JP2012010728A - Magnetic resonance imaging apparatus, and t2 map acquisition method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an MRI apparatus, and a T2 map acquisition method, capable of acquiring a T2 map with favorable precision while reducing SAR.SOLUTION: A plurality of echo signals are measured using at least one flip angle set at less than 180° in a plurality of re-converging RF pulses in a multiple SE sequence based on a CPMG method. Using only even-numbered echo signals among a plurality of echo signals measured, the T2 map is acquired.

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. In particular, the present invention relates to a technique for obtaining a T2 map by reducing a specific absorption rate (SAR).

  The MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In recent years, in the above MRI apparatus, a color mapping (hereinafter referred to as T2 mapping) technique for obtaining a T2 value of a subject tissue and coloring it into an image (T2 map) according to the obtained T2 value has been used for articular cartilage evaluation. (For example, Non-Patent Document 1). Multi-spin echo measurement sequence based on fast spin echo (FSE) sequence based on CPMG (Carr-Purcell-Meiboom-Gill) method is used to measure echo signal used to calculate T2 value when T2 mapping is performed ( Hereinafter, a multi-SE sequence) is used.

  The FSE sequence is a pulse sequence for high-speed imaging that measures a plurality of echo signals with a short repetition time (TR). Specifically, in order to refocus the phase of the nuclear spin by inverting the excited nuclear spin by 180 ° after a radio frequency magnetic field (hereinafter referred to as RF) pulse for exciting the nuclear spin of the subject by 90 ° The refocus RF pulses are applied to the subject by the number of echo signals to be measured. Each echo signal is measured with a different phase encoding amount according to its echo number.

  The CPMG method that is the basis of this FSE sequence is a pulse sequence that changes the phase (applied axis) of an RF pulse that is excited by 90 ° and an RF pulse that is inverted by 180 ° by 90 °. Therefore, the odd-numbered echo signal is affected by the imperfection of the RF pulse, but the even-numbered echo signal is not affected by this. The pulse train of the CPMG method is described in Patent Document 1, for example.

U.S. Pat. Japanese Patent No. 2570957

Journal of Magnetic Resonance Vol.28, pp.175-180, 2008

  In the MRI apparatus, the NMR frequency increases in proportion to the magnetic field strength. Along with this, RF power absorption to the human body called SAR (Specific Absorption Rate) increases, and countermeasures against it have become issues. SAR means the amount of energy absorbed by a tissue of unit mass per unit time when a subject (human body) is exposed to electromagnetic waves. In an MRI apparatus, the square of the excitation angle (flip angle) of an RF pulse. It is proportional to the time integral value of.

  As described above, when a multi-SE sequence is used as a pulse sequence for measuring the echo signal used to calculate the T2 value, SAR may increase because multiple refocus RF pulses are applied after the excitation RF pulse. There is. In particular, in a multi-SE sequence, a plurality of refocusing RF pulses greatly contribute to SAR.

  In order to reduce the SAR in the multi-SE sequence, for example, it is conceivable to reduce the flip angle of the refocus RF pulse. However, if the refocusing RF pulse has an incomplete flip angle other than 180 °, as described above, the odd-numbered echo signal is affected by the imperfection of the RF pulse. Therefore, it is difficult to obtain the T2 value with high accuracy.

  Therefore, the present invention has been made in view of the above problems, and an object of the present invention is to provide an MRI apparatus and a T2 map acquisition method capable of accurately obtaining a T2 map while reducing SAR.

  To achieve the above object, the present invention measures a plurality of echo signals by setting at least one flip angle of a plurality of refocusing RF pulses of an FSE sequence based on the CPMG method to less than 180 °, A T2 map is acquired using only the even-numbered echo signals measured.

  Specifically, the MRI apparatus of the present invention uses a multi-SE sequence that applies a plurality of refocus RF pulses to a subject based on the CPMG method, and controls measurement of a plurality of echo signals from the subject. A control unit and an arithmetic processing unit that acquires a T2 map of the subject using a plurality of echo signals, and at least one flip angle of the plurality of refocusing RF pulses is less than 180 °, and the arithmetic processing The unit is characterized in that the T2 map is acquired using only the even-numbered echo signals measured from the plurality of echo signals.

  The T2 map acquisition method of the present invention includes a step of measuring a plurality of echo signals from a subject using a multi-SE sequence in which at least one flip angle of a plurality of refocusing RF pulses is less than 180 °; Obtaining a T2 map using only even-numbered echo signals measured from a plurality of echo signals.

  According to the MRI apparatus and the T2 map acquisition method of the present invention, it is possible to accurately obtain a T2 map while reducing SAR.

The block diagram which shows the whole basic structure of the MRI apparatus which concerns on this invention Sequence chart showing multi-echo measurement using a multi-SE sequence based on the CPMG method according to the first embodiment A flowchart showing a processing flow according to the first embodiment. Sequence chart showing multi-echo measurement using multi-SE sequence based on CPMG method according to Example 2 Sequence chart showing multi-echo measurement using multi-SE sequence based on CPMG method according to Example 3

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. In all the drawings for explaining the embodiments of the invention, those having the same function are given the same reference numerals, and their repeated explanation is omitted.

  First, an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention.

  This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject 101. As shown in FIG. 1, a static magnetic field generating magnet 102, a gradient magnetic field coil 103, a gradient magnetic field power supply 109, and an RF transmission coil 104, an RF transmitter 110, an RF receiver coil 105, a signal detector 106, a signal processor 107, a measurement controller 111, an overall controller 108, a display / operation unit 113, and a subject 101 are mounted. And a bed 112 for taking the top plate into and out of the static magnetic field generating magnet 102.

  The static magnetic field generating magnet 102 generates a uniform static magnetic field in the direction perpendicular to the body axis of the subject 101 in the vertical magnetic field method and in the body axis direction in the horizontal magnetic field method. A permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the.

  The gradient magnetic field coil 103 is a coil wound in the three-axis directions of X, Y, and Z that are the real space coordinate system (stationary coordinate system) of the MRI apparatus, and each gradient magnetic field coil is a gradient magnetic field that drives it. A current is supplied to the power source 109. Specifically, the gradient magnetic field power supply 109 of each gradient coil is driven according to a command from the measurement control unit 111 described later, and supplies a current to each gradient coil. Thereby, gradient magnetic fields Gx, Gy, and Gz are generated in the three-axis directions of X, Y, and Z.

  When imaging a two-dimensional slice plane, a slice gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, orthogonal to the slice plane and orthogonal to each other. Phase encoding gradient magnetic field pulse (Gp) and frequency encoding (leadout) gradient magnetic field pulse (Gf) are applied in the remaining two directions, and position information in each direction is encoded in the NMR signal (echo signal). .

  The RF transmission coil 104 is a coil that irradiates the subject 101 with an RF pulse, and is connected to the RF transmission unit 110 and supplied with a high-frequency pulse current. As a result, an NMR phenomenon is induced in the spins of atoms constituting the living tissue of the subject 101. Specifically, the RF transmission unit 110 is driven in accordance with a command from the measurement control unit 111 described later, and the RF transmission coil 104 is disposed in the vicinity of the subject 101 after the high frequency pulse is amplitude-modulated and amplified. , The subject 101 is irradiated with an RF pulse.

  The RF receiving coil 105 is a coil that receives an echo signal emitted by the NMR phenomenon of spin that constitutes the living tissue of the subject 101. The received echo signal is connected to the signal detecting unit 106 and is received by the signal detecting unit 106. Sent.

  The signal detection unit 106 performs processing for detecting an echo signal received by the RF receiving coil 105. Specifically, the echo signal of the response of the subject 101 induced by the RF pulse irradiated from the RF transmission coil 104 is received by the RF receiving coil 105 disposed in the vicinity of the subject 101, and measurement control described later is performed. In accordance with a command from the unit 111, the signal detection unit 106 amplifies the received echo signal, divides it into two orthogonal signals by quadrature detection, and samples each by a predetermined number (for example, 128, 256, 512, etc.) Each sampling signal is A / D converted into a digital quantity and sent to a signal processing unit 107 described later. Therefore, the echo signal is obtained as time-series digital data (hereinafter referred to as echo data) composed of a predetermined number of sampling data.

  The signal processing unit 107 performs various processes on the echo data, and sends the processed echo data to the measurement control unit 111.

  The measurement control unit 111 mainly transmits various commands for collecting echo data necessary for reconstruction of the tomographic image of the subject 101 to the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106. And a control unit for controlling them. Specifically, the measurement control unit 111 operates under the control of the overall control unit 108 described later, and controls the gradient magnetic field power source 109, the RF transmission unit 110, and the signal detection unit 106 based on a predetermined pulse sequence. The echo necessary for reconstructing the image of the imaging region of the subject 101 is repeatedly executed by applying an RF pulse and applying a gradient magnetic field pulse to the subject 101 and detecting an echo signal from the subject 101. Control data collection. In the repetition, the application amount of the phase encoding gradient magnetic field is changed in the case of two-dimensional imaging, and the application amount of the slice encoding gradient magnetic field is further changed in the case of three-dimensional imaging. Values such as 128, 256, and 512 are usually selected as the number of phase encodings, and values such as 16, 32, and 64 are normally selected as the number of slice encodings. With these controls, echo data from the signal processing unit 107 is output to the overall control unit 108.

  The overall control unit 108 controls the measurement control unit 111 and controls various data processing and processing result display and storage, and includes an arithmetic processing unit 114 having a CPU and a memory, an optical disc, And a storage unit 115 such as a magnetic disk. Specifically, the measurement control unit 111 is controlled to execute the collection of echo data, and when the echo data is input from the measurement control unit 111, the arithmetic processing unit 114 converts the encoded information applied to the echo data. Based on this, it is stored in an area corresponding to the K space in the memory. Hereinafter, the description that the echo data is arranged in the k space means that the echo data is stored in an area corresponding to the K space in the memory. A group of echo data stored in an area corresponding to the K space in the memory is also referred to as K space data. Then, the arithmetic processing unit 114 performs processing such as signal processing and image reconstruction by Fourier transform on the K space data, and displays the resulting image of the subject 101 on the display / operation unit 113 described later. And is recorded in the storage unit 115.

  The display / operation unit 113 includes a display unit for displaying the reconstructed image of the subject 101, a trackball or a mouse and a keyboard for inputting various control information of the MRI apparatus and control information for processing performed by the overall control unit 108. Etc., and an operation unit. The operation unit is disposed in the vicinity of the display unit, and an operator interactively controls various processes of the MRI apparatus through the operation unit while looking at the display unit.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging the information on the spatial distribution of proton density and the spatial distribution of the relaxation time of the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged in two or three dimensions.

  Next, a first embodiment of the MRI apparatus and the T2 map acquisition method of the present invention will be described. In the present embodiment, in the multi-SE sequence, the flip angle of the refocus RF pulse is set to less than 180 °, and the T2 value and the T2 map are calculated using the even-numbered echo signals. Hereinafter, this embodiment will be described with reference to FIG.

  FIG. 2 is a sequence chart showing a multi-SE sequence based on the FSE sequence based on the CPMG method of the present embodiment. RF, Gs, Gp, Gf, and Signal indicate the application timings of the RF pulse, slice selection gradient magnetic field, phase encoding gradient magnetic field, and frequency encode gradient magnetic field, respectively, and the measurement timing of the echo signal. The measurement control unit 111 controls echo signal measurement based on the sequence chart shown in FIG. Specifically, the measurement control unit 111 performs the following control. That is, the 90 ° excitation RF pulse 201 is applied simultaneously with the first slice selection gradient magnetic field 200 for selecting a predetermined cross section, and the nuclear spins in the cross section are excited. Further, a phase encoding gradient magnetic field 203 is applied. Thereafter, the slice selective gradient magnetic field 200 'and an α ° (for example, 140 °) refocus RF pulse 202 of less than 180 ° are sequentially applied at an application time interval τ to generate echo signals, respectively. Every time the echo signal is generated, the frequency encode gradient magnetic field 204 is applied to measure the echo signal. With respect to the gradient magnetic field in the frequency encoding direction, the phase gradient magnetic field 204 'is applied prior to the generation of the echo signal. 2 shows an example in which six echo signals are measured, the number of multi-SE sequences in this embodiment is not limited to six, and may be five or less or seven or more.

