JP3135592B2 - Magnetic resonance imaging - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic resonance imaging apparatus, and more particularly, to a high resonance imaging apparatus using a uniform coil and a surface coil.
The present invention relates to a magnetic resonance imaging apparatus that acquires / N images.

[0002]

2. Description of the Related Art A magnetic resonance imaging apparatus is considered to be almost completed even if it takes several minutes to image ¹ H. In clinically imaging a region accompanied by stillness or slow movement, a high quality image is provided to the extent that there is practically no problem.

However, in recent years, high-speed imaging (imaging time of about 50 ms) which enables imaging of a fast-moving part (such as the heart) and ³¹ P, ¹⁹ F, ¹³ C, and ²³ other than ¹ H have been proposed.
There is an increasing demand for imaging of nuclides such as N. In these cases, technically, improving the S / N is a major issue. For example, in high-speed imaging, S / N deteriorates due to shortening of the imaging time, and S / N deficiency due to the extremely small amount of ³¹ P in the body as about 10 ^{-4 of 1} H is mentioned. .

[0004] In order to improve the S / N, a surface coil is conventionally used for receiving a magnetic resonance signal. The surface coil is placed in close contact with the site of interest of the subject, and can detect signals around the contact site with high S / N. However, there is a drawback that only an image around the contact site can be obtained. A predetermined cross section cannot be imaged with high S / N over the entire area.

On the other hand, a so-called uniform coil (for example, a saddle type coil, a slotted tube resonat) which generates a substantially uniform high-frequency magnetic field in a plane perpendicular to the static magnetic field in a desired region of the subject.
or a birdcage coil) for receiving magnetic resonance signals, an image of the desired cross section of the subject can be obtained. If this uniform coil is used in the quadrature receiving system, the S / N can be improved by a factor of 2 ^{1/2 over the} entire image. However, the S / N of the magnetic resonance signal obtained using a uniform coil
However, it is not possible to obtain an S / N as high as that around the close contact portion when the surface coil is used.

In order to solve the above problems, for example, in US Pat. No. 4,825,162, a plurality of surface coils are arranged in a desired region of an object to be imaged, and these plurality of surface coils are arranged. After detecting magnetic resonance signals from the subject via the surface coil and performing imaging processing on each of the detected magnetic resonance signals to generate a plurality of series of image data, pixel data corresponding to the same spatial position ( A single complex signal or a one-dimensional complex signal = spectral signal) is multiplied by a predetermined weighting function based on the distribution of a high-frequency magnetic field generated by each surface coil to add data to each pixel, By combining one image of the desired area, a high S
A technique for obtaining a / N image has been disclosed.

In this known technique, magnetic resonance signals are simultaneously observed by a plurality of surface coils within a time required to obtain one image, so that the surface coils do not constantly interfere with each other, that is, Decoupling means for preventing mutual coupling of coils is provided so that even if a high-frequency current of a predetermined frequency flows through one surface coil, the high-frequency current does not flow through another surface coil.

However, in this known technique, in order to determine the weighting function, the distribution of the high-frequency magnetic field generated by each surface coil must be obtained in advance. If this high-frequency magnetic field distribution is determined by computer simulation, it is necessary to know the structure, electric conductivity and dielectric constant of the subject, which is practically impossible. Further, in order to experimentally obtain a high-frequency magnetic field distribution, it is necessary to acquire a uniform image of the entire desired region to be imaged in advance using, for example, a uniform coil used for transmission, which requires extra time. And Further, at that time, it is necessary that each surface coil is further actively made insensitive to a predetermined frequency by a decoupling circuit using a pin diode or the like, which complicates the circuit.

Further, when a plurality of surface coils are arranged so as to cover the region of interest of the subject, the S / N of the obtained magnetic resonance signal is similar to the case where a uniform coil is used in the quadrature reception system near the coil. However, the theoretical confirmation that the S / N at the center of the subject, that is, the point farthest from any surface coil, is equal to or greater than that obtained when a uniform coil is used in the quadrature reception method. Not yet obtained.

[0010]

As described above, a magnetic resonance signal is simultaneously collected using a plurality of surface coils , and the obtained image data is weighted and added to obtain a high S / N image. In the technology, the weighting function must be determined in advance based on the high-frequency magnetic field distribution generated by each surface coil, which requires extra time and complicated circuits. there were.

When a surface coil is used, an image can be formed with a high S / N ratio in the vicinity of the contact portion of the subject, but the S / N decreases rapidly as the distance from the contact portion increases.

On the other hand, when a uniform coil using the quadrature receiving method is used, an image of the entire desired cross section can be obtained, but it does not reach the S / N ratio around the contact portion when the surface coil is used.

An object of the present invention is to obtain a weighting function when a high S / N image is obtained by simultaneously detecting magnetic resonance signals via a plurality of surface coils and weighting and adding the obtained image data. Is to provide a magnetic resonance imaging apparatus capable of acquiring data for the same within a time for detecting a magnetic resonance signal via a surface coil.

