JP2017023485A - Magnetic resonance imaging apparatus and slice selective excitation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a slice selective excitation method for suppressing extension of an imaging time and selectively exciting a target slice without any positional deviation of the excitation slice due to chemical shift regardless of a simple configuration.SOLUTION: A magnetic resonance imaging apparatus divides an RF pulse for exciting two frequency bands, into multiple times of pulses obtained by dividing an intended flip angle at a ratio following the binominal distribution and excites them respectively. The two frequency bands accord to a frequency difference of a chemical shift between intended two substances. Further, application interval of the divided RF is defined as a time interval when transverse magnetization of both substances become inverse phase. In the RF pulses to be applied odd-number-th time, phases of the respective excitation RF pulses achieving two frequency bands are same, however, in the RF pulses to be applied even-number-th time, the phases of the two-frequency-band excitation RF pulses are inverted.SELECTED DRAWING: Figure 4

Description

  The present invention relates to an imaging technique of a tomographic imaging apparatus (hereinafter referred to as “MRI apparatus”) using a nuclear magnetic resonance phenomenon, and relates to a technique for preventing excitable slice position shifts in a plurality of substances having different resonance frequencies.

  Magnetic Resonance Imaging (MRI), which is now widely used, mainly uses the nuclear magnetic resonance phenomenon of hydrogen nuclei in water molecules to image differences in hydrogen nucleus density and relaxation time that vary depending on the biological tissue. To do. For this reason, in general, a sequence is designed to excite a slice at a predetermined position with an excitation RF pulse having a center frequency matched to the resonance frequency of water.

  However, the living body is composed of various substances, and the resonance frequency slightly changes for each substance. This change is called chemical shift. Since this chemical shift increases in frequency units in proportion to the static magnetic field strength, the higher the static magnetic field strength, the more the positional shift caused by the chemical shift with respect to the slice direction increases. As a result, depending on the substance, the target slice cannot be selectively excited, resulting in artifacts such as artifacts.

  For example, by increasing the intensity of the slice selective gradient magnetic field applied together with the excitation RF pulse and widening the frequency band per slice, it is possible to absorb the position shift and suppress the influence. However, since the output of the RF pulse is limited, it cannot be reliably suppressed.

  In addition, there is a method of collecting data a plurality of times and using them to suppress the influence of chemical shift in post-processing (see, for example, Patent Document 1).

JP-A-6-327648

  In the technique of Patent Document 1, water and fat are listed as substances having different resonance frequencies, and a first excitation pulse having a center frequency matched to the resonance frequency of water is used to excite a slice at a predetermined position. Next, a second excitation pulse having a center frequency matched with the resonance frequency of fat is used to excite the slice at the same position to collect second data. At this time, the gradient of the gradient magnetic field applied when the second excitation pulse is applied is reversed from the gradient of the gradient magnetic field applied when the first excitation pulse is applied, and an offset corresponding to a chemical shift is added. . Finally, the first data and the second data are added, and an image is reconstructed from the added data.

  Therefore, it is necessary to collect measurement data twice, and further, post-processing of acquired data is required.

  The present invention has been made in view of the above circumstances, and provides a technique for selectively exciting a target slice with a simple configuration and without a displacement of the excitation slice due to a chemical shift while suppressing an increase in imaging time.

  In the present invention, by changing the RF pulse at the time of slice excitation, data from a desired slice is acquired with high accuracy without multiple imaging and post-processing.

  In the present invention, an RF pulse for exciting two frequency bands is divided into a plurality of pulses obtained by dividing a target flip angle at a ratio according to a binomial distribution, and excitation is performed. The two frequency bands are adjusted to the frequency difference of the chemical shift between the two target substances.

  The divided RF application interval is set to a time interval in which the transverse magnetizations of both substances are in opposite phases (shifted by an odd multiple of π). During this time, no slice gradient magnetic field is applied, and both substances are precessed at their respective resonance frequencies. In addition, the phase of each excitation RF pulse that realizes the two frequency bands is applied in the same phase in the RF pulse applied at the odd number of times, and the phase of the excitation RF pulse in the two frequency bands is opposite in phase in the RF pulse applied at the even number of times. Apply with.

  According to the present invention, it is possible to selectively excite a target slice with a simple configuration and without a displacement of the excitation slice due to a chemical shift while suppressing an increase in imaging time.

It is a block diagram of the whole structure of the MRI apparatus of 1st embodiment. (A) is a conventional pulse sequence diagram, and (b) and (c) are explanatory diagrams for explaining excitation positions of water and fat protons. It is explanatory drawing for demonstrating the pulse sequence of embodiment of this invention. It is explanatory drawing for demonstrating the excitation slice position by the slice selection excitation pulse of embodiment of this invention. (A) And (b) is explanatory drawing for demonstrating the motion of the magnetization on the objective slice of embodiment of this invention. (A) And (b) is explanatory drawing for demonstrating the motion of magnetization other than the objective slice of embodiment of this invention.

