KR20170076462A

KR20170076462A - Method for suppressing water signal using double on resonance RF pulse in MR scanning

Info

Publication number: KR20170076462A
Application number: KR1020150186749A
Authority: KR
Inventors: 정준영; 최상한
Original assignee: (의료)길의료재단; 가천대학교 산학협력단
Priority date: 2015-12-24
Filing date: 2015-12-24
Publication date: 2017-07-04

Abstract

The present invention discloses a method for suppressing a water signal using a double-on resonance radio frequency pulse in a magnetic resonance scanning process. According to the water signal suppression method of the present invention, the first on-resonance RF pulse and the second on-resonance RF pulse having a phase difference of 180 degrees are applied, and the first and second on- And at least one auxiliary gradient magnetic field for canceling the non-uniformity of the main magnetic field generated by the auxiliary gradient magnetic field is appropriately adjusted. Therefore, it is possible to remove artifacts caused by non-uniformity of the main field in an imaging technique using on-resonance RF pulses such as water suppression, and apply it to an imaging technique using a space-charge-saturated RF pulse , The spindles excited by the pre-saturating RF pulses can be excited once again by the on-resonance RF pulses, thereby eliminating the phenomenon that the signals are returned and the undesired signals appear superimposed on the finally obtained signal.

Description

[0001] The present invention relates to a method for suppressing a water signal using a double-on-resonance radio frequency pulse,

The present invention relates to a method of suppressing a water signal using a double-on resonance radio frequency pulse, and more particularly to a method for suppressing a water signal using a resonance radio frequency pulse (On Resonance RF Pulse) in magnetic resonance scanning (Water Suppression) technique removes artifacts that are sensitive to the non-uniformity of the main field, and simultaneously uses both on-resonance RF pulses and spatially pre-saturation RF pulses Lt; RTI ID = 0.0 > a < / RTI > on-resonance RF pulse.

The present invention can be applied to various diagnostic methods using magnetic resonance such as MRI (Magnetic Resonance Imaging), MRS (magnetic resonance spectroscopy), and MRSI (magnetic resonance spectroscopy).

There are various types of imaging devices such as X-ray, CT, ultrasound, RI, and MRI. Among them, MRI is not harmful to the human body as compared with other imaging diagnostic devices, and it is an important measurement device in clinical examination because it images the characteristics of the internal constituent materials of the human body. MRI apparatus can obtain tissue parameters such as spin density, T1 and T2, chemical shift, magnetization transition, chemical shift saturation transition, blood flow, spectroscopy, and the like, which are intrinsic information of a living body. .

MRS is an imaging technique using an MRI device, and the chemical characteristics inside the human body can be directly confirmed through the MRS. MRS can be imaged based on the resonance frequency of various types of proton, but most MRS is based on the resonance frequency of a hydrogen atom (proton) like MRI. However, since most of the hydrogen atoms in the living body are attributed to water (H2O), the distribution characteristic of water is not valuable as information of chemical characteristics in the living body. Therefore, need.

The most common method used to solve this problem is to selectively excite water using a frequency selective RF pulse. 1 and 2 are diagrams for explaining the principle of a water saturation (Suppression) method using an on-resonance RF pulse. The chemical shifts are measured in parts per million (ppm) relative to the water, the water molecule is 4.7 ppm and other substances of interest such as NAA, Cr, and Cho have values between 4.7 ppm and 0 ppm. The bandwidth of the water molecule signal is about 0.2 ppm to 1 ppm, which is about 13 Hz as f = (64 MHz) * (0.2 ppm) when the external magnetic field is 1.5 Tesla (resonance frequency 64 MHz). That is, 1H of a water molecule finds a higher frequency as it has a larger chemical shift and experiences a slightly more effective magnetic field than 1H of other constituents other than water.

The chemical shift selective imaging sequence (CHESS) technique can unilaterally suppress a signal at a specific frequency by using an RF pulse, wherein the RF pulse used only has a specific frequency of the measurement organization Frequency RF pulses are applied to selectively exclude only water or other specific materials so that the signal can be removed.

A CHESS method for selectively applying a saturated RF pulse corresponding to a resonance frequency of water or fat or other materials will be described with reference to FIG. When the magnetization of water or other material is selectively excited and placed on the XY plane, the magnetization signal of the water is effectively removed by dispersing the XY component of the materialization by applying a spoiler gradient, and the next imaging RF pulse Respectively.

