JP5300279B2 - Magnetic resonance imaging system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which can collect data by a high-speed spin echo method to generate a better quality of componential image data or componential suppression image data. <P>SOLUTION: The magnetic resonance imaging apparatus includes: a data collection means and an image data generation means, wherein the data collection means loads a high-frequency excitation pulse to excite a magnetization spin in a photographed site and then loads repeatedly a high-frequency inversion pulse to inverse the phase of the magnetization spin, thereby to generate a plurality of echoes and then to collect a first data and a second data in accordance with the first sequence and the second sequence which are different from each other in t2, i.e. a time interval between the loaded high-frequency excitation pulse and the earliest echo (Sb1); and the image data generation means based on the first data and the second data to generate the componential image data or the componential suppression image data. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention magnetically excites a subject's nuclear spin with a radio frequency (RF) signal of a Larmor frequency, and re-images the nuclear magnetic resonance (NMR) signal generated by this excitation. The present invention relates to a magnetic resonance imaging (MRI) apparatus, and more particularly to a magnetic resonance imaging apparatus that separates a water component and a fat component of a received signal or collects a fat suppression image.

  Magnetic resonance imaging is an imaging method in which a nuclear spin of a subject placed in a static magnetic field is magnetically excited with an RF signal having a Larmor frequency, and an image is reconstructed from an MR signal generated by the excitation.

  As a data collection method in this magnetic resonance imaging, a fast spin echo (FSE) method is known. In the FSE method, after applying a 90 ° RF excitation pulse, multiple echo signals are generated by repeatedly applying a 180 ° reversal (refocus) RF pulse to reverse the phase of the magnetized spin. This is a data collection method that performs independent phase encoding.

In this FSE method, a method for separating a water component and a fat component of an echo signal without being affected by magnetic field inhomogeneity and a fat suppression method for suppressing a fat signal have been proposed (for example, see Non-Patent Document 1). ). This fat component and water component separation method and fat suppression method are affected by J coupling, which is an interaction caused by electron cloud distortion in one molecule, and the shorter the ESP (echo space), the more the fat tissue. It uses the property that the lateral relaxation (T2) value increases . Specifically, this method separates and extracts the water component and the fat component by comparing the FSE images obtained by executing each of the sequence having a short echo interval and the sequence having a long echo interval, and taking a difference. (See Non-Patent Document 2, for example). Then, an image in which a fat component and a water component are separated or a fat suppression image is created as a difference image.

  FIG. 1 is a diagram for explaining a conventional FSE pulse sequence for separating a fat signal and a water signal and a method for separating a fat signal and a water signal.

  1 (a) and 1 (b), the horizontal axis represents time, RF represents an RF pulse, Gss represents a gradient magnetic field pulse for slice selection (SS), and Gpe represents a phase encode (PE). GRO indicates a gradient magnetic field pulse, Gro indicates a gradient magnetic field pulse for readout (RO), and ECHO indicates an echo signal.

  As shown in FIG. 1 (a), in the first FSE sequence, an RF signal is continuously applied at an echo interval ESPa by repeatedly applying an RF refocusing pulse following an RF excitation pulse at a constant echo interval ESPa. It is a sequence to collect. As a result of the execution of the first FSE sequence, first echo data Ea for one image data on the k space (Fourier space) is collected, and image reconstruction processing such as inverse Fourier transform of the first echo data Ea is performed. Thus, the first image data Ia is generated. On the other hand, the execution of the second FSE sequence as shown in FIG. 1B having an echo interval ESP b longer than the echo interval ESP a of the first FSE sequence shown in FIG. Second echo data Eb for one image data is collected. Then, second image data Ia is generated by image reconstruction processing of the second echo data Eb.

  In order to make the contrast factors other than the J coupling the same, it is desirable that the effective echo times (TEeff) of the first FSE sequence and the second FSE sequence be as identical as possible. Effective TE (echo time) is TE corresponding to an echo signal that contributes most to image contrast in the FSE method. In general, the image contrast is governed by low-frequency component echo data near the center of the k-space.

As described above, when the first image data Ia and the second image data Ib are generated, a formula (1) is obtained from the first image data Ia and the second image data Ib based on a predetermined threshold value Ithreshold. -1) and equation (1-2), fat image data If and water image data Iw can be calculated.
[Equation 1]
If = Ia IF Ia-Ib ≧ Ithreshold
= 0 IF Ia-Ib <Ithreshold (1-1)
Iw = sqrt (Ia ² + Ib ² ) IF Ia-Ib ≦ Ithreshold
= 0 IF Ia-Ib> Ithreshold (1-2)
That is, as shown in Expression (1-1) and Expression (1-2), the difference value between the first image data Ia and the second image data Ib is compared with the threshold value Ithreshold. When the difference value is equal to or larger than the threshold value Ithreshold, the first image data Ia is set as fat image data If. When the difference value is equal to or smaller than the threshold value Ithreshold, the first image data Ia and the second image data Ib are used. Is the water image data Iw.

  Here, the influence on the signal intensity by J coupling is about 10 ppm at most, and it hardly changes depending on the influence of the magnetic field inhomogeneity observed with a normal MRI apparatus. For this reason, each signal intensity of the water component data and the fat component data included in the first image data Ia and the second image data Ib is hardly affected by the magnetic field inhomogeneity. As a result, the obtained water image data Iw and fat image data If are hardly affected by the magnetic field inhomogeneity.

On the other hand, data collection is performed using an FSE sequence in which the interval from when an RF excitation pulse is applied until the first echo signal is obtained is set to be an odd multiple of 3 or more of the interval between two adjacent echoes. Thus, a technique for reducing an increase in signal intensity in adipose tissue due to J coupling has been disclosed (for example, see Patent Document 1).
JP-A-7-155309 R. Todd Constable et al., “Distinguishing Coupled versus Non-coupled Spins using Fast Spin Echo Imaging: A Fat-Water Separation Technique”, ISMRM No. 1196, 1993. Matt A. Bernstein et al., "Handbook of MRI pulse sequences", ISBN-0-12-092861-2, p795-797.

