Imaging device

Abstract

To perform imaging in a short time in comparison to a conventional case where imaging of MRE and T1-map is performed individually.SOLUTION: An imaging apparatus comprises: MRE image creation means (57) which creates an MRE image (141) on the basis of an MR phase image; and map creation means (58) which derives a relaxation time (T1) on the basis of the intensities (S1, S2) of signals generated respectively in a case of using two or more different values of flip angles (α1, α2) being angles that change the direction of a magnetization vector and creates a (T1) map (142) being the distribution of the relaxation time from the relaxation time (T1) for each position.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an imaging device, and more particularly to an imaging device capable of measuring tissue stiffness using MR elastography (Magnetic Resonance Elastography: MRE), and acquiring a T1-map image obtained by quantitatively evaluating the longitudinal relaxation time (T1) of protons for each pixel in the tissue. [Background technology]

[0002] In medical settings, MRI (Magnetic Resonance Imaging) devices, which have minimal effects on patients such as radiation exposure, are used. In an MRI device, an alternating (high-frequency) magnetic field corresponding to the spin of the protons is applied to the hydrogen nuclei (protons) contained in each cell of the human body to excite them. Based on the electromagnetic waves emitted when the excited protons return to their original state (relax), it is possible to obtain an image that represents areas with different proton relaxation times (for example, water and fat) as shades of gray. MRE (Magnetic Resonance Elastography) is an imaging technique that uses MRI to image an object while applying vibration (approximately 50 Hz to 100 Hz in the case of the torso), utilizing the differences in vibration wave propagation due to differences in the "hardness" inside the object to create an image of its hardness (see Patent Documents 1 and 2).

[0003] In MRI, T1-weighted and T2-weighted images, which are based on relatively enhanced T1 and T2 relaxation times, are often used. However, techniques using T1-maps and T2-maps, which are distribution images using the absolute (quantitative) values ​​of T1 and T2, are also known (see Non-Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Special Publication No. 2005-507691 (“0008”, “0014” ~ “0023”) [Patent Document 2] Japanese Patent Publication No. 2011-98158 ("0003" to "0018", "0026" to "0029") [Non-patent literature]

[0005] [Non-Patent Document 1] Masahiro Okada, "MRI Image Analysis of Liver Dysfunction," Nihon University Medical Society, Nihon University Medical Journal 78(3):173-176 (2019) [Non-Patent Document 2] Tomoya Kobayashi, et al., "Evaluation of Measurement Accuracy in T1 and T2 Mapping Tools," Japanese Journal of Magnetics and Medicine, Vol. 32, No. 2 (2012), pp. 66-75. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] (Problems with conventional technology) T1-weighted images are relative values, meaning they can only be evaluated within the image they were acquired in, and cannot be simply compared between different images. However, T1-maps, which use quantitative values, are absolute values, making it possible to compare and quantitatively evaluate images from different devices or taken on different days. For example, in T1-weighted images, areas where contrast agent has permeated and areas where it has not appear in different colors, making it possible to determine whether the tissue is normal or cancerous. However, with T1-maps, it is possible to evaluate the extent to which contrast agent has permeated and whether the organ is functioning normally based on absolute values, potentially allowing for prediction and estimation of the degree of malignancy.

[0007] Conventional T1-map imaging techniques, such as those described in Non-Patent Documents 1 and 2, have the problem of being time-consuming to perform (approximately one hour for accurate measurement). On the other hand, during MRE imaging, patients and subjects are sometimes required to hold their breath to minimize the impact of respiratory movements on vibrations. However, the average patient can only hold their breath for a few seconds to about 20 seconds, and prolonged breath-holding poses a significant burden on the patient. Therefore, conventionally, when performing MRE and T1-map imaging, it was necessary to perform the imaging separately, which resulted in long overall imaging and examination times, and a significant burden on patients and subjects.

[0008] The technical challenge of this invention is to perform imaging in a shorter time compared to the conventional method of performing MRE and T1-map imaging separately. [Means for solving the problem]

[0009] To solve the aforementioned technical problems, the imaging apparatus of the invention described in claim 1 is: A magnetic field generator that generates a static magnetic field, a gradient magnetic field whose magnetic field changes according to the position, and an alternating magnetic field preset based on the proton magnetic resonance conditions, on the part of the subject being examined. A vibration-applying member that applies vibration to the part under inspection, An MR phase image acquisition means acquires an MR phase image corresponding to the phase of the electromagnetic wave signal based on the electromagnetic waves acquired according to the timing of the application of the gradient magnetic field, An MRE image creation means for creating an MR elastography image based on an MR phase image, magnetic The flip is the angle that changes the direction of the vector. corner In the parameters, when two or more of the aforementioned parameters with different values ​​are used, a map creation means derives the longitudinal relaxation time based on the signal intensity generated for each, and creates a map of the distribution of longitudinal relaxation times from the longitudinal relaxation times for each location. Equipped with ru Imaging device And, An alternating magnetic field applying means for generating an alternating magnetic field that changes the direction of the magnetization vector at a flip angle as the parameter, comprising: a first alternating magnetic field that changes the direction of the magnetization vector by a predetermined first flip angle; and the alternating magnetic field applying means for generating a second alternating magnetic field that changes the direction of the magnetization vector by a second flip angle different from the first flip angle. The map creation means derives the longitudinal relaxation time based on the first signal intensity generated after the application of the first alternating magnetic field and the second signal intensity generated after the application of the second alternating magnetic field, and creates a map of the distribution of longitudinal relaxation times from the longitudinal relaxation times for each position. It is characterized by having the following features.

