JPH0368342A - Magnetic resonance imaging device - Google Patents

Abstract

PURPOSE:To obtain an image having high S/N over a wide range by carrying out the image formation processing by detecting the magnetic resonance signals through a plurality of signal detecting coils and multiplying each determined weight function for each coil and adding the data for each image element. CONSTITUTION:The signal detecting coils 9a-9c of a multisurface coil 9 are arranged so as to surrounded a desired image generating region for an inspected body 5. The signal obtained in each signal detecting coil 9a-9c is put into the two-dimensional fourier transformation, and a two-dimensional image data for the coils is obtained. The weight function in proportion to the spacial distribution of the high frequency magnetic fields is previously determined for each image element data of the image data, and after the weight function is multiplied with each image data, addition calculation is performed for the number (n) of the signal detecting coils 9a-9c, and the image data for (n) pieces is reconstructed to the image data for one piece.

Description

[Detailed Description of the Invention] [Purpose of the Invention (Industrial Application Field) The present invention relates to a magnetic resonance imaging apparatus, and particularly to a magnetic resonance imaging apparatus equipped with an image composition means for acquiring a high S/N image using a surface coil. It relates to a magnetic resonance imaging device.

(Prior Art) Magnetic resonance imaging apparatuses are considered to be nearly perfect for +H imaging, even if the imaging time takes several minutes. Clinically, when imaging a site that is stationary or that moves slowly, it provides images of such high quality that there is almost no problem in practical use.

However, in recent years, there has been a growing demand for high-speed imaging (imaging time ~5 hs) that enables imaging of fast-moving parts (such as the heart), and for imaging of nuclides other than IH, such as 31p and +9p2-port C123N. . In this case, technically speaking, improving the S/N becomes a big issue. For example, in high-speed imaging, there is a deterioration in S/N due to shortened imaging time, and for 31p, the S/N is insufficient due to the extremely small amount of 31p in the body, about 10-6 of IH.

In order to improve the S/N ratio, surface coils have been conventionally used as high frequency receiving coils. Surface coils are placed in close contact with the area of interest of the subject and can detect signals around the area of close contact with a high S/N ratio, but they have the disadvantage that only images around the area of close contact can be obtained, and High S/N over the entire predetermined cross section
cannot be imaged. Another method is to sequentially change the arrangement of one surface coil and take images, and then synthesize the images obtained from each arrangement to synthesize an image of a predetermined cross section. However, this method requires adjustment of the device to change the arrangement of the surface coil. is required, making the work complicated.

(Problems to be Solved by the Invention) When trying to obtain a high S/N image in high-speed imaging or imaging of trace amounts of nuclides, conventional techniques using a single surface coil cannot There was a problem in that it was difficult to easily obtain images over the area.

An object of the present invention is to provide a magnetic resonance imaging apparatus that can obtain images with high S/N over a wide area in high-speed imaging or imaging of trace amounts of nuclides.

[Besides invention! 21 (Means for Solving the Problems) In order to achieve the above object, the present invention arranges a plurality of signal detection coils (surface coils) so as to surround a desired area of a subject to be imaged, After each of the magnetic resonance signals from the subject is detected through the plurality of signal detection coils, and the detected magnetic resonance signals are subjected to imaging processing to generate multiple series of image data,
Pixel data (single complex signal or one-dimensional complex signal or vector signal) corresponding to the same spatial position is multiplied by a predetermined weighting function according to the arrangement of each signal detection coil, and then added or averaged. By doing this, data for each pixel is created and one image of the desired area is synthesized.

- In order to simultaneously observe magnetic resonance signals using multiple signal detection coils within the time required to obtain one image, it is necessary to prevent the signal detection coils from regularly interfering with each other, that is, to detect one signal. It is desirable to provide decoupling means for preventing mutual coupling of the coils so that even if a high frequency current of a predetermined frequency is passed through the signal detection coil, the high frequency current will not flow through the other signal detection coils.

If the sensitivity regions of each signal detection coil do not overlap significantly, there is little correlation between the noise induced in each coil from the subject, so the magnetic resonance signals of multiple signal detection coils can be simultaneously sampled and observed. However, the noise is canceled out. If the sensitivity regions largely overlap, there will be a correlation between the noise induced by the subject in each coil, so the sampling interval (sampling time) determined by the image band
It is desirable to shift the sampling timing of the magnetic resonance signals detected by each coil within the range of .

