JP3167038B2 - Magnetic resonance imaging equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus (hereinafter, referred to as an MRI apparatus) for obtaining a tomographic image of a subject utilizing a magnetic resonance phenomenon before measuring a signal to be imaged. The center frequency of the magnetic resonance phenomenon,
Tuning point of the tuning circuit of the receiving system, subject to adjustment of the high-frequency magnetic field
The present invention relates to an MRI apparatus which is performed every time a desired imaging region is set to the center of a static magnetic field .

[0002]

2. Description of the Related Art An MRI apparatus uses a nuclear magnetic resonance (NMR) phenomenon to measure a density distribution, a relaxation time distribution, and the like of a nuclear spin (hereinafter, referred to as a spin) at a desired examination site in a subject. Then, an image is reconstructed from the measurement data, and the cross section of the subject is displayed as an image.

In this apparatus, as shown in FIG.
The subject 7 is placed on a bed 22 movable at least in the body axis direction in a static magnetic field generator 4 that generates a static magnetic field of about 2 Tesla. At this time, the spins in the subject perform precession about the direction of the static magnetic field at a frequency determined by the strength H ₀ of the static magnetic field. This frequency is called the Larmor frequency. The Larmor frequency ν ₀ is expressed by ν ₀ = γ / 2π · H ₀ (1). Here, γ is a gyromagnetic ratio and has a unique value for each type of atomic nucleus. If the angular velocity of the Larmor precession is ω ₀ , there is a relation of ω ₀ = 2πν ₀ ... (2), so that ω ₀ = γH ₀ .

Here, when a high-frequency magnetic field (electromagnetic wave) having a frequency equal to the Larmor frequency ν ₀ of the nucleus to be measured by the high-frequency irradiation coil 11 is applied, spins are excited and transit to a high energy state. When the high-frequency magnetic field is terminated, the spin returns to its original low energy state. The electromagnetic waves emitted at this time are transmitted to the high-frequency receiving coil 1
4, amplifies the signal by the amplifier 15, and
, And digitized by an A / D converter 17 (hereinafter, referred to as an ADC) to be processed by a central processing unit 1 (hereinafter, referred to as a CP).
Called U. ). The CPU 1 performs an image reconstruction calculation based on the data, and the calculated data is displayed on the display 18 as a tomographic image of the subject 7. The above high-frequency magnetic field is applied to the sequencer 2 controlled by the CPU 1.
Are amplified by the high-frequency transmission coil amplifier 10 and sent to the high-frequency transmission coil 11.

[0005] In addition to the static magnetic field 4 and the high-frequency magnetic field, the MRI apparatus includes a gradient coil group 13 for generating a gradient magnetic field for obtaining positional information in space.
The gradient magnetic field coils 13 are supplied with current from a gradient magnetic field coil power supply 12 operated by a signal from the sequencer 2 to generate a gradient magnetic field. Where MRI
The photographing principle of the device will be described. As shown in FIG. 10A, the nucleus placed in the static magnetic field H ₀ in the Z direction behaves like a single bar magnet in classical physics, and at the Larmor frequency ν ₀ described above. Precessing around the Z axis. This frequency is given by equation (2) and is proportional to the strength of the static magnetic field. Γ in the equations (1) and (3) is called a gyromagnetic ratio, and has a value specific to an atomic nucleus. In general, the number of nuclei to be measured is enormous, and each is rotating at an arbitrary phase.
In-plane components cancel each other, and macroscopic magnetization of only the Z-direction component remains. In this state, a high-frequency magnetic field H ₁ having a frequency f ₀ equal to the Larmor frequency ν ₀ is applied in the X direction (FIG. 10).
(B)), the macroscopic magnetization starts to fall in the Y direction. This falling angle is almost proportional to the product of the amplitude of H ₁ and the application time,
The H ₁ when fall 90 ° with respect to pulse application time point 90 ° pulse, the H ₁ when fall 180 degrees is referred to as a 180 ° pulse.

[0006] A two-dimensional Fourier imaging method is a method generally used at present for imaging with an MRI apparatus. FIG. 9 shows a typical pulse sequence of a typical spin echo method among these methods. In this pulse sequence, a 90 ° pulse 25 is applied first, and then a 180 ° pulse 26 is added after a time of Te / 2 where the echo time is Te. After applying a 90 ° pulse 25 (FIG. 1)
0 (b)), each spin is XY at its own speed
Since rotation starts in the plane, a phase difference occurs between the spins over time. Here, when the 180 ° pulse 26 is applied, the spins are inverted symmetrically to the x ′ axis, and thereafter, at the time Te, the spins are focused again to continue the rotation at the same speed to form the echo signal 30.

