JP5469855B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention applies a pre-saturation pulse (hereinafter referred to as pre-pulse) in a magnetic resonance imaging (hereinafter referred to as MRI) device to modulate the nuclear magnetization of the subject, and adds a striped or grid-like tag on the MRI image. The present invention relates to imaging technology.

  Tagging is a technique for applying a pre-pulse to modulate the spatial distribution of longitudinal magnetization and adding a striped or lattice tag on the MRI image. A pre-pulse used for tagging (hereinafter, tagging pre-pulse) is mainly used as a pre-pulse for a cardiac cine imaging sequence, and is used for simple evaluation of the deformation behavior of the heart wall.

  A typical tagging pre-pulse is composed of two high-frequency magnetic field (RF) pulses with a flip angle of 90 °, a dephase gradient magnetic field between RF pulses, and a spoiler gradient magnetic field in the slice direction. There is what is called 1 SPAMM (for example, see Patent Document 1). In 1-1 SPAMM, the dephase gradient magnetic field is applied in either the phase encoding direction or the readout direction, and the tag is generated in a direction orthogonal to the gradient magnetic field application direction in the slice.

  In addition, there is one using a high frequency burst pulse (hereinafter referred to as BURST) as an improvement in the retention time of the 1-1 SPAMM tag (see, for example, Patent Document 2). BURST is a high-frequency burst pulse formed of sub-pulses, which are a plurality of RF pulses whose amplitude is modulated by the SINC function, formed at equal intervals on the time axis, a dephase gradient magnetic field applied between the sub-pulses, and a spoiler gradient magnetic field It consists of. In 1-1 SPAMM, a profile that is a trigonometric function is a rectangular function in BURST. For this reason, the contrast of the tag is improved.

US Pat. No. 5,054,489 US Pat. No. 6,546,274

  However, in the case of BURST, the number of RF pulses (sub-pulses) to be applied increases, so the pre-pulse application time becomes long. Therefore, the subsequent main imaging is affected. For example, as described above, when the subsequent main imaging is a cardiac cine imaging sequence, the number of cardiac time phases that can be acquired during the cardiac cycle decreases. In addition, since the start time of the main imaging is delayed from the R wave, it is difficult to acquire data at an early contraction time necessary for comparing the distortion of the tag in the MRI image. Therefore, the accuracy of the evaluation of the deformation behavior of the heart wall is lowered.

  The present invention has been made in view of the above circumstances, and in tagging imaging for adding a tag to an MR image, while maintaining the contrast of the tag, the prepulse application time is shortened and an image capable of highly accurate evaluation is obtained. The purpose is to provide technology.

  In the magnetic resonance imaging apparatus for imaging a subject, the present invention uses a high-frequency burst pulse composed of a sub-pulse amplitude-modulated by an asymmetric SINC function as a tagging pre-pulse.

  Specifically, a static magnetic field generating means for generating a static magnetic field, a gradient magnetic field generating means for generating gradient magnetic fields in three directions orthogonal to each other, a high frequency magnetic field generating means for generating a high frequency magnetic field, and a nucleus generated from an inspection object A signal detection means for detecting a magnetic resonance signal, an image reconstruction means for reconstructing an image from the signal detected by the signal detection means, the gradient magnetic field generation means, the high-frequency magnetic field generation means, and the signal detection means are subjected to predetermined pulses. And a control means that operates according to the sequence, wherein the pulse sequence includes a main imaging sequence in which the image reconstruction means acquires a signal for reconstructing an image, and the main imaging sequence. A pre-pulse sequence that gives an additional effect to the signal used, the pre-pulse sequence being equally spaced on the time axis A high-frequency pulse applying unit that applies a high-frequency burst pulse composed of a plurality of formed sub-pulses, and a gradient magnetic field applying unit that applies a gradient magnetic field in the same one of the three directions between the sub-pulses, The magnetic resonance imaging apparatus is characterized in that the amplitudes of the plurality of subpulses are modulated by an asymmetric SINC function.

  According to the present invention, since the pre-pulse application time can be shortened while maintaining the contrast of the tag, an image that can be evaluated with high accuracy can be obtained.

  Hereinafter, a first embodiment to which the present invention is applied will be described. Hereinafter, in all the drawings for explaining the embodiments of the present invention, those having the same function are denoted by the same reference numerals, and repeated explanation thereof is omitted.

  First, a magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) of this embodiment will be described. FIG. 1 is a configuration diagram of an MRI apparatus 100 according to this embodiment. As shown in the figure, an MRI apparatus 100 of the present embodiment includes a static magnetic field generator 12 that generates a static magnetic field in an imaging space, a bed 22 that is mounted with an imaging target 11 such as a patient, and is placed in the imaging space. An RF coil 14 for applying a high frequency magnetic field (RF) pulse to the imaging object 11, an RF probe 15 for detecting a nuclear magnetic resonance (NMR) signal generated by the imaging object 11, and an X space, an X direction in the imaging space, A gradient magnetic field coil 13 for generating a gradient magnetic field in the Z direction is provided. Furthermore, the MRI apparatus 100 includes a gradient magnetic field power supply 19, an RF transmission unit 20, a control unit 21, a signal detection unit 16, a signal processing unit 17, and a display unit 18.