  In addition, since the multi-SE sequence of the present embodiment shown in the drawing is based on the FSE sequence based on the CPMG method, the application axis y of the α ° refocus RF pulse 202 is applied to the application axis x ′ of the 90 ° excitation RF pulse 201. 'Is orthogonal, and the application time interval (τ) of the α ° refocus RF pulse is set to be twice the time interval between the 90 ° excitation pulse and the first α ° inversion pulse. The measurement control unit 111 repeatedly executes a multi-SE sequence for measuring a plurality of spin echoes based on such a CPMG method for each slice within a repetition time TR, that is, executed by multi-slice imaging, The measurement of the number of echo signals necessary for image reconstruction is controlled for each slice.

  As described above, the multi-SE sequence of the present embodiment uses an α ° refocusing RF pulse having a flip angle of less than 180 ° instead of a 180 ° inversion RF pulse that inverts the nuclear spin by 180 ° as the refocusing RF pulse. , SAR can be reduced. The degree of SAR reduction depends on the flip angle. The smaller the flip angle, the more the SAR can be reduced. However, the flip angle α ° may be determined according to the obtained signal intensity and the accuracy with which the T2 value is obtained.

  If the flip angle of the refocusing pulse 202 is reduced, the application time of the refocusing pulse 202 can be shortened, so that the application time of the slice selective gradient magnetic field 200 'can also be shortened. If the application time of the slice selective gradient magnetic field 200 'is shortened, the application time of the gradient magnetic field 204 in the frequency direction can be afforded and the reception bandwidth can be reduced, so that the S / N can be improved.

  As described above, in the CPMG method, odd-numbered echo signals (hereinafter referred to as odd echo signals) are affected by imperfections in the RF pulse, but even-numbered echo signals (hereinafter referred to as even echo signals) are Not affected by imperfections. Even if the flip angle of the refocus RF pulse is not 180 °, the phase refocuses even in the even echo signal. For this reason, it is possible to obtain the T2 value and T2 map of the imaging tissue using the even echo signals 205, 206, and 207, preferably using only these even echo signals, and based on the temporal change of these signal intensities. It becomes possible. That is, the T2 value and the T2 map can be obtained with high accuracy while reducing the SAR using the refocus RF pulse having a lip angle of less than 180 °.

  The outline of the process for obtaining the T2 value and the T2 map is as follows. That is, the data of each even echo signal is arranged in different k spaces. That is, the echo data of the same echo time (the time from the 90 ° excitation RF pulse 201 to the peak position of the echo signal) is arranged in the same k space to create k space data for each echo time. Then, k-space data for each echo time is Fourier-transformed to obtain an image for each echo time. Finally, image values at the same pixel position are acquired from the images for each echo time, and the T2 value of the tissue at the pixel position is obtained from the temporal change of these image values. This process is performed for all the pixels, and the T2 map is acquired by mapping the T2 value to the pixel position. The method for calculating the T2 value is described in Patent Document 2, for example, and since this calculation method can be used, detailed description thereof is omitted.

Next, the processing flow for creating the T2 map of the present embodiment will be described based on the flowchart shown in FIG.
In step 301, the operator sets imaging conditions (TR, TE, flip angle, FOV, slice thickness, number of slices, etc.) of the above-described multi-SE sequence via the display / operation unit 113, and starts imaging. The arithmetic processing unit 114 specifically determines the parameter values such as the amplitude and application timing of each pulse of the multi-SE sequence based on the set imaging conditions, and notifies the measurement control unit 111 of these parameter values. And instructing the start of imaging.

  In step 302, the measurement control unit 111 controls the imaging by executing the multi-SE sequence based on the parameter values of the multi-SE sequence notified from the arithmetic processing unit 114 in step 301, and image reconstruction of the imaging region The required number of echo signals is measured, and the echo data is notified to the arithmetic processing unit 114.

