KR101577235B1 - Sar measuring device for mri and method of the same - Google Patents

Abstract

The present invention relates to an apparatus for measuring the absorption rate of electromagnetic waves in a magnetic resonance imaging apparatus, and more particularly, to an apparatus for measuring the absorption rate of electromagnetic waves in a magnetic resonance imaging apparatus capable of accurately measuring a SAR value of electromagnetic waves emitted from an MRI apparatus, To a measuring device.
An apparatus for measuring an electromagnetic wave absorptivity of a magnetic resonance imaging apparatus according to the present invention comprises a body having a receiving portion filled with a phantom solution, a lid sealing the upper portion of the body, and an electric field measuring portion placed in the receiving portion of the body The electric field measuring unit includes a plate, a support member spaced apart from the upper portion of the plate, and an electric field measurement sensor fixed to the support member and measuring an electric field in the phantom.
According to another aspect of the present invention, there is provided an apparatus for measuring the rate of absorption of electromagnetic waves, comprising: a body having a receiving portion filled with phantoms; a cover for sealing the upper portion of the body; and a temperature measuring portion mounted on the receiving portion of the body, A plurality of fixing bars mounted on the upper portion of the plate so as to be spaced apart from each other at a predetermined interval, and an optical fiber temperature sensor fixed to the fixing frame and measuring a temperature change in the phantom.

Description

TECHNICAL FIELD [0001] The present invention relates to an apparatus and method for measuring the absorption rate of electromagnetic waves in a magnetic resonance imaging apparatus,

The present invention relates to an apparatus and method for measuring the absorption rate of electromagnetic waves in a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus capable of accurately measuring a SAR value of electromagnetic waves emitted from an MRI apparatus and a temperature change of a human body To an apparatus for measuring the absorption rate of an electromagnetic wave human body and a method thereof.

With the development of MR engineering and MRI technology, ultra high magnetic field magnetic resonance imaging devices of 3T or more have been developed and provide high resolution medical images. Magnetic resonance imaging (MRI) is a diagnostic system that uses the principle of nuclear magnetic resonance to image the spatial distribution of hydrogen nuclei present in the human body and to show the state of the human body.

Magnetic Resonance Imaging (MRI) is divided into a magnetizing unit for forming a primary magnetic field and a magnetizing unit for forming a secondary magnetic field in order to induce a resonance phenomenon. The magnetizing unit for forming a secondary magnetic field includes a seaming coil, , And a high-frequency coil. In particular, the high-frequency coil receives the RF signal that emits as it loosens after the float is excited. At this time, the state of the human body is imaged based on the received RF signal.

In order to display a high-resolution image, a high-field or ultra-high field is required. However, as the magnetic field strength increases, strong RF energy due to RF (Radio Frequency) coil can accumulate in the body of the patient. Strong RF energy accumulated at this time increases the temperature of the skin and body tissues of the patient, causing a sense of warmth or severe burns. Moreover, aged MRI equipment that has passed its service life can cause unexpected serious safety accidents.

For this reason, the limits of electromagnetic waves for MRI equipment have been established by establishing the allowance criteria for each country. The international health guidelines of the International Health Organization (WHO) provide a set of international guidelines, and in Korea, the Ministry of Information and Communication The ministry is setting standards for electromagnetic wave protection.

However, currently used MRI equipment, MRI equipment calculates the SAR value and displays the calculated SAR value when the MRI image is taken. Therefore, the SAR value calculated and the SAR value actually felt by the patient show a significant difference.

Korean Patent Application Publication No. 10-2009-0091897, Korean standard body type phantom creation method for breast cancer diagnostic apparatus

A Study on the Electromagnetic Stability of Medical Imaging Devices (Korean Journal of Safety Management and Science,

An object of the present invention is to provide an apparatus and a method for measuring a SAR value actually received by a human body, which is irrelevant to an incorrect SAR value to be reported by a magnetic resonance imaging apparatus.

It is also an object of the present invention to provide an apparatus and method for precisely measuring a SAR value of electromagnetic waves emitted from an MRI apparatus and a temperature change experienced by a human body.

An apparatus for measuring an electromagnetic wave absorptivity of a magnetic resonance imaging apparatus according to the present invention comprises a body having a receiving portion filled with a phantom solution, a lid sealing the upper portion of the body, and an electric field measuring portion placed in the receiving portion of the body The electric field measuring unit includes a plate, a support stand spaced apart at a predetermined interval from the plate, and an electric field measurement sensor fixed to the support stand and measuring an electric field in the phantom.

