JPH07327954A - Resonance frequency measuring method and magnetic resonance imaging system - Google Patents

Abstract

PURPOSE:To obtain an image with high S/N by accurately measuring the resonance frequency of a proton atomic nucleus of a water molecule in an HRI system and applying a high frequency pulse of accurate resonance frequency. CONSTITUTION:An echo signal is measured by a spin echo method, and it is frequency-decomposed by applying one-dimensional Fourier transformation, and the resonance frequency can be found from the maximum value of spectrum. At this time, echo time from the generation of spin to the measurement of the echo signal is set as a time sufficient for the attenuation of the signal of a proton other than the water molecule, for example, fat is measured selectively. The proton other than the water molecule can be effectively suppressed by using the resonance frequency found as the frequency of a pre-pulse of an MTC method. An inversion recovery method can be employed as the sequence of resonance frequency measurement.

Description

Detailed Description of the Invention

[0001]

The present invention relates to nuclear magnetic resonance (hereinafter referred to as NM
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging (hereinafter, referred to as MRI) apparatus utilizing a phenomenon (R), and more particularly, to a measurement method for measuring a resonance frequency of proton nuclei of water molecules to be imaged and an MRI apparatus including the measurement sequence.

[0002]

2. Description of the Related Art An MRI apparatus uses a magnetic resonance phenomenon to measure the density distribution, relaxation time distribution, etc. of nuclear spins (hereinafter simply referred to as "spins") at a desired inspection site in a subject, and the measurement is performed. From the data, a tomographic image of the examination site can be displayed. In order to obtain this measurement data, the MRI apparatus applies a high frequency pulse having a spin resonance frequency and a gradient magnetic field pulse for imparting position information to a subject placed in a static magnetic field in a predetermined sequence, and this sequence is applied. Is repeated several times.

Various sequences are known as this sequence. Among them, a high contrast image is obtained by utilizing the difference in the magnetization exchange structure between the proton nucleus of water and the proton nucleus in the polymer such as fat. Get MTC (Magnet
ization transfer contrast method has been proposed (Journal of Magnetic Resonance, No. 55,
283-300, 1983, Magnetic Resonance
Quarterly No. 8, No. 2, pp. 116-137).

In this method, as a prepulse, a binomial pulse (binomial pulse) is used to semi-selectively excite proton nuclei of substances other than water to make the signal non-signal. Substance (water molecule)
A high frequency pulse having a resonance frequency of In this case, if the frequency of the binomial pulse is not the exact resonance frequency of the proton nuclei of the water molecule, it is completely zero for this frequency.
Since the pulse does not occur, the longitudinal magnetization of water molecule protons at this frequency is rather reduced, and the signal intensity is reduced, resulting in an image with deteriorated S / N. Therefore, it is necessary to obtain an accurate resonance frequency of the proton nucleus of the water molecule.

Conventionally, in the method of obtaining the resonance frequency of the object to be imaged, the object to be imaged is excited by a high-frequency pulse, the signal is one-dimensional Fourier transformed, and the frequency corresponding to the peak of the spectrum is obtained.

[0006]

However, when there are two signals whose resonance frequencies are very close to each other in the object to be photographed, they are sometimes observed as one signal due to the combination with the sampling resolution. In that case, the observed resonance frequency is not a true value but an intermediate value between the two signals.

In general, the proton nucleus of fat has a resonance frequency very close to the resonance frequency of the proton nucleus of water molecule, which is only 3.4 ppm lower. Therefore, in the conventional measurement of the resonance frequency, there is no way to distinguish between the signals of the proton nuclei of water molecules contained in the object to be imaged and the proton nuclei of non-water molecules, especially the signals from the proton nuclei of fat in the body It was not possible to determine the exact resonance frequency of the proton nucleus of the water molecule because the signals of different resonance frequencies were mixed in.

The present invention attenuates or eliminates signals other than the signals from the proton nuclei of water molecules, particularly the signals from the proton nuclei of fats, and makes it possible to accurately determine the resonance frequency of the proton nuclei of water molecules. It is an object to provide a frequency measuring method.

[0009]

In order to achieve the above object, a resonance frequency measuring method according to the present invention uses a gradient for measuring a magnetic resonance signal from a proton nucleus by applying a high frequency pulse to excite a proton nucleus spin. By repeating the sequence of applying a magnetic field and performing a one-dimensional Fourier transform on the magnetic resonance signal obtained from this sequence,
When determining the resonance frequency of the proton nuclei, select the magnetic resonance signal from the proton nuclei of the water molecule by using the difference in the decay rate between the signal from the proton nuclei of the water molecule and the signal from the proton nuclei other than the water molecule. Measure.