Figure 3 shows a pulse sequence of preparations generally used in fat suppression methods. 3, the RF pulse 310 is applied to select a very small frequency band to selectively excite the fat, and the ripple 320 and the spoiler 330 oblique magnetic field are used for phase recovery and dispersion of the fatigue magnetization . If the RF pulse is selectively excited by water instead of fat, the water signal is suppressed instead of the fat signal.

However, in the case of the fat suppression method, the degree of suppression varies depending on the inhomogeneous state of the magnetic field (B0) in the local region. In a region deviating from the center of the magnetic field, uneven fat or water clearing occurs .

Also, in this type of suppression method, a problem is caused by the interference signal due to the pre-saturation RF pulse. 4 shows a pulse sequence used in an imaging technique using resonance RF pulses and pre-saturated RF pulses from the MRS. In this imaging technique, a pre-saturating RF pulse 405 is used before the on resonance RF pulse 410 is applied, as shown in Fig. At this time, the spindle excited by the pre-saturating RF pulse 405 is again excited by the on-resonance RF pulse 410, so that the signals are restored and an undesired signal appears superimposed on the finally obtained signal . This phenomenon occurs when exciting the frequency present within the frequency bandwidth of the pre-saturating RF pulse with an on-resonance RF pulse.

Water saturation technique Although the frequency bandwidth is small and the excitation frequency depends on the frequency of the free water proton and the size of the main field, in most cases the frequency present in the frequency bandwidth of the pre-saturating RF pulse is considered as the on resonant RF pulse Problems arise from interference signals due to pre-saturated RF pulses.

Finally, in the MRS method of removing the water signal by the CHESS method, residual signal due to signal recovery occurs over time in the RF-applied water, which makes water signal suppression incomplete. The WET (water suppression enhanced through-T1 effects) method removes these residual signals effectively and eliminates water signal suppression incompleteness due to the residual signal, and is a commonly used method for water signal suppression of MRS. (WET, a T1 and B1 insensitive water suppression method for in vivo localized H NMR spectroscopy. J Magn Reson B (1994))

The WET method generally applies RF three times around 90 degrees, and the exact RF flip angle is determined by the interval between the set RFs. The RF flip angle is 89.2, 83.4, and 160.8 degrees, respectively.

However, since the CHESS method applies RF once, the WET method uses three on-resonance RF pulses, so that there is a problem that a SAR (Specific Absorption Rate) value, which is an evaluation index of the energy absorption amount to the living body, becomes high. In addition, since a time interval between RFs is also determined, it takes a certain time.

Another method for water signal suppression is the spectral suppression technique, which is effective for water signal suppression, but it has the disadvantage of increasing the TE because RF must be applied between the 180 ° RF after the excitation signal.

The present invention has been made in view of the above technical background, and it is an object of the present invention to provide an image technique in which a water signal is effectively suppressed in consideration of a sequence load at the time of human body photographing through MR scanning.

Another object of the present invention is to provide a water scavenging (inhibiting) method which is not sensitive to the uniformity of the main magnetic field.

A further object of the present invention is to provide a method for eliminating the appearance of unwanted signal reoccurrence by on-resonance RF pulses in imaging techniques using both on-resonance RF pulses and spatially pre-saturated RF pulses.

In order to solve the above-described problems, according to the present invention, a first on-resonance RF pulse and a second on-resonance RF pulse having a phase difference of 180 degrees are applied to the first on- And at least one auxiliary gradient magnetic field for canceling the non-uniformity of the magnetic field is applied.

That is, a method according to one aspect of the present invention is a method of suppressing a water signal using a double-on resonance radio frequency pulse in a magnetic resonance scanning process, comprising: applying a first on-resonance RF pulse; Applying a second on-resonance RF pulse that offsets a non-uniformity of the main field generated by the first on-resonance RF pulse; And applying at least one auxiliary oblique magnetic field for canceling a non-uniformity of a main magnetic field generated by applying the first and second on-resonance RF pulses, wherein the step of applying the auxiliary oblique magnetic field comprises: Adjusting a combination of the auxiliary inclined magnetic fields; And changing the sign of the at least one auxiliary oblique magnetic field.

Wherein adjusting the combination of the at least one auxiliary gradient magnetic field comprises adjusting the combination of the at least one auxiliary gradient magnetic field so that the sum of the at least one auxiliary gradient magnetic field is zero, The step of changing the code may be a step of changing the sign of the at least one auxiliary gradient magnetic field so that the sum of the at least one auxiliary gradient magnetic field is zero.