[0014]

[0015]

SUMMARY OF THE INVENTION The present invention provides a method for detecting a magnetic resonance signal through a plurality of surface coils within a same time period through a uniform coil conventionally used exclusively for transmission. Detecting magnetic resonance signals and performing image processing using a plurality of channels obtained by performing imaging processing including Fourier transform on the magnetic resonance signals obtained through the surface coil and the uniform coil, to generate an image. The data required to determine the weighting function used for, for example, the high-frequency magnetic field distribution of the surface coil is obtained.

That is, a magnetic resonance imaging apparatus according to the present invention comprises a means for applying a static magnetic field to a subject and a gradient magnetic field pulse, and a plane perpendicular to the static magnetic field in a desired region of the subject to be imaged. A uniform coil for generating a high-frequency magnetic field in the object, a plurality of surface coils arranged in close proximity to the object, and a plurality of objects from the object via the uniform coil and the plurality of surface coils, respectively. Signal detection means for detecting magnetic resonance signals of the channels; image data generation means for performing imaging processing including Fourier transform on the magnetic resonance signals of the plurality of channels to generate image data of a plurality of channels; Using image data, a high-frequency magnetic field distribution of at least the plurality of surface coils is obtained, and Weight function determining means for determining a weight function defined as a function of a pixel position for at least each of the plurality of surface coils, and using the weight function to obtain the weight function via at least the plurality of surface coils. Image generating means for obtaining an image of the desired area by weighting and adding pixel data corresponding to the same spatial position of the image data. Further, another magnetic resonance imaging apparatus according to the present invention includes a means for applying a static magnetic field to a subject and applying a gradient magnetic field pulse, and a plane perpendicular to the static magnetic field in a desired region of the subject to be imaged. A uniform coil for generating a high-frequency magnetic field, a plurality of surface coils disposed close to the subject, and a plurality of channels from the subject via the uniform coil and the plurality of surface coils, respectively. Signal detection means for detecting magnetic resonance signals of the plurality of channels, image data generation means for performing imaging processing including Fourier transform on the magnetic resonance signals of the plurality of channels to generate image data of a plurality of channels, and the image of the plurality of channels. Using the data, a weighting function defined as a function of pixel position for at least each of the plurality of surface coils is determined. Weighting function determining means for performing the convolution integration of a plurality of channels of the magnetic resonance signals of the plurality of channels and the result of the inverse Fourier transform of the weighting function to obtain convolution integration results of the plurality of channels; And an image generating means for generating an image of the desired area by Fourier-transforming the addition / synthesis result obtained by the addition / synthesis means. I do.

[0017]

The magnetic resonance signal is received through the uniform coil within the same time as the detection of the magnetic resonance signal through the plurality of surface coils. Along with the image, uniform image data of the entire desired region of the subject to be imaged is obtained, and the high-frequency magnetic field distribution of each surface coil is determined using these image data.

Therefore, after multiplying the magnetic resonance signal of each channel by a weighting function corresponding to the high-frequency magnetic field distribution, data is added for each pixel, or the result of inverse Fourier transform of the magnetic resonance signal of a plurality of channels and the weighting function is obtained. By performing a Fourier transform on the result of addition and synthesis of the convolution integration results of a plurality of channels, a high S / N image of a desired area can be obtained.

[0019]

[0020]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a magnetic resonance imaging apparatus according to one embodiment of the present invention.

In FIG. 1, a static magnetic field magnet 1 is excited by an excitation power supply 2 and applies a uniform static magnetic field to a subject 5. The gradient magnetic field coil 3 is driven by a drive circuit (drive amplifier) 4 controlled by a system controller 16, and moves a subject 5 (for example, a human body) on a bed 6 at a right angle X, X in a desired tomographic plane of interest. Gradient magnetic fields Gx, Gy, Gz whose magnetic field strength changes linearly in the Y direction and the Z direction perpendicular thereto are applied. Under the control of the system controller 16, a high-frequency magnetic field generated by applying a high-frequency signal from the transmission unit 7 to the uniform coil 9, which is a transmission / reception coil, via the duplexer 8 is applied to the subject 5. You.

Inside the uniform coil 9, a multi-surface coil 1 which is a signal detection coil close to the subject 5.
0 is arranged. Then, a magnetic resonance signal from the subject 5 is received by the uniform coil 9 and the multi-surface coil 10. The magnetic resonance signal received by the uniform coil 9 is guided to the receiving unit 11 via the duplexer 8, and the magnetic resonance signal received by the multi-surface coil 10 is directly guided to the receiving unit 11. The duplexer 8 is used for switching the uniform coil 9 between transmission and reception, and transmits a high-frequency signal from the transmission unit 7 to the uniform coil 9 during transmission, and transmits the high-frequency signal from the uniform coil 9 during reception. It functions to guide the reception signal to the reception unit 11.

The magnetic resonance signal input to the receiving section 11 is amplified and detected, and then sent to the data collecting section 12 under the control of the system controller 16. The data collection unit 12 collects the magnetic resonance signals input via the reception unit 11 under the control of the system controller 16, A / D converts the data, and sends the data to the computer 13 as image reconstruction data.