In the first aspect of the present invention, the sequence for signal measurement includes the spin echo method, the echo time from spin excitation to measurement is extended from the original echo time, and the signal of fat proton nuclei is attenuated. After that, the signal is measured, and thereby the signal of the proton nucleus of the water molecule is selectively measured. In the second aspect of the present invention, the sequence for signal measurement uses an inversion recovery method (hereinafter referred to as IR method), and the inversion time from the first high-frequency pulse application to the next high-frequency pulse application Select and measure when the proton nucleus signal becomes zero. As a result, only the signal from the proton nucleus of the water molecule is measured.

The MRI apparatus of the present invention includes such a measurement sequence of resonance frequencies, performs this measurement sequence before the sequence for imaging, and obtains the resonance frequency from a high frequency pulse or fat or the like required for imaging. The irradiation frequency of the preparation pulse that suppresses the signal is used.

[0012]

The longitudinal relaxation time (T ₁ ) and transverse relaxation time (T ₂ ) of excited proton nuclear spins of fat are generally shorter than those of water molecules. The signal strength of the signal obtained by the spin echo method is the transverse relaxation time (T ₂ )
Is a function whose time constant is, the echo signal in which the signal of the proton nucleus of fat is attenuated can be measured by extending the echo time from the excitation to the measurement. Since this signal is almost the signal from the proton nucleus of the water molecule, the resonance frequency of the proton nucleus of the water molecule can be obtained from the maximum value of the frequency spectrum by the one-dimensional Fourier transform.

In the IR method, the first excited spin returns to the original magnetization state after the state in which the longitudinal magnetization is zero.
Since the longitudinal relaxation time (T ₁ ) of the proton nucleus of water molecule is different from that of fat proton nucleus, the longitudinal magnetization of the proton nuclear spin of water molecule is not zero and the longitudinal magnetization of the proton nuclear spin of fat is zero. Inversion time can be selected for the state. As a result, the signal from the proton nucleus of the fat becomes zero, and the signal from the proton nucleus of the water molecule can be selectively measured. Therefore, also in this case, the resonance frequency of the proton nucleus of the water molecule can be obtained.

By performing such resonance frequency measurement prior to imaging, the resonance frequency of the proton nuclei of water molecules must be accurately irradiated, and fat suppression can be effectively performed in imaging such as the MTC method. A pulse can be applied, and an image with higher S / N can be obtained.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 is a block diagram showing a schematic configuration of an MRI apparatus for carrying out the magnetic resonance imaging method of the present invention. This MRI apparatus includes a magnet 201 for generating a uniform static magnetic field in a space in which the subject is placed, and a subject Excitation system 202 for generating a high-frequency magnetic field for producing each magnetic resonance in the nuclear spins of the atoms constituting the tissue, reception system 203 for receiving and detecting a signal generated from the subject, and then performing A / D conversion, magnetic field Image based on the measurement data from the gradient magnetic field generation system 204 and the reception system 203 capable of independently changing the strength of the magnetic field into a linear system in three axial directions (X, Y, Z directions) orthogonal to each other. An image processing system 205 that performs various calculations necessary for reconstruction, a sequence control system 206 that controls the operation timing of each system in the above configuration, Probe 207 to be used for sending and receiving frequency, and operation console 2 for operations
It is equipped with 08.

In such an MRI apparatus, a gradient magnetic field for giving positional information to a high frequency pulse and an NMR signal is repeatedly applied to a subject placed in a measurement space in which a static magnetic field is formed, at a predetermined intensity and timing. Then, the NMR signals necessary for image formation are collected. The sequence from application of the high frequency pulse and gradient magnetic field to signal measurement is set in the sequence control system 206. The sequence control system 206 incorporates a sequence for resonance frequency search described below as a preceding stage of the sequence for such imaging.

FIG. 1 shows a pulse sequence for measuring the resonance frequency by the spin echo (SE) method as one embodiment of the present invention. In this sequence, two high frequency pulses 101, 1 are used to excite the proton nuclei to be imaged.
02 is used, and a gradient magnetic field 10 for selectively exciting the imaging target simultaneously with these high frequency pulses 101 and 102.
3 is added. The gradient magnetic field may not need to be used depending on the object. The echo signal 104 is received by the A / D converter of the receiving system 203 in a section 105 after a predetermined echo time (TE) from the first application of the high frequency pulse 101.