  The gradient magnetic field coil 13 generates a gradient magnetic field in each direction in accordance with a signal from the gradient magnetic field power supply 19, and the RF coil 14 generates an RF pulse in response to a signal from the RF transmission unit 20. The RF transmission unit 20 includes a high-frequency oscillator, a modulator, and a high-frequency amplifier. The RF pulse output from the high-frequency oscillator is amplitude-modulated by the modulator at a timing according to a command from the control unit 21, and the amplitude-modulated high-frequency magnetic field pulse Is supplied to the RF coil 14 after being amplified by a high frequency amplifier. The signal detected by the RF probe 15 is detected by the signal detection unit 16 and converted into an image signal by the signal processing unit 17 by signal processing or calculation. The obtained image signal is displayed as an image on the display unit 18.

  Operations of the gradient magnetic field power source 19, the RF transmission unit 20, and the signal detection unit 16 are controlled by the control unit 21. The control time chart is generally called a pulse sequence. The control unit 21 is realized by an information processing apparatus including a CPU, a memory, and a storage device, and a pulse sequence is created in advance and stored in the storage device.

  In imaging, different phase encodings are given depending on the gradient magnetic field, and NMR signals (echo signals) obtained by the respective phase encodings are detected. As the number of phase encodings, values such as 128, 256, and 512 are usually selected per image. Each echo signal is usually obtained as a time-series signal composed of 128, 256, 512, and 1024 sampling data. These data are two-dimensionally Fourier transformed to reconstruct one MR image.

  At present, the MRI imaging target is a proton that is a main constituent of the subject that is the imaging target 11 as being widely used in clinical practice. By imaging the spatial distribution of the proton density and the spatial distribution of the relaxation phenomenon in the excited state, two-dimensional or three-dimensional imaging of the form or function of the human head, abdomen, limbs, etc. is performed.

  In the present embodiment, the time required for the tagging sequence for applying the tagging pre-pulse is reduced while maintaining the contrast of the tag. Hereinafter, the pulse sequence of this embodiment for realizing this will be described. As described above, in the MRI apparatus 100 of the present embodiment, the control unit 21 controls the operation of each unit in accordance with this pulse sequence, and the signal processing unit 17 reconstructs an image from the obtained echo signal, thereby shortening the time. An MR image to which a high-contrast tag is added is obtained by the tagging sequence.

  First, the entire imaging sequence will be described by taking as an example a pulse sequence when the tagging pre-pulse is used as a pre-pulse for cardiac cine imaging synchronized with the cardiac cycle. FIG. 2 is a diagram illustrating execution timings of an R waveform 200 of an electrocardiogram, a tagging sequence 210, and a pulse sequence (cine imaging sequence) 220 for main imaging. Here, an example of two heartbeats when the cardiac phase number is 7 is shown. As shown in the figure, a tagging sequence 210 and a cine imaging sequence 220 are executed during the cardiac cycle between the R wave 200 and the R wave 200. These sequences are executed in synchronization with the R wave 200. For example, an SSFP sequence can be applied to the cine imaging sequence 220. In this embodiment, the control unit 21 controls the operations of the gradient magnetic field power source 19, the RF transmission unit 20, and the signal detection unit 16 according to this pulse sequence, thereby obtaining a tagged cine image of the heart.

  Here, the case where the cine imaging sequence 220 is executed as an example of the pulse sequence of the main imaging has been described as an example, but the pulse sequence of the main imaging is not limited thereto. Single phase imaging may be used.

  Next, details of the tagging sequence 210 will be described. FIG. 3A shows an example of the tagging sequence 210 using 1-1 SPAMM as a tagging pre-pulse. As shown in the figure, a tagging sequence 210A using 1-1 SPAMM as a pre-tagging pulse is applied between two RF pulses 311 and 312 having a flip angle of 90 degrees, and between the RF pulse 311 and the RF pulse 312. The dephase gradient magnetic field 321 and the spoiler gradient magnetic field 331 in the slice direction are provided. As described above, the dephase gradient magnetic field is applied in either the phase encoding direction or the readout direction. Here, an example of applying in the lead-out direction is shown.

  When the cine imaging sequence 220 is executed after executing the tagging sequence 210A, an MRI image to which a striped tag orthogonal to the readout direction is added as shown in FIG. 3B is obtained.

This fringe is composed of a region where the signal is suppressed (black line portion in FIG. 3B) and a region where the signal is not suppressed, and the tag direction and interval are the application direction of the dephase gradient magnetic field 321. It is controlled in combination with the applied amount. That is, the tag is generated in the direction (here, the phase encoding direction) orthogonal to the application direction of the dephase gradient magnetic field 321 on the slice plane determined by the slice gradient magnetic field (not shown), The application amount S of the phase gradient magnetic field 321 has the relationship of the following formula (1).
d = 1 / γS (1)
Here, γ is a magnetic rotation ratio.

