JPH08112267A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PURPOSE: To enable measuring of a chemical shift image free from the effect of unevenness of a magnetic field in a short time by applying the gradient polarity of a magnetic field inclined at least in one direction being inverted cyclically to adjust a shim coil current based on a peak position of a water spectrum extracted. CONSTITUTION: In the measuring of a magnetic resonance signal pertaining to a hydrogen atomic nucleus from a specimen 1, the gradient polarity of a magnetic field inclined at least in one direction is applied being inverted cyclically among the inclined magnetic fields in a plurality of directions. A chemical shift image of the specimen 1 is measured by using a multiple encoding method to extract the peak position of water spectrums at pixels of the image. In the extraction, the position at which the maximum intensity is gained is defined as peak position of the water spectrum in all spectrums at the pixels. Then, current flowing through shim coils 11 is changed based on the results of computation with an arithmetic device 5 so that the center position of the peak is uniform among the water spectrums at the pixels thereby improving the uniformity of a magnetostatic field.

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, and more particularly to a magnetic resonance imaging apparatus suitable for measuring the spatial distribution of information on chemical shift.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus causes a magnetic resonance phenomenon by irradiating a subject placed in a static magnetic field with a high frequency magnetic field of a specific frequency, and utilizes a magnetic resonance signal generated from the subject. Thus, the chemical / physical information of the substance can be imaged. Especially, Magnetic Resonance Spectroscopic Imaging (Magnetic Resonance
Spectroscopic Imaging (hereinafter referred to as MRSI) method is
This is a method of measuring a difference in magnetic resonance frequency (hereinafter, referred to as chemical shift) due to a difference in chemical bond of various molecules and obtaining a spatial distribution image (hereinafter, referred to as chemical shift image) of each molecule. Since the magnitude of the chemical shift is very small, in the order of parts per million (ppm), it is important to adjust the magnetic field homogeneity that affects the magnetic resonance frequency in the MRSI method.

In general, a subject such as a living body has a considerable influence on the homogeneity of the magnetic field. Therefore, it is necessary to improve the homogeneity of the magnetic field by including the subject. Therefore, a method of adjusting the magnetic field homogeneity by changing the amount of current flowing through the shim coil and superimposing the magnetic field generated by the shim coil on the static magnetic field has been proposed. This method requires reference data for determining the shim current, and a chemical shift image of the subject is used as this reference data. From the viewpoint of S / N, when the subject is a living body, the chemical shift of water in the subject is often used.

Further, without improving the magnetic field homogeneity, the data of the peak position of the water spectrum of each pixel extracted from the reference data is stored in a memory, and each spectrum of the chemical shift image is corrected based on the data. A method of doing so is proposed in JP-A-59-148854.

[0005]

However, there is a problem that a considerably long measurement time is required to obtain a chemical shift image of water in the subject used as the reference data. For example, when a chemical shift image having a matrix number of 16 × 16 and a repetition time of 1.5 seconds is measured by using the 3D-CSI method which is currently known as the most general MRSI method, the measurement time is at least 6 times. When the number of matrices is increased to 32 × 32, the measurement time is at least 25.6 minutes.

Further, in the case of ¹ H-MRSI, the magnetic resonance signal of the metabolite to be observed is smaller than the magnetic resonance signal of water by one thousandth or less, so that a chemical shift image of the metabolite is usually obtained. When obtaining, a large water signal is pseudo-saturated before measurement. Therefore, the peak position of the water spectrum of each pixel extracted from the chemical shift image of water is stored in the memory, and each spectrum of the chemical shift image obtained after the measurement of the data is corrected based on the data of the peak position. In such a case, it is necessary to perform the measurement twice, that is, the chemical shift image of water and the chemical shift image of the metabolite. It is an object of the present invention to provide a magnetic resonance imaging apparatus capable of obtaining a chemical shift image without the influence of magnetic field inhomogeneity in a short time.

[0007]

According to a first device of the present invention, when acquiring a magnetic resonance signal from a subject, a gradient magnetic field of at least one of the gradient magnetic fields of a plurality of directions is periodically changed. The magnetic field uniformity is adjusted by the shim coil based on the peak position of the water spectrum extracted from the obtained magnetic resonance signal by inverting and applying.