In addition, the support base is installed at the center point of the rectangular plate, the central point of the long side and the corner point, and the three points are formed in a triangular shape with each other.

The apparatus for measuring the absorptance of electromagnetic waves in a magnetic resonance imaging apparatus according to the present invention comprises a body having a receiving part filled with phantoms, a lid for sealing the upper part of the body, and a temperature measuring part seated in the receiving part of the body, The temperature measuring unit includes a plate, a plurality of fixing bars spaced apart at a predetermined interval from the top of the plate, and an optical fiber temperature sensor fixed to the fixing frame and measuring a temperature change in the phantom.

Further, the fixing table is arranged in a round zigzag shape.

In addition, the fixture has a top layer, a middle layer and a bottom layer, and the optical fiber temperature sensor starts from the bottom layer to the middle layer, and finally to the top layer.

The distance between the top layer and the middle layer and the distance between the middle layer and the bottom layer are 2.5 cm.

The optical fiber temperature sensor may be disposed along a zigzag shape with rounded corners, and an electric field within the phantom may be provided between the respective columns. The optical fiber temperature sensor may be disposed on the upper surface of the plate in any one of four rows, five columns, An electric field measuring sensor for measuring an electric field is further installed.

A method for measuring an electromagnetic wave absorption rate of a magnetic resonance imaging apparatus includes the steps of: placing an electric field measurement sensor and / or a temperature measurement unit in a receiving portion of a square-shaped body; placing the electric field measurement sensor and / Filling the upper portion of the body with a cover having a vacuum valve, and removing the bubbles of the phantom by sucking the vacuum valve provided on the upper portion of the cover, After the vacuum pump connected to the valve is operated, the vacuum valve is opened little by little to check the vacuum state of the phantom container. Then, the vacuum valve is fully opened and held for 24 hours. After the cover is opened, Remove.

In addition, the opening of the lid is checked by inserting air into the closed body and the lid according to the opening of the vacuum valve, and then the lid of the vacuum container is separated.

According to the present invention, it is possible to diagnose and evaluate the SAR safety of the MRI equipment in use in a medical institution, and to detect a temperature change of a human body, thereby preventing a safety accident in advance.

Accurate measurements can also be made by removing air bubbles from the phantom solution in the phantom vessel using a vacuum.

1 shows a vacuum phantom container according to an embodiment of the present invention.
FIG. 2 shows the SAR distribution in the HEC gel phantom as a simulation result.
FIG. 3 shows the optical fiber magnetic field measuring sensor and the optical fiber temperature sensor installed on the plate.
FIG. 4 shows temperature distribution of three layers measured from the MRI apparatus according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an apparatus for measuring the absorptance of electromagnetic waves in a magnetic resonance imaging apparatus according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

≪ Preparation of phantom solution of human body and phantom container >

1 shows a vacuum phantom container according to an embodiment of the present invention.

The vacuum phantom container according to the present invention was made of ABS (Acrylonitrile-Butadiene Styrene) with a thickness of 3.5 cm and a size of 650 × 420 × 100 mm ³ . The thickness applied in the design of the phantom container should be sufficient to ensure that the appearance of the phantom container does not change during subsequent vacuum formation. The dimensions of the phantom container were measured with a measuring instrument having length traceability.

Referring to FIG. 1, a phantom container 100 includes a body 110 and a lid 120 for opening and closing the upper side of the body 110. Also, an O-ring 130 for sealing the body and the cover is interposed in the groove. In the drawing, reference character H denotes a handle for moving the phantom container.

A vacuum valve 140 for forming a vacuum is formed inside the phantom container at an upper portion of the lid. The vacuum valve removes air bubbles inside the phantom by discharging air existing inside the phantom container.

On the other hand, an apparatus for measuring the SAR and the temperature change is mounted on the receiving portion of the phantom container. The measuring device is mounted on a rectangular plate made of ABS and is equipped with a fiber optic electric field measuring sensor for measuring the electric field and an optical fiber temperature sensor for measuring the temperature change. The types, characteristics, arrangement, etc. of the optical fiber electric field measurement sensor and the optical fiber temperature sensor will be described later.

The receiving portion of the phantom container is filled with the phantom solution. The phantom solution includes HEC (Hydroxy-ethylcellulose) having a capacity of 650 x 420 x 90 mm < ^{3 &} gt ;. The material of the phantom solution is prepared by stirring 38.13 g of sodium chloride and 762.6 g of HEC powder in 24.6 L of deionized water, from which both hydrogen ions and oxygen ions have been removed. The prepared phantom solution is contained in the receiving portion of the phantom container.