[0011] Claim 2 The invention described in Claim 1 In the imaging device described in For each of the first flip angle and the second flip angle, the longitudinal relaxation time derived is the arithmetic mean of the longitudinal relaxation times when the phase of the vibration applied by the vibration applying member is different by 180 degrees. It is characterized by this.

Effect of the Invention

[0012] According to the invention described in claim 1, by performing a series of elastography imaging while applying vibration, an MR elastography image and a relaxation time distribution map can be created, and compared with the conventional case where MRE and T1-map imaging are performed separately, the imaging can be performed in a shorter time. Also, claim 1 According to the invention described in Claim 2 According to the invention described in

Brief Description of the Drawings

[0013] [Figure 1] FIG. 1 is an explanatory diagram of a magnetic resonance imaging apparatus according to Embodiment 1 of the present invention. [Figure 2] FIG. 2 is a functional block diagram of a computer main body in the magnetic resonance imaging apparatus according to Embodiment 1. [Figure 3] FIG. 3 is an explanatory diagram of the application of a magnetic field and the application of vibration according to Embodiment 1, and is a graph with time on the horizontal axis. [Figure 4] FIG. 4 is an explanatory diagram of the derivation of the slope m (Ernst angle) according to Embodiment 1. [Figure 5]Figure 5 is an explanatory diagram of an example of the T1 map of Example 1. Figure 5A is the T1 map obtained when the signal strengths at vibration phases of 0° and 180° are added and averaged when the flip angle is 5°, and when the signal strengths at vibration phases of 90° and 270° are added and averaged when the flip angle is 10°. Figure 5B is the T1 map obtained when the signal strengths at vibration phases of 90° and 270° are added and averaged when the flip angle is 5°, and when the signal strengths at vibration phases of 0° and 180° are added and averaged when the flip angle is 10°. Figure 5C shows the T1 map when the flip angle is 5° and the signal strengths at vibration phases of 0° and 90° are added and averaged, and when the flip angle is 10° and the signal strengths at vibration phases of 180° and 270° are added and averaged, and Figure 5D shows the T1 map when the flip angle is 5° and the signal strengths at vibration phases of 180° and 270° are added and averaged, and when the flip angle is 10° and the signal strengths at vibration phases of 0° and 90° are added and averaged. [Figure 6] Figure 6 is an explanatory diagram of the phantom used in Experimental Example 1. [Figure 7] Figure 7 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the SR method in an aqueous solution phantom with T1 of 505 ms. [Figure 8] Figure 8 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the SR method in an aqueous solution phantom with a T1 of 642 ms. [Figure 9] Figure 9 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the SR method in an aqueous solution phantom with a T1 of 861 ms. [Figure 10] Figure 10 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the SR method in an aqueous solution phantom with T1 of 1288 ms. [Figure 11] Figure 11 is a table of relative errors in experimental results for combinations of T1 values ​​and flip angles using the IR method with an aqueous solution phantom with T1 of 353 ms. [Figure 12] Figure 12 is a table of relative errors in experimental results for combinations of T1 values ​​and flip angles using the IR method with an aqueous solution phantom with T1 of 398 ms. [Figure 13] Figure 13 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the IR method in an aqueous solution phantom with a T1 of 518 ms. [Figure 14] Figure 14 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the IR method in an aqueous solution phantom with T1 of 648 ms. [Figure 15] Figure 15 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the IR method with an aqueous solution phantom with T1 of 874 ms. [Figure 16] Figure 16 is an explanatory diagram of the phantom used in Experimental Example 3. [Figure 17] Figure 17 is an explanatory diagram for Experimental Example 3, illustrating the relationship between vibration phase, MR phase image, MR intensity image, flip angle, MRE image, and T1 map. [Figure 18] Figure 18 is an explanatory diagram of the MR phase image; Figure 18A is an explanatory diagram of a comparative example when the flip angle is 10°, and Figure 18B is an explanatory diagram of the MR phase image of experimental example 3. [Figure 19] Figure 19 is an explanatory diagram of the MRElastogram imaging results, with Figure 19A being an explanatory diagram of the phantom and Figure 19B being an image of the imaging results. [Figure 20] Figure 20 is an explanatory diagram of the effect of the flip angle on the MRE, Figure 20A is an explanatory diagram of a comparative example when the flip angle is 10°, and Figure 20B is an MRElastogram image of Experimental Example 3. [Figure 21] Figure 21 is an explanatory diagram showing the case where MRE images are repeatedly acquired using the method of Experimental Example 3. [Figure 22] Figure 22 is an explanatory diagram of the T1 map results; Figure 22A is an explanatory diagram of the T1 map when prepared using the IR method, and Figure 22B is an explanatory diagram of the T1 map when prepared using the method of Experimental Example 3. [Figure 23] Figure 23 is an explanatory diagram of the results of confirming the effect of vibration on the T1 map. Figure 23A shows the T1 map in a vibration-free state, and Figure 23B shows the T1 map in a vibration-induced state. [Figure 24]Figure 24 is an explanatory diagram showing the case where the T1 map is repeatedly created using the method of Experimental Example 3. [Modes for carrying out the invention]