  However, the conventional method for obtaining water image data and fat image data by calculation processing from two images obtained by executing two types of FSE sequences with different echo intervals described above has the following problems.

  As a first problem, there is a problem that the imaging time becomes long. That is, in the conventional method, it is necessary to collect echo data with substantially the same effective TE by executing FSE sequences of two types of echo intervals. For this reason, the number of echo signals obtained within one repetition time (TR) with the FSE sequence with the longer echo interval is greater than the number of echo signals obtained within 1TR with the FSE sequence with the shorter echo interval. Less. For example, if the echo interval of the FSE sequence with the longer echo interval is three times the echo interval of the FSE sequence with the shorter echo interval, the echo signal obtained in 1TR by the FSE sequence with the longer echo interval The number is 1/3 of the number of echo signals obtained in 1TR by the FSE sequence having the shorter echo interval. As a result, there is a problem that imaging time becomes long and sufficient speed cannot be increased.

  The second problem is that the bandwidth of the collected echo signal is limited by the FSE sequence with a short echo interval. For example, in an FSE sequence having a longer echo interval, wasted time other than echo collection increases and data collection efficiency decreases. For this reason, the signal-to-noise ratio (SNR) of the FSE sequence with the longer echo interval is lower than the sequence optimized with the number of echoes. Also, if the bandwidth of the echo signal is set separately for each of two different echo interval FSE sequences, the amount of positional deviation due to chemical shift in the readout direction of the image data obtained by executing each FSE sequence will be different. . As a result, the position shift on the calculation image such as the water image and the fat image becomes remarkable, leading to deterioration of the image quality.

  As a third problem, when an image with a low S / N is generated or due to the influence of other disturbances, separation of the water image and the fat image and extraction of the fat suppression image are not performed well, and the failure occurs. There is a problem that there is a possibility.

  In addition, there is a problem that artifacts are likely to occur when creating differential image data because blur due to T2 relaxation differs between image data obtained by two FSE sequences having different echo intervals.

  The present invention has been made to cope with such a conventional situation, and by collecting data by the high-speed spin echo method, it is possible to generate component image data or component-suppressed image data with better image quality. An object is to provide a magnetic resonance imaging apparatus. Another object of the present invention is to provide a magnetic resonance imaging apparatus capable of collecting data necessary for generating component image data or component-suppressed image data in a shorter time by the high-speed spin echo method. It is to be.

In order to achieve the above-described object, the magnetic resonance imaging apparatus according to the present invention repeats a high-frequency inversion pulse for inverting the phase of the magnetization spin following the high-frequency excitation pulse for exciting the magnetization spin in the imaging region. Data collection for generating a plurality of echoes by applying and collecting first and second data according to first and second sequences, respectively, in which the time from the application of the high frequency excitation pulse to the first echo is different from each other Means,
Image data generating means for generating component image data or component-suppressed image data based on the first and second data , wherein the data collecting means is effective for each of the first and second sequences. The first and second data are collected with the same echo time as each other .

  In the magnetic resonance imaging apparatus according to the present invention, it is possible to generate component image data or component-suppressed image data with better image quality by collecting data by the high-speed spin echo method. In the magnetic resonance imaging apparatus according to the present invention, data necessary for generating component image data or component-suppressed image data can be collected in a shorter time by the high-speed spin echo method.

  Embodiments of a magnetic resonance imaging apparatus according to the present invention will be described with reference to the accompanying drawings.

  FIG. 2 is a block diagram showing an embodiment of the magnetic resonance imaging apparatus according to the present invention.

  The magnetic resonance imaging apparatus 20 includes a cylindrical static magnetic field magnet 21 that forms a static magnetic field and a shim coil 22, a gradient magnetic field coil 23, and an RF coil 24 that are provided inside the static magnetic field magnet 21. This is a built-in configuration.

  In addition, the magnetic resonance imaging apparatus 20 includes a control system 25. The control system 25 includes a static magnetic field power supply 26, a gradient magnetic field power supply 27, a shim coil power supply 28, a transmitter 29, a receiver 30, a sequence controller 31, and a computer 32. The gradient magnetic field power source 27 of the control system 25 includes an X-axis gradient magnetic field power source 27x, a Y-axis gradient magnetic field power source 27y, and a Z-axis gradient magnetic field power source 27z. In addition, the computer 32 includes an input device 33, a display device 34, an arithmetic device 35, and a storage device 36.

  The static magnetic field magnet 21 is connected to a static magnetic field power supply 26 and has a function of forming a static magnetic field in the imaging region by a current supplied from the static magnetic field power supply 26. In many cases, the static magnetic field magnet 21 is composed of a superconducting coil, and is connected to the static magnetic field power supply 26 at the time of excitation and supplied with current. It is common. In some cases, the static magnetic field magnet 21 is composed of a permanent magnet and the static magnetic field power supply 26 is not provided.

  A cylindrical shim coil 22 is coaxially provided inside the static magnetic field magnet 21. The shim coil 22 is connected to the shim coil power supply 28, and is configured such that a current is supplied from the shim coil power supply 28 to the shim coil 22 to make the static magnetic field uniform.

  The gradient magnetic field coil 23 includes an X-axis gradient magnetic field coil 23 x, a Y-axis gradient magnetic field coil 23 y, and a Z-axis gradient magnetic field coil 23 z, and is formed in a cylindrical shape inside the static magnetic field magnet 21. A bed 37 is provided inside the gradient magnetic field coil 23 as an imaging region, and the subject P is set on the bed 37. The RF coil 24 may not be built in the gantry but may be provided near the bed 37 or the subject P.