  A first embodiment of the present invention will be described. Note that in all the drawings for explaining the embodiments of the invention, the same reference numerals are given to components having the same function unless otherwise specified, and the repeated description thereof is omitted.

[Device configuration]
First, an overall outline of an example of the MRI apparatus of the present embodiment will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of one embodiment of the MRI apparatus of this embodiment.

  The MRI apparatus 100 of the present embodiment obtains a tomographic image of a subject using an NMR phenomenon. As shown in FIG. 1, a static magnetic field generation unit 120, a gradient magnetic field generation unit 130, a sequencer 140, A high-frequency magnetic field generation unit (hereinafter referred to as a transmission unit) 150, a high-frequency magnetic field detection unit (hereinafter referred to as a reception unit) 160, and a control processing unit 170 are provided.

  The static magnetic field generator 120 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 101 if the vertical magnetic field method is used, and in the body axis direction if the horizontal magnetic field method is used. The apparatus includes a permanent magnet type, normal conduction type or superconducting type static magnetic field generation source disposed around the subject 101.

  The gradient magnetic field generation unit 130 is a gradient magnetic field coil 131 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (device coordinate system) of the MRI apparatus 100, and a gradient magnetic field power source that drives each gradient magnetic field coil 132, and the gradient magnetic field power supply 132 of each gradient magnetic field coil 131 is driven in accordance with a command from the sequencer 140, which will be described later, to apply gradient magnetic fields Gx, Gy, Gz in the three axial directions of X, Y, and Z. . At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other. A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction to encode position information in each direction into an echo signal.

  The transmitter 150 causes the subject 101 to generate a high frequency magnetic field pulse (hereinafter referred to as “RF pulse”) in order to cause a nuclear magnetic resonance (NMR) phenomenon to occur in the nuclear spins of the atoms constituting the living tissue of the subject 101. A high frequency oscillator (synthesizer) 152, a modulator 153, a high frequency amplifier 154, and a high frequency coil (transmission coil) 151 on the transmission side are provided. The high frequency oscillator 152 generates an RF pulse. The modulator 153 amplitude-modulates the output RF pulse in accordance with a command from the sequencer 140. The high-frequency amplifier 154 amplifies the amplitude-modulated RF pulse and supplies the amplified RF pulse to the transmission coil 151 disposed close to the subject 101. The transmission coil 151 irradiates the subject 101 with the supplied RF pulse.

  The receiving unit 160 detects a nuclear magnetic resonance signal (echo signal, NMR signal) emitted by nuclear magnetic resonance of the nuclear spin constituting the living tissue of the subject 101, and receives a high-frequency coil (receiving coil) on the receiving side. 161, a signal amplifier 162, a quadrature detector 163, and an A / D converter 164. The reception coil 161 is disposed in the vicinity of the subject 101 and detects an NMR signal of the response of the subject 101 induced by the electromagnetic wave irradiated from the transmission coil 151. The detected NMR signal is amplified by the signal amplifier 162 and then divided into two orthogonal signals by the quadrature phase detector 163 at the timing according to the command from the sequencer 140, and each is digitally converted by the A / D converter 164. It is converted into a quantity and sent to the control processing unit 170.

  The transmission coil 151 and the gradient magnetic field coil 131 are opposed to the subject 101 in the static magnetic field space of the static magnetic field generating unit 120 into which the subject 101 is inserted, if the vertical magnetic field method is used, and if the horizontal magnetic field method is used. It is installed so as to surround the subject 101. Further, the receiving coil 161 is installed so as to face or surround the subject 101.

  The sequencer 140 instructs each unit to apply an RF pulse and a gradient magnetic field pulse and measure an echo signal generated by the subject 101 in accordance with an instruction from the control processing unit 170. Specifically, in accordance with instructions from the control processing unit 170, various commands necessary for collecting tomographic image data of the subject 101 are transmitted to the transmission unit 150, the gradient magnetic field generation unit 130, and the reception unit 160.

  The control processing unit 170 controls the entire MRI apparatus 100, performs various data processing operations, displays and stores processing results, and includes a CPU 171, a storage device 172, a display device 173, and an input device 174. The storage device 172 includes an internal storage device such as a hard disk and an external storage device such as an external hard disk, an optical disk, and a magnetic disk. The display device 173 is a display device such as a CRT or a liquid crystal. The input device 174 is an interface for inputting various control information of the MRI apparatus 100 and control information of processing performed by the control processing unit 170, and includes, for example, a trackball or a mouse and a keyboard. The input device 174 is disposed in the vicinity of the display device 173. The operator interactively inputs instructions and data necessary for various processes of the MRI apparatus 100 through the input device 174 while looking at the display device 173. Note that the display device 173 such as a touch panel may also function as the input device 174.