  Further, FIG. 4A shows an example of a tagging sequence 210B that generates a lattice tag in two directions. For the grid-like tag, the tagging sequence 210A for generating the striped tag is executed twice in succession. However, in the first time, the application direction of the dephase gradient magnetic field is one of the readout direction and the phase encoding direction, and the second time is a direction that is not applied in the first time.

  When the tagging sequence 210B is executed, as shown in FIG. 4B, striped tags orthogonal to the readout direction and striped tags orthogonal to the phase encoding direction are respectively generated on the MRI image, Both intersect and a portion with a high signal intensity is distributed in the form of lattice points.

  Furthermore, it is possible to generate a tag in three directions. In the three-direction tagging sequence, the tagging sequence 210A for generating a striped tag is continuously executed three times. At this time, the application direction of the dephase gradient magnetic field is sequentially changed to the readout direction, the phase encoding direction, and the slice direction in the first, second, and third times.

  Next, FIG. 5A shows a tagging sequence 210C when BURST is used as a tagging prepulse. A tagging sequence 210C by BURST is formed at equal intervals on the time axis, and a high-frequency burst pulse 510 composed of a plurality of sub-pulses 511a, 511b, 511c, 511d, 511e, 511f, and 511g whose amplitude is modulated by the SINC function 540, Dephase gradient magnetic fields 520a, 520b, 520c, 520d, 520e, and 520f applied between the sub-pulses, and a spoiler gradient magnetic field 530 are provided. The dephase gradient magnetic field 520 is applied in either the readout direction or the phase encoding direction. FIG. 5A shows a case where the voltage is applied in the lead-out direction as an example. Here, as an example, the case where there are seven subpulses (including the case where the amplitude is 0) is shown, but the number of subpulses is not limited to this. Further, when it is not necessary to particularly distinguish each subpulse and each dephase gradient magnetic field, they are represented by a subpulse 511 and a dephase gradient magnetic field 520, respectively.

  When the cine imaging sequence 220 is executed after executing the tagging sequence 210C, an MRI image to which striped tags orthogonal to the readout direction as shown in FIG.

As in the case of the tagging sequence 210A based on 1-1 SPAMM, this fringe is composed of a region where the signal is suppressed (black portion in FIG. 5B) and a region where the signal is not suppressed. The relationship between the tag interval d and the application amount S of each dephase gradient magnetic field 520 is expressed by the following equation (2).
d = 1 / γS (2)
Further, in the tagging sequence 210C by BURST, not only the tag interval d but also the tag width w can be adjusted. That is, if the period of the SINC function 540 used for amplitude modulation is T, the period of the δ function, which is the application interval of the subpulse 511, is τ, and the application intensity of the dephase gradient magnetic field 520 is G, the tag width w and the tag interval d Are represented by the following formulas (3) and (4), respectively.
w = 1 / γGT (3)
d = 1 / γGτ (4)
That is, the width w of the tag and the interval d of the tag can be controlled by the value of the period T of the SINC function 540 and the period of the δ function (application interval of the subpulse 511) τ. The ratio of the tag width w to the tag interval d is w: d = τ: T.

  Note that the SINC function 540 used for amplitude modulation of the subpulse 511 in BURST generally includes a main wave (main lobe) having the largest amplitude and a plurality of sub waves (side lobes) connected to both sides thereof. Due to the nature of the Fourier transform, the profile approaches the rectangular function shape as the number of side lobes increases, and a high-quality tag can be obtained. However, in the case of heartbeat-synchronized cine imaging as in the present embodiment, if the number of side lobes is too large, the time required for the tagging pre-pulse 210 becomes longer, and the time of the cine imaging sequence 220 that is main imaging decreases.

For this reason, in implementation, the SINC function 540 used for modulation is one with one left and right side lobe as shown in FIG. The SINC function 540 is obtained by multiplying a general SINC function by a window function represented by the following expression (5).
w (x) = 1 (−2π ≦ x ≦ 2π),
w (x) = 0 (x <-2π, 2π <x) (5)

  Here, the time required for the tagging sequence 210A shown in FIG. 3 (a) and the tagging sequence 210C shown in FIG. 5 (a) when the MRI image to which the tag with the interval d is added is compared. Here, the application time of the RF pulses 311 and 312 and the sub-pulse 511 is t1, the application time of the dephase gradient magnetic fields 321 and 520 is t2, and the application time of the spoiler gradient magnetic fields 331 and 530 is t3.