  The gradient magnetic field coil 23 is connected to a gradient magnetic field power supply 27. The X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z of the gradient magnetic field coil 23 are respectively an X-axis gradient magnetic field power source 27x, a Y-axis gradient magnetic field power source 27y, and a Z-axis gradient magnetic field coil 27z. It is connected to the magnetic field power supply 27z.

  The X-axis gradient magnetic field power source 27x, the Y-axis gradient magnetic field power source 27y, and the Z-axis gradient magnetic field power source 27z are supplied with currents supplied to the X-axis gradient magnetic field coil 23x, the Y-axis gradient magnetic field coil 23y, and the Z-axis gradient magnetic field coil 23z, respectively. In the imaging region, a gradient magnetic field Gx in the X-axis direction, a gradient magnetic field Gy in the Y-axis direction, and a gradient magnetic field Gz in the Z-axis direction can be formed, respectively.

  The RF coil 24 is connected to the transmitter 29 and the receiver 30. The RF coil 24 receives the RF signal from the transmitter 29 and transmits it to the subject P, and receives the NMR signal generated along with the excitation of the nuclear spin inside the subject P by the RF signal to the receiver 30. Has the function to give.

  On the other hand, the sequence controller 31 of the control system 25 is connected to the gradient magnetic field power source 27, the transmitter 29, and the receiver 30. The sequence controller 31 has control information necessary for driving the gradient magnetic field power supply 27, the transmitter 29, and the receiver 30, for example, operation control information such as the intensity, application time, and application timing of the pulse current to be applied to the gradient magnetic field power supply 27. And the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 are driven according to the stored predetermined sequence to drive the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy, and the Z-axis gradient magnetic field. It has the function of generating Gz and RF signals.

  In addition, the sequence controller 31 is configured to receive raw data, which is complex data obtained by detection of an NMR signal and A / D (analog to digital) conversion in the receiver 30, and supply the raw data to the computer 32. The

  For this reason, the transmitter 29 is provided with a function of applying an RF signal to the RF coil 24 based on the control information received from the sequence controller 31, while the receiver 30 detects the NMR signal received from the RF coil 24. Then, by executing required signal processing and A / D conversion, a function of generating raw data that is digitized complex data and a function of supplying the generated raw data to the sequence controller 31 are provided.

  Further, the computer 32 is provided with various functions by executing the program stored in the storage device 36 of the computer 32 by the arithmetic unit 35. However, a specific circuit having various functions may be provided in the magnetic resonance imaging apparatus 20 regardless of the program.

  FIG. 3 is a functional block diagram of the computer 32 shown in FIG.

  The computer 32 functions as an imaging condition setting unit 40, a sequence controller control unit 41, a k-space database 42, an image reconstruction unit 43, an image database 44, and an image processing unit 45 by a program.

  The imaging condition setting unit 40 has a function of setting imaging conditions including a pulse sequence based on instruction information from the input device 33 and providing the set imaging conditions to the sequence controller control unit 41. In particular, the imaging condition setting unit 40 has a function of setting two types of FSE sequences for generating image data obtained by separating a water component and a fat component or fat suppression image data.

  For this purpose, the imaging condition setting unit 40 has a function of causing the display device 34 to display imaging condition setting screen information. Then, the user refers to the imaging condition setting screen displayed on the display device 34 and operates the input device 33 to perform imaging from a plurality of imaging protocols for each imaging region and imaging conditions prepared in advance. An imaging protocol to be used can be selected, and imaging conditions such as necessary parameter values can be set. GUI (Graphical User Interface) technology can be used as an interface for setting such shooting conditions.

  The two types of FSE sequences set in the imaging condition setting unit 40 are equal to each other, but the time intervals from the application time of the RF excitation pulse to the collection time of the first echo signal are different from each other. Is done. That is, the imaging condition setting unit 40 sets the first FSE sequence on the short side and the second FSE sequence on the long side from the application time of the RF excitation pulse to the first echo signal acquisition time as the imaging conditions. Configured to be.

  FIG. 4 is a pulse sequence chart showing an example of the first FSE sequence set in the imaging condition setting unit 40 shown in FIG. 2, and FIG. 5 shows the first set in the imaging condition setting unit 40 shown in FIG. 2 is a pulse sequence chart showing an example of an FSE sequence of FIG.

  4 and 5, the horizontal axis represents time, RF represents an RF pulse, Gss represents a gradient magnetic field pulse for slice selection, Gpe represents a gradient magnetic field pulse for phase encoding, and Gro represents a gradient magnetic field pulse for readout. ECHO represents an echo signal.

  As shown in FIG. 4, in the first FSE sequence A, an RF refocusing pulse is repeatedly applied following an RF excitation pulse at a constant echo interval ESP, so that echo signals Sa1, Sa2, continuously at an echo interval ESP. This is a sequence to collect Sa3,…. In the first FSE sequence A, the first echo signal Sa1 is collected after a time t1 arbitrarily determined in advance after applying the RF excitation pulse. This time t1 is customarily set to be the same as the echo interval ESP, but can be changed.

  By executing the first FSE sequence A, the echo signals Sa1, Sa2, Sa3,... Are collected in a state of being phase-encoded with different phase-encoding amounts by application of phase-encoding gradient magnetic field pulses. As a result, all echo signals Sa1, Sa2, Sa3,... Necessary for generating one piece of image data are collected. In general, multi-shot imaging is often performed in which an RF excitation pulse is repeatedly applied a plurality of times while changing the phase encoding amount. FIG. 4 shows an example of performing multi-shot imaging. However, it is also possible to perform single shot imaging that collects all echo signals necessary for generating image data for one sheet by one high frequency excitation.

  The first echo data Ea for one image data collected as a result of the execution of the first FSE sequence A is arranged in the k space as will be described later, and the inverse Fourier transform is performed on the first echo data Ea. The first image data Ia can be generated by the image reconstruction process.