Although the signal is measured as described above, the spatial distribution of the signal must be obtained in order to form a tomographic image. For this purpose, a linear gradient magnetic field is used. A spatial magnetic field gradient can be created by superimposing a gradient magnetic field on a uniform static magnetic field. As described above, the spin rotation frequency is proportional to the magnetic field strength, so that the spin rotation frequency is spatially different when a gradient magnetic field is applied. Therefore,
By examining this frequency, the position of each spin can be known. For this purpose, a phase encoding gradient magnetic field 28 and a frequency encoding gradient magnetic field 29 are used.

The pulse sequence described above is used as a basic unit, and the intensity of the phase encoding gradient magnetic field 28 is changed every time, and is repeated a predetermined number of times, for example, 256 times, at a constant repetition time (TR). The measurement signal obtained in this way is
The spatial distribution of the macroscopic magnetization is obtained by performing the dimensional inverse Fourier transform. In the above description, the three types of gradient magnetic fields may be any of X, Y, and Z, or may be a composite of them, as long as they do not overlap each other. The above-mentioned basic principle of MRI is described in detail in "NMR Medicine" (Basic and Clinical) (edited by Nuclear Magnetic Resonance Medical Research Society, Maruzen Co., Ltd., published on January 20, 1984).

By the way, when performing NMR imaging as described above, search for the center frequency of the magnetic resonance signal,
It is necessary to tune the receiving coil and adjust the high-frequency magnetic field strength. Here, the center frequency search (hereinafter, abbreviated as frequency lock) will be described first. Resonance frequency f ₀ of the static magnetic field H ₀ by the equation (3) are proportional. Therefore, the static magnetic field H
_It can be seen that if ₀ does not change, f ₀ is constant. However, the magnitude of the static magnetic field usually varies, albeit slightly, due to various causes. Therefore, the resonance frequency also changes little by little. Therefore, prior to the main measurement, a magnetic resonance signal is obtained while sequentially changing the frequency of the high-frequency magnetic field (see FIG. 5). In FIG. 5, the frequency f ₀ at the point where the measurement signal is the maximum is defined as the resonance frequency.

Next, tuning of the receiving coil will be described with reference to FIGS. Receiving circuit 14, a capacitor 24 as shown in FIG. 8, the variable capacitance diode 23, constitute a resonant circuit in the receiving coil 14 is receiving a signal of the magnetic resonance frequency f _0. The resonance circuit of the receiving circuit 14 is affected by the capacity of the subject 7. For this reason, when the subject 7 or the examination site changes, resynchronization must be performed.
To tune the receiving coil, the capacitance of the variable capacitance diode 23 is controlled by voltage. Therefore, a magnetic resonance signal is obtained while controlling the variable capacitance diode 23 as described above in a state where the frequency lock has been performed prior to the main measurement (see FIG. 7). The applied voltage Vx of the variable capacitance diode 23 at the point where the measurement signal is maximum in FIG. 7 is applied during the actual measurement.

Next, adjustment of the intensity of the high-frequency magnetic field applied to the subject (hereinafter referred to as irradiation intensity adjustment) will be described. As described above, a high-frequency magnetic field that defeats macroscopic magnetization by 90 ° is called a 90 ° pulse.
Is a load. Therefore, when the subject 7 changes, the current flowing through the irradiation coil 11 for the 90 ° pulse must be changed. Here, in the spin echo method described above, for example, when the 90 ° pulse is applied, the spin fall angle is 80 °, and when the 180 ° pulse is
Suppose that it becomes 0 °. At that time, the received magnetic resonance signal is smaller than the magnetic resonance signal received at the time of the 90 ° and 180 ° pulses. 90 without considering the relaxation phenomenon
At the time of the {, 180} pulse, the magnetic resonance signal becomes maximum. In order to find the maximum point, a magnetic resonance signal is obtained while changing the current flowing through the irradiation coil 11, and the current when the resonance signal becomes maximum is measured, and this is used in the main measurement.

Here, the procedure of the conventional imaging method will be described.
This will be described with reference to FIG. FIG. 4 shows the flow of the imaging procedure. The subject is placed on a bed 22 movable at least in the body axis direction, and the center of the imaging region of the subject is determined by a light localizer (optical positioning device) or the like (step 41). Next, the center of the imaging region is moved to the center of the static magnetic field (step 42). Next, various imaging conditions are determined, and a measurement start button is pressed (step 4).
3). After that, pre-measurement (steps 44 to 46) is performed, main measurement (step 47) is performed, and an image is created. Further, when images having different image qualities are required (Step 48), the imaging start condition, the position, and the like are determined, and then the measurement start button (Step 43) is pressed. As described above, in order to perform the measurement, the measurement start button is pressed, the pre-measurement is performed and the main measurement is performed every time.