In the second apparatus according to the present invention, when measuring the magnetic resonance signal from the subject, the gradient polarity of the gradient magnetic field in at least one direction among the gradient magnetic fields in a plurality of directions is periodically inverted and applied, The peak position of the water spectrum of each pixel extracted from the obtained magnetic resonance signal is stored in the memory, and the spectrum of each pixel of the chemical shift image is corrected based on the data of the peak position. . The spectrum correction can be performed by obtaining a transformation that aligns the peak positions of the water spectrum in all pixels to a fixed position, and applying this transformation to the spectrum of each pixel.

A third apparatus according to the present invention selectively excites only the magnetization of water to measure the magnetic resonance signal of water, then pseudo-saturates the magnetization of water, and then continuously magnetizes the substance other than water. By exciting the magnetic resonance signal of a substance other than water by measuring, the chemical shift images of water and a substance other than water are acquired separately in one measurement, and the data of the chemical shift image of the obtained water is obtained. Based on the obtained chemical shift image of substances other than water, the spectrum of each pixel is corrected. The correction of the spectrum can be performed by performing conversion for aligning the peak positions of the water spectrum at a constant position in all the pixels described above.

In a fourth device according to the present invention, in the third device, when measuring a magnetic resonance signal relating to hydrogen nuclei from an object, a gradient of a gradient magnetic field in at least one direction among gradient magnetic fields in a plurality of directions. It is characterized in that the polarity is periodically inverted and applied. The magnetic resonance signal can be a signal relating to hydrogen nuclei, but may be a signal relating to other nuclei.

[0011]

When the magnetic resonance signal from the subject is acquired, the gradient polarity of the gradient magnetic field in at least one direction among the gradient magnetic fields in a plurality of directions is periodically inverted and applied to excite the magnetization once. Since the chemical shift information and the spatial information in at least one direction can be obtained at the same time by measuring the signal and the signal, the reference data for determining the current flowing in the shim coil or the spectrum of each pixel of the chemical shift image can be corrected. The chemical shift data of water in the subject, which is the reference data, can be obtained at high speed.

Further, only the magnetization of water is selectively excited to measure the magnetic resonance signal of water, and then the magnetization of water is pseudo-saturated. By measuring the magnetic resonance signal of a substance and separately obtaining the chemical shift data of water and a substance other than water in one measurement, the reference data for correcting the spectrum of each pixel of the chemical shift image can be obtained. The chemical shift data of water in the subject can be obtained at high speed.

At this time, the gradient polarities of the gradient magnetic fields in at least one direction among the gradient magnetic fields in a plurality of directions are periodically inverted and applied, so that the chemical shift information and at least the chemical shift information can be obtained by one-time magnetization excitation and signal measurement. Since spatial information in one direction can be obtained at the same time, chemical shift images of water and substances other than water can be acquired at high speed, and based on the obtained data of chemical shift images of water, chemical shift images of substances other than water can be obtained. Each spectrum of the image can be corrected.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a configuration example of a magnetic resonance imaging apparatus to which the present invention is applied. A subject 1 such as a living body is placed in a space to which a static magnetic field generated by a static magnetic field generating magnet 2 and gradient magnetic fields in three different directions generated by a gradient magnetic field generating coil 3 are applied. There may be a case where the shim coil 11 capable of adjusting the homogeneity of the static magnetic field is provided by changing the current passed through each coil. Subject 1
On the other hand, a high frequency magnetic field generated by the probe 4 is irradiated to cause a magnetic resonance phenomenon, and the magnetic resonance signal generated from the subject 1 is detected by the probe 4. The detected magnetic resonance signal is sent to the arithmetic unit 5 via the receiver 9, and the arithmetic unit 5 generates image information from the magnetic resonance signal and displays it on the display 6. Power supply unit 1 for driving the shim coil 11
2, power supply unit 7 for driving the gradient magnetic field generating coil 3, transmitter 8
And the receiver 9 is controlled by the sequence controller 10. Further, the measurement signal and the measurement condition are stored in the storage medium 13 as necessary.