  On the other hand, the second FSE sequence B shown in FIG. 5 is similar to the first FSE sequence A shown in FIG. 4 by repeatedly applying an RF refocusing pulse at a constant echo interval ESP following the RF excitation pulse. In this sequence, echo signals Sb1, Sb2, Sb3,... Are continuously collected at an echo interval ESP. However, in the second FSE sequence B, the time t2 from when the RF excitation pulse is applied until the first echo signal Sb1 is obtained is the first echo signal after the RF excitation pulse is applied in the first FSE sequence A. It is set longer than the time t1 until Sa1 is obtained. FIG. 5 shows an example in which time t2 until the first echo signal Sb1 is obtained by the first several echoes is increased in CPMG (Carr-Purcell Meiboom-Gill sequence).

  By executing the second FSE sequence B, the echo signals Sb1, Sb2, Sb3,... Are collected in a state of being phase-encoded with a different phase encoding amount by applying a phase encoding gradient magnetic field pulse. As a result, all echo signals Sb1, Sb2, Sb3,... Necessary for generating one piece of image data are collected. In general, multi-shot imaging is often performed, but single-shot imaging can also be performed.

  Then, the second echo data Eb for one image data collected as a result of the execution of the second FSE sequence B is arranged in the k space as will be described later, and the inverse Fourier transform is performed on the second echo data Eb. The second image data Ib can be generated by the image reconstruction process.

  When the first echo data Ea and the second echo data Eb are collected by executing the first FSE sequence A and the second FSE sequence B as shown in FIGS. 4 and 5, as shown in FIG. As compared with the case of collecting echo data using two types of FSE sequences set so that one of the echo intervals is longer, the time efficiency of collecting the echo data can be improved. That is, as shown in FIGS. 4 and 5, the first echo is executed by executing the first FSE sequence A and the second FSE sequence B with different times t1 and t2 until the first echo signals Sa1 and Sb1 are obtained. By collecting the data Ea and the second echo data Eb, it is possible to collect two sets of echo data with different characteristics related to J coupling more efficiently than before without extending the echo interval. is there.

Here, as a preferred example, in the second FSE sequence B, the time t2 from when the RF excitation pulse is applied until the first echo signal Sb1 is obtained is expressed by two adjacent two as shown in the equation (2). It is set to be an odd multiple of 3 or more of the echo interval ESP between the echo signals.
[Equation 2]
t2 = (2m + 1) ・ ESP (2)
However, in Formula (2), m is an integer (natural number) of 1 or more. FIG. 5 shows an example in which m = 1, that is, the time t2 from when the RF excitation pulse is applied until the first echo signal Sb1 is obtained is set to three times the echo interval ESP.

  For example, the gradient magnetic field pulse Gss for slice selection and the gradient magnetic field pulse Gro for readout can be determined as follows so that the expression (2) is satisfied. However, each gradient magnetic field pulse may be adjusted by other known gradient magnetic field determination methods.

  The area of the gradient magnetic field pulse Gss for slice selection applied with the first 180 ° RF refocus pulse, that is, the time integral value obtained by the gradient magnetic field pulse intensity x application time, is the second and subsequent 180 ° RF refocus pulse. And an odd multiple of 3 or more of the time integral value of the gradient magnetic field pulse Gss for slice selection applied together. In other words, the time integral value of the gradient magnetic field pulse Gss for slice selection applied between the first 180 ° RF refocusing pulse and the second 180 ° RF refocusing pulse is set to the adjacent 180 ° for the second and subsequent times. It is set to an integer multiple of 2 or more of the time integration value of the gradient magnetic field pulse Gss for slice selection applied between the RF refocus pulses.

  The echo signal is read out by applying a gradient magnetic field pulse Gro for readout. The area of the readout gradient magnetic field pulse Gro to be applied when the first echo signal Sb1 is read, that is, the time integral value obtained by the intensity of the gradient magnetic field pulse × the application time is the second and subsequent echo signals Sb2, Sb3, Sb4, It is an integer multiple of 2 or more of the time integral value of the readout gradient magnetic field pulse Gro applied at the time of reading.

  In addition, in order to control the time integral value of the gradient magnetic field, it is possible to adjust the application time under a constant magnetic field strength, and it is also possible to adjust the magnetic field strength under a constant application time. . It is also possible to adjust both the magnetic field strength and the application time.

  Next, the effect obtained by setting the time t2 from when the RF excitation pulse is applied until the first echo signal Sb1 is obtained in the second FSE sequence B as shown in Expression (2) will be described.

  As in the second FSE sequence B, the time t2 is extended so that the time t2 between the application time of the RF excitation pulse and the acquisition time of the first echo signal Sb1 is longer than the time t1 in the first FSE sequence A. Then, the reduction degree of the fat signal when the second FSE sequence B is executed is larger than the reduction degree of the fat signal when the first FSE sequence A is executed. Therefore, the contrast of the image data obtained by executing the second FSE sequence B is an image obtained by a normal spin echo (SE) method than the contrast of the image data obtained by executing the first FSE sequence A. Close to data contrast.

  On the other hand, from the viewpoint of shortening the imaging time and obtaining a large number of echo signals, it is preferable that the time t2 until the first echo signal Sb1 is obtained in the second FSE sequence B is shorter. Therefore, m = 1 in the equation (1), that is, the time t2 until the first echo signal Sb1 is obtained in the second FSE sequence B is ideally three times the echo interval ESP. In the experiment, even when the time t2 until the first echo signal Sb1 is obtained in the second FSE sequence B is set to be three times the echo interval ESP, the image data has the same level of contrast as when the image data is collected by the normal SE method. It has been confirmed that can be obtained.

  In this way, if the time t2 from when the RF excitation pulse is applied until the first echo signal Sb1 is obtained is set to an odd multiple of 3 or more of the echo interval ESP, an excitation echo (STE: stimulated echo is described later). ) And indirect echoes can be generated at the same timing as the main echoes and used for image construction.