  The CPU 171 implements each process of the control processing unit 170 such as control of the operation of the MRI apparatus 100 and various data processes by executing a program previously stored in the storage device 172 in accordance with an instruction input by the operator.

  In the present embodiment, the CPU 171 functions as a measurement control unit that instructs the sequencer 140 in accordance with a pulse sequence previously stored in the storage device 172 by executing a program. When data from the receiving unit 160 is input to the control processing unit 170, signal processing, image reconstruction processing, and the like are executed, and a tomogram of the subject 101 as a result is displayed on the display device 173, and It functions as an image reconstruction unit stored in the storage device 172.

  Currently, the nuclide to be imaged by the MRI apparatus, which is widely used clinically, is a hydrogen nucleus (proton) that is a main constituent material of the subject 101. In the MRI apparatus 100, information on the spatial distribution of the proton density and the spatial distribution of the relaxation time of the excited state is imaged so that the form or function of the human head, abdomen, limbs, etc. can be expressed two-dimensionally or three-dimensionally. Take an image.

[Description of slice position displacement]
Prior to the description of the pulse sequence of the present embodiment, it will be described using FIG. 2A to FIG. 2C that the slice position is shifted (displaced) when the resonance frequency of the substance is different. Here, water and fat will be described as examples. The resonance frequency of water is Fw, and the resonance frequency of fat is Ff. The difference between the resonance frequencies of both is ΔF.

  In the MRI sequence 200, as shown in FIG. 2A, the selective excitation RF pulse 210 and the slice selective gradient magnetic field 220 are simultaneously applied to determine the position of the slice from which data is collected. That is, the slice position is determined by the frequency of the selective excitation RF pulse 210 and the gradient of the slice selective gradient magnetic field 220. Here, in this specification, a slice range having a predetermined thickness centered on a predetermined slice position is simply referred to as a slice position.

  Here, the center frequency of the selective excitation RF pulse 210 is adjusted to the resonance frequency Fw of water, and the gradient of the gradient magnetic field 220 is determined so that the slice at the desired position Zw is excited. Then, as shown in FIGS. 2B and 2C, for the proton of water (hereinafter simply referred to as water), the slice position at the desired position Zw is excited and data is collected from the slice position. can do. However, for fat protons (hereinafter simply referred to as fat), a position Zf shifted by a distance ΔZ corresponding to the frequency difference ΔF is excited, and data from the slice position is collected.

  As described above, when a slice is selected by a combination of the normal selective excitation RF pulse 210 and the slice selective gradient magnetic field 220, data from different positions is collected for each substance having a different resonance frequency, and the artifact It is a factor. Conventionally, the resonance frequency of the selective excitation RF pulse 210 is changed, data is collected a plurality of times, and post-processing is devised.

[Pulse sequence]
In the present embodiment, the configuration of the selective excitation RF pulse is devised, and data from the target slice is obtained with high accuracy by collecting data once in a subject composed of a plurality of substances.

  In the present embodiment, a two-band excitation RF pulse that excites two frequency bands is used as the selective excitation RF pulse. The two frequency bands excited by the two-band excitation RF pulse are adjusted to the frequency difference of the chemical shift between the two target substances. That is, the two-band excitation RF pulse is applied together with the slice selective gradient magnetic field pulse, and thereby has a frequency band having a frequency band for exciting protons in the first substance of the target slice (hereinafter simply referred to as the first substance). It is obtained by superimposing one RF pulse and a second RF pulse having a frequency band for exciting protons (hereinafter simply referred to as a second substance) in the second substance of the same slice. Then, the two-band excitation RF pulse is divided and applied to a plurality of RF pulses. Each of the divided RF pulses has a total intensity equal to a predetermined excitation excitation RF pulse intensity, and is synchronized with a different precession due to a difference in resonance frequency between the magnetization of the first substance and the magnetization of the second substance. Thus, the magnetization of each material is designed to cancel each other at the time of excitation in a slice other than the desired slice.

  Details of the pulse sequence of the present embodiment for realizing this will be described. FIG. 3 is a pulse sequence 300 of the present embodiment. Hereinafter, in the present embodiment, a case where a two-band excitation RF pulse used as an excitation RF pulse is divided and applied in two will be described as an example.

  As shown in the figure, the pulse sequence 300 of the present embodiment includes a slice selection unit 301 that selects a target slice that is a target slice position, and an echo measurement unit 360 that measures an echo signal generated from the target slice. .

