JP2012070964A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus capable of suppressing signals from a predetermined area in an imaging area with few excitation pulses.SOLUTION: In the imaging area including a subject, two partially overlapping areas S11 and S12 are determined. At the time, a signal suppression area is included in the non-overlapping area of the two areas, and a signal read area is included in the overlapping area of the two areas. Then, by applying an inversion pulse to each of the two areas, magnetic resonance signals are suppressed by inverting longitudinal magnetization of a spin once for the non-overlapping area, and the longitudinal magnetization of a spin is inverted twice to be practically returned to the original for the overlapping area W1. Thereafter, at the point of time when the longitudinal magnetization of the excited spin in the non-overlapping area becomes a null point, a pulse sequence for collecting signals is started, and the magnetic resonance signals from the overlapping area W1 are collected.

Description

  The present invention relates to a magnetic resonance imaging apparatus that suppresses magnetic resonance signals from a predetermined region of a subject.

As one of diagnostic techniques using a magnetic resonance imaging apparatus, magnetic resonance spectroscopy (MRS)
Resonance Spectroscopy) is known. This MRS is a technique for obtaining information about a metabolite based on the frequency of a magnetic resonance signal by utilizing the fact that the spin resonance frequency varies slightly depending on the type of metabolite. For example, chemical shift imaging (CSI) is performed to image the frequency of magnetic resonance signals and obtain information such as identification of each metabolite, its concentration, and relaxation time.

  By the way, the signal of the metabolite to be measured in the proton MRS such as the brain is very small compared to the signal from free water (hereinafter referred to as water signal) and the signal from fat (hereinafter referred to as fat signal). Yes, if these free water and fat are mixed in the measurement site, it becomes an obstacle to observation. For this reason, in such an MRS, water signals and fat signals are usually suppressed.

  Since fat is abundantly contained in tissues other than the brain in the head, conventionally, the OVS (Outer Volume Suppression) method is used to suppress fat signals (see Patent Document 1, FIG. 1, FIG. 2, etc.). That is, after exciting a slice perpendicular to the imaging section, a gradient magnetic field is applied to dephase the transverse magnetization of spins in this slice. This process is repeated while sequentially changing the slices until the fat region is covered by each slice. The OVS method is also called a spatial pre-saturation method, a spatial pre-saturation method, or Spatial-SAT.

  FIG. 7 is a diagram illustrating an example in which the OVS method is applied to MRS of the head. To explain with reference to this figure, in the imaging region B including the head 41 of the subject, the central portion of the head is the region of interest R from which the magnetic resonance signal is read out, and is distributed in a substantially elliptical shape around the head, that is, near the skull. A plurality of slices (eight sheets S1 to S8 in this example) are set so as to cover the fat region to be processed. Then, spin excitation and dephase are performed on these slices.

  FIG. 8 is a diagram showing an example of a CSI pulse sequence including a conventional fat signal suppression sequence by the OVS method as a pre-process. In the previous step of suppressing the fat signal, excitation is repeated for the number of slices necessary to cover the fat region.

JP 2006-346055 A

  As described above, in the conventional signal suppression method, excitation is performed so as to cover a region where the magnetic resonance signal is to be suppressed with a plurality of slices. As a result, the spin excited by the high-frequency pulse starts to recover the signal intensity immediately, so that the recovery of the longitudinal magnetization varies, and it is difficult to sufficiently suppress the magnetic resonance signal from the region to be suppressed. Further, when the number of slices increases, not only the setting of slices becomes complicated, but also the number of excitation pulses irradiated to the subject increases, so the burden on the subject also increases.

  Under such circumstances, there is a demand for a magnetic resonance imaging apparatus that can suppress unnecessary magnetic resonance signals from a predetermined region in a subject with a small number of excitation pulses.

  According to the first aspect of the present invention, an inversion pulse is applied to two regions that are determined to partially overlap in an imaging region including a subject, whereby the two regions do not overlap. For the region, the longitudinal magnetization of the spin is reversed once to suppress the magnetic resonance signal from the non-overlapping region, and for the overlapping region of the two regions, the longitudinal magnetization of the spin is reversed twice to substantially return to the original region. Then, a magnetic resonance imaging apparatus is provided that collects magnetic resonance signals from the overlap region by performing a pulse sequence for signal collection.