[0014] Next, specific examples of embodiments 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 explanation using diagrams, diagrams of components other than those necessary for the explanation have been omitted as appropriate for ease of understanding. [Examples]

[0015] Figure 1 is an explanatory diagram of a magnetic resonance imaging apparatus according to Embodiment 1 of the present invention. In Figure 1, the magnetic resonance imaging apparatus 1 of Embodiment 1, which is an example of an imaging apparatus of the present invention, has a magnet section 2 as an example of a magnetic field generator. The magnet section 2 has a through hole 3 that penetrates horizontally through its interior. A bed 6 that supports a patient 4 in a lying position can pass through the through hole 3. The magnet section 2 has a static magnetic field generating magnet 11 as an example of a static magnetic field applying member. Superconducting magnets or permanent magnets can be used as the static magnetic field generating magnet. Inside the static magnetic field generating magnet 11, a gradient magnetic field generating coil 12 is arranged as an example of a gradient magnetic field applying member. Inside the gradient magnetic field generating coil 12, an alternating (high-frequency) magnetic field generating coil 13 is arranged as an example of an excitation magnetic field applying member. Inside the alternating magnetic field generating coil 13, a receiving coil 14 is arranged as an example of a receiving section for receiving electromagnetic waves.

[0016] In Example 1, a vibration pad 16, an example of a vibration-applying member, is supported on the body surface on the spine side of subject 4 in order to measure the MRE of the psoas muscle, which is an example of a body part to be examined. The vibration pad 16 is generally a sound pressure type (a method that transmits sound pressure generated by a voice coil to the target), but is not limited to this. For example, a mechanical type using a rod or a vibration method using an induction coil can be used.

[0017] A computer device 21, which is an example of an information processing device, is electrically connected to the magnet unit 2 via a cable Cb. Therefore, the computer device 21 is configured to send and receive control signals such as those from the static magnetic field generating magnet 11 and detection signals from the receiving coil 14 to the magnet unit 2. The computer device 21 includes a computer main unit 22, a display 23 as an example of a display unit, and a keyboard 24 and mouse 25 as an example of an input unit. In Embodiment 1, a configuration in which the computer device 21 and the magnet unit 2 are connected by a cable Cb is illustrated, but the invention is not limited to this, and it is also possible to send and receive information using any wireless communication method such as a mobile phone line, Bluetooth®, or wireless LAN.

[0018] (Description of the control unit of the computer main unit 22 in Example 1) Figure 2 is a functional block diagram of the computer unit in the magnetic resonance imaging apparatus of Example 1. In Figure 2, the control unit 41 of the computer main unit 22 of Embodiment 1 is composed of a computer device having an I / O (input / output interface) that performs input / output of signals to and from the outside and adjustment of input / output signal levels, a ROM (read-only memory) that stores programs and data for performing necessary startup processing, a RAM (random access memory) for temporarily storing necessary data and programs, a CPU (central processing unit) that performs processing according to the startup program stored in the ROM, etc., and a clock oscillator, etc. Various functions can be realized by executing the programs stored in the ROM and RAM, etc. The control unit 41 stores basic software that controls basic operations, so-called operating system OS, an image capture device control program AP1 as an example of an application program, and other software not shown.

[0019] (Element connected to the control unit 41 of Example 1) The control unit 41 receives output signals from signal output elements such as the keyboard 24, mouse 25, and receiving coil 14. Furthermore, the control unit 41 of Embodiment 1 outputs control signals to controlled elements such as the display 23, the static magnetic field generating magnet 11, the gradient magnetic field generating coil 12, and the alternating magnetic field generating coil 13.

[0020] (Functions of the control unit 41) The imaging device control program AP1 of the control unit 41 in Example 1 has the following functional means (program modules) 51 to 59.

[0021] The magnetic field control means 51 controls the magnetic field for MR imaging of the subject 4. The magnetic field control means 51 of Embodiment 1 includes a repetition time memory means 51a, an echo time memory means 51b, a static magnetic field application means 51c, a gradient magnetic field application means 51d, and an alternating magnetic field application means 51e as an example of an alternating magnetic field application means.

[0022] Figure 3 is an explanatory diagram of the magnetic field application and vibration induction in Example 1, with time on the horizontal axis. The repetition time storage means 51a stores the repetition time TR, which is the interval at which a high-frequency magnetic field is applied as an example of an alternating magnetic field applied to excite protons contained in the body part being examined of the subject 4. In other words, the repetition time TR is the time corresponding to the interval of one cycle (a series of imaging operations).

[0023] The echo time storage means 51b stores the echo time TE, which is the interval from when an alternating magnetic field is applied until the electromagnetic waves emitted when the excited protons return to their original state (relax) are acquired. In Embodiment 1, the repetition time TR and echo time TE are preset, but it is also possible to configure the magnetic resonance imaging apparatus 1 so that users can manually input and set or change them.

