Magnetic resonance imaging apparatus and magnetic resonance imaging method

Abstract

To improve the image quality.SOLUTION: A magnetic resonance imaging apparatus includes a generation unit and a reconstruction unit. The generation unit executes a pulse sequence including a plurality of repetition times TRs each of which includes an echo train and a driven equilibrium pulse applied following the echo train. The apparatus further includes a varying unit which varies a flip angle of the driven equilibrium pulse. The reconstruction unit obtains magnetic resonance image data collected by executing the pulse sequence, and reconstructs a magnetic resonance image by using the magnetic resonance image data.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] Embodiments disclosed herein and in the drawings relate to magnetic resonance imaging apparatus and magnetic resonance imaging method. [Background technology]

[0002] MRI T1WI-VFA (variable flip angle) 3D is currently the most important pulse sequence for intracranial vascular wall imaging in black blood imaging techniques, allowing for characterization of plaque size, location, morphology, and contrast. Furthermore, different techniques can be used to suppress blood and cerebrospinal fluid to better visualize the vascular wall. Examples of commonly used sequences include MSDE (Motion-senstized driven equilibrium), DANTE (Delay alternating with nutation for tailored excitation), and IR-FSE3D (Inversion recovery - Fast Spin Echo 3D).

[0003] MSDE has a motion-sensitive gradient waveform, consisting of one 90° excitation pulse, several 180° refocusing pulses, and one -90° drive equilibrium pulse. By applying a gradient magnetic field between pulses, fluid signals can be suppressed. MSDE enhances the effectiveness of black blood imaging by suppressing fluid signals such as blood signals, but the signal-to-noise ratio decreases as the effective TE (echo delay time) increases.

[0004] DANTE consists of multiple low-frequency flip excitations and can suppress fluid signals. DANTE improves imaging effects of blood vessel walls by suppressing signals from fluids such as blood and cerebrospinal fluid, but the signal-to-noise ratio decreases as the flip angle and excitation frequency increase.

[0005] IR-FSE3D belongs to the inversion recovery method, which optimizes the duration between the flip pulse and the next excitation to obtain better contrast of the blood vessel wall. However, this method also reduces the signal-to-noise ratio (SNR) of the image. Furthermore, in imaging of certain specific regions, such as intracranial blood vessel wall imaging, high resolution (0.5 mm resolution in each direction) is required, making it even more difficult to obtain a good SNR. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2015-116474 [Patent Document 2] Japanese Patent Publication No. 2015-188635 [Patent Document 3] Japanese Patent Publication No. 2019-126727 [Overview of the project] [Problems that the invention aims to solve]

[0007] One of the problems that the embodiments disclosed in this specification and drawings aim to solve is to improve image quality. However, the problems that the embodiments disclosed in this specification and drawings aim to solve are not limited to the above problem. Problems corresponding to the effects of each configuration shown in the embodiments described later can also be positioned as other problems. [Means for solving the problem]

[0008] The magnetic resonance imaging apparatus according to this embodiment comprises a generation unit and a reconstruction unit. The generation unit executes a pulse sequence in which each repetition time TR includes an ecotrain and a drive equilibrium pulse applied following the ecotrain. The reconstruction unit acquires magnetic resonance image data collected by executing the pulse sequence with a modification unit that changes the flip angle of the drive equilibrium pulse, and reconstructs a magnetic resonance image using the magnetic resonance image data. [BRIEF DESCRIPTION OF THE DRAWINGS]

[0009] [Figure 1] FIG. 1 is a diagram showing a magnetic resonance imaging apparatus according to a first embodiment. [Figure 2] FIG. 2 is a diagram showing a pulse sequence generated by a sequence generation unit in the magnetic resonance imaging apparatus according to the first embodiment. [Figure 3] FIG. 3 is a diagram showing signal calibration processing in the magnetic resonance imaging apparatus according to the first embodiment. [Figure 4] FIG. 4 is a flowchart showing procedures of a magnetic resonance imaging method in the magnetic resonance imaging apparatus according to the first embodiment. [Figure 5] FIG. 5 is an effect comparison diagram for a case where magnetic resonance imaging is performed using different pulse sequences. [Figure 6] FIG. 6 is a diagram showing a magnetic resonance imaging apparatus according to a second embodiment. [Figure 7] FIG. 7 is a diagram showing signal calibration processing in the magnetic resonance imaging apparatus according to the second embodiment. [Figure 8] FIG. 8 is a flowchart showing procedures of a magnetic resonance imaging method in the magnetic resonance imaging apparatus according to the second embodiment. [MODES FOR CARRYING OUT THE INVENTION]

[0010] Hereinafter, embodiments of a magnetic resonance imaging apparatus and a magnetic resonance imaging method will be described in detail with reference to the drawings.

[0011] Hereinafter, preferred embodiments of the magnetic resonance imaging apparatus and the magnetic resonance imaging method of the present invention will be described in detail with reference to the drawings. Herein, common reference numerals are assigned to identical or similar configurations, and duplicate description will be omitted as appropriate.