The time TS required for the tagging sequence 210A using 1-1 SPAMM shown in FIG. 3A as the pre-tagging pulse is as shown in the following equation (6).
TS = 2 × t1 + t2 + t3 (6)
On the other hand, the time TB required for the tagging sequence 210C using BURST as the pre-tagging pulse shown in FIG. 5A is as shown in the following equation (7).
TB = 7 × t1 + 6 × t2 + t3 (7)
As described above, when BURST is used, the pre-pulse application time becomes longer by 5 × t1 + 5 × t2 than when 1-1 SPAMM is used. For example, when t1 = 0.3 ms, t2 = 0.4 ms, and t3 = 1.5 ms, the length becomes 2.4 times longer. In addition, when generating a tag in two directions, the pre-pulse application time is about twice, so in the above example, it is about 4.8 times longer.

  As described above, in heart rate-synchronized cine imaging, the interval between the R waves 200 is constant, so that the execution time of the cine imaging sequence 220 decreases correspondingly as the time of the tagging sequence 210 increases. Therefore, the number of cardiac phases that can be acquired in the cine imaging sequence 220 is reduced. Further, the period from the generation of the R wave 200 to the start time of the cine imaging sequence 220 becomes long, and data at the early contraction immediately after the R wave, which is important for comparing the distortion of the tag, cannot be acquired.

  Therefore, in the present embodiment, the tagging prepulse is modulated so that the profile maintains a rectangular function shape and the time required for the tagging sequence is further shortened. Hereinafter, the tagging prepass and the tagging sequence of this embodiment will be described.

  FIG. 6 shows a tagging sequence 210 of this embodiment. FIG. 6A shows an example in which the ratio between the application interval τ of the sub-pulse 611 and the period T of the modulation function is 1 to 2 (τ: T = 1: 2). FIG. Is an example in the case of 1: 4 (τ: T = 1: 4). The tagging sequence 210 of this embodiment is basically the same as the tagging sequence 210C. That is, in the case of FIG. 6A, the high frequency burst pulse 610 is composed of sub-pulses 611a, 611b, 611c, 611d, 611e, and 611f (hereinafter, representatively sub-pulse 611). A dephase gradient magnetic field 620 is applied between the sub-pulses 611, and a spoiler gradient magnetic field 630 is applied after application of the dephase gradient magnetic fields 620a, 620b, 620c, and 620d (hereinafter, representatively 620). However, the amplitude of the subpulse is modulated by an asymmetric SINC function 640 in which the right side lobe of the SINC function 540 is deleted. The same applies to FIG.

The asymmetric SINC function 640 used for amplitude modulation in this embodiment is obtained by multiplying a general SINC function by a window function represented by the following equation (8).
w (x) = 1 (-2π ≦ x ≦ π)
w (x) = 0 (x <−2π, π <x) (8)

  Here, a profile for one period by the tagging sequence 210C and a profile for one period by the tagging sequence 210 of the present embodiment are shown in FIGS. 7A, 7B, 7C, and 7D, respectively. Show. FIGS. 7A and 7C are a conventional example in the case of τ: T = 1: 2, respectively, and an example of the present embodiment. FIGS. The case of 1: 4. In this figure, the horizontal axis represents the offset frequency and the vertical axis represents the longitudinal magnetization. Each profile was derived by numerical integration of the Bloch equation.

  As shown in this figure, the profile by the tagging sequence 210C that modulates the amplitude of the sub-pulse 511 with the symmetrical SINC function 540 is compared with the profile shown in FIGS. 7A and 7B. In (c) and (d), the rise and fall of the profile are slightly smooth, and the area where the signal is suppressed increases. However, there is no significant difference in the shape. This is presumably because the flip angle of 511g is small and the contribution to the modulation of longitudinal magnetization is small.

  In fact, when τ: T = 1: 2, the half width for one tag period is about 32% according to the tagging sequence 210C, about 35% according to the tagging sequence 210 of this embodiment, and only 2%. It is a difference. Even in the case of τ: T = 1: 4, the difference is only 15% in the tagging sequence 210C and approximately 14% in the tagging sequence 210 of the present embodiment, which is a difference of only 1%. Further, in FIGS. 7 (c) and 7 (d), the ripple waveform (ringing) seen at both ends of the profile FIGS. 7 (a) and 7 (b) according to the tagging sequence 210C is shown in FIGS. It can be seen that there is a reduction.

  From the above, it can be said that even if the amplitude of the sub-pulse 611 is modulated using the asymmetric SINC function 640, the profile shape is hardly affected. Therefore, according to the tagging sequence 210 of the present embodiment, a profile having a rectangular function shape substantially the same as that of the tagging sequence 210C can be obtained, and a tag with good contrast can be obtained.

  Here, the time required for the tagging sequence 210 </ b> C and the tagging sequence 210 of the present embodiment when the MRI image to which the tag with the interval d is added is generated is compared. Here, similarly to the above, the application time of the subpulses 611 and 511 is t1, the application time of the dephase gradient magnetic fields 620 and 520 is t2, and the application time of the spoiler gradient magnetic fields 630 and 530 is t3.