  The invention according to the second aspect is the magnetism according to the first aspect, in which a crusher gradient pulse is applied after each application of the inversion pulse to dephase the transverse magnetization of the spin in the non-overlapping region. A resonance imaging apparatus is provided.

  The invention according to a third aspect provides the magnetic resonance imaging apparatus according to the first aspect or the second aspect, wherein the non-overlapping region includes a fat region.

  According to a fourth aspect of the invention, in the magnetic resonance according to any one of the first to third aspects, the signal acquisition pulse sequence includes a magnetic resonance spectroscopy (MRS) pulse sequence. An imaging apparatus is provided.

  A fifth aspect of the invention provides the magnetic resonance imaging apparatus according to any one of the first to fourth aspects, wherein the imaging region is a region including the head of the subject.

  The invention of the sixth aspect is based on the first aspect, wherein one of the two areas is a non-selective area in the imaging area, and the other of the two areas is a selective area in the imaging area. A magnetic resonance imaging apparatus according to any one of the fifth aspects is provided.

  A seventh aspect of the invention provides the magnetic resonance imaging apparatus according to any one of the first to sixth aspects, wherein the inversion pulse has a flip angle of substantially 180 °. To do.

  The invention according to an eighth aspect provides the magnetic resonance imaging apparatus according to the seventh aspect, wherein the inversion pulse is an adiabatic pulse (adiabatic pulse).

  According to a ninth aspect of the present invention, there is provided the eighth aspect from the first aspect, wherein the signal collection pulse sequence is started when the longitudinal magnetization of the spin in the non-overlapping region becomes a null point. A magnetic resonance imaging apparatus according to any one of the above aspects is provided.

  In the invention of the above aspect, the “subject” includes not only a human body but also an animal. Further, the magnetic resonance imaging apparatus according to the above aspect of the invention is not limited to medical use but can be applied to industrial use.

  According to the invention of the above aspect, in the preceding step, by applying an inversion pulse to two regions that partially overlap each other, magnetic resonance signals from non-overlapping regions of the two regions are suppressed, and the 2 Magnetic resonance signals from overlapping regions of two regions can be obtained, and unnecessary magnetic resonance signals from predetermined regions in the subject can be suppressed with a small number of excitation pulses.

1 is a diagram schematically illustrating a configuration of a magnetic resonance imaging apparatus according to a first embodiment. It is a figure for demonstrating suppression of the fat signal by 1st embodiment. It is a figure which shows an example of the pulse sequence implemented by 1st embodiment. It is a figure for demonstrating suppression of the fat signal by 2nd embodiment. It is a figure which shows an example of the pulse sequence implemented by 2nd embodiment. It is a figure which shows the change of the signal strength in the case where it applies only once inversion pulse, and the case where it applies twice. It is a figure for demonstrating the conventional signal suppression by an OVS method. It is a figure which shows an example of the conventional signal suppression sequence by OVS method.

  Embodiments of the invention will be described below.

(First embodiment)
FIG. 1 is a diagram schematically showing the configuration of the magnetic resonance imaging apparatus of the present embodiment.

  As shown in FIG. 1, the magnetic resonance imaging apparatus 1 includes a static magnetic field magnet (magnet) unit 12, a gradient coil unit 13, an RF (Radio Frequency) coil unit 14, an RF drive unit 22, a gradient drive unit 23, data (data). ) It has a collection unit 24, a subject transport unit 25, a control unit 30, a storage unit 31, an operation unit 32, an image reconstruction unit 33, and a display unit 34.

  The static magnetic field magnet unit 12 generates a static magnetic field and generates a static magnetic field space 11.

  The gradient coil unit 13 is supplied with a current and independently generates gradient magnetic fields in three axis directions, ie, a slice axis direction, a phase encode direction, and a frequency encode direction. Here, the frequency encoding direction, the phase encoding direction, and the slice axis direction correspond to the x direction, the y direction, and the z direction shown in FIG. 1, respectively.