[0012] (First Embodiment) Figure 1 is a structural block diagram of a magnetic resonance imaging apparatus according to the first embodiment. As shown in Figure 1, the magnetic resonance imaging apparatus 100 mainly comprises a bed 110, a static magnetic field magnet 120, a gradient magnetic field coil 130, a transmitting coil 140, a receiving coil 150, and a control unit 160.

[0013] The bed 110 is for placing the subject P within the acquisition range of the magnetic resonance imaging apparatus. The static magnetic field magnet 120 is for generating a static magnetic field in the subject detection space of the magnetic resonance imaging apparatus 100.

[0014] The gradient magnetic field coil 130 is positioned inside the static magnetic field magnet 120. It is formed by combining three coils corresponding to the mutually orthogonal X, Y, and Z axes, and these three coils can individually generate gradient magnetic fields along the X, Y, and Z axes under the control of the control unit 160, with the magnetic field strength changing along each axis. The gradient magnetic fields for the X, Y, and Z axes generated by the gradient magnetic field coil 130 are, for example, the readout gradient magnetic field Gx, the phase encoding gradient magnetic field Gy, and the slicing gradient magnetic field Gz.

[0015] The transmitting coil 140 is positioned inside the gradient coil 130 and generates a high-frequency magnetic field. The control unit 160 supplies RF pulses corresponding to the Larmor frequency, which is determined by the type of atom being targeted and the strength of the magnetic field, to the transmitting coil 140.

[0016] The receiving coil 150 is positioned inside the gradient magnetic field coil 130 and receives magnetic resonance signals (hereinafter sometimes referred to as echo signals or acquired signals) emitted from the subject P under the influence of a high-frequency magnetic field. When the receiving coil 150 receives a magnetic resonance signal, it outputs the received magnetic resonance signal to the control unit 160.

[0017] The control unit 160 controls various parts of the magnetic resonance imaging apparatus 100, including the processor, memory, transmitting circuit, and receiving circuit.

[0018] This embodiment primarily concerns a configuration and method for processing pulse sequences used in a magnetic resonance imaging apparatus and acquired magnetic resonance image data, which is realized by the processor in the magnetic resonance imaging apparatus executing each functional module stored in memory. Therefore, in the control unit 160 in Figure 1, only the functional modules related to the first embodiment of this embodiment are shown, and the control unit 160 is assumed to be composed of each functional module, while other configurations are omitted. In addition, the magnetic resonance imaging apparatus may include other hardware structures and other control modules in the control unit 160, but these can all be realized by the configuration of the prior art, so their explanation is omitted here.

[0019] Furthermore, each functional module described herein corresponds to a process executed by a processor (processing circuit) and may be implemented by installing it as software in the memory of the magnetic resonance imaging apparatus, with the processor reading and executing the software program in the memory. Alternatively, it may be implemented as hardware by creating a dedicated circuit with the corresponding function and incorporating it into the magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus can transmit and receive data and collect data via a network such as the Internet.

[0020] The term "processor" used in the above explanation refers to circuits such as CPU (Central Processing Unit), GPU (Graphical Processing Unit), or ASIC (Application Specific Integrated Circuit), or PLD (e.g., SPLD (Simple Programmable Logic Device), CPLD (Complex Programmable Logic Device), and FPGA (Field Programmable Gate Array)).

[0021] Alternatively, instead of storing the software program in memory, the processor may be configured to directly integrate the software program into its circuitry. In this case, the processor performs its functions by executing the program integrated into the circuitry.

[0022] Returning to the explanation in Figure 1, as shown in Figure 1, the functional module of the control unit 160 includes a sequence generation unit 10, a signal simulation unit 20, a signal change calibration unit 30, and a magnetic resonance image reconstruction unit 40.

[0023] The sequence generation unit 10, flip angle changing unit 11, addition unit 12, signal simulation unit 20, signal change calibration unit 30, and magnetic resonance image reconstruction unit 40 shown in Figure 1 are implemented by the aforementioned processor (processing circuit) as sequence generation function, flip angle changing function, addition function, signal simulation function, signal change calibration function, and magnetic resonance image reconstruction unit 40, respectively. The sequence generation unit 10, flip angle changing unit 11, addition unit 12, signal simulation unit 20, signal change calibration unit 30, and magnetic resonance image reconstruction unit 40 are examples of a generation unit, changing unit, addition unit, simulation unit, calibration unit, and reconstruction unit, respectively.

[0024] Of these, the sequence generation unit 10 generates a pulse sequence for acquiring magnetic resonance imaging data. A pulse sequence is a pulse process and combination used by the magnetic resonance apparatus during examination. In the first embodiment, the sequence generated by the sequence generation unit 10 must satisfy the following predetermined conditions: the pulse sequence includes a plurality of repetition times TR, and each repetition time includes an ector train and a driven equilibrium pulse that follows the ector train. Currently, for example, the T1WI, a longitudinal relaxation time T1-weighted sequence commonly used for intracranial vascular wall imaging, satisfies the above conditions. Therefore, here we will explain using the T1WI sequence as the base sequence to acquire images of the intracranial vascular wall as an example.