The time TBa required for the tagging sequence 210 (when τ: T = 1: 2) shown in FIG. 6A of the present embodiment is as shown in the following formula (9).
TBa = 5 × t1 + 4 × t2 + t3 (9)
Further, the time TBb required for the tagging sequence 210 ′ in FIG. 6B is as shown in the following formula (10).
TBb = 11 × t1 + 10 × t2 + t3 (10)

On the other hand, the ratio of τ to T is the same as that in FIGS. 6A and 6B, respectively, and the tagging prepulse is modulated by the symmetrical SINC function 540 (τ: T = 1: 4). The time TBa ′ and TBb ′ required for the tagging sequence 210C in the case) are as shown in the following equations (11) and (12), respectively.
TBa = 7 × t1 + 6 × t2 + t3 (11)
TBb = 15 × t1 + 14 × t2 + t3 (12)

  Therefore, the time required for the tagging sequence 210 is 2 (t1 + t2) when τ: T = 1: 2, and 4 (t1 + t2) when τ: T = 1: 4, compared to the time required for the tagging sequence 210C. Shortened. For example, when t1 = 0.3 ms, t2 = 0.4 ms, and t3 = 1.5 ms, it can be shortened by about 23% in FIG. 6A and about 24% in FIG. 6B. That is, when the amplitude of the sub-pulse 611 is modulated by the asymmetric SINC function 640, the time required for the tagging sequence can be greatly shortened compared to the case where the amplitude is modulated by the left and right SINC functions 540.

  As described above, according to the present embodiment, when BURST is used as a tagging pre-pulse, the amplitude of the sub-pulse is modulated by an asymmetric SINC function. As a result, the time required for the tagging sequence can be greatly shortened with only a slight degradation of the profile. Accordingly, the start time of the main imaging after the tagging sequence can be advanced. Further, in imaging performed in synchronization with a predetermined periodic signal, the time for actual imaging can be increased. In particular, in heartbeat-synchronized cine imaging, the number of cardiac phases acquired in the main imaging can be increased, and early contraction data immediately after the R wave, which is important for comparing tag distortion, can be acquired.

In the above embodiment, the case where one peak after the main lobe is deleted from the SINC function 540 having three peaks with the side lobe having one peak before and after the main lobe has been described as an example. However, it may be configured to delete one mountain before the main lobe. In this case, the amplitude of the subpulse is modulated by an asymmetric SINC function 740 obtained by multiplying a general SINC function by a window function represented by the following equation (13).
w (x) = 1 (−π ≦ x ≦ 2π)
w (x) = 0 (x <π, 2π <x) (13)

  Even when the amplitude of the sub-pulse is modulated by the asymmetric SINC function 740, since the flip angle of 511g is small and the contribution to the modulation of the longitudinal magnetization is small, substantially the same profile can be obtained. Therefore, the time required for the tagging sequence can be shortened, and the start of the main imaging can be accelerated. Further, in imaging performed in synchronization with a predetermined periodic signal, it is possible to increase the time of main imaging performed after the tagging sequence. In particular, in cardiac cine imaging synchronized with heartbeats, the number of cardiac phases acquired in the main imaging can be increased, and early contraction data necessary for comparing distortions can be acquired. However, in order to reduce the influence of lateral relaxation during pre-pulse application, it is desirable to delete the right side lobe.

  Note that the number of side lobes of the asymmetric SINC function that modulates the amplitude of the sub-pulse is not limited to either one of the left and right peaks. In general, when amplitude modulation of a sub-pulse is performed with an asymmetric SINC function in which any one of the left and right m mountains (m is a positive integer less than or equal to n) of the left and right symmetric SINC functions of (2n + 1) mountains (n is a positive integer), 2n + 1) The time required for the tagging sequence can be shortened while substantially maintaining the shape of the profile as compared with the case where the amplitude of the sub-pulse is modulated by the symmetric SINC function of the mountain. That is, a tag with good contrast can be obtained in a short time.

The SINC function of the above (2n + 1) mountain, the asymmetric SINC function in which the right side lobe is removed from the m mountain (m is a positive integer satisfying m ≦ n), and the asymmetric SINC function in which the left side lobe is removed from the m mountain are respectively The general SINC function is obtained by multiplying the following window functions (14), (15), and (16).
w (x) = 1 (− (n + 1) π ≦ x ≦ (n + 1) π)
w (x) = 0 (x <− (n + 1) π, (n + 1) π <x) (14)
w (x) = 1 (− (n + 1) π ≦ x ≦ (n + 1−m) π)
w (x) = 0 (x <− (n + 1) π, (n + 1−m) π <x) (15)
w (x) = 1 (− (n + 1−m) π ≦ x ≦ (n + 1) π)
w (x) = 0 (x <− (n + 1−m) π, (n + 1) π <x) (16)

  In addition, the window function to be multiplied when the SINC function for amplitude-modulating the subpulse is asymmetric is not limited to the one that leaves the basic shape of the SINC function and deletes the side lobe as described above. For example, various window functions such as a Hamming window function, a Hanning window function, and a Gauss window function can be applied.