[0024] The static magnetic field applying means 51c controls the static magnetic field generating magnet 11 to generate a static magnetic field. In Example 1, the static magnetic field applying means 51c generates a static magnetic field of 3[T] as an example. The gradient magnetic field application means 51d controls the gradient magnetic field generating coil 12 to generate a gradient magnetic field (gradient magnetic field) for MRE that changes according to position. Therefore, the gradient magnetic field is a magnetic field called the vibration detection gradient magnetic field MEG (motion encoding gradient). As shown in Figure 3, the gradient magnetic field application means 51d of Example 1 generates a gradient magnetic field 91 in the slice direction (slice axis) in three mutually orthogonal axis directions: the slice direction, the readout direction, and the phase direction. In addition, the gradient magnetic field application means 51d of Example 1 generates gradient magnetic fields N1-N4 in the readout direction, which generate an echo signal 102 by acting on the transverse magnetization generated by the alternating magnetic field 101 described later. Furthermore, the gradient magnetic field application means 51d of Example 1 applies a spoiler gradient magnetic field 92, which is a gradient magnetic field that has the effect of eliminating residual transverse magnetization so that residual transverse magnetization is not carried over to the next cycle.

[0025] In Example 1, the alternating magnetic field application means 51e applies an alternating magnetic field 101 that changes the magnetization vector by a predetermined first flip angle α1. For second flip angle imaging, a second alternating magnetic field 101' can be applied that changes the magnetization vector by a predetermined second flip angle α2. In Example 1, as an example, the first flip angle α1 is set to 5° and the second flip angle α2 to 10°, but the values ​​of the flip angles α1 and α2 are not limited to the exemplified values ​​and can be changed as appropriate. Furthermore, while Example 1 exemplified the use of two types of flip angles α1 and α2, it is not limited to this, and it is possible to use three or more types of flip angles.

[0026] The alternating magnetic field application means 51e controls the alternating magnetic field generating coil 13 to generate a high-frequency magnetic field (sometimes referred to as RF or high-frequency pulse), which is an alternating magnetic field corresponding to the frequency that excites protons (based on the magnetic resonance conditions of protons). In Example 1, the alternating magnetic field application means 51e uses the alternating magnetic field 101 that excites protons to change the magnetization vector of the protons to an arbitrary angle (tilt it to an arbitrary angle). Multiple echo signals 102 are generated by applying gradient magnetic fields N1-N4 to the transverse magnetization generated by the alternating magnetic field 101.

[0027] The vibration application control means 52 applies vibration to the part under inspection using the vibration pad 16. As an example of a receiving means, the signal acquisition means 53 acquires the electromagnetic wave signal generated when the subject's proton relaxes via the receiving coil 14 at the time of the echo signal 102. Therefore, in Embodiment 1, with the vibration shown in Figure 3 applied by the vibration application control means 52, the signal is measured in the receiving coil 14 at the echo time TE, thereby capturing an MR image while applying vibration.

[0028] The MR image acquisition means 54 creates an MR image based on the electromagnetic wave signal (echo signal) acquired by the signal acquisition means 53. The signal processing means 54a performs signal processing on the received electromagnetic wave signal. In Embodiment 1, the signal processing means 54a uses the signal acquired by the signal acquisition means 53 as the real part r, and the signal acquired by the signal acquisition means 53 with a phase delay of π / 2 as the imaginary part i. That is, it generates a complex number based on the received electromagnetic wave signal, a signal in k-space (frequency space) in the field of MRI. Then, it performs an inverse Fourier transform (fast inverse Fourier transform in this embodiment) on the real part r and the imaginary part i to convert them into signals R and I in real space. Then, based on the real part R and the imaginary part I in real space, the intensity M = (R 2 +I 2 ) 1 / 2 And the phase φ = tan -1The (I / R) ratio is calculated. Since this calculation is already implemented in conventional MRI devices and is publicly known, further detailed explanation is omitted.

[0029] The means for creating MR intensity images (means for acquiring MR intensity images) 54b creates an MR intensity image based on the intensity M calculated by the signal processing means 54a. The MR intensity image is generally used as an MRI image for diagnosis.

[0030] The MR phase image creation means (MR phase image acquisition means) 54c creates an MR phase image (used as a Wave Image in MRE) based on the phase φ calculated from the electromagnetic wave signal. In this example, the MR phase image creation means 54c of Example 1 creates an MR phase image based on the phase φ calculated from the electromagnetic wave signal measured at the echo time TE.

[0031] The wavelength acquisition means 55 acquires the wavelength λ of the vibration wave propagating within the subject 4's body due to vibrations applied by the vibration pad 16, based on the MR phase image, for each region of interest (ROI). The hardness estimation means 56 estimates the hardness μ of the part under test based on the wavelength λ of the vibration wave, the frequency f of the vibration wave applied by the vibration pad 16, and the density ρ of the part under test. The frequency f of the vibration wave is known from the frequency f of the vibration wave applied by the vibration pad 16, and the density ρ of the part under test is approximately 1 [g / cm³], which is the density of the human body. 3 ]. And the hardness (elastic modulus) μ is μ = ρ·(λ·f) 2 It is calculated from.

[0032] The MRE image creation means 57 creates an MRElastogram image (MRE image) that visualizes hardness using the MR phenomenon. The MRE image creation means 57 of Example 1 creates an MRE image that is color-coded according to the hardness μ calculated for each target area (pixel). For example, it is possible to display hard parts (pixels with a large hardness μ value) in red, and as it becomes softer (hardness μ value decreases), the display changes to yellow, green, blue, and purple.