[0025] The sequence generation unit 10 can generate a normal T1WI sequence, in which each repetition time TR is followed by the same drive balance pulse. That is, the sequence generation unit 10 executes a longitudinal relaxation time T1 enhanced sequence (T1WI sequence) by the sequence generation function of the processing circuit. The sequence generation unit 10 generates a pulse sequence in which each repetition time TR includes an eco-train and a drive balance pulse applied following the eco-train. The sequence generation unit 10 also includes a flip angle changing unit 11 and an addition unit 12 that can correct the pulse sequence. That is, the sequence generation unit 10 executes a pulse sequence in which each repetition time TR includes an eco-train and a drive balance pulse applied following the eco-train, while changing the flip angle of the drive balance pulse.

[0026] Specifically, the flip angle changing unit 11 changes the flip angle of the drive equilibrium pulse provided at the tail of each repetition time TR in the T1WI sequence, thereby generating a difference between the flip angles of drive equilibrium pulses present at different repetition times TR. In other words, the flip angle changing unit 11 changes the flip angle of the drive equilibrium pulse included in the longitudinal relaxation time T1 enhancement sequence (T1WI sequence) through the flip angle changing function of the processing circuit.

[0027] For example, the flip angle changing unit 11 sets the flip angle of the drive equilibrium pulse at each repetition time TR such that the angle decreases as the repetition time TR progresses. Alternatively, the flip angle changing unit 11 may use the flip angle changing function of the processing circuit to set the flip angle of the drive equilibrium pulse at each repetition time TR such that the flip angle of the drive equilibrium pulse decays exponentially as the repetition time TR progresses. Furthermore, the flip angle changing unit 11 can change the flip angle of the drive equilibrium pulse randomly, rather than following a fixed rule. However, in order to optimize the signal-to-noise ratio, it is preferable that the flip angle changing unit 11 gradually decays the flip angle of the drive equilibrium pulse over time using the flip angle changing function of the processing circuit.

[0028] Furthermore, the additional section 12 adds a single IR (Inversion Recovery) pulse with a time TI placed before the entire T1WI sequence.

[0029] Figure 2 shows a schematic diagram of the pulse sequence (RF) finally generated by the sequence generation unit 10 through the changes made by the flip angle changing unit 11 and the addition unit 12.

[0030] First, a schematic time series diagram representing the pulse sequence is shown at the top of Figure 2. As shown in Figure 2, the pulse sequence begins with a single IR pulse, followed by a time TI and then multiple repetition time TRs. Thus, when viewed on the time axis, the pulse sequence consists of TI and multiple TRs. Here, TI is approximately 2 seconds, but TI is not limited to 2 seconds and may be of other lengths. TR is usually less than 1 second, but when TI is approximately 2 seconds, the excitation signal level increases in TI compared to TR.

[0031] Each repetition time TR starts with excitation by one flip pulse and mainly comprises an echo train and a flip angle set. The echo train in Fig. 2 is enclosed in a dashed frame and contains a plurality of refocusing pulses, and the configuration of the echo train is identical to that of a T1WI sequence. To put it simply, each columnar bar within the dashed frame in Fig. 2 represents one refocusing pulse, and different heights of the columnar bars correspond to different pulse intensities (also referred to as pulse angles). Since a magnetic resonance imaging apparatus generally collects echo signals after each refocusing pulse, the number of refocusing pulses is the same as the number of signal acquisitions, and the combination of echoes after a plurality of refocusing pulses is referred to as an echo train.

[0032] The flip angle set includes flip angles β y , γ y , θ x , and the part enclosed by a solid frame after the echo train in Fig. 2 represents the flip angle set, wherein the three columnar bars respectively represent the flip angles β y , γ y , θ x , and different heights of the columnar bars correspond to different angles. Among them, the flip angles β y , γ y are flip angles for shifting the magnetization vector remaining after applying the echo train to the transverse plane. The flip angle θ x is provided at the tail of the repetition time TR, and is a flip angle for flipping the remaining transverse magnetization vector to the negative Z-axis to improve T1 contrast, which is referred to as the flip angle of a driven equilibrium pulse. Here, the flip angles θ in a plurality of repetition times TR are respectively θ x to θ 1,x to θ m,x (where m is a natural number of 1 or more, and represents the m-th repetition time TR). Accordingly, for example, the flip angle θ in the first repetition time TR after TI is θ x is θ 1,x .

[0033] Furthermore, the flip angles β y , γ yBy providing this, the effect of the flip angle of the drive equilibrium pulse can be enhanced. However, the flip angle β y gamma y Alternatively, only the flip angle of the drive equilibrium pulse may be provided without providing the flip angle. In Figure 2, the flip angle changing unit 11 changes the flip angle θ of the drive equilibrium pulse as shown in the chart at the bottom of Figure 2. 1,x ~θ m,x This is set to decay exponentially. In other words, the flip angle θ of the drive equilibrium pulse. 1,x ~θ m,x As indicated by the arrow, this corresponds to a point on the exponentially decaying curve in the chart in Figure 2. The horizontal coordinate of this chart represents m, and the vertical coordinate represents the signal strength. From the time series diagram of the pulse sequence in Figure 2, the flip angle θ of the drive equilibrium pulse at the repetition time TR can be seen. m,x It is intuitively clear that the height of the columnar bars decreases as they are further back.