On the other hand, in the step of changing the sign of the at least one auxiliary gradient magnetic field, the sign of the first auxiliary gradient magnetic field in the at least one auxiliary gradient magnetic field may be positive (+) or negative (-).

The method may further include the step of applying the spoiler gradient magnetic field together with the step of applying the auxiliary gradient magnetic field.

The auxiliary gradient magnetic field may be generated before application of the first on-resonance RF pulse, after application of the first on-resonance RF pulse, before application of the second on-resonance RF pulse, At least one of the time points may be applied.

Preferably, the second on-resonance RF pulse has a phase difference of 180 degrees with the first on-resonance RF pulse.

The method according to the present invention may further comprise the step of applying a Spatial Pre-Saturation RF Pulse prior to the step of applying the first on-resonance RF pulse.

According to the present invention, it is possible to remove artifacts caused by non-uniformity of the main field in an imaging technique using an on-resonance RF pulse, and to apply the method of the present invention to an imaging technique using a spatially- , The spindles excited by the pre-saturating RF pulses can be excited once again by the on-resonance RF pulses, thereby eliminating the phenomenon that the signals are returned and the undesired signals appear superimposed on the finally obtained signal.

Since the artifacts generated by the non-uniformity of the main magnetic field can be removed, a post-treatment process as in the prior art is not required. In order to solve the non-uniformity of the main magnetic field, By using a combination of oblique magnetic fields, the SAR value can be reduced. Or the TE is not increased since it is not processed after the excitation signal as in the spectral suppression method.

Figs. 1 and 2 are diagrams for explaining the principle of a water erasing (suppressing) method using an on resonance RF pulse. Fig.
Figure 3 shows a pulse sequence of preparations generally used in fat suppression methods.
Figure 4 shows a pulse sequence used in an imaging technique that uses pre-saturating RF pulses with on-resonance RF pulses.
FIG. 5 is a view for explaining the principle of an image technique using a double-on resonance RF pulse according to an embodiment of the present invention for solving artefacts occurring in imaging techniques using a general on-resonance RF pulse.
FIG. 6 illustrates a preparation pulse sequence used in an imaging technique using a double-on resonance RF pulse according to an embodiment of the present invention. Referring to FIG. 5, the double-resonance RF pulse And the inclined magnetic field is used in combination.
FIG. 7 is a diagram illustrating an imaging technique using a double-on resonance RF pulse according to an embodiment of the present invention for solving artifacts occurring in a conventional imaging technique using an on-resonance RF pulse and a pre-saturating RF pulse.
FIG. 8 shows a preparation pulse sequence according to an embodiment of the present invention for applying an oblique magnetic field to an unwanted signal in an imaging technique using an on-resonance RF pulse and a pre-saturation RF pulse.
FIG. 9 shows an example of application to the MRS water signal suppression technique using a pulse sequence including a combination of a double-on resonance RF pulse and a gradient magnetic field according to an embodiment of the present invention.

In general, there is a water saturation (Supression) method as an image technique using an on resonance RF pulse, which is mainly applied to an MRS image which does not require water signal information. In addition to this on-resonance RF pulse, a method of using an RF pulse for pre-saturation to attenuate a signal of a specific position of an image by exciting a region locally is also used.

In this method, a signal due to artifacts is generated by the non-uniformity of the main magnetic field because the phase of the selectively excited spindle is not constant due to the non-uniformity of the main field due to the use of the on- And the signals obtained from the spindles of the selected frequency do not have the same phase due to this difference in phase, resulting in non-uniform artifacts in the reconstructed image.

In order to solve the problem of artifacts, a double on resonance RF pulse is used in the present invention.

FIG. 5 is a view for explaining the principle of an image technique using a double-on resonance RF pulse according to an embodiment of the present invention for solving artefacts occurring in imaging techniques using a general on-resonance RF pulse.

As shown in FIG. 5, one on-resonance RF pulse 510 is applied followed by a second on-resonance RF pulse 520 having an opposite phase, i.e., a 180 degree phase difference.

In the imaging technique using the on resonance RF pulse 510, since the on resonance RF pulse 510 is used alone without an oblique magnetic field, the phase information of the spindle selectively excited by the non-uniformity of the main magnetic field is constant . Because of these nonuniform phase arcs, the signals obtained from the spindles of the selected frequency do not have the same phase, indicating an artifact of a non-uniform shape in the reconstructed image. One way to compensate for this phase difference is to apply additional gradient fields.

