KR100447802B1 - Highpass coil for magnetic resonance imager - Google Patents

Abstract

The present invention relates to a radio frequency (RF) highpass coil, which is a component of a magnetic resonance imaging apparatus (MRI), and particularly for animal and clinical use, even in a common magnetic field of 1.5T, which is generally used in clinical practice. It relates to a coil that can be used. The stepped coil is a spiral stair-shaped legs of equal length and width are arranged at equal intervals in a cylindrical material of a predetermined length, and both ends of each leg are attached to both ends of the cylindrical material to form a closed loop. Each end ring is connected to each other at right angles, and capacitors are joined at equal intervals on the end ring between the conductor and the conductor, and a current inflow cable (BNC) is connected to the left and right sides of one of the capacitors located on the end ring. do.

Description

High frequency coil for magnetic resonance imaging {HIGHPASS COIL FOR MAGNETIC RESONANCE IMAGER}

The present invention relates to a radio frequency coil which is a component of a magnetic resonance imaging apparatus (MRI), and more particularly, to a coil structure that can be used for animal experiments and clinical use even in a common magnetic field of 1.5T, which is generally used in clinical practice. will be.

A radiofrequency high frequency coil, a component of the magnetic resonance imaging device, consists of a plurality of equally spaced straight conductors called legs on the surface of a plastic cylinder, connected to two closed loops called endrings at the two ends of the cylinder. It has a shape that is connected by. And condensers are placed on the legs and the end rings on the cylinder. Generalized as the structure of such a coil is the high frequency cage coil shown in FIG. 1 and the high frequency spiral coil shown in FIG. 1 and 2, reference numeral 10 denotes a leg as a conductor attached at equal intervals on the acrylic cylinder, and reference numeral 20 is an endring attached to both ends of the acrylic cylinder to form a closed loop. ). And reference numeral 35 shows a current inflow cable (BNC) coupled to the end ring, although not shown, the capacitors are located in the middle of the end ring between the adjacent legs.

In general, a coil is a single current loop that branches in a number of short parallel paths surrounding one axis and then recombines on the opposite side. According to the resonance condition, the closed loop has resonance when the length is equal to an integral multiple of the entire wavelength. The end rings of the coil form a closed loop and thus have an ideal sinusoidal current. The current flowing through each loop is either in sine or cosine mode, and the current going up through one side of the end ring again descends half of the other end ring. The total current is therefore the sum of all currents flowing in the same direction over half of the cylindrical surface. As a result, in the cage coil shown in FIG. 1, the ending ring current is a discrete sum that is approximately proportional to the integration of the surface current.

On the other hand, in the case of a circuit having N repeating components, that is, legs, there are end ring resonance modes in which current does not flow along the legs but only in the end rings, and N / 2 −1 degenerate resonance modes. The most dominant of these degenerate modes is selected as the resonance mode.

The cage coil has an ideal current distribution by using a capacitor with the same value at each capacitor position, so that one of the natural resonance modes of the coil can form a uniform transverse radio frequency magnetic field in the volume and the image volume. It can be said that a quadrature configuration with good uniformity can be achieved.

The spiral coil as shown in FIG. 2 is essentially a helical version of the cage coil and refers to a configuration that uses slanted legs instead of the straight axial legs of the cage coil. However, since the spiral coil also creates discrete rotational symmetry around the axis, the circuit model applied to the cage coil is also applied to the spiral coil, creating resonance modes with sinusoidal current distribution like cage cages. The difference from cage coils is the variation of the current phase with distance along the axis, in which the rotation of the spiral legs as a function of the axial distance leads to the phase rotation of the radiofrequency field. Thus, spiral coils can maintain efficiency and field uniformity by realizing linear phase changes along the coil axis.

As described above, the above-mentioned cage coils are mainly used in magnetic resonance imaging apparatus, which is widely used as a diagnostic device in clinical practice, and some spiral coils are experimentally applied. Their magnetic fields are 1.5T in size and require high sensitivity. Unsuitable for medical and clinical trials. Therefore, in experiments requiring high sensitivity, a high magnetic field magnetic resonance imaging device should be used.

However, as the external static magnetic field increases, the uniformity of the magnetic field becomes more difficult to achieve, while the specific absorption rate (SAR) becomes a problem because the amount of radio frequency absorbed by the sample increases.

Therefore, there is a need for a coil structure that can be used for animal and clinical applications requiring high sensitivity even under a normal magnetic field such as 1.5T.

Accordingly, an object of the present invention is to provide a high frequency coil for magnetic resonance imaging that can maintain the image uniformity and improve the signal-to-noise ratio even under a normal magnetic field of 1.5T.

1 is an external view of a highpass birdcage coil generally used.

2 is an external view of a highpass spiral coil generally used.

3 is an external view of a high frequency staircase coiol according to an embodiment of the present invention.

Figure 4 is an illustration of an experimental image obtained using a high frequency cage coil, a high frequency spiral coil and a high frequency stepped coil.

FIG. 5 is a diagram illustrating a contour, three-dimensional side view, and histogram for each of FIG. 4 obtained with each coil to show uniformity improvement of the high frequency stepped coil according to an embodiment of the present invention. FIG.

6 to 8 are obtained using a high frequency cage coil having eight legs and eight leg high frequency stepped coils. With emphasis Exemplary results of enhanced sagittal images and gradient echo axial images.

FIG. 9 is a diagram for showing a signal-to-noise ratio improvement of a stepped coil in comparison with a caged coil according to an embodiment of the present invention. FIG.

Coil for magnetic resonance imaging according to an aspect of the present invention for achieving the above object;

End legs are arranged at equal intervals in the shape of a spiral staircase with the same length and width of the legs in a spiral step shape, each end of each leg is attached to both ends of the cylindrical material to form a closed loop, respectively It is bonded at right angles, the condenser is bonded at equal intervals on the end ring between the leg and the leg, characterized in that the current inflow cable is connected to the left and right of one of the condenser on the end ring.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following description of the present invention, if it is determined that a detailed description of related known functions or configurations may unnecessarily obscure the subject matter of the present invention, the detailed description thereof will be omitted.

First, Figure 3 shows an appearance of the stepped coil according to an embodiment of the present invention.

As shown in FIG. 3, the corkscrew staircase coil rotates the leg 40 to the right in a spiral step instead of the straight axial legs 10 of the cage coil shown in FIG. 1. It is a new type of coil. However, this coil also maintains discrete rotational symmetry around its axis, similar to caged or helical coils, and therefore has resonance modes with ideal sine current distribution as in the case of caged coils. Therefore, the circuit model that was applied to the cage coil can be used again.

The differences are, firstly, the legs 10 of the cage coils are rectangular shapes parallel to each other, and the legs 10 of the helical coil shown in FIG. 2 are parallelogram shapes lying about 45 degrees laterally, but embodiments of the present invention According to the stepped coil is wound in the shape of a square spiral staircase of the same length and width of the leg 40. That is, the legs 10 of the cage or spiral coil form a long rectangular or parallelogram shape, but the stepped coil forms a shape that is subdivided into small squares to compensate for the weak magnetic field. And secondly, in the case of transmitting the radio frequency, the overall effect on the magnetic field formed in the sample located in the coil is examined in a vector form. The cage coil and the spiral coil form a magnetic field of almost the same size, but the stepped coil is transverse Since the vector sum of the magnetic field components formed by the directional and longitudinal legs must be taken, the magnitude of the magnetic field formed in the coil creates a larger magnetic field than the previous two coils.

Referring to Figure 3 with reference to the manufacturing process of the stepped coil, the stepped coil according to an embodiment of the present invention 8 as shown in Figure 3 on the acrylic cylinder having an outer diameter of 120mm to 190mm in length A coil with two legs 40 was produced. As the legs 40, thin copper tape was used. End rings 50 and 60 were bonded to both ends of the acrylic cylinder with a width of 13 mm. Then, the eight legs 40 having a width of 5 mm joined at right angles to one end ring 60 are equal in length and width with equal intervals, and then rotated to the right in a spiral step shape to the opposite end ring. And 50. At this time, the rotation angle of each leg 40 to achieve 145 degrees. The rotation angle of the leg is 145 degrees refers to a position where both ends of the leg 40 are rotated relative to the cylinder when both ends of the leg 40 are joined to the end rings 60 and 50, respectively. Meanwhile, a current inflow BNC cable 70 having a impedance of 50 Ω is bonded to only one end ring 60 for single excitation and reception, and the end rings 50 and 60 between the adjacent legs 40 are connected. A capacitor (not shown) was bonded to the intermediate point.

Hereinafter, experimental results for comparing the performance of the stepped coil having the above-described structure, the general cage coil and the spiral coil will be described. For reference, Table 1 shows the tuning frequency (MHz) and matching value (dB) of each coil.

Tuning frequency Matching value Unloading loading Unloading loading 8-leg high pass cage 63.7 63.9 -8 -24 8-leg high-pass spiral coil 63.9 63.7 -11 -24 Eight Leg High Stair Coil 63.7 63.9 -12 -20

First, image uniformity and signal-to-noise ratio were measured for each experimental result. First, image uniformity is obtained by using Equation 1 below. Measurements were made on phantom images.

In Equation 1 Wow Are the maximum and minimum values of the signal strength, respectively, and are measured values for the phantom image. The signal-to-noise ratio is then calculated using Equation 2 Phantom images and clinical results were measured.

In Equation 2 Wow Are the average of the signal and noise, respectively. Represents the average deviation of the noise. The mean value of the signal is the average of the values measured in the phantom image, and the noise signals are the mean and standard deviation of the values measured in the portion other than the phantom image in the output image.

Hereinafter, the phantom experiment will be described.

First, as an experimental condition, 10mM in a plastic cylinder of 6cm diameter using each coil from GE Signa Horizon (USA), the most widely used 1.5T magnetic field MRI instrument in the clinic. Cross-sectional images of phantoms were obtained. The pulse sequence uses the FSE sequence, TR is 3000ms, TE is 15ms, the bow angle is 90 degrees, and the frequency width is 16 kHz. The image area (FOV) is 16 × 16, the thickness is 5.0 × 256 × 192, excitation The number NEX was set to 1 to obtain an image.

An image obtained using three kinds of high frequency coils having eight legs and the same phantom under the same experimental conditions is shown in FIG. 4. In FIG. 4, a) is a cage coil, b) a helical coil, and c) is a cross sectional image of a phantom obtained using a stepped coil. Referring to FIG. 4, in the case of a), portions that appear slightly dark at the leg positions of the coil are shown, which shows the nonuniformity of the image, but in the case of b), the nonuniformity along the leg is not seen, and in the case of c) It can be seen that the image uniformity is much improved than the other two coils. This uniformity improvement can be confirmed again through a contour, three-dimensional side view, and histogram for each coil shown in FIG. 5. The histogram of FIG. 5 shows that the magnitude of the signal increases while the magnitude of the noise decreases from a) to c). This indicates that the signal-to-noise ratio of the stepped coil is improved.

Meanwhile, the image uniformity and the signal-to-noise ratio measured from the phantom image with three high frequency coils having eight legs are shown in Table 2 below.

Uniformity (%) Signal-to-noise ratio 8-leg high frequency cage coil 87 128 8-leg high frequency spiral coil 85 156 8-leg high frequency stepped coil 90 173

When comparing the results for each coil with reference to Table 2,

First, the high frequency spiral coil has 2% less image uniformity than the high frequency cage coil, but the signal-to-noise ratio has been improved by about 22%.

Second, compared to high frequency cage coils, high frequency stepped coils have improved image uniformity and signal-to-noise ratio by 3% and 35%, respectively.

Third, the high-frequency stepped coils have improved image uniformity and signal-to-noise ratio by 5% and 11%, respectively, compared to high-frequency spiral coils.

In other words, it can be seen that the signal-to-noise ratio is increasing from the cage coil to the spiral coil and from the step coil. Also, the image uniformity was reduced in spiral coils compared to cage coils, but increased in stepped coils. This is a result showing that the image uniformity of the helical coil can only be increased by quadrature configuration.

Therefore, the experimental results show that the high frequency stepped coil is a coil with improved image uniformity and signal-to-noise ratio compared to a high frequency cage coil or a high frequency spiral coil.

Hereinafter, the results of clinical trials will be described.

First, in order to confirm the results obtained through the phantom experiment in the clinical experiments, a high frequency cage coil with 8 legs and a high frequency step coil with 8 legs were tested on the human body under the same experimental conditions. The part of the human body used is the wrist joint, which is the most delicate part of the movement. emphasis , Highlighted sagittal images and gradient echo cross-sectional images were obtained under the following experimental conditions.

The enhanced sagittal image was obtained with a TR of 3000 ms, a TE of 98 ms, a bow angle of 90 degrees, a frequency width of 16 kHz, an image area of 16 × 16, a thickness of 3 mm, and an excitation number of 2 in a 256 × 192 matrix. , Highlighted sagittal images were obtained with a 450 ms TR, 9 ms TE, 90 degrees lean angle, 16 kHz frequency width, 16 x 16 image area, 3 mm thickness, and 2 x excitation counts in a 256 x 192 matrix in the SE sequence. In addition, the gradient echo cross-section image has a 20-degree bow angle in the GRE sequence, 450 ms in TR, 18 ms in TE, and 18 ms in 16 kHz, with an image area of 15 × 15 and a thickness of 4 mm and a 256 × 192 matrix. Obtained as 2.

Obtained using 8-leg high frequency cage coils and 8 leg high frequency stepped coils Emphasis and Highlighted sagittal images and gradient echo cross-sectional images are shown in FIGS. 6, 7, and 8, respectively. Looking at each of the images shown, it can be seen that the contrast of the image obtained by using the stepped coil than the image obtained by the cage coil is larger. In order to obtain the signal-to-noise ratio for the images obtained with each coil, the comparison was made as follows.

first, With emphasis Table 3 shows the signal and noise values for bone and muscle to compare the weighted sagittal image.

Second, the signal and noise values for bone, muscle and blood vessels are shown in Table 4 to compare the gradient echo cross-sectional images. In addition, the signal-to-noise ratio calculated using these values is shown in Table 5. The signal-to-noise ratio improvement of the stepped coil relative to the cage coil is measured and shown in FIG. 9.

Emphasis and The signal-to-noise ratio improvement of the stepped coil relative to the cage coil in the highlighted sagittal image is

First, about bones Emphasis and About 40% or more in all of the highlighted images,

Second, about muscles About 39% in the highlighted image In the highlighted image, the improvement was about 48%.

Next, cross-sectional images using gradient echoes improved signal-to-noise ratios by first, about 69% for bones, about 80% for muscles, and about 52% for blood vessels.

Therefore, the results of the above experiment show the performance improvement of the high frequency stepped coil having 8 legs, which shows the possibility of use in clinical application.

8-leg high frequency cage coil 8-leg high frequency stepped coil Highlight video Highlight video Highlight video Highlight video signal bone 584.7 373.8 767.7 522.7 muscle 187.9 69.2 238.4 95.3 noise Average 12.8 13.7 11.9 13.4 Standard Deviation 8.6 7.0 8.0 7.0

8-leg high frequency cage coil 8-leg high frequency stepped coil signal bone 166.5 206.5 muscle 83.0 106.5 blood vessel 307.0 350 noise Average 11.6 8.7 Standard Deviation 5.8 4.4

8-leg high frequency cage coil 8-leg high frequency stepped coil T_1 highlighted sagittal plane bone 66.5 94.5 muscle 20.4 28.3 T_2 highlighted sagittal plane bone 51.4 72.8 muscle 7.9 11.7 Gradient echo cross section bone 26.7 45.0 muscle 12.3 22.2 blood vessel 50.9 77.6

In the above description, the phantom and the human body were compared for the high frequency cage coil, the high frequency spiral coil, and the high frequency stepped coil having eight legs. Here, the factors that evaluate the performance of the coil are two factors: image uniformity and signal-to-noise ratio. The results obtained through phantom experiments for each of the three coils show that the image uniformity is 87%, 85%, and 90%, respectively. The image uniformity of the pass stepped coil was the highest and the signal-to-noise ratio increased to 128, 156, and 173, respectively.

Therefore, the image uniformity and signal-to-noise ratio of the high frequency stepped coils to the high frequency cage coils increased by 3% and 35%, respectively. The increase was 11%. This means that the performance of the high frequency stepped coil is improved. The results of this performance improvement are also shown in clinical trials of human wrists. The signal-to-noise ratio of high frequency stepped coils to high frequency cage coils emphasis, More than 40% improvement in the enhanced sagittal image and 52% in the gradient echo cross-sectional image.

As a result, the stepped coil according to the embodiment of the present invention may be referred to as a coil having only the advantages of the conventional high frequency cage coil or spiral coil. In addition, using quadrature with two ports will improve the performance of the coil. In other words, it is possible to make an optimal coil that can approach the signal-to-noise ratio of the surface coil while maintaining the field uniformity of the high frequency cage coil. Therefore, this coil configuration will be very effective when used for animal or clinical use in existing 1.5T magnetic resonance imaging apparatus, and in functional magnetic resonance imaging (FMRI) and magnetic resonance spectroscopy (MRS) where high signal-to-noise ratio is required. It would be a particularly useful coil if used.

As described above, since the present invention can make an optimal coil that can approach the signal-to-noise ratio of the surface coil while maintaining the field uniformity of the high frequency cage coil, it is possible to obtain an optimal coil as in the conventional 1.5T magnetic resonance imaging apparatus. Even in the magnetic field, there is an advantage of maintaining image uniformity, and an advantage in that the signal-to-noise ratio is improved compared to the cage coil and the spiral coil.

On the other hand, the present invention has been described with reference to the embodiments shown in the drawings but this is only illustrative, it will be understood by those skilled in the art that various modifications and equivalent other embodiments are possible. Therefore, the true technical protection scope of the present invention should be defined only by the appended claims.

Claims (3)

In the high frequency coil for magnetic resonance imaging, Each of the stepped conductor legs of equal length in the transverse and longitudinal directions is joined from one end to the other end of the cylindrical cylindrical surface, each end of each leg being attached to both ends of the cylinder, respectively. While joining the first and second endring conductors at right angles to each other, the legs are connected at equal intervals, and the capacitors are joined at equal intervals to the endring conductors between adjacent legs and the legs. High frequency coil for magnetic resonance imaging, characterized by connecting the current inflow cable to the left and right. The method of claim 1, wherein the number of legs arranged at equal intervals is any one of eight and sixteen, and both ends of each of the legs coupled to the first and second end ring conductors are 145 degrees on the cylindrical cylinder. High frequency coil for magnetic resonance imaging, characterized in that present in each of the positions rotated 180 to 180 degrees. delete