The computer 13 is controlled by the console 14, and performs image reconstruction processing including two-dimensional Fourier transform on the image reconstruction data input from the data collection unit 12. By this image reconstruction, image data of the number of channels equal to the number of coils of the multi-surface coil 10 is obtained, and these image data are weighted and added to synthesize image data corresponding to one image. The computer 13 also controls the system controller 16. The image data obtained by the electronic computer 13 is supplied to the image display 15, where the image is displayed.

FIG. 2 is a schematic sectional view showing the configuration and arrangement of the uniform coil 9 and the multi-surface coil 10. The uniform coil 9 applies a high-frequency magnetic field to the entire region of the subject 5 to be imaged, and further receives and detects a magnetic resonance signal from the subject 5, and is arranged so as to cover the subject 5. ing. The uniform coil 9 is a coil that can apply a uniform high-frequency magnetic field to a region of the subject 5 to be imaged. Specifically, for example, a saddle coil, a distributed constant coil, or a configuration using these coils A quadrature transmitting and receiving coil is used. The multi-surface coil 10 is provided inside the uniform coil 9, and a plurality (6 in this example) disposed close to the subject 5 so as to surround a desired region of the subject 5 to be imaged.
) Of the surface coils 10a to 10f.

FIG. 3 shows details of the receiving section 11 in FIG. A first signal detecting means including a preamplifier 21, a detection circuit (DET) 22, and a low-pass filter (LPF) is provided corresponding to the uniform coil 9, and a preamplifier 31a is provided corresponding to each of the surface coils 10a to 10f.
To 31f, detection circuits 32a to 32f, and low-pass filters 33a to 33f. That is, the magnetic resonance signals received by the uniform coil 9 and the surface coils 10a to 10f respectively correspond to the preamplifiers 21 and 31a to 31f.
And the detection circuits 22 and 32a to 32f
After being detected by the low-pass filters 23 and 33
The unnecessary components are removed in a to 33f, and then input to the data collection unit 12.

The data collection unit 12 samples and holds the detection output of the magnetic resonance signal input from the reception unit 11 and digitizes the detection output with an A / D converter.
Collect data for image reconstruction. The digitized image reconstruction data is taken into the computer 13 of FIG. As a data collection method in the data collection unit 12, all low-pass filter outputs in the reception unit 11 are sampled and held at every sampling time (Δt) determined by a band of an image to be acquired, and each sample hold is performed during Δt. A method of scanning and digitizing the output, and Δt
There is a method in which each low-pass filter output is sequentially sampled and held and digitized, and any of these methods may be used.

Uniform coil 9 and surface coil 10a
Decoupling between both coils is required because of being disposed close to 10 to 10f. For this decoupling, for example, a differential coil 40 shown in FIG. 4 is used as a surface coil. As shown in FIG. 5, the principle is that two identical ring-shaped coils 41 and 42 are coaxially and parallelly arranged and connected so that currents spatially opposite to each other flow. This is the same as that used as a differential coil in a squid magnetometer. One surface coil is constituted by the differential coil 40 and a tuning / matching circuit including capacitance elements 43 to 45 connected to both ends thereof. tuning·
The matching circuit is connected to a preamplifier in the receiving unit 11.

In the surface coil composed of the differential coil configured as described above, when the spatially uniform high-frequency magnetic field B ₁ generated by the uniform coil 9 interlinks, an induction force equal to both coils 41 and 42 is generated. Although power is about to be generated, since the coils 41 and 42 are connected as described above, the electromotive forces induced in the coils 41 and 42 cancel each other out, and no high-frequency current flows. Therefore, this surface coil is always decoupled from the outer uniform coil 9. However, in this case, as a property of the uniform coil 9, it is necessary to generate a spatially sufficiently uniform high-frequency magnetic field in a region where the surface coil composed of the differential coil is arranged.

The S / N when a differential coil is used as a surface coil is described in the literature: Magn. Re
son. Med. 3, 590-603 (1986). That is, the S / N of the surface coil composed of the differential coil is equivalent to the S / N of the normal surface coil composed of a one-turn coil if the noise component from the subject is dominant. In a situation where its own high-frequency loss cannot be neglected, the S / N of a normal surface coil is inferior. Therefore, in actual production and arrangement of the coil, the distance between the surface coil and the subject and the two coils 41 and 42 are required. It is necessary to carefully consider the interval of.

Next, the surface coils 10a to 10f
A method of decoupling between adjacent surface coils when a differential coil is used will be described with reference to FIGS. FIGS. 6 and 7 show surface coils (10a, 10a in FIG.
FIGS. 8 and 9 show the decoupling method when b) is arranged in a plane.
4 shows a decoupling method when b is arranged on the circumference.