As an example, a tagging sequence in which the amplitude of a subpulse is modulated with an asymmetric SINC function obtained by multiplying a general SINC function by the asymmetric Hamming window function shown in the following equation (17), with the right side lobe removed. Is shown in FIG. FIG. 8 shows an example where n = m = 1.
w (x) = 0.54 + 0.46 cos (x / (n + 1))
(− (N + 1) π ≦ x ≦ 0)
w (x) = 0.54 + 0.46 cos (x / (n + 1−m))
(0 <x ≦ (n + 1−m) π)
w (x) = 0
(X <− (n + 1) π, (n + 1−m) π <x) (17)
(N is a positive integer, m is a positive integer less than or equal to n)

  FIG. 8A shows an example in which the ratio between the application interval τ of the sub-pulse 811 and the period T of the asymmetric SINC function to be modulated is 1: 2 (τ: T = 1: 2), and FIG. In this example, the ratio is 1: 4 (τ: T = 1: 4). Similar to the above embodiment, the tagging sequence shown in FIGS. 8A and 8B includes a high-frequency burst pulse 810 composed of subpulses 811, a dephase gradient magnetic field 820 applied between the subpulses 811, and a dephase gradient. And a spoiler gradient magnetic field 830 applied after application of the magnetic field 820.

  FIGS. 9A and 9B show profiles for one period according to the tagging sequence shown in FIGS. 8A and 8B when other conditions are the same as those in the above embodiment. As shown in this figure, the rise and fall of the profile are slightly smoother than the profiles shown in FIGS. 7 (c) and 7 (d). However, it can be seen that ringing is reduced.

  That is, if a high frequency burst pulse in which a subpulse is amplitude-modulated using an asymmetric SINC function obtained by multiplying a Hamming window function is used as a tagging prepulse, not only the time required for the tagging sequence is shortened, but also ringing. Reduction is possible. Therefore, it is possible to improve the contrast between the region where the signal is not suppressed and the suppressed region. That is, it is possible to obtain an image to which a tag with good contrast is given in a short time.

  Furthermore, in the tagging sequence, the time required for the tagging sequence may be shortened by reducing the application time of the sub-pulse 611 and / or the application time of the dephase gradient magnetic field 620 constituting the high-frequency burst pulse 610. . An example of the tagging sequence in this case is shown in FIG. 10A is a modified version of the pulse sequence of FIG. 6A, and FIG. 10B is a modified version of the pulse sequence of FIG. 6B. Here, FIG. 10A will be described as an example. In FIG. 10A, each sub-pulse 1011 (1011a, 1011c, 1011d, 1011e) constituting the high-frequency burst pulse 1010 has the last character set to the same sub-pulse as the last character of each sub-pulse 611 in FIG. It shall correspond. The same applies to each dephase gradient magnetic field 1020 (1020a ', 1020c, 1020d).

  As shown in FIG. 10A, the application amount of each sub-pulse 1011 is determined by the application time and the application intensity (transmission gain). When the applied intensity is constant, the flip angle is proportional to the sub-pulse application time. Therefore, when the application intensity of all the subpulses 1011 is maximized, the application time can be shortened for subpulses having a small flip angle, for example, 1011a, 1011c, and 1011e. The same applies to FIG. 10B.

  The same applies to the dephase gradient magnetic field 1020, and the application amount is determined by the application time and the application intensity. Therefore, the application time of each dephase gradient magnetic field 1020 can be shortened by increasing the application intensity. Therefore, the application intensity of the dephase gradient magnetic field 1020 is set so that the application time of each dephase gradient magnetic field 1020 is the shortest. Thereby, the application time of each dephase gradient magnetic field 1020 can be shortened. Note that the application interval of the sub-pulse 1011 can be shortened as much as the application time of the dephase gradient magnetic field 1020 is shortened.

  Further, two dephase gradient magnetic fields (620a and 620b in FIG. 6) sandwiching a sub-pulse (611b in FIG. 6) whose amplitude is 0 by amplitude modulation by an asymmetric SINC function are defined as one dephase gradient magnetic field 1020a ′. At this time, the application amount of the dephase gradient magnetic field 1020a 'is set to be equal to the total application amount of the dephase gradient magnetic field 620a and the dephase gradient magnetic field 620b. Thereby, the time of the tagging sequence can be shortened by the interval between the dephase gradient magnetic field 620a and the dephase gradient magnetic field 620b.

  The tagging sequence 210 according to the present embodiment and the modifications thereof have been described by taking the case of generating a striped tag as an example. The tagging sequence 210 and its modification of the present embodiment can be used to generate a two-way (lattice-like) tag and a three-way tag as in the conventional tagging sequence 210C.