[0033] The map creation means 58 derives the longitudinal relaxation time based on the signal intensity generated at each of the following parameters when two or more different values ​​are used for any of the parameters: the time (TI) from the time between applying an alternating magnetic field (reversal RF pulse) that reverses the magnetization to applying an excitation alternating magnetic field (excitation RF pulse) that changes the direction of the magnetization vector; the repetition time (TR), which is the interval at which the excitation alternating magnetic field is applied; and the flip angle (FA), which is the angle at which the direction of the magnetization vector is changed. The map creation means 58 then creates a T1 map, which is a distribution of longitudinal relaxation times, from the longitudinal relaxation times at each location. The map creation means 58 of Example 1 uses a first flip angle α1 and a second flip angle α2 as an example of two different parameters to derive the longitudinal relaxation time T1 based on the signal intensity generated at each flip angle α1, α2 using the VFA (Variable Flip Angle) method.

[0034] In the VFA method, the inherent longitudinal relaxation time (T1 value) of a material is estimated from the signal intensity of MR intensity images obtained at multiple flip angles α1 and α2. Generally, when performing imaging using the VFA method on conventional MRI devices, a sequence to remove residual transverse magnetization (Spoiled GRE sequence) is executed at the end of each cycle. In other words, since the VFA method is a method for estimating the longitudinal relaxation time (T1 value), it is desirable to remove residual transverse magnetization so that factors other than the longitudinal relaxation time do not affect the signal intensity. For this reason, the SPGR sequence, which removes (spoiled) residual transverse magnetization, is used.

[0035] Furthermore, if we denote the Ernst angle as E1, the longitudinal relaxation time as T1, the repetition time as TR, the flip angle as α, the net magnetization vector as M0, and the signal intensity of the MR intensity image obtained by the SPGR sequence as S, then the following equations (1) and (2) are known to hold. E1=exp(-TR / T1) …Formula (1) S / sin(α)=E1 S / tan(α) + M0(1-E1) …Equation (2) Equation (2) is S / sin(α) = y, When S / tan(α) = x, It can be considered as a linear equation of the form y=E1x+M0·(1-E1) with two variables (y=mx+k). Here, E1 corresponds to the slope (m) of this equation, and M0·(1-E1) can be considered as the intercept (k). Therefore, the slope m can be considered as Ernst angle = exp(-TR / T1). In other words, by finding the slope m of the two-variable linear equation, the value of T1 can be calculated from the equation slope m = exp(-TR / T1).

[0036] Figure 4 is an explanatory diagram for the derivation of the inclination m (Ernst angle) in Example 1. The calculation of the slope m is as follows: In Figure 4, if we plot the values ​​(X1, Y1) at the first flip angle α1 and the values ​​(X2, Y2) at the second flip angle α2 on a graph with "S / sin(α)" from equation (2) on the vertical axis (Y axis) and "S / tan(α)" on the horizontal axis (X axis), then E1 can be calculated from equation (2) as the slope m (=(Y2-Y1) / (X2-X1)) of the line segment connecting the two points on the graph. Specifically, the slope m (=E1=exp(-TR / T1)) can be calculated using the following equation (3). m={(S2 / sin(α2)-(S1 / sin(α1))} / {(S2 / tan(α2)-(S1 / tan(α1))}...Equation (3) The longitudinal relaxation time T1 is calculated from this slope m, equation (1), and equation (4) below. T1=-TR / (ln(m)) …Equation (4) Then, the longitudinal relaxation time T1 is calculated for each location (ROI), and a T1 map is created, which is the location distribution of the longitudinal relaxation time T1.

[0037] FIG. 5 is an explanatory diagram of an example of the T1 map of Example 1. FIG. 5A is a T1 map when the signal intensities are averaged by addition at the vibration phases of 0° and 180° when the flip angle is 5° and the signal intensities are averaged by addition at the vibration phases of 90° and 270° when the flip angle is 10°. FIG. 5B is a T1 map when the signal intensities are averaged by addition at the vibration phases of 90° and 270° when the flip angle is 5° and the signal intensities are averaged by addition at the vibration phases of 0° and 180° when the flip angle is 10°. FIG. 5C is a T1 map when the signal intensities are averaged by addition at the vibration phases of 0° and 90° when the flip angle is 5° and the signal intensities are averaged by addition at the vibration phases of 180° and 270° when the flip angle is 10°. FIG. 5D is a T1 map when the signal intensities are averaged by addition at the vibration phases of 180° and 270° when the flip angle is 5° and the signal intensities are averaged by addition at the vibration phases of 0° and 90° when the flip angle is 10°.

[0038] In Example 1, as shown in FIG. 5A, in the vibration applied by the vibration pad 16, at the first flip angle α1, when the phase of the vibration is the reverse phase (when it is 180° different), as an example, the values S of the signal intensity S at 0° and 180° 0° , S 180° are used as the value obtained by averaging by addition. That is, S1 = (S 0° + S 180° ) / 2. And at the second flip angle α2 as well, when the phase of the vibration is the reverse phase, as an example, the values S of the signal intensity S at 90° and 270° 90° , S 270° are used as the value obtained by averaging by addition. That is, S2 = (S 90° + S 270° ) / 2. Note that it is not limited to the exemplified numerical values and can be appropriately changed according to the design, specifications, etc. For example, as shown in FIG. 5B, it is also possible to use the data with the vibration phase of 90° and 270° at the first flip angle α1 and the vibration phase of 0° and 180° at the second flip angle α2. Although it is not very preferable because the accuracy decreases, it is not impossible to use the data with the vibration phase of 0° and 180° at the first flip angle α1 and the vibration phase of 0° and 1so° at the second flip angle α2.

