JPH09238921A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic resonance imaging apparatus which eliminates the need for arranging excess trap circuits to limit the number thereof to the necessary minimum whether a high magnetic field with a higher frequency of an RF magnetic field or a large receiving coil is used. SOLUTION: A coil 31 for RF reception is provided with a trap circuit in which a trap frequency varies so that resonation occurs at a trap frequency having a specified quantity of frequency offset with respect to the frequency of an RF excited magnetic field to present a higher impedance under the normal temperature and with a rise in the temperature, the frequency offset increases while the impedance lowers.

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 (hereinafter sometimes abbreviated as MRI apparatus).

[0002]

2. Description of the Related Art Conventionally, in this type of MRI apparatus, a transmitting RF coil for generating an RF excitation magnetic field (RF pulse) that causes a magnetic resonance phenomenon in a subject and an MR signal from the subject are received. It is known that the receiving RF coil for this purpose is configured separately. A resonance circuit commonly called a trap circuit is inserted in the reception RF coil. The trap circuit protects fragile elements such as a varactor diode and a preamplifier connected to the receiving RF coil from the induced electromotive force induced in the receiving RF coil due to the RF excitation from the transmitting RF coil. In addition, it is used to suppress the induced current induced in the receiving RF coil by the electromotive force so as not to disturb the excitation RF magnetic field.

FIG. 5 is a diagram showing an example of a concrete circuit configuration of such a trap circuit. The trap circuit 40 is
A parallel resonance circuit in which a coil 41 having an inductance Lt and a capacitor 42 having a capacitance Ct are connected in parallel, and two diodes are back-to-back (bac
and a so-called cross diode circuit 43 connected by k to back). The values of Ct and Lt are selected so that when the cross diode circuit 43 is in a conducting state, the resonance occurs at a frequency that is substantially tuned to the frequency of the excitation RF (hereinafter referred to as a trap frequency). That is, the trap circuit 43 exhibits a high parallel resonance impedance during RF excitation.

FIG. 6 is a diagram showing an example of a concrete circuit configuration of a receiving RF coil provided with the above-mentioned trap circuit. The receiving RF coil 44 has an R of inductance L
An F coil 45, a trap circuit 40, a variable capacitor 46 for tuning adjustment, a variable capacitor 47 for matching adjustment, and a preamplifier 48 are provided.

When a strong RF magnetic field for excitation is exposed from the transmitting RF coil during transmission, an induced electromotive force is generated in the RF coil 45 according to the frequency. In this case, the cross diode circuit 43 is applied with a voltage equal to or higher than a predetermined value and becomes conductive, and the coil 4
The resonance circuit composed of 1 and the capacitor 42 resonates.

As a result, the trap circuit 40 exhibits a high parallel resonance impedance, a large current does not flow in the RF coil 45, and the variable capacitor 4 for tuning adjustment is used.
6. The voltage due to the induced electromotive force is hardly applied to the variable capacitors 47 for matching adjustment (these capacitors are often fragile elements like varactor diodes) and the preamplifier 48. Similarly, since the current due to the induced electromotive force hardly flows, the RF magnetic field due to the induced current does not disturb the RF magnetic field for excitation.

On the other hand, during reception, RF from the subject is detected.
The magnetic field (magnetic resonance signal, MR signal) is weak, and the electromotive force that induces it in the RF coil 45 is also weak, so that the cross diode circuit 43 does not become conductive. Therefore, the coil 41 and the cross diode circuit 43 are similar to those which do not exist at the time of reception, and the weak signal induced in the coil 45 is normally transmitted to the preamplifier 48.

FIG. 7 is a diagram showing another configuration example of the trap circuit. This circuit is provided with a PIN diode 49 instead of the above-mentioned cross diode circuit, and at the time of RF excitation, coils 501 and 502 of the inductance Lb inserted in the bias line of the PIN diode 49 (or for low frequency signals, Low impedance,
The DC current is supplied to the RF signal via any circuit as long as it has a high impedance.

By biasing the direct current in this manner, the PIN diode 49 becomes conductive during transmission. Therefore, as in the case of the cross diode, the trap circuit shown in FIG. 7 exhibits a high impedance at the frequency of the excitation RF magnetic field (trap frequency). Further, at the time of reception, the PIN diode 49 is reversely biased, and in this state, the PIN diode 49 does not conduct, and the PIN diode 49 and the coil 41 are the same as not existing.

Although rarely used, the coil 41
The circuit shown in FIG. 8 in which the resonance circuit is configured by replacing the capacitor 42 with the capacitor 42 can also function as a trap circuit.

By the way, in adjusting the trap circuit as described above, the values of the inductance Lt of the coil 41 and the capacitance Ct of the capacitor 42 are selected so as to resonate at a predetermined frequency. The circuit condition is different from that during use. For example, an object to be inspected has individual differences and is innumerable, and at high frequencies, some kind of interference is caused to the trap circuit. Further, the high-frequency environment is slightly different between the inside of the magnet of the MRI apparatus and the place of adjustment, and therefore, adjustment can be made only roughly, and the resonance frequency deviates slightly from the frequency of the excitation RF magnetic field during actual use.

As another feature, the diode and the coil of the resonance circuit have resistance loss, and while the diode functions as a trap (that is, while resonating by the RF excitation magnetic field), a considerable amount is induced by the induced current. I have a fever.

A large problem does not occur in the RF coil using only one such trap circuit. However, in a high magnetic field MRI apparatus that uses a large receiving coil and has a high frequency of the excitation RF magnetic field, it may not be enough to arrange only one trap circuit for one RF coil. That is, the heat generation is too great and the trap circuit is damaged.
There is a problem that if the subject (patient) is transmitted to the surface of the F-coil and is locally touched by the subject, there is a risk of burns. Therefore, as shown in FIG. 9, a plurality of trap circuits are distributed and arranged for one RF coil (here, 40
(Two trap circuits consisting of 1 and 402) are performed. The RF coil 45 having the inductance L shown above with reference to FIG. 5 is arranged in a divided manner in the circuit shown in FIG. 9 and corresponds to the coils 451 to 453 having the inductances L1 to L3, respectively.

By disposing a plurality of trap circuits in a distributed manner as described above, it is preferable that the heat generation per location of the trap circuit can be reduced according to the number of the trap circuits provided. However, as described above, since the adjustment of the trap circuit is not perfect, it is possible that at the time of resonance, the impedance of one trap circuit is sufficiently high, but the impedance of another trap circuit is low. In such an unbalanced state, there is a problem that the induced electromotive force concentrates on the trap circuit with high impedance.

As a result, the heat generated in the trap circuit, to which the induced electromotive force is concentrated and applied, becomes excessive, the trap circuit is destroyed first, and subsequently, one of the remaining trap circuits is induced again. There may be a problem that electric power is concentrated and sequentially broken. In order to avoid such a problem, if the impedance of each trap circuit is well balanced, only a few traps are needed. More traps should be provided to further distribute the load so that it is within the damage limit.

Inserting a large number of trap circuits in the RF coil is not preferable because of the following disadvantages. (1) As the number increases, the adjustment of each trap circuit becomes difficult. (2) The RF coil is bulky and difficult to use. Also RF
The manufacturing cost of the coil increases. (3) The performance of the RF coil deteriorates. This is due to the high frequency loss of the trap circuit. The cross diode has some leakage current even when a minute voltage is applied (that is, when receiving). This appears as resistance loss, which lowers the Q value of the RF coil. Therefore, if a large number of trap circuits are provided, the Q value of the RF coil significantly decreases, and as a result, the SNR of the MRI image decreases. In the trap circuit using the PIN diode instead of the cross diode, such a decrease in Q value is relatively small, but the PIN diode is expensive and costly. Further, since the PIN diode cannot completely block high frequency waves in its bias line, the received signal leaks to some extent. In this respect as well, the trap circuit using the PIN diode also has a decrease in the performance of the RF coil, and again, the number of traps increases, which is a problem.

[0017]

SUMMARY OF THE INVENTION The present invention has been made to address the above-mentioned circumstances, and its purpose is to use a high magnetic field having a high RF magnetic field frequency or to use a large receiving coil. Even so, it is an object of the present invention to provide a magnetic resonance imaging apparatus in which the number of extra trap circuits is not required and the number thereof can be minimized.

[0018]

According to the magnetic resonance imaging apparatus of the first aspect of the present invention, R to which an RF excitation magnetic field is applied.
Connected to the F coil and the RF coil, at room temperature,
The impedance is increased by resonating at a trap frequency having a predetermined frequency offset with respect to the frequency of the RF excitation magnetic field, and the frequency offset increases and the impedance decreases as the temperature rises. And a resonance means for changing the trap frequency.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of a magnetic resonance imaging apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of this embodiment. In the gantry 20, the static magnetic field magnet 1, X-axis, Y-axis,
A Z-axis gradient magnetic field coil 2, a transmission RF coil 31, and a reception RF coil 32 are provided.

The static magnetic field magnet 1 as the static magnetic field generator is constructed by using, for example, a superconducting coil or a normal conducting coil. The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 is a coil for generating an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, and a Z-axis gradient magnetic field Gz. The transmission RF coil 31 is for generating a radio frequency (RF) pulse as a selective excitation pulse for selecting a slice, and the reception RF coil 32 is for a magnetic resonance signal (MR) generated by magnetic resonance.
Signal). The object M placed on the tabletop of the bed 13 is inserted into an imageable region (a spherical region where an imaging magnetic field is formed and a diagnosis is possible only in this region) in the gantry 20. It

The static magnetic field magnet 1 is driven by the static magnetic field controller 4. The transmitting RF coil 31 is driven by the transmitter 5 when magnetic resonance is excited, and the receiving RF coil 32 is
When the magnetic resonance signal is detected, it is coupled to the receiver 6. The X-axis / Y-axis / Z-axis gradient magnetic field coil 2 includes an X-axis gradient magnetic field power supply 7,
It is driven by a Y-axis gradient magnetic field power supply 8 and a Z-axis gradient magnetic field power supply 9.

The X-axis gradient magnetic field power source 7, the Y-axis gradient magnetic field power source 8, the Z-axis gradient magnetic field power source 9 and the transmitter 5 are driven by a sequencer 10 according to a predetermined sequence, and an X-axis gradient magnetic field Gx, a Y-axis gradient magnetic field Gy, A Z-axis gradient magnetic field Gz and a radio frequency (RF) pulse are generated in a predetermined pulse sequence. In this case, the X-axis gradient magnetic field Gx, the Y-axis gradient magnetic field Gy,
The Z-axis gradient magnetic field Gz is mainly used as, for example, the phase encoding gradient magnetic field Ge, the reading gradient magnetic field Gr, and the slice gradient magnetic field Gs. The computer system 11 drives and controls the sequencer 10, captures the magnetic resonance signal received by the receiver 6 via the receiving RF coil 32, and performs predetermined signal processing to generate a tomographic image of the object. It is displayed on the display unit 12.

The present embodiment is constructed as described above. Further, the receiving coil 32 of the present embodiment has a trap circuit 40 having the same structure as that shown in FIG. 5 (however, the inductance Lt of the coil 41 and the capacitance Ct of the capacitor 42, which form a resonance circuit, are different from those of the conventional one. (Set to different values), a plurality of them are connected as shown in FIG. The trap circuit 40
Shows not only the structure shown in FIG. 5 but also FIG.
The same structure may be used.

The trap frequency (resonance frequency) of the trap circuit shifts to either the higher direction or the lower direction as the temperature rises. This is due to the temperature characteristics of Ct and Lt. Further, in this case, whether the trap frequency shifts to the higher side or the lower side is determined by these Ct and Lt.
Depends on the characteristics of.

Further, the capacitor C of the capacitor 42
Since t is directly involved in tuning on reception,
It is common to select a material whose reactance does not change significantly with temperature. Therefore, in many cases, for example, in the trap circuit 40 of the present embodiment, the inductance Lt of the coil 41 determines the temperature characteristic of the trap frequency. When the trap circuit is configured as shown in FIG. 8 described above, Lt of the coil 41 is directly involved in tuning during reception. Therefore, in this case, similarly, the capacitance Ct of the capacitor 42 determines the temperature characteristic of the trap frequency.

The present embodiment positively utilizes the temperature characteristics of the inductance Lt of the coil 41 or the capacitance Ct of the capacitor 42 as described above.

If the trap frequency has a characteristic of increasing with temperature rise, the trap frequency is set to be slightly high so as to have a predetermined amount of frequency offset with respect to the frequency of the RF excitation magnetic field in initial adjustment at room temperature, The impedance at the RF excitation magnetic field frequency (hereinafter, referred to as trap impedance) is set to decrease as the temperature rises. Such setting of the trap frequency is performed by setting the inductance L of the coil 41.
t or the capacitance Ct of the capacitor 42 is initially adjusted at room temperature.

FIG. 2 is a graph schematically showing the relationship between the trap frequency and trap impedance of the trap circuit and the RF excitation magnetic field frequency. Further, the same figure shows a case where the trap frequency increases as the temperature rises. When the trap frequency increases as the temperature rises, the characteristic curve of the trap impedance moves to the right as a whole. This reduces the trap impedance at the frequency of the RF excitation field. In other words, the trap circuit exhibits a high impedance by resonating at a trap frequency having a predetermined amount of frequency offset with respect to the frequency of the RF excitation magnetic field at room temperature, and the frequency offset increases as the temperature rises, and The trap frequency changes so that the impedance decreases.

It should be noted that how much the trap frequency is set to be "slightly higher" with respect to the frequency of the RF excitation magnetic field, that is, what value the above-mentioned frequency offset amount is specifically set, is the Q of the parallel resonance of the trap circuit. Based on value. More specifically, the frequency offset amount is set to a value that does not exceed the bandwidth determined by multiplying the reciprocal of the Q value of the resonance circuit by the frequency of the RF excitation magnetic field with the trap frequency as the center. For example, when the excitation RF magnetic field frequency is 63.9 MHz (1.5 Tesla proton resonance frequency) and the Q value is about 100, the trap bandwidth is 0.64 MHz. The amount of frequency offset with respect to the frequency is a value in the vicinity of this trap bandwidth, which is expressed by the formula 10
0 kHz. This is much larger, for example 1MHz
If the initial setting is performed with a certain offset amount, the trap impedance will drop too much, and the induced current due to the RF excitation magnetic field cannot be suppressed sufficiently effectively.

Further, it is preferable that the frequency offset amount is a value larger than at least 1/10 of the bandwidth. This is because, with an offset amount of about several tens of KHz, frequency adjustment accuracy of this degree cannot be originally obtained, and even if it is obtained, the trap frequency shifts due to temperature rise in the portion close to the peak of the resonance characteristic (trap impedance). Even if it does, the impedance does not change much.

If the trap frequency has a characteristic of decreasing with temperature rise, the trap frequency in the initial adjustment is adjusted to be lower than the frequency of the excitation RF magnetic field, although the adjustment is similar.

As described above, in this embodiment, when transmitting the excitation RF by the transmission RF coil 31,
The trap functions in the RF coil 32 for reception, and even if the temperature of the trap circuit rises due to the concentration of the induced electromotive force in any of the trap circuits due to the difference in the characteristics of the individual trap circuits, the temperature of the resonance frequency increases. Since the frequency offset in anticipation of change is set in the initial state, the trap resonance frequency appropriately deviates from the trap band (frequency band) of the RF excitation magnetic field according to the temperature rise, that is, the trap impedance decreases, The induced electromotive force is not concentrated only on the trap circuit and is distributed to other trap circuits, so that the trap circuit can be prevented from being damaged due to heat generation due to an excessive induced current. Therefore, for example, R for reception
Even if the F coil 32 is a large receiving coil to which a high-frequency magnetic field is applied, it is not necessary to provide a large number of trap circuits, and the number of trap circuits can be minimized.

Furthermore, the reliability of the trap circuit is improved, its adjustment is easy, and the cost of the RF coil is reduced.
RF coil is not bulky and easy to handle,
The performance of the RF coil as a whole is improved, and the SN of MRI images is improved.
The effect of contributing to the improvement of R can be expected.

Next, a second embodiment of the present invention will be described. The second embodiment is similar to the first embodiment described above in that the initial adjustment of the resonance frequency of the trap circuit is set slightly higher or lower than the RF excitation magnetic field frequency according to the temperature coefficient of the trap frequency. It is a composition. The second embodiment further positively introduces an element for varying the reactance in the trap circuit according to the temperature.

In the first embodiment, when Lt and Ct do not have necessary and sufficient temperature coefficients, the trap impedance may not change significantly even if the temperature rises. Therefore, in the second embodiment, as shown in FIG. 3, the capacitor 5 of the type having a large temperature dependence of capacitance is used.
2 (capacitance Ctmp) is inserted in series with the coil 41 having an inductance Lt.

The combined reactance of Ctmp and Lt is given by equation (1), where ω is the angular frequency. Combined reactance = ωLt −1 / (ωCtmp) (1) Then, the circuit formed by Ctmp and Lt has one inductance (this is Lt which has a reactance equal to the above combined reactance, as shown in Expression (2). ’)
Can be regarded as

Lt ′ = Lt · [1-1 / (ω2 · Ctmp · Lt)] (2) Then, when the RF excitation magnetic field frequency is ω0, Lt ′
Select Ctmp so that is positive.

By the way, as shown in the following equation (3), Ctmp
May be expressed. In the following equation (3), K is a positive value larger than 1. Ctmp = K.multidot. [1 / (. Omega.2 .multidot.Lt)] (3) In such a case, Lt 'is given by the following equation (4).

Lt ′ = Lt · [1− (1 / K)] (4) Then, Lt and K (that is, Ctmp) so that the resonance frequencies of Lt ′ and Ct are values near approximately ω 0. Is selected.

Here, when the temperature coefficient of the capacity of Ctmp is α and the temperature rise from the initial state is ΔT, Ctmp, L
t'is as shown in equations (5) and (6). Ctmp = K · [1 / (ω2 · Lt)] · (1 + α · ΔT) (5) Lt ′ = Lt · [1-1 / (K · (1 + α · ΔT))]… (6) α · ΔT Since L is sufficiently smaller than 1, Lt 'may be approximated by the following equation (7).

Lt ′ = Lt · [1- (1-α · ΔT) / K] (7) When this is divided by Lt ′ of ΔT = 0, the following formula (8) is obtained. Lt '(. DELTA.T) / Lt' (. DELTA.T = 0) = 1 + [. Alpha..multidot..DELTA.T / (K-1)] ... (8) That is, the new combined inductance Lt 'has a temperature coefficient .alpha ./ (K-1). Is introduced.

Generally, the resonance frequency of the LC resonance circuit is L
Since it is determined by the square root of the product of C and C, the trap frequency of the trap circuit has a temperature coefficient of α / (K-1) / 2. It is easy to obtain a capacitor having a relatively large α, for example, +1000 ppm / deg, and it is not difficult to set a small value for K, for example, K = 2, and then the trap frequency is +500 pp. The third embodiment will be described. The third embodiment relates to the modification of the second embodiment, and the second point is that the trap circuit resonance frequency has an intentional temperature coefficient by introducing an element whose reactance changes with temperature in the trap circuit. However, as shown in FIG. 4, Ctmp is inserted in parallel with Lt as shown in FIG.

In the present embodiment, when the angular frequency is ω, the combined reactance of Lt and Ctmp is such that Ctmp is 1 /
Only when the value is smaller than (ω2 · Lt), is it the same as the inductance Lt 'as in the following equation (9).

Lt ′ = Lt / (1-ω2 · Ctmp · Lt) (9) With respect to such Lt ′, the temperature coefficient due to Ctmp can be intentionally introduced as in the second embodiment.

The characteristic point of the present invention as described above is that R
In a circuit that uses a resonant circuit to suppress the induced current flowing in the F coil, intentionally shift the initial state from the beginning to an appropriate degree in the direction in which the resonant frequency will shift, in anticipation of temperature changes. , And intentionally introducing the reactance with a known temperature coefficient into the resonant circuit to intentionally create the temperature characteristic of the resonant circuit. Therefore, the present invention is not limited to the trap circuits as shown in the first to third embodiments, but can be implemented with various modifications.

[0046]

As described above, according to the present invention, R
Provided is a magnetic resonance imaging apparatus which does not require an extra trap circuit even when a high magnetic field having a high F magnetic field frequency is used or a large receiving coil is used, and the number of trap circuits can be minimized. be able to.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a magnetic resonance imaging apparatus according to a first embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the trap frequency and trap impedance of the trap circuit and the RF excitation magnetic field frequency.

FIG. 3 is a diagram showing a circuit configuration example of a trap circuit according to a second embodiment of the present invention.

FIG. 4 is a diagram showing a circuit configuration example of a trap circuit according to a third embodiment of the present invention.

FIG. 5 is a diagram showing a circuit configuration example of a conventional trap circuit.

FIG. 6 is a diagram showing a circuit configuration example of a receiving RF coil including a conventional trap circuit.

FIG. 7 is a diagram showing another circuit configuration example of a conventional trap circuit.

FIG. 8 is a diagram showing still another circuit configuration example of the conventional trap circuit.

FIG. 9 is a diagram showing a circuit configuration example of a receiving RF coil in which a plurality of conventional trap circuits are distributed and arranged.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Static magnetic field magnet, 2 ... X-axis / Y-axis / Z-axis gradient magnetic field coil, 31 ... RF coil for transmission, 32 ... RF coil for reception 4 ... Static magnetic field control device, 5 ... Transmitter, 6 ... Receiver, 7 ... X-axis gradient magnetic field amplifier, 8 ... Y-axis gradient magnetic field amplifier, 9 ... Z-axis gradient magnetic field amplifier, 10 ... Sequencer, 11 ... Computer system, 12 ... Display part.

Claims (4)

[Claims] 1. An RF coil to which an RF excitation magnetic field is applied, which is connected to the RF coil and resonates at a trap frequency having a predetermined frequency offset with respect to the frequency of the RF excitation magnetic field at room temperature. A magnetic resonance imaging apparatus comprising: a resonance unit that exhibits a high impedance and that changes the trap frequency so that the frequency offset increases and the impedance decreases as the temperature rises. 2. The frequency offset amount is within a bandwidth centered on the trap frequency and determined by multiplying the reciprocal of the Q value of the resonance means by the frequency of the RF excitation magnetic field. The magnetic resonance imaging apparatus according to. 3. The frequency offset amount is greater than one-tenth of the bandwidth determined by multiplying the reciprocal of the Q value of the resonance means by the frequency of the RF excitation magnetic field with the trap frequency as the center. The magnetic resonance imaging apparatus according to claim 1, wherein the magnetic resonance imaging apparatus is a magnetic resonance imaging apparatus. 4. The resonance means comprises an inductance for resonance and a first capacitance, is connected in series or in parallel with the inductance, and has a desired temperature characteristic of the resonance means. 2. A second capacitance for changing the frequency according to the temperature characteristic is further provided.
The magnetic resonance imaging apparatus according to claim 3.