  In the case of generating a two-dimensional tag, before executing the pulse sequence of the main imaging, as described above, the amplitude of the sub-pulse 611 constituting the high-frequency burst pulse 610 is modulated by the asymmetric SINC function 640 of the above-described embodiment, and is also processed. Following the tagging sequence 210 in which the phase gradient magnetic field 620 is applied in the readout direction, a tagging sequence in which only the application direction of the dephase gradient magnetic field 620 in the tagging sequence 210 is changed to the same phase encoding direction is executed. Further, when generating a three-dimensional tag, a tagging sequence in which only the application direction of the dephase gradient magnetic field 620 of the tagging sequence 210 is changed to the slice encoding direction is executed. Note that the order of execution of these tagging sequences does not matter.

  At this time, in order to reduce the influence of longitudinal relaxation during the application of the high frequency burst pulse, the high frequency burst pulse of the second axis rather than the first axis and the third axis than the second axis is applied to the dephase gradient magnetic field application axis. A shorter application time is desirable. Therefore, in FIG. 11, for example, in a pulse sequence for generating a two-dimensional tag, the amplitude of the sub-pulse of the first-axis high-frequency burst pulse is modulated by a symmetric SINC function, and that of the second axis is the asymmetric SINC of the above embodiment. It may be configured to modulate with an asymmetric SINC function which is a function and which deletes the side lobe after the main lobe. A tagging sequence in this case is shown in FIG. In the pulse sequence for generating a three-dimensional tag, the amplitude of the sub-pulse of the first-axis high-frequency burst pulse is modulated by the symmetric SINC function, and that of the second axis is the asymmetric SINC function of the above-described embodiment. The third side axis may be modulated with a symmetric SINC function in which both side lobes before and after the main lobe are deleted. A tagging sequence in this case is shown in FIG. In addition to this example, an asymmetric SINC function and a symmetric SINC function can be used in any combination in the amplitude modulation of subpulses constituting a high-frequency burst pulse.

  As described above, in the present embodiment, the time required for the tagging sequence is shortened by shortening the application time of the high frequency burst pulse. Therefore, the start time of the main imaging can be advanced. In addition, even when there is a restriction on the time of the main imaging after the tagging sequence, a sufficient time for the main imaging can be obtained. For example, when cardiac cine imaging synchronized with heart rate is performed, the number of cardiac phases acquired by cine imaging can be increased. In addition, an image of early contraction immediately after the R wave can be acquired, and a change in distortion between cardiac phases can be measured more accurately.

It is an apparatus block diagram of the MRI apparatus 100 of embodiment of this invention. It is a pulse sequence of the embodiment of the present invention. (A) is a tagging sequence in the case of using one-dimensional SPAMM as a pre-pulse. (B) is a figure which shows the example of the tag on MR image by 1-dimensional SPAMM. (A) is a tagging sequence when two-dimensional SPAMM is used for pre-pulses. (B) is a figure which shows the example of the tag on the MR image by two-dimensional SPAMM. (A) is a tagging sequence when BURST is used for the pre-pulse. (B) is a figure which shows the example of the tag on the MR image by BURST. (A) And (b) is a tagging sequence of the embodiment of the present invention. (A) And (b) is a figure for demonstrating the profile by the conventional BURST, (c) And (d) is the profile by the tagging sequence of embodiment of this invention. (A) And (b) is a tagging sequence in the case of modulating the amplitude of a sub-pulse with an asymmetric SINC function multiplied by an asymmetric Hamming window, which is another example of the embodiment of the present invention. (A) And (b) is a figure for demonstrating the profile by a tagging sequence which is the other example of embodiment of this invention. (A) and (b) are other examples of the embodiment of the present invention, and are pulse sequences in the case of adjusting the sub-pulse application time, the application interval, and the application time of the dephase gradient magnetic field to shorten the tagging sequence. is there. (A) is a sequence example when the embodiment of the present invention is applied to a tagging sequence for generating a two-dimensional tag, and (b) is a sequence when applied to a tagging sequence for generating a three-dimensional tag. It is an example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11: Imaging object, 12: Static magnetic field generator, 13: Gradient magnetic field coil, 14: RF coil, 15: RF probe, 16: Signal detection part, 17: Signal processing part, 18: Display part, 19: Gradient magnetic field power supply , 20: RF transmitter, 21: controller, 22: bed, 100: MRI apparatus, 200: R waveform, 210: tagging sequence, 210 ′: tagging sequence, 210 ″: tagging sequence, 210A: tagging sequence, 210B : Tagging sequence, 210C: tagging sequence, 220: cine imaging sequence, 311: RF pulse, 312: RF pulse, 321: dephase gradient magnetic field, 331: spoiler gradient magnetic field, 510: high frequency burst pulse, 511: sub-pulse, 511a: Sub pulse 511b: Sub pulse 511c: Sub pulse 511d: Subpulse, 511e: Subpulse, 511f: Subpulse, 511g: Subpulse, 520: Dephase gradient magnetic field, 520a: Dephase gradient magnetic field, 520b: Dephase gradient magnetic field, 520c: Dephase gradient magnetic field, 520d: Dephase Gradient magnetic field, 520e: Dephase gradient magnetic field, 520f: Dephase gradient magnetic field, 530: Spoiler gradient magnetic field, 540: SINC function, 611: Subpulse, 611a: Subpulse, 611b: Subpulse, 611c: Subpulse, 611d: Subpulse, 611e: Subpulse, 611f: Subpulse, 620: Dephase gradient magnetic field, 620a: Dephase gradient magnetic field, 620b: Dephase gradient magnetic field, 620c: Dephase gradient magnetic field, 620d: Dephase Gradient magnetic field, 630: Spoiler gradient magnetic field, 640: Asymmetric SINC function, 740: Asymmetric SINC function, 811: Subpulse, 820: Dephase gradient magnetic field, 830: Spoiler gradient magnetic field, 1011: Subpulse, 1011a: Subpulse, 1011b: Subpulse, 1011c: Subpulse, 1011d: Subpulse, 1011e: Subpulse, 1020: Dephase gradient magnetic field, 1020a ′: Dephase gradient magnetic field, 1020c: Dephase gradient magnetic field, 1020d: Dephase gradient magnetic field