  The slice selection unit 301 includes a selective excitation RF pulse 310 and a slice selection gradient magnetic field pulse 320. The selective excitation RF pulse 310 is a two-band excitation RF pulse. The two-band excitation RF pulse is applied a plurality of times (here, twice) at a predetermined time interval Δt (311 and 312). Hereinafter, they are referred to as a first two-band excitation RF pulse 311 and a second two-band excitation RF pulse 312.

  The slice selective gradient magnetic field pulse 320 is applied only when the two-band excitation RF pulses 311 and 312 are applied, and the application is stopped during a period in which these are not applied (during the application interval Δt). Thereby, during this time, the transverse magnetization of both substances precess at their respective resonance frequencies.

  The two-band excitation RF pulses 311 and 312 are applied together with the slice selective gradient magnetic field pulse 320 to thereby excite the magnetization of the first substance in the target slice, and the first one-band excitation RF pulse (RF pulse 01). And the slice selective gradient magnetic field pulse 320 to be applied together with the second one-band excitation RF pulse (RF pulse 02) for exciting the magnetization of the second substance in the target slice. . That is, the two-band excitation RF pulses 311 and 312 have waveforms obtained by adding the RF pulse 01 and the RF pulse 02, respectively.

  Note that, as described above, the two-band RF pulse of the present embodiment excites two different slice positions for the same substance.

  Further, when the RF pulse 01 and the RF pulse 02 are added, the phase of these RF pulses is such that the magnetization of the first substance and the magnetization of the second substance are respectively applied at the time of excitation other than the target slice. Are determined to be alternately reversed.

  Specifically, in the first two-band excitation RF pulse 311, the RF pulse 01 and the RF pulse 02 are added in the same phase. On the other hand, in the second two-band excitation RF pulse 312, the RF pulse 01 and the RF pulse 02 are added in opposite phases. That is, one of the RF pulse 01 and the RF pulse 02 is changed (inverted) by π and added. In addition, an antiphase means the state from which two phase differences become an odd multiple of (pi).

For example, when the center frequencies of the excitation bands are f ₁ and f ₂ , respectively, the first two-band excitation RF pulse 311RFp1 and the second two-band excitation RF pulse 312RFp2 are expressed by the following equations (1) and (2), respectively. An RF having a carrier frequency represented by At this time, for slice selective excitation, the amplitude A (t) that changes according to time t is, for example, a sinc shape.
RFp1 = A (t) * exp (i2πf ₁ t) + A (t) * exp (i2πf ₂ t) (1)
RFp2 = A (t) * exp (i2πf ₁ t) -A (t) * exp (i2πf ₂ t) (2)

  In the present embodiment, in the second two-band excitation RF pulse 312, among the RF pulse 01 and the RF pulse 02, a substance having a small resonance frequency is selected from the first substance and the second substance in the target slice. The phase of the RF pulse to be excited is inverted and added. For example, when the resonance frequency of the second substance is smaller, the phase of the RF pulse 02 is inverted and added.

  In the second two-band excitation RF pulse 312, the phase of one of the RF pulse 01 and the RF pulse 02 is inverted, so that the phase is inverted by π between the excitations in the two bands.

  It should be noted that the two RF pulses to be superimposed need only have the same frequency selection profile, and may not have the same shape. The limit of the RF output depends on the hardware performance of the MRI apparatus 100 and the specific absorption rate (SAR). For this reason, the RF shape may be changed so that the maximum output becomes as small as possible, or the relative shift in the time direction may be performed.

  In addition, the application interval Δt between the first two-band excitation RF pulse 311 and the second two-band excitation RF pulse 312 is a time during which the transverse magnetization of the first substance and the transverse magnetization of the second substance are in opposite phases. Interval. Application of the first two-band excitation RF pulse 311 causes precession of the transverse magnetization of the first substance and the transverse magnetization of the second substance at the respective resonance frequencies. At this time, since the resonance frequencies of the two are different, both transverse magnetizations are gradually shifted. A time during which the deviation amount is an odd multiple of π is set as an application interval Δt. The application interval Δt is calculated from the difference in resonance frequency between the two substances.

For example, if the resonance frequency difference is ΔF, the application interval Δt can be calculated by the following equation (3).
Δt = 1 / (2 · ΔF) (3)

  Note that the change in the phase of transverse magnetization due to precession is affected by the uniformity of the static magnetic field and the characteristics of the gradient magnetic field. That is, it is affected by the residual magnetic field. For this reason, in calculating the application interval Δt, the residual magnetic field may be added to the resonance frequency difference.

  Further, the intensity of the first two-band excitation RF pulse 311 and the second two-band excitation RF pulse 312 is 1: 1. In addition, the total intensity is determined so as to be the intensity when initially applied as one two-band excitation RF pulse (selective excitation RF pulse 310).