[0034] Furthermore, the exponential equation for adjusting the flip angle is, for example, as shown in equation (1) below.

[0035]

number

[0036] In equation (1), α0 represents the initial (n=0) flip angle, which is usually 90°, and the parameter n is a natural number greater than or equal to 0, ranging from 0 to the number of flip angles of the drive equilibrium pulse. The parameter N controls the decay rate and is generally set to half the total repetition time TR in order to maintain a balance between the signal-to-noise ratio and contrast.

[0037] As a result, the sequence generation unit 10 generates and outputs a pulse sequence in which the flip angle of the drive equilibrium pulse at the tail of the repetition time TR changes exponentially, as shown in Figure 2. The control unit 160 controls the configuration of the patient bed 110, gradient coil 130, transmitting coil 140, and receiving coil 150, etc., according to the pulse sequence finally generated by the sequence generation unit 10, to collect echo signals from the subject P and transmit them to the magnetic resonance image reconstruction unit 40 as raw data of a magnetic resonance image. These collected echo signals are sometimes called acquired signals.

[0038] In the first embodiment, a higher signal-to-noise ratio (SNR) is obtained compared to the conventional technique by setting the flip angle of the drive equilibrium pulse to change exponentially. On the other hand, point diffusion or artifacts may occur due to the change in signal. Therefore, the magnetic resonance image can be further optimized by calibrating the acquired signal.

[0039] Specifically, the signal simulation unit 20 simulates the excitation signal at the repetition time TR using the signal simulation function of the processing circuit and obtains the simulated signal. For example, the signal simulation unit 20 can calculate the amplitude of the simulated signal by simulating the amplitude of the flip signal at the beginning of each repetition time TR using the Bloch equation or the Extended Phase Graph (EPG) method using the signal simulation function of the processing circuit. For example, the Bloch equation is a set of macroscopic equations, and if the pulse sequence is known, the simulated signal can be evaluated using the Bloch equation. For example, if there are relaxation times T1 and T2, the Bloch equation is used to calculate the nuclear magnetization intensity M=(Mx,My,Mz) as a function of time, thereby representing the change in the vector under specific conditions such as RF magnetic field, gradient magnetic field, and relaxation time. The Extended Phase Graph (EPG) algorithm is also commonly used for signal simulation. Naturally, the signal simulation algorithms for pulse sequences are not limited to the two types mentioned above; it is possible to simulate pulse sequences using various existing simulation methods.

[0040] Furthermore, the signal change calibration unit 30 uses the simulated signal generated by the signal simulation unit 20 to calibrate the magnetic resonance image data acquired by the magnetic resonance image reconstruction unit 40, based on the signal change calibration function of the processing circuit. Specifically, the signal change calibration unit 30 calculates a reference value for the simulated signal from the simulated signal generated by the signal simulation unit 20, uses the ratio of this reference value to the simulated signal as a weighting value, and applies this weighting value to the acquired signal obtained when the magnetic resonance imaging device executes this sequence, thereby calibrating the acquired signal and transmitting the calibrated acquired signal to the magnetic resonance image reconstruction unit 40.

[0041] Figure 3 is a schematic diagram showing the signal calibration process in a magnetic resonance imaging apparatus according to the first embodiment. Here, the upper part of Figure 3 shows the pulse sequence generated by the sequence generation unit 10, and the lower part of Figure 3 shows a schematic chart of the calibration process by the signal change calibration unit 30. In the chart, the solid circles represent the amplitude of the simulation signal obtained by simulating the flip pulse F provided at the beginning of the repetition time TR connected to the solid arrows, and correspond to the intensity of the flip pulse F. Based on the simulation results, multiple solid circles are formed by mapping the simulation results of each flip pulse F to a chart like the one in Figure 3, where the horizontal axis is time and the vertical axis is amplitude. Furthermore, based on the amplitude of each simulated simulation signal, the average value of each amplitude is taken to form an average line at position 0.3, that is, the average value of the simulation signals is 0.3, and this average value is divided by the amplitude of each simulation signal to obtain the calibration ratio r. Furthermore, the hollow circle in the icon represents the amplitude of the acquired signal that was actually collected. By multiplying the amplitude of the acquired signal by the calibration ratio r, the acquired signal is brought closer to the average line, as shown by the dashed line in the figure, thereby calibrating the acquired signal. The calibration process in Figure 3 is schematic; in reality, an echo train in a typical pulse sequence usually includes multiple refocusing pulses, and echoes are collected after each refocusing pulse. Therefore, there are multiple acquired signals at each repetition time TR, and calibration is performed by multiplying all of these multiple acquired signals by the calibration ratio r for that repetition time TR.

[0042] Let the calibration ratio be r, and the simulation signal amplitude be S. S Assuming that the acquired signal amplitude is Sa, the calibration relation is as follows, as described above.

[0043] Calibration ratio (r)=average value / Ss

[0044] Calibrated signal value = Sa*r = Sa*(mean value / Ss)

[0045] Here, the average value was explained as the reference value for the simulation signal, but the reference value is not limited to the average value; other calculation methods, such as using the median value, may also be used.