In the case of FIG. 6, decoupling is performed by overlapping two differential type surface coils 10a and 10b in the plane by an area S determined by the area surrounding the coils. A similar decoupling method for a one-turn coil is described, for example, in U.S. Pat. No. 4,825,162. In FIG. 7, decoupling is performed by shifting the coils 10a and 10b in the axial direction (the direction perpendicular to the plane surrounded by the coils). FIG. 8 is a modification of the method of FIG. That is, when the differential coil type surface coils 10a and 10b are arranged on the circumference after being curved, the area S 'of the overlapping portion of the coil 41 on the outer peripheral side and the area S' of the overlapping portion of the coil 42 on the inner peripheral side are determined. However, decoupling becomes possible by disposing the coils 10a and 10b at specific positions that satisfy the condition of S ">S> S '. FIG. 9 is a modification of FIG. 7, in which the coils 41 and 42 are arranged at an angle θ. In this case, decoupling can be realized by adjusting one differential type surface coil 10b in the x and y directions in the drawing.

In these decoupling methods, all of the magnetic fields generated when a current flows through one differential coil
This is a state in which the sum of the magnetic fields linked to the other differential coil is zero. As described above, the possibility of decoupling in various arrangements is another feature of the surface coil using the differential coil.

Next, a decoupling method between non-adjacent surface coils will be described. Coupling between non-adjacent surface coils is less than coupling between adjacent surface coils. Therefore, it is sufficient to perform decoupling between surface coils that are not adjacent to each other, without paying attention to the fact that the influence of coupling can be suppressed by lowering the apparent Q without using a precise decoupling method. In particular,
What is necessary is just to add a Q dump circuit to each of the differential type surface coils.

FIG. 10 shows a specific example of the Q dump circuit. It is assumed that the coil 50 resonates at a specific frequency fo by an inductance L and a capacitance element C. The parallel resistance Rp indicates the impedance of the coil 50 in the resonance state, and is expressed as Rp = 2πfo LQ using the Q value.

An inverting input terminal and a non-inverting input terminal of an amplifier 51 having a gain K times are connected to both ends of the coil 50,
Further, a feedback resistor 52 (referred to as a resistance value Rf) is connected between the output terminal and the inverting input terminal of the amplifier 51 so that the amplifier 5
The first output terminal and the non-inverting input terminal are external connection terminals.
The amplifier 51 is actually the preamplifier 31a in FIG.
To 31f are used.

The Q dump circuit composed of the coil 50, the amplifier 51 and the feedback resistor 52 shown in FIG. 10 is represented by an equivalent circuit shown in FIG. The resistance Rd in FIG. 11 is given by Rd = Rf / (K + 1).

Therefore, if the gain K of the amplifier 51 is made sufficiently large, Rp >> Rd, and the impedance at both ends is reduced, so that the apparent Q is reduced.

Depending on the actual arrangement of the differential type surface coils, the coupling between surface coils that are not adjacent may be sufficiently small. In such a case, such a Q dump circuit is unnecessary.

If decoupling between the uniform coil 9 and each of the surface coils 10a to 10f or between adjacent surface coils cannot be sufficiently performed due to the coil shape and arrangement, the surface coils 10a to 1f
It is better to use a Q dump circuit for 0f.

Next, the procedure for high S / N imaging in this embodiment will be described in detail. FIG. 12 shows an imaging sequence for obtaining a two-dimensional image as an example. This imaging sequence is 90 ° as a high-frequency magnetic field (high-frequency pulse).
A pulse sequence for obtaining a two-dimensional image by a known spin echo method using a pulse -180 ° pulse, where Gs is a gradient magnetic field in the slice direction, Gr is a gradient magnetic field in the read direction, and Ge is a gradient magnetic field in the encode direction. The application timing is shown.

As shown in FIG. 12, the encoding gradient magnetic field Ge
The magnetic resonance signal is collected while changing the amplitude of. The application of the high-frequency pulse is performed using the uniform coil 9, and the reception of the magnetic resonance signal is performed using the uniform coil 9 and all the surface coils 10a to 10f (multi-surface coil 10). The magnetic resonance signals detected by the receiving unit 11 via the coils 9, 10 a to 10 f are taken into the computer 13 as image reconstruction data via the data collection unit 12, and are subjected to a two-dimensional Fourier transform in the computer 13. By the conversion, image reconstruction is performed. By this image reconstruction, image data of one channel obtained through the uniform coil 9 and image data of a plurality of channels (six channels in this example) obtained through the surface coils 10a to 10f are obtained. And the surface coil 10
The image data of 6 channels obtained through a to 10 and the image data of a total of 7 channels obtained by adding 1 channel obtained through the uniform coil 9 are determined so that the S / N is maximized. Are weighted and added by the weight function of (1), image data of one image is synthesized.

Generally, when it is considered that the noise detected by the coil is almost inductive noise from the subject 5,
The above weight function is given by [Equation 1] to [Equation 3].

Here, R is a position vector indicating a position (pixel position) on the image, r is a vector indicating a position in a space where the subject is located, and k _i (R) is image data obtained by the i-th surface coil. Weight function for h
_i (R) is a high-frequency magnetic field distribution generated by the i-th surface coil, λ (R) is a correction function for correcting unevenness in sensitivity of image data after weighting and addition caused by the high-frequency magnetic field distribution of the surface coil, E _i ( r) is an electric field generated when a unit high-frequency current is applied to the i-th surface coil. The integration of H _ij is performed for the entire subject.