  By the way, it is practically impossible for the 180 ° RF refocusing pulse to have only a 180 ° flip angle component. For this reason, every time a 180 ° RF refocus pulse is applied, the magnetization spin is not affected by the first component whose phase is reversed as scheduled, the second component that enters longitudinal magnetization, and the 180 ° RF refocus pulse. Thus, it is differentiated into a third component in which the phase is steadily dispersed as it is. The first component is generated as a main echo, the second component is generated as an STE, and the third component is generated as an indirect echo. That is, STE is an echo generated via longitudinal magnetization after application of a 90 ° RF excitation pulse. The indirect echo is an echo generated by accidentally (unsteady) receiving phase inversion by a 180 ° RF refocus pulse.

  Here, by setting the time t2 from when the 90 ° RF excitation pulse is applied until the first echo signal Sb1 is obtained to an odd multiple of 3 or more of the echo interval ESP, the STE or indirect echo is used as the main echo. The principle that can be generated at the same timing will be described. For convenience of explanation, it is assumed that m = 1 in equation (2). The application timing of the 180 ° RF refocus pulse in the second FSE sequence B is 180 ° RF refocus when the time t1 until the first echo signal Sa1 is obtained in the first FSE sequence A is set to the echo interval ESP. This corresponds to the pulse application timing obtained by removing the first 180 ° RF refocus pulse application timing and the third 180 ° RF refocus pulse application timing. The interval of 180 ° RF refocusing pulse in the first FSE sequence A in which the time t1 until the first echo signal Sa1 is obtained is set to the echo interval ESP is such that STE and indirect echo are generated at the same timing as the main echo. Has been adjusted. Here, the phase dispersion of the STE, indirect echo, and main echo is constant regardless of the interval of the 180 ° RF refocusing pulse. Therefore, if the time t2 from when the 90 ° RF excitation pulse is applied until the first echo signal Sb1 is obtained is set to an odd multiple of 3 or more of the echo interval ESP, the STE and indirect echoes have the same timing as the main echo. Can be generated.

  As described above, in the second FSE sequence B, when the time t2 from when the 90 ° RF excitation pulse is applied until the first echo signal Sb1 is obtained so that the expression (2) is satisfied, the normal SE method is used. An image having a contrast equivalent to that of the image can be obtained.

  Note that the image quality difference due to factors other than J coupling between the first image data Ia collected by the execution of the first FSE sequence A and the second image data Ib collected by the execution of the second FSE sequence B. It is important to reduce as much as possible. For this reason, the first several echo signals collected by the execution of the first FSE sequence A are not collected at the timing when the echo signals are not collected by the execution of the second FSE sequence B, or the first image data Ia is generated. It is desirable not to use it. FIG. 4 shows an example in which the first two echo signals collected by executing the first FSE sequence A are not used for generating the first image data Ia.

  In addition, when performing multi-shot imaging, it is desirable to reduce the amount of positional deviation caused by the collection time difference between the first image data Ia and the second image data Ib as much as possible. For this reason, it is desirable to alternately execute the first FSE sequence A and the second FSE sequence B for each TR in terms of time.

  Such setting of the first FSE sequence A and the second FSE sequence B can be performed by operating the input device 33 through the imaging condition setting screen displayed on the display device 34 as described above. In particular, in the second FSE sequence B, the time t2 until the first echo signal Sb1 is obtained can be arbitrarily adjusted by operating the input device 33. Further, the value of m and time t2 in the expression (2) may be directly input by operating the input device 33, or may be selected from a plurality of values prepared in advance. The time t1 until the first echo signal Sa1 is obtained in the first FSE sequence A can be arbitrarily adjusted similarly to the time t2 until the first echo signal Sb1 is obtained in the second FSE sequence B. The interval may be a default value or a fixed value.

  Next, other functions of the computer 32 will be described.

  When the sequence controller control unit 41 receives the scan start instruction information from the input device 33, the sequence controller control unit 41 gives the sequence controller 31 with imaging conditions including the first FSE sequence and the second FSE sequence from the imaging condition setting unit 40. Has a function of controlling the drive. The sequence controller control unit 41 receives the first and second echo data collected from the sequence controller 31 by executing the first FSE sequence and the second FSE sequence, respectively, and is formed in the k-space database 42. It has the function to arrange in k space. Therefore, each echo data is stored as k-space data in the k-space database 42, and the first and second echo data are arranged in the k-space formed in the k-space database 42 as described above.

  The image reconstruction unit 43 takes in the first and second echo data from the k-space database 42 and performs image reconstruction processing including inverse Fourier transform, respectively, thereby performing first and second object P as real space data. A function of reconstructing the second image data, and a function of writing the first and second image data obtained by the reconstruction into the image database 44. Therefore, the image database 45 stores the first and second image data reconstructed by the image reconstruction unit 44.

  The image processing unit 45 reads the first and second image data from the image database 44 and performs image processing to thereby separate image data and fat from water components and fat components such as water image data and fat image data. It has a function of generating suppressed image data and a function of giving the generated image data to the display device 34 for display.

  FIG. 6 is a schematic diagram for explaining an example of a method for generating water image data and fat image data from the first image data and the second image data in the image processing unit 45 shown in FIG.

  As shown in FIG. 6, the image processing unit 45 reads the first image data Ia and the second image data Ib from the image database 44, and both the first image data Ia and the second image data Ib are read. The water image data Iw and the fat image data If are generated.