(Function) As in the present invention, magnetic resonance signals are detected through a plurality of signal detection coils consisting of surface coils, each image is processed, and each coil is multiplied by a predetermined weighting function. After that, by adding the data for each pixel, while taking advantage of the high S/N of the image by using a surface coil, it is possible to obtain a high S/N image covering a wide area that cannot be detected with a single surface coil.
Obtained by N.

If a coupling means is provided to prevent multiple signal detection coils from regularly interfering with each other, magnetic resonance signals can be simultaneously output from each coil without deterioration of S/N due to coupling between the coils. Detected. Therefore, magnetic resonance signals are detected by multiple signal detection coils in the same time as it takes to obtain one image data via one signal detection coil, and high S /N imaging is achieved.

Further, when the sensitivity regions of the respective coils overlap, deterioration of the S/N ratio can be prevented by shifting the sampling timing of the magnetic resonance signals from each coil within the range of the sampling interval determined by the image band.

(Embodiment) FIG. 1 is a block diagram showing the configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention.

In the figure, a static magnetic field magnet 1 is excited by an excitation type Ig, 2, and provides a --like static magnetic field to a subject 5.

The gradient magnetic field coil 3 is driven by a drive circuit 4 controlled by a system controller 11, and is directed to a subject 5 (for example, a human body) on a bed 6 in orthogonal X, Y directions, and within a desired tomographic plane of interest. Gradient magnetic fields Gx, Gy, Gz whose magnetic field strength changes linearly in the Z direction perpendicular to these
Apply. Further, under the control of the system controller 11 , a high-frequency magnetic field generated from a transmitting coil 8 is applied to the subject 5 in response to a high-frequency signal from a transmitter 7 .

The signal detection multi-surface coil 9 receives magnetic resonance signals from the subject 5. The received magnetic resonance signal is amplified and detected by the receiving section 10, and then sent to the data collecting section 12 under the control of the system controller 11. The data collection unit 12 collects the magnetic resonance signals inputted through the reception unit 0 under the control of the system controller 11, converts them from A/D, and then sends them to the electronic computer 13.
send to

The electronic computer 13 is controlled by the console 14, performs image reconstruction processing on the magnetic resonance signals input from the data acquisition section 12, and obtains multiple series of image data. Further, the electronic computer 13 also controls the system controller 11. Image data obtained by the electronic computer 13 is supplied to an image display 15, and the image is displayed.

The transmitting coil 8 is for applying a high frequency magnetic field to the entire region of the subject 5 to be imaged, and is arranged so as to cover the subject 5. As the transmitting coil 8, a saddle type coil or a distributed constant type coil is used.

On the other hand, the multi-surface coil 9 has a plurality of (in this example, five) surface coils (
(hereinafter referred to as signal detection coils) 9a to 9e are the test object 5.
are arranged so as to surround the desired area to be imaged.

FIG. 3 shows details of the receiving section 10 in FIG. 1. Each signal detection coil 9a to 9e includes a tuning/matching circuit 16a.
~16e1 preamplifier 17 a~17e1 detection circuit 18
Signal detection means consisting of a to 18e are respectively provided. The detection outputs of the magnetic resonance signals outputted from the detection circuits 18a to 18e are digitized by A/D converters 19a to 19e in the data acquisition section 12, and are input into the electronic computer 13 in FIG.

Further, decoupling circuits 20a to 20j are provided between the signal detection coils 98 to 9e to prevent the coils from constantly cambling with each other. As the decoupling circuits 20a to 20j, for example, a bridge circuit including a reactance element ZZ2 having two types of values as shown in FIG. 4(a) is used.
) shows an equivalent circuit of the bridge circuit of FIG. 4(a). Reactance element 2. ．． A capacitor 22.22 is connected to the capacitor 22.22 as shown in FIG. C2 or, as shown in FIG. 6, inductors L, .L2 and their values adjusted to cancel the coupling effect.

FIG. 7 shows an example of a state in which a decoupling circuit using such a bridge circuit is actually connected to a surface coil. Two signal detection coils (9a in the example in the figure) to decouple terminals a, b and terminal C1d, respectively.
, 9b), and adjust the value of the capacitor 21 to perform deca..l bling. One circuit is required between the coils to be decoupled, and if the number of signal detection coils is n, then n(n-2)/2 are required.

