JPH0376136B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention utilizes nuclear magnetic resonance (hereinafter abbreviated as NMR) to image a desired location of a subject.
This relates to NMR imaging equipment.

[Conventional technology]

NMR imaging equipment excites atomic nuclei by irradiating them with high-frequency waves, and detects high-frequency signals (called NMR signals) emitted from the resonating atomic nuclei. Coils are generally used for irradiation and detection of high-frequency signals, and saddle-shaped, solenoid-shaped, and various modified coils have been considered. Since the irradiation and detection are performed in different time periods, a method is also known in which a single coil is used for both.
However, NMR imaging equipment that targets the human body requires spatially wide and uniform irradiation and high sensitivity.
To achieve good detection of SN, a relatively large irradiation coil and a relatively small detection coil placed close to the human body are often used. If the irradiation coil is placed in the same direction as the detection coil, the two will be coupled at high frequencies and the detection sensitivity will be reduced, so they are usually placed on orthogonal axes.

Incidentally, since the sensitivity of the detection coil directly affects the SN ratio of the reconstructed image, many studies and improvements have been made to the detection coil. The excited spins behave as if small magnetic pole pieces were rotating on the same plane. Focusing on this point, CNCHEN et al. Coil (Quadrature)
CNCHEN and others, ``J OF MAGNETIC
In principle, it is said that the S/N ratio can be improved by √2 times. FIG. 6 is a diagram showing the principle of this orthogonal coil, and the tuning circuit and the like are omitted here to simplify the explanation. In the figure, magnetization rotating in one plane induces identical signals in coil 1 and coil 2 with a phase difference of 90°. Here, since the axial directions of the coils 1 and 2 are perpendicular to each other, signals are detected with mutually independent random noise. Possible noise sources include the resistance of the coils 1 and 2, and the equivalent resistance from the subject due to magnetic coupling and electrical coupling between the coils 1 and 2 of the subject. When the phases of the signals from both coils 1 and 2 are matched using a phase shifter 3 and added together using a synthesizer 4, the signal becomes twice as large, and the noise becomes multiplied by √2.
As a result, the S/N ratio is improved by √2 times.

However, this result is valid when the sensitivities of coil 1 and coil 2 are equal, and for this purpose, it is necessary that the dimensions and shapes of coils 1 and 2 are equal, and that the equivalent resistance from the test object described above is also equal. .
The equivalent resistances from the test objects are not actually equal due to the difference in shape of each test object, and in reality, an imbalance occurs between the coils 1 and 2 that are perpendicular to each other. Also the 7th
As shown in the figure, if a solenoid-type coil 5, which is said to have the highest sensitivity, is adopted on the first axis side in a shape close to the subject 6, the axis perpendicular to it will have virtually the same sensitivity. It is not possible to set the coils, for example, a saddle-shaped coil 7, and the unbalance between the coils 5 and 7 becomes even greater. As a result, the SN ratio of both coils 5 and 7 actually reaches, for example, 1.4 to 1.8 times.
In addition, because coils 1 and 2 or 5 and 7 have spatially different sensitivity characteristics, it is not possible to satisfy the above optimal condition (the sensitivities of coils 1 and 2 or 5 and 7 are equal) over a wide area. .

[Problem that the invention seeks to solve]

In the conventional technology, no consideration was given to the unbalance, and the SN ratio finally obtained was low. For example, in Fig. 7, the coil 5
When the SN ratio was 80 and the SN ratio of coil 7 was 50, when the signal levels of both were aligned and added, the final SN ratio was 84.8, an improvement of only 6%. For this reason, there was a problem that high quality images could not be obtained.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a nuclear magnetic resonance imaging apparatus that has a high signal-to-noise ratio and can obtain high-quality images.

[Means for solving problems]

The above-mentioned problem lies in the fact that information with different SN ratios are added with the same weighting. Therefore, in the present invention, signals are weighted according to the difference in the amount of information between the two. This allows a higher signal-to-noise ratio to be achieved.

In FIG. 7, the signal component obtained by the coil 5 is S ₁ and the SN ratio is SN ₁ , and the coil 7 is similarly S ₂ ,
Let's say SN ₂ . Also, let the absolute sensitivity ratio S ₂ /S ₁ be G,
If the ratio of weighted addition is 1 on the coil 5 side and K on the coil 7 side, the SN ratio after addition is: becomes. Here, the condition for maximizing the SN ratio is K=1/G(SN ₂ /SN ₁ ) ² ...(2), and the SN ratio SN _nax at that time is SN _nax =√ ² ₁ + ² ₂ ... …(3) becomes.

Taking into account the imbalance and applying the above equation (2) focusing on the difference in information amount, and adding it using equation (3), the SN ratio can be increased to, for example, 94.3, which is approximately
This would be an 18% improvement.

By the way, the optimization of the SN ratio using equation (2) can be applied by the following three means. The simplest first
This method measures the SN ratio near the center of each orthogonal coil, uses that value in equation (2) to find K, and then adjusts the output gain of each coil using a semi-fixed attenuator. This method can substantially improve the SN ratio without increasing the cost of the device.

The second method is to measure the mapping of the SN ratio using the spatial sensitivity distribution of each coil, find K in the region of interest of the image to be obtained, and optimize the output of each coil using a variable attenuator. It is a method of measurement. This method does not increase the cost of the equipment and can be applied to multi-slice measurements unique to nuclear magnetic resonance imaging equipment.

The third method uses the same SN ratio mapping as the second method to obtain a map of K (K map) using equation (2), and then handles the signals of each coil separately to obtain each image. In this method, the two images are synthesized pixel by pixel or local pixel set according to the K map. Although this method increases cost, it can maintain the highest signal-to-noise ratio in all regions.

[Effect]

The outputs of each coil are weighted by an attenuator according to the SN ratio of the output, and the phase shifter is used to correct and add the phase difference, or each output is weighted according to the SN ratio of the image from each output. By weighting and adding the signals, the final signal-to-noise ratio can be increased and a high-quality image can be obtained.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an example of the overall configuration of a nuclear magnetic resonance imaging apparatus according to the present invention. This nuclear magnetic resonance imaging apparatus obtains a tomographic image of a subject 6 by using the nuclear magnetic resonance (NMR) phenomenon, and includes a static magnetic field generating magnet 10, a central processing unit (hereinafter referred to as CPU) 11, and a sequencer. 1
2, a transmission system 13, a magnetic field gradient generation system 14, a reception system 15, and a signal processing system 16. The static magnetic field generating magnet 10 generates a strong and uniform static magnetic field around the subject 6 in the body axis direction or in a direction perpendicular to the body axis, and is used to generate a strong and uniform static magnetic field in a certain extent around the subject 6. A magnetic field generating means of a permanent magnet type, a normal conduction type, or a superconducting type is arranged in the magnetic field. The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for data collection of tomographic images of the subject 6 to the transmission system 13, magnetic field gradient generation system 14, and reception system 15. The transmission system 13 includes a high frequency oscillator 17, a modulator 18, a high frequency amplifier 19, and a high frequency coil 20a on the transmitting side.
The high-frequency pulse outputted from the high-frequency oscillator 17 is amplitude-modulated by the modulator 18 in accordance with the command from the sequencer 12, and after this amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 19, it is delivered close to the subject 6. A high frequency coil 20a arranged as
By supplying the electromagnetic waves to the subject 6, the electromagnetic waves are irradiated onto the subject 6. The magnetic field gradient generation system 14 is composed of gradient magnetic field coils 21 wound in the three axial directions of X, Y, and Z, and a gradient magnetic field power supply 22 that drives each coil.
By driving the gradient magnetic field power supply 22 of each coil in accordance with the command from 2, gradient magnetic fields Gx, Gy, Gz in three axial directions of X, Y, and Z are applied to the subject 6. . Depending on how this gradient magnetic field is applied, a slice plane for the subject 6 can be set. The receiving system 15 includes a receiving side high frequency coil 20b, an amplifier 23, and a quadrature phase detector 24.
and an A/D converter 25, and the electromagnetic wave (NMR signal) of the response of the subject 6 due to the electromagnetic wave irradiated from the high frequency coil 20a on the transmission side is transmitted to the subject 6.
The signal is detected by a high frequency coil 20b placed close to the high frequency coil 20b, is inputted to an A/D converter 25 via an amplifier 23 and a quadrature phase detector 24, and is converted into a digital quantity. Two series of collected data are sampled by the quadrature phase detector 24, and the signals are sent to the signal processing system 16. This signal processing system 16 includes a CPU 11, a recording device such as a magnetic disk 26 and a magnetic tape 27,
It consists of a display 28 such as a CRT, and the above
The CPU 11 performs processing such as Fourier transformation, correction coefficient calculation, and image reconstruction, and the signal intensity distribution of an arbitrary cross section or the distribution obtained by performing appropriate calculations on multiple signals is converted into an image and displayed on the display 28. ing. In addition, in FIG. 1, the high-frequency coils 20a, 20b and the gradient magnetic field coil 21 on the transmitting side and the receiving side are arranged in the magnetic field space of the static magnetic field generating magnet 10 arranged in the space around the subject 1. .

Here, the high frequency coil 20b according to the present invention is orthogonal coils 5 and 7 as shown in FIG. Further, as described above, the high frequency coil 20a for irradiation
is also used as a receiving coil on the 1-axis side, and a receiving coil orthogonal to it is provided to create orthogonal coils 5 and 7.
may be configured. Furthermore, both coil 5 and coil 7 in FIG. 7 may be used for both irradiation and reception. In either case, it is important to extract signals from the two coils 5 and 7 that are perpendicular to each other.

FIG. 2 shows an embodiment using the first method of the present invention described above, in which the outputs of orthogonal coils 5 and 7 tuned to the resonance frequency are connected to inputs 30 and 31.
Each signal is amplified by preamplifiers 32 and 33, and the output of the preamplifier 33 is passed through a semi-fixed attenuator 34 and a phase shifter 35 for making the phases of both outputs the same, then added by an adder 36, and output to an output 37. do. The semi-fixed attenuator 34 is
The value is set in advance using the above equation (2) based on the SN ratio and absolute sensitivity ratio G of each output when measuring a reference object (not shown). Since the purpose of the present invention is to adjust the relative sensitivity balance obtained by equation (2), the same effect can be achieved even if the configuration illustrated in FIG. 2 is modified. For example, changing the arrangement order of the circuit sections 33 to 35, using a variable gain amplifier that integrates the circuit sections 33 and 34, or changing the impedance of each coil 5 and 7 after tuning to perform synthesis. . In either method, according to this embodiment, it is possible to improve the SN ratio near the center, which is most important in terms of image quality.

FIG. 3 shows an embodiment using the second method of the present invention described above. Semi-fixed attenuator 34 in Fig. 2
is set as a variable attenuator 38, and the CPU is adjusted so that the optimum value of K corresponding to the target spatial position is obtained.
Controlled by 11. This CPU 11 may be replaced by the sequencer 12. According to this embodiment, the best SN ratio can be obtained at the target spatial position.

FIG. 4 shows an embodiment using the third method of the present invention described above. In the overall configuration shown in FIG. 1, two receiving systems 15 are provided corresponding to orthogonal coils 5 and 7, and the CPU 11 calculates two images based on the respective outputs. The obtained images are combined according to the above-mentioned K map to finally form a single image. Figure 5 shows the calculation procedure at this time, and the K map K is added to the obtained image A(i,j)
The final image I(i,j) is obtained by multiplying by (i,j) and adding the resulting image B(i,j).
The K map may be obtained for all three-dimensional pixels, or three-dimensional representative points with a reduced number of points may be prepared, and pixels between the representative points may be obtained by interpolation. According to this embodiment, it is possible to maintain the best signal-to-noise ratio at all target points.

〔Effect of the invention〕

As described above, the present invention weights the NMR signals in consideration of the imbalance in characteristics that occurs between the detection means (orthogonal coils) that detect the NMR signals on two orthogonal axes. So
This has the effect of increasing the signal-to-noise ratio and obtaining high-quality images.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of the overall configuration of the device of the present invention, FIGS. 2 to 4 are block diagrams showing examples of the main part configuration of the device, and FIG. Diagram for explanation, Figure 6 is NMR
FIG. 7 is a diagram showing the principle of an orthogonal coil used for NMR signal detection in an imaging apparatus, and is a perspective view showing a specific example of the coil. 5, 7... Orthogonal coil, 6... Subject, 10... Static magnetic field generating magnet, 11... Central processing unit (CPU), 13
... Transmission system, 14... Magnetic field gradient generation system, 15... Receiving system, 16... Signal processing system, 20b... High frequency coil (NMR signal detection means), 34, 38... Attenuator, 35... Phase shifter, 36... Adder.

Claims (1)

[Scope of Claims] 1. A means for applying a static magnetic field and a gradient magnetic field to a subject; a means for applying a high frequency to cause nuclear magnetic resonance in the nuclei of atoms constituting the tissue of the subject; A nuclear magnetic resonance imaging apparatus comprising magnetic resonance signal detection means for detecting signals on two orthogonal axes, respectively, and arithmetic means for performing an image reconstruction operation using the nuclear magnetic resonance signals. an attenuator that weights the two nuclear magnetic resonance signals detected by the signal detection means according to the SN ratio of the two nuclear magnetic resonance signals; and a phase shifter that corrects the phase difference between the two nuclear magnetic resonance signals. and an addition means for adding the two weighted and phase difference corrected nuclear magnetic resonance signals. 2 means for applying a static magnetic field and a gradient magnetic field to the subject, means for applying high frequency to cause nuclear magnetic resonance in the nuclei of atoms constituting the tissue of the subject, and 2 means for orthogonal to the signals caused by the nuclear magnetic resonance In a nuclear magnetic resonance imaging apparatus, the nuclear magnetic resonance imaging apparatus includes nuclear magnetic resonance signal detection means for detecting signals on two axes, and calculation means for performing image reconstruction calculation using the nuclear magnetic resonance signals. An image is obtained for each of the two nuclear magnetic resonance signals obtained, and each subset of pixels of the obtained image is
A nuclear magnetic resonance imaging apparatus characterized by comprising means for performing weighted addition according to an SN ratio.