FIG. 2 shows an example of a pulse sequence of a multi-encoding method, which is one of the high-speed MRSI methods for obtaining a chemical shift image at high speed, which is used in the present invention. In this pulse sequence, first, only the nuclear magnetization in the selected slice is rotated (excited) by simultaneously applying the first gradient magnetic field Gs1 for slice selection and the first high-frequency magnetic field RF1 called 90 ° pulse. ) Let me. Next, when the echo time is Te, Te / 2
After that, the second gradient magnetic field Gs2 for slice selection and 180 °
A second high-frequency magnetic field RF2 called a pulse is applied at the same time to invert the nuclear spin excited by the 90 ° pulse by 180 °, thereby generating an echo signal Sig centered at a time point after Te / 2. .

Then, when measuring the echo signal Sig, a gradient magnetic field Gr for read-out in which the gradient of the gradient magnetic field Gx is periodically inverted is applied, so that continuous information including spatial information in the X-axis direction is applied. An echo signal (hereinafter referred to as an echo train signal) can be generated. In addition,
The spatial information in the Y-axis direction is added to the echo signal Sig by the gradient magnetic field Ge for encoding. And the Ge
Is gradually changed, and excitation of magnetization and measurement of a signal are repeated at a repeating time Tr. A chemical shift image can be obtained by subjecting the obtained series of signals to three-dimensional inverse Fourier transform. The multiple encoding method is described in, for example, Japanese Patent Laid-Open No. 61-13143.

According to this multi-encoding method, the 3D-CSI currently known as the most general MRSI method is used.
Compared with the method, the dimension of measurement can be reduced by one dimension,
Higher speed is possible. This is because two-dimensional information including chemical shift information and spatial information in the X-axis direction can be simultaneously extracted from an echo train signal that can be acquired by one-time excitation of magnetization and measurement of a signal.

[First Embodiment] The first embodiment will be described below. First, the apparatus of FIG. 1 is used to measure the chemical shift image of the subject using the multiplex encoding method whose pulse sequence is shown in FIG. 2, and the peak position of the water spectrum of each pixel of the chemical shift image is extracted. . In extraction, the position of maximum intensity is generally taken as the peak position of the water spectrum in the entire spectrum of each pixel. Depending on the site, the peak intensity of the fat spectrum may exceed the peak intensity of the water spectrum. In that case, the difference between the chemical shifts of fat and water (about 3.5 ppm) should be utilized in advance to obtain water. The fat spectrum can be kept separate. Then, in the water spectrum of each pixel, the current flowing through each shim coil is changed based on the calculation result of the calculation device so that the center position of the peak becomes uniform, thereby improving the homogeneity of the static magnetic field. You can

For example, as shown in FIG. 3, when the peak position of the water spectrum of each pixel is shifted along the X-axis, it is found that there is magnetic field inhomogeneity in the X-axis direction within the subject, and the chemical shift is Since it is proportional to the magnetic field strength, the magnetic field strength at each pixel position, that is, the magnitude of the nonuniform magnetic field can be known. Therefore, the uniformity of the static magnetic field can be improved by adjusting the shim coil that can adjust the magnetic field distribution in the X-axis direction. As described in JP-A-5-7569, if the effect of each shim coil on the center position of the peak is measured in advance, the current flowing in each shim coil from the chemical shift image of water is calculated by an arithmetic unit. Can be calculated by

[Second Embodiment] A second embodiment will be described below. First, the chemical shift image of the subject is measured using the multiplex encoding method, and the peak position of the water spectrum of each pixel of the chemical shift image is extracted. In extraction, the position of maximum intensity is generally taken as the peak position of the water spectrum in the entire spectrum of each pixel. Depending on the site, the peak intensity of the fat spectrum may exceed the peak intensity of the water spectrum. In that case, the difference between the chemical shifts of fat and water (about 3.5 ppm) should be utilized in advance to obtain water. The fat spectrum can be kept separate. Then, the peak position of the water spectrum of each pixel is stored in the memory.
The spectrum of each pixel of the chemical shift image can be corrected based on the peak position data.

