JPH07303622A - Rf coil for mri and mri apparatus - Google Patents

Abstract

PURPOSE:To provide an RF coil for MRI which can improve the SNR of receiving signals and an MRI apparatus which can improve the SNR by using such RF coil for MRI. CONSTITUTION:The coil for MRI has at least six elements and is constituted with a birdcage coil 1 structured so as to detect a magnetic field in the direction crossing at a right angle with the main magnetic field and a solenoid coil 2 to detect a magnetic field in the direction crossing at a right angle with the main magnetic field and the magnetic field detected by the birdcage coil 1 as well. The MRI apparatus is provided with an RF signal supply means, a compensating means to adjust the phases of receiving signals of the coil 1 and the coil 2 so as to make them equal and an adding means to add the adjusted receiving signals.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RF coil and an MRI apparatus used in a magnetic resonance imaging (MRI) apparatus, and more particularly to an MRI RF coil having a main magnetic field in a direction orthogonal to a body axis of a subject and the MRI RF coil. The present invention relates to an MRI apparatus using a coil.

[0002]

2. Description of the Related Art An MRI apparatus measures the density distribution, relaxation time distribution, etc. of nuclear spins at a desired examination site in a subject by utilizing the nuclear magnetic resonance phenomenon, and the cross section of the subject is determined from the measured data. The image is displayed.

Nuclear spins of a subject placed in a uniform and strong static magnetic field generator perform precession around the direction of the static magnetic field at a frequency (Larmor frequency) determined by the strength of the static magnetic field. Therefore, when a high frequency pulse having a frequency equal to this Larmor frequency is irradiated from the outside, spins are excited and transition to a high energy state. This is called a nuclear magnetic resonance phenomenon. When the irradiation of this high-frequency pulse is stopped, the spin returns to the original low energy state with a time constant corresponding to each state, and at this time, the electromagnetic wave is emitted to the outside. A high-frequency receiver coil (R
F coil) to detect. At this time, a triaxial gradient magnetic field is applied to the static magnetic field space for the purpose of adding position information to the space. As a result, position information in the space can be captured as frequency information.

FIG. 10 is a configuration diagram showing a conventional configuration of this type of coil. In FIG. 10, in the vertical magnetic field type MRI apparatus using the opposed permanent magnets, the SN ratio (hereinafter,
A quadrature receiving coil for improving SNR (Signal to Noise Ratio) will be described.

FIG. 10 (a) shows a saddle coil 21 used as a first coil of a quadrature receiving coil. This saddle coil 21 has a circular arc portion in the XY plane, and has 4 arcs in the Z direction.
It has a book element. The saddle coil 21 generates a magnetic field in the X direction. The saddle coil 21 is shown in FIG. 10 when viewed from the XY plane.
As shown in (b), the elements are arranged at an angle of 120 °.

Further, FIG. 10 (c) shows a solenoid coil 22 used as a second coil of the quadrature receiving coil. The solenoid coil 22 is a coil formed by loops on the XY plane and having loops stacked in the Z direction. The solenoid coil 22 generates a magnetic field in the Z direction.

By combining the two types of coils as described above, two types of magnetic field components orthogonal to each other in the X and Z directions can be detected without mutual interference. Therefore, the received signal of each coil is amplified by a preamplifier or the like, then phase-corrected by a phase correction circuit to make it in phase, and then added by an adder circuit. When adding in this way, the SN of both coils
When R is equal, the signal is doubled (+6 dB), but since noise has a random phase, it is √2 times (+3 dB).
B). Therefore, the SNR of the output of the adder circuit is 3d.
B is increased ((+6 dB)-(+ 3 dB)), and the noise component can be reduced.

[0008]

However, as shown in FIG.
The two types of coils shown in
, The SNR improvement effect as calculated above cannot be obtained.

Here, suppose that the SNR of the first coil is a, the SNR of the second coil is b, the gain ratio of the first coil to the second coil is x, and the noise level of the first coil is N1. , SNR after in-phase addition (SNR (x)) is as follows.

[0010]

[Equation 1]

X that maximizes this SNR (x) is SN
It is x that satisfies the condition that the differential value SNR ′ (x) of R (x) becomes zero. When this condition is obtained, x = b / a,
At this time, the SNR after the in-phase addition becomes maximum. X =
The SNR under the condition of b / a is √ (a ² + b ² ).

Assuming that the SNR of both coils alone is a> b, the improvement ratio of the SNR by combining both coils is √ (a ² + b ² ) / a. SN of both coils
FIG. 11 shows a plot of what the SNR improvement ratio (√ (a ² + b ² ) / a) becomes using the R ratio a / b as a parameter.

In the example shown in FIG. 11, the saddle coil 2
In the region where the relative SNR of the solenoid coil 22 is higher than 1 (the region where a / b is larger than 1), the SNR improvement when quadrature reception is performed by both coils is an ideal value √2 ( It can be read that it is well below +3 dB). It should be noted that such a state occurs because the efficiency of the solenoid coil 22 is higher than that of the saddle coil 21 in a low magnetic field state.

The present invention has been made in view of the above points, and an object thereof is MRI capable of improving the SNR of a received signal.
RF coil and an MRI apparatus capable of improving SNR by using such an MRI RF coil.

[0015]

A first means for solving the above-mentioned problems is a birdcage coil having at least 6 elements and configured to detect a magnetic field in a direction orthogonal to the main magnetic field. , In the direction orthogonal to the main magnetic field,
An RF coil for MRI, comprising: a solenoid coil that detects a magnetic field in a direction orthogonal to the detection magnetic field of the birdcage coil.

A second means for solving the above problems is used for transmission and reception, has at least 6 elements, and is configured to detect a magnetic field in a direction orthogonal to the main magnetic field at the time of reception. An RF for MRI comprising a birdcage coil and a solenoid coil for detecting a magnetic field in a direction orthogonal to a main magnetic field at the time of reception and also in a direction orthogonal to the detection magnetic field of the birdcage coil.
It is a coil.

A third means for solving the above-mentioned problems is a birdcage coil having at least 6 elements and configured to detect a magnetic field in a direction orthogonal to the main magnetic field, and a birdcage coil orthogonal to the main magnetic field. Direction, and a solenoid coil configured to have a required coil length by a large number of parallel conductors so as to detect a magnetic field in a direction orthogonal to the detection magnetic field of the birdcage coil. It is the RF coil for MRI.

A fourth means for solving the above-mentioned problems is to have a birdcage coil having at least 6 elements and configured to detect a magnetic field in a direction orthogonal to the main magnetic field, and orthogonal to the main magnetic field. Direction, a phase correction for correcting the solenoid coil for detecting a magnetic field in a direction orthogonal to the detection magnetic field of the birdcage coil and for correcting the received signal of the birdcage coil and the received signal of the solenoid coil to be in phase with each other. An MRI apparatus comprising: means and an adding means for adding received signals whose phases are corrected.

A fifth means for solving the above problems is used for transmission and reception, has at least 6 elements, and is configured to detect a magnetic field in a direction orthogonal to the main magnetic field at the time of reception. A birdcage coil, a solenoid coil for detecting a magnetic field in a direction orthogonal to the main magnetic field and also in a direction orthogonal to the detection magnetic field of the birdcage coil, and an RF signal for transmission is generated and supplied to the birdcage coil. An RF signal supply means, a phase correction means for correcting the received signal of the birdcage coil and a received signal of the solenoid coil so as to have the same phase, and an adding means for adding the received signals with the corrected phases. It is an MRI apparatus characterized by the above.

[0020]

In the RF coil for MRI of the first means for solving the problems, the birdcage coil has at least 6 elements, so that its SNR is close to that of the solenoid coil. The SNR of the received signal is improved by combining the magnetic fields in the orthogonal directions detected by.

MRI of the second means for solving the problem
Since the birdcage coil is used for transmission in the RF coil for use, the decoupling mechanism required when using the transmission-dedicated coil becomes unnecessary.

RF for MRI of third means for solving the problems
In the coil, the eddy current is prevented from being generated by configuring the solenoid coil so as to have a required coil length by a large number of parallel conductors.

In the MRI apparatus of the fourth means for solving the problem, the birdcage coil has at least 6 elements and the SNR is close to the SNR of the solenoid coil. Then, the signals detected by the respective coils are phase-corrected and added to obtain S of the received signal.
NR is improved.

[0024]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a configuration diagram showing a configuration example of an MRI RF coil according to an embodiment of the present invention. In this FIG.
The birdcage coil 1 has at least 6 elements and is configured to detect a magnetic field in a direction orthogonal to the main magnetic field. Further, the solenoid coil 2 is configured to detect a magnetic field in a direction orthogonal to the main magnetic field and also in a direction orthogonal to the detection magnetic field of the birdcage coil. The birdcage coil 1 is adapted to be used for transmission and reception, and the solenoid coil 2 is used only for reception.

FIG. 2 is a configuration diagram showing an example of the configuration of the MRI apparatus of one embodiment of the present invention. In FIG. 2, the weak received signals at the birdcage coil 1 and the solenoid coil 2 are passed through protective means such as a λ / 4 protection unit 3 and 4 for protecting the preamplifier from the transmission RF power at the time of reception. Amplified to a predetermined level by the preamplifiers 5 and 6. Here, the output of the preamplifier 5 is gain-adjusted by the gain adjusting unit 7 including an amplifier or an attenuator having a variable gain, and the phase adjusting unit 8 that performs phase adjustment so that the output of the preamplifier 6 and the signal are in phase. The phase is adjusted by. The output of the phase adjusting unit 8 and the output of the preamplifier 6 are combined by an adding unit 9 such as a combiner.
Are supplied to respective input terminals of and are subjected to in-phase addition. After that, the added reception signals are subjected to various signal processing necessary for a predetermined MRI in the signal processing device 10. At the time of transmission, the RF signal from the transmitter 11 is supplied to the birdcage coil 1 and transmitted to the subject.

In the MRI apparatus configured as described above, quadrature reception is performed, and the reception signals from the birdcage coil 1 and the solenoid coil 2 for detecting mutually orthogonal magnetic fields are added in phase to improve the SNR. It is configured to work.

The birdcage coil shown in FIG. 1 is called the low-pass type shown in FIG. 3 (a), but in addition to this, a capacitor is provided in the ring portion shown in FIG. 3 (b). It is also possible to use the high-pass type birdcage coil which has.

The difference from the conventional MRI apparatus for performing such quadrature reception is that a birdcage coil is used instead of the conventional saddle coil. Therefore,
The advantages of performing quadrature reception using the birdcage coil and the solenoid coil will be described below.

First, R by the birdcage coil 1
The advantage is that the uniformity of the F magnetic field is good. That is, since the birdcage coil 1 performs transmission and the birdcage coil 1 and the solenoid coil 2 perform quadrature reception, the uniformity of the RF magnetic field is improved as compared with the case of using a conventional saddle coil. Image shading (sh
ading: There is an effect that it is possible to improve the phenomenon that the average brightness decreases due to the sensitivity decrease in the peripheral part of the image.

The homogeneity of the RF magnetic field of the birdcage coil 1 will be described with reference to FIGS. 4 and 5. The RF magnetic field strength of the birdcage coil 1 having a cross section as shown in FIG. 4 is shown in FIG.
It shows in (b). In FIG. 5, the number of elements of the birdcage coil 1 is 4, 8, 16, 32. The characteristic of the number of elements of 4 in FIG. 5 matches the characteristic of the conventional saddle coil. This is because the saddle coil also has four elements in the Z direction. Further, in FIG. 5, it can be seen that the uniformity of the RF magnetic field is improved as the number of elements is increased. Although not shown in FIG. 5, when the number of elements reaches 6, the uniformity of the RF magnetic field of the birdcage coil 1 exceeds the characteristics of the saddle coil. Therefore, by using the birdcage coil 1 having at least 6 elements, an RF magnetic field with good uniformity can be obtained.

Further, as the number of elements of the birdcage coil 1 is increased, there is an effect that the uniform area is expanded. For example, ± 1 from the magnetic field strength at the center of the birdcage coil 1.
When a region in which 0% change appears is measured as a uniform region, the ratio of the minimum radius r i of the uniform region to the radius r 0 of the birdcage coil 1 (uniform region occupancy ratio: r i / r
Fig. 6 shows 0) in relation to the number of elements. That is, as shown in FIG. 6, it can be seen that the uniform region expands as the number of elements increases.

From the above, there is also an advantage that a large coil dedicated to transmission for reducing shading is unnecessary. Therefore, as described above, the birdcage coil 1
Is used for transmission and reception, and quadrature reception is performed by the birdcage coil 1 and the solenoid coil 2, whereby an MRI apparatus capable of improving SNR can be realized.

Furthermore, by increasing the number of elements of the birdcage coil 1 to improve the uniformity of the RF magnetic field, there is an effect that the noise figure (NF: Noise Figure) of the coil can be improved. This improvement in NF means improvement in SNR.

Where NFc of the coil is

[0035]

[Equation 2]

Is an index of how much the SNR is lowered due to the thermal resistance noise of the coil. FIG. 7 shows the relationship between the resonance frequency f of the birdcage coil 1 and NF using the number of elements as a parameter. For example, at a resonance frequency of 5 MHz, the number of elements is 4 (equivalent to a saddle coil),
Take 8 and 12 as an example. When the number of elements is 4, the SNR relative value in this case is +10 when the number of elements is 8.
%, The SNR improvement effect of + 22% is obtained.

On the other hand, although not shown, the solenoid coil 2
Is the same as the birdcage coil 1, the NF at 5 MHz is about 1.2 dB, and it is known that the SNR is 50% better than that of the saddle coil (same as the number of elements of the birdcage coil is 4). There is.

Based on these data, the SNR improvement ratio (using solenoid coil alone) in the case of solenoid coil 2 and saddle coil and solenoid coil 2 and birdcage coil 1 (12 elements) is comparatively examined using the graph of FIG. I will try.

In the case of orthogonal reception of the solenoid coil and the saddle coil, since a / b = 1.5, the SNR ratio (√ (a ² + b ² ) / a) becomes 1.20 from FIG. Therefore, it can be seen that the SNR improving effect of 20% is obtained. On the other hand, in the case of orthogonal reception of the solenoid coil and the birdcage coil, a / b = 1.5 / 1.22 = 1.
2, and the SNR ratio (√ (a ² + b ² ) /
Since a) becomes 1.30, it can be seen that an SNR improving effect of 30% can be obtained. That is, when performing quadrature reception together with the solenoid coil, by using a new birdcage coil instead of the conventional saddle coil,
The SNR improving effect of 20% is improved to the SNR improving effect of 30%.

As described above, quadrature reception is performed by the solenoid coil and the birdcage coil, so that the SNR improving effect can be obtained. At this time, the number of elements of the birdcage coil must be 6 or more,
The improvement effect increases as the number of elements increases.

The solenoid coil 2 is made of copper foil.
A turn coil is often used, but in addition to this, a large number of parallel conductors (for example, a plurality of flat cables or the like) may be wound and each conductor may be connected by a capacitor or the like.

As described above, when a large number of parallel conductors are used, the effect of reducing the loss due to the eddy current can be obtained. FIG. 8 is a configuration diagram showing a schematic configuration of the solenoid coil 2 in which the required coil length is configured by the flat cable as described above. Here, the conductor portion 2a made of a flat cable
And a connecting portion 2b formed of a capacitor. The effects of such a configuration will be described in more detail.

FIG. 9 shows the loss when a solenoid coil made of a copper sheet of 35 μm is used by using an LP element birdcage coil of 8 elements, and a solenoid of 50 flat cables by using an LP element birdcage coil of 8 element. The loss when a coil is used is shown. Qcopper here

[0044]

[Equation 3]

Is an index showing the degree of loss in the solenoid coil due to the combination with the solenoid coil. That is, the higher the Qcopper, the less the loss. From the results shown in FIG. 9, it can be read that Qcopper is significantly improved by using the solenoid coil of the flat cable. Here, the number has increased more than three times. It was also found that the resonance frequency f0 hardly changed by using the flat cable. As an example, when an 8 element LP type birdcage coil is used and a solenoid coil made of a 35 μm / 135 mm copper sheet is used, f0 of 0.334 MHz is obtained.
There was a change, but with a flat cable 120 mm
0.0001MH when the solenoid coil is configured
The fluctuation was f0 below z.

The Qunload of the birdcage coil at 5 MHz is about 350, but when combined with the copper sheet solenoid coil, it decreases to about 305. This becomes about 340 when combined with a solenoid coil using a flat cable. Therefore, this result also shows that the loss in the solenoid coil is small.

As described above, when performing quadrature reception together with the birdcage coil, the solenoid coil is formed of a flat cable, so that the RF loss for MRI is reduced.
A coil can be realized.

[0048]

As described above in detail, according to the present invention, by combining the birdcage coil and the solenoid coil when performing quadrature reception, the RF for MRI that can improve the SNR of the received signal. By using a coil and such an MRI RF coil, an MRI apparatus capable of improving SNR can be realized.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing details of a configuration example of an MRI RF coil according to an embodiment of the present invention.

FIG. 2 is an RF coil for MRI and M according to an embodiment of the present invention.
It is a block diagram which shows the structural example of RI apparatus.

FIG. 3 is a configuration diagram showing details of a configuration example of an MRI RF coil according to an embodiment of the present invention.

FIG. 4 is a configuration diagram showing details of a configuration example of an MRI RF coil according to an embodiment of the present invention.

FIG. 5 is a characteristic diagram showing the uniformity of the magnetic field of the birdcage coil.

FIG. 6 is a characteristic diagram showing the homogeneity of the magnetic field of the birdcage coil.

FIG. 7 is a characteristic diagram showing noise figure characteristics of a birdcage coil.

FIG. 8 is a configuration diagram showing an example of a configuration of a solenoid coil.

FIG. 9 is an explanatory diagram illustrating characteristics of a solenoid coil.

FIG. 10 is an explanatory diagram showing a configuration of a coil used for quadrature reception.

FIG. 11 is an explanatory diagram showing an example of SNR improvement in orthogonal reception.

[Explanation of symbols]

 1 Bird Cage Coil 2 Solenoid Coil 3,4 λ / 4 Protection Section 5,6 Preamp 7 Gain Adjusting Section 8 Phase Adjusting Section 9 Addition Section 10 Signal Processing Device 11 Transmitting Section

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location G01N 24/04 530 C

Claims (4)

[Claims] 1. A birdcage coil having at least 6 elements, the birdcage coil being configured to detect a magnetic field in a direction orthogonal to the main magnetic field, and the birdcage coil in a direction orthogonal to the main magnetic field. An RF coil for MRI, comprising: a solenoid coil that detects a magnetic field in a direction orthogonal to the detection magnetic field. 2. Used for transmitting and receiving, at least 6
A birdcage coil having a book element and configured to detect a magnetic field in a direction orthogonal to the main magnetic field at the time of reception, and a direction perpendicular to the main magnetic field at the time of reception and orthogonal to the detection magnetic field of the birdcage coil. An RF coil for MRI, comprising: a solenoid coil that detects a magnetic field in the direction of. 3. A birdcage coil having at least 6 elements, the birdcage coil being configured to detect a magnetic field in a direction orthogonal to the main magnetic field, and the birdcage coil in a direction orthogonal to the main magnetic field. An RF coil for MRI, comprising: a solenoid coil configured to have a required coil length by a large number of parallel conductors so as to detect a magnetic field in a direction orthogonal to the detection magnetic field. 4. Used for transmitting and receiving, at least 6
A birdcage coil having a book element and configured to detect a magnetic field in a direction orthogonal to the main magnetic field at the time of reception, and a direction orthogonal to the main magnetic field and also orthogonal to the detection magnetic field of the birdcage coil. Direction, a solenoid coil for detecting a magnetic field, an RF signal supply means for generating and supplying a transmission RF signal to the birdcage coil, and a phase of a reception signal of the birdcage coil and a reception signal of the solenoid coil are equal to each other. An MRI apparatus comprising: a phase correction unit that corrects the received signal and an addition unit that adds the received signals whose phases have been corrected.