[0046] The magnetic resonance image reconstruction unit 40 can acquire magnetic resonance image data by executing the pulse sequence generated by the sequence generation unit 10, and reconstruct a magnetic resonance image using the magnetic resonance image data. In other words, the magnetic resonance image reconstruction unit 40 acquires magnetic resonance image data by executing the pulse sequence using the magnetic resonance image reconstruction function of the processing circuit, and reconstructs a magnetic resonance image using the magnetic resonance image data.

[0047] If a calibrated acquired signal is available, the magnetic resonance imaging reconstruction unit 40 uses the calibrated acquired signal to reconstruct the magnetic resonance image. For example, the magnetic resonance imaging reconstruction unit 40 reconstructs the image by filling the k-space with echo signals acquired by the magnetic resonance imaging apparatus. Existing magnetic resonance imaging methods may be used for image reconstruction, and a detailed explanation is omitted here.

[0048] Figure 4 is a flowchart of the magnetic resonance imaging method in the magnetic resonance imaging apparatus according to the first embodiment.

[0049] First, the sequence generation unit 10 generates a T1WI sequence using an existing sequence generation method (step S401). Here, the sequence generation unit 10 may also generate a T1WI sequence by directly obtaining a T1WI sequence that satisfies the requirements via external equipment or a network.

[0050] Next, in step 402, the flip angle changing unit 11 changes the flip angle of the drive equilibrium pulse provided at the tail of each repetition time TR in the T1WI sequence acquired in step S401 so that it decays exponentially as the repetition time TR progresses. The addition unit 12 also adds a single IR pulse with time TI before the entire T1WI sequence (step S403). That is, the sequence generation unit 10 generates a pulse sequence to be executed by adding a single IR pulse before the entire pulse sequence composed of multiple repetition times TR using the sequence generation function of the processing circuit.

[0051] Next, in step S404, the magnetic resonance imaging apparatus 100 collects echo signals according to the sequence pulses finally generated by the sequence generation unit 10 through the changes in the flip angle changing unit 11 and the addition unit 12, and uses the collected signals as magnetic resonance image data.

[0052] Furthermore, in step S405, the signal simulation unit 20 simulates the excitation signal at the repetition time TR using the Bloch equation and obtains the simulated signal.

[0053] Next, the signal change calibration unit 30 acquires the acquired signal collected in step 404 and the simulation signal calculated in step S405, calculates a calibration ratio using the simulation signal, and applies this calibration ratio as a weighting value to the acquired signal to obtain the calibrated acquired signal (step S407).

[0054] Next, in step S407, the magnetic resonance image reconstruction unit 40 reconstructs a magnetic resonance image based on the calibrated acquired signal.

[0055] Furthermore, in the flowchart of Figure 4, the execution order of steps S402 and S403 is not limited to this order; a single IR pulse may be added before changing the flip angle of the drive balance pulse.

[0056] Furthermore, by combining steps S401, S402, and S403 into a single step, the sequence generation unit 10 can directly generate a pulse sequence in which the flip angle of the drive balance pulse has been changed, in accordance with the T1WI sequence generation rule and the rule for changing the flip angle of the drive balance pulse.

[0057] In the first embodiment, by changing the flip angle of the drive equilibrium pulse in the pulse sequence over time, the signal-to-noise ratio (SNR) of the image can be increased while maintaining image contrast, thereby improving the quality of image reconstruction. Furthermore, the SNR can be further improved by adding a single IR pulse before the pulse sequence. In addition, by simulating a univariate and calibrating the acquired signal using the simulated signal, image defects such as artifacts caused by signal fluctuations can be reduced.

[0058] (Modified version of the first embodiment) In the first embodiment, the flip angle of the drive equilibrium pulse in the pulse sequence is gradually changed over time, and a single IR pulse is added before the sequence. However, it is also possible to change only the flip angle of the drive equilibrium pulse and omit the addition of the single IR pulse. When only the flip angle of the drive equilibrium pulse in the pulse sequence is gradually changed over time, it is possible to improve the signal-to-noise ratio of the image while maintaining the contrast of the image, compared to the conventional technique.

[0059] Figure 5 is a comparative diagram of the effects of performing magnetic resonance imaging using different sequences. Figure 5(a) shows an image reconstructed based on signals acquired with a conventional T1WI sequence. Due to the low signal-to-noise ratio, the depiction of the blood vessel wall is not clear, as indicated by the blank arrow.

[0060] Figure 5(b) shows an image acquired and reconstructed after changing only the flip angle of the drive equilibrium pulse in the T1WI sequence to decay exponentially. In this image, the signal-to-noise ratio (SNR) of the vascular wall is improved compared to the SNR of the image shown in Figure 5(a), and the vascular wall is clearly visible. Figure 5(c) shows an image acquired and reconstructed after changing the flip angle of the drive equilibrium pulse in the T1WI sequence to decay exponentially, and adding a single IR pulse to the beginning of the sequence. In this image, the SNR of the vascular wall is significantly improved compared to the SNR of the image shown in Figure 5(a), and the vascular wall is more clearly visible. From the above comparison, the different effects exhibited by different modification methods are intuitively apparent. As mentioned above, a similarly significant improvement can be obtained by simply changing the flip angle of the drive equilibrium pulse.