FIG. 8 is an example of a method for decoupling without using a special decoupling circuit, in which adjacent signal detection coils 98 to 9e are partially overlapped and the degree of overlap is adjusted to decouple the decoupling. doing the ring.

Also, if the signal detection coils 9a to 9e are not made insensitive to the transmission coil 8 during transmission, when the high frequency current flows through the transmission coil 8, the signal detection coils 9a to 9e
A current also flows through the transmitting coil 8, and the high frequency magnetic field distribution formed by the transmitting coil 8 is disturbed. Figure 9 shows an example of a method to prevent this.
e) A circuit including a trap capacitor 22, a trap inductor 23, and a cross diode 24 is inserted. Capacitor 22 and inductor 23 are adjusted to resonate at exactly a predetermined magnetic resonance frequency fo.

25 and 26 are matching capacitors. During transmission, a part of the high frequency magnetic field generated from the transmitting coil 8 is transmitted to the inductor 2.
3, a high voltage is applied across the cross diode 24, and the cross diode 24 is turned on.

Then, both ends of the capacitor 22 become high impedance, and the signal detection coil 9 (9a to 9e) is cut off to the preamplifier 17 (17a to 17e) in FIG. 3. When a magnetic resonance signal is detected, the voltage across the cross diode 24 is low and the cross diode 24 is turned off, so that the signal detected by the signal detection coil 9 is transmitted to the preamplifier 17.

Note that a bin diode may be used instead of the cross diode 24, and the bin diode may be in an ON state only during transmission and in an OFF state in other cases.

During reception, the transmitting coil 8 is connected to the signal detecting coil 9.
It is also necessary to make it insensitive, but to do this, a parallel circuit of an inductor and a capacitor using a vinyl diode is inserted in series with the transmitting coil 8, and it is turned off only when transmitting and turned on at other times. Good.

Next, the imaging procedure in this example will be explained. First, magnetic resonance signals are observed using each of the signal detection coils 9a to 9e according to a normal imaging sequence. An example of an imaging sequence is shown in FIG. This imaging sequence consists of a 90° pulse - 180° pulse as a radiofrequency magnetic field.
This is a pulse sequence for obtaining a two-dimensional image by a known spin echo method using pulses, where Gs is the gradient magnetic field in the slice direction, Gr is the gradient magnetic field in the read direction, and Ge is the application timing of the gradient magnetic field in the encode direction. show. The signals obtained from each of the signal detection coils 9a to 9e are respectively subjected to two-dimensional Fourier transform to obtain two-dimensional image data for five images.

Next, a reconstruction method will be described using the obtained image data for five images as image data for one image.

- within the shell, for each pixel data of the image data obtained via the signal detection coil i, a complex weighting function F
1(x, y) in advance and convert it into each pixel data (
After multiplying I (x, y) by I (complex signal), and then adding I" (X, y) for the number n of signal detection coils 9a to 9e, image data for n images is divided into image data for one image. Reconstruct into image data.This can be expressed as the following equation.

1' (x, y) - Σ F i (x, y) ・I
t(x, y) - (1) Note that instead of taking the addition, the addition average (x, y) may be taken as follows.

I ” (x, y) = Σ F i (x, y) ・
I j (x, y) / Σ F 1 (x, y)
(2) If there is no correlation between the noises detected by the signal detection coils 9a to 9e, the image S/N of I'(x, y) or I'(x, y) is the same as that of the signal. When defined by the ratio of the size and the standard deviation σl of noise, it is expressed by the following formula.

S/N = P (X, y) (Σ F I (x, y
) ・S i'(x,y)]/(ΣF
i 2 (X, /) a I 21
-(3) Here, I I (x, y) = P (x, y) ・S
'I (x, y) - (4) However, S "i (x
, y) is the high-frequency magnetic field distribution when a high-frequency current is passed through each signal detection coil 9, and P(x, y) is a quantity that does not depend on the signal detection coil, that is, the spin density, longitudinal relaxation time, and transverse relaxation time. T1° represents a quantity dependent on T2.

From equation (3), the conditions for Fl(x, y) that maximize the S/N are determined as follows.

(1≦1≦n1 or A is a constant that does not depend on i) This (5
) formula shows that Fi (x, y) is proportional to the high-frequency magnetic field distribution normalized by the square of the standard deviation of the image noise produced by the coil I. In other words, it is shown that if the gain of each image is adjusted to be σ1-...-σj-...-σn, FI(x, y) is proportional to the high-frequency magnetic field distribution.