In general, the image data Ise obtained by the SE method is basically composed of water component image data Iw and fat component image data If, as shown in Expression (3).
[Equation 3]
Ise = Iw + If (3)
Accordingly, the first image data Ia and the second image data Ib obtained by executing the first FSE sequence A and the second FSE sequence B, which have the same effective TE, are converted into the fat component image data If. Considering the influence of J coupling, it can be expressed as equations (4-1) and (4-2), respectively.
[Equation 4]
Ia = Iw + ka ・ If (4-1)
Ib = Iw + kb ・ If (4-2)

However, in the equations (4-1) and (4-2), ka and kb are signals of the fat component image data If in the first image data Ia and the second image data Ib by J coupling. This is the rate of change in the intensity of the signal component (signal intensity ratio) when it rises under the influence. The change rate ka, kb of the fat component signal is a known parameter that can be measured in advance by imaging using a phantom. Since the time t1 until the first echo signal Sa1 is obtained in the first FSE sequence A is shorter than the time t2 until the first echo signal Sb1 is obtained in the second FSE sequence B, the change in the fat component signal The relationship shown in Equation (5) is established for the rates ka and kb.
[Equation 5]
ka>kb> 1 (5)

From the equation (4-1), the equation (4-2), and the equation (5), the water image data Iw and the fat image data If are obtained as, for example, the equation (6-1) and the equation (6-2), respectively. be able to.
[Equation 6]
Iw = (Ia + Ib) / 2-1 / 2 × (ka + kb) / (ka-kb) × (Ia-Ib) (6-1)
If = (Ia-Ib) / (ka-kb) (6-2)

Alternatively, the water image data Iw can be obtained, for example, as in Expression (7-1) or Expression (7-2). [Equation 7]
Iw = Ia-ka × (Ia-Ib) / (ka-kb) (7-1)
Iw = Ib-kb × (Ia-Ib) / (ka-kb) (7-2)

  In this way, the degree of signal rise due to the effect of J coupling of the fat component signal is obtained in advance, and it is considered that the water component signal and the fat component signal are mixed in the same pixel of the image data I. By the calculation process, the water image data Iw and the fat image data If can be separated and extracted from the first image data Ia and the second image data Ib. In other words, the water image data Iw and the fat image data If can be obtained by calculation based on the difference in J coupling characteristics. As a result, as shown in the equations (1-1) and (1-2), the first pixel can be obtained without considering that the water component signal and the fat component signal are mixed in the same pixel of the image data. Compared with the case where the difference value between the image data Ia and the second image data Ib is simply compared with the threshold value Ithreshold to separate and extract the fat image data If and the water image data Iw, the influence of noise on the image quality is reduced. It is possible to improve the unnaturalness of the fat image data If and the water image data Iw due to a low SNR (signal to noise ratio).

  However, without obtaining the water image data Iw and the fat image data If by the calculation processing from the first image data Ia and the second image data Ib separately reconstructed, the first FSE sequence A and the second image data Ib are obtained. By controlling the arrangement method of the first echo data Ea and the second echo data Eb obtained by the execution of the FSE sequence B in the k space, the fat image data If and the water image data Iw can be obtained by simpler processing. It can also be generated.

  FIG. 7 shows fat image data If and by controlling the arrangement method of the echo data Eab obtained by executing the first FSE sequence A and the second FSE sequence B shown in FIGS. It is a figure explaining the method to produce | generate water image data Iw.

  FIG. 7 (a) shows the echo data Eab composed of the first echo data Ea obtained by the first FSE sequence A and the second echo data Eb obtained by the second FSE sequence B in the k space. The arrangement method is shown. As shown in FIG. 7A, echo signals Sa1, Sa2, Sa3,... Collected by the execution of the first FSE sequence A and Sb1, Sb2, Sb3 collected by the execution of the second FSE sequence B The echo signals Sa1, Sa2, Sa3,..., Sb1, Sb2, Sb3,... Are arranged by the sequence controller control unit 41 so that they are alternately arranged in the phase encoding (kpe) direction on the common k space. The In other words, in the imaging condition setting unit 40, echo signals Sa1, Sa2, Sa3,... Collected by the execution of the first FSE sequence A and Sb1, Sb2, collected by the execution of the second FSE sequence B. .., Sb1 by the first FSE sequence A and the second FSE sequence B so that Sb3,... Are alternately arranged in the phase encoding (kpe) direction on the common k space. , Sb2, Sb3,... Are determined.

  7B shows an image reconstruction unit for echo data Eab composed of echo signals Sa1, Sa2, Sa3,..., Sb1, Sb2, Sb3,. 43 is image data Iab obtained by performing image reconstruction processing including inverse Fourier transform at 43. When the image data Iab is reconstructed from the echo data Eab shown in FIG. 7A, the water image data and the fat image data are separated in the phase encoding (PE) direction in the real space as shown in FIG. 7B. Appearing image data Iab is generated.

  This is because the signal intensity of the fat signal component changes for each echo data line due to the effect of J coupling, and amplitude modulation corresponding to a period of two data points occurs in the k space. Yes. That is, the image data Iab is reconstructed so that the fat signal component that is strongly influenced by J coupling is shifted in the PE direction by half of the field of view (FOV). Therefore, if the image processing unit 45 is configured to divide the image data Iab in half by a line perpendicular to the PE direction, the water image data Iw and the fat image If can be obtained. FIG. 7 (c) shows water image data Iw and fat image If obtained by dividing the image data Iab shown in FIG. 7 (b).

  Thus, the echo signals Sa1, Sa2, Sa3,... Collected by the execution of the first FSE sequence A and the Sb1, Sb2, Sb3,. The echo data Eab is generated by alternately arranging them in the phase encode (kpe) direction in space, and the water image data Iw and the fat image If are obtained by separating the image data Iab obtained by image reconstruction processing of the echo data Eab. According to the second image generation method to be generated, the SNR for the water image data Iw corresponds to the echo data collection time. For this reason, from the first image data Ia and the second image data Ib generated individually, the equation (6-1), the equation (6-2), the equation (7-1), or the equation (7-2) is used. It is considered that the SNR is improved as compared with the first image generation method that generates the water image data Iw and the fat image If. As a result, better image quality can be expected.