The noise of the coil includes high-frequency resistance noise caused by the coil itself and dielectric or inductive noise caused by the object. When the coil is mounted on the object, noise from the object is generated. It is expected to make up the majority. If the noise due to the coil itself cannot be neglected, this effect may be considered by changing the diagonal elements of the matrix [H _ij ]. However, since the diagonal element of [H _ij ] is actually smaller than the off-diagonal element, a weight function in which the off-diagonal element of [H _ij ] is set to 0 may be used for simplicity.

To determine the weight function k (R), the high-frequency magnetic field distribution h of each of the surface coils 10a to 10f is determined.
(R) needs to be determined. Hereinafter, this will be described. h (r) can be approximated by h (R). First, the phase of the image data obtained via the uniform coil 9 and the surface coils 10a to 10f is corrected. Here, it is assumed that an image as shown in FIG. 13 is obtained using the uniform coil 9. In FIG. 13, the thick line indicates the position of the uniform coil 9 and the broken line indicates the surface coil 10.
The position of a is shown.

FIG. 14A shows a histogram of the image on the dashed line A in FIG. 13, and FIG. 14B shows a histogram corresponding to the same position of the image obtained by the surface coil 10a. FIGS. 14A and 14B
In the horizontal axis represents the position, and the vertical axis represents the signal intensity S _T, S _S in each image. According to FIG. 14B, it can be seen that the sensitivity of the image obtained by the surface coil 10a decreases as the distance from the position of the surface coil 10a increases. In addition, S /
When N is bad, it is desirable to appropriately perform a smoothing process such as a moving average.

Next, the signal intensity ratio ha of each part of the subject 5
(= S _S / S _T ). FIG. 14 (c) shows the result of calculating the signal intensity ratio ha from the histogram of the image on the dashed line A shown in FIGS. 14 (a) and 14 (b).
Since there is no signal source or no signal is output due to the effect of the relaxation time, the image data is lost. Therefore, processing such as interpolation is performed. Since the high-frequency magnetic field distribution generated by the surface coil can be expanded by an orthogonal function, a method of determining coefficients of each term of the orthogonal function system using the obtained image data by the least square method or the like may be used. By these methods, a high-frequency magnetic field distribution ha (R) as shown in a histogram in FIG. 14D over the entire imaging region of the surface coil 10a can be obtained.

As the high frequency magnetic field distribution ha (R), a signal intensity ratio ha (= S _S / S _T ) of a portion of the subject 5 is simply used as shown in a histogram of FIG. It does not matter.

On the other hand, if the high-frequency magnetic field distribution of the uniform coil 9 becomes non-uniform due to the subject 5 and the like in the image obtained through the uniform coil 9,
It is necessary to obtain the high-frequency magnetic field distribution of.

In the above description, raw data of a magnetic resonance signal obtained from each surface coil is used as image reconstruction data, and the Fourier-transformed image data is weighted and added to obtain high S / N of a desired area. Is obtained, but the following method can be used.
That is, convolution integration of the raw data of the magnetic resonance signal from each surface coil before Fourier transformation and the inverse Fourier transformation result of the weight function corresponding to each is performed, and the convolution integration results of the obtained multiple channels are added and synthesized. After that, a high S / N image of the desired area can be obtained similarly by performing Fourier transform.

For example, in the case of a two-dimensional image, the raw data of the magnetic resonance signal obtained by the i-th surface coil is I _fi (Kx, Ky), the image data after Fourier transform is I _i (X, Y), and the i-th the weighting function k _i of the image
(X, Y), the inverse Fourier transform of the weight function k _i (X, Y) is k _fi (Kx, Ky), and the high S
Assuming that the / N image is I (X, Y), the relationship shown in [Equation 4] holds.

As shown in [Equation 4], the raw data I _fi (Kx, Ky) of the magnetic resonance signal and the weight function k
_The sum of the convolution integral result of _i (X, Y) and the inverse Fourier transform k _fi (Kx, Ky) for each surface coil corresponds to the inverse Fourier transform of the finally obtained image I (X, Y). You can see that Therefore, if this is Fourier-transformed, the image I (X, Y) will be obtained.

In the above embodiment, decoupling between the surface coils 10a to 10f was performed by using a differential coil as the surface coil and devising the arrangement thereof. Can be For example, a decoupling circuit as shown in FIGS. 15 to 17 may be used. FIG. 15A shows a decoupling circuit composed of a bridge circuit composed of two kinds of reactance elements Z1 and Z2, and its equivalent circuit is shown in FIG.
This is shown in FIG. Terminals a and b and terminals c and d are respectively connected to two surface coils 10 to be decoupled.
a, 10b. Reactance element Z1
, Z2, the capacitance elements C1 and C2 are used as shown in FIG. 16, or the inductance elements L1 and L2 are used as shown in FIG. 17 and their values are adjusted to cancel the coupling effect. Can be.

FIG. 18 shows an example in which such a decoupling circuit using a bridge circuit is applied to decoupling between the surface coils 10a and 10b formed of one-turn coils, and FIG. 19 shows a differential type as shown in FIG. The example applied to the decoupling between the surface coils 10a and 10b using a coil is shown.