As the resonance frequency of the high frequency pulses 101 and 102, a theoretical frequency obtained from the magnetic field strength is used. That is, the resonance frequency ω of the proton nuclear spin is an equation ω = γH (γ is a gyromagnetic ratio and is a unique value for each type of nucleus when the magnetic field strength (static magnetic field and gradient magnetic field superimposed thereon) is H. There is) required. The high-frequency pulse 101 is a 90-degree pulse in which the longitudinal magnetization of the spin is inverted by 90 degrees, and the high-frequency pulse 102 is a 180-degree pulse in which the longitudinal magnetization of the spin is inverted, and is applied after TE / 2 from the application of the high-frequency pulse 101. Here, the echo time TE is set to be long enough to attenuate the signal from fat. This will be further explained.

First, the spin is tilted by 90 degrees by applying the high-frequency pulse 101, but the phase becomes more prominent over time due to the nonuniformity of the magnetic field. If the 180-degree pulse 102 is applied after t time to invert the magnetization, the phases will be aligned after a further t time. Therefore 9
The echo signal can be observed 2t after the 0 degree pulse. The intensity of this echo signal, when attenuated with a constant T _2, the time constant T _2, i.e. transverse relaxation time is different from the proton nuclei of proton nuclei and the water molecules of the fat signal of fat attenuates faster than the other signal . Therefore, the time (T
By setting E) to a time sufficient for the fat signal to be attenuated to some extent, the water molecule signal can be selectively measured.

Table 1 shows the longitudinal relaxation T ₁ value and the transverse relaxation T ₂ value of the internal tissues at a magnetic field strength of 0.5T. As you can see from the table, the relaxation time of fatty protons is
Shorter than other organizations.

[0021]

[Table 1]

FIG. 3 is a graph showing the temporal change in the signal intensity of each tissue in the SE method. As described above, since each tissue attenuates at the T ₂ value, the fat signal decays faster than the other signals, and becomes about half or less of the gray matter (which can be considered as a water molecule) at about 100 ms. Therefore, in the sequence of FIG. 1, if the echo time TE up to signal measurement is set to about 100 ms, the signal from the fat tissue can be attenuated and the signal from the proton nucleus of the water molecule can be measured.

Strictly speaking, the echo signal measured in this manner is a signal in which various chemical shift components are combined, and A /
After D conversion, one-dimensional Fourier transform is performed for frequency decomposition. By obtaining the maximum value of the obtained frequency spectrum, the resonance frequency of the proton nucleus of the water molecule can be obtained. Although the 90-degree pulse is used as the high-frequency pulse 101 in the above embodiment, a pulse having a smaller flip angle may be used.

Next, a resonance frequency measuring method which is another embodiment of the present invention will be described. FIG. 2 shows a pulse sequence of the IR method according to this embodiment. In this sequence, a 180-degree pulse 401 for inverting the longitudinal magnetization of spins of proton nuclei is used as a high-frequency pulse, and a 90-degree pulse 402 and a 180-degree pulse 403 for measuring the excited proton nucleus signal as a spin echo signal are used. Similar to the SE method, a gradient magnetic field 404 for selectively exciting the imaging target is applied at the same time as these high frequency pulses. The gradient magnetic field 404 may not need to be used depending on the object. The echo signal 405 is measured in the A / D conversion section 406.

In this sequence, the inversion time (T
I) is the time when the fat signal becomes almost zero. That is, the longitudinal magnetization reversed by applying the high frequency pulse 401 is recovered by the longitudinal relaxation over time,
Return to the original point through the zero point. In this relaxation process, a 90 degree pulse is applied t hours after the application of the high frequency pulse 401, and a 180 degree pulse is further applied, whereby the magnitude of longitudinal magnetization can be measured as an echo signal 405. Here, if the longitudinal magnetization is just zero after t hours, no signal is obtained. Time t when longitudinal magnetization becomes zero
₀ depends on the longitudinal relaxation time T ₁ (t ₀ = T ₁ ln2), and Table 1
As shown in, the protons of fat and the protons of water molecules are different. Therefore, by setting the time t ₀ when the longitudinal magnetization of fat is zero to the inversion time (TI), the fat signal can be zero and the water molecule signal can be selectively measured.

FIG. 5 is a graph showing the change over time in the signal intensity (magnetic field intensity 0.5 T) of each tissue in the IR method. Since each tissue recovers at the T ₁ value, the fat signal recovers faster than the other tissue signals, and the longitudinal magnetic field becomes zero in about 100 ms. Therefore, when the magnetic field strength is 0.5T,
In the sequence of FIG. 4, the inversion time TI is set to 1
At around 00 ms, the signal from the adipose tissue has almost disappeared, and only the signal from the proton nucleus of the water molecule can be measured. When the magnetic field strength is different, the inversion time is also different.

The echo signal measured in this way is frequency-decomposed by one-dimensional Fourier transform after A / D conversion, as in the case of the SE method, and the maximum value of the obtained frequency spectrum is obtained. The resonance frequency of the proton nucleus of the water molecule can be obtained. The sequence for searching the resonance frequency of the proton nuclei of water molecules has been described above. However, the resonance frequency differs depending on the magnetic field strength and is affected by the patient and the imaging site. Therefore, this sequence is executed as a pre-stage of imaging in the MRI apparatus. It is preferable to do so.

FIG. 6 shows a flowchart for performing resonance frequency search before photographing. Here, for example, when the patient or the imaging region is changed (601) before the imaging, the sequence (602) for the resonance frequency search starts automatically or manually, and then the original imaging sequence (603) starts. , An image is displayed (604). The sequence for imaging is not particularly limited, but in the MTC method, the frequency of the pre-pulse that semi-selectively excites the spin of the proton nucleus of fat is semi-selectively determined in advance in the sequence (602) to effectively output the fat signal. It is possible to suppress and obtain a high-contrast image.

[0029]

As is apparent from the above description, according to the resonance frequency measuring method in the MRI apparatus of the present invention, the signal of the proton nucleus other than the target substance (water molecule) is attenuated or disappeared. Since the signal is measured, the signal of the water molecule can be selectively measured and the resonance frequency can be accurately obtained.

Further, according to the MRI apparatus of the present invention, the sequence for measuring the resonance frequency is performed before the imaging so that the resonance frequency of the proton nucleus of the water molecule must be accurately irradiated. For example, fat suppression. A pulse or the like can be effectively applied, and an image with good S / N can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a pulse sequence of an SE method which is an embodiment of a resonance frequency measuring method according to the present invention.

FIG. 2 is a block diagram showing the overall configuration of an MRI apparatus to which the present invention is applied.

FIG. 3 is a graph showing vertical relaxation of each tissue.

FIG. 4 is a diagram showing a pulse sequence of an IR method which is another embodiment of the resonance frequency measuring method according to the present invention.

FIG. 5 is a graph showing lateral relaxation of each tissue.

FIG. 6 is a flowchart showing an imaging method to which the present invention is applied.

[Explanation of symbols]

202 ... High-frequency magnetic field generating means (excitation system) 203 ... Measuring means (reception system) 204 ... Gradient magnetic field generating means (gradient magnetic field generating system) 205. ..Signal processing means (image processing system) 206 ... Sequencer (sequence control system)

Claims (4)

[Claims] 1. A magnetic resonance imaging apparatus for exciting a proton nuclear spin that constitutes a specific tissue in a subject by nuclear magnetic resonance and reconstructing an image of the tissue using a magnetic resonance signal generated from the tissue. A method of measuring the resonance frequency of proton nuclei of a molecule, in which a high-frequency pulse is applied to excite proton nucleus spins,
To selectively measure the magnetic resonance signal from the proton nucleus of the water molecule by utilizing the difference in the decay rate between the signal from the proton nucleus of the water molecule and the signal from the proton nucleus other than the water molecule, A resonance frequency measuring method characterized in that a resonance frequency of a proton nucleus of the water molecule is obtained by performing a sequence in which the application is repeated and performing a one-dimensional Fourier transform on a magnetic resonance signal obtained from the sequence. 2. A sequence for measuring the magnetic resonance signal is a spin echo method, and an echo time from excitation of spins to measurement of an echo signal in the sequence attenuates a signal from a proton nucleus other than the water molecule. The resonance frequency measuring method according to claim 1, wherein the resonance frequency measuring time is sufficient. 3. The sequence for measuring the magnetic resonance signal is an inversion recovery method, and the inversion time from the first high-frequency pulse application to the next high-frequency pulse application in the sequence has a The resonance frequency measuring method according to claim 1, wherein the time from which the signal from the atomic nucleus becomes zero is set. 4. A high-frequency magnetic field generating means for exciting spins at a specific site in the body, a gradient magnetic field generating means for applying a phase-encoding gradient magnetic field that gives a phase change to the excited spins, and an echo signal while applying a gradient magnetic field. In a magnetic resonance imaging apparatus having a measuring means for measuring, a sequencer for controlling the procedure from the excitation to the echo signal measurement, and a signal processing means for reconstructing an image using the echo signal, the sequencer is a water molecule. A sequence for measuring the resonance frequency of the proton nuclei of the water molecule, wherein the sequence utilizes the difference between the signal from the proton nucleus of the water molecule and the signal from the proton nucleus other than the water molecule and the decay rate The magnetic resonance signal from the magnetic resonance signal obtained by the sequence is selectively measured. A magnetic resonance imaging apparatus, wherein the resonance frequency of the proton nuclei of the water molecule is obtained by performing a one-dimensional Fourier transform on the signal.