected than during imaging. Therefore, in this receiving section 12, the MR signal level is lowered by lowering the gain of the amplifier or by installing an appropriate attenuator to obtain an appropriate level.
It is necessary to receive MR signals. for example,
In order to lower the gain of the amplifier, as shown in FIG. Also, in order to lower the MR signal level, as shown in Figure 5b,
An attenuator 24 can be provided between the first stage amplifier 22 and the second stage amplifier 23. The operation of this circuit system will be explained. The setting of the transmission pulse power, which is performed prior to photographing the object, is performed by the operator's manual operation, and after rough adjustment of the 90° pulse condition and the 180° pulse condition, the transmission pulse power is set based on the control signal from the transmission control means 10. Transmission section 11
By applying a 90° pulse and a 180° pulse to the high frequency coil 3, a rotating magnetic field for excitation of the region to be measured is applied from the high frequency coil 3. The MR signal from the measurement site is received by the receiving section 12 via the high frequency coil 3, and the MR signal is received by the transmitting pulse control means 10.
is input.

The transmitting coil control means 10 holds the peak value of the input MR signal. Thereafter, the attenuator in the APC 16 is varied to change the transmission pulse power, and as described above, MR signals at different transmission pulse powers are received and the peak value of the MR signal is maintained.

Based on the plurality of MR signals collected by changing the transmission pulse power in this way, the transmission pulse control means 10
The transmission pulse power when the peak value of the MR signal is maximum, that is, the control state of the attenuator in the APC 16 is recognized. Thereby, the transmission pulse power is set according to the attributes of the subject. In other words, the conditions of the 90° pulse and the 180° pulse are properly set according to the attributes of the subject, and the inclination angle of the spin-based magnetic moment is adjusted to predetermined values of 90° and 180°.

Moreover, in the case of this embodiment, the slice width is further increased to include the uniform sensitivity region of the maximum high-frequency coil, so the intensity of the received MR signal increases, and the relative Since the change in the slice position is smaller than the slice thickness and the change in the MR signal intensity is smaller, each pulse condition can be set more accurately.

In the above embodiment, the power controller (APC) 16 was used when changing the transmission pulse power, but the selection means 15 for selecting and setting the amplitude and pulse width of the pulse wave is used to change the amplitude and transmit pulse power. Various modifications can be made, such as changing the .

In addition, the characteristics of the high-frequency coil that sends the transmission pulses (size of uniform region, efficiency of rotating magnetic field generation, etc.), the transmission pulse power values of 90° pulses and 180° pulses after coarse adjustment, etc., and the amplifier in the receiver 12. It is also possible to have the gain of the attenuator, the amount of attenuation of the attenuator, etc. stored in advance in a computer system or the like.

In addition, in the above example, the maximum value of the MR signal generated by the 90° pulse - 180° pulse sequence was used, but the maximum value obtained when using only the 180° pulse
A method using the minimum value of the MR signal can also be used.

[Effects of the Invention] As described above, according to the present invention, it is possible to provide a high frequency output adjustment method that can accurately set the conditions for high frequency output of 90° pulse or 180° pulse and obtain an accurate signal. .

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing the configuration of a magnetic resonance imaging apparatus used in an embodiment of the present invention, Fig. 2 is a characteristic diagram for explaining an embodiment of the present invention, and Fig. 3a is a slice diagram in this embodiment. FIG. 3b is a graph showing the pulse sequence in this embodiment, FIG. 4 is a block diagram showing the system circuit used in this embodiment, and FIGS. 5a and 5b are FIG. 2 is a circuit diagram showing the main parts of a receiving section in a circuit used in an example. DESCRIPTION OF SYMBOLS 1... Magnetic resonance imaging device, 2... Gradient magnetic field generation coil, 3... High frequency coil, 10... Transmission pulse control means, 11... Transmission means, 12... Receiving means.

Claims (1)

[Claims] 1. A step of applying a transmission pulse that generates an excitation rotating magnetic field to the predetermined region in order to generate an inclination angle in the spin-based magnetic moment in a predetermined region in a static magnetic field, and a step for generating the inclination angle of the spin-based magnetic moment. Therefore, the steps include: detecting a magnetic resonance signal induced from the predetermined region with a high-frequency coil; and adjusting the magnitude of the transmission pulse so that the angle of inclination of the spin-based magnetic moment becomes a predetermined value. In the high frequency output adjustment method, when adjusting the magnitude of the transmission pulse, the slice thickness of the predetermined region where the magnetic resonance signal is detected is
A high frequency output adjustment method characterized in that the sensitivity of the high frequency coil is set to be approximately equal to a region where the sensitivity is substantially uniform and maximum.