[0061] Furthermore, although the first embodiment was described based on the conventional T1WI, this embodiment is not limited to this, and only an ecotrain and a drive balance pulse are required in the sequence, and this embodiment may be applied to other pulse sequences. Also, in the first embodiment, the acquired signal is calibrated using the signal simulation unit 20 and the signal change calibration unit 30 to eliminate image defects such as artifacts, but the signal simulation unit 20 and the signal change calibration unit 30 may be omitted, and the corrected pulse sequence may be used directly to acquire the signal. The acquired signal still has a certain technical advantage compared to signals acquired by conventional methods.

[0062] (Second embodiment) The second embodiment will be described with reference to Figures 6 and 7. The difference between the magnetic resonance imaging apparatus of the second embodiment and the first embodiment lies in the method of signal calibration. The differences will be explained below, with redundant explanations omitted where appropriate.

[0063] Figure 6 is a structural block diagram of a magnetic resonance imaging apparatus according to the second embodiment. As shown in Figure 6, the magnetic resonance imaging apparatus 100a mainly comprises a bed 110, a static magnetic field magnet 120, a gradient magnetic field coil 130, a transmitting coil 140, a receiving coil 150, and a control unit 160a. Functional modules included in the control unit 160 are a sequence generation unit 10, a magnetic resonance image reconstruction unit 40a, a template echo extraction unit 50, and a signal change calibration unit 30a.

[0064] Furthermore, similar to Figure 1, the sequence generation unit 10, flip angle changing unit 11, addition unit 12, signal change calibration unit 30a, magnetic resonance image reconstruction unit 40a, and template echo extraction unit shown in Figure 6 are implemented by a processor (processing circuit) as sequence generation function, flip angle changing function, addition function, signal change calibration function, magnetic resonance image reconstruction function, and template echo extraction function, respectively. Note that the sequence generation unit 10, flip angle changing unit 11, addition unit 12, signal simulation unit 20, signal change calibration unit 30a, magnetic resonance image reconstruction unit 40a, and template echo extraction unit 50 are examples of a generation unit, modification unit, addition unit, simulation unit, calibration unit, reconstruction unit, and extraction unit, respectively.

[0065] The sequence generation unit 10 generates the T1WI sequence and further includes a flip angle changing unit 11 and an addition unit 12. The flip angle changing unit 11 changes the flip angle of the drive balance sequence provided at the tail of each repetition time TR in the T1WI sequence, and the addition unit 12 adds a single IR pulse by placing time TI before the entire T1WI sequence.

[0066] Furthermore, the magnetic resonance image reconstruction unit 40a can execute the pulse sequence generated by the sequence generation unit 10 to acquire the collected magnetic resonance image data, and reconstruct the magnetic resonance image using the magnetic resonance image data. If a calibrated acquired signal is available, the magnetic resonance image reconstruction unit 40a reconstructs the magnetic resonance image using the calibrated acquired signal. Therefore, a detailed explanation is omitted here.

[0067] In the second embodiment, the magnetic resonance imaging apparatus 100a performs calibration of the acquired signal using a template echo extraction unit 50 and a signal change calibration unit 30a. The echo train of the pulse sequence has multiple refocus pulses, and one echo acquisition is performed after each refocus pulse. The template echo extraction unit 50 selects the signal acquired by echo acquisition as the template echo from the echo acquisition corresponding to each refocus pulse. For example, the echo signal acquired in the echo acquisition corresponding to the first refocus pulse of the echo train is used as the template echo.

[0068] If a template echo is selected from the echo acquisition signal, the control unit 160a controls the gradient coil 130 so as not to phase encode the template echo when the gradient coil 130 executes the pulse sequence, and furthermore, does not use the template echo in image reconstruction.

[0069] The template echo extraction unit 50 uses the echo signal acquired in echo acquisition corresponding to the first refocus pulse of the echo train as a template echo, and uses this template echo only for signal calibration. The magnetic resonance image reconstruction unit 40a uses the echo signals acquired in echo acquisition corresponding to the second and subsequent refocus pulses as raw data of the magnetic resonance image, and uses it for image reconstruction.

[0070] Furthermore, the signal change calibration unit 30a uses the template echo extracted by the template echo extraction unit 50 to calibrate other magnetic resonance image data other than the template echo acquired by the magnetic resonance image reconstruction unit 40. Specifically, the signal change calibration unit 30a calculates a reference value for the template echo signal from the template echo extracted by the template echo extraction unit 50, uses the ratio of this reference value to the template echo signal as a weighting value, and performs calibration by applying this obtained weighting value to the raw data of the magnetic resonance imaging device.

[0071] Figure 7 is a schematic diagram showing the signal calibration process in a magnetic resonance imaging apparatus according to the second embodiment. Here, the upper part of Figure 7 shows the pulse sequence generated by the sequence generation unit 10, and the group after the columnar bar indicating the first refocus pulse of the echo train represents the echo acquisition performed after the first refocus pulse, and the echo signal acquired in this echo acquisition is used as the template echo. The lower part of Figure 7 shows a schematic chart of the calibration process by the signal change calibration unit 30a. In the chart, the solid circles represent the amplitude of the template echo connected to the solid arrows and correspond to the intensity of the template echo. When each template echo is mapped to a chart with time on the horizontal axis and amplitude on the vertical axis, as shown in Figure 7, multiple solid circles are formed. Furthermore, based on the amplitude of the template echo at each repetition time TR, the average value of each amplitude is taken to form an average line at position 0.3, for example, i.e., the average value of the amplitude of the template echo is 0.3, and this average value is divided by the amplitude of each template echo to obtain the calibration ratio r. Furthermore, the hollow circles in the icons represent the amplitudes of acquired signals other than the template echo that was actually collected. By multiplying the amplitudes of these acquired signals by the calibration ratio r, the acquired signals are calibrated by bringing them closer to the average line, as shown by the dashed line in the figure. The calibration process in Figure 7 is schematic; in reality, an echo train in a typical pulse sequence usually includes multiple refocusing pulses, and echoes are collected after each refocusing pulse. Therefore, there are multiple acquired signals at each repetition time TR, and calibration is performed by multiplying all of these multiple acquired signals other than the template echo by the calibration ratio r for that repetition time TR.

[0072] When the calibration ratio is r, the amplitude of the template echo is St, and the amplitude of the acquired signal is Sa, the calibration relationship is as follows, as described above.

[0073] Calibration ratio (r)=average value / St

[0074] Calibrated signal value = Sa * r = Sa * (mean value / St)

[0075] Here, the mean was used as the reference value for template echo, but the reference value is not limited to the mean; other calculation methods, such as using the median, can also be used.

[0076] Figure 8 is a flowchart of the magnetic resonance imaging method in a magnetic resonance imaging apparatus according to the second embodiment. First, the sequence generation unit 10 generates a T1WI sequence using an existing sequence generation method (step S901). Here, the sequence generation unit 10 can also generate a T1WI sequence by directly acquiring a T1WI sequence that satisfies the requirements via external equipment or a network.

[0077] Next, in step 902, the flip angle changing unit 11 changes the flip angle of the drive equilibrium pulse provided at the tail of each repetition time in the T1WI sequence acquired in step S901 so that it decays exponentially as the repetition time TR progresses. In addition, the addition unit 12 adds a single IR pulse with time T1 placed before the entire T1WI sequence (step S903).

[0078] Next, in step S904, the magnetic resonance imaging apparatus 100a collects echo signals according to the sequence pulses finally generated by the sequence generation unit 10 through the changes in the flip angle changing unit 11 and the addition unit 12, and the magnetic resonance image reconstruction unit 40a collects the magnetic resonance image data collected according to the generated sequence as an acquired signal.

[0079] Next, in step S905, the template echo extraction unit 50 extracts the echo collected after the first refocus pulse at each repetition time TR from the echo signals collected in step 904 as a template echo. The signal change calibration unit 30a then calculates a calibration ratio using the template echo signal and applies this calibration ratio as a weighting value to the other acquired signals other than the template echo to perform calibration, thereby obtaining the calibrated acquired signal (step S906). That is, the template echo extraction unit 50 extracts one echo from each echo train at each repetition time TR to be used as a template echo by the template echo extraction function of the processing circuit. The signal change calibration unit 30a uses the template echo to calibrate the magnetic resonance image data acquired by the magnetic resonance image reconstruction unit 40a by the signal change calibration function of the processing circuit.

[0080] Next, in step S907, the magnetic resonance image reconstruction unit 40a reconstructs a magnetic resonance image based on the acquired signal after calibration.

[0081] Furthermore, in the flowchart of Figure 8, the execution order of steps S902 and S903 is not limited to this order; a single IR pulse may be added before changing the flip angle of the drive balance pulse.

[0082] Furthermore, by combining steps S901, S902, and S903 into a single step, the sequence generation unit 10 may directly generate a pulse sequence in which the flip angle of the drive equilibrium pulse has been changed, in accordance with the T1WI sequence generation rule and the rule for changing the flip angle of the drive equilibrium pulse.

[0083] In the second embodiment, by directly using one of the collected echo signals to calibrate the other echo signals, a large amount of computation during echo simulation is avoided, saving computational resources. Furthermore, by calibrating the collected signals, image defects such as artifacts caused by signal fluctuations can be reduced.

[0084] Each functional module in the magnetic resonance imaging apparatus described in the above-described embodiment is executable as a software program. A general-purpose computer can obtain the same effects as those of the magnetic resonance imaging apparatus of the above-described embodiment by pre-storing and loading this program. This program can be distributed via a network such as the Internet. Furthermore, by recording this program on a computer-readable non-temporary recording medium such as a hard disk, floppy disk (FD), CD-ROM, MO, or DVD, and reading and executing it from the recording medium by a computer, the same operation as the magnetic resonance imaging apparatus of the above-described embodiment can be achieved.

[0085] According to at least one embodiment described above, image quality can be improved.

[0086] While several embodiments have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be implemented in a variety of other forms, and various omissions, substitutions, modifications, and combinations of embodiments are possible without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of symbols]

[0087] 10 Sequence generation unit 11. Flip angle change section 12 Additional Sections 20 Signal Simulation Section 30 Signal change calibration section 40 Magnetic Resonance Imaging Reconstruction Unit 50 Template Echo Extraction Unit

Claims

1. Each repetition time TR, including a first repetition time and a second repetition time, is a generation unit that generates a pulse sequence including an ecotrain and a plurality of pulses, including a drive balance pulse, applied following the ecotrain. A reconstruction unit that acquires magnetic resonance image data collected by executing the aforementioned pulse sequence and reconstructs a magnetic resonance image using the aforementioned magnetic resonance image data, Equipped with, The set of flip angles corresponding to the plurality of pulses in the first repetition time and the set of flip angles corresponding to the plurality of pulses in the second repetition time include a first flip angle corresponding to the drive balance pulse and a second flip angle corresponding to a pulse other than the drive balance pulse among the plurality of pulses, which is a flip angle that shifts the magnetization vector to the transverse plane. A magnetic resonance imaging apparatus wherein the first flip angle corresponding to the drive equilibrium pulse in the first repetition time is different from the first flip angle corresponding to the drive equilibrium pulse in the second repetition time.

2. The magnetic resonance imaging apparatus according to claim 1, wherein the generation unit adds a single IR pulse before the entire pulse sequence including the first repetition time and the second repetition time.

3. The generation unit executes an enhancement sequence with a longitudinal relaxation time T1. The magnetic resonance imaging apparatus according to claim 1.

4. A simulation unit that simulates the excitation signal at the aforementioned repetition time TR and acquires the simulated signal, A calibration unit that uses the simulation signal to calibrate the magnetic resonance image data acquired by the reconstruction unit, The magnetic resonance imaging apparatus according to claim 1, further comprising the following:

5. The magnetic resonance imaging apparatus according to claim 4, wherein the simulation unit simulates the excitation signal using the Bloch equation or the Extended Phase Graph method.

6. An extraction unit extracts one echo from each echo train of each repetition time TR and uses it as a template echo. A calibration unit that uses the template echo to calibrate the magnetic resonance image data acquired by the reconstruction unit, The magnetic resonance imaging apparatus according to claim 1, further comprising the following:

7. Further comprising a modification unit for changing the first flip angle, The magnetic resonance imaging apparatus according to claim 1, wherein the modification part attenuates the first flip angle included in the pulse sequence over time.

8. The magnetic resonance imaging apparatus according to claim 4, wherein the simulation unit simulates the excitation signal at the first repetition time to obtain a simulation signal value, and calibrates the acquired magnetic resonance image data corresponding to the first repetition time based on the ratio of the obtained simulation signal value to a predetermined value.

9. The magnetic resonance imaging apparatus according to claim 1, wherein the first flip angle in the first repetition time is greater than the first flip angle in the second repetition time following the first repetition time.

10. The magnetic resonance imaging apparatus according to claim 8, wherein the predetermined value is the average value of the simulation signal values ​​obtained during the first repetition time.

11. The device further includes a modification unit for changing the first flip angle, The magnetic resonance imaging apparatus according to claim 1, wherein the modification unit changes the first flip angle in the first repetition time and the first flip angle in the second repetition time, respectively.

12. Each repetition time TR, including a first repetition time and a second repetition time, is executed as a pulse sequence including an ecotrain and a plurality of pulses, including a drive balance pulse, applied following the ecotrain, while changing the flip angle of the drive balance pulse. By executing the aforementioned pulse sequence, magnetic resonance image data is acquired, and the magnetic resonance image data is used to reconstruct the magnetic resonance image. The set of flip angles corresponding to the plurality of pulses in the first repetition time and the set of flip angles corresponding to the plurality of pulses in the second repetition time include a first flip angle corresponding to the drive balance pulse and a second flip angle corresponding to a pulse other than the drive balance pulse among the plurality of pulses, which is a flip angle that shifts the magnetization vector to the transverse plane. The first flip angle corresponding to the drive equilibrium pulse in the first repetition time is different from the first flip angle corresponding to the drive equilibrium pulse in the second repetition time. Magnetic resonance imaging method.

13. Each repetition time TR, including a first repetition time and a second repetition time, is a generation unit that generates a pulse sequence including an ecotrain and a plurality of pulses, including a drive balance pulse, applied following the ecotrain. A reconstruction unit that acquires magnetic resonance image data collected by executing the aforementioned pulse sequence and reconstructs a magnetic resonance image using the aforementioned magnetic resonance image data, Equipped with, The set of flip angles corresponding to the plurality of pulses in the first repetition time and the set of flip angles corresponding to the plurality of pulses in the second repetition time include a first flip angle corresponding to the drive balance pulse and a second flip angle corresponding to a pulse other than the drive balance pulse among the plurality of pulses, which is a flip angle that shifts the magnetization vector to the transverse plane. The generating unit randomly changes the first flip angle corresponding to the drive equilibrium pulse in the first repetition time and the first flip angle corresponding to the drive equilibrium pulse in the second repetition time, in a magnetic resonance imaging apparatus.

Images