[0013]

Conventionally, as described above, frequency lock performed prior to measurement, tuning of a receiving coil, and the like are measured multiple times without moving the same subject.
In order which has been performed for each measurement in the case of shooting the same position at different photographing conditions, to carry out the pre-measurement for each measurement
The entire imaging time is longer and the time for restraining the subject, that is, the patient, is longer, which is not only painful, but also causes image quality deterioration such as artifacts due to body movement. Also, this has been an obstacle to improving the throughput.

The present invention solves these problems. After setting an object, the frequency lock and the tuning of the receiving coil are automatically performed, and the frequency locking and the tuning of the receiving coil are not performed for each measurement. Therefore, the object of the present invention is to shorten the restraint time of the patient and improve the throughput.

[0015]

In order to achieve the above object, frequency locking, tuning of a receiving coil, and the like are not performed for each measurement, but are performed when a part to be imaged of a patient is set at the center of a static magnetic field. Such means are provided.

[0016]

According to the present invention, frequency lock, tuning of the receiving coil, and the like performed prior to each measurement are conventionally performed for each measurement, so that the measurement time before each measurement is long and the subject, that is, the patient is restrained. Not only is it long and painful,
Image quality was degraded, such as artifacts due to body movement. Also, this has been an obstacle to improving the throughput. The present invention solves these problems. After setting a subject, automatically performs pre-measurement such as frequency lock and reception coil tuning, and does not perform frequency lock, reception coil tuning and the like for each measurement. Thereby, the restraint time of the patient can be shortened, and the throughput can be improved.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will now be described with reference to FIGS.
This will be described below. As shown in FIG. 3, the magnetic resonance imaging apparatus is roughly classified into a central processing unit (CPU) 1
, Sequencer 2, transmission system 3, and static magnetic field generating magnet 4
, A receiving system 5, and an image display storage system 6. The central processing unit (CPU) 1 controls each of the sequencer 2, the transmission system 3, the reception system 5, and the image display storage system 6, and reconstructs an image using output data of the reception system 5 according to a predetermined program. Things. The sequencer 2 operates based on a control command from the central processing unit 1, and
The transmission system 3, the gradient magnetic field generation system 21 of the static magnetic field generation magnet 4, and the reception system 5
The transmission system 3 includes a high-frequency oscillator 8, a modulator 9, and an irradiation coil 11 as a high-frequency coil. The modulator 9 amplitude-modulates a high-frequency pulse from the high-frequency oscillator 8 according to a command from the sequencer 2. Amplifying the amplitude-modulated high-frequency pulse via the high-frequency amplifier 10 and supplying the amplified pulse to the irradiation coil 11 irradiates the subject 7 with a predetermined pulsed electromagnetic wave. This is for generating a uniform static magnetic field in a predetermined space area accommodating the magnetic field 7. Inside the static magnetic field generating magnet 4, in addition to the irradiation coil 11, a gradient magnetic field coil 13 for generating a gradient magnetic field in a plurality of directions within the static magnetic field, and a receiving coil 14 of the receiving system 5.
Is installed. The gradient magnetic field generation system 21 includes a gradient magnetic field power supply 12 that supplies a current to a gradient magnetic field coil 13 that independently generates a gradient magnetic field in a plurality of Cartesian coordinate axes orthogonal to each other.

The receiving system 5 has a receiving coil 14, an amplifier 15 connected to the receiving coil 14, a quadrature detector 16, and an A / D converter 17.
After the MR signal is detected by the receiving coil 14 and the signal is amplified by the amplifier 15, it is converted into two series of collected data by the quadrature phase detector 16, and the data is converted into a digital amount at a timing according to a command of the sequencer 2. The image display / storage system 6 has an external storage device such as a magnetic disk 20 and an optical disk 19 and a display 18 such as a CRT. When input to the device 1, the central processing unit 1 executes processes such as signal processing and image reconstruction, displays a desired cross-sectional image of the subject 7 on the display 18, and displays a magnetic image of the external storage device. The information is recorded on the disk 20 or the like.