  For example, when the flip angle of the selective excitation RF pulse 310 is initially set to α, the flip angles of the first two-band excitation RF pulse 311 and the second two-band excitation RF pulse 312 are respectively α / 2 and To do.

  In the pulse sequence 300 of the present embodiment, the first two-band excitation RF pulse 311, the second two-band excitation RF pulse 312, and two slice selection gradient magnetic field pulses 320 are applied to excite the target slice, and then echo measurement is performed. The unit 360 applies the phase encode pulse 330 and the frequency encode pulse 340 as in normal imaging, and acquires the echo signal 350.

[Excited slice position]
A state of excitation of the target slice by the selective excitation RF pulse 310 and the slice selection gradient magnetic field 320 of the slice selection unit 301 of the pulse sequence 300 will be specifically described with reference to FIG. Here, the first substance will be described as water and the second substance as fat.

  Let the target slice position be Z2. The excitation intensity of the first two-band excitation RF pulse 311 and the second two-band excitation RF pulse 312 are both α / 2, and the second two-band excitation RF pulse 312 Are reversed and superimposed.

  First, the first two-band excitation RF pulse 311 is applied. Here, the first two-band excitation RF pulse 311 excites the water of the target slice position (target slice) Z2 by α / 2 and the fat of the target slice Z2 by α / 2. The RF pulse 02 is added. Therefore, first, water and fat at these positions are excited by α / 2, respectively (411, 422). In the following, in this figure, 411 to 425 schematically indicate the excitation position (horizontal axis direction) and the flip angle (vertical axis direction).

  By the RF pulse 01 of the first two-band excitation RF pulse 311, the fat at the position Z1 displaced by ΔZ is also excited by α / 2 from the target slice Z2 (421). Also, the RF pulse 02 excites the water at the position Z3 displaced by ΔZ in the opposite direction to the fat from the target slice position Z2 by α / 2 (412).

  The distance ΔZ between the centers of the slice position Z1 and the slice position Z2 is equal to the distance ΔZ between the centers of the slice position Z2 and the slice position Z3, and the difference in excitation position due to the resonance frequency difference ΔF between water and fat. be equivalent to.

  Next, after Δt has elapsed, the second two-band excitation RF pulse 312 is applied. The second two-band excitation RF pulse 312 is overlapped by changing the phase of the RF pulse 02 by π. Therefore, the water of the target slice Z2 is further excited by α / 2 by the RF pulse 01 of the second two-band excitation RF pulse 312 (413), and the water at the position Z3 displaced by ΔZ is reversed by the RF pulse 02. Excited by α / 2 in the direction (414).

  Similarly, the fat at the position Z1 displaced by ΔZ from the target slice Z2 is excited by α / 2 by the RF pulse 01 of the second two-band excitation RF pulse 312 (423). Thus, the fat of the target slice Z2 is excited by α / 2 in the opposite direction (424).

  However, as described above, between the application of the first two-band excitation RF pulse 311 and the application of the second two-band excitation RF pulse 312 (Δt), the transverse magnetization of fat and the transverse magnetization of water Since the phase difference is an odd multiple of π, when considering water excitation as a reference, the fat is α / 2 in the reverse direction at the position Z1 by the RF pulse 01 of the second two-band excitation RF pulse 312. The fat of the target slice Z2 is further excited by α / 2 in the same direction as the water at the same position by the same RF pulse 02 (424).

  As a result, for the target slice Z2, both water and fat continue to be tilted in a certain direction, and only α is excited (415, 425), and for the other positions Z1 and Z3, the phase is inverted at every excitation for both water and fat. The net flip angle becomes 0 and is canceled (426, 417).

[Movement of magnetization]
The movement of magnetization at each slice position Z1, Z2, and Z3 by the pulse sequence 300 will be described with reference to FIGS. 5 (a), 5 (b), 6 (a), and 6 (b). Here, it is assumed that the flip angle of the selective excitation RF pulse 310 is set to 90 degrees. That is, the flip angles of the first two-band excitation RF pulse 311 and the second two-band excitation RF pulse 312 are 45 degrees, respectively.

  In addition, during the application interval Δt between the first two-band excitation RF pulse 311 and the second two-band excitation RF pulse 312, the transverse magnetization of water is rotated by 2π, and the transverse magnetization of fat is rotated by π. To do. Furthermore, it is assumed that the phase of the RF pulse 02 is reversed in the second two-band excitation RF pulse 312. The static magnetic field direction is the z-axis direction. In these figures, the magnetization is represented by a rotating system based on the resonance frequency at the slice position Z1.