Here, what the complex weighting function F i (x, y) indicates in the high-frequency magnetic field distribution will be explained. Signal detection coils 93-9
A phantom is arranged symmetrically about the axis e, and when the phantom is excited uniformly by the transmitting coil 8, the real part and imaginary part of the magnetic resonance signals detected by the signal detecting coils 9a to 8e are measured at the echo beak. When imaged after performing phase correction, the real part of the complex weighting function F I (x, y) indicates the distribution of high-frequency magnetic field components in the axial direction of the signal detection coil,
The imaginary part shows the distribution of components in the direction orthogonal to the axis of the signal detection coil.

In addition, when performing phase correction in two dimensions so that each pixel data of the obtained image data becomes 2 and the imaginary part becomes 0,
The same phase correction must be performed for the complex weighting function. Furthermore, the symmetry of the signal detection coil is poor,
If the position of the phantom is not symmetrical with respect to the axis of the signal detection coil, the axes representing the real and imaginary parts will change, and the complex weighting function will also have to be changed accordingly.

However, in reality, even if the complex weighting function is not determined so strictly, the effect of improving the S/N ratio does not deteriorate that much. The high-frequency magnetic field distribution due to the signal detection coil can be determined by computer simulation or measurement using magnetic resonance imaging.

Note that instead of the above complex weighting function Fl(x, y), a function proportional to the absolute value of the magnetic field strength of the high-frequency magnetic field may be used, in other words, a weighting function proportional to the spatial distribution of the high-frequency magnetic field. That's fine.

When calculating the high-frequency magnetic field distribution by computer simulation, it is necessary to know the position of the signal detection coil. One method is to measure the position after fixing the signal detection coil. In addition, as another method, the first
As shown in FIG. 1, one phantom 32 is installed at a fixed position relative to each signal detection coil 9a, for example, on the signal detection coil fixing stand 31, and the phantom 32 is imaged at the same time as the subject 5. , the position of the signal detection coil may be determined from the position of the phantom on the image. The contents of the phantom 32 include high S/N in the subject 5;
You may use a different nuclide from the '1IP1 constant symmetric +& interpolation to be imaged. For example, in a 31p high S/N imaging experiment, an IH phantom may be used because a 1a signal detection coil is also placed for positioning the subject.

If the sensitivity regions of the signal detection coils 9a to 9e do not overlap significantly, there is little correlation between the noise induced from the subject 5 in each of the coils 9a to 9e.
The magnetic resonance signals detected by 9e may be observed simultaneously. However, when the sensitivity regions overlap, a correlation appears between the noises induced from the subject 5 in each of the signal detection coils 9a to 9e. In order to avoid this, each signal detection coil 9a must be
It would be better to shift the sampling timing of the signal from ~9e. This will be explained using FIG. 12. For example, in the signal detection coil 9a, as shown in FIG. 12(a), the sampling interval (sampling time) Δt1
! When observing the magnetic resonance signal for each encode at 111 time T, the sampling start time of the coil 9b is slightly shifted while the sampling time between the observation ends remains the same, as shown in FIG. 3(b). The sampling start times for the coils 9C to 9e are also shifted little by little as shown in (C) to (e) in the figure. This reduces the correlation between the five noises detected by each of the coils 9a to 9e, and improves the S/N when weighting is calculated later.

In this embodiment, a case has been described in which the magnetic resonance signals detected by each signal detection coil are simultaneously combined, but when images obtained individually are combined without using each signal detection coil at the same time. Addition or averaging processing using a weighting function is also effective.

Furthermore, although this embodiment has been described with respect to the case of obtaining a two-dimensional image, the present invention can also be applied to the case of obtaining a spatial three-dimensional image. In this case, the weighting function also becomes a function of three variables corresponding to three dimensions.

Furthermore, the present invention can also be applied to chemical shift imaging. For example, as shown in FIG.
When using a three-dimensional imaging sequence including the chemical shift axis (ωδ) and the chemical shift axis (ωδ), the weighting function is a quantity that has a value in space, so it becomes a function that has two spatial two-dimensional variables as variables. The actual weighting is expressed in the following equations for the case of simple addition and the case of averaging.

1'(x,y,ω6)-)i? , F i(x
,y) ・I j(x, y, ω6)...(8) 1"(x, y, ωδ) -F l(x, y) ・I
i(x, y, ωδ)/Σ F 1(x, y>
...(7) [Effects of the Invention] According to the present invention, magnetic resonance is detected through a plurality of signal detection coils consisting of so-called surface coils arranged so as to surround a desired region of a subject to be imaged. Images obtained by using surface coils are obtained by detecting signals, performing imaging processing on each of these magnetic resonance signals, multiplying them by a predetermined weighting function for each coil, and performing addition or averaging. While taking advantage of the feature of high S/N, it is possible to obtain images covering a wide area that cannot be detected with a single surface coil at high S/N.

In addition, at this time, by decoupling the multiple signal detection coils so that they do not regularly interfere with each other, magnetic resonance signals can be detected simultaneously from each coil without deterioration of S/N due to coupling between the coils. I can do it. Therefore, magnetic resonance signals are detected by multiple signal detection coils within the same time as it takes to obtain a single image signal, achieving high S/N imaging of a desired wide area in a short time. be done.

Furthermore, if the sensitivity areas of each signal detection coil overlap,
By shifting the timing of signal sampling from each coil within the sampling time range determined by the image band, high S/N imaging of a wide area can be achieved.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of a magnetic resonance imaging apparatus according to an embodiment of the present invention, FIG. 2 is a diagram showing an example of the multi-surface coil for signal detection in FIG. 1, and FIG. 4(a) and 4(b) are block diagrams showing the details of the receiving section in FIG. 3, and FIG.
Figures 6 and 6 are diagrams showing the bridge circuit in more detail, and Figure 7 shows the bridge circuit in more detail.
The figure shows a connection diagram of a decoupling circuit using the bridge circuit shown in Fig. 5, Fig. 8 shows another example of the decoupling means, and Fig. 9 shows how the signal detection coil is connected to the transmitting coil during transmission. FIG. 10 is a diagram showing an example of a pulse sequence for imaging in the same embodiment; FIG. 11 is a diagram showing a method for positioning the signal detection coil; FIG. 13 is a timing diagram for explaining the operation of another embodiment of the present invention, and FIG. 13 is a diagram showing an example of a pulse sequence for imaging in still another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Static magnetic field magnet, 2... Power source for excitation, 3... Gradient magnetic field generation coil, 4... Drive circuit, 5... Subject, 6... Bed, 7... Transmission Part, 8... Transmission coil, 9... Multi-surface coil for signal detection, 9
a to 9b... Signal detection coil, 10... Receiving section,
11. System coil controller, 12. Data collection unit, 13. Computer, 14. Console.
15... Image display, 16a-16e... Tuning/matching circuit, 17a-17e... Preamplifier, 18
a to 18e...detection circuit, 19a to 19e-=A/D
Transducers, 20a, 20b. 2 Q j... Decoupling circuit, 21... Capacitor for decoupling adjustment, 22... Coil for trap, 23... Capacitor for trap, 24... Cross diode, 31... For signal detection Coil positioning phantom, 32... Coil fixing stand for signal detection.

Claims (4)

[Claims] (1) It includes a means for applying a static magnetic field, a gradient magnetic field pulse, and a high-frequency magnetic field to the subject, and a plurality of signal detection coils arranged so as to surround a desired region of the subject to be imaged. , a plurality of signal detection means for detecting magnetic resonance signals from a subject through each of these signal detection coils, and performing imaging processing on the magnetic resonance signals respectively detected by these plurality of signal detection means, means for generating a series of image data; and a weighting function predetermined as a function of spatial position for each of the plurality of signal detection coils, for dividing pixel data corresponding to the same spatial position of the plurality of series of image data. A magnetic resonance imaging apparatus characterized by comprising means for obtaining an image of the desired region by multiplying and adding or averaging. (2) The magnetic resonance imaging apparatus according to claim 1, wherein the weighting function is proportional to the spatial distribution of a high-frequency magnetic field generated when a high-frequency current is passed through each signal detection coil. (3) The magnetic resonance imaging apparatus according to claim 1, further comprising means for preventing mutual coupling of the plurality of signal detection coils. (4) The plurality of signal detection means includes a plurality of A, which samples and digitizes the magnetic resonance signals detected via the plurality of signal detection coils at predetermined sampling intervals.
/D converter, the operation of sequentially sampling the magnetic resonance signals from the plurality of signal detection coils during the sampling period is repeated during the signal observation time of the magnetic resonance signals, or the magnetic resonance signals are simultaneously sampled. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus detects.