For example, when there is magnetic field inhomogeneity in the X-axis direction inside the subject, the peak position of the water spectrum of each pixel shifts along the X-axis as shown in FIG. 4A. Therefore,
A conversion F (P ₁ ) that aligns the peak positions of all pixels with the peak position of a certain pixel P ₁ can be obtained [FIG. 4 (b)]. Further, in the spectrum of each pixel of the chemical shift image other than water (for example, choline or creatine having a low S / N) [FIG. 4 (c)], the peak position shift due to the similar magnetic field inhomogeneity also occurs. Therefore, the conversion F (P
By carrying out ₁ ), it becomes possible to obtain the chemical shift image FIG. 4 (d) in which the influence of magnetic field inhomogeneity is removed.

[Third Embodiment] The third embodiment will be described below. FIG. 5 shows the 3D-CSI to which this embodiment is applied.
It is an example of the pulse sequence of the method. In this pulse sequence, first, only the magnetization of water is selected by applying a first high-frequency magnetic field RF3 called CHESS (Chemical Shift Selective) pulse having a limited excitation band, which is described in JP-A-60-168041. To excite.
Next, assuming that the echo time is Te1, after Te1 / 2, the first gradient magnetic field Gs3 for slice selection and the second high-frequency magnetic field RF4 called 180 ° pulse are simultaneously applied,
180 ° nuclear spin excited by RF3
By reversing, an echo signal Sig1 centered at a time point after Te1 / 2 is generated. The spatial information in the X-axis direction and the spatial information in the Y-axis direction are added to the echo signal Sig1 by the gradient magnetic fields Ge1 and Ge2 for encoding.

After acquisition of the echo signal Sig1, the magnetization of water is saturated by applying the gradient magnetic fields Gp1, Gp2 and Gp3 for spoiling, and subsequently, the second gradient magnetic fields Gs1 and 3 for slice selection are used. By applying the high-frequency magnetic field RF1 at the same time, only the nuclear magnetization of substances other than water in the selected slice is rotated (excited). Next, assuming that the echo time is Te2, a third gradient magnetic field Gs2 for slice selection and a second pulse called 180 ° pulse after Te2 / 2
The high frequency magnetic field RF2 is simultaneously applied, and the nuclear spins excited by the RF1 are inverted by 180 °, so that the echo signal Sig2 is further generated after Te2 / 2. The spatial information in the X-axis direction and the spatial information in the Y-axis direction are the gradient magnetic fields Ge3 and Ge4 for encoding.
Is added to the echo signal Sig2.

As a result, within one repetition time Tr,
It is possible to separately acquire magnetic resonance signals of water and substances other than water. And, the Ge1, Ge2, Ge
The gradients of 3 and Ge4 are changed stepwise, and excitation of magnetization and measurement of a signal are repeated at a repeating time Tr. By applying a three-dimensional inverse Fourier transform to each of the obtained series of signals Sig1 and Sig2,
Chemical shift images of water and substances other than water can be obtained. Then, in the same manner as in the second embodiment, based on the obtained data of the chemical shift image of water, a conversion was found to align the peak positions of the water spectrum with respect to all the pixels, and the obtained water was obtained. By subjecting the chemical shift image of a substance other than the above to the conversion, it is possible to correct the influence of the nonuniform magnetic field on the spectrum of each pixel.

[Fourth Embodiment] The fourth embodiment will be described below. FIG. 6 is an example of a pulse sequence of the multi-encoding method to which this embodiment is applied. In this pulse sequence, first, only a magnetic field of water is selectively excited by applying a first high-frequency magnetic field RF3 called a CHESS pulse having a limited excitation band. Next, assuming that the echo time is Te1, after Te1 / 2, the first gradient magnetic field Gs3 for slice selection and the second high-frequency magnetic field RF4 called a 180 ° pulse are simultaneously applied, and the nuclear spin excited by the RF3 is applied. Is inverted by 180 ° to generate an echo signal Sig1 centered at a time point after Te1 / 2. The spatial information in the X-axis direction is based on the gradient magnetic field Gr1 for readout, and the spatial information in the Y-axis direction is based on the gradient magnetic field Ge for encoding.
1 is added to the echo signal Sig1.