  First, the movement of water and fat magnetization in the target slice Z2 will be described.

  As shown in FIG. 5A, the magnetization 511 of water in the target slice Z2 is initially directed in the z-axis direction. Here, when the first two-band excitation RF pulse 311 is applied, it is excited by 45 degrees by the RF pulse 01 (512). Thereafter, the transverse magnetization rotates by 2π during Δt (513). Next, when the second two-band excitation RF pulse 312 is applied, the RF pulse 01 further excites 45 degrees, and as a result, the state is tilted 90 degrees from the initial state (514).

  As shown in FIG. 5B, the fat magnetization 521 in the target slice Z2 is initially oriented in the z-axis direction. Here, when the first two-band excitation RF pulse 311 is applied, it is excited 45 degrees by the RF pulse 02 (522). Thereafter, the transverse magnetization rotates by π during Δt (523). Next, when the second two-band excitation RF pulse 312 is applied, the RF pulse 02 whose phase is inverted by π is further tilted 45 degrees, and as a result, the state is tilted 90 degrees from the initial state. (425).

  As shown in FIG. 6A, the magnetization 531 of the water in the slice Z3 is initially directed in the z-axis direction. Here, when the first two-band excitation RF pulse 311 is applied, it is excited 45 degrees by the RF pulse 02 (532). Thereafter, the transverse magnetization rotates 2π during Δt (533). Next, when the second two-band excitation RF pulse 312 is applied, the RF pulse 02 whose phase is inverted by π is returned 45 degrees in the z-axis direction, and as a result, the initial state is restored (534).

  As shown in FIG. 6B, the fat magnetization 541 in the slice Z1 is initially oriented in the z-axis direction. Here, when the first two-band excitation RF pulse 311 is applied, it is excited 45 degrees by the RF pulse 01 (542). Thereafter, the transverse magnetization rotates by π during Δt (543). Next, when the second two-band excitation RF pulse 312 is applied, the RF pulse 02 inverted by the phase π is returned 45 degrees in the z-axis direction, and as a result, the initial state is restored (544).

  As described above, the MRI apparatus 100 according to the present embodiment includes the measurement control unit that performs measurement by controlling each unit according to the predetermined pulse sequence 300, and the pulse sequence 300 is a target slice that is a target slice position. The slice selection unit 301 includes a selective excitation RF pulse 310 and a slice selection gradient magnetic field pulse 320, and the selective excitation RF pulse 310 is applied twice at a predetermined application interval. Two-band excitation RF pulses 311 and 312 are provided.

  The two-band excitation RF pulses 311 and 312 are applied together with the slice selective gradient magnetic field pulse 320 to excite the magnetization of the first substance of the target slice, and the slice selective gradient magnetic field. It is obtained by adding together with a second RF pulse 02 which excites the magnetization of a second substance having a resonance frequency different from that of the first substance in the target slice by applying with the pulse 320.

  In addition, the phases of the first RF pulse 01 and the second RF pulse 02 are determined so that the magnetization of the first substance and the magnetization of the second substance are applied each time the excitation is performed except for the target slice. The amplitude ratio of each of the two-band excitation RF pulses 311 and 312 that are determined to be inverted alternately and applied twice is set to follow a binomial distribution.

  Thus, according to the present embodiment, by devising the selective excitation RF pulse 310 applied for slice selection, protons of two substances having different resonance frequencies can be excited only for the target slice position. Can do. For this reason, unintended slices are not excited due to different resonance frequencies.

  Therefore, according to the present embodiment, data from a desired slice can be acquired with high accuracy without multiple imaging and post-processing. Therefore, according to the present embodiment, it is possible to selectively excite a target slice with a simple configuration and without a displacement of the excitation slice due to a chemical shift while suppressing an increase in imaging time.

  According to the present embodiment, since the target slice can be excited with high accuracy, data from non-target slices is not mixed, and the accuracy of image quality is improved accordingly. Further, since the imaging time is not extended, the burden on the subject is small. Furthermore, since multiple data collections are not required, artifacts due to movement of the subject during multiple data collections do not occur.

<Modification of number of divisions>
In the above embodiment, the selective excitation RF pulse 310 that is a two-frequency band RF pulse is divided and applied to the first two-band excitation RF pulse 311 and the second two-band excitation RF pulse 312. However, the number of divisions is not limited to 2 and may be 3 or more. In this case, the intensity (amplitude) ratio of each divided two-band excitation RF pulse is a ratio (value) according to the binomial distribution. Then, the total intensity of the divided two-band excitation RF pulses is set to be the target flip angle of the selective excitation RF pulse 310.

  For example, when dividing into two two-band excitation RF pulses, the intensity ratio is 1: 2: 1. Further, when dividing into four two-band excitation RF pulses, the intensity ratio is set to 1: 3: 3: 1.