  However, the echo signals Sa1, Sa2, Sa3,... Collected by the first FSE sequence A and Sb1, collected by the second FSE sequence B, due to the influence of the movement of the subject P during the collection of echo data. Compared to the second image generation method in which Sb2, Sb3,... Are alternately arranged on a common k space to generate the water image data Iw and the fat image If, the first image data Ia generated individually The first image data Iw and the fat image data If are generated from the second image data Ib by the equation (6-1), the equation (6-2), the equation (7-1), or the equation (7-2). The image generation method may reduce the appearance of artifacts.

  Accordingly, depending on the imaging situation such as the amount of movement of the subject P, image processing is performed to determine whether water image data, fat image data, or fat suppression image data is generated by the first image generation method or the second image generation method. It can be determined as a condition. This image processing condition can be set through the above-described shooting condition setting screen. For example, one of the first image generation method and the second image generation method can be selected.

  The first image generation method and the second image generation method use water image data, fat image data, or fat suppression images from echo data collected by two FSE sequences with different echo intervals as shown in FIG. It can also be used when generating data.

  Next, the operation and action of the magnetic resonance imaging apparatus 20 will be described.

  FIG. 8 is a flowchart showing a flow when a water image and a fat image of the subject P are picked up by the FSE method in the magnetic resonance imaging apparatus 20 shown in FIG. 2, and the reference numerals with numerals in FIG. Each step is shown.

  First, in step S1, first and second SSFPs having different times t1, t2 until the first echo signals Sa1, Sb1 as shown in FIG. 4 and FIG. A sequence is set as a shooting condition. Also, a method for generating water image data and fat image data is selected. Here, a case where the first image generation method described above is selected will be described.

  Next, in step S2, scanning is executed according to the set imaging conditions, and echo data is collected.

  For this purpose, the subject P is set on the bed 37 in advance, and a static magnetic field is formed in the imaging region of the static magnetic field magnet 21 (superconducting magnet) excited by the static magnetic field power supply 26. Further, a current is supplied from the shim coil power supply 28 to the shim coil 22, and the static magnetic field formed in the imaging region is made uniform.

  Then, when an instruction to collect a water image and a fat image at the imaging region of the subject P is given from the input device 33 to the sequence controller control unit 41, the sequence controller control unit 41 receives the first and second from the imaging condition setting unit 40. The SSFP sequence is acquired and given to the sequence controller 31. The sequence controller 31 tilts the imaging region where the subject P is set by driving the gradient magnetic field power source 27, the transmitter 29, and the receiver 30 in accordance with the first and second SSFP sequences received from the sequence controller control unit 41. A magnetic field is formed and an RF signal is generated from the RF coil 24.

  Therefore, an NMR signal generated by nuclear magnetic resonance inside the subject P is received by the RF coil 24 and given to the receiver 30. The receiver 30 receives the NMR signal from the RF coil 24, performs necessary signal processing, and then performs A / D conversion to generate raw data that is an NMR signal of digital data. The receiver 30 gives the generated raw data to the sequence controller 31. The sequence controller 31 provides the raw data to the sequence controller control unit 41, and the sequence controller control unit 41 arranges the raw data as k-space data in the k-space formed in the k-space database 42.

  As a result, the k-space database 42 stores the first echo data Ea collected by the first SSFP sequence A and the second echo data Eb collected by the second SSFP sequence B.

  Next, in step S3, the image reconstruction unit 43 takes the first and second echo data Ea, Eb from the k-space database 42 and performs image reconstruction processing, thereby performing the first and second image data Ia, Reconfigure Ib. The first and second image data Ia and Ib are written and stored in the image database 45.

  Next, in step S4, the image processing unit 45 reads the first and second image data Ia and Ib from the image database 44, and the rate of change in the intensity of the fat component signal due to the influence of the previously acquired J coupling. Based on ka and kb, the water image data Iw and the fat image data If are generated by Expression (6-1), Expression (6-2), Expression (7-1), or Expression (7-2). The generated water image data Iw and fat image data If are given to the display device 34, and a water image and a fat image are displayed on the display device 34. The displayed image is used for diagnosis.

  In other words, the magnetic resonance imaging apparatus 20 as described above has two sets of different characteristics relating to J coupling by executing two FSE sequences in which the echo interval is constant but the time until the first echo signal is collected is changed. Echo data is collected, and water image data, fat image data, or fat suppression image data is generated from the collected echo data and displayed.

  In addition, the magnetic resonance imaging apparatus 20 considers that a water component signal and a fat component signal are mixed in the same pixel of image data by using the rate of change in signal increase due to the effect of J coupling of the fat component signal. Water image data, fat image data, or fat suppression image data is generated by the above calculation processing. Alternatively, the magnetic resonance imaging apparatus 20 separates image data reconstructed by alternately arranging data lines of echo signals obtained by executing the above two FSE sequences in the k space, Fat image data or fat suppression image data is generated.

  For this reason, according to the magnetic resonance imaging apparatus 20, a water image, a fat image, or a fat with a better image quality can be obtained by adopting an image processing method for obtaining constant image time or obtaining higher quality image data. A suppressed image can be generated. That is, it is possible to obtain a water image, a fat image, or a fat-suppressed image in which the water component and the fat component are well separated and the influence of movement is small.

  Furthermore, according to the magnetic resonance imaging apparatus 20, since it is not necessary to unnecessarily extend the echo interval of the FSE sequence, a stable water image and fat image that are not affected by the magnetic field inhomogeneity in a shorter time than before. Or a fat suppression image can be obtained. In particular, as the bore for setting the subject P is increased in recent years, there is an increasing number of cases where the subject P needs to be imaged even in an imaging region where the magnetic field is not uniform. For this reason, an imaging technique of a water image, a fat image, or a fat suppression image that is not affected by the magnetic field non-uniformity is important.