FIGS. 20 to 22 show decoupling coils 43 to 4 formed of figure-shaped or loop-shaped coils.
An example using No. 5 is shown. In FIG. 20, a decoupling coil 43 is coupled to the surface coils 10a and 10b formed of one-turn coils. This allows coil 1
The magnetic flux linked to one of 0a and 10b can be set to 0, and decoupling becomes possible. FIG. 21 and FIG.
Is a surface coil 10a, 1 composed of a differential coil
The decoupling coils 44 and 45 are coupled to 0b, and decoupling is performed in the same manner.

The decoupling coils 43, shown here,
Reference numeral 44 denotes a butterfly coil which can realize a decoupling state with the uniform coil 9 itself. However, FIG.
In the second example, since the decoupling coil 45 couples with the uniform coil 9 depending on the direction of the high-frequency magnetic field generated by the uniform coil 9, it is desired to consider the arrangement or to separate the coil from the subject in application.

In the previous embodiment, the differential coil was used as the surface coil in order to perform decoupling between the uniform coil 9 and the surface coils 10a to 10f. The decoupling circuit shown in FIG. 15 to FIG.

Next, another embodiment of the high-frequency coil section composed of a uniform coil and a surface coil will be described. FIG. 23 shows an example using a birdcage coil 61 as a uniform coil and a differential coil 64 as a surface coil. The birdcage coil 61 shown here is of a so-called high-pass type, and includes an inductance element 63 arranged in parallel to the axial direction and at a predetermined interval in the circumferential direction, and a capacitance element 62 for connecting the adjacent inductance element inductance elements 63. It is composed of That is, the birdcage coil 61 is configured by arranging the inductance element 63 and the capstan element 62 in a ladder shape.

On the other hand, the differential coil 64 is provided in a space surrounded by the inductance element 63 and the capacitance element 62 constituting the birdcage coil 61.
And two conductor loops arranged side by side in the radial direction. The upper and lower conductor loops are connected by a capacitance element 65 so that currents flow in opposite directions. In this case, if the magnetic fluxes linked to the two conductor loops are equal, no induced electromotive force is generated for the change in the magnetic flux. Accordingly, decoupling between the birdcage coil 61 and the differential coil 64 is possible.

It is desired to consider the decoupling between the differential coils 64 as well. As for the differential coil 64, the coupling magnetic flux can be cut off by disposing the conductive plate 66 between the adjacent coils as shown in FIGS. The conductive plate 66 is desirably disposed at a position where the coils on both sides of the conductive plate 66 are symmetrical. The larger the area, the greater the decoupling effect.

Next, FIGS. 26 to 28 show examples in which the decoupling method described with reference to FIGS. 24 and 25 is actually applied. In FIG. 26, the inductance element parallel to the axial direction (corresponding to the inductance element 63 in FIG. 23) of the high-pass birdcage coil 61 is constituted by the conductive plate 67. Also serves as a decoupling role.

FIGS. 27 and 28 show examples in which a low-pass birdcage coil 71 is used. FIG.
In 7, an inductance element parallel to the axial direction is formed by a conductive plate 72, and a capacitance element formed by an electrode plate 73 and a dielectric plate 74 is inserted in the middle of the conductive plate 72. . In this case, the dielectric plate 7
It is desirable to bend the end of the electrode plate 73 so as to cover a part of the dielectric plate 74 so that the magnetic flux does not leak from a small portion corresponding to the thickness of the dielectric plate 74. FIG. 28 shows an example in which inductance elements parallel to the axis are respectively connected to two conductive plates 7.
5 and 76, each of which is opposed to each other with the dielectric plate 74 interposed therebetween.

FIG. 29 shows an example in which the decoupling circuit described in FIG. 16 is applied to decoupling between adjacent differential coils 64 in FIG.

When the decoupling between adjacent differential coils is insufficient by the above-described method (for example, when the width of the conductive plate 66 in FIG. 26 cannot be made too large).
If there is coupling between differential coils that are not adjacent to each other, it is desirable to reduce the apparent Q by using a Q dump circuit as shown in FIG. 10 in order to suppress them.

FIGS. 30 to 33 show another example of the configuration of the high-frequency coil section comprising a birdcage coil and a differential coil. FIG. 30 shows a configuration in which a conductive plate 67 forms an inductance element parallel to the axial direction in a high-pass birdcage coil 61, and a conductive plate 68 extending in the circumferential direction between the central portions of adjacent conductive plates 67. To divide each space surrounded by the inductance element (conductive plate 67) of the birdcage coil 61 and the capacitance element 62 into two spaces in the axial direction, and to form a differential coil 64 in each of these divided spaces. Are arranged respectively. In this case, decoupling between the adjacent differential coils 64 is performed by the conductive plates 67 and 68.

FIG. 31 shows a low-pass birdcage coil 7.
30 is an example in which the same configuration as that of FIG. 30 is applied. In this example, an annular conductor plate 77 is passed through the center of a capacitance element 78 disposed at the center in the axial direction, thereby forming an inductance element (a conductive plate) of the birdcage coil 71. Each space surrounded by 72) and the capacitance element 78 is divided into two spaces in the axial direction, and the differential coil 64 is arranged in each divided space.