In the process of filling the phantom container with the phantom solution to form the phantom, a phantom solution is filled in the receiving part of the phantom container on which the measuring device is placed after placing the electric field and temperature measuring device in the receiving part of the phantom container, For 24 hours to complete the phantom. At this time, if the bubbles are present in the completed phantom, an error may occur in the measured value. A more accurate measurement can be obtained by completely removing bubbles during phantom formation.

Specifically, the phantom solution is filled in the receiving portion of the phantom container, and the lid is closed to seal the receiving portion. Next, the silicon tube is connected to the vacuum valve formed on the top of the lid to connect the vacuum pump to the air in the phantom container. At this time, after operating the vacuum pump, the vacuum valve is slightly opened to check the vacuum state of the phantom container, and then the vacuum valve is fully opened. The sudden opening of the vacuum valve can cause the phantom solution to flow into the vacuum pump through the silicon tube, to remove only the bubbles near the surface of the phantom solution, and to remove the bubbles at the bottom.

Then, after 24 hours after the vacuum valve is completely opened, the bubbles in the phantom solution move to the upper side of the phantom solution to form bubbles. Thereafter, the vacuum pump is stopped and the vacuum valve is completely closed. Finally, after removing the silicone tube, slowly turn the vacuum valve to make sure that the air enters the vacuum vessel, then remove the cover of the vacuum vessel and remove the air bubbles on the surface of the phantom solution.

<MRI SAR simulation>

As described above, a measurement device for detecting an electric field and temperature is installed in the phatum, and a SAR simulation is performed to determine the installation position of the measurement device.

In the present invention, a SAR simulation is performed using a three-dimensional high frequency time domain electromagnetic field simulation software CST microwave studio. In the simulation, a 3T @ 128 MHz MRI using a 2 channel volume coil with a length of 1m and a diameter of 0.8m was used and a finite difference time domain method (FDTD) Electromagnetic field and SAR were calculated.

The electrical properties, density and specific heat of the HEC gel phantom obtained from the calibration test were used.

- electrical conductivity = 0.47 S / m

- dielectric constants (? 'and? ") = 78.68 / 63.39

- magnetic permeability = 1

- heat capacity = 0.0151 kJ / K / kg

- density = 1.00805 g / cm &lt; ³ &gt;

On the other hand, calibration tests are performed to measure the exact absorption rate of electromagnetic waves. Using a measurement device that maintains calibration traceability, necessary data such as electrical characteristics, density and specific heat of the gel phantom are measured, Is used to calculate the electromagnetic wave absorption rate.

That is, the electromagnetic wave absorptivity of the sample is calculated by the following equation.

Where σ = electrical conductivity, and ρ = the mass density of HEC gel phantom.

The simulation results obtained through such conditions and settings are shown in Fig. FIG. 2 shows the SAR distribution in the HEC gel phantom as a simulation result.

Referring to the drawing, the lowest value of the SAR value of the center of the phantom solution is shown, and the SAR value of the edge surface is high. These results are consistent with the experimental results of ASTM F2182-11a and the contents of existing published research papers. The SAR values obtained from the simulation are divided into three regions. Specifically, it is divided into a central region of the phantom, four side regions, and a boundary region formed between the middle region and the four side regions. Among the four side regions, the SAR value in the central region of the long side is larger than the central side region of the short side.

The SAR values in these areas are the lowest in the middle and the highest in the center area of the opposite pair of long sides. And the edge area is the outer surface of the phantom, which means the part protruding from the human body, so the edge area is important for SAR measurement.

Therefore, the measurement positions are the three characteristic areas shown in the simulation, with the center, left edge (center of one long side) and edge of the phantom determined.

<MRI SAR measurement>

Based on the simulation results, three optical fiber electric field measurement sensors were placed at the center of the phantom solution (Position # 1) to set the reference point, and the other sensor was positioned at the left edge of the SAR , And the other sensor is placed at the corner (Position # 2). After placing the measuring device in which the three optical fiber electric field measuring sensors are placed in the receiving portion of the phantom container, the phantom solution is filled to complete the human phantom.

FIG. 3 shows the optical fiber magnetic field measuring sensor and the optical fiber temperature sensor installed on the plate.

Referring to the drawing, the optical fiber electric field measuring sensor 220 is coupled to a support 221 and mounted on the top of the plate. The optical fiber electric field measuring sensor in the support is installed to have a predetermined angle.

The optical fiber electric field measurement sensor is an isotropic miniaturized 3-axis optical electric field sensor (manufactured by Seikoh Giken Ltd.), Model No. SH-10EL was used.

Then, human phantom phantom is installed in MRI equipment and SAR value is measured. The MRI apparatus used in the experiment was 3RF sequences at 3T, a 2-channel volume coil as a transmit coil, and a Torso coil as a receive coil. It was also performed with three sequences of T1w TSE, T1w SPIR, and T2w TSE.

On the other hand, the measured value of the electric field using the phantom differs from the actual electric field received by the human body. To overcome this difference, the correction factor should be calculated and the correction factor applied to the measured value should be supplemented. The correction factor was obtained by a fiber optic electric field measurement sensor calibration test using a TEM cell set to input standard E-fields of 10, 50, and 100 (V / m).

A TEM (Transverse Electromagnetic) cell is a device used to create a standard electromagnetic field in a shielded space. It calibrates an electric field measurement sensor used for measurement and measures the exact electric field.

Then, the electric field is measured, the actual electric field is calculated by multiplying by the correction factor, and the calculated electric field is applied to the following SAR equation.

The SAR calculation formula calculated using the optical fiber electric field measurement sensor installed in the phantom is as follows.

Where σ = electrical conductivity, and ρ = the mass density of HEC gel phantom. That is, the SAR is proportional to the square of the sum of the electric field scalar amounts of the three axes.

The results of SAR measurements are shown in Table 1. Table 1 below compares the SAR values measured by a 3T MRI scanner and the SAR values measured by a fiber optic electric field sensor at the center of a HEC gel phantom using T1w TSE and T1w SPIR sequences.

Using the T1w TSE sequence, the average SAR value measured by the fiber optic field sensor is lower than the SAR value (<1.4 W / kg) reported by MRI equipment, but the maximum SAR value (= 2.77 W / kg) Respectively.

The average SAR values (= 2.51 and 1.68 W / kg) measured at the left edge and corner when using the T1w SPIR sequence are higher than the SAR values reported by the MRI equipment (<1.4 W / kg). In addition, the maximum value at the left edge (= 8.27 W / kg) is higher than the SAR limit specified by the International Electrotechnical Commission (IEC) (= 4.0 W / kg).

Table 2 below compares the SAR value calculated by a 3T MRI scanner and the SAR value measured by a fiber optic electric field sensor at the center of a HEC gel phantom using T2w TSE.

Referring to Table 2, when using the T2w TSE sequence, the average SAR values (6.84 and 5.48 W / kg) measured at the left edge and corner are higher than the SAR values reported by the MRI apparatus (<1.5 W / kg) May exceed safety limits specified by the International Electrotechnical Commission (IEC) (= 4.0 W / kg). Also, the maximum value (= 14.34 W / kg) tends to be 3.5 times greater than the SAR limit, which may lead to serious patient safety problems. These results are consistent with the MRI SAR simulation results.

<Temperature change measurement>

The electromagnetic waves of MRI make the human body feel warm. By measuring the temperature change that the patient can feel, it can show the safety of the MRI equipment. In the present invention, the temperature change of the phantom solution was measured using a fiber Bragg grating (FBG) optical fiber temperature sensor. The temperature is in accordance with Bragg's Law.

Referring back to FIG. 3, a plurality of fixing tables 231 are spaced apart from each other at a predetermined interval on a plate 210 mounted on a phantom receiving portion, and three optical fiber temperature sensors 230 are installed on the fixing table . The plurality of fixing bars are arranged in a zigzag shape with rounded portions.

In the figure, the optical fiber temperature sensors are arranged so as to form five rows, and an electric field measuring sensor for measuring an electric field in the phantom is further provided between the respective rows.

Preferably, the optical fiber temperature sensors may be arranged in four or six rows. When arranged in four rows, an electric field measuring sensor is disposed between the two rows and the three columns and disposed at the center of the plate. When arranged in six rows, an electric field measuring sensor is provided between the third row and the fourth row.

In the experiment, 75 FBG fiber optic temperature sensors were used. The wavelength band of the FBG fiber optic temperature sensor was 1524.904 ~ 1564.882 nm, the measurable range was 0.01 ± 0.0002nm / ℃, and the resolution was 0.1 ℃.

The FBG optical fiber temperature sensor has three layers of top layer, middle layer and bottom layer, and each of them has 25 layers. Also, the fiber optic temperature sensor starts from the bottom layer to the middle layer according to the arrangement of the fixing table, and finally, along the top layer. The separation between each layer is 2.5 cm.

Temperature change values obtained from 75 optical fiber temperature sensors were converted into images by applying linear interpolation method using MATLAB code. The temperature change value obtained from the optical fiber temperature sensor is shown in Fig.

FIG. 4 shows temperature distribution of three layers measured from the MRI apparatus according to the present invention. Referring to the drawings, the temperature change measurement values of the three MRI sequences show the highest temperature rise at the edge of the four sides and the lowest temperature change in the middle. T1w TSE sequence showed maximum temperature rise of 1.5 ℃, T1w SPIR of 2.6 ℃ and T2w TSE of 5.2 ℃.

If the temperature change is more than 5, the patient can feel a sense of warmth and feel a higher sense of warmth depending on the patient's body shape and body fat mass. Therefore, serious safety problems may occur depending on the patient's condition.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the present invention is not limited to the disclosed exemplary embodiments, but various changes and modifications may be made by those skilled in the art without departing from the scope of the present invention.

100: Phantom container
110: Body
120: Cover
130: O ring
140: Vacuum valve
200: Measuring device
210: Plate
220: Fiber optic electric field measuring sensor
221: Support
230: Fiber optic temperature sensor
231:
H: Handle

Claims (10)

An apparatus for measuring the absorptance of electromagnetic waves in a magnetic resonance imaging apparatus,
The electromagnetic wave absorptivity measuring apparatus may further comprise:
A body having a receiving portion filled with the phantom solution, a lid sealing the upper portion of the body, and an electric field measuring portion seated in the receiving portion of the body,
Wherein the electric field measuring unit comprises a plate, a support provided at an upper portion of the plate so as to be spaced apart at a predetermined interval, and an electric field measurement sensor fixed to the support and measuring an electric field in the phantom. Electromagnetic wave absorptivity measuring device.
The method according to claim 1,
Wherein the support is provided at a center point of a rectangular plate, at a central point of a long side and at a corner point, and the three points are formed in a triangular shape with respect to each other.
The method according to claim 1,
Wherein the electromagnetic wave absorptivity of the magnetic resonance imaging apparatus is calculated according to the following equation.

Where σ = electrical conductivity, ρ = mass density of HEC gel phantom, and E = electric field.
An apparatus for measuring the absorptance of electromagnetic waves in a magnetic resonance imaging apparatus,
The electromagnetic wave absorptivity measuring apparatus may further comprise:
A body having a receiving portion filled with a phantom, a lid sealing the upper portion of the body, and a temperature measuring portion seated in the receiving portion of the body,
Wherein the temperature measuring unit comprises a plate, a plurality of fixing bars mounted on the top of the plate so as to be spaced apart from each other at a predetermined interval, and an optical fiber temperature sensor fixed to the fixing table and measuring a temperature change in the phantom. Apparatus for measuring the absorptivity of electromagnetic waves in a video apparatus.
The method of claim 4,
Wherein the fixing table is arranged in a round zigzag shape.
The method of claim 5,
Wherein the fixture has a top layer, a middle layer, and a bottom layer, the optical fiber temperature sensor starts from the bottom layer, continues to the middle layer, and finally is installed along the top layer. .
The method of claim 6,
Wherein the distance between the top layer and the middle layer and the distance between the middle layer and the bottom layer are 2.5 cm.
The method of claim 5,
The optical fiber temperature sensor is installed along a zigzag shape with rounded corners, and the electric field in the phantom is measured between the respective columns. The optical fiber temperature sensor is installed on the upper surface of the plate in any one of four rows, five columns or six columns. And an electric field measuring sensor for measuring the magnetic field of the magnetic resonance imaging apparatus.
Placing an electric field measuring sensor and / or a temperature measuring part in a receiving part of a square shaped body,
Filling the phantom solution in the receiving part on which the electric field measuring sensor and / or the temperature measuring part are placed,
Closing the upper portion of the body with a cover having a vacuum valve,
And removing air bubbles from the phantom by sucking from a vacuum valve provided at an upper portion of the lid,
The bubbles were removed by operating a vacuum pump connected to a vacuum valve, opening the vacuum valve gradually to check the vacuum state of the phantom container, then fully opening the vacuum valve for 24 hours, opening the cover, And removing bubbles formed on the upper side of the magnetic resonance imaging apparatus.
The method of claim 9,
Wherein the opening of the cover is made by inserting air into the enclosed body and the cover after opening the vacuum valve, and then separating the cover of the vacuum container.

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