  Each of the divided two-band excitation RF pulses is obtained by adding the RF pulse 01 and the RF pulse 02 described above. At this time, they are added in the same phase when odd-numbered times are applied, and are added in opposite phases when even-numbered times are applied. In addition, at the time of addition for the even number, the phase on the RF pulse side that excites the substance having the smaller resonance frequency in the target slice is inverted by π to obtain an opposite phase.

  In addition, as described above, in each divided two-band excitation RF pulse, the transverse magnetization of the first substance and the transverse magnetization of the second substance are in opposite phases (the phase difference between the two substances is an odd multiple of π). Apply at time interval Δt.

  As described above, the application interval Δt is calculated from the frequency difference between the chemical shifts of both substances. Therefore, when the static magnetic field inhomogeneity is sufficiently small, the phase difference between the first substance and the second substance (for example, water and fat) is actually π at the time interval of Δt. However, since there is actually a static magnetic field inhomogeneity, the phase difference between them does not become π at this time interval Δt.

When the number of divisions of the selective excitation RF pulse 310 is N (N is an integer of 2 or more), n = N−1 and the phase error θ due to the static magnetic field inhomogeneity are used, and the first substance (for example, water) The signal strength S is expressed by the following formula (4).
S (θ) = | cos ⁿ (θ) | (4)
Therefore, as the number of divisions is increased, the selectivity for static magnetic field inhomogeneity is improved by multiplying by cos (θ). This is because as the number of divisions N increases and the power of | cos (θ) | increases, S (θ) approaches a rectangle and resistance to static magnetic field inhomogeneity increases.

<Modification 2>
Moreover, in the said embodiment, when the selective excitation RF pulse 310 of this embodiment is used with the slice selection gradient magnetic field 320, a specific slice is excited, and the image data of the said slice is acquired using arbitrary pulse sequences after that. However, the use of the selective excitation RF pulse 310 of the present embodiment is not limited to this.

  The selective excitation RF pulse 310 of the present embodiment may be used together with the slice selective gradient magnetic field pulse 320 and used as pre-saturation for suppressing a signal from the selected region in an arbitrary pulse sequence.

  In this case, for example, the selective excitation RF pulse 310 of this embodiment is used together with the slice selection gradient magnetic field 320, and after exciting a specific slice, the transverse magnetization is spoiled using the gradient magnetic field pulse or the like. This suppresses the signal from this slice.

  In addition, by using the selective excitation RF pulse 310 of the present embodiment as a presaturation pulse, for widely grasping hemodynamics by removing body movements and pulsation artifacts and suppressing blood flow signals as widely used in general. Can be used.

<Substance species>
In the above embodiment and the modification, the case where the first substance is water and the second substance is fat has been described as an example, but the first substance and the second substance are not limited to this. . Any substance having a different resonance frequency may be used.

  Further, in the above embodiment and the modification, the case where the target slice position is excited with high accuracy has been described as an example for two substances having different resonance frequencies. For example, when there are three substances having different resonance frequencies, the same processing as in the case of two substances is applied to two substances having a small chemical shift difference by using the average resonance frequency. In some cases, the slice position can be excited with high accuracy.

100: MRI apparatus, 101: subject, 120: static magnetic field generation unit, 130: gradient magnetic field generation unit, 131: gradient magnetic field coil, 132: gradient magnetic field power supply, 140: sequencer, 150: transmission unit, 151: transmission coil, 152: High-frequency oscillator, 153: Modulator, 154: High-frequency amplifier, 160: Receiver, 161: Receiver coil, 162: Signal amplifier, 163: Quadrature detector, 164: D converter, 170: Control processor, 171 : CPU, 172: storage device, 173: display device, 174: input device, 200: conventional pulse sequence, 210: selective excitation RF pulse, 220: slice selective gradient magnetic field, 220: gradient magnetic field, 300: pulse sequence, 301 : Slice selection unit, 310: selective excitation RF pulse, 311: two-band excitation RF pulse, 312: two-band excitation F pulse, 320: slice selective gradient magnetic field pulse, 330: phase encode pulse, 340: frequency encode pulse, 350: echo signal, 360: echo measurement unit, 411: water by RF pulse 01 of the first two-band excitation RF pulse 412: excitation slice position of water by RF pulse 02 of the first two-band excitation RF pulse, 413: excitation slice position of water by RF pulse 01 of the second two-band excitation RF pulse, 414: first Excitation slice position of water by RF pulse 02 of two two-band excitation RF pulses, 415: Excitation slice position of water by selective excitation RF pulse, 417: Excitation mode of water outside target slice, 421: First two-band excitation Excitation slice position of fat by RF pulse 01 of RF pulse 422: first two-band excitation RF path Excitation position of fat by RF pulse 02 of the pulse 423: Excitation slice position of fat by the RF pulse 01 of the second two-band excitation RF pulse 424: Fat fat slice by the RF pulse 02 of the second two-band excitation RF pulse Excitation slice position, 425: Excitation position of fat by selective excitation RF pulse, 426: Excitation mode of fat outside target slice, 511: Magnetization of water in target slice, 512: Magnetization of water in target slice, 513: Target slice 514: magnetization of water in the target slice, 521: magnetization of fat in the target slice, 522: magnetization of fat in the target slice, 523: magnetization of fat in the target slice, 524: magnetization of fat in the target slice, 531: Magnetization of water outside the target slice, 532: Magnetization of water outside the target slice, 533: Magnetism of water outside the target slice 534: magnetization of water outside the target slice, 541: magnetization of fat outside the target slice, 542: magnetization of fat outside the target slice, 543: magnetization of fat outside the target slice, 544: fat magnetization outside the target slice Magnetization

Claims (9)

A measurement control unit that performs measurement by controlling each unit according to a predetermined pulse sequence,
The pulse sequence includes a slice selection unit that selects a target slice that is a target slice position,
The slice selection unit includes a selective excitation RF pulse and a slice selection gradient magnetic field pulse,
The selective excitation RF pulse includes a two-band excitation RF pulse applied a plurality of times at a predetermined application interval,
Each of the two-band excitation RF pulses is applied together with the slice selective gradient magnetic field pulse, the first RF pulse for exciting the magnetization of the first substance of the target slice, and the slice selective gradient magnetic field pulse. And having a waveform obtained by adding a second RF pulse for exciting the magnetization of a second substance having a resonance frequency different from that of the first substance in the target slice,
The phases of the first RF pulse and the second RF pulse are such that the magnetization of the first substance and the magnetization of the second substance are alternately reversed at each application time during excitation other than the target slice. Determined
The magnetic resonance imaging apparatus, wherein an amplitude ratio of each of the two-band excitation RF pulses applied a plurality of times is set to follow a binomial distribution. The magnetic resonance imaging apparatus according to claim 1,
The first RF pulse and the second RF pulse are added in the same phase at the time of odd-numbered application and in the opposite phase at the time of even-numbered application,
The magnetic resonance imaging apparatus, wherein the application interval is a time interval in which the transverse magnetization of the first substance and the transverse magnetization of the second substance are in opposite phases. The magnetic resonance imaging apparatus according to claim 2,
During the even-numbered application of the two-band excitation RF pulse, among the first RF pulse and the second RF pulse, among the first substance and the second substance in the target slice, A magnetic resonance imaging apparatus characterized in that the phase of an RF pulse for exciting a substance having a low resonance frequency is inverted and added. The magnetic resonance imaging apparatus according to claim 2 or 3,
The application interval is calculated by adding a residual magnetic field to a resonance frequency difference between the first substance and the second substance. The magnetic resonance imaging apparatus according to any one of claims 1 to 4,
The amplitude of each of the two-band excitation RF pulses applied a plurality of times is set such that the sum of the amplitudes of all the two-band excitation RF pulses becomes a value of a flip angle set in the selective excitation RF pulse. Magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to any one of claims 1 to 5,
The pulse sequence includes an echo measurement unit that measures an echo signal generated from the target slice after the slice selection unit. The magnetic resonance imaging apparatus according to any one of claims 1 to 5,
2. The magnetic resonance imaging apparatus according to claim 1, wherein the pulse sequence includes a spoil unit that spoils the magnetization of the target slice after the slice selection unit. The magnetic resonance imaging apparatus according to any one of claims 1 to 7,
The first substance is water;
The magnetic resonance imaging apparatus, wherein the second substance is fat. Selectively excite a desired slice by applying a two-band excitation RF pulse together with a slice selective gradient magnetic field pulse a plurality of times at a predetermined application interval,
Each of the two-band excitation RF pulses is applied together with the slice selective gradient magnetic field pulse, the first RF pulse for exciting the magnetization of the first substance of the target slice, and the slice selective gradient magnetic field pulse. And having a waveform obtained by adding a second RF pulse for exciting the magnetization of a second substance having a resonance frequency different from that of the first substance in the target slice,
The phases of the first RF pulse and the second RF pulse are determined in accordance with the phase of the transverse magnetization of the first substance and the transverse magnetization of the second substance at the time of application at the time of excitation other than the target slice. Alternately determined to have opposite phases,
The slice selective excitation method, wherein an amplitude ratio of each of the two-band excitation RF pulses applied a plurality of times is set to follow a binomial distribution.

Images