Claims (11)

A static magnetic field generating means for generating a static magnetic field, a gradient magnetic field generating means for generating gradient magnetic fields in three directions orthogonal to each other, a high frequency magnetic field generating means for generating a high frequency magnetic field, and a nuclear magnetic resonance signal generated from an inspection object are detected. Signal detecting means, image reconstructing means for reconstructing an image from signals detected by the signal detecting means, control for operating the gradient magnetic field generating means, the high-frequency magnetic field generating means and the signal detecting means in accordance with a predetermined pulse sequence A magnetic resonance imaging apparatus comprising:
The pulse sequence is
A main imaging sequence in which the image reconstruction means obtains a signal for reconstructing an image;
A pre-pulse sequence that gives an additional effect to the signal used in the main imaging sequence,
The prepulse sequence is:
A high-frequency pulse applying unit that applies a high-frequency burst pulse composed of a plurality of sub-pulses formed at equal intervals on the time axis;
A gradient magnetic field applying unit that applies a dephase gradient magnetic field in the same one of the three directions between the sub-pulses,
The amplitude of the plurality of sub-pulses is modulated by asymmetric S INC function,
Each of the sub-pulses is applied without simultaneously applying a gradient magnetic field for slice selection . The magnetic resonance imaging apparatus according to claim 1,
The asymmetric SINC function is obtained by multiplying the SINC function by an asymmetric window function. The magnetic resonance imaging apparatus according to claim 2,
The window function is a function in which − (n + 1) π or more and (n + 1−m) π or less (n is a positive integer, m is a positive integer of n or less) is 1, and the others are 0, and A magnetic resonance imaging apparatus characterized in that one of the functions of (n + 1−m) π and (n + 1) π is 1 and the others are 0. The magnetic resonance imaging apparatus according to claim 2,
The window function is either a function that is 1 when it is −2π or more and π or less and 0 otherwise, and a function that is 1 when it is −π or more and 2π or less and is 0 otherwise. Magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 2,
The magnetic resonance imaging apparatus, wherein the window function is an asymmetric Hamming window function. The magnetic resonance imaging apparatus according to any one of claims 1 to 5,
Before SL Each sub-pulses, the magnetic resonance imaging apparatus, characterized in that the transmission gain is obtained by the maximum. The magnetic resonance imaging apparatus according to any one of claims 1 to 6,
The dephasing gradient is applied intensity is obtained by the maximum,
Applying interval before Symbol each sub-pulses, the magnetic resonance imaging apparatus characterized by the application intensity of the dephase gradient magnetic field is the dephasing gradient magnetic field application time after being shortened by maximizing. The magnetic resonance imaging apparatus according to any one of claims 1 to 7,
Two said dephasing gradient magnetic field to be applied before or after the amplitude is modulated to 0 sub-pulses, characterized in that it is replaced by dephasing gradient of 1 with an applied amount of the sum of the two dephasing gradient Magnetic resonance imaging apparatus. A magnetic resonance imaging apparatus according to any one of claims 1 to 8,
The magnetic resonance imaging apparatus according to claim 1, further comprising a biological signal detection unit that detects a periodic biological signal to be examined and generates a synchronization signal, wherein the prepulse sequence is executed in synchronization with the synchronization signal. The magnetic resonance imaging apparatus according to claim 9,
The biological signal is an R wave,
The prepulse sequence and the main imaging sequence are executed during the cardiac cycle between the R wave and the R wave.
A magnetic resonance imaging apparatus . The magnetic resonance imaging apparatus according to claim 10,
One prepulse sequence and a plurality of the main imaging sequences are executed during one cardiac cycle,
The main imaging sequence is a cine imaging sequence
A magnetic resonance imaging apparatus .

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