After acquisition of the echo signal Sig1, the magnetization of water is saturated by applying the gradient magnetic fields Gr1, Gr2 and Gr3 for spoil, and subsequently the second gradient magnetic field Gs1 for slice selection and the third high frequency. By applying the magnetic field RF1 at the same time, only the nuclear magnetization of substances other than water in the selected slice is rotated (excited). Next, assuming that the echo time is Te2, after Te2 / 2, a third gradient magnetic field Gs2 for slice selection and a second high-frequency magnetic field RF2 called a 180 ° pulse are simultaneously applied, and the nuclear spin excited by the RF1 is applied. By 180 ° inversion, an echo signal Sig2 is further generated after Te2 / 2. The spatial information in the X-axis direction is added to the echo signal Sig2 by the gradient magnetic field Gr2 for readout, and the spatial information in the Y-axis direction is added to the echo signal Sig2 by the gradient magnetic field Ge2 for encoding.

This makes it possible to separately obtain magnetic resonance signals of water and substances other than water within one repetition time Tr. Then, the gradients of Ge1 and Ge2 are changed stepwise, and excitation of magnetization and measurement of a signal are repeated for a repeating time Tr. A chemical shift image of water and a substance other than water can be obtained by applying a three-dimensional inverse Fourier transform to each of the obtained series of signals Sig1 and Sig2. Then, in the same manner as in the second embodiment, based on the obtained data of the chemical shift image of water, a conversion was found to align the peak positions of the water spectrum with respect to all the pixels, and the obtained water was obtained. By subjecting the chemical shift image of a substance other than the above to the conversion, it is possible to correct the influence of the nonuniform magnetic field on the spectrum of each pixel.

As the high-speed MRSI method, in addition to the multi-encoding method, the Magnetic Resonance in Medicine No. 1
The same effect can be obtained when the EPSM method and the PREP method proposed by Mansfield described in Vol. 370-386 (1984) are used.

Further, in each of the above embodiments, the case of three-dimensional measurement (chemical shift axis, X axis, Y axis) was described, but four-dimensional measurement (chemical shift axis, X axis, Y axis, Z).
In the case of (axis), the same effect can be obtained. Also, when observing magnetic resonance signals other than hydrogen atoms, the same effect can be obtained by improving the magnetic field homogeneity in advance by using the method of the above-described embodiment.

[0030]

According to the present invention, it is possible to provide a magnetic resonance imaging apparatus capable of measuring a chemical shift image in a short time without the influence of magnetic field inhomogeneity.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a magnetic resonance imaging apparatus.

FIG. 2 is a diagram showing a pulse sequence of a multiple encoding method.

FIG. 3 is an explanatory diagram of the first embodiment.

FIG. 4 is an explanatory diagram of the second embodiment.

FIG. 5 is a diagram showing a pulse sequence of the third embodiment.

FIG. 6 is a diagram showing a pulse sequence of the fourth embodiment.

[Explanation of symbols]

RF ... high-frequency magnetic field, Gz ... Z-axis gradient magnetic field, Gx ...
X-axis gradient magnetic field, Gy ... Y-axis gradient magnetic field, RF
1 ... 90 ° pulse, RF2 ... 180 ° pulse, Gs1 ...
90 ° slice magnetic field, Gs2 ... 180 ° slice magnetic field,
Gr ... Readout magnetic field, Ge ... Encode magnetic field, Si
g ... Magnetic resonance echo signal, Tr ... Repetition time, Te ... Echo time, RF3 ... CHESS pulse, RF4 ... 180
Pulse, Gs3 ... 180 ° slice magnetic field, Gp1, G
p2, Gp3 ... spoil magnetic field, Ge1, Ge2, Ge
3, Ge4 ... Encoding magnetic field, Sig1, Sig2 ... Magnetic resonance echo signal

Claims (4)