However, the on-resonance RF pulse 510 can be regarded as having no selective gradient magnetic field applied at the same time, and that a small amount of oblique magnetic field due to the non-uniformity of the main magnetic field affects the phase, Further, since the on resonance RF pulse 510 uses the principle of selecting and exciting the frequency with respect to the entire volume without using the selective gradient magnetic field, another identical on-resonance RF pulse 520 is used for the phase correction . Another on resonance RF pulse 520 used at this time has a phase difference of 180 degrees from the first applied resonance RF pulse 510 and can be corrected by returning the phase that is distorted by the unevenness of the main magnetic field have.

FIG. 6 illustrates a preparation pulse sequence used in an imaging technique using a double-on resonance RF pulse according to an embodiment of the present invention. Referring to FIG. 5, the double-resonance RF pulse And the inclined magnetic field is used in combination.

When two consecutive RF pulses are applied, the phase is corrected by the on-resonance RF pulse, but all the spindles must be seeded again before adding successive imaging techniques for the image. In embodiments of the present invention as a method for eliminating such residual spindles, additional gradient magnetic fields are applied.

At this time, unlike in the method according to the prior art, the combination of the rewinder and the spoiler gradient magnetic fields can be optimized to minimize the time stretched by the double-on resonance RF pulse.

That is, in the image acquisition method using the double-on resonance RF pulse according to the embodiment of the present invention, the first on-resonance RF pulse is applied and the non-uniformity of the main field generated by the first on- Applying a second on-resonance RF pulse and applying one or more auxiliary gradient magnetic fields for canceling the non-uniformity of the main magnetic field generated by applying the first and second on-resonance RF pulses, The combination of the auxiliary gradient magnetic fields is adjusted and the sign is changed to obtain the optimum auxiliary gradient magnetic field combination.

In the combination of the optimal auxiliary gradient magnetic field, the combination and sign of the auxiliary gradient magnetic field can be adjusted so that the sum of the applied one or more auxiliary gradient magnetic fields is zero.

The polarity of the auxiliary oblique magnetic field does not matter whether it starts from plus (+) or minus (-), for example (+, -, - 0, 0, -), (-, 0, 0, +).

FIG. 6 shows a pulse sequence of an image technique according to an embodiment of the present invention.

6, a first on-resonance RF pulse 610 and a second on-resonance RF pulse 620 that applies a second on-resonance RF pulse 620, which is 180 degrees out of phase with the first on-resonance RF pulse 610, The application of the first on-resonance RF pulse 610 may be performed before application of the first on-resonance RF pulse 610, between the first and second on-resonance RF pulses 610 and 620, 630 are applied.

The oblique magnetic field 630 can be applied equally to the x, y, and z axes, and its value and sign are adjusted so that the sum of the oblique magnetic fields for each axis is zero.

6, a gradient magnetic field 630 is applied between the first and second on-resonance RF pulses 610 and 620 before application of the first on-resonance RF pulse 610. In other words, And apply the spoiler gradient magnetic field 640 after application of the second on-resonance RF pulse 620. [

In this case also, the gradient magnetic field 530 can be applied equally to the x, y, and z axes, and the value and sign are adjusted so that the sum of the angular gradient magnetic fields for each axis becomes zero.

On the other hand, the frequency bandwidth, center frequency and pulse shape of the first and second on-resonance RF pulses can be adjusted as needed. The frequency bandwidth, the center frequency, and the pulse shape of the on resonance RF pulse can be appropriately selected according to the used imaging technique, and are not particularly limited in the present invention.

Since an image having no main-field nonuniformity is automatically obtained using the combination of the first and second on-resonance RF pulses and the gradient magnetic field, a post-processing process for compensating for the unevenness of the main magnetic field And additional image acquisition for this is not necessary.

Figure 7 shows that in a typical imaging technique using on-resonant RF pulses and pre-saturated RF pulses, the spindles excited by the pre-saturating RF pulses are excited once again by the on-resonant RF pulses so that the signals are returned to the finally obtained signal FIG. 2 is a diagram showing an embodiment of the present invention for eliminating the phenomenon that undesired signals appear overlapping. FIG.

As shown in FIG. 7, two on-resonance RF pulses 710 and 720 are used with a pre-saturating RF pulse 705.