  The RF coil unit 14 receives an electric current and applies an excitation RF pulse for exciting the spin of the subject 40 in the static magnetic field space 11.

  The RF drive unit 22 drives the RF coil unit 14 based on a control signal from the control unit 30 and applies an excitation RF pulse in the static magnetic field space 11.

  The gradient driving unit 23 drives the gradient coil unit 13 based on a control signal from the control unit 30 to generate a gradient magnetic field in the static magnetic field space 11.

  The data collection unit 24 performs phase detection on the echo (echo) signal (magnetic resonance signal) received by the RF coil unit 14 and performs AD (Analog-Digital) conversion to generate echo signal data. The data of the generated echo signal is output to the storage unit 31.

  The subject transport unit 25 transports the subject 40 into and out of the static magnetic field space 11 based on a control signal from the control unit 30.

  Based on the operation signal from the operation unit 32, the control unit 30 controls each unit of the RF drive unit 22, the gradient drive unit 23, the data collection unit 24, and the subject transport unit 25 so as to execute a predetermined pulse sequence. Send a signal to control.

  The storage unit 31 stores echo signal data collected by the data collection unit 24, image data obtained by image reconstruction processing by the image reconstruction unit 33, and the like.

  Under the control of the control unit 30, the image reconstruction unit 33 reads the echo signal data from the storage unit 31, and performs image reconstruction processing on the echo signal data to generate image data. The image data is output to the storage unit 31.

  The display unit 34 displays information necessary for operation of the operation unit 32, an image represented by the image data, and the like.

  The control unit 30, the storage unit 31, and the image reconstruction unit 33 are configured by a computer, for example.

  The operation of the magnetic resonance imaging apparatus of this embodiment will be described below.

  The magnetic resonance imaging apparatus 1 of the present embodiment uses a magnetic resonance spectroscopy (hereinafter referred to as MRS) pulse sequence designed to suppress echo signals (hereinafter referred to as fat signals) from a fat region of a subject. The echo signal is collected by executing, and an image is generated based on the echo signal. Thereby, an image of CSI in which the fat signal is suppressed is generated.

  In this example, the head MRS of the subject is assumed. In practice, as signal suppression, in addition to suppression of fat signals, echo signals from brain free water (hereinafter referred to as water signals) are suppressed by, for example, the CHESS (chemical shift selective) method. However, since suppression of the water signal is generally performed, illustration and description of a pulse sequence relating to suppression of the water signal are omitted here.

  FIG. 2 is a diagram for explaining fat signal suppression according to the first embodiment. FIG. 3 is a diagram illustrating an example of a pulse sequence performed according to the first embodiment.

  The pulse sequence shown in FIG. 3 is based on the MRS pulse sequence for signal collection, and the feature is in the pre-process for suppressing the fat signal surrounded by a broken line.

  First, in the imaging region B including the subject 40, two regions to which an inversion pulse for inverting the longitudinal magnetization of the spin is applied are determined so as to partially overlap. At this time, the region of interest for reading the echo signal is included in the overlapping region. Further, the non-overlapping region, that is, the region where only one of the two regions is present includes the fat region for which the echo signal is to be suppressed. Here, as shown in FIG. 2A, a rectangular region at the center of the brain in the head 41 of the subject 40 is set as a region of interest R, and the fat region F distributed in a substantially elliptical shape near the skull of the head 41 is used. Assume that the echo signal is suppressed. As shown in FIG. 2B, the first region S11, which is one of the two regions, substantially includes the region of interest R, and extends along the diagonally lower right direction (+ x direction and −y direction) in the drawing. The slice area extending in a selective manner is determined. Further, as shown in FIG. 2C, the second region S12, which is the other of the two regions, substantially includes the region of interest R and extends along the upper right diagonal direction (+ x direction and + y direction) in the drawing. A slice region is selectively determined. Thereby, most of the region of interest R is included in the overlapping region W1 of the first and second regions S11, S12 having the “X” shape, and the non-overlapping region K1 of the first and second regions S11, S12 is included. Most of the fat region F is included.

  Next, a pulse sequence for suppressing fat signals is performed. First, as shown in FIG. 3, gradient pulses Sx11 and Sy11 and a first inversion pulse RF11 are applied to select a slice of the first region S11, and an inversion pulse is applied to this region. This excites spins in the first region S11 to reverse its longitudinal magnetization. Note that this inversion pulse usually has a flip angle of substantially 180 ° so that the direction of longitudinal magnetization of the spin can be accurately reversed. Among them, an adiabatic pulse (adiabatic pulse) capable of uniformly reversing the longitudinal magnetization of the spin in a wide space is preferable. This is to eliminate the non-uniformity of the longitudinal magnetization excitation of the spin due to the non-uniformity of the static magnetic field, which is a problem particularly in a high magnetic field. After applying the first inversion pulse RF11, a crusher gradient pulse C11 is applied to dephase the transverse magnetization of the spin excited in the first region S11. Thereafter, as shown in FIG. 3, by applying gradient pulses Sx12 and Sy12 and a second inversion pulse RF12, the second region S12 is sliced and an inversion pulse is applied to this region. . This excites spins in the second region S12 to reverse its longitudinal magnetization. At this time, in the first region S11, the longitudinal magnetization of the spin has already been reversed once by the application of the first inversion pulse RF11. Therefore, in the overlapping region W1 of the first region S11 and the second region S12, the application of the second inversion pulse RF12 causes the second inversion of the spin longitudinal magnetization, and the direction of the longitudinal magnetization is based on the original direction. Will return. That is, in the overlapping region W1 of the first region S11 and the second region S12, as shown in FIG. 6A, the longitudinal magnetization of the spin remains substantially the same, and the echo signal suppressing effect hardly acts. On the other hand, in the non-overlapping region K1 of the first region S11 and the second region S12, as shown in FIG. 6 (b), the longitudinal magnetization of the spin is reversed from the initial state, so that the MRS signal is collected at the null point. Starts, the suppression effect of the echo signal works.

  As a result, the echo signal from the region of interest R can be almost obtained, and the echo signal from the fat region F can be substantially suppressed. After the application of the second inversion pulse RF12, a crusher gradient pulse C12 is applied to dephase the transverse magnetization of the spin excited in the second region S12.

Next, the longitudinal magnetization of the excited spin in the non-overlapping region K1 recovers depending on the T1 value and waits for a time Tw until the null point is reached. Then, when the longitudinal magnetization reaches the null point, the implementation of the MRS pulse sequence for signal collection is started. Thereby, it is possible to collect a spectrum in which the intensity of the echo signal is reduced depending on the T1 value of the portion to be suppressed. As the MRS, for example, a PRESS (point-resolved) as shown in FIG.
spectroscopy) or the like. The PRESS method is a method of acquiring a spin echo signal by a slice selection pulse of 90 ° -180 ° -180 °.

(Second embodiment)
FIG. 4 is a diagram for explaining fat signal suppression according to the second embodiment. FIG. 5 is a diagram illustrating an example of a pulse sequence performed according to the second embodiment.

  First, in the imaging region 50 including the subject 40, two regions to which an inversion pulse for inverting the longitudinal magnetization of the spin is applied are determined so as to partially overlap. Here, as shown in FIG. 4B, a region including the entire region of the imaging region B is non-selectively determined as the first region S21 which is one of the two regions. Further, as shown in FIG. 4C, a region including the region of interest R and extending in the vertical direction (y direction) in the drawing is selectively determined as the second region S22 which is the other of the two regions. . Thereby, the region of interest R is included in the overlapping region W2 of the first and second regions S21, S22, and most of the fat region F is included in the non-overlapping region K2 of the first and second regions S21, S22. It is.

  Next, a sequence for suppressing fat signals is performed. First, as shown in FIG. 5, the first inversion pulse RF21 is applied, and the inversion pulse is applied to the first region S21. Thereby, the longitudinal magnetization of the spin in the first region S21 is excited and reversed. After applying the first inversion pulse RF21, a crusher gradient pulse C21 is applied to dephase the transverse magnetization of the spin excited in the first region S21. After that, as shown in FIG. 5, the gradient pulse Sx22 and the second inversion pulse RF22 are applied, the second region S22 is slice-selected, and the inversion pulse is applied to this region. As a result, the longitudinal magnetization of the spin in the second region S22 is excited and reversed. That is, in the overlapping region W2 of the first region S21 and the second region S22, the longitudinal magnetization of the spin remains as it is, and the echo signal suppressing effect does not work. On the other hand, in the non-overlapping region K2 of the first region S21 and the second region S22, since the longitudinal magnetization of the spin is reversed from the initial state, the effect of suppressing the echo signal works. As a result, the echo signal from the region of interest R can be almost obtained, and the echo signal from the fat region F can be substantially suppressed. After applying the second inversion pulse RF22, a crusher gradient pulse C22 is applied to dephase the transverse magnetization of the spin excited in the second region S22.

  Next, the process waits until the longitudinal magnetization of the excited spin recovers and reaches the null point, and when the longitudinal magnetization reaches the null point, the implementation of the MRS pulse sequence is started.

  As described above, according to the above-described embodiment, in the previous step, the echo signal from the non-overlapping region of the two regions is suppressed only by applying an inversion pulse to each of the two regions partially overlapping, An echo signal from an overlapping region of the two regions can be obtained, and an unnecessary echo signal from a predetermined region in the subject can be suppressed with a small number of excitation pulses.

  The embodiments of the invention are not limited to those described above, and various additions and changes can be made without departing from the spirit of the invention.

  For example, in the above embodiment, the crusher gradient pulse is applied after the inversion pulse is applied and the transverse magnetization of the spin is dephased. However, when the transverse magnetization signal is read, the crusher gradient pulse may not be applied. Conceivable.

  In the above-described embodiment, signal suppression is performed using only two inversion pulses in the previous process, but signal suppression can be performed more accurately by applying a conventional OVS method or tissue selective presaturation method. You can also do it often.

  In the above embodiment, fat signals are suppressed, but signals from other tissues and substances may be suppressed.

DESCRIPTION OF SYMBOLS 1 Magnetic resonance imaging apparatus 12 Static magnetic field magnet part 13 Gradient coil part 14 RF coil part 22 RF drive part 23 Gradient drive part 24 Data collection part 25 Subject transport part 30 Control part 31 Storage part 32 Operation part 33 Image reconstruction part 34 Display unit 40 Subject

Claims (9)

  By applying an inversion pulse to each of the two regions that are determined to overlap in the imaging region including the subject, the longitudinal magnetization of the spin is reversed once for the non-overlapping regions of the two regions. The magnetic resonance signal from the non-overlapping region is suppressed, and for the overlapping region of the two regions, the longitudinal magnetization of the spin is reversed twice to substantially return to the original state, and then the signal collection pulse sequence is performed. A magnetic resonance imaging apparatus that collects magnetic resonance signals from the overlapping region.   2. The magnetic resonance imaging apparatus according to claim 1, wherein a transverse magnetization of a spin in the non-overlapping region is dephased by applying a crusher gradient pulse after each application of the inversion pulse.   The magnetic resonance imaging apparatus according to claim 1, wherein the non-overlapping region includes a fat region.   4. The magnetic resonance imaging apparatus according to claim 1, wherein the signal acquisition pulse sequence includes a magnetic resonance spectroscopy (MRS) system sequence. 5.   The magnetic resonance imaging apparatus according to any one of claims 1 to 4, wherein the imaging region is a region including a head of the subject.   The one of the two regions is a non-selective region in the imaging region, and the other of the two regions is a selective region in the imaging region. Magnetic resonance imaging equipment.   The magnetic resonance imaging apparatus according to claim 1, wherein the inversion pulse has a flip angle of substantially 180 °.   The magnetic resonance imaging apparatus according to claim 7, wherein the inversion pulse is an adiabatic pulse.   9. The magnetic resonance imaging apparatus according to claim 1, wherein execution of the signal acquisition pulse sequence is started at a time point when the longitudinal magnetization of the spin in the non-overlapping region becomes a null point.