[Claims] 1. A static magnetic field generating means, a shim magnetic field generating means for adjusting the homogeneity of the static magnetic field, a gradient magnetic field generating means for generating gradient magnetic fields in three different directions, a high frequency magnetic field generating means, and An arithmetic means including a signal detection means for detecting a magnetic resonance signal from the specimen, an arithmetic means for performing an arithmetic operation on the magnetic resonance signal obtained from the signal detection means, and a sequence control device for controlling the operation sequence of each means. In a magnetic resonance imaging apparatus for generating a chemical shift image by means of a sequence controller, the sequence controller has a function of periodically inverting and applying the gradient polarity of the gradient magnetic field in at least one direction among the gradient magnetic fields generated by the gradient magnetic field generating means. Then, the calculating means extracts the peak position of the chemical shift signal due to the hydrogen nucleus of water from the magnetic resonance signal for each pixel, Click a magnetic resonance imaging apparatus characterized by having a function of adjusting the shim magnetic field generating means to be the same position. 2. A static magnetic field generating means, a gradient magnetic field generating means for generating gradient magnetic fields in three different directions, a high frequency magnetic field generating means, a signal detecting means for detecting a magnetic resonance signal from a subject, and a signal detecting means. Computing means for computing the magnetic resonance signal obtained by the means, data storage means,
In a magnetic resonance imaging apparatus that includes a sequence control unit that controls the operation sequence of each unit, generates a chemical shift image by the calculation unit, and stores the generated image data in the storage unit, the sequence control unit is configured by the gradient magnetic field generation unit. Of the generated gradient magnetic field, it has a function of periodically inverting the gradient polarity of the gradient magnetic field in at least one direction and applying it, and the calculation means is the peak position of the chemical shift signal due to the hydrogen nucleus of water from the magnetic resonance signal. A magnetic resonance imaging apparatus having a function of extracting each pixel for each pixel, obtaining a conversion for aligning the peak positions of all the pixels to a constant position, and performing the conversion on the peak position of the chemical shift signal of each pixel. . 3. Static magnetic field generation means, gradient magnetic field generation means for generating gradient magnetic fields in three different directions, high frequency magnetic field generation means, signal detection means for detecting magnetic resonance signals from the subject, and signal detection. A magnetic resonance imaging apparatus for generating a chemical shift image by the calculating means, which includes a calculating means for calculating a magnetic resonance signal obtained from the means, and a sequence controlling means for controlling an operation sequence of each means. Selectively excites only the hydrogen nuclei of water and measures the magnetic resonance signal, then pseudo-saturates the magnetization of the hydrogen nuclei of water, and subsequently excites the magnetization of the nuclei of substances other than water to generate the magnetic resonance signal. A sequence that separately acquires the chemical shift signal caused by hydrogen nuclei of water and other chemical shift signals by one measurement. The calculation means extracts the peak position of the chemical shift signal resulting from the hydrogen nuclei of water from the magnetic resonance signal for each pixel, and obtains the conversion for aligning the peak positions of all the pixels to a fixed position. A magnetic resonance imaging apparatus having a function of performing the conversion at a peak position of a chemical shift signal other than a chemical shift signal caused by hydrogen nuclei of water in a pixel. 4. A static magnetic field generating means, a gradient magnetic field generating means for generating gradient magnetic fields in three different directions, a high frequency magnetic field generating means, a signal detecting means for detecting a magnetic resonance signal from a subject, and a signal detecting means. A magnetic resonance imaging apparatus for generating a chemical shift image by the calculating means, which includes a calculating means for calculating a magnetic resonance signal obtained from the means, and a sequence controlling means for controlling an operation sequence of each means. Is generated by selectively exciting only the hydrogen nuclei of water and applying the gradient magnetic field generated by the gradient magnetic field generating means by periodically inverting the gradient polarity of the gradient magnetic field in at least one direction. After measuring the magnetic resonance signal, the magnetization of hydrogen nuclei in water is pseudo-saturated, and then the nuclei of substances other than water are excited, The chemical shift caused by hydrogen nuclei in water is measured once by measuring the magnetic resonance signal caused by the nuclei of substances other than water by periodically inverting the gradient polarity of the gradient magnetic field in one direction and applying it. Performs sequence control to acquire the image and other chemical shift images, and the calculating means extracts the peak position of the chemical shift signal resulting from the hydrogen nuclei of water from the magnetic resonance signal for each pixel, and the peak in all the pixels. A magnetic resonance imaging apparatus having a function of obtaining a conversion for aligning positions to a fixed position and performing the conversion at a peak position of a chemical shift signal other than the chemical shift signal caused by hydrogen nuclei of water in each pixel.