FIG. 8 shows a preparation pulse sequence according to an embodiment of the present invention for applying an oblique magnetic field to an unwanted signal in an imaging technique using an on-resonance RF pulse and a pre-saturated RF pulse.

First, as shown in FIG. 8, after a pre-saturating RF pulse 805 is applied, a voltage is applied between the first and second on-resonance RF pulses 810 and 820 The oblique magnetic field 830 is applied. In addition, a spoiler gradient magnetic field 850 may be applied after application of the second on-resonance RF pulse 820. It is preferable that the magnitude and sign of the gradient magnetic field are adjusted so as to be zero.

FIG. 9 is a graph comparing results of water suppression by applying a pulse sequence including a combination of a double-on resonance RF pulse and a gradient magnetic field to an MRS according to an embodiment of the present invention.

The photographic material is a phantom for MRS. The material contains 1Kg H20 dist 8.2g NaC2H3O2 (Sodium Acetate) and 9.6g C3H5O3Li (Lithium Lactate).

FIG. 9A is a spectrum result of data obtained by MRS photographed without water suppression, FIG. 9B is a spectral result of data obtained by MRS photographed by water suppression of a photographic material in a conventional WET system, 9C is a spectral result of data obtained by MRS photographed by water suppression of a photographing material by a double-on-RF (double RF) method of the present invention.

In Fig. 9a without water suppression, the water signal is absolute and other signals are not observed correctly. On the other hand, strong water signals were suppressed in the cases of FIGS. 9B and 9C in which water was suppressed, and peaks of interest were observed. 9c) dl Although the water signal suppression power is somewhat lower than that of the conventional WET system (Fig. 9b), the existing system uses three RFs, while the proposed system uses the existing system It is possible to reduce the Specific Absorption Rate (SAR) value, which is an index of the energy absorption amount to the living body, by using two RFs less than one RF. In particular, the time required can be reduced by about 30% in the conventional method.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention as defined in the appended claims. It will be understood that the present invention can be changed.

Claims

CLAIMS What is claimed is: 1. A method of suppressing a water signal using a double-on resonance radio frequency pulse in a magnetic resonance scanning process,
Applying a first on-resonance RF pulse;
Applying a second on-resonance RF pulse that offsets a non-uniformity of the main field generated by the first on-resonance RF pulse; And
Applying at least one auxiliary gradient magnetic field for canceling the non-uniformity of the main magnetic field generated by applying the first and second on-resonance RF pulses,
Wherein the step of applying the auxiliary gradient magnetic field comprises:
Adjusting a combination of the at least one auxiliary gradient magnetic field; And
And changing the sign of the at least one auxiliary gradient field.

The method according to claim 1,
Wherein adjusting the combination of the at least one auxiliary oblique magnetic field comprises:
And adjusting the combination of the at least one auxiliary gradient field so that the sum of the at least one auxiliary gradient field is zero.

The method according to claim 1,
Wherein changing the sign of the at least one auxiliary gradient magnetic field comprises:
And changing the sign of the at least one auxiliary tilting magnetic field so that the sum of the at least one auxiliary tilting magnetic field is zero.

The method of claim 3,
In the step of changing the sign of the at least one auxiliary gradient magnetic field,
Wherein the sign of the first sub-gradient magnetic field of the at least one sub-gradient magnetic field is positive (+).

The method of claim 3,
In the step of changing the sign of the at least one auxiliary gradient magnetic field,
Wherein the sign of the first sub-gradient magnetic field of the at least one sub-gradient magnetic field is negative (-).

The method according to claim 1,
And applying a spoiler gradient magnetic field after applying the second on-resonance RF pulse.

The method according to claim 1,
Wherein the auxiliary inclined magnetic field comprises:
Is applied at least one of before the application of the first on-resonance RF pulse, after the application of the first on-resonance RF pulse and before the application of the second on-resonance RF pulse and after the application of the second on- Water signal suppression method.

The method according to claim 1,
Wherein the second on-resonance RF pulse has a phase difference of 180 degrees with the first on-resonance RF pulse.

The method according to claim 1,
After the step of applying the auxiliary gradient magnetic field,
And applying a pulse sequence to use the water signal erase method.

The method according to claim 1,
Prior to applying the first on-resonance RF pulse,
Further comprising applying a Spatial Pre-Saturation RF Pulse. &Lt; Desc / Clms Page number 17 >