  In step 303, the arithmetic processing unit 114 selects even echo signal data (hereinafter referred to as even echo data) among the plurality of echo signals measured in step 302. Preferably, only even echo data is selected. As for the selection of even echo data, the arithmetic processing unit 114 may automatically select even echo data. Alternatively, the operator can specify the echo time (TE) range of the echo signal used for calculating the T2 value via the operation unit, and the arithmetic processing unit 114 selects even echo data within the specified TE range. May be.

In step 304, the arithmetic processing unit 114 calculates the T2 value of the imaging region using the even echo data selected in step 303, preferably using only the even echo data, and obtains a T2 map. Specifically, even echo data of the same echo time is collected to create k-space data for each echo time, each k-space data is Fourier transformed to create an image for each echo time, and image values at the same pixel position From this, the T2 value at the pixel position is calculated, this process is performed for all the pixels, and the T2 value is mapped to the pixel position to obtain the T2 map. Then, the acquired T2 map is displayed on the display unit.
The above is the description of the processing flow for creating the T2 map of the present embodiment.

  As described above, the MRI apparatus and the T2 map acquisition method of the present embodiment set the flip angle of the refocusing RF pulse to less than 180 ° in the multi-SE sequence and are not affected by imperfections of the RF pulse. The T2 value is calculated using the echo signal measured in step 1. As a result, it is possible to obtain the T2 value with high accuracy while reducing the SAR as compared with the case of using the 180 ° inverted RF pulse, and the T2 map can be a highly accurate image.

  Next, a second embodiment of the MRI apparatus and the T2 map acquisition method of the present invention will be described. In the present embodiment, the flip angle of the refocus RF pulse of the multi-SE sequence is set to less than 180 ° as in the first embodiment. Then, by using a plurality of multi-SE sequences with different echo times (TE) of the even echo signals, more even echo signals are measured compared to the first embodiment, and these more even echo signals are measured. The echo signal is used to obtain the T2 value and T2 map with higher accuracy. Hereinafter, the present embodiment will be described with reference to FIG.

  FIG. 4 shows a sequence chart of only the RF pulse (RF) and the echo signal (Signal) in the multi-SE sequence of the present embodiment. The other Gs, Gp, and Gf are omitted because their shapes are the same as those in FIG. 2 except that their application timings change corresponding to the application timing of each RF pulse.

  (a) part shows a first multi-SE sequence in which the application time interval of α ° (less than 180 °) refocus RF pulse is τ as in FIG. 2, and (b) part is α ° (less than 180 °) A second multi-SE sequence in which the application time interval of the refocus RF pulse is set to τ ′ (≠ τ) is shown. That is, in the multi-SE sequence in part (b), after the 90 ° excitation RF pulse 401, a plurality of α ° refocus RF pulses 402 are sequentially applied with a time interval τ ′ / 2 after the 90 ° excitation RF pulse 401. Then, even echo signals 405, 406, and 407 are generated. By changing the application time interval of the α ° refocus RF pulse, the echo time (that is, the measurement timing) of the even echo signal can be varied.

  That is, the first multi-SE sequence and the second multi-SE sequence have different time intervals from the 90 ° excitation RF pulse until each even echo signal is measured, that is, the echo time of each even echo signal. Therefore, the even echo signal of the first multi-SE sequence and the even echo signal of the second multi-SE sequence have different degrees of T2 attenuation, and the T2 value and T2 map are calculated using these even echo data. By calculating, it is possible to calculate the T2 value and the T2 map with higher accuracy than in the case of the first embodiment. In addition, it is possible to calculate the T2 value and T2 map with higher accuracy by using the even echo data measured using three or more multi-SE sequences with different application time intervals of α ° refocusing RF pulses. Is possible. The number of multi-SE sequences with different α ° refocusing RF pulse application time intervals and the α ° refocusing RF pulse application time interval for each multi-SE sequence may use preset values or operations. The person may set it.

  Next, a processing flow for creating a T2 map according to the present embodiment will be described. The processing flow of the present embodiment is basically the same as the flowchart of FIG. 3 showing the processing flow of the first embodiment described above, but the processing contents of steps 301 and 302 are different, so only the different processing steps will be described below. To do.

  In Step 301 of the present embodiment, in addition to the processing content of Step 301 of Embodiment 1, the number of multi-SE sequences having different application time intervals of α ° refocus RF pulses, and α ° reconvergence for each multi-SE sequence The RF pulse application time interval is set. The arithmetic processing unit 114 may set predetermined values, or the operator may set these values via the operation unit.

In step 302 of the present embodiment, in addition to the processing content of step 302 of the first embodiment, the measurement control unit 111 executes each multi-SE sequence in which the application time interval of the α ° refocus RF pulse is different, Controls echo signal measurement for each SE sequence.
Since the subsequent steps are the same as in the first embodiment, description thereof is omitted.
The above is the description of the processing flow for creating the T2 map of the present embodiment.

  As described above, the MRI apparatus and the T2 map acquisition method of the present embodiment are a plurality of multi-SEs in which the echo time (TE) of the even echo signal is made different by the refocus RF pulse having a flip angle of less than 180 °. The echo signal is measured using the sequence, and the T2 value and the T2 map are calculated using the even echo data. As a result, compared to the first embodiment, it is possible to calculate the T2 value and the T2 map with higher accuracy using more even echo data having different echo times.

  Next, a third embodiment of the MRI apparatus and the T2 map acquisition method of the present invention will be described. In this embodiment, the flip angles of all refocus RF pulses are not set to a constant α ° as in the first and second embodiments, but a pair of adjacent odd refocus RF pulses and even refocus RF pulses are paired. And the flip angle of at least one pair of refocus RF pulses is less than 180 °. The SAR may be further reduced by setting the flip angle of multiple pairs of refocusing RF pulses to less than 180 °. At that time, the flip angles of the two pairs of refocusing RF pulses may be different from each other. Thereby, SAR reduction can be flexibly handled according to the imaging purpose and imaging conditions. Hereinafter, the present embodiment will be described with reference to FIG.

  FIG. 5 shows a sequence chart of only the RF pulse (RF) and the echo signal (Signal) in the multi-SE sequence of this embodiment. Specifically, a plurality of pairs of refocus RF pulses (n = 1 to 3) 502 having a 90 ° excitation RF pulse 501 and a flip angle of αn ° are sequentially applied to generate even echo signals 505, 506, and 507. The other Gs, Gp, and Gf are omitted because their shapes are the same as those in FIG. 2 except that their application timings change corresponding to the application timing of each RF pulse.

  The multi-SE sequence in FIG. 5 generates an even echo signal 505 with an odd refocus RF pulse 502-1 and an even refocus RF pulse 502-2 as a pair and a flip angle of α1 °. A pair of converged RF pulse 502-3 and even refocus RF pulse 502-4 is used to generate an even echo signal 506 with a flip angle of α2 °, and the next odd refocus RF pulse 502-5 and even refocus RF pulse. An even echo signal 507 is generated with a pair of 502-6 and a flip angle of α3 °. For example, any one or more of α1 ° to α3 ° may be less than 180 °, and the other may be 180 °. Alternatively, any one of α1 ° to α3 ° may be the same less than 180 °, and the other may have different flip angles. Alternatively, all may have different flip angles of less than 180 °.

  Thus, the flip angles of the refocus RF pulses in the multi-SE sequence of the present embodiment are not all constant, and the adjacent odd refocus RF pulses and even refocus RF pulses are paired to have the same flip angle, and at least The flip angle of the pair of refocus RF pulses is less than 180 °. At that time, the flip angles of the two pairs of refocusing RF pulses may be different from each other. However, the application axis of each refocus RF pulse is determined based on the CPMG method, and can be the same as that of the first embodiment, for example. In this way, by flexibly controlling the flip angle of each refocus RF pulse, it is possible to flexibly cope with SAR reduction according to the imaging purpose and imaging conditions.

  However, if the flip angle of each refocus RF pulse is varied without being constant, the signal intensity of the echo signal varies depending on the variation, and simple T2 attenuation does not occur. Therefore, assuming that T2 = ∞ (infinite) in advance, the signal intensity of each echo signal measured when the flip angle of the refocus RF pulse is changed as in this embodiment is calculated by theoretical calculation or T2 Is obtained as a reference value for each echo time based on an echo signal measured using a very long member (for example, pure water). Then, the intensity of each echo signal obtained by measuring an actual subject using a multi-SE sequence in which the flip angle of each pair of refocusing RF pulses in this embodiment is not constant is set at the same echo time reference value. Division and normalization are performed, and a T2 value and a T2 map are calculated using each normalized echo signal. Accordingly, it is possible to cancel the fluctuation of the echo signal intensity based on the fact that the flip angle of each pair of refocusing RF pulses is not constant, and to calculate a correct T2 value and T2 map.

  Next, a processing flow for creating a T2 map according to the present embodiment will be described. The processing flow of the present embodiment is basically the same as the flowchart of FIG. 3 showing the processing flow of the first embodiment described above, but since the processing of step 304 is different, only different processing steps will be described below.

In step 304 of the present embodiment, first, as described above, the arithmetic processing unit 114 uses the reference value acquired in advance, and standardizes the reference value of the same echo time and each measured echo signal. Process. Then, the arithmetic processing unit 114 performs the processing content of step 304 of the first embodiment using each normalized echo signal.
The above is the description of the processing flow for creating the T2 map of the present embodiment.

  As described above, the MRI apparatus and the T2 map acquisition method of the present embodiment set the flip angle of at least one pair of refocusing RF pulses to less than 180 °. Alternatively, the flip angles of a plurality of pairs of refocusing RF pulses are made different from each other. As a result, it is possible to obtain the T2 value and the T2 map while flexibly dealing with the reduction of SAR according to the imaging purpose and imaging conditions.

  101 subject, 102 static magnetic field generating magnet, 103 gradient magnetic field coil, 104 transmission RF coil, 105 reception RF coil, 106 signal detection unit, 107 signal processing unit, 108 overall control unit, 109 gradient magnetic field power source, 110 RF transmission unit, 111 Measurement control unit, 112 beds, 113 Display / operation unit, 114 Arithmetic processing unit, 115 Storage unit

Claims (5)

A measurement controller that controls measurement of a plurality of echo signals from the subject using a multi-SE sequence that applies a plurality of refocus RF pulses to the subject based on the CPMG method;
An arithmetic processing unit for obtaining a T2 map of the subject using the plurality of echo signals;
A magnetic resonance imaging apparatus comprising:
The measurement control unit sets at least one flip angle of the plurality of refocusing RF pulses to less than 180 °,
The magnetic resonance imaging apparatus, wherein the arithmetic processing unit acquires the T2 map using only even-numbered echo signals measured from the plurality of echo signals. The magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the measurement control unit makes a flip angle of all the refocus RF pulses constant at less than 180 °. The magnetic resonance imaging apparatus according to claim 1,
The measurement control unit controls the measurement of echo signals using a plurality of multi-SE sequences having different time intervals between refocusing RF pulses,
The arithmetic processing unit acquires the T2 map using an even-numbered echo signal measured in each multi-SE sequence. The magnetic resonance imaging apparatus according to claim 1,
The measurement control unit sets a pair of adjacent odd refocus RF pulses and even refocus RF pulses as the same flip angle, sets the flip angle of at least one pair of refocus RF pulses to less than 180 °, and sets two pairs of refocus RF. A magnetic resonance imaging apparatus characterized in that pulse flip angles are different from each other. A T2 map acquisition method for acquiring a T2 map of a subject using a multi-SE sequence in which a magnetic resonance imaging apparatus operates and applies a plurality of refocusing RF pulses to the subject based on a CPMG method,
Measuring a plurality of echo signals from the subject using a multi-SE sequence in which at least one flip angle of the plurality of refocusing RF pulses is less than 180 °;
Obtaining the T2 map using only even-numbered echo signals measured from the plurality of echo signals;
The T2 map acquisition method characterized by having.

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