FIG. 32 shows that, in order to dispose three or more differential coils 64 in the axial direction, decoupling between the differential coils 64 adjacent in the axial direction is performed by partially overlapping each coil. This is done by reducing the intersecting magnetic flux to zero. Note that even if the decoupling method described with reference to FIGS. 15 and 16 is used, three or more differential coils can be arranged in the axial direction.

FIG. 33 shows an example in which the differential coil 64 and the figure-eight coil 69 are used in combination as the surface coil. Both the differential coil 64 and the figure-eight coil 69 can be decoupled from the birdcage coil 61. Also, since the coils 64 and 69 can be arranged so that the total of the magnetic fluxes interlinking with each other becomes zero, the coils 64 and 69 are alternately arranged in the circumferential direction as shown in FIG. Decoupling between adjacent surface coils can be easily achieved, and there is no need to make the inductance element 63 parallel to the axial direction of the birdcage coil 61 into a plate-like shape also serving as decoupling.

In the above embodiment, the birdcage type coil is used as the uniform coil. However, since the slotted tube resonator also has four gaps, a surface coil composed of a differential coil or the like is used by utilizing this gap. It is possible to place it in close proximity to the coil.

FIG. 34 shows a case where a birdcage coil is used as the uniform coil 9 as described with reference to FIGS.
It is a figure which shows arrangement | positioning of the high frequency coil part which used the differential coil as the surface coils 10a-19h, and the sampling coil 10a was set in the space surrounded by the inductance element and the capacitance element in the birdcage coil which comprises the uniform coil 9. To 10h are arranged. In this way, both the uniform coil 9 and the sampling coils 10a to 10h can be arranged close to the subject 5,
An image with a high S / N can be obtained both at the surface portion and at the deep portion of the subject 5.

[0072]

According to the present invention, a magnetic resonance signal is conventionally used only for transmitting a high-frequency magnetic field within the same time as simultaneously detecting magnetic resonance signals with a plurality of surface coils and collecting data for imaging. By detecting magnetic resonance signals and collecting data for imaging using the uniform coil, the high S / N image data of each surface coil and the uniformity of the entire desired region of the subject to be imaged can be obtained. Image data is obtained, and the high-frequency magnetic field distribution of each surface coil can be obtained using the image data.

Then, a high S / N image obtained from each surface coil is weighted and added using a weighting function determined based on the high-frequency magnetic field distribution, or magnetic resonance from each surface coil before Fourier transform is performed. The convolution of the raw data of the signal and the result of the inverse Fourier transform of the weighting function corresponding to each of them is performed, and the convolution and integration results of the obtained multiple channels are added and synthesized. S / Images can be combined.

When the image data obtained by simply weighting and adding is non-uniform, it can be made uniform by correcting the sensitivity using the high-frequency magnetic field distribution obtained by each surface coil.

[0075]

[0076]

(Equation 1)

[0077]

(Equation 2)

[0078]

(Equation 3)

[0079]

(Equation 4)

[Brief description of the drawings]

FIG. 1 is a block diagram of a magnetic resonance imaging apparatus according to one embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view showing an arrangement example of a uniform coil and a multi-surface coil with respect to a subject in FIG.

FIG. 3 is a block diagram showing details of a receiving unit in FIG. 1;

FIG. 4 is a diagram showing a specific example of the surface coil in FIG. 3;

FIG. 5 is a view for explaining the principle of a differential coil used in the surface coil of FIG. 4;

FIG. 6 is a diagram showing a decoupling method between surface coils when the surface coil of FIG. 4 is used.

FIG. 7 is a diagram showing an example of a decoupling method between each surface coil when the surface coil of FIG. 4 is used.

FIG. 8 is a diagram showing an example of a decoupling method between surface coils when the surface coil of FIG. 4 is used.

FIG. 9 is a diagram showing an example of a decoupling method between surface coils when the surface coil of FIG. 4 is used.

FIG. 10 is a diagram illustrating an example of a Q dump circuit for decoupling;

FIG. 11 is a diagram showing an equivalent circuit of the circuit of FIG. 10;

FIG. 12 is a diagram showing a pulse sequence for imaging in the embodiment.

FIG. 13 is a view showing an image obtained through a uniform coil in the embodiment.

FIG. 14 is a view for explaining how to obtain a high-frequency magnetic field distribution generated by a surface coil in the embodiment.

FIG. 15 is a diagram showing an example of a decoupling method between surface coils using a capacitance element and an inductance element.

FIG. 16 is a diagram showing an example of a decoupling method between surface coils using a capacitance element and an inductance element.

FIG. 17 is a diagram showing an example of a decoupling method between surface coils using a capacitance element and an inductance element.

FIG. 18 is a diagram showing an example of a decoupling method between surface coils using a capacitance element and an inductance element.

FIG. 19 is a diagram showing an example of a decoupling method between surface coils using a capacitance element and an inductance element.

FIG. 20 is a diagram showing an example of a decoupling method between surface coils using a decoupling coil.

FIG. 21 is a diagram showing an example of a decoupling method between surface coils using a decoupling coil.

FIG. 22 is a diagram showing an example of a decoupling method between surface coils using a decoupling coil.

FIG. 23 is a perspective view of a high-frequency coil unit using a high-pass birdcage coil and a differential coil.

24 is a view for explaining a decoupling method between adjacent differential coils in FIG. 23;

FIG. 25 is a view for explaining the decoupling principle of FIG. 24;

26 is a perspective view of a high-frequency coil unit to which the decoupling method of FIG. 23 is applied.

FIG. 27 shows a part of a low-pass birdcage coil.

FIG. 28 shows a part of a low-pass birdcage coil.

FIG. 29 is a diagram showing an example in which the decoupling circuit of FIG. 16 is applied to decoupling between differential coils in FIG. 23;

FIG. 30 is a diagram showing another configuration example of a high-frequency coil unit including a birdcage coil and a differential coil.

FIG. 31 is a diagram illustrating another configuration example of a high-frequency coil unit including a birdcage coil and a differential coil.

FIG. 32 is a diagram illustrating another configuration example of a high-frequency coil unit including a birdcage coil and a differential coil.

FIG. 33 is a diagram illustrating another configuration example of a high-frequency coil unit including a birdcage coil and a differential coil.

FIG. 34: Using a birdcage coil as a uniform coil,
The figure which shows arrangement | positioning of the high frequency coil part which used the differential type coil as a multi-surface coil.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet 3 ... Gradient magnetic field generating coil 5 ... Subject 7 ... Transmitting part 8 ... Duplexer 9 ... Uniform coil 10a-10f ... Surface coil 11 ... Receiving part 12 ... Data collecting part 13 ... Electronic computer 40 ... Differential type Coil 43-45 ... Decoupling coil 51 ... Q dumping amplifier 61,71 ... Bird cage coil 62 ... Capacitance element 63 ... Inductance element 64 ... Differential coil 65,78 ... Capacitance element 66-68,72,77 ... Conductivity Board 69… 8-shaped coil

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-132645 (JP, A) JP-A-2-13432 (JP, A) JP-A-1-250236 (JP, A) JP-A-3- 68342 (JP, A) JP-A-4-180733 (JP, A) JP-A-2-23507 (JP, U) JP-A-2-500175 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (5)

(57) [Claims] A means for applying a static magnetic field to a subject and applying a gradient magnetic field pulse; and a means for generating a high-frequency magnetic field in a plane perpendicular to the static magnetic field in a desired region of the subject to be imaged. Signal, a plurality of surface coils arranged in close proximity to the subject, and signal detection for detecting a plurality of channels of magnetic resonance signals from the subject via the uniform coil and the plurality of surface coils, respectively. means, wherein performs image processing including each Fourier transform for a plurality channel magnetic resonance signal, an image data generation means for generating image data of a plurality of channels, using the image data of the plurality of channels, at least prior to
Calculate the high-frequency magnetic field distribution of multiple surface coils.
A weight function determining means for determining a weight function defined as a function of a pixel position for at least each of the plurality of surface coils depending on the high-frequency magnetic field distribution; and A magnetic resonance imaging apparatus comprising: an image generating unit that obtains an image of the desired area by weighting and adding pixel data corresponding to the same spatial position of image data obtained via a surface coil. . 2. A means for applying a static magnetic field to a subject and applying a gradient magnetic field pulse, and one means for generating a high-frequency magnetic field in a desired region of the subject to be imaged in a plane perpendicular to the static magnetic field. Signal, a plurality of surface coils arranged in close proximity to the subject, and signal detection for detecting a plurality of channels of magnetic resonance signals from the subject via the uniform coil and the plurality of surface coils, respectively. Means, image data generating means for performing imaging processing including Fourier transform on the magnetic resonance signals of the plurality of channels to generate image data of a plurality of channels, and at least the plurality of image data using the image data of the plurality of channels. Weighting function determining means for determining a weighting function defined as a function of the pixel position for each of the surface coils, A convolution integrator for performing convolution integration of a plurality of channels with the magnetic resonance signals of the plurality of channels and an inverse Fourier transform result of the weight function to obtain a convolution integration result of the plurality of channels; and adding and combining the convolution integration results of the plurality of channels. A magnetic resonance imaging apparatus, comprising: an addition / synthesis unit that performs the Fourier transform of an addition / synthesis result obtained by the addition / synthesis unit; and an image generation unit that generates an image of the desired region. 3. The weight function determining means obtains a high frequency magnetic field distribution of at least the plurality of surface coils using the image data of the plurality of channels, and determines the weight function depending on the high frequency magnetic field distribution. 3. The magnetic resonance imaging apparatus according to claim 2, wherein: 4. Preventing at least one of mutual coupling between at least adjacent coils among the plurality of surface coils and mutual coupling between at least a part of the plurality of surface coils and the uniform coil. The magnetic resonance imaging apparatus according to claim 1, further comprising a unit. 5. At least a part of the plurality of surface coils has a pair of coils of the same shape arranged substantially coaxially and substantially parallel, and these paired coils are connected such that currents flow in opposite directions. 5. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus comprises: