JPH11253416A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately calculate a specific absorption ratio(SAR) by providing a control data calculation means for controlling allowable exitation energy in taking images on the basis of the total amount of proton obtained based on detection of a magnetic resonance signal from an subject. SOLUTION: A CPU 1 controls a coil for static magnetic field through a static magnetic field control part 2 to generate a uniform static magnetic field in an image-taking area, and generates an inclined magnetic field by controlling a coil 5 for inclined magnetic field for performing superposing onto the static magnetic field. An RF coil 8 for transmission and receiption is placed in the vicinity of a portion of subject body to be measured which is on a top plate, and applies an RF pulse to the portion to detect an echo signal generated by exciting the corresponding atomic nucleus. A matching circuit 9 composed of variable capacitor is parallely connected to the coil 8 to calculate SAR as data for controlling the allowable excitation energy in the image taking operation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a magnetic resonance imaging apparatus for obtaining a magnetic resonance image of a subject using a magnetic resonance phenomenon.

[0002]

2. Description of the Related Art Magnetic resonance (MR) in which a certain nucleus resonantly absorbs an RF pulse energy of a specific wavelength in a magnetic field, and then emits this as an echo signal.
Magnetic resonance imaging (MRI: Magnetic Resonan) that obtains a magnetic resonance image of an imaging target (subject) using the phenomenon
ce Imaging) devices are known.

In this MRI apparatus, a subject is irradiated with a high-frequency magnetic field (RF pulse), and a physical action such as heat is generated inside the subject by the RF pulse. Such a physical action increases in accordance with the amount of the RF pulse, and can be almost ignored when the physical action is small. However, when the physical action is large, there is a possibility that the subject is adversely affected.

[0004] Therefore, the FDA in the United States shows a recommended value of the upper limit of the RF pulse energy for irradiating the human body as SAR ≤ 0.4 [W / kg] as a recommendation standard. Although the interpretation and management method of this Recommendation Criteria generally do not always completely match,
As a calculation method of AR (Specific Absorption Ratio), for example, the following (Equation 1) using a nominal weight is known.

[0005]

(Equation 1)

Here, in (Equation 1), Pw is the output power (pulse excitation energy) [W] at the time of load (at the time of photographing the subject), and Pv is at the time of no load (there is no loss in place of the subject). Output power [W] when capturing an extremely small object.
D is a duty ratio (%) of the output power. W is the nominal weight [kg] of the subject.

Therefore, the SAR as data for managing the allowable excitation energy in radiography is obtained by dividing the energy of the RF pulse absorbed by the subject by the nominal weight of the subject. This nominal weight is substantially equivalent to normal weight.

In recent years, the speed of the photographing sequence has been increased (for example, R
With the ARE method and the FSE method, the above-mentioned recommendation standards are being reviewed. However, conventionally, doctors and technicians who are engaged in MRI apparatuses calculate the above-mentioned SAR, and when imaging, the SAR becomes larger than the calculated SAR. In order to prevent this, the number of slices to be photographed was managed. If the number of slices to be photographed within a certain period of time is reduced, the output power Pw at the time of load decreases, so that the SAR decreases.

[0009]

As described above, in the conventional SAR management, the output power Pw at the time of load, the output power Pv at the time of no load, the output power Pv, Power duty ratio D, nominal weight W
Has calculated the SAR by the doctor or technician.
Is calculated based on the nominal weight W, there is a problem that an error may occur.

That is, if the irradiation area of the RF pulse for exciting even the same subject changes, the absorbed RF pulse energy per unit weight changes. In addition, the weight of the portion corresponding to the irradiation area does not always coincide with the nominal weight W.

Further, the SAR has a problem that a doctor or a technician involved in the MRI apparatus has to calculate the SAR.
Therefore, an object of the present invention is to provide a magnetic resonance imaging apparatus capable of accurately grasping the weight of a portion corresponding to an irradiation area of a subject and accurately calculating SAR.

[0012]

(1) A magnetic resonance imaging apparatus according to the present invention is a magnetic resonance imaging apparatus for obtaining a magnetic resonance image of a subject by utilizing a magnetic resonance phenomenon. A management data calculating means for detecting a magnetic resonance signal from the subject, obtaining a total amount of protons based on the detection, and calculating data for managing allowable excitation energy in imaging based on the total amount of protons. It is characterized by having been provided. (2) The magnetic resonance imaging apparatus according to the present invention is the magnetic resonance imaging apparatus according to (1), wherein the management data calculation means performs excitation for inclining the spin of the nucleus of the imaging region by a predetermined angle without selecting a slice. The peak value of the obtained echo signal is measured, and the total amount of protons is obtained based on the peak value. (3) The magnetic resonance imaging apparatus according to the present invention is the magnetic resonance imaging apparatus according to (2), wherein the management data calculation means obtains a total amount of protons from an echo signal obtained by a spin echo method. I do. (4) The magnetic resonance imaging apparatus of the present invention is the magnetic resonance imaging apparatus according to (2), wherein the management data calculation means obtains the total amount of protons from an echo signal obtained by the gradient echo method. I do. (5) The magnetic resonance imaging apparatus of the present invention is the magnetic resonance imaging apparatus according to any one of (2) to (4), wherein the management data calculation means detects a reflected signal voltage with respect to frequency. Is used to determine the total amount of protons based on the Q value measured using (6) The magnetic resonance imaging apparatus according to the present invention is the magnetic resonance imaging apparatus according to any one of (2) to (5), wherein the management data calculation means is connected to a power supply circuit of the excitation coil and outputs the output. It is characterized by having a matching circuit for improving efficiency. (7) The magnetic resonance imaging apparatus according to the present invention is the magnetic resonance imaging apparatus according to any one of (2) to (6), wherein the management data calculation means is provided before the amplifier, and An attenuator whose attenuation value or attenuation rate is known to attenuate the electric signal so that an amplifier that amplifies the electric signal obtained by detecting the echo signal does not saturate is provided. (8) The magnetic resonance imaging apparatus according to the present invention is the magnetic resonance imaging apparatus according to any one of (2) to (6), wherein the management data calculation means detects and obtains an echo signal from the imaging site. The excitation energy to the imaging region is reduced so that the amplifier that amplifies the electric signal does not saturate, and the peak value of the echo signal radiated from the imaging region is estimated to increase in accordance with the decrease in the excitation energy. The total amount of protons is determined based on the estimated peak value. (9) The magnetic resonance imaging apparatus of the present invention is the magnetic resonance imaging apparatus according to any one of (2) to (8), wherein the management data calculation means automatically sets the spin excitation angle to be constant. An absorption power measuring means for measuring the transmission power at no load and at the time of loading that satisfies the condition that the excitation angle becomes 90 ° by the power control. After the measurement by the absorption power measurement means, the examination of the imaging part of the subject The characteristic excitation is performed after the spin is completely recovered from the influence of the automatic power control of the absorption power measuring means. (10) The magnetic resonance imaging apparatus according to the present invention is the magnetic resonance imaging apparatus according to any one of (2) to (9), wherein the management data calculation means transmits and receives the excitation region almost uniformly. It is characterized by having a dual-purpose coil. (11) The magnetic resonance imaging apparatus according to the present invention is the magnetic resonance imaging apparatus according to any one of (1) to (10), and is for managing the allowable excitation energy in the imaging calculated by the management data calculating means. The data is displayed and output. (12) The magnetic resonance imaging apparatus according to the present invention is the magnetic resonance imaging apparatus according to any one of (1) to (11), and is for managing the allowable excitation energy in the imaging calculated by the management data calculating means. It is characterized in that photographing conditions are changed based on data.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a main configuration of the MRI apparatus according to the first embodiment of the present invention. Although not shown in the block diagram, an operation panel, a top plate on which the subject is placed, a mechanism for driving the top plate, a monitor for displaying the captured MR image, and data of the captured MR image (For example, a hard disk device) or the like for storing and storing the data.

The central controller 1 controls the MRI apparatus as a whole, and includes a CPU (central processing unit),
ROM (read only memory), RAM (random access
memory), various interfaces, various controllers, and the like.

The central control unit 1 controls the static magnetic field coil 3 through the static magnetic field control unit 2 to generate a uniform static magnetic field in the imaging area, and controls the gradient magnetic field coil through the gradient magnetic field control unit 4. A gradient magnetic field (gradient magnetic field) is generated by being superimposed on the static magnetic field under control.

The gradient magnetic field is generated in three directions of X-axis, Y-axis and Z-axis in order to give three-dimensional positional information to nuclear spins. The central controller 1 is output from a signal analyzer (equivalent to a spectrum analyzer) 7 for analyzing a signal (voltage signal) from a pickup coil 6 for detecting an echo signal from a subject excited by an RF coil. The analysis result signal is taken in.

The RF coil for transmission / reception 8 is installed near a measurement site of the subject placed on the top plate, applies an RF pulse to the measurement site of the subject, and irradiates a corresponding nucleus of the measurement site. Upon excitation, an echo signal generated from the nucleus is detected.

The transmission / reception RF coil 8 is a coil for both transmitting an RF pulse and receiving an MR signal.
A QD (orthogonal) -WB coil is used for the whole body, and a QD (orthogonal) -BR coil is used for the head.

A matching circuit 9 for matching the transmission / reception RF coil 8 is controlled by the central control unit 1. That is, the matching circuit 9
Is to adjust the power applied to the transmission / reception RF coil 8 based on its electrical characteristics so that the power efficiency is maximized.

The central control unit 1 controls the switching of the RF switch 10, and by the switching of the RF switch 10,
The matching circuit 9 (the transmitting / receiving RF coil 8) and the RF
The pulse generator 11 is connected. The RF pulse generator 11 is controlled by the central controller 1 and supplies power for generating an RF pulse to the transmission / reception RF coil 8.

When the RF switch 11 is switched, the transmission / reception RF coil 8 and the Q value measuring unit 12 are connected. The Q value measurement unit 12 is controlled by the central control unit 1 to measure and calculate a Q value from the echo signal detected by the transmitting / receiving RF coil 8 and output the Q value data to the central control unit 1. I do.

When the RF switch 11 is switched, the transmission / reception RF coil 8 and the MR signal detection unit 13 are connected. The MR signal detection unit 13 is controlled by the central control unit 1, processes the echo signal detected by the transmission / reception RF coil 8, converts it into MR image data, and converts the MR image data into the central control unit 1. Output to

FIG. 2 is a block diagram showing the configuration of the RF pulse generator 11 and its surroundings. The RF pulse generator 11 includes an RF pulse generator 11-1 that generates an RF pulse waveform that determines a duty D of an RF pulse applied to the transmitting / receiving RF coil 8, and an RF pulse generator 11-1 An RF power amplifier 11-2 amplifies a pulse waveform to a predetermined power, and an RF power meter 11-3 that measures the power output from the RF power amplifier 11-2 and supplies the measured power to the RF switch 10. ing.

In the RF power amplifier 11-2, the APC (autopower control) unit 1-1 provided in the central control unit 1 adjusts the RF pulse 90 ° condition (the condition that the spin excitation angle becomes 90 °). And the transmission power is controlled accordingly. Furthermore, the central control unit 1 can also control the amplification / reduction of the amplitude of the transmission power (RF pulse) to a desired magnification (for example, 30 °) based on the 90 ° condition.

The power measured by the RF power meter 11-3 is read as data by a power reading unit 1-2 provided in the central control unit 1. For example,
The power reading unit 1-2 reads the no-load transmission power Pv and the no-load transmission power Pw when the 90 ° condition is satisfied.

FIG. 3 is a block diagram showing the structure of the Q value measuring section 12 and its peripherals. The Q value measuring unit 12
Supplied an oscillator 12-1 that generates a signal of each frequency to be sampled under the control of the central control unit 1, and a signal from the oscillator 12-1 to the transmitting / receiving RF coil 8 via the RF switch 10. And a directional coupler section 12-2 for inputting a reflected signal at the time.

The reflection signal input by the directional coupler unit 12-2 is taken as data into a Q value calculation unit 1-3 provided in the central control unit 1, and the Q value is calculated based on the data. Is calculated. For example, the Q value Qv under no load and the Q value Qw under load when the 90 ° condition is satisfied are calculated.

FIG. 4 is a block diagram showing the configuration of the MR signal detecting section 13 and its peripherals. The MR signal detection unit 13 includes a preamplifier 13-1 for amplifying an MR signal (echo signal) from the subject received by the transmission / reception RF coil 8, and a preamplifier 13-1 for amplifying the MR signal received by the transmission / reception RF coil 8. An attenuator 13-2 for attenuating a predetermined amount of attenuation (attenuation amount) so as not to saturate 13-1 and the preamplifier 13-1 and the transmitting / receiving RF coil 8 (matching circuit 9) are connected to the central control unit 1 And a changeover switch 13-3 connected directly or via the attenuator 13-2 in conjunction with the RF switch 10 and an MR signal amplified by the preamplifier 13-1 and processed as an MR image signal. The MR image signal is processed by the central control unit 1 and is monitored by a monitor or a memory (not shown). And so on.

The attenuation amount of the attenuator 13 is read as data A by an attenuation amount reading section 1-4 provided in the central control section 1. Also,
The peak value of the MR signal processed by the detection unit 13-4 is read by a peak value reading unit 1-5 provided in the central control unit 1.
Is read as data S.

FIG. 5 is a circuit diagram showing an example of the matching circuit 9. The matching circuit 9 is provided with the transmission and reception R
It comprises variable capacitors 9-1, 9-2, 9-3 in parallel with the F coil 8.

In this embodiment having such a configuration, the SAR as data for managing the allowable excitation energy in imaging is based on the consideration described below.
Is calculated.

RF coil 8 for transmission / reception during transmission of RF pulse
6 is shown in FIG. The capacitor 9-4 is a variable capacitor obtained by equivalently converting the capacitors 9-1, 9-2, and 9-3.

RF power amplifier 11 as power supply source
-2, when power is supplied from RF-2
A high-frequency magnetic field Bw is generated in proportion to a high-frequency current Irf flowing through the high-frequency current Irf. At this time, even if the equivalent series resistance 21 existing depending on the subject changes, after the APC is controlled (re-adjusted) to satisfy the 90 ° condition, the high-frequency current I
rf is constant. Therefore, at this time, the following (Equation 2) holds for the transmission power P.

[0034]

(Equation 2)

Here, r is the series equivalent resistance 21 representing the sum of the loss of the transmitting and receiving RF coil 8 and the loss due to the subject.
And Q is the inductance L of the transmitting / receiving RF coil 8, the resonance angular frequency ω,
The Q value obtained from the resistance value r,
Assuming that the capacitance of the variable capacitor 9-1 is C, the conditional expression (Expression 4) is satisfied. From the above considerations, the transmission power Pv and Q value Qv at no load and the transmission power Pw and Q value Qw at load are:

[0036]

(Equation 3) (Equation 5) holds.

FIG. 7 shows an equivalent circuit of the transmitting / receiving RF coil 8 when receiving the MR signal. Induced electromotive force induced in the transmitting / receiving RF coil 8 by an echo signal generated from the subject (22 is a virtual power supply that generates this induced electromotive force)
Is Es, the voltage Vx detected across the variable capacitor 9-1 is

[0038]

(Equation 4) And the detection voltage at the transmitting / receiving RF coil 8 is (Equation 7)
From Q · Es.

Here, when a small amount of phantom with a small high-frequency power loss is set so that it can be regarded as no load, and its weight is set to Mv [kg], transmission / reception is performed by an echo signal generated from protons of this Mv [kg]. Assuming that the electromotive force induced in the RF coil 8 is Esv, the electromotive force Esv is proportional to the total amount Mv of protons. At no load, the Q value is Qv, the attenuation amount Av of the attenuator 13-2, and the gain from the preamplifier 13-1 to the detector 13-4 is Bv. Then, the detection voltage at the transmitting / receiving RF coil 8 becomes Qv · Esv, and the output voltage of the attenuator 13-2 becomes Av · Qv · Esv.
The output voltage of 3-4 is Av.Bv.Qv.Esv = Sv, and this output voltage Sv is the peak value of the measured MR signal. This is a value proportional to the total amount of protons. Therefore,

[0040]

(Equation 5) And the conversion coefficient k are defined and determined in advance.

The above discussion applies to the case of load.
At the time of load, an electromotive force induced in the transmitting / receiving RF coil 8 by an echo signal generated from protons of Mw [kg] is Esw, a Q value is Qw, and the attenuator 13-2
, And the gain from the preamplifier 13-1 to the detector 13-4 is Bv.

Then, the detected voltage of the transmitting / receiving RF coil 8 is also Qw · Esw, the output voltage of the attenuator 13-2 is Aw · Qw · Esw, and finally the output voltage of the detector 13-4 is under load. Aw, Bw, Q
w · Esw = Sw

By the way, from equation (8), Mv = k · Av
Bv, Qv, and Esv, Mv and Esv are in a proportional relationship, and Mw and Esw are in the same proportional relationship. Therefore, Mw = k ・ Av ・ Bv ・ Qv ・ Esw holds. Therefore, based on the definition of Sw and (Equation 5),

[0044]

(Equation 6) From this (Equation 9), the total amount of protons can be obtained. Unless the attenuator amount A of the attenuator 13-2 and the gain B from the preamplifier 13-1 to the detection unit 13-4 are changed under no load and at load,
(Equation 10) is obtained.

As a result, the total amount of protons in place of the nominal weight can be obtained. Thereafter, the SAR can be obtained by obtaining the energy amount of the RF pulse to be absorbed. Therefore, the transmission power supplied from the RF power amplifier 11-2 to the transmission / reception RF coil 8 is Pv when there is no load, and Pw when there is a load. At this time, the power Pd absorbed by the subject
[W] is

[0046]

(Equation 7)

This is obtained by (Equation 11). In addition,
D is the duty of the pulse sequence determined by the RF pulse oscillator 11-1. As a result of the above, instead of the SAR obtained by the formula using the conventional nominal weight,

[0048]

(Equation 8)

An accurate SAR can be obtained by using (Equation 12) the total amount of protons (at the site excited by transmitting the RF pulse) corresponding to the imaging site. In this MRI apparatus, the conversion coefficient k of the proton weight with respect to the MR signal is obtained in accordance with the flow of the conversion coefficient k calculation process performed by the central control unit 1 shown in FIG. First, as a process of step 1 (ST1), it is necessary to confirm that a sample (for example, an oil phantom) made of a substance having a substantially same (proton number / weight) ratio of an imaging region is set in the MRI apparatus. It goes into a standby state. The amount of the sample is determined by the Q value at the time of complete no-load measured by the transmitting / receiving RF coil 8 and the Q value at the time of placing the sample.
An amount (a small amount) such that the values are approximately equal.

When it is confirmed that the sample has been set in the MRI apparatus, the processing in step 2 (ST2) is as follows:
A matching process for controlling the matching circuit 9 for the transmitting and receiving RF coil 8 to adjust the transmission output characteristic satisfactorily is performed.

When this matching process is completed, the process of step 3 (ST3) is performed to obtain the Q value at the time of no load in which the sample is set.
The output of various frequencies is supplied to the F coil 8 for transmission, and as a result, the voltage of the reflected signal received by the transmission / reception RF coil 8 is measured by the Q value measurement unit 12 (directional coupler unit 12−).
The measurement is performed according to 2), and the Q value is calculated from the measurement result by the Q value calculation unit 1-3 as the process of step 4 (ST4).

With respect to the Q value of this embodiment, as shown in FIG. 9, from the graph of the relationship between the frequency and the reflected signal voltage obtained by the Q value measuring unit 12, the lowest value at which the reflected signal voltage changes from decreasing to increasing. The frequency of the value is expressed as f0 (= ω / (2
π)), and the difference between the frequency when the reflected signal voltage is increased and the frequency when the reflected signal voltage is decreased by -3 dB from the maximum value of the reflected signal voltage is Δf, and the Q value of the transmitting and receiving RF coil 8 alone is Q = 2f0 It is calculated by the definition formula of / Δf.

In this embodiment, the corresponding RF
The Q value can be obtained by the pickup coil 6 loosely magnetically coupled to the coil and the signal analyzer 7 as a device equivalent to a spectrum analyzer. By the signal analyzer 7,
As shown in FIG. 10, power of various frequencies is supplied to the transmission / reception RF coil 8 and transmitted. As a result, a graph of the relationship between the frequency and the signal voltage of the pickup coil is obtained. Is the highest frequency of f0, and the difference between the frequency at the time of decrease and the frequency at the time of increase of the signal voltage of -3 dB from the highest value is .DELTA.f, and the Q value of the RF coil 8 for transmission / reception alone is Q = f0 It is calculated by the definition formula of / Δf. Therefore, the Q value measuring unit 1
2 (directional coupler section 12-2) and pickup coil 6
(Signal analyzer 7) may be provided, and one of them may be deleted.

Further, since the Q value is related to the transmission power Pv at the time of no load and the transmission power Pw at the time of load by (Equation 5), it is not essential to obtain the Q value, Step 4 can be omitted.

Next, as a process of step 5 (ST5), the APC unit 1-1 executes A5 for obtaining the 90 ° condition.
Perform PC control. In this APC control, an appropriate slice thickness is set to excite an imaging region (sample) of a subject,
The MR signal is collected by changing the transmission power stepwise (the transmission power is read by the power reading unit 1-2 with the detection signal from the RF power meter 11-3). When this APC control is completed, this APC is performed as a process in step 6 (ST6).
From the MR signals collected by the control, the transmission power (transmission power at no load Pv) satisfying the 90 ° condition is read.

That is, a graph of the relationship between the transmission power and the MR signal voltage is created by curve fitting, and the transmission power at which the MR signal is maximized is read. This maximum transmission power is transmission power satisfying the 90 ° condition (transmission power Pv at no load). At this time, the RF pulse is 90 °
Pulse and 180 ° pulse.

Next, as the processing in step 7 (ST7), the slice thickness is set to infinity, that is, the spin echo method without applying the slice gradient magnetic field and the phase encoding gradient magnetic field is used to transmit and receive with the transmission power satisfying the 90 ° condition. Pulse from the RF coil 8 (90 ° pulse and 18
0 ° pulse) to collect the MR signal.

The spin echo method without applying the slice gradient magnetic field and the phase encoding gradient magnetic field is executed by a pulse sequence as shown in FIG. In the drawing, RF is R which is output from the transmitting / receiving RF coil 8.
F indicates a pulse, GS indicates a slice gradient magnetic field, and GE
Indicates a phase encoding gradient magnetic field, GR indicates a read gradient magnetic field, and SIG indicates an MR signal. Where R
The 90 ° pulse of the F pulse is output at a sufficiently long time, for example, 3000 msec or more, at which the spin is considered to be sufficiently recovered from the previous (final) RF pulse, and until the MR signal is detected from the 90 ° pulse. Is controlled to be as short as possible, for example, 15 msec or less, in order to reduce the influence of lateral relaxation (T2 attenuation).

In this embodiment, the spin echo method is used. However, the present invention is not limited to this. For example, a gradient echo method executed by a pulse sequence as shown in FIG. Good thing. In this gradient echo method, 90
° pulse is used, but 180 ° pulse is not used.

The reason why the spin echo method is used in this embodiment is that the uniformity of the static magnetic field is reduced in the gradient echo method because the MR signal is collected over a wide area so that the slice is infinite. This is because the spin echo method only needs to ensure the uniformity of the static magnetic field at a practical level, while achieving it at a high level.

When the acquisition of the MR signals is completed, the peak value Sv of the acquired MR signals is read as the process of step 8 (ST8). Next, as the process of step 9 (ST9), the weight W of the sample is measured by a weight measuring unit (not shown).

Next, as a process of step 10 (ST10), the weight W measured in step 9 is divided by the peak value Sv of the MR signal read in step 8 to calculate a conversion coefficient k.

K = W / Sv The calculated conversion coefficient k is set as a parameter in a memory (not shown) in the central control unit 1.

When the setting of the conversion coefficient k is completed, the central control unit 1 ends the calculation processing of the conversion coefficient k. When the MRI apparatus finishes the conversion coefficient k calculation process, the central control unit 1 shown in FIG.
The value of the SAR is obtained according to the flow of the AR calculation process.

First, as a process of step 11 (ST11), a standby state is established until it is confirmed that the subject has been set on this MRI apparatus. When it is confirmed that the subject is set on the MRI apparatus, step 12 (ST
In the process 12), it is determined whether or not a dedicated receiving (RF) coil is used separately from the transmitting and receiving RF coil 8. Here, when it is determined that the receiving coil is used,
In the process of step 13 (ST13), the receiving coil is decoupled (or detuned) so as not to affect electromagnetically, and the process proceeds to the next process of step 14 (ST14).

When it is determined that the receiving coil is not used, the process proceeds to step S14. In the process of step 14, the output of various frequencies is supplied to the transmission / reception RF coil 8 and transmitted in the same manner as the process of step 3 in order to obtain the Q value at the time of load in which the subject is set. As a result, the voltage of the reflected signal received by the transmitting / receiving RF coil 8 is
(Directional coupler unit 12-2), and the measurement is performed in step 15
In the process of (ST15), the Q value is calculated from the measurement result, similarly to the process of step 4 described above. Note that the processing in step 14 and the processing in step 15 are also similar to the processing in step 3 and the processing in step 4 described above.
Obtaining the value is not essential and can be omitted.

Next, as the processing of step 16 (ST16), the APC section 1-1 is executed in the same manner as the processing of step 5 described above.
Performs APC control for obtaining the 90 ° condition.
When this APC control is completed, step 17 (ST1)
As the process of 7), similar to the process of step 6 described above,
From the MR signal acquired by this APC control, 90 °
The transmission power (transmission power under load Pw) satisfying the condition is read.

Next, as a process of step 18 (ST18), a standby is performed for a sufficiently long time (for example, 3000 msec or more) until the magnetization of the subject is sufficiently recovered. When the waiting for the magnetization recovery time is completed, step 19 (ST1)
In the process of 9), it is determined whether a 90 ° pulse is used.

If it is determined that a 90 ° pulse is to be used, the attenuator 1 is turned on by the RF switch 10 and the changeover switch 13-3 in step 20 (ST20).
Switching to the circuit path in which 3-2 operates, the slice thickness is made infinite, that is, by the spin echo method (see FIG. 11) in which the slice gradient magnetic field and the phase encoding gradient magnetic field are not applied in the same manner as in the processing in step 7 described above. An RF pulse (a 90 ° pulse and a 180 ° pulse) is transmitted from the transmitting / receiving RF coil 8 with a transmission power satisfying the 90 ° condition to collect an MR signal. It should be noted that the gradient echo method (see FIG. 12) may also be used in the processing in step 20.

When the acquisition of this MR signal is completed, the peak value Sw of the collected MR signal is read as the process of step 21 (ST21) as in the process of step 8 described above. Next, as the processing of step 22 (ST22), the attenuator 13-2 is used by the attenuation amount reading unit 14.
Is read, the voltage gain Bw in the preamplifier 13-1 and the detector 13-4 is read, and the duty ratio D is read from the pulse sequence.

Next, as the processing of step 23 (ST23), the parameter conversion coefficient k set in advance, the transmission power Pv at no load, the peak value Sw of the MR signal, the attenuation amount Av at no load, the no load The SAR value is calculated from the voltage gain Bv, the attenuation amount Aw, the voltage gain Bw, and the duty ratio D based on (Equation 12), and the SAR calculation process is terminated.

If it is determined in step 19 that the 90 ° pulse is not used, step 24 (ST2)
In the process 4), the RF switch 10 and the changeover switch 13-3 are used to switch the circuit path in which the attenuator 13-2 operates, and as shown in FIG. 14, the slice thickness is made infinite and the slice gradient magnetic field and the phase encode gradient magnetic field are changed. The desired transmission power (transmission power for transmitting an α ° pulse, 0 <α ° <90 °, and a pre-amplifier 13-1 is determined so as not to be saturated with an expected MR signal by a pulse sequence that is not applied. Α), the transmission and reception RF coil 8
Transmit F pulses (α ° pulse and 180 ° pulse) to acquire MR signals. In the process of step 24, if the uniformity of the static magnetic field is achieved at a high level, a pulse sequence as shown in FIG.
An RF pulse (only an α ° pulse) may be transmitted from the transmission / reception RF coil 8 at a desired transmission power to collect an MR signal. The desired transmission power Px for using the α ° pulse is the transmission power Px for the 90 ° pulse.
w gives

[0073]

(Equation 9) (Equation 13).

When the acquisition of the MR signal is completed, the processing in step 25 (ST25) is performed as in step 21 described above.
As in the above processing, the peak value Sx of the collected MR signal is read. When the peak value Sx of the MR signal at the α ° pulse is read, the MR value is read as the process of step 26 (ST26).
The peak value Sw of the MR signal at a 90 ° pulse is calculated from the signal peak value Sx by estimation.

That is, the relationship of Sw = (1 / sin α °) · Sx is established between the peak value Sx of the MR signal at the α ° pulse and the peak value Sw of the MR signal at the 90 ° pulse. This equation is used to estimate and calculate the peak value Sw of the MR signal at the 90 ° pulse.

When the estimation and calculation of the peak value Sw of the MR signal at the 90 ° pulse are completed, step 27 (ST2)
7), the preamplifier 13-1 and the detector 13-4
, And the duty ratio D is read from the pulse sequence.

Next, as the processing of step 28 (ST28), a preset parameter conversion coefficient k, transmission power Pv at no load, peak value Sw of the MR signal, voltage gain Bv at no load, and voltage gain Based on Bw and the duty ratio D, based on (Equation 12), the value of SAR is calculated with (Av / Aw) = 1, and the SAR calculation processing is terminated.

In the SAR calculation process, the SAR calculation process proceeds from step 20 to step 23 using the 90 ° pulse and the attenuator 13-2, and the α ° pulse determined so as not to saturate the preamplifier 13-1 is used. 90 °
Two methods, SAR calculation processing, which proceeds from step 24 to step 28 for estimating and calculating the peak value of the MR signal when a pulse is used, can be selected. However, the present invention is not limited to this. It is not necessary that only one of them be used.

For example, if there is only the SAR calculation processing that proceeds from step 24 to step 28, the attenuator 13-2 and the changeover switch 13-3 become unnecessary, and they can be eliminated from the configuration.

When the SAR calculation processing is completed, the MRI apparatus displays the value of the SAR and sets the number of slices in imaging according to the flow of the SAR display processing performed by the central control unit 1 shown in FIG.

First, as the process of step 29 (ST29), the SAR value calculated in the above-described SAR calculation process is displayed on a display device (not shown).
In the process 0), the number of slices that can be photographed is calculated based on the value of the SAR. After calculating the number of slices that can be photographed, the calculated number of slices that can be photographed is displayed in parallel with the SAR value on the display device as a process of step 31 (ST31). In this case, press the return key. In the case of manual setting, enter the number of slices. ", And determine whether or not the automatic setting is performed in the process of step 32 (ST32).

Here, for example, when the return key is pressed,
If the automatic setting is determined, the number of photographable slices calculated in step 30 is set as the number of slices to be photographed in step 33 (ST33).
The AR display processing ends.

If, for example, a numerical value is input and it is determined that the setting is not automatic setting, the process enters a standby state until the input of the number is completed as a process of step 34 (ST34). The number of slices is set as the number of slices to be photographed, and the SAR display process ends.

After the SAR display process, the MRI apparatus performs a normal imaging process. About this shooting process,
Here, the description is omitted. In this photographing processing, photographing is performed based on the number of slices to be photographed set in the above-described SAR display processing.

As described above, according to this embodiment, the matching circuit 9 for increasing the efficiency of the power supplied to the transmitting / receiving RF coil 8 and the APC section 1-1 are used to satisfy the 90 ° condition. An RF power meter 11-3 and a power reading unit 1-2 for measuring the transmission power of the RF pulse at the time of no load and at the time of the control, and a directional coupler unit 12-2 for obtaining a Q value; A Q value calculator 1-3, an attenuator 13-2 for preventing the preamplifier 13-1 from being saturated, and an MR
A peak value reading unit 1-5 for detecting a peak value of a signal is provided, and a pulse sequence in which a 90 ° pulse is transmitted without slice encoding or an α ° pulse is transmitted to perform a peak of an MR signal proportional to the total amount of protons. The value is determined and, instead of the SAR value using the nominal weight,
By automatically calculating the value of SAR using the total amount of protons based on the SAR, it is possible to accurately grasp the weight equivalent of the portion corresponding to the irradiation area of the subject,
Can be accurately calculated, and SAR management can be performed more fully and optimally.

[0086]

As described in detail above, according to the present invention,
By obtaining the total amount of protons from the MR signal and obtaining the SAR value using the total amount of protons instead of the nominal weight, the weight of the portion corresponding to the irradiation area of the subject can be accurately grasped. Can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a main configuration of an MRI apparatus according to an embodiment of the present invention.

FIG. 2 is an exemplary block diagram showing a configuration of an RF pulse generator and its periphery in the MRI apparatus according to the embodiment;

FIG. 3 is an exemplary block diagram showing a configuration of a Q value measurement unit and peripheral components of the MRI apparatus according to the embodiment;

FIG. 4 is an exemplary block diagram showing the configuration of an MR signal detection unit and its periphery of the MRI apparatus according to the embodiment;

FIG. 5 is a circuit diagram showing an example of a matching circuit of the MRI apparatus according to the embodiment;

FIG. 6 is a circuit diagram showing an equivalent circuit of a transmission / reception RF coil at the time of RF pulse transmission of the MRI apparatus of the embodiment.

FIG. 7 is a circuit diagram showing an equivalent circuit of a transmitting / receiving RF coil when the MR signal of the MRI apparatus of the embodiment is received.

FIG. 8 is a diagram showing a flow of a calculation process of a conversion coefficient k performed by the central control unit of the MRI apparatus according to the embodiment.

FIG. 9 is a diagram showing a graph of a relationship between a frequency and a reflected signal voltage in a directional coupler of a Q value measuring unit of the MRI apparatus according to the embodiment and numerical values relating to a Q value;

FIG. 10 is a graph showing a relationship between a frequency and a signal voltage by a pickup coil in the signal analyzer of the MRI apparatus according to the embodiment, and a numerical value relating to a Q value.

FIG. 11 is a diagram showing a pulse sequence of a spin echo method without applying a slice gradient magnetic field and a phase encoding gradient magnetic field in the MRI apparatus of the embodiment.

FIG. 12 is a diagram showing a pulse sequence of a gradient echo method without applying a slice gradient magnetic field and a phase encoding gradient magnetic field in the MRI apparatus of the embodiment.

FIG. 13 is a diagram showing a flow of an SAR calculation process performed by the central control unit of the MRI apparatus according to the embodiment.

FIG. 14 is a diagram showing an example of a pulse sequence in which a slice gradient magnetic field using an α ° pulse and a phase encoding gradient magnetic field are not applied in the MRI apparatus of the embodiment.

FIG. 15 is a diagram showing another example of the pulse sequence in which the slice gradient magnetic field using the α ° pulse and the phase encoding gradient magnetic field are not applied in the MRI apparatus of the embodiment.

FIG. 16 is a diagram showing a flow of SAR display processing performed by the central control unit of the MRI apparatus according to the embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Central control part, 1-1 ... APC part, 1-2 ... Power reading part, 1-3 ... Q value calculation part, 1-4 ... Attenuation amount reading part, 1-5 ... Peak value reading part, 8 ... RF coil for transmission / reception, 9 matching circuit, 10 RF switch, 11 RF pulse generating section, 11-2 RF power amplifier, 11-3 RF power meter, 12 Q value measuring section, 12-2 direction 13: MR signal detector, 13-1: preamplifier, 13-2: attenuator, 13-4: detector

Claims (12)

[Claims] In a magnetic resonance imaging apparatus for obtaining a magnetic resonance image of a subject by utilizing a magnetic resonance phenomenon, a high-frequency pulse is transmitted to the subject, a magnetic resonance signal from the subject is detected, and a magnetic resonance signal from the subject is detected. A magnetic resonance imaging apparatus comprising: a management data calculating unit that calculates a total amount of protons based on the total amount of protons and calculates data for managing allowable excitation energy in imaging based on the total amount of protons. 2. The management data calculation means measures a peak value of an echo signal obtained when excitation is performed by inclining a spin of a nucleus of an imaging region by a predetermined angle without selecting a slice, and a proton value is determined based on the peak value. The magnetic resonance imaging apparatus according to claim 1, wherein the total amount of the magnetic resonance imaging is determined. 3. The magnetic resonance imaging apparatus according to claim 2, wherein said management data calculating means calculates a total amount of protons from an echo signal obtained by a spin echo method. 4. The magnetic resonance imaging apparatus according to claim 2, wherein said management data calculating means obtains a total amount of protons from an echo signal obtained by a gradient echo method. 5. The management data calculation unit calculates a total amount of protons based on a Q value measured using a directional coupler that detects a reflected signal voltage with respect to a frequency. A magnetic resonance imaging apparatus according to claim 4. 6. The control data calculating unit according to claim 2, further comprising a matching circuit connected to a power supply circuit of the excitation coil and increasing the output efficiency. Magnetic resonance imaging equipment. 7. The control data calculating means is provided in front of the amplifier and attenuates the electric signal so that the amplifier for amplifying an electric signal obtained by detecting an echo signal from an imaging part is not saturated. The magnetic resonance imaging apparatus according to any one of claims 2 to 6, further comprising an attenuator whose value or attenuation factor is known. 8. The management data calculation means reduces excitation energy to an imaging part so that an amplifier for amplifying an electric signal obtained by detecting an echo signal from the imaging part does not saturate. 7. The method according to claim 2, wherein a peak value of an echo signal radiated from the imaging region is increased and estimated in accordance with the decrease, and a total amount of protons is obtained based on the increased estimated peak value. The magnetic resonance imaging apparatus according to claim 1. 9. The management data calculating means measures transmission power under no load and under load that satisfies the condition that the excitation angle is 90 ° by auto power control that automatically sets the spin excitation angle to be constant. After the absorption power measurement means is provided, the test excitation of the imaging part of the subject after the measurement by the absorption power measurement means should be performed after the spin is completely recovered from the influence of the auto power control of the absorption power measurement means. The magnetic resonance imaging apparatus according to any one of claims 2 to 8, wherein: 10. The magnetic device according to claim 2, wherein the management data calculating means includes a coil for both transmitting and receiving, which excites the imaging region substantially uniformly. Resonance imaging device. 11. The magnetic resonance apparatus according to claim 1, wherein data for managing the allowable excitation energy in the imaging calculated by the management data calculating means is displayed and output. Imaging device. 12. The imaging condition according to claim 1, wherein the imaging condition is changed based on data for managing the allowable excitation energy in the imaging calculated by the management data calculating means. Item 7. The magnetic resonance imaging apparatus according to Item 1.