[0039] In Figures 5C and 5D, when the phases of the vibrations are adjacent phases, for example, combinations of 0° and 90°, 180° and 270° (see Figure 5C), or combinations of 180° and 270°, 0° and 90° (see Figure 5D), wave artifacts may appear in the resulting T1 map. In contrast, in the case of out-of-phase vibrations, as shown in Figures 5A and 5B, the effects of the vibrations are canceled out, and a T1 map without wave artifacts is obtained.

[0040] In Example 1, the flip angles α1 and α2 were used as parameters, and the T1 map was created using the VFA method as an example, but the method is not limited to this. For example, it is possible to use the time (TI: Inversion Time) from the time the alternating magnetic field that reverses the magnetization is applied until the excitation alternating magnetic field is applied as a parameter, perform measurements at two or more inversion times TI (four or more points are preferable), and calculate the longitudinal relaxation time T1 using the IR method (Inversion Recovery method) to create a T1 map. It is also possible to use the repetition time (TR: Repetition Time), which is the interval at which the alternating magnetic field 101 is applied, as a parameter, perform measurements at two or more repetition times TR, and create a T1 map using the SR method (Saturation Recovery method). Other methods such as the Morley method based on the IR method and the Smart method based on the SR method are also applicable. Furthermore, since the IR and SR methods tend to take longer to measure than the VFA method, the VFA method, which allows measurement with a relatively short repetition time (TR), is preferable for shortening the time during which the subject holds their breath while being scanned for MRE.

[0041] The image display means 59 displays images created by the MR image acquisition means 54 and the MRE image creation means 57, and the T1 map formed by the map creation means 58, on the display 23. That is, the MR intensity image (the image used in normal diagnosis), which is a cross-sectional image of the area under examination, the MR phase image (used as a Wave Image in MRE), the elastogram image (MRE image), and the T1 map are displayed on the display 23. In the first embodiment, since the anatomical structure is difficult to understand from the Wave Image and Elastogram images alone, it is also possible to provide a display mode that overlays the MR intensity image with the Wave Image and Elastogram images. It is also possible to display images overlaid, or not display all images on the display 23, and to switch between displaying the MR intensity image, Wave Image, Elastogram image, and T1 map according to the input. Furthermore, it is possible to arrange the MR intensity image and Elastogram image, etc. side by side in the same cross-section, or to display Elastogram images, etc., from different cross-sections side by side, and any other changes are possible.

[0042] (Effect of Example 1) In the magnetic resonance imaging apparatus 1 of Embodiment 1, which has the above configuration, when performing MRE imaging using the vibration pad 16, a sequence to remove residual transverse magnetization is executed, and the sequence to remove residual transverse magnetization is executed at two different flip angles α1 and α2. From the measured data, not only the MRE image but also the T1 map can be obtained. Therefore, compared to the conventional method in which MRE and T1-map imaging are performed separately, imaging can be performed in a shorter time. Thus, compared to the conventional configuration in which imaging takes a long time, the burden on the patient is also reduced.

[0043] (Experimental Example 1) Figure 6 is an explanatory diagram of the phantom used in Experimental Example 1. Experiment Example 1 investigated the appropriate combination of flip angles α1 and α2. In the experiment, the relative error [%] of the values ​​derived using the VFA method was calculated while varying the flip angle relative to the reference SR method. As shown in Figure 6, the experiment used Falcon tubes 106-109, each containing four manganese chloride aqueous solutions with different T1 values, as phantoms (aqueous solution phantoms). The T1 values ​​were 1288 ms, 861 ms, 642 ms, and 505 ms, respectively. The flip angle was set between 5° and 100° in 5° increments for the experiment. The experimental results are shown in Figures 7-10.

[0044] Figure 7 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the SR method in an aqueous solution phantom with T1 of 505 ms. Figure 8 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the SR method in an aqueous solution phantom with a T1 of 642 ms. Figure 9 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the SR method in an aqueous solution phantom with a T1 of 861 ms. Figure 10 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the SR method in an aqueous solution phantom with T1 of 1288 ms. As shown in Figures 7 to 10, although it varies depending on the phantom, the overall trend was that combinations of flip angles α1 and α2 with small values ​​(25° or less from experiments) tend to have higher accuracy in calculating the T1 value (smaller relative error).

[0045] (Experimental Example 2) In Experiment Example 2, building upon the results of Experiment Example 1, we conducted an experiment to investigate appropriate combinations of flip angles α1 and α2 by calculating the relative error of the VFA method compared to the IR method, which is considered to have relatively high accuracy in calculating the longitudinal relaxation time T1. In Experiment Example 2, an experiment was conducted to simulate the T1 values ​​of a normal liver and a liver after contrast agent injection. The T1 value of a normal liver is approximately 900 ms, and the T1 values ​​of a normal liver after contrast agent injection are approximately 600 ms at 3 minutes, 500 ms at 8 minutes, and 400 ms at 18 minutes. In Experiment Example 2, to simulate these conditions, a Falcon tube containing five manganese chloride aqueous solutions with different T1 values ​​was used as a phantom (aqueous solution phantom). The T1 values ​​were 874 ms, 648 ms, 518 ms, 398 ms, and 353 ms, respectively. Based on Experiment Example 1, the flip angle was set in 5° increments between 5° and 25°. The experiment was performed three times. The experimental results are shown in Figures 11 to 15.