Next, the flow of imaging according to an embodiment of the present invention will be described with reference to FIG. The subject 7 is placed on a bed 22 movable at least in the body axis direction, and the subject 7 is moved by a light localizer (optical positioning device) (not shown).
Is determined (step 11). next,
The center of the imaging region is moved to the center of the static magnetic field (step 12). Thereafter, frequency locking (step 13), tuning of the receiving coil (step 14), and irradiation intensity adjustment (step 15) are performed. Then, the imaging condition is determined and the button for starting the measurement is pressed (step 16). When the measurement start button is pressed, the main measurement (step 17) is performed, and an image based on the measurement data is created. When images having different image qualities are required, the imaging start condition, the position, and the like are changed, and then the measurement start button is pressed again. In this case, the irradiation intensity adjustment may be omitted because the change due to the subject 7 is small.

Next, another embodiment of the present invention will be described with reference to FIG. FIG. 2 shows a flow of imaging according to another embodiment of the present invention. As in the embodiment shown in FIG.
Is placed on a bed 22 movable at least in the body axis direction, and by a light localide (optical positioning device) or the like,
The center of the imaging region of the subject 7 is determined (Step 21).
Next, the center of the imaging region is moved to the center of the static magnetic field (step 22). After that, frequency lock (Step 2)
3) Perform tuning of the receiving coil (step 24) and adjustment of irradiation intensity (step 25). Then, the imaging condition is determined, and the measurement start button is pressed (step 26). Thereafter, frequency lock is performed in step 27 for each measurement. Here, the frequency lock to be performed is performed in such a manner that the frequency variable range is narrowed because the frequency lock is performed when the subject 7 is set.
3 can be performed in a shorter time than the frequency lock. Thereafter, the main measurement (step 28) is performed to create an image. And if you need a different quality image,
After determining the imaging condition, position, etc., the measurement start button is pressed. In this case, the irradiation intensity adjustment may be omitted because the change due to the subject 7 is small.

[0021]

According to the present invention, a desired imaging section of a subject can be obtained.
When the position is set to the center of the static magnetic field, frequency lock, tuning of the receiving coil, and irradiation intensity adjustment are performed.
Frequency lock, tuning of receiving coil, and adjustment of irradiation intensity are not performed for each measurement even when performing multiple measurements with one subject
Therefore, to reduce the duty time of the patient, it is effective to improve the throughput.

[Brief description of the drawings]

FIG. 1 is a flowchart of an imaging process according to an embodiment of the present invention.

FIG. 2 is a flowchart of an imaging process according to another embodiment of the present invention.

FIG. 3 is a block diagram showing a configuration of a magnetic resonance imaging apparatus to which the present invention is applied.

FIG. 4 is a flowchart of an imaging process performed by a conventional device.

FIG. 5 is a diagram illustrating a relationship between a frequency of a high-frequency magnetic field and a magnetic resonance signal at the time of frequency locking.

FIG. 6 is a diagram illustrating a relationship between a current flowing through an irradiation coil and a magnetic resonance signal when adjusting a high-frequency magnetic field intensity.

FIG. 7 is a diagram showing a relationship between a voltage applied to a variable capacitance diode and a magnetic resonance signal when tuning a receiving coil.

FIG. 8 is a schematic equivalent circuit diagram of a receiving coil.

FIG. 9 is a diagram showing a pulse sequence of the spin echo method.

FIG. 10 is a diagram illustrating the behavior of spin.

[Explanation of symbols]

 2 Sequencer 7 Subject 8 High frequency transmitter 11 Irradiation coil 14 Receiving coil 22 Bed 23 Movable capacitance diode 30 Echo signal

Claims (2)

(57) [Claims] 1. A means for applying a static magnetic field to a subject, and causing magnetic resonance in a slice direction gradient magnetic field, a frequency encoding gradient magnetic field, a phase encoding gradient magnetic field, and nuclei of atoms constituting the tissue of the subject. Magnetic resonance comprising: means for repeatedly applying a high-frequency pulse to be applied in a predetermined pulse sequence; receiving means including a receiving coil for detecting a magnetic resonance signal; and means for obtaining a reconstructed image to be used for diagnosis based on the detection signal. In the imaging apparatus, each time a desired imaging region of the subject is set to the center of the static magnetic field,
What Sakiritsu this measurement, magnetic resonance signals from the predetermined frequency band
A magnetic resonance imaging apparatus comprising: a center frequency at which the maximum value is obtained, tuning of the receiving coil under the obtained center frequency, and adjustment means for adjusting the intensity of the high-frequency pulse. 2. The magnetic resonance imaging apparatus according to claim 1, wherein said adjusting means obtains a center frequency in a frequency band narrower than said frequency band for each measurement.