JP6548257B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging apparatus, and more particularly, to a magnetic resonance imaging apparatus capable of magnetic resonance elastography (MRE) for measuring the hardness of tissue.

In the medical field, an MRI apparatus (Magnetic Resonance Imaging apparatus: magnetic resonance imaging apparatus) which is less affected by radiation exposure and the like to a patient is used. In an MRI apparatus, a hydrogen magnetic field (proton) contained in each cell of the human body is excited by applying a high frequency magnetic field corresponding to the spin of proton, and the excited proton is emitted when it returns to its original state (relaxation) Based on the electromagnetic waves, it is possible to obtain an image in which portions with different proton density (for example, water and fat) are represented by shading.
MRE (Magnetic Resonance Elastography) is the difference in "hardness" inside the target part by imaging with an MRI device while applying vibration (about 50 Hz in the case of a trunk) to the target It is an imaging method of imaging hardness by utilizing the difference in propagation of vibration waves due to the above (see Patent Documents 1 and 2).

  In general, lesions become harder as the grade of malignancy increases, and "palpation" for diagnosing the grade of malignancy has been practiced since ancient times. Although palpation is a simple and effective diagnostic method, it is difficult to palpate deep parts of the body and parts surrounded by bones and the like. In MRE, forcible vibration is generated on the human body surface, and the difference in local hardness is imaged from the appearance that the vibration wave propagates in the body. This enables the MRE to measure the hardness of the part that is difficult to palpate, and may provide an image with an entirely different diagnostic value.

As a technology related to MRE, the technology described in Non-Patent Document 1 below is known.
Non-Patent Document 1 describes a multi-echo type sequence in which a plurality of timings (echo time: TE) for obtaining a signal are obtained during an interval (repeat time TR) to which a high frequency magnetic field is applied when acquiring MR elastography images. Describes a technique for obtaining an image emphasizing the propagation of an oscillatory wave in the body by applying a readout gradient magnetic field, which is a gradient magnetic field for reading an MR signal, in synchronization with each echo time. .

  On the other hand, in the conventional MRI (Magnetic Resonance Imaging) apparatus, the Dixon method has long been used to obtain an MR image in which water or fat is emphasized (see Patent Documents 3 and 4). The Dixon method makes use of the resonance frequency difference between water and fat to produce a water image and a fat image reflecting only the signal from water in the tissue or the signal from fat. Since the ratio of water to fat changes due to lesions or tissue degeneration, water images and fat images have high utility value in image diagnosis using MR images. Thus, the Dixon method is an effective imaging method in current MRI technology.

The technique described in Non-Patent Document 2 below is known as a technique for performing MR elastography and Dixon in parallel.
In Non-Patent Document 2, when obtaining an image of MR elastography, after applying a high frequency magnetic field, three directions of a slice direction, a read out direction, and a phase encoding direction of a gradient magnetic field are generated until an echo time elapses. By applying a gradient magnetic field for motion detection (MEG: motion encoding gradient (also referred to as MSG: Motion Sensitizing gradient)) to obtain an image of MR elastography and a signal obtained at echo time On the basis of the above, a technology for obtaining MR images using the Dixon: Dixon method is described.

Japanese Patent Application Publication No. 2005-507691 ("0008", "0014" to "0023") JP, 2011-98158, A ("0003"-"0018", "0026"-"0029") JP, 2015-93108, A ("0003"-"0015") JP 2012-90867 A ("0003" to "0016")

Numano T, et. Al, “A simple method for MR elastography: a gradient-echo type multi-echo sequence”, Magn Reson Imaging. 2015; 33 (1): 31-37 Joshua Trzasko, et. Al, “Simultaneous MR elastography and Fat + Water Imaging”, Proceeding of Internation Sciety of Magnetic Resonances Medicine 23 (2015) 1061

(Problems of the prior art)
Although Non-Patent Document 1 describes a method for performing MRE, there is no description regarding an MRI image and no description regarding simultaneous measurement of MRI.
Although the technique described in Non-Patent Document 2 uses the MRE and Dixon in combination, it is necessary to set the echo time after applying the MEG necessary for performing the MRE measurement, so the MEG is applied. As a result, the echo time can not be set short, and the echo time is long. As a result, the signal strength of the image is reduced due to the influence of the lateral relaxation phenomenon. That is, after protons are excited by applying a high frequency magnetic field, it is easy to obtain an image in a state close to a completely returned state, not on the way back to the original state. Therefore, the difference between the signals of water and fat tends to be small, and the contrast of the obtained MRI image tends to be unclear.

  The present invention makes it a technical subject to obtain a clear MRI image and an MRE image in a short echo time in a magnetic resonance imaging apparatus.

In order to solve the above technical problems, the magnetic resonance imaging apparatus of the invention according to claim 1 is:
A magnetic field generator for generating a test subject of a subject with a static magnetic field, a gradient magnetic field whose magnetic field changes according to the position, and an alternating magnetic field preset based on magnetic resonance conditions of protons;
Means for applying an alternating magnetic field for applying the alternating magnetic field with predetermined repetition time;
The echo time, which is the time to receive the electromagnetic wave emitted when the protons of the test part excited by the alternating magnetic field relax, is set several times between the previous alternating magnetic field and the next alternating magnetic field. And receiving means for receiving the electromagnetic wave based on the echo time set based on a shift in resonance frequency between protons contained in water and protons contained in fat;
The gradient magnetic field is set in the readout direction in synchronization with the echo time set a plurality of times with respect to a slice direction, a readout direction, and a phase encoding direction, which are three intersecting axes set for the portion to be inspected. Means for applying a gradient magnetic field to be applied;
Based on the electromagnetic wave acquired according to the echo time, from the signal strength in the case where the spin of the proton contained in water and the spin of the proton contained in the fat are in phase and the signal strength in the opposite phase, Means of acquiring an intensity image for acquiring and calculating a first image based on water and a second image based on fat;
A vibration applying member for applying vibration to a portion to be inspected, the vibration applying member synchronized with the echo time and applying a plurality of vibrations having different phases;
Means for acquiring an MR phase image for acquiring an MR phase image corresponding to the phase of an electromagnetic wave signal based on the electromagnetic wave acquired according to the echo time, for each phase of vibration applied by the vibration applying member;
The hardness of the portion to be inspected is estimated based on the wavelength of the vibration wave obtained from the MR phase image, the frequency of the vibration wave applied by the vibration applying member, and the density of the portion to be inspected. Hardness estimation means,
It is characterized by having.

  According to the invention described in claim 1, clear MRI and MRE images can be obtained with a short echo time as compared with the prior art using MEG. As a result, the Dixon method can be used in combination with a signal noise ratio higher than that of Non-Patent Document 2.

FIG. 1 is an explanatory view of a magnetic resonance imaging apparatus according to a first embodiment of the present invention. FIG. 2 is a functional block diagram of a computer main body in the magnetic resonance imaging apparatus of the first embodiment. FIG. 3 is an explanatory view of application of a magnetic field and application of vibration in Example 1, and is a graph in which time is taken on the horizontal axis. FIG. 4 is an explanatory view of the fact that the resonance frequency of protons is shifted between water and fat. FIG. 5 is an explanatory diagram of a process of creating an MR image according to the first embodiment. FIG. 6 is an explanatory diagram of an example of the first image and the second image in the MR intensity image of the first embodiment. FIG. 7 is an explanatory view of echo time, applied gradient magnetic field and vibration in the case of conventional MRE imaging. FIG. 8 is an explanatory view of the relationship between the number of times the gradient magnetic field is applied and the phase shift, FIG. 8A is an explanatory view when the gradient magnetic field continues to be positive, and FIG. 8B is an explanation when the gradient magnetic field continues to be negative FIG.

Next, specific examples of the embodiment of the present invention (hereinafter referred to as examples) will be described with reference to the drawings, but the present invention is not limited to the following examples.
In the following description using the drawings, illustration of members other than members necessary for the description is appropriately omitted for easy understanding.

FIG. 1 is an explanatory view of a magnetic resonance imaging apparatus according to a first embodiment of the present invention.
In FIG. 1, a magnetic resonance imaging apparatus 1 of Example 1 has a magnet unit 2 as an example of a magnetic field generation apparatus. The magnet portion 2 is formed with a through hole 3 which penetrates the inside in the horizontal direction. The bed 6 on which the subject 4 in the sleeping state is supported can pass through the through hole 3.
The magnet unit 2 includes a static magnetic field generating magnet 11 as an example of a static magnetic field application member. It is possible to use a superconducting magnet or a permanent magnet as the static magnetic field generating magnet. A gradient magnetic field generating coil 12 as an example of a gradient magnetic field application member is disposed inside the static magnetic field generating magnet 11. Inside the gradient magnetic field generating coil 12, a high frequency magnetic field generating coil 13 as an example of an excitation magnetic field applying member is disposed. Inside the high frequency magnetic field generating coil 13, a receiving coil 14 for receiving an electromagnetic wave is disposed as an example of a receiving unit.

  Further, in the first embodiment, a diaphragm (passive driver as an example of a vibration applying member for the MRE measurement in the portion of the body surface corresponding to the position of the liver as an example of the subject to be examined). ) 16 is supported. The diaphragm 16 is a member that applies vibration to the subject 4 at a preset frequency, and can adopt, for example, any conventionally known configuration described in Patent Document 2 and the like.

  A computer device 21 as an example of an information processing apparatus is electrically connected to the magnet unit 2 via a cable Cb. Therefore, the computer device 21 is configured to be able to transmit and receive the control signal of the static magnetic field generating magnet 11 or the like, the detection signal of the receiving coil 14 and the like with the magnet unit 2. The computer device 21 has a computer main body 22, a display 23 as an example of a display unit, and a keyboard 24 and a mouse 25 as an example of an input unit. Although the configuration in which the computer device 21 and the magnet unit 2 are connected by the cable Cb is exemplified in the first embodiment, the present invention is not limited to this, and any wireless communication such as a cellular phone line, Bluetooth (registered trademark), wireless LAN, etc. It is also possible to transmit and receive information by a communication method.

(Description of Control Unit of Computer Body 22 of First Embodiment)
FIG. 2 is a functional block diagram of a computer main body in the magnetic resonance imaging apparatus of the first embodiment.
In FIG. 2, the control unit 41 of the computer main body 22 according to the first embodiment performs I / O (input / output interface) for performing input / output of signals with the outside, adjustment of input / output signal levels, etc., necessary startup processing. Perform processing according to the boot program stored in ROM (read only memory) in which the program and data etc. are stored, RAM (random access memory) for temporarily storing necessary data and programs, and ROM etc It is comprised by computer apparatus which has CPU (central processing unit), a clock oscillator, etc., and various functions can be implement | achieved by executing the program memorize | stored in said ROM, RAM, etc.
The control unit 41 stores basic software for controlling basic operations, an operating system OS, a magnetic resonance apparatus control program AP1 as an example of an application program, and other software (not shown).

(Elements Connected to Control Unit 41 of First Embodiment)
An output signal from a signal output element such as the keyboard 24, the mouse 25, and the reception coil 14 is input to the control unit 41.
Further, the control unit 41 of the first embodiment outputs a control signal to controlled elements such as the display 23, the static magnetic field generating magnet 11, the gradient magnetic field generating coil 12, and the high frequency magnetic field generating coil 13.

(Function of control unit 41)
The magnetic resonance apparatus control program AP1 of the control unit 41 of the first embodiment has the following functional means (program modules) 51 to 58.

  The magnetic field control unit 51 controls the magnet unit 2 to control a magnetic field for performing MR imaging of the subject of the subject 4. The magnetic field control means 51 according to the first embodiment includes repetitive time storage means 51a, echo time storage means 51b, static magnetic field application means 51c, gradient magnetic field application means 51d, and high frequency magnetic field application as an example of alternating magnetic field application means. And means 51e.

FIG. 3 is an explanatory view of application of a magnetic field and application of vibration in Example 1, and is a graph in which time is taken on the horizontal axis.
The repetition time storage means 51a stores a repetition time TR which is an interval for applying a high frequency magnetic field as an example of an alternating magnetic field applied to excite protons contained in the test part of the subject 4.

The echo time storage means 51b stores echo times TE1 to TE6 which are intervals from the application of the high frequency magnetic field to the acquisition of the electromagnetic wave emitted when the excited protons return (relax) to the original state. In FIG. 3, in the first embodiment, the echo time storage unit 51b acquires a first echo time TE1 which is a period from the application of a high frequency magnetic field to acquisition of a first electromagnetic wave, and the first electromagnetic wave is acquired. The second echo time TE2, which is the period from the acquisition of the second electromagnetic wave to the second one,..., The sixth period from the acquisition of the fifth electromagnetic wave to the acquisition of the sixth electromagnetic wave The echo time TE6 is stored. That is, in the first embodiment, the electromagnetic wave is set to be acquired six times between the previous alternating magnetic field and the next alternating magnetic field (repetition time TR).
In the first embodiment, the repetition time TR and the echo times TE1 to TE6 are set in advance, but the user of the magnetic resonance imaging apparatus 1 can manually input and set or change. It is possible.

FIG. 4 is an explanatory view of the fact that the resonance frequency of protons is shifted between water and fat.
In the first embodiment, the second echo time TE2 to the sixth echo time TE6 are set based on the difference in resonance frequency between the proton contained in water and the proton contained in fat. In FIG. 4, as an example, when the static magnetic field is 3 [T], the resonance frequency of protons is 127.5 [MHz], and water and fat are shifted by 3.5 [ppm]. The deviation of the resonance frequency is 127.5 [MHz] × 3.5 [ppm] = 446.3 [Hz]. Therefore, in time, it becomes 1 / 446.3 = 2.2 [ms]. Therefore, every 0 [ms], 2.2 [ms], 4.4 [ms], ..., the direction of the spins of protons contained in water and the direction of the spins of protons contained in fat have the same phase (in It becomes the phase (opposed phase or out of phase) in the case of 1.1 [ms], 3.3 [ms], 5.5 [ms],.

  The implementation of the MRE requires a plurality of imagings synchronized with the different phase vibrations applied from the diaphragm 16. In the first embodiment, four types of vibrations whose phases are shifted by 90 ° are given from the diaphragm 16, and when the vibration phase is divided into four, each imaging is vibration phase 1 (0 °), vibration phase It can be defined as 2 (90 °), vibration phase 3 (180 °) and vibration phase 4 (270 °). Here, the first echo time TE1 at the time of imaging of the vibration phase 1 and the vibration phase 3 is set to 2.2 [ms] (in phase), and the first echo at the time of imaging of the vibration phase 2 and the vibration phase 4 The time TE1 is set to 3.3 [ms] (opposed phase). As a result, two images of the in phase image and the opposed phase are obtained. By averaging each of the two images of in phase and exposed phase, an in phase image and an exposed phase image are obtained in which the signal-to-noise ratio is improved by √2. The signal-to-noise ratio of the image obtained by the Dixon method is also improved by using the in-phase image and the opposed-phase image with the improved signal-to-noise ratio. The first echo time TE1 (3.3 ms) of the vibration phase 2 and the vibration phase 4 is different from the first echo time TE1 (2.2 ms) of the vibration phase 1 and the vibration phase 3 Therefore, the synchronization with the vibration phase is deviated. Therefore, it is necessary to correct the vibration phase. For example, in the case of the 50 Hz oscillation, TE1 of the opposed phase image is delayed by 1.1 [ms] relative to TE1 of the in phase image. Converting this delay into a vibration phase difference corresponds to 19.8 °. When the number of vibration phase divisions is 4, the vibration phase is imaged four times at 0 °, 90 °, 180 °, and 270 °, but in the posed phase image, a delay of 19.8 ° from the in phase occurs. The phase delay is corrected, and imaging is performed four times at 0 ° (in phase), 70.2 ° (opposed phase), 180 ° (in phase) 250.2 ° (opposed phase).

The static magnetic field application unit 51 c controls the static magnetic field generating magnet 11 to generate a static magnetic field. The static magnetic field application unit 51c of the first embodiment generates a static magnetic field of 3 [T] as an example.
The gradient magnetic field application unit 51 d controls the gradient magnetic field generating coil 12 to generate a gradient magnetic field (gradient magnetic field) in which the magnetic field changes according to the position. As shown in FIG. 3, the gradient magnetic field applying unit 51d of the first embodiment has a repetition direction TR for each of three axial directions of slice, read out and phase orthogonal to each other. , A gradient magnetic field 61 is generated. Further, as shown in FIG. 3, in the first embodiment, an alternating magnetic field 62 in which the positive and negative polarities periodically change is applied in the readout direction corresponding to the echo times TE1 to TE6.

  The high frequency magnetic field application unit 51e controls the high frequency magnetic field generating coil 13 to generate a high frequency magnetic field 63 which is an alternating magnetic field corresponding to a frequency for exciting protons. As an example, the high frequency magnetic field application unit 51e of the first embodiment applies a magnetic field that inclines the spins aligned in the direction of the static magnetic field by 90 °. The high frequency magnetic field application unit 51e of the first embodiment generates a high frequency magnetic field at a time corresponding to the repetition time TR.

The vibration application control means 52 controls the diaphragm 16 to apply vibration to the inspected portion. In FIG. 3, the period of the vibration applied by the vibration applying control means 52 of the first embodiment is synchronized with the period of the alternating magnetic field of the gradient magnetic field, that is, the echo times TE1 to TE6. In the first embodiment, the vibration application control means 52 has a function capable of arbitrarily changing the phase of vibration with respect to the vibration shown in FIG. 3 and, using this function, for example, the number of vibration phase divisions is four. In one case, vibration with phase shifted by 90 °, 180 ° and 270 ° from 0 ° is also applied.
The signal acquiring unit 53 as an example of the receiving unit is an electromagnetic wave signal generated when the proton of the subject 4 is relaxed via the receiving coil 14 at the timing of the first echo time TE1 to the sixth echo time TE6. To get Therefore, in the first embodiment, in the state where the vibration shown in FIG. 3 is applied by the vibration applying control means 52, the signal is measured by the receiving coil 14 in the echo time TE1 to TE6, and the vibration whose phase is shifted 90 ° is applied. In this state, the signal is measured by the receiving coil 14 at echo times TE1 to TE6, and the signal is similarly measured even in the state of 180 ° and 270 °, thereby changing the phase of vibration and imaging MR images multiple times. Do.

FIG. 5 is an explanatory diagram of a process of creating an MR image according to the first embodiment.
The MR image acquisition unit 54 includes a signal processing unit 54 a, an MR intensity image generation unit 54 b, and an MR phase image generation unit 54 c, and based on the electromagnetic wave signal acquired by the signal acquisition unit 53, Create
The signal processing means 54a performs signal processing on the received electromagnetic wave signal. The signal processing unit 54a according to the first embodiment sets the signal acquired by the signal acquisition unit 53 as a real part r (real part) and delays the signal acquired by the signal acquisition unit 53 by π / 2 phase as an imaginary unit i Imagine an imaginary part). That is, a complex number based on the received electromagnetic wave signal, that is, a signal of k space (frequency space) in the technical field of MRI is generated. Then, an inverse Fourier transform (in the embodiment, an inverse fast Fourier transform) is performed on the real part r and the imaginary part i to convert them into real space signals R and I. Then, based on the real part R and the imaginary part I in the real space, the intensity M = (R ² + I ² ) ^1/2 in the complex plane and the phase φ = tan ⁻¹ (I / R) are calculated.

FIG. 6 is an explanatory diagram of an example of the first image and the second image in the MR intensity image of the first embodiment.
The MR intensity image creating unit 54b creates an MR intensity image based on the intensity M calculated by the signal processing unit 54a. The MR intensity image is an image generally used for diagnosis as an MRI image. In FIG. 5, in the MR intensity image creating means 54b of Example 1, the intensity M calculated from the electromagnetic wave signal measured at the first echo time TE1, that is, the direction of the spin of protons contained in water, and fat The intensity M calculated from the image (in phase image) when the direction of the spins of protons contained in the phase is in phase (in phase) and the electromagnetic wave signal measured at the second echo time TE2, ie, water Based on the direction of spins of contained protons and an image (opposed phase image) in which the direction of spins of protons contained in fat is out of phase or exposed phase, water based 1. Obtain and calculate an image of 1 and a second image based on fat.

Specifically, in the in-phase image, since the water signal (Iwater) and the fat signal (Ifat) are in phase, the image is an image (Iwater + Ifat) in which the respective signals are added. On the other hand, in the opposed phase image, since the water signal (Iwater) and the fat (Ifat) signal are in antiphase, an image (Iwater-Ifat) is obtained by subtracting the fat signal from the water signal.
Therefore, by adding the in phase image and the exposed phase image, (Iwater + Ifat) + (Iwater-Ifat) = 2Iwater, and a first image based on water (brighter as the water content increases) is created . Also, by subtracting the exposed phase image from the in phase image, (Iwater + Ifat)-(Iwater-Ifat) = 2Ifat, a fat-based (a fat-rich portion is brighter) second image is created. That is, in the first embodiment, the first image and the second image are created by the so-called Dixon method.

The MR phase image creation unit 54c creates an MR phase image (wave image image) based on the phase φ calculated from the electromagnetic wave signal. Here, in the MR phase image creating means 54c of the first embodiment, the MR phase image is calculated based on the phase φ calculated from the electromagnetic wave signal measured in the first echo time TE1 to the sixth echo time TE6. Create a type. In addition, MR phase images are created in the cases where the phase of the vibration applied by the vibration application control means 52 is 0 °, 90 °, 180 °, and 270 °.
The wavelength acquisition means 55 acquires the wavelength λ of the vibration wave propagating inside the subject 4 by the vibration applied by the diaphragm 16. The wavelength acquiring unit 55 of the first embodiment changes the phase φ, which varies according to the vibration wave, for each pixel of the image based on the MR phase image in each case of 0 °, 90 °, 180 °, and 270 °. The wavelength λ of the oscillatory wave is obtained from Specifically, the wavelength λ of the vibration wave is estimated for each local region of the image from the state of propagation of the vibration wave obtained from the images of vibration phases 0 °, 90 °, 180 °, and 270 °.

The hardness estimation means 56 estimates the hardness μ of the portion to be inspected based on the wavelength λ of the vibration wave, the frequency f of the vibration wave applied by the diaphragm 16, and the density ρ of the portion to be inspected. . The frequency f of the vibration wave is known from the frequency f of the vibration wave applied by the diaphragm 16, and the density 被 of the portion to be inspected is approximately 1 [g / cm ³ ]. And hardness (elastic modulus) μ is calculated from μ = ρ · (λ · f) ² .
The MRE image creation means 57 creates an MR elastogram image (MRE image) in which the hardness is imaged using the MR phenomenon. The MRE image creation unit 57 of the first embodiment creates an MRE image that is color-coded according to the hardness μ calculated for each local region (pixel). As an example, a hard part (a pixel with a large value of hardness μ) is displayed in red, and as softness (a value of hardness μ decreases), it is displayed as changing to yellow, green, blue, and purple. It is possible.

  The image display means 58 displays the image created by the MR image acquisition means 54 or the MRE image creation means 57 on the display 23. That is, the first image (water image) and the second image (fat image), which are cross-sectional images of the examination part, the MR phase image (wave image image), and the elastogram image (MRE image) are displayed Displayed at 23. In Example 1, since the anatomical structure is difficult to understand only with the Wave Image image and the elastogram image, the intensity image (the first image or the second image) and the Wave Image image and the elastogram image are used. It also has a display mode for overlapping display. Note that the images are displayed overlappingly or not all images are displayed on the display 23, and the intensity image (the first image and the second image) and the Wave Image image or the elastogram image are switched according to the input. Can also be displayed. In addition, it is also possible to arrange the intensity image (the first image and the second image) and the elastogram image side by side in the same cross section, or to arbitrarily change the elastogram image in different cross sections. Is possible.

(Operation of Example 1)
In the magnetic resonance imaging apparatus 1 of the first embodiment having the above-described configuration, a plurality of echo times TE1 to TE6 are set during one repetition time TR, and the first echo times TE1 and second echo times TE1 to TE6 are set. An MRI image (water image and fat image) is created based on the measurement result at the echo time TE2, and an MRE image is created based on the measurement result at the sixth echo time TE6.
In the magnetic resonance imaging apparatus currently on the market, the MRI image and the MRE image are not taken in parallel but the imaging of the MRI image and the imaging of the MRE image are performed separately. Therefore, it takes a long time to take an MRI image and an MRE image, and there is a problem that the subject 4 is burdened. On the other hand, in the magnetic resonance imaging apparatus 1 according to the first embodiment, the MRI image and the MRE image are captured in parallel during one repetition time TR, as compared with the case where the imaging is performed separately. The required time can be shortened.

FIG. 7 is an explanatory view of echo time, applied gradient magnetic field and vibration in the case of conventional MRE imaging.
In FIG. 7, in the case of imaging a conventional MRE as described in Patent Documents 1 and 2 and Non-patent Document 2, three directions of a slice direction, a lead-out direction, and a phase direction from a diaphragm are used. An alternating magnetic field synchronized with vibration, so-called MEG (or MSG) is applied. Then, after the application of the alternating magnetic field, the echo time is set. Therefore, in the prior art, the echo time is increased by the application of the MEG. That is, since the radio frequency magnetic field is applied and the protons are excited, the time from the measurement to the measurement becomes long. As a result, the signal strength of the image is reduced due to the influence of the lateral relaxation phenomenon. That is, after protons are excited, an image in a state close to a completely returned state is likely to be acquired, not on the way back to the original state. Therefore, the difference between the signals of water and fat tends to be small, and the contrast of the obtained MRI image tends to be unclear.

As an example, in the prior art, if the vibration of the diaphragm is 50 Hz, it is necessary to apply MSG 20 ms (= 1 second / 50 Hz), and the echo time may be set to 20 ms or less. Have difficulty. Therefore, as described in Non-Patent Document 2, even if an MRI image is generated by the Dixon method based on an electromagnetic wave signal measured with a long echo time, there is a problem that the contrast tends to be unclear.
On the other hand, in the first embodiment, as described above, the first echo time TE1 can be set to 2.2 [ms] or 3.3 [ms], and the echo time is shorter than that in the related art. It is shortened and less susceptible to lateral relaxation phenomena. Therefore, compared to the prior art, an MRI image with sharp contrast can be obtained.

FIG. 8 is an explanatory view of the relationship between the number of times the gradient magnetic field is applied and the phase shift, FIG. 8A is an explanatory view when the gradient magnetic field continues to be positive, and FIG. 8B is an explanation when the gradient magnetic field continues to be negative FIG.
Further, as described in Patent Documents 1 and 2 and Non-patent Document 2, in Example 1, in contrast to the case where an alternating magnetic field of one positive and one negative (one cycle) is applied as MEG, In response to the echo times TE1 to TE6, alternating magnetic fields of positive and negative three times (for three cycles) are applied.
In FIGS. 8A and 8B, assuming that the resonance frequency in the static magnetic field to which the gradient magnetic field is not applied is taken as a reference resonance frequency, when the gradient magnetic field continues to be positive, the magnetic field strength in the space is The resonant frequency continues to increase because of the magnetic field strength. On the contrary, if the gradient magnetic field continues to be negative, the resonance frequency continues to decrease. Therefore, the resonance frequency when the gradient magnetic field is applied is higher or lower than the reference resonance frequency. Here, if the direction in which the gradient magnetic field is applied matches the displacement caused by the vibration (FIG. 8A), the resonance frequency continues to be higher. On the other hand, when the direction in which the gradient magnetic field is applied is opposite to the direction of displacement caused by the vibration (FIG. 8B), the resonance frequency continues to decrease. As a result, the frequency difference with the reference resonance frequency is measured as the amount of phase shift (phase shift amount).
Here, in the case where excitation is not performed, when a positive (positive) gradient magnetic field and a negative (negative) gradient magnetic field are applied to the inspected portion for one cycle, the phase continues to decrease in the positive gradient magnetic field, and minus The phase continues to rise in the gradient magnetic field of Since the areas of the positive and negative gradient magnetic fields are the same, the phase returns to the original position after the positive and negative gradient magnetic fields are applied.

Then, when vibration is applied by the vibrating plate 16, the portion to be inspected receives a gradient magnetic field while being displaced, and the phase shift amount becomes larger than that in the case where it is not vibrating. In addition, even if the gradient magnetic field is reversed (positive to negative or negative to positive), the phase is not returned to the original state because it is further displaced while being subjected to the gradient magnetic field. In MRE, since the oscillation synchronized with the gradient magnetic field is applied, the phase shift amount continues to be accumulated. Therefore, compared to the prior art in which the gradient magnetic field synchronized with the vibration is applied for only one cycle, in the first embodiment in which three cycles are applied, the portion with a small phase shift in the MR phase image (the portion with a small accumulation amount) The difference is likely to be large at large portions (the accumulation amount is large), and the image (contrast) tends to be clear.
Therefore, according to the first embodiment in which the MRE image is created based on the phase φ measured in the sixth echo time TE6 in which gradient magnetic fields of three periods are applied, a clear MRE image is obtained as compared with the prior art. be able to. That is, in the first embodiment in which a plurality of echo times are set and a gradient magnetic field is applied corresponding to the echo times, a clear MRE image can be obtained as compared with the prior art.

  In the first embodiment, any MRE image of echo times TE1 to TE6 can be selected after imaging. List the benefits of this. Although the MRE image of the sixth echo time TE6 has high sensitivity to vibration, the echo time is extended, so there is a possibility that the signal-to-noise ratio is reduced due to the transverse relaxation phenomenon, and the MRE image of the first echo time TE1. Although the vibration sensitivity is weak, the signal-to-noise ratio obtained is high. As described above, there is a trade-off relationship between the vibration sensitivity and the signal-to-noise ratio. In the first embodiment, an MRE image with an appropriate sensitivity can be selected after imaging.

  In particular, in the conventional magnetic resonance imaging apparatus, it was often used in the bone soft part (muscle and tendon), but in the MRI image alone, it is diagnosed whether the tendon etc. is connected, broken, or inflammation occurs. I could only do it. That is, although the tendon etc. were connected, it could not be judged that it became hard or was about to break. On the other hand, in the first embodiment, not only the water image of the portion to be inspected but also a fat image and an image of hardness can be acquired. In particular, it has been known that when the degeneration due to damage or the like starts, fat etc. accumulates, and by combining the fat image by the Dixon method and the information of hardness by MRE, the muscle and the like are cut at the stage before the damage. It is expected to contribute to the diagnosis of whether or not so. For example, it is expected to contribute to the diagnosis of shoulder muscles of baseball pitchers and tendons of athletes' legs.

(Modification example)
As mentioned above, although the Example of this invention was explained in full detail, this invention is not limited to the said Example, A various change is performed within the range of the summary of this invention described in the claim. It is possible. Modifications (H01) to (H04) of the present invention are exemplified below.
(H01) In the above embodiment, the number of echo times is six, but is not limited thereto. It is also possible to set four times or eight times or more.
(H02) The specific numerical values exemplified in the above embodiment can be arbitrarily changed in accordance with the design, use and the like. For example, since the resonance frequency changes depending on the strength of the static magnetic field, when the strength of the static magnetic field is changed, the echo time, the frequency of the diaphragm 16 and the like are also changed in conjunction.

(H03) In the above embodiment, a so-called tunnel type magnetic resonance imaging apparatus in which the magnet unit 2 has a ring shape is exemplified, but the present invention is not limited to this. For example, the present invention is also applicable to a so-called open magnetic resonance imaging apparatus in which the magnet unit 2 is U-shaped.
(H04) In the above embodiment, the vibration frequency is 50 Hz, but it is not limited thereto. Excitation at 75 Hz or 100 Hz is also possible.

1 Magnetic resonance imaging system
2 ... Magnetic field generator,
16: Vibration applying member,
51 d: application means of gradient magnetic field,
51e: application means of alternating magnetic field,
53 ... receiving means,
54b ... acquisition means of intensity image,
54c ... means for acquiring an MR phase image
56 · · · Hardness estimation means,
f ... frequency of vibrational wave,
TE1 to TE6 ... echo time,
TR ... repetition time,
λ: wavelength of vibrational wave,
ρ ... the density of the inspection area,
μ ... hardness of the test area.

Claims (1)

A magnetic field generator for generating a test subject of a subject with a static magnetic field, a gradient magnetic field whose magnetic field changes according to the position, and an alternating magnetic field preset based on magnetic resonance conditions of protons;
Means for applying an alternating magnetic field for applying the alternating magnetic field with predetermined repetition time;
The echo time, which is the time to receive the electromagnetic wave emitted when the protons of the test part excited by the alternating magnetic field relax, is set several times between the previous alternating magnetic field and the next alternating magnetic field. And receiving means for receiving the electromagnetic wave based on the echo time set based on a shift in resonance frequency between protons contained in water and protons contained in fat;
The gradient magnetic field is set in the readout direction in synchronization with the echo time set a plurality of times with respect to a slice direction, a readout direction, and a phase encoding direction, which are three intersecting axes set for the portion to be inspected. Means for applying a gradient magnetic field to be applied;
Based on the electromagnetic wave acquired according to the echo time, from the signal strength in the case where the spin of the proton contained in water and the spin of the proton contained in the fat are in phase and the signal strength in the opposite phase, Means of acquiring an intensity image for acquiring and calculating a first image based on water and a second image based on fat;
A vibration applying member for applying vibration to a portion to be inspected, the vibration applying member synchronized with the echo time and applying a plurality of vibrations having different phases;
Means for acquiring an MR phase image for acquiring an MR phase image corresponding to the phase of an electromagnetic wave signal based on the electromagnetic wave acquired according to the echo time, for each phase of vibration applied by the vibration applying member;
The hardness of the portion to be inspected is estimated based on the wavelength of the vibration wave obtained from the MR phase image, the frequency of the vibration wave applied by the vibration applying member, and the density of the portion to be inspected. Hardness estimation means,
A magnetic resonance imaging apparatus characterized by comprising: