KR102364991B1 - Magnetic resonance imaging system and radio frequency coil apparatus therefor - Google Patents

Abstract

The present invention relates to a magnetic resonance imaging system and an RF coil device therefor. The magnetic resonance imaging system according to an embodiment of the present invention is a magnetic resonance imaging system using the head of a human body as a subject, and a plurality of RFs spaced apart from each other at equal intervals so that the head of the human body can be positioned in an internal space. (Radio Frequency) It is formed of a coil and a high dielectric material, and includes a plurality of high dielectric pads respectively provided on one surface of at least a portion of the plurality of RF coils on the inner space side, wherein the plurality of RF coils have an amplitude or a phase. At least one may operate as a plurality of channels set differently from each other. According to the present invention, interference effects can be minimized and high-quality image quality can be secured.

Description

Magnetic resonance imaging system and RF coil device therefor {MAGNETIC RESONANCE IMAGING SYSTEM AND RADIO FREQUENCY COIL APPARATUS THEREFOR}

The present invention relates to a magnetic resonance imaging system and an RF coil device therefor.

Magnetic resonance imaging (MRI) systems are widely used in the medical diagnosis field because of their advantages such as harmlessness to the human body, 3D imaging, and high-resolution images. In particular, the 7T (7 Tesla) high-field MRI system can provide images with a higher signal-to-noise ratio (SNR) and higher resolution than the existing 1.5T or 3T low-field MRI systems. , and even images of the cerebral cortex can be obtained, attracting attention in that it can provide high-quality medical services to patients with brain diseases.

In general magnetic resonance imaging (MRI) systems, to acquire images of the human head, coils for both transmission and reception are mainly used.

In the low-field magnetic resonance imaging (MRI) system, the magnetic field generated by the head-only RF coil is uniform throughout. However, when using the conventional high-field magnetic resonance imaging (MRI) system with a frequency of 300 MHz, the dielectric constant ( Permittivity) shortens the effective RF-wavelength, so there is a limit that a uniform magnetic field cannot be formed inside the head.

In addition, when RF waves pass through a wide space between the RF coil and the human head as a subject in a high magnetic field magnetic resonance imaging (MRI) system, interference occurs, which severely degrades image quality.

Prior Literature 1: J. T. Vaughan and J. R. Griffiths, RF coils for MRI. John Wiley & Sons, 2012. Prior Document 2: S. Sohn, L. DelaBarre, A. Gopinath, and JT Vaughan, “Rf head coil design with improved rf magnetic near-fields uniformity for magnetic resonance imaging (mri) systems,” IEEE Transactions on Microwave Theory and Techniques , vol. 62, no. 8, pp. 1784-1789, Aug 2014.

The present invention is to solve the above-mentioned problems, by setting a plurality of RF coils as multi-channel and applying a high-dielectric pad to one surface of the RF coil to be inspected, thereby minimizing the interference effect and ensuring high-quality image quality. An object of the present invention is to provide a magnetic resonance imaging system and an RF coil device therefor.

In addition, the present invention provides a magnetic resonance imaging system capable of optimizing and improving the performance of a 7T high magnetic field MRI system by optimizing the thickness, size and material of a high dielectric pad applied to the RF coil and an RF coil device therefor to do it for the purpose of work.

The present invention for achieving this object is a magnetic resonance imaging system using the head of a human body as a subject, a plurality of RF (Radio Frequency) devices spaced apart from each other at equal intervals so that the head of the human body can be positioned in an internal space. a plurality of high-dielectric pads formed of a coil and a high dielectric material, each of which is provided on one surface of at least a portion of the plurality of RF coils on the inner space side, wherein the plurality of RF coils have at least one of an amplitude or a phase with each other It is characterized in that it operates with a plurality of channels set differently.

Another feature of the present invention is that the plurality of RF coils are a plurality of pairs of RF coils separated from each other at a predetermined angle and disposed to face each other around the inner space.

Another feature of the present invention is that the plurality of RF coils are divided at an angle of 45 degrees from each other and are eight RF coils disposed at positions of each side of a regular octagon surrounding the inner space.

In addition, the present invention does not place the RF coils in the forward direction toward which the face faces among the heads of the human body in which the eight RF coils are located in the inner space, and 4 pairs of RF coils are arranged so as to be symmetrical with respect to the forward direction, Each of the four pairs of RF coils is characterized in that a high dielectric pad is provided on one surface of the inner space.

In addition, the present invention is a first RF coil and the first RF coil and the first RF coil disposed in the forward direction facing the face of the head of the human body located in the inner space is divided and arranged at an equal angle to each other, the Another feature is to include second to seventh coils each having high dielectric pads on one surface of the inner space.

In another aspect of the present invention, each of the plurality of RF coils includes a microstrip transmission line, a first capacitor having one end connected to one end of the microstrip transmission line, the other end connected to a ground terminal, and a first capacitor connected in parallel with the first capacitor. Another feature is that it includes two capacitors and a third capacitor having one end connected to the other end of the microstrip transmission line and the other end connected to the ground terminal.

In another aspect of the present invention, the first capacitor has 8.2 pF, the second capacitor has 3.4 pF, and the third capacitor has 2.73 pF.

Another feature of the present invention is that the specific permittivity ε _r value of the high dielectric pad is 78.

Another feature of the present invention is that the specific dielectric constant ε _r of the high dielectric pad is 300.

Another feature of the present invention is that the thickness of the high-dielectric pad is 0.5 cm.

The present invention for achieving this object is an RF (Radio Frequency) coil device applied to a magnetic resonance imaging system using the head of the human body as a subject, it is formed of an RF coil and a high dielectric object, and the RF coil of the human body It is characterized in that it includes a high-dielectric pad provided on one surface of the head, and a specific permittivity ε _r value of the high-dielectric pad is 78 or 300.

In addition, the present invention provides a microstrip transmission line in which the RF coil has one end connected to one end of the microstrip transmission line, a first capacitor having the other end connected to a ground terminal, a second capacitor connected in parallel with the first capacitor, and Another feature is that it includes a third capacitor having one end connected to the other end of the microstrip transmission line and the other end connected to the ground terminal.

The present invention is also characterized in that the first capacitor has 8.2 pF, the second capacitor has 3.4 pF, and the third capacitor has 2.73 pF.

Another feature of the present invention is that the thickness of the high-dielectric pad is 0.5 cm.

Another feature of the present invention is that the high dielectric pad is made of barium samarium titanium (BaSmTi) oxide.

According to the present invention as described above, the interference effect can be minimized and high-quality image quality can be secured by setting a plurality of RF coils to be multi-channel and applying a high-dielectric pad to one surface of the RF coil to be inspected.

In addition, according to the present invention, by optimizing the thickness, size, and material of the high dielectric pad applied to the RF coil, the performance of the 7T high magnetic field MRI system can be optimized and improved.

1 is a view for explaining a magnetic resonance imaging system according to an embodiment of the present invention.
2 is a diagram illustrating an example of an RF coil unit of a magnetic resonance imaging system according to an embodiment of the present invention.
3 is a diagram illustrating an RF transmission coil apparatus of a magnetic resonance imaging system according to an embodiment of the present invention.
4 is a view showing the B ₁ ⁺ field of the uniform head imitation cylindrical phantom applied in an experimental example of the present invention.
5 is a diagram illustrating an example of a measurement result of the magnetic resonance imaging system according to the present invention.
6 is a diagram comparing Txeff at three pad positions of an 8-channel TEM coil as a simulation result of a magnetic resonance imaging system according to the present invention.
7 is a graph showing the change in TXeff intensity when the pad thickness is changed from 1 cm to 0.1 cm as a simulation result of the magnetic resonance imaging system according to the present invention.
8 is a view showing Txeff of an 8-channel TEM coil of a horizontal slice (slice 1, 2, 3, 4, 5) of a subject according to an experimental example of the present invention.
9 shows the results of an experiment in which ε _r values are set to 78 and 300 as an experimental example of the present invention.
10 is a diagram showing the SAR distribution with respect to the dielectric constant ε _r of the high dielectric pad.
11 is a view showing measurement settings and results of an example in which a high dielectric pad is mounted and an example in which the high dielectric pad is not mounted, respectively.
12 shows the noise correlation with and without pad and the optimal SNR.
13 is a diagram illustrating an example of a magnetic resonance image to which a T1 weight and a T2 weight are applied.

The above-described objects, features and advantages will be described below in detail with reference to the accompanying drawings, and accordingly, those of ordinary skill in the art to which the present invention pertains will be able to easily implement the technical idea of the present invention. In describing the present invention, if it is determined that a detailed description of a known technology related to the present invention may unnecessarily obscure the gist of the present invention, the detailed description will be omitted. Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to indicate the same or similar elements, and all combinations described in the specification and claims may be combined in any manner. And unless otherwise provided, it is to be understood that references to the singular may include one or more, and references to the singular may also include plural expressions.

1 is a block diagram illustrating a magnetic resonance imaging system according to an embodiment of the present invention.

Referring to FIG. 1 , a Magnetic Resonance Imaging (MRI) system 100 installs a Radio Frequency (RF) coil 110 on the head 11 of a patient 10 and within the whole body coil 120 . MRI scans were performed.

The magnetic resonance imaging system 100 may transmit a signal from the whole body coil 120 to photograph the head of the human body as a subject, and receive a signal from the multi-channel RF coil 110 . For example, a Transverse Electromagnetic (TEM) coil may be used as the RF coil and a separate receiving coil may be covered on the head of the human body to perform transmission and reception.

The Magnetic Resonance Imaging (MRI) system 100 may acquire a tomographic image of the subject by using a resonance phenomenon of a radio frequency (RF) coil. The RF coil combines with the static magnetic field B ₀ to generate a time varying magnetic field B ₁ , and receives the returned MR signal.

The magnetic resonance imaging system 100 is a conventional 1.5T or 3T low-field magnetic resonance imaging (MRI) system in order to ensure a higher signal-to-noise ratio (SNR) and higher resolution image than that of the conventional magnetic resonance imaging (MRI) system. A high magnetic field of 7T (7 Tesla) can be used.

The magnetic resonance imaging system 100 is formed of a plurality of RF (Radio Frequency) coils spaced apart from each other at equal intervals so that the head of the human body can be positioned in an internal space, and a high dielectric object, and the plurality of RF coils A plurality of high-dielectric pads respectively provided on one surface of at least a portion of the inner space may be included. Here, the plurality of RF coils may operate as a plurality of channels in which at least one of an amplitude and a phase is set to be different from each other.

As the RF coil, a BC (Birdcage) coil, a TEM (transverse electromagnetic) coil, a surface coil, and the like are used. Multiple channels may be used in the RF coil to improve RF efficiency and B1 shimming. Hereinafter, an example in which a TEM coil is applied as an RF coil will be described, but is not necessarily limited thereto.

The TEM coil includes Microstrip Transmission Lines (MTL), which can be separated from each other to operate as independent transmission coils.

Each or at least some of the plurality of RF coils may form a multi-channel by forming different channels. For example, a multi-channel may be configured by differently setting at least one of an amplitude or a phase of a channel in which the RF coil operates.

A desired time-varying magnetic field B1 distribution can be achieved. In high-performance high-field magnetic resonance imaging (MRI), the TEM coil is effective in overcoming the homogeneity problem compared to the BC coil. Multiple channels can be applied to the RF coil to improve RF efficiency.

However, in the case of such a multi-channel coil, when the RF wave passes through a large space of the air medium between the coil and the head, the size of the B1 field may drop. Interference effects in the B1+ field may seriously degrade image quality in high-area magnetic resonance imaging (MRI).

In the present invention, in order to overcome the above-described problem, by applying a high-dielectric (HD) pad to the RF coil, the local sensitivity and/or homogeneity of the RF magnetic field (B1 field) can be improved.

Hereinafter, a multi-channel RF coil and a magnetic resonance imaging system to which a high dielectric pad is applied to improve transmission efficiency (Txeff) will be described.

2 is a view showing an example of the RF coil unit of the magnetic resonance imaging system according to the present invention.

In the example shown in FIG. 2, the magnetic resonance imaging system according to the present invention shows an example in which eight RF coils are applied. However, this is only an example, and the number of RF coils may be implemented as various other even numbers such as 2, 4, 6, 10, and the like.

As shown in Figure 2, the plurality of RF coils 111 are spaced apart from each other at equal intervals so that the head of the human body can be positioned in the inner space.

The plurality of RF coils 111 are separated from each other at a predetermined angle, and are composed of a plurality of pairs disposed to face each other around the inner space. For example, the plurality of RF coils 111 may be eight RF coils separated from each other at an angle of 45 degrees and disposed at positions of each side of a regular octagon surrounding the inner space.

In one embodiment, as shown in Figure (a), the eight RF coils 111 are a first RF coil 7 disposed in the forward direction toward which the face of the head of the human body is located in the internal space, and the first RF It may include the coil 7 and second to seventh coils 1 to 6 and 8 that are arranged to be separated from each other at equal angles. Here, the first RF coil 7 may not include a high dielectric pad, and the second to seventh coils 1 to 6 and 8 may each have a high dielectric pad on one surface of the inner space.

In one embodiment, as shown in Figure (b), eight RF coils 111 may not be disposed in the forward direction toward which the face faces among the heads of the human body located in the internal space. Four pairs of RF coils are disposed so as to be left and right symmetrical with respect to the forward direction, and each of the four pairs of RF coils may be provided with a high dielectric pad on one surface of the inner space.

The high dielectric pad 112 is formed of a high dielectric material. The high dielectric pad 112 is provided on one surface of at least a portion of the plurality of RF coils 111 on the inner space side, respectively.

As an example, the high dielectric pad may have a relative permittivity ε _r value of 78 or 300.

<Dielectric Pad>

According to Maxwell's equation and Ampere's law, in the case of human brain tissue, which is a conductive dielectric material, the RF field inside an object or body is distributed by the conductive current (Jc) and the displacement current (Jd), and Equation 1 and Equation 2 is satisfied.

[Equation 1]

[Equation 2]

는 허수로서, 두 방정식에서 모두 전도 전류(Jc)와 변위 전류(Jd) 사이에 90도의 위상 차이를 도입한다. Here, E is the electric field strength, H is the magnetic field strength, B is the magnetic flux density, ω is the angular frequency, σ is the conductivity of the object, and ε _r is the relative permittivity of the dielectric.

is an imaginary number, introducing a phase difference of 90 degrees between conduction current (Jc) and displacement current (Jd) in both equations.

와

의 영향을 받는다. 자기장의 1차 소스는 송신 코일 내부의 전도성 전류(Jc)이며, 다른 소스는 RF 장 전파를 샘플 매체를 통해 90° 위상 이동 시 전파할 수 있는 변위 전류(Jd)이다. B1 필드에 대한 이 두 개의 상대적 소스는 수학식 3에 따라 Jc와 Jd의 비율로 표시할 수 있다.The propagation of magnetic field B inside the human body is

Wow

is affected by The primary source of the magnetic field is a conductive current (Jc) inside the transmitting coil, and the other source is a displacement current (Jd) that can propagate RF field propagation in a 90° phase shift through the sample medium. These two relative sources for the B1 field can be expressed as the ratio of Jc and Jd according to Equation (3).

[Equation 3]

Therefore, materials with high ε _r and low σ can be used as Jd in the RF field, and the local electric field B ₁ strength can be increased.

범위는 일반적으로 수용액과 같은 액체 물질이나 젤 기반 물질이 사용될 수 있고,

은 고체 물질이 사용될 수 있다.In the field of magnetic resonance imaging (MRI), high dielectric materials with higher resistivity than biological tissues tend to be used. The high dielectric material may be determined differently according to the range of the dielectric constant ε _r .

In general, liquid substances such as aqueous solutions or gel-based substances can be used,

A silver solid material may be used.

By applying a high-k material to the RF coil, the distribution of the displacement current can be adjusted and the RF field can be adjusted to uniformly distribute in the region-of-interest (ROI).

According to the complexity of the RF coil configuration, a three-dimensional solver as in Equation 4 was used in the computation domain to determine the RF field at a desired frequency.

[Equation 4]

where N is a media matrix composed of two components C and M, C contains the conductivity value, and M contains the transmittance value of the computational domain. The total number of grid edges in the computational domain for field distribution evaluation is equal to the order of N.

Npad is the pad media matrix after inserting the pad into the domain, added to the total order N. However, in a multi-channel coil environment, Npad is composed of a plurality of terms. These multiple terms define a final matrix Npad that depends on the number of pads and their position relative to the coil. Accordingly, the multi-channel pad media matrix is given by the following Equation (5).

[Equation 5]

Here, j represents the total number of pads, and δn makes it possible to change the characteristics of each pad. In particular, in the case of an 8-channel RF coil, the pad matrix satisfies Equation 6 below.

[Equation 6]

The pad matrix can be controlled by the total number of pads and their individual properties. Changes in the B ₁ ⁺ field can also be observed when pad properties such as transmittance, conductivity, size, and, more importantly, the position of the pad relative to the human head and RF coil, change in the computational domain within the human body.

<Modeling method of TEM coil and dielectric pad>

8-Channel TEM RF Coil System

Two circularly polarized components of magnetic fields B ₁ ⁺ and B ₁ ^- having opposite rotation directions may be expressed by Equations 7 and 8 below.

[Equation 7]

[Equation 8]

Here, Bx and By are the transverse complex components of the magnetic field in the x and y directions, and the asterisks indicate complex conjugates, respectively. In addition, Txeff satisfies the following Equation (9).

[Equation 9]

는 B₁ ⁺ 필드의 평균값이고, P_in는 총 입력전력 (kW)이다. 총 입력 전력을 1로 노말라이즈 하면, TEM RF 코일의 8개 채널 모두의 입력 전력이 추가되었고, 그 다음 총 전력을 그 계수로 나누어 정상 입력 전력을 얻을 수 있다. 따라서, 모든 B₁ ⁺ 필드는 정규화된 입력 전력을 기준으로 계산될 수 있다. 필드는 코일에 전달된 전력의 1W에 해당하며, B₁ ⁺ 필드 분포 값은 모든 결과에서 TXeff를 나타낼 수 있다. From here,

is the average value of B ₁ ⁺ field, and P _in is the total input power (kW). If the total input power is normalized to 1, the input power of all eight channels of the TEM RF coil is added, and then the total power is divided by the factor to obtain the normal input power. Accordingly, all B ₁ ⁺ fields can be calculated based on the normalized input power. The field corresponds to 1W of power delivered to the coil, and the value of B ₁ ⁺ field distribution can represent TXeff in all results.

On the other hand, the center of the brain is usually considered to be a deep tissue, and it is difficult to achieve a homogeneous field distribution in the center. Therefore, in the present invention, a target area of 5 cm x 5 cm (ROI) is set in the center of the human head for performance analysis.

A Duke body model is placed in a magnetic resonance imaging (MRI) device including an 8-channel RF transmitting coil device and an experiment is performed. The Duke model consists of all the various tissues of the male body obtained from the virtual family dataset.

3 is a diagram illustrating an RF transmission coil apparatus of a magnetic resonance imaging system according to an embodiment of the present invention.

Referring to Figure 3 (a), the RF coil may include a micro-strip transmission line (MTL) and capacitors connected thereto.

For example, the RF coil has a first capacitor (CP) having one end connected to one end of the microstrip transmission line and the other end connected to a ground terminal, a second capacitor (Cm) connected in parallel with the first capacitor, and one end connected to the first capacitor. It may include a third capacitor (Ct) connected to the other end of the microstrip transmission line, the other end is connected to the ground terminal. Here, the first capacitor CP may have 8.2 pF, the second capacitor Cm may have 3.4 pF, and the third capacitor Ct may have 2.73 pF.

As shown in FIG. 3 , the RF transmission coil device for magnetic resonance imaging (MRI) may include a ground substrate, a dielectric substrate, and a microstrip transmission line (MTL) for transmitting an RF field of a human head.

As an example, as the parameters shown in FIG. 3 , a = 15 cm, b = 5 cm, c = 0.5 cm, h = 15 cm, l = 5 cm, p = 2 cm, w = 1.8 cm may be satisfied.

For example, Teflon may be applied as the dielectric substrate, and the ε _r value of Teflon may be 2.1. A conductor such as copper may be used as the grounding substrate or the microstrip transmission line.

The elements of the TEM coil are independently controlled by modifying the phase and amplitude of the excitation. Therefore, the TEM coil includes several channels of a transmission line that satisfies the optimal excitation of the target site of the human body.

Referring to Figure 3 (a), a single TEM element composed of a microstrip transmission line (MTL) and a ground substrate is disclosed.

By using capacitors (Cp and Ct) at the ends of the microstrip, the resonance length of the microstrip is shortened to λ/2, and a capacitor (Cm) matched in series with the load can be used to match the TEM coil and the load . As an example, all channels of the TEM RF coil can be tuned to fit the 7T resonance frequency (298 MHz) using specific capacitor values (Cp = 8.2 pF, Ct = 2.73 pF, Cm = 3.4 pF). The return loss value of all channels is less than -20dB, allowing RF coil matching where the head is.

To verify the performance of magnetic resonance imaging (MRI), one channel of an RF coil array was simulated using a high-frequency structural simulator (HFSD) software. RF shimming techniques can also be applied with the TEM coil to minimize the problem of inhomogeneity of higher magnetic fields in the ROI. This RF shimming technique can control the non-uniformity by designing the phase and amplitude of the RF coil. Here, RF shimming was applied before and after insertion of the high-k pad. After finding the optimal values for the input power (W) and the phase of the input source, simulations were performed again for the two input parameter values. In the calculation domain, the refinement criterion was set to "very fine" state. The grid edge of the computational domain was defined as 11.249 x 10 ⁶ cells (megacells or MCells).

TEM RF Coil with HD Pad

To investigate the performance of the high-dielectric (HD) pad, the high-dielectric pad for 7T MR imaging disclosed in Fig. 3 (a) and Fig. 3 (b) was evaluated. The simulation configuration is as illustrated in Figure 2, and consists of an 8-channel RF transmitting coil and a high-dielectric (HD) pad. The B ₁ ⁺ field inside the head is illustrated in Fig. 3(c).

4 is a view showing the B ₁ ⁺ field of the uniform head imitation cylindrical phantom applied in an experimental example of the present invention.

As shown in Fig. 4 (a), a uniform head-shaped cylindrical phantom with a length of 30.5 cm and a diameter of 17.8 cm was simulated. One channel of the RF coil and one element of a high-dielectric (HD) pad with the suggested dimensions were placed 2.5 cm from the phantom. The RF coil, high-dielectric (HD) pad, and phantom were aligned with each other for accurate and proper field distribution.

Figure 4 (b) shows the experimental results of the comparative example to which the high-dielectric (HD) pad is not applied, and Fig. 4 (c) shows the experimental results of the example to which the high-dielectric (HD) pad is applied. As shown in the illustrated example, it can be confirmed that a more uniform H field value is guaranteed in an embodiment to which a high-dielectric (HD) pad is applied.

In one embodiment, a high-dielectric (HD) pad is set to a size of 5 cm * 15 cm * 0.5 cm and placed in front of all coil elements of an 8-channel coil assembly as indicated by the rectangular block area in front of all channels in FIG. can be applied Frequency and size analysis was performed based on the resonance mode of the high-dielectric (HD) pad, and it was confirmed that when the thickness was 0.5 cm, it resonated at 298 MHz corresponding to 7T.

For comparison, two different values of ε _r were investigated: 78 and 300. After loading the high-dielectric (HD) pad, the pad's high permittivity value tuned the resonant frequency. Therefore, the RF coil was adjusted to 298 MHz by changing the variable capacitor Ct value.

Number, size and location of pads

To investigate the effect of the B ₁ ⁺ field, high-dielectric (HD) pads were simulated with various positions and dimensions. A series of electromagnetic simulations were performed to achieve TXeff improvement at the center of the human head (ROI) and to perform sensitivity analysis for optimal location and size.

To emulate a real patient setup, simulations were performed with two distinct high-dielectric (HD) pads measuring 5 cm x 15 cm x 1 cm and 5 cm x 15 cm x 0.5 cm. A pad thickness of 0.5 cm can put less strain on the patient's head and provide more space with the head compared to a thickness of 1 cm. A high-dielectric (HD) pad was simulated at three different positions close to the head and the coil and between the head and the RF coil.

In the simulation setup, 2, 4, 6, 7, and 8 high-dielectric (HD) pads were applied in front of all RF coils.

An 8-channel RF coil is configured around the subject's head, and the configuration of the 8-channel RF coil is possible at two different angles or configurations. For example, the 8-channel RF coil is an example in which the RF coil is fixed to the front of the subject as shown in Fig. 2 (a), or the RF coil is not fixed to the front of the subject as shown in Fig. 2 (b). Yes Both experiments were performed.

When two pads are applied, each pad is separated by 180 degrees, when four pads are applied, each pad is separated by 90 degrees, and in the case of six pads, each pad is separated by 60 degrees.

When the 8-channel RF coil is fixed to the front of the subject as shown in Fig. 2 (a), the pad is omitted so that a total of 7 pads are applied to the RF coil on the front of the subject, and 8-channel When the RF coil is not fixed to the front of the subject as shown in Fig. 2 (b), a total of 8 pads were applied.

Virtual setup of magnetic resonance imaging (MRI)

As the simulator, an ABloch-based magnetic resonance imaging (MRI) system simulator (SYSSIM) was used. As a simulation setup, T1-weighted and T2-weighted MR images on an 8-channel TEM coil were used to simulate.

To this end, we used a specific gradient called gradient recalled echo (GRE) to obtain x-weighted and B-weighted images of both configurations (A and B) and RFshimmed MR images with and without pads. The purpose of this measurement is to evaluate SNR, signal strength and TXeff across selected slices. The field distributions of the 8-channel coil assemblies were obtained using Sim4Life software and entered into a magnetic resonance imaging (MRI) system simulator to verify the distributions in the selected order. Two different locations were used to target transverse slices, one at the center of the anatomy and the other at 34 mm below the central slice. Both fragments and all tissues contain tissue with different T1T2 and proton density (PD) values depending on the fieldstrength. In addition, both slices contain, inter alia, the frontal sinus in the central slice and the sphenoid space in the lower slice.

<Experiment result>

8-channel RF coil device with high dielectric pad

Table 1 below summarizes the cases in which 2, 4, 6, 7 and 8 high-dielectric pads are applied to the 8-channel RF coil device shown in Figures (a) and (b) of Fig. 2, respectively.

[Table 1]

Hereinafter, Configuration A means the embodiment of Figure 2 (a), Configuration B means the embodiment of Figure 2 (b).

5 is a diagram illustrating an example of a measurement result of the magnetic resonance imaging system according to the present invention. Figure 5 shows the 8-channel Txeff for the two coil configurations tested with and without HD pads.

Compared to the padless case, the penetration depth inside the head phantom of the shield distribution of the head is increased and the uniform distribution covers the entire target slice. These results show that more accurate and higher results are obtained by using a high-dielectric (HD) pad equipped with a TEM coil. The RF coil and high-dielectric (HD) pad were implemented in a state placed around the head as shown in Fig. 2 (a) and (b).

6 is a diagram comparing Txeff at three pad positions of an 8-channel TEM coil. For comparison, pads having a thickness of 1 cm and 0.5 cm were used.

As can be seen from Fig. 6, both cases of Configuration A - Fig. 2 (a)- and Configuration B - Fig. 2 (b)- improved TXeff in the ROI and minimized the interference of the RF field of the head. Able to know. In addition, the RF field in the brain region (ROI) was larger in the example to which the HD pad was applied, compared to the example in which the HD pad was not applied.

Also, as a result of applying the HD pad in the case of Configuration A, it can be seen that the sum value of the B ₁ ⁺ field of the ROI increased from 1.90 mT to 2.295 mT. In the Configuration B case, it can also be seen that the B ₁ ⁺ field in the ROI is improved from 1.95 mT to 2.325 mT. It can be seen that the high-dielectric (HD) pad additionally provides a B ₁ ⁺ field duet to the Jd current providing constructive interference to the ROI, resulting in improved TXeff in the target region.

When the number of high-dielectric (HD) pads is different (2, 4, 6), the ROI is further improved as the number of pads increases. In addition, the pad with a thickness of 0.5 cm showed better improvement than the pad with a thickness of 0.1 cm.

7 is a graph showing the change in TXeff strength when the pad thickness is changed from 1 cm to 0.1 cm. Here, the TXeff value is derived from the summation of the fields of the ROI.

We also simulated different thicknesses of high-dielectric (HD) pads with a difference of 10 mm to 1 mm to further justify the decision on the choice of 0.5 cm high-dielectric (HD) pad. For all simulations and B ₁ ⁺ field distributions, a pad with an optimal thickness of 0.5 cm was selected for further investigation of RF coil performance. The difference in magnetic field strength is shown in Fig. 6 as a function of pad thickness, where the figure shows that a 0.5 cm thick pad yielded the maximum field distribution in the ROI.

6 and 7 show that when the high-dielectric (HD) pads near the RF coil are set as in Configurations A and B, the field improvement of the ROI in both cases is higher than in the other cases (different positions and different number of pads). clearly show that Based on these results, it is observed that the high-dielectric (HD) pad improves the selected ROI of the human head by 21%.

RF seaming applied

8 is a view showing Txeff of an 8-channel TEM coil of a horizontal slice (slice 1, 2, 3, 4, 5) of a subject according to an experimental example of the present invention. The top row shows the resulting results without pads and the bottom row shows the results with pads.

To minimize the problem of inhomogeneity at higher magnetic fields in the ROI of the study subject, RF shimming techniques can be applied with multi-channel TEM coils. The objective of this technique was to achieve TXeff by designing the phase and amplitude of the RF power generated by each RF coil element. Optimal solutions for phase and amplitude can be solved using freely available software packages within a reasonable amount of time. The software was repeated several times to find the optimal values of amplitude and phase. For a uniform field distribution in the ROI, the optimal values of the applied source and phase are selected for each channel in the multi-channel configuration environment. Eight channels of a TEM coil with a total input power of 5.64 W and an optimal phase angle for each channel are shown in Table 2 below.

[Table 2]

With and without high-dielectric (HD) pads, the total input power was 5.64 and 5.77 W, respectively. The input power was normalized to 1 for Txeff, and each channel input power was divided into the total input power with and without high-dielectric (HD) pad case. In the simulation results, it is observed that inserting a high-dielectric (HD) pad into the RF coil setup improves the ROI of the human head by 38%. The field distribution of the target slice was more intense in the ROI, and the TXeff value improved from 3.11 mT to 4.265 mT. An optimized field distribution was also observed for the other four transverse slices.

8 is a diagram showing the B ₁ ⁺ distribution of a comparative example in the target slice according to an experimental example of the present invention and the other four slices of the human head model. B ₁ ⁺ distributions with and without pads in the target slice (slice #3) and the other four slices of the human head model are shown in FIG. 8 . The results show that high-dielectric (HD) pads are effective for all four slices in different locations. As a result, it can be seen that the distribution is improved for every slice of the subject human head. High-dielectric (HD) pads provide a higher B ₁ ⁺ distribution, and it can be seen from the figure that the pads have improved TXeff in the selected ROI of the human head.

SAR and Field Comparison of ε _r = 78, and 300

For higher ε _r values, it produces a higher magnetic field distribution due to the high Jd current of the object. To evaluate this effect, experiments were conducted by setting the ε _r values to 78 and 300.

9 shows the results of an experiment in which ε _r values are set to 78 and 300 as an experimental example of the present invention.

To highlight the best solution for the TEM multi-channel coil, the results obtained from the two ε _r values were compared based on the performance of TXeff and SAR. In the simulation results, we observed that ε _r = 78 and ε _r = 300 produced almost identical TXeff in the ROI. A value of ε _r = 300 produced a better field distribution in the lateral region of the human head, but both ε _r values in the ROI gave the same field distribution.

Also, considering the pad fabrication process, it is easier to fabricate the pad with ε _r = 78 compared to ε _r = 300. Moreover, ε _r = 300 shifted the return loss curve, and the resonant frequency shifted to 290 MHz. We set the coil's Ct capacitor to 2.33ppm to retune at 298MHz.

The comparison of TXeff for ε _r = 78 and ε _r = 300 is shown in Fig. 8. Furthermore, different values were used to select the optimal Er for the loss and lossless conditions of the high-dielectric (HD) pad (0.0001, 0.5, 1). ).

In the simulations, we observed that both ε _r values gave improved TXeff under the lossless condition (σ = 0.0001), whereas the in situ improvement became less effective under the loss condition (σ = 0.5, 1). When σ = 1, the field improvement is negligible.

In the design of an RF coil for magnetic resonance imaging (MRI), the SAR calculation is an important factor in the final decision to evaluate the performance of the coil. SAR analysis using ε _r = 78 and 300 pads was performed for 1 g peak SAR. A TEM coil without a high-dielectric (HD) pad produced a 0.71 W/kg SAR in the ROI, which is the limit of the head SAR in magnetic resonance imaging (MRI). For pad SAR analysis, both ε _r 78 and 300 values were included in the TEM RF coil configuration. It can be observed that ε _r 78 and 300 are generated in the SAR distribution of the sensor. The peak SAR values are 0.693 W/kg and 0.632 W/kg, respectively. Both of these values show that the peak SAR values in the ROI decrease when using high-dielectric (HD) pads.

10 is a diagram showing the SAR distribution with respect to the dielectric constant ε _r of the high dielectric pad. The comparative normalized SAR distribution for the two ε _r values is shown in Fig. 10 (a). An RF shimming technique can be used to minimize the SAR in the ROI. As discussed above, RF shimming can reduce the peak SAR value by reducing the total input power (5.64 W) with more field distribution in the ROI. After using RF shimming, the SAR of the ROI was minimized and compared with and without pads. The normalized SAR distribution is shown in Fig. 10 (b), showing the case of using the pad to calculate the minimum SAR value in the ROI. Also, both B+1 distribution and SAR were observed.

In the end, it can be seen that the analysis for ε _r 78 and 300 provides in situ improvement and SAR reduction effects.

S-parameter Measurements with Pad and Human Phantom

12 is a diagram showing measurement settings and results of an example in which a high dielectric pad is mounted and an example in which the high dielectric pad is not mounted, respectively. The TEM RF coil was mounted on a Teflon substrate and two V9000 variable capacitors (Cm and Ct) were used. On the back side of the Teflon, a current loop was provided to the coil by attaching the ground to the capacitor and the input power.

To verify the simulation results, a commercial high-dielectric (HD) pad having a size of 5 cm x 15 cm and a thickness of 0.5 cm was applied. The high-dielectric pad is made of barium samarium titanium (BaSmTi) oxide, which can be used in a stable, high-quality, reduced-dimensional system. By changing the knob of the V9000 variable kerf pad, the matching and tuning of the RF coil in the 7T was confirmed.

The RF coil was tested and tuned at each position of the 8-channel measuring head, and the tuning of the resonant frequency at 7T (298 MHz) was achieved using the V9000 capacitor again. The measurement setup gave the desired reflection coefficient results.

Noise Correlation and Optimal SNR

MUSAIK V2.0 was used for simulation to evaluate the effect of channel noise and coil array. 12 shows the noise correlation with and without pad and the optimal SNR.

As can be seen from FIG. 12 , the use of a high-dielectric (HD) pad can improve the noise correlation matrix of all channels.

For high-dielectric (HD) pads, the adjacent color of all small rectangular areas is the same as for the padless case. It can be seen that the color of the two adjacent rectangles in the central diagonal is the same in the pad case, but the color is different in the case where the pad is not applied. The same trend is observed for the other rectangles on the central diagonal. The square pattern of these colors shows that the noise correlation pattern is better when using the pad.

In addition, the optimal SNR of the 8-channel RF coil is calculated by MUSAIK V2.0, and the field slice shows that the optimal SNR is improved and increased in all areas of the average field slice. Numerical values of the different regions across all transverse slices are indicated in FIG. 12 .

12, it can be seen that the optimal SNR is improved by inserting a high-dielectric (HD) pad, and the effect of the pad is clearly shown. It can be seen that the pad obtained from both color and numerical values of the SNR intensity provides a more optimal SNR for all regions with one horizontal one.

Magnetic Resonance Imaging (MRI) Virtual Measurement

13 is a diagram illustrating an example of an MR image to which a T1 weight and a T2 weight are applied.

13 is a single GRE sequence with 3 ms echo time (TE), 200 kHz bandwidth (BW), 500 ms relaxation/repetition time (TR), 30 degree flip angle, 128x128 image acquisition matrix, 2 mm slice thickness, 230 mm x 230 mm field of view (FOV). The results obtained by applying The described sequence is used for all simulations of both configurations with and without pads.

The results clearly show that the pad provides a higher signal intensity and that the deeper tissue of the human head produces a stronger signal distribution compared to the absence of the pad. In addition, the ROI was visualized in the MR image using the RF shimmering algorithm, and the intensity of the central image was stronger than that of the padless image. 13 shows a comparison of high-dielectric (HD) pad and non-use magnetic resonance imaging (MRI) images for both slice configurations. This figure also compares the RF shimmering images for configuration A. Magnetic resonance imaging (MRI) images using high-dielectric (HD) pads show relatively better SNR and gain compared to those without pad images. It also uses RF shimming techniques to generate better SNR in deep tissue (central head).

In this study, an 8-channel TEM head coil of 7T can be used with a high-dielectric (HD) pad. The proposed RF coil and high-dielectric (HD) pad were placed around the head in two different configurations (A and B). A microstrip transmission line (MTL) was used as the resonant element of the TEM RF coil. High-dielectric (HD) pads with relative transmittances of 78 and 300, = 0.0001 were used for simulation, and multiple dimensions of pad thickness were simulated to select the optimal thickness. In addition, different numbers of pads were evaluated by placing pads around the head based on the quantified field distribution effects in the selected ROIs. These passive high-dielectric materials have been shown to improve magnetic resonance imaging (MRI) imaging and modify the distribution of RF electromagnetic fields.

In addition, a high-dielectric (HD) pad was applied to verify the simulation results, and it can be seen that the high-dielectric (HD) pad improved TXeff by 21% in the ROI (center of the head) in all simulation and measurement results. RF shimmering technology can also be applied to HD materials, and after applying this technique, the ROI improvement is 38%. It can be seen that the head SAR was calculated for different ε _r values of the pad, and the peak value of the ROI was minimized. As a result, it can be seen that, according to the present invention, RF interference in measurement of a subject can be minimized by applying a high-k pad to multi-channel RF.

Some embodiments omitted in this specification are equally applicable when the implementing subject is the same. In addition, since the present invention described above can be various substitutions, modifications and changes within the scope without departing from the technical spirit of the present invention for those of ordinary skill in the art to which the present invention pertains, the above-described embodiments and accompanying It is not limited by the drawings.

10: subject human body
11: Subject head
100: magnetic resonance imaging system
110: RF coil unit
111: RF coil
112: high dielectric pad

Claims (15)

It is a magnetic resonance imaging system that operates based on the high magnetic field of 7T (Tesla) with the head of the human body as the subject.
A plurality of RF (Radio Frequency) coils spaced apart from each other at equal intervals so that the head of the human body can be positioned in the internal space; and
a plurality of high-dielectric pads formed of a high dielectric material and respectively provided on one surface of the RF coil on the inner space side with respect to at least a portion of the plurality of RF coils;
including,
The plurality of RF coils operate with a plurality of channels in which at least one of an amplitude or a phase is set to be different from each other,
The plurality of high-dielectric pads have a thickness of 0.4 cm to 0.6 cm,
The specific permittivity ε value of the high dielectric pad is 78 or 300,
The RF coil is
A TEM (Transverse ElectroMagnetic) coil comprising:
microstrip transmission line,
first and second capacitors connected to the ends of the microstrip transmission line to shorten the resonance length of the microstrip transmission line to λ/2;
a third capacitor matching the TEM coil and load; and
including a grounding board;
A magnetic resonance imaging system in which all channels of the TEM RF coil correspond to a 7T resonance frequency (298 MHz) by the first to third capacitors.
According to claim 1, wherein the plurality of RF coils
A magnetic resonance imaging system comprising a plurality of pairs of RF coils separated from each other at a predetermined angle and disposed to face each other around the inner space.
According to claim 1, wherein the plurality of RF coils
A magnetic resonance imaging system in which eight RF coils are separated from each other at an angle of 45 degrees and disposed at positions of each side of a regular octagon surrounding the inner space.
The method of claim 3, wherein the eight RF coils are
Four pairs of RF coils are arranged so as to be symmetrical with respect to the forward direction, and the RF coils are not arranged in a forward direction in which the face faces among the heads of the human body located in the internal space, and the four pairs of RF coils are located in the internal space A magnetic resonance imaging system in which each high dielectric pad is provided on one side of the side.
The method of claim 3, wherein the eight RF coils are
a first RF coil disposed in the forward direction toward which the face of the head of the human body is located in the inner space; and
second to seventh coils disposed at an equal angle to the first RF coil and each having a high dielectric pad on one surface of the inner space;
Magnetic resonance imaging system comprising a.
According to claim 1, wherein each of the plurality of RF coils
microstrip transmission line;
a first capacitor having one end connected to one end of the microstrip transmission line and the other end connected to a ground terminal;
a second capacitor connected in parallel with the first capacitor; and
a third capacitor having one end connected to the other end of the microstrip transmission line and the other end connected to a ground terminal;
A magnetic resonance imaging system comprising a.
7. The method of claim 6,
The first capacitor has 8.2 pF, the second capacitor has 3.4 pF, and the third capacitor has 2.73 pF.
According to claim 1,
A magnetic resonance imaging system in which a specific permittivity ε _r value of the high dielectric pad is 78.
According to claim 1,
A magnetic resonance imaging system in which a specific permittivity ε _r value of the high dielectric pad is 300.
According to claim 1,
The thickness of the high dielectric pad is 0.5 cm magnetic resonance imaging system.
An RF (Radio Frequency) coil device applied to a magnetic resonance imaging system that operates based on a high magnetic field of 7T (Tesla) with the head of the human body as the subject,
RF coil; and
a high dielectric pad formed of a high dielectric material and provided on one surface of the RF coil toward the head of the human body;
including,
The plurality of high-dielectric pads have a thickness of 0.4 cm to 0.6 cm,
The specific permittivity ε value of the high dielectric pad is 78 or 300,
The RF coil is
A TEM (Transverse ElectroMagnetic) coil comprising:
microstrip transmission line,
first and second capacitors connected to the ends of the microstrip transmission line to shorten the resonance length of the microstrip transmission line to λ/2;
a third capacitor matching the TEM coil and load; and
including a grounding board;
All channels of the TEM RF coil correspond to a 7T resonance frequency (298 MHz) by the first to third capacitors.
12. The method of claim 11, wherein the RF coil is
microstrip transmission line;
a first capacitor having one end connected to one end of the microstrip transmission line and the other end connected to a ground terminal;
a second capacitor connected in parallel with the first capacitor; and
a third capacitor having one end connected to the other end of the microstrip transmission line and the other end connected to a ground terminal;
RF coil device comprising a.
13. The method of claim 12,
The first capacitor has 8.2 pF, the second capacitor has 3.4 pF, and the third capacitor has 2.73 pF.
12. The method of claim 11,
The thickness of the high dielectric pad is 0.5 cm RF coil device.
12. The method of claim 11,
The high dielectric pad is an RF coil device composed of barium samarium titanium (BaSmTi) oxide.

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