[0046] Figure 11 is a table of relative errors in experimental results for combinations of T1 values ​​and flip angles using the IR method with an aqueous solution phantom with T1 of 353 ms. Figure 12 is a table of relative errors in experimental results for combinations of T1 values ​​and flip angles using the IR method with an aqueous solution phantom with T1 of 398 ms. Figure 13 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the IR method in an aqueous solution phantom with a T1 of 518 ms. Figure 14 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the IR method in an aqueous solution phantom with T1 of 648 ms. Figure 15 is a table of relative errors from experimental results for combinations of T1 values ​​and flip angles using the IR method with an aqueous solution phantom with T1 of 874 ms. As shown in Figures 11 to 15, in Figures 11, 12, 13, and 15, the combination of flip angles 5° and 10° yielded high accuracy, while in Figure 14, the combinations of flip angles 5° and 10° and 5° and 15° yielded high accuracy. In Experimental Example 2, it was also confirmed that combinations with smaller flip angle values ​​α1 and α2 tended to yield higher accuracy in calculating the T1 value. However, there was variability in the accuracy of calculating the longitudinal relaxation time T1, suggesting that the S / R ratio (signal-to-noise ratio, image graininess) may be a contributing factor.

[0047] (Experimental Example 3) Figure 16 is an explanatory diagram of the phantom used in Experimental Example 3. In Experiment Example 3, we conducted an experiment on simultaneous acquisition of MRE and T1 map. In Figure 16, in Experimental Example 3, an agarose phantom 110 was used as the phantom. The agarose phantom 110 in Experimental Example 3 has an upper layer 111 with 1% agarose (T1 value: approximately 400 ms) and a lower layer 112 with 0.75% agarose (T1 value: approximately 800 ms).

[0048] Figure 17 is an explanatory diagram for Experimental Example 3, illustrating the relationship between vibration phase, MR phase image, MR intensity image, flip angle, MRE image, and T1 map. In Figure 17, in Experiment Example 3, Elastogram image (MRE image) 141 is created based on MR phase images 121-124 with vibration phase 0° and flip angle 5°, vibration phase 90° and flip angle 10°, vibration phase 180° and flip angle 5°, and vibration phase 270° and flip angle 10°. In Experiment Example 3, T1 map 142 is created based on the summation and Gaussian filter of MR intensity image 131 with vibration phase 0° and flip angle 5° and MR intensity image 133 with vibration phase 180° and flip angle 5°, and MR intensity image 132 with vibration phase 90° and flip angle 10° and MR intensity image 134 with vibration phase 270° and flip angle 10°. Note that in Experiment Example 3, values ​​with summation and a 5x5 Gaussian filter are used to improve the S / R ratio (signal-to-noise ratio, image graininess). The experimental results are shown in Figures 18 to 24.

[0049] Figure 18 is an explanatory diagram of the MR phase image; Figure 18A is an explanatory diagram of a comparative example when the flip angle is 10°, and Figure 18B is an explanatory diagram of the MR phase image of experimental example 3. The results of Experimental Example 3, shown in Figure 18B, confirmed that, similar to the conventional comparative example with a fixed flip angle shown in Figure 18A, it is possible to investigate the locations where vibrations are transmitted cleanly.

[0050] Figure 19 is an explanatory diagram of the MRElastogram imaging results, with Figure 19A being an explanatory diagram of the phantom and Figure 19B being an image of the imaging results. Figure 20 is an explanatory diagram of the effect of the flip angle on the MRE, Figure 20A is an explanatory diagram of a comparative example when the flip angle is 10°, and Figure 20B is an MRElastogram image of Experimental Example 3. Note that the areas enclosed by frames in Figures 19 and 20 correspond to the areas enclosed by frames in Figure 18. In Experimental Example 3, as shown in Figure 19B, elastogram images 141 of the upper layer 111 and the lower layer 112, as well as the elastic modulus, were acquired, confirming that the MRE was performed normally. Furthermore, in Experimental Example 3, shown in Figure 20B with two different flip angles, it was confirmed that almost the same results were obtained as in the conventional case shown in Figure 20A with a fixed flip angle. In other words, it was confirmed that performing MRE with two different flip angles had little effect on the calculation results of the elastic modulus.

[0051] Figure 21 is an explanatory diagram showing the case where MRE images are repeatedly acquired using the method of Experimental Example 3. In Figure 21, the MRE imaging in Experimental Example 3 was repeated three times, and it was confirmed that similar results were obtained, confirming the high reproducibility of MRE.

[0052] Figure 22 is an explanatory diagram of the T1 map results; Figure 22A is an explanatory diagram of the T1 map when prepared using the IR method, and Figure 22B is an explanatory diagram of the T1 map when prepared using the method of Experimental Example 3. Figure 22 compares the T1 map created using the IR method (Figure 22A) and the T1 map created using the VFA method (Figure 22B) for the same agarose phantom 110. Comparing Figure 22A and Figure 22B, the time required to create the T1 map 142 in Figure 22B was significantly reduced from approximately 1 hour to approximately 1 minute. On the other hand, while the accuracy of the T1 map 142 in Figure 22B decreased at the phantom edges compared to Figure 22A, the values ​​in the central part of the phantom were almost the same.