  In the above-described embodiment, the separation of the image data of the water component and the image data of the fat component has been described. However, the component image is obtained by separating or extracting image data from substance components other than water and fat by the same method. It is also possible to generate data and component-suppressed image data. That is, it is possible to separate or extract image data of various components having different influences due to J coupling and form an image.

The figure explaining the separation method of the FSE pulse sequence for performing the separation of the conventional fat signal and a water signal, and a fat signal and a water signal. 1 is a configuration diagram showing an embodiment of a magnetic resonance imaging apparatus according to the present invention. FIG. 3 is a functional block diagram of the computer shown in FIG. 2. 3 is a pulse sequence chart showing an example of a first FSE sequence set in the imaging condition setting unit shown in FIG. The pulse sequence chart which shows an example of the 2nd FSE sequence set in the imaging condition setting part shown in FIG. The schematic diagram explaining an example of the method of producing | generating water image data and fat image data from 1st image data and 2nd image data in the image process part shown in FIG. A method of generating fat image data and water image data by controlling the arrangement method of echo data obtained by executing the first FSE sequence and the second FSE sequence shown in FIGS. 4 and 5 on the k-space, respectively. Illustration to explain. The flowchart which shows the flow at the time of imaging the water image and fat image of the subject P by FSE method in the magnetic resonance imaging apparatus shown in FIG.

Explanation of symbols

20 Magnetic Resonance Imaging Device 21 Magnet for Static Magnetic Field 22 Shim Coil 23 Gradient Magnetic Field Coil 24 RF Coil 25 Control System 26 Static Magnetic Field Power Supply 27 Gradient Magnetic Field Power Supply 28 Shim Coil Power Supply 29 Transmitter 30 Receiver 31 Sequence Controller 32 Computer 33 Input Device 34 Display Device 35 arithmetic device 36 storage device 37 bed 40 imaging condition setting unit 41 sequence controller control unit 42 k-space database 43 image reconstruction unit 44 image database 45 image processing unit P subject

Claims (10)

A plurality of echoes are generated by repeatedly applying a radio frequency inversion pulse for inverting the phase of the magnetization spin following the radio frequency excitation pulse for exciting the magnetized spin in the imaging region, and from the application of the radio frequency excitation pulse Data collection means for collecting the first and second data according to first and second sequences, respectively, with different times until the first echo;
Image data generating means for generating component image data or component-suppressed image data based on the first and second data;
Have
The data collecting means is configured to collect the first and second data by making the effective echo times in the first and second sequences identical to each other.
Magnetic resonance imaging apparatus characterized by. The magnetic resonance imaging apparatus according to claim 1, wherein the image data generation unit is configured to generate water image data and fat image data from differences in J coupling characteristics. 2. The data collection unit is configured to collect the first and second data with the same echo interval in the first and second sequences being the same as each other. Magnetic resonance imaging equipment. The image data generation means uses the rate of change of the signal strength of at least one of the first and second data due to J coupling to generate the component image data or the component-suppressed image data from the first and second data. The magnetic resonance imaging apparatus of claim 1, wherein the apparatus is configured to generate A plurality of echoes are generated by repeatedly applying a radio frequency inversion pulse for inverting the phase of the magnetization spin following the radio frequency excitation pulse for exciting the magnetized spin in the imaging region, and from the application of the radio frequency excitation pulse Data collection means for collecting the first and second data according to first and second sequences, respectively, with different times until the first echo;
Image data generating means for generating component image data or component-suppressed image data based on the first and second data;
Have
The data collection means is configured to collect the first and second data by alternately executing the first and second sequences every repeated time .
A magnetic resonance imaging apparatus. A plurality of echoes are generated by repeatedly applying a radio frequency inversion pulse for inverting the phase of the magnetization spin following the radio frequency excitation pulse for exciting the magnetized spin in the imaging region, and from the application of the radio frequency excitation pulse Data collection means for collecting the first and second data according to first and second sequences, respectively, with different times until the first echo;
Image data generating means for generating component image data or component-suppressed image data based on the first and second data;
Have
The image data generation means includes the component image data or the component data from the image data reconstructed by alternately arranging the first and second data in a phase encoding direction for each echo data line in a common k space. Configured to generate component-suppressed image data ;
A magnetic resonance imaging apparatus. The data collection means is configured to collect the second data by setting the time from application of the high frequency excitation pulse to the first echo in the second sequence as an odd multiple of 3 or more of the echo interval. The magnetic resonance imaging apparatus according to claim 1. 2. The magnetic resonance imaging apparatus according to claim 1, further comprising setting means for setting a time from application of a high-frequency excitation pulse to the first echo in at least one of the first and second sequences. A plurality of echoes are generated by repeatedly applying a radio frequency inversion pulse for inverting the phase of the magnetization spin following the radio frequency excitation pulse for exciting the magnetized spin in the imaging region, and from the application of the radio frequency excitation pulse Data collecting means for collecting first and second data according to first and second sequences, respectively, wherein at least one of the time to the first echo and the echo interval is different from each other;
Image data generating means for generating component image data or component-suppressed image data from the first and second data using a parameter representing a degree of influence of J coupling on at least one of the first and second data; ,
Have
The data collecting means is configured to collect the first and second data by making the effective echo times in the first and second sequences identical to each other.
Magnetic resonance imaging apparatus characterized by. A plurality of echoes are generated by repeatedly applying a radio frequency inversion pulse for inverting the phase of the magnetization spin following the radio frequency excitation pulse for exciting the magnetized spin in the imaging region, and from the application of the radio frequency excitation pulse Data collecting means for collecting first and second data according to first and second sequences, respectively, wherein at least one of the time to the first echo and the echo interval is different from each other;
Image data for generating component image data or component-suppressed image data from image data reconstructed by alternately arranging the first and second data for each echo data line in the phase encoding direction on a common k space Generating means;
A magnetic resonance imaging apparatus comprising:

Images