[0053] Figure 23 is an explanatory diagram of the results of confirming the effect of vibration on the T1 map. Figure 23A shows the T1 map in a vibration-free state, and Figure 23B shows the T1 map in a vibration-induced state. In Figures 23A and 23B, compared to the state without vibration (Figure 23A), a decrease in accuracy was observed at the edges of the T1 map 142 with vibration applied (Figure 23B). The results in Figures 22 and 23 suggest that the decrease in accuracy at the edges may be due to the influence of magnetic field inhomogeneity, and that vibrations reflecting off the walls of the phantom case may cause artifacts in the MR intensity image, leading to a decrease in the accuracy of calculating the longitudinal relaxation time T1.

[0054] Figure 24 is an explanatory diagram showing the case where the T1 map is repeatedly created using the method of Experimental Example 3. In Figure 24, the T1 map 142 was created by repeating the imaging method of Experimental Example 3 three times, similar to Figure 21. As shown in Figure 24, although there is some variability, almost identical results were obtained, confirming high reproducibility.

[0055] (Example of change) Although embodiments of the present invention have been described in detail above, the present invention is not limited to the embodiments described above, and various modifications can be made within the scope of the gist of the present invention as described in the claims. Examples of modifications to the present invention (H01) to (H05) are shown below. (H01) In the above embodiment, a magnetic resonance imaging apparatus in which the magnet section 2 is ring-shaped, a so-called tunnel type, was illustrated, but the invention is not limited thereto. For example, it can also be applied to a magnetic resonance imaging apparatus in which the magnet section 2 is U-shaped, a so-called open type. (H02) In the above embodiment, the specific numerical values ​​exemplified, such as the flip angle and vibration phase, can be arbitrarily changed according to the design and specifications.

[0056] (H03) In the above embodiment, the specific shape, length, width, etc. of the vibration pad 16 can be changed according to the design and specifications. (H04) In the above embodiment, it is desirable that the vibration phase combination be in opposite phase, but if there are circumstances such as the part and size of the object to be photographed, the elastic modulus, etc. that make artifacts less likely to appear, it is also possible to use adjacent phases. (H05) In the above embodiment, the shear wave information obtained from the wave image includes not only the wavelength but also the amplitude. In MR elastography, in addition to the method of determining the elastic modulus from the "wavelength of the shear wave" as exemplified in the embodiment (Local frequency estimation: LFE), a method of determining the elastic modulus from the amplitude (Algebraic inversion of the differential equation: AIDE) is known. Therefore, it is not limited to the LFE method but can also be applied to the AIDE method. Furthermore, to determine the elastic modulus from the amplitude, assuming that the volume does not change as the shear wave propagates through the object being observed, the following equations can be used to express the relationship between the elastic modulus and the amplitude using the wave equation. TIFF0007898717000001.tif11165 Here, ω is the angular frequency of the passive driver, u is the displacement (amplitude) caused by the shear wave, and Δ is the Laplacian (an operator that spatially takes the second partial derivative of the displacement). The angular frequency can be reinterpreted from the vibration frequency of the vibration device (i.e., it is known), and the modulus of elasticity can be calculated by reading the amplitude from the wave image. [Explanation of Symbols]

[0057] 1... Imaging device, 2... Magnetic field generator, 4... Subject, 16…Vibration-applying member, 51d...Gradient magnetic field applying means, 54c...Means for creating MR phase images, 57…Method for creating MRE images, 58...Methods for creating maps, 91...Gradient magnetic field 92... Spoiler gradient magnetic field 121-124...MR phase images, 141…MRE image, 142…(T1) map, S1, S2... Intensity of residual (transverse) magnetization, T1…(vertical) relaxation time, α1... First flip angle, α1, α2... Flip angle, α2... Second flip angle.

Claims

1. A magnetic field generator that generates a static magnetic field, a gradient magnetic field whose magnetic field changes according to the position, and an alternating magnetic field preset based on the proton magnetic resonance conditions, on the part of the subject being examined. A vibration-applying member that applies vibration to the part under inspection, An MR phase image acquisition means acquires an MR phase image corresponding to the phase of the electromagnetic wave signal based on the electromagnetic waves acquired according to the timing of the application of the gradient magnetic field, An MRE image creation means for creating an MR elastography image based on an MR phase image, A map creation means that, when two or more parameters with different values ​​are used for the flip angle parameter, which is the angle at which the direction of the magnetization vector is changed, derives the longitudinal relaxation time based on the signal intensity generated for each parameter and creates a map of the distribution of longitudinal relaxation times from the longitudinal relaxation times for each position, A photographic device equipped with, An alternating magnetic field applying means for generating an alternating magnetic field that changes the direction of the magnetization vector at a flip angle as the parameter, comprising: a first alternating magnetic field that changes the direction of the magnetization vector by a predetermined first flip angle; and the alternating magnetic field applying means for generating a second alternating magnetic field that changes the direction of the magnetization vector by a second flip angle different from the first flip angle. The map creation means derives the longitudinal relaxation time based on the first signal intensity generated after the application of the first alternating magnetic field and the second signal intensity generated after the application of the second alternating magnetic field, and creates a map of the distribution of longitudinal relaxation times from the longitudinal relaxation times for each position. A photographic device characterized by having the following features.

2. The longitudinal relaxation time derived in each case of the first and second flip angles is calculated by using the average of the longitudinal relaxation times when the phase of the vibration applied by the vibration-applying member differs by 180 degrees. The imaging device according to feature 1.

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