JP2013031633A - Magnetic resonance imaging apparatus and method for sar prediction - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an MRI apparatus capable of preventing reduction of throughput of a routine examination by easily and accurately performing prediction and monitoring of SAR (Specific Absorption Ratio) values based on the IEC standard.SOLUTION: An RF absorption amount of a subject is calculated for each subject, and the SAR value of a prescribed pulse sequence is calculated based on the RF absorption amount. In calculating the RF absorption amount for each subject, SAR measurement for each subject is implemented, or past data are used, for example.

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. In particular, it relates to the accurate prediction of SAR values.

  The MRI device measures NMR signals generated by the spins of the subject, especially the tissues of the human body, and visualizes the form and function of the head, abdomen, limbs, etc. in two or three dimensions Device. In imaging, the NMR signal is given different phase encoding depending on the gradient magnetic field, frequency-encoded, and measured as time series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In the MRI apparatus, the temperature rise or SAR (Specific Absorption Ratio) derived from the high frequency pulse (hereinafter referred to as RF pulse) in the subject must be suppressed to a certain threshold value or less according to the IEC (International Electrotechnical Commission) regulations. As a technique for complying with this standard, a temperature measurement technique for an object (for example, Patent Document 1) and a human body model SAR simulation (for example, Patent Document 2) are known.

Japanese Unexamined Patent Publication No. 9-13824 US Patent No. 7282914

  The temperature measurement technique for an object is a technique for measuring a temperature change from a small phase change, and tends to have a large error. Furthermore, since it is necessary to perform measurements other than the MRI imaging required for diagnosis, the throughput in routine inspection is reduced.

  The human body model SAR simulation takes time to perform the SAR simulation of the human body region with a resolution of about 1 cm. Furthermore, it is necessary to accurately model the body shape of the subject for each examination, but the modeling takes time, and the throughput in the routine examination decreases.

  Therefore, an object of the present invention is made in view of the above problems, and it is possible to prevent a decrease in throughput in routine inspection by simply and accurately predicting and monitoring SAR values based on the IEC regulations. It is to provide a possible MRI device.

  In order to achieve the above object, in the present invention, a transmission unit that irradiates a subject placed in a static magnetic field with an RF pulse, and an imaging that controls imaging using a predetermined pulse sequence including the irradiation of the RF pulse. Magnetic resonance comprising a control unit, a SAR calculation unit for obtaining a SAR value absorbed by the subject by the RF pulse, and a SAR monitoring unit for monitoring whether the SAR value exceeds a predetermined limit value In the imaging apparatus, the SAR calculation unit obtains an RF absorption amount of the subject for each subject, and obtains a SAR value of the predetermined pulse sequence based on the RF absorption amount.

  According to the MRI apparatus and the SAR prediction method of the present invention, it is easy to predict and monitor the SAR according to the body shape of the subject and the relative position to the device, that is, for each subject, before starting the examination for each examination. And can be done accurately. As a result, the predicted value of the SAR and the measured value almost match, so it is possible to avoid unintentionally setting imaging conditions that would cause the SAR to exceed the limit before inspection. It is possible to avoid stopping imaging during the inspection by setting the imaging conditions. Therefore, a reduction in throughput can be avoided.

The block diagram which shows the whole structure of an example of the MRI apparatus which concerns on this invention Diagram showing the power flow in each part of the transmission system It shows an example of an imaging sequence of irradiating a plurality of RF pulses _{(RF 1, RF 2, ···} RF w, ···) a set of Functional block diagram showing each function of the arithmetic processing unit of the first embodiment Flowchart showing the overall processing flow of the first embodiment The flowchart which shows the detailed processing flow of step 502 of Example 1. FIG. The figure for demonstrating the effect of Example 1 The flowchart which shows the detailed processing flow of step 502 of Example 2. FIG. Graph showing the relationship between the reference RF pulse output and RF absorption measured by multiple subjects The flowchart which shows the detailed processing flow of step 502 of Example 3. Graph showing the relationship between the weight of an object measured with multiple subjects and the amount of RF absorption The flowchart which shows the detailed processing flow of step 502 of Example 4.

  Hereinafter, a first embodiment to which the present invention is applied will be described with reference to the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, the overall configuration of the MRI apparatus of the present embodiment will be described. FIG. 1 is a block diagram showing the overall configuration of the MRI apparatus 100 of the present embodiment. The MRI apparatus 100 of the present embodiment obtains a tomographic image of a subject using an NMR phenomenon, and includes a static magnetic field generation system 2, a gradient magnetic field generation system 3, a transmission system 5, a reception system 6, and a signal A processing system 7, a sequencer 4, and an arithmetic processing unit (CPU) 8 are provided.

  If the static magnetic field generation system 2 is a vertical magnetic field system, the static magnetic field is uniform in the direction perpendicular to the body axis in the space around the subject 1, and if it is a horizontal magnetic field system, the static magnetic field is uniform in the body axis direction of the subject 1. Is generated. This is realized by a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source arranged around the subject 1.

  Gradient magnetic field generation system 3 drives gradient magnetic field coils 9 for applying gradient magnetic fields in X, Y, and Z axes, which are the coordinate system (stationary coordinate system) of MRI apparatus 100, and the respective gradient magnetic field coils. And a gradient magnetic field power supply 10 to be provided. Gradient magnetic fields Gx, Gy, and Gz are applied in the three axial directions of X, Y, and Z by driving the gradient magnetic field power supply 10 of each gradient coil according to a command from the sequencer 4 described later.

  At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded into an echo signal.

  The transmission system 5 irradiates the subject 1 with a high-frequency magnetic field (RF) pulse in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and the high-frequency oscillator 11 and the modulator 12, a high frequency amplifier 13, and a high frequency coil (transmission coil) 14 a on the transmission side. The RF pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4 to be described later, amplified by the high-frequency amplifier 13, and transmitted from the transmission coil 14a disposed close to the subject 1. The specimen 1 is irradiated.

  The receiving system 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of the nuclear spins of the atoms constituting the living tissue of the subject 1, and receives signals from the high-frequency coil (receiving coil) 14b on the receiving side. An amplifier 15, a quadrature detector 16, and an A / D converter 17 are provided. The response NMR signal induced by the RF pulse irradiated from the transmission coil 14a is detected by the reception coil 14b arranged close to the subject 1, amplified by the signal amplifier 15, and sent from the sequencer 4 described later. Are divided into two orthogonal signals by the quadrature detector 16 and converted into digital quantities by the A / D converter 17 and sent to the signal processing system 7 as measurement data.

  The sequencer 4 controls to repeatedly apply the RF pulse and the gradient magnetic field pulse according to a predetermined imaging sequence. The sequencer 4 operates under the control of the CPU 8 and sends various commands necessary for collecting measurement data to the transmission system 5, the gradient magnetic field. Send to generation system 3 and reception system 6. The imaging sequence is a time chart that defines the on / off timing of RF pulses, gradient magnetic field pulses, etc., excitation RF pulse application interval (TR), bandwidth (BW), number of additions, number of phase encoding steps, etc. These measurement conditions (measurement parameters) are combined to define the temporal change of the magnetic field acting on the measurement target being measured. The imaging sequence is created in advance according to the purpose of measurement, and stored as a program and data in a storage device 18 or the like described later.

  The signal processing system 7 performs various data processing and display and storage of processing results, and includes a CPU 8, a storage device 18 such as a ROM and a RAM, an external storage device 19 such as an optical disk and a magnetic disk, and a display device 20. It consists of. When measurement data from the receiving system 6 is input to the CPU 8, the CPU 8 executes processing such as signal processing and image reconstruction, and displays the tomographic image of the subject 1 as a result on the display device 20 and also stores it. Record in device 18 or external storage device 19.

  The operation unit 25 receives various control information of the MRI apparatus 100 itself and various control information of processing performed by the signal processing system 7, and includes an input device 21 such as a trackball or a mouse and a keyboard. The operation unit 25 is disposed close to the display device 20, and the operator interactively inputs information necessary for various processes of the MRI apparatus 100 via the operation unit 25 while viewing the display device 20.

  At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.

  In FIG. 1, the transmission coil 14a and the gradient magnetic field coil 9 are opposed to the subject 1 in the static magnetic field space of the static magnetic field generation system 2 into which the subject 1 is inserted, in the case of the vertical magnetic field method, If it is a horizontal magnetic field system, it is installed so as to surround the subject 1.

  Further, the receiving coil 14b is disposed so as to face the subject 1 or surround the subject 1. In this embodiment, as this receiving coil 14b, a surface coil that receives NMR signals from the measurement target region of the subject 1 with a plurality of coils, and an NMR signal from the measurement target region can be received with one coil, And a whole body coil having a uniform sensitivity distribution.

  The MRI apparatus 100 of the present embodiment continuously executes a plurality of imaging sequences including different imaging sequence types. Depending on the imaging sequence type, in order to keep the SAR value within the range of the limit value defined in IEC (hereinafter referred to as the IEC limit value), wait until the SAR value drops to the specified value after the previous execution of the imaging sequence There is a need. In the present embodiment, the execution order of the imaging sequence is determined and the inspection is executed so as to minimize the total waiting time within one inspection.

First, the outline of the present invention will be described.
The present invention predicts, for each subject, the SAR value absorbed by the subject by irradiation of the RF pulse to the subject placed in a static magnetic field, and obtains the RF absorption amount of the subject, A SAR value of a predetermined pulse sequence is obtained based on the RF absorption amount. When obtaining the RF absorption amount for each subject, for example, SAR measurement for each subject and past data are used.

  Preferably, when a plurality of examinations are performed on the subject, the SAR calculation unit obtains the RF absorption amount of the subject for each examination, and obtains the SAR value of the predetermined pulse sequence based on the RF absorption amount .

  Here, the inspection means execution of a protocol (hereinafter referred to as an inspection protocol) combining at least one imaging sequence for the purpose. Then, at least one examination protocol with different examination sites is performed on one subject. Therefore, in the present invention, in one subject, the RF absorption amount of the reference RF pulse is further measured for each examination, and the SAR value is predicted and monitored for each examination.

  Each embodiment of the present invention is different in how to measure the RF absorption amount, but in order to predict the SAR value from the RF absorption amount, a common method described below is used.

本実施例1では、ある基準となる基準RFパルスでのRF吸収量(エネルギー)を検査毎に算出される。具体的には、基準とする基準RFパルスを用いて被検体に照射した場合での、RF吸収量をP_{obj_standard}、RF送信パワーをP_{fwd_standard}、RF反射パワーをP_{rfl_standard}、高周波コイルでのRF消費量をP_{coil_standard}とすると、これらの間には式[1]と同じ関係が成立する。基準RFパルスでのRF吸収量の測定のためには、基準RFパルス強度設定シーケンスを用いる。基準RFパルス強度設定シーケンスは、エコー信号を最大強度にするフリップ角90度のRFパルス照射強度を探索するためのパルスシーケンスである。具体的には、RFパルスを照射してエコー信号を計測する処理を、RFパルスの照射強度を変えながら複数回繰り返し、計測するエコー信号が最大強度となるRFパルスを基準RFパルスとする。 First, the SAR value of an arbitrary imaging sequence will be described based on FIG. FIG. 2 shows the flow of power in each part of the transmission system 5. As shown in FIG. 2, when RF pulses of RF transmission power P _fwd1 and P _fwd2 are transmitted to the transmission coil 14a, the RF reflection powers P _rft1 and P _rft2 reflected from the transmission coil 14a are measured, and the transmission coil 14a Considering the RF consumption (power consumption) P _coil , the RF absorption P _obj absorbed by the subject is calculated by the equation [1].

In the first embodiment, the RF absorption amount (energy) at a reference RF pulse that is a reference is calculated for each inspection. Specifically, when the subject is irradiated with a reference RF pulse as a reference, the RF absorption amount is P _{obj_standard} , the RF transmission power is P _{fwd_standard} , the RF reflection power is P _{rfl_standard} , and RF consumption in the high frequency coil When the amount is P _{coil_standard} , the same relationship as in equation [1] is established between them. A reference RF pulse intensity setting sequence is used to measure the amount of RF absorption with the reference RF pulse. The reference RF pulse intensity setting sequence is a pulse sequence for searching for an RF pulse irradiation intensity with a flip angle of 90 degrees that makes the echo signal maximum intensity. Specifically, the process of measuring the echo signal by irradiating the RF pulse is repeated a plurality of times while changing the irradiation intensity of the RF pulse, and the RF pulse having the maximum intensity of the measured echo signal is set as the reference RF pulse.

Then, based on the calculated P _{obj_standard} , when the subject is imaged in an arbitrary imaging sequence, the RF absorption amount in all the RF pulses used in the imaging sequence is estimated. Specifically, as shown in FIG. 3, in an imaging sequence that irradiates a set of a plurality of RF pulses (RF ₁ , RF ₂ ,... RF _w ,...), Each RF pulse including intensity ( RF ratio of each RF pulse RF _w in the set of RF ₁ , RF ₂ ,... RF _w ,...: RF absorption amount in all RF pulses used in the imaging sequence using RfWaveRatio _w Is estimated based on Equation [2].

なお、IECで規定される頭部SARおよび身体部分SARを求める場合には、体重Weightはそれぞれ頭部又は身体部分の体重とする。 Then, the SAR value of the imaging sequence is predicted based on Equation [3] from the RF absorption amount of Equation [2] and the weight Weight of the subject.

When obtaining the head SAR and body part SAR specified by IEC, the weight Weight is the weight of the head or body part, respectively.

As shown in the above formulas [1] to [3], since the SAR value is predicted based on the actual RF absorption amount P _{obj_standard} in the subject, the subject is _independent of the subject's body type and imaging position. The SAR value can be accurately predicted for each specimen. Since the predicted SAR value is almost the same as the SAR value of the imaging sequence to be executed in the future, it is not necessary to give an excessive likelihood to the SAR predicted value with respect to the IEC limit value. However, in order not to exceed the IEC limit value, an appropriate likelihood may be set in view of safety.

  As described above, by accurately predicting the SAR value for each examination (that is, for each subject), the imaging sequence is unintentionally interrupted during execution due to SAR restrictions, or waiting time between imaging sequences in the examination Can be predicted before testing begins. If it is predicted that an interruption or waiting time of the imaging sequence will occur against the will of the operator, the operator can change the imaging conditions so that the interruption or waiting time of the imaging sequence does not occur. Thus, it is possible to avoid a decrease in throughput due to interruption of the imaging sequence during the inspection and occurrence of a waiting time, and therefore it is possible to carry out the inspection smoothly.

  Next, a first embodiment of the MRI apparatus and the SAR prediction method of the present invention will be described. In the first embodiment, the RF absorption amount of the reference RF pulse is measured for each inspection, and the SAR value is accurately predicted using the RF absorption amount of the reference RF pulse.

  This eliminates a waiting time during which the imaging sequence due to the SAR restriction is not executed during the inspection, and avoids a decrease in throughput due to the waiting time.

  First, each function of the arithmetic processing unit 8 of the first embodiment for measuring the RF absorption amount of the reference RF pulse and predicting the SAR value using the RF absorption amount of the reference RF pulse is shown in FIG. This will be described based on the functional block diagram. The arithmetic processing unit 8 in the first embodiment includes an object information setting unit 401, an imaging condition setting unit 402, a SAR calculation unit 403, an imaging control unit 404, and a SAR monitoring unit 405.

  The subject information setting unit 401 displays an input screen for inputting subject information on the display device 20, accepts input of subject information by the operator via the input device 21, and the inputted subject information Is notified to the SAR calculation unit 403. In particular, age, sex, weight, etc. are required as the subject information related to the calculation of the SAR value. Age, gender, and weight are used in calculating the weight of Equation [3].

  The imaging condition setting unit 402 displays an input screen for inputting imaging conditions (specifically, specific parameter values of the imaging sequence) on the display device 20, and the imaging conditions set by the operator via the input device 21. Is input, and the input imaging conditions are notified to the SAR calculation unit 403 and the imaging control unit 404. In particular, when the imaging sequence exceeds the SAR limit value, a warning to that effect is displayed on the display device 20, and a new display for prompting the user to change the imaging condition or set the waiting time is performed. The change of the imaging condition or the waiting time setting by the operator is accepted via the, and the contents of the accepted imaging condition change or waiting time setting are notified to the SAR calculation unit 403 and the imaging control unit 404.

  The SAR calculation unit 403 sets the RF absorption amount of the subject based on the subject information notified from the subject information setting unit 401.

  Further, when the head SAR and the body part SAR defined by IEC are obtained, the corresponding partial body weights of the subject in the transmission coil 14a are set. The SAR includes a 6-minute average whole body SAR, a 6-minute average body part SAR, a 6-minute average head SAR, a 10-second average whole body SAR, a 10-second average body part SAR, and a 10-second average head SAR.

  In addition, based on the imaging condition notified from the imaging condition setting unit 402, the time-series SAR value of each imaging sequence constituting the examination protocol is calculated, and the calculated SAR value is notified to the SAR monitoring unit 405 described later.

Further, the SAR value is obtained as needed based on the values of P _fwd and P _rfl notified from the sequencer 4 during imaging, and the obtained SAR value is notified to the SAR monitoring unit 405.

  Based on the imaging condition notified from the imaging condition setting unit 402, the imaging control unit 404 specifically applies control data such as an application timing and an application waveform such as an RF pulse and a gradient magnetic field pulse for executing an arbitrary pulse sequence. To the sequencer 4 to execute the pulse sequence.

  Further, when the SAR monitoring unit 405 notifies that the SAR value exceeds the IEC limit value, the imaging sequence being executed is immediately stopped.

  The SAR monitoring unit 405 monitors so that the SAR value notified from the SAR calculation unit 403 does not exceed the IEC limit value. When it is determined that the SAR value (predicted value or actual measurement value) notified from the SAR calculation unit 403 exceeds the IEC limit value, the imaging control unit 404 is notified so and the currently executed imaging sequence is stopped. At the same time, a display indicating that the imaging sequence is stopped due to the SAR value restriction is displayed on the display device 20.

  Next, the processing flow of the first embodiment performed in cooperation with the above functional units will be described based on the flowchart shown in FIG. This processing flow is stored in advance in the storage device 18 as a program, and is executed by the arithmetic processing unit 8 reading the program from the storage device 18 and executing it. Hereinafter, the processing contents of each processing step will be described in detail.

  In step 501, the subject information setting unit 401 displays an input screen for inputting subject information on the display device 20, and accepts input of subject information such as the age, sex, and weight of the subject by the operator. . Then, the input subject information is notified to the SAR calculation unit 403.

  In step 502, the SAR calculation unit 403 sets the RF absorption amount of the subject based on the subject information notified from the subject information setting unit 401. Details of this step will be described later.

  In step 503, the SAR calculation unit 403 obtains a head SAR and a body part SAR specified by the IEC based on the subject information notified from the subject information setting unit 401, and the subject in the transmission coil 14a. Set the weight of each part of the specimen.

  In step 504, the imaging condition setting unit 402 displays an input screen for inputting imaging conditions of one imaging sequence constituting the examination protocol on the display device 20, and the imaging conditions set by the operator via the input device 21 are displayed. (That is, the imaging sequence parameter value) is input, and the input imaging conditions are notified to the SAR calculation unit 403 and the imaging control unit 404.

  In step 505, the SAR calculation unit 403 calculates the SAR values of the imaging sequence in time series based on the imaging conditions notified from the imaging condition setting unit 402. Then, the calculated time-series SAR value is notified to the SAR monitoring unit 405.

  If each of the notified time-series SAR values exceeds the IEC limit value and the execution of the imaging sequence cannot be permitted, the SAR monitoring unit 405 notifies the imaging condition setting unit 402 to that effect.

  Upon receiving the notification, the imaging condition setting unit 402 displays an input screen for accepting a change in imaging conditions or a waiting time setting on the display device 20, accepts a change in imaging conditions or a waiting time setting by the operator, and has been changed. The imaging condition or idle time setting is notified to the SAR calculation unit 403.

  Then, the SAR calculation unit 403 recalculates the time-series SAR value based on the changed imaging condition or the set waiting time.

  Thereafter, the above series of processing is repeated until the time series SAR value falls within the IEC limit value.

  Then, the imaging condition setting unit 402 notifies the imaging control unit 404 of the imaging condition and waiting time setting when the time-series SAR value is within the IEC limit value.

  The imaging control unit 404 specifies control data such as the application timing and application waveform of RF pulses and gradient magnetic field pulses for executing the imaging sequence set to the imaging conditions in which the time-series SAR value is within the IEC limit value. Generated and notified to the sequencer 4 to execute the imaging sequence.

In step 506, the SAR calculation unit 403 calculates the SAR value based on the equation [3] using P _fwd and P _rfl actually measured and notified by the sequencer 4 during the execution of the imaging sequence. The SAR value is notified to the SAR monitoring unit 405.

  Then, the SAR monitoring unit 405 compares the SAR value notified from the SAR calculation unit 403 with the IEC limit value, and monitors whether the SAR value exceeds the IEC limit value. When it is determined that the SAR value exceeds the IEC limit value, the imaging control unit 404 is notified accordingly.

  Then, when notified that the SAR value exceeds the IEC limit value, the imaging control unit 404 issues a command to the sequencer 4 to immediately stop the execution of the imaging sequence, and stops the execution of the imaging sequence. However, in step 505 described above, the imaging conditions and waiting time are set so that the SAR value does not exceed the IEC limit value. Therefore, the SAR monitoring in step 506 and the SAR value exceed the IEC limit value. In this case, the stop of the imaging sequence does not basically occur, and the monitoring of the SAR value in step 506 is only a preventive monitoring.

  In step 507, the imaging control unit 404 determines whether or not the execution of all imaging sequences constituting the inspection protocol has been completed. If completed, the process proceeds to step 508. If not completed, the process returns to step 504. Thus, the above-described steps 504 to 506 are repeated for other imaging sequences constituting the inspection protocol.

In step 508, the routine inspection ends.
The above is the outline of the processing flow of the first embodiment.

Next, the detailed processing flow of step 502 will be described based on the flowchart shown in FIG.
In step 601, the SAR calculation unit 403 temporarily sets the RF absorption amount of the reference RF pulse with a large likelihood so that the imaging sequence can be executed in almost all cases. The reason why the RF absorption amount is set with the likelihood is that the actual RF absorption amount is still unknown, so priority is given to the safety in the reference RF pulse intensity setting sequence and the SAR measurement-dedicated sequence that will be performed later. .

  In step 651, the SAR calculation unit 403 predicts the SAR value of the reference RF pulse intensity setting sequence from the RF absorption amount temporarily set in step 601 based on Equation [3].

  In step 652, the imaging control unit 404 executes a reference RF pulse intensity setting sequence to determine the intensity (RFGain) of the reference RF pulse. Thereby, the intensity of the RF pulse at which the flip angle is 90 degrees is set as the reference RF pulse.

  In step 602, the SAR calculation unit 403 predicts the SAR value of the SAR measurement-dedicated sequence from the waveform ratio with the reference RF pulse whose intensity is set in step 652. Since the predicted SAR value is based on the RF absorption amount temporarily set in step 601, it is a considerably larger value than the actual value. Here, the SAR measurement-dedicated sequence is a sequence in which the RF pulse intensity and the imaging conditions are optimized so that the RF power measurement error is reduced. For example, you may use the pulse energy method (IEC60601-2-33) for SAR measurement prescribed | regulated by IEC.

  In step 603, the imaging control unit 404 executes a SAR measurement dedicated sequence.

In step 604, the sequencer 4 measures P _{fwd_standard} and P _{rfl_standard} during execution of the SAR measurement dedicated sequence, and notifies the SAR calculation unit 403 of these measured values.

In step 605, the SAR calculation unit 403 calculates the absorption amount P _{obj_standard} of the reference RF pulse of the subject based on Equation [1] using the measurement values of P _{fwd_standard} and P _{rfl_standard} notified in step 604. . The absorption amount P _{obj_standard} of the reference RF pulse is a substantially accurate RF absorption amount of the measured subject.

In step 606, the SAR calculation unit 403 _resets the RF absorption amount temporarily set in step 601 with P _{obj_standard} calculated in step 605. Therefore, the reset RF absorption amount P _{obj_standard} is approximately exactly the RF absorption amount of the subject.

The above is the description of the detailed processing flow of step 502. Thereafter, the processing after Step 503 is performed using the _reset RF absorption amount P _{obj_standard} . In the processing after step 503, as described above, the SAR calculation unit 403 uses the P _{obj_standard} calculated in step 605 to calculate the SAR values of the imaging sequences that constitute the inspection protocol based on the equations [2] and [3]. calculate.

  Then, the SAR calculation unit 403 notifies the SAR monitoring unit 405 of the calculated SAR value of the imaging sequence.

  Then, the SAR monitoring unit 405 compares the notified SAR value with the IEC limit value, and monitors whether the SAR value exceeds the IEC limit value. If the SAR value is within the IEC limit value, the imaging control unit 404 is notified accordingly.

  Then, the imaging control unit 404 generates control data of the imaging sequence and notifies the sequencer 4 to cause the sequencer 4 to execute the imaging sequence.

  As described above, according to the MRI apparatus and the SAR prediction method of the first embodiment, the RF absorption amount of the reference RF pulse is obtained for each subject, and based on the RF absorption amount of the reference RF pulse, an arbitrary amount is obtained. Predict the SAR value of the imaging sequence. This makes it possible to accurately predict the SAR value for each imaging sequence regardless of the body shape of the subject and the relative positional relationship with the transmission coil. As a result, it is possible to determine whether waiting time is required before the start of inspection, and when waiting time is required, waiting time can be reduced by changing imaging conditions, etc., and throughput reduction due to unexpected waiting time can be avoided Become.

  In particular, the method according to the present Example 1 can correctly cope with the fact that the permeability and conductivity are different even when the weight and partial weight of the subject are the same, and the absorbed energy is different even when the RF irradiation magnetic field is the same.

  This can be schematically shown in FIG. In FIG. 7, the horizontal axis represents the weight of the subject, and the vertical axis represents the absorbed energy at each weight. As indicated by the black circles in FIG. 7, each absorbed energy is distributed with a dispersion with respect to the average value. Conventionally, in order to make imaging safer, based on the assumption that the absorbed energy of the subject has such a certain degree of dispersion, the SAR was estimated with the value indicated by a white star slightly larger, In order to increase the degree of freedom of imaging, more accurate estimation indicated by a black star mark was required. According to the present Example 1, since it is possible to obtain the actual absorbed energy (SAR) corresponding to each subject, the site to be examined, it is possible to correctly estimate the SAR, and it is not necessary to make a large and uniform estimate. Highly flexible imaging is possible.

  Next, a second embodiment of the MRI apparatus and the SAR prediction method of the present invention will be described. In Example 1 described above, an example in which the SAR measurement dedicated sequence is used to measure the RF absorption amount with the reference RF pulse has been described. However, in this Example 2, the reference RF pulse intensity setting can be performed without using the SAR measurement dedicated sequence. The sequence itself is used to measure the RF absorption at the reference RF pulse. Hereinafter, only portions different from the above-described first embodiment will be described, and description of the same portions will be omitted.

  Each function of the arithmetic processing unit 8 for predicting the SAR value of the second embodiment is the same as the function block diagram shown in FIG. 4, but the processing content of each functional unit is different. Hereinafter, the processing content of each functional unit will be described through the description of the processing flow of the second embodiment.

  In the present embodiment 2, the portions different from the above-described embodiment 1 do not execute the SAR measurement-dedicated sequence as described above, and the other portions are the same as the above-described embodiment 1, and therefore, in the embodiment 1 shown in FIG. Only the processing flow of step 502 is different.

FIG. 8 is a flowchart showing the process flow of step 502 in the second embodiment. In the flowchart shown in FIG. 8, the portions different from the flowchart shown in FIG. In step 604, the sequencer 4 measures P _{fwd_standard} and P _{rfl_standard} during execution of the reference RF pulse measurement sequence, and notifies the SAR calculation unit 403 of the measured values. The subsequent processing is the same as in FIG. As a result, the RF absorption amount for each subject is determined based on the reference RF pulse measurement sequence, and the RF absorption amount determined in step 606 is reset.

  As described above, in the second embodiment, the RF absorption amount in the reference RF pulse is measured using the reference RF pulse intensity setting sequence itself without using the SAR measurement dedicated sequence. As a result, since the execution step of the SAR measurement dedicated sequence can be omitted, the measurement of the RF absorption amount for each subject can be simplified and shortened.

  Also in the second embodiment, since the actual absorbed energy (SAR) can be obtained as in the first embodiment, it is possible to contribute to enabling imaging with a higher degree of freedom.

  Next, a third embodiment of the MRI apparatus and the SAR prediction method of the present invention will be described. In the above-described Example 2, the RF absorption amount at the reference RF pulse was measured using the reference RF pulse intensity setting sequence, but in this Example 3, the RF absorption amount at the reference RF pulse was not measured, and the past The RF absorption amount for each subject is estimated using the reference RF pulse intensity based on the relationship between the reference RF pulse intensity and the RF absorption amount obtained during the examination of a plurality of persons.

First, it will be described with reference to FIG. 9 that the RF absorption amount can be estimated from the reference RF pulse intensity. FIG. 9 is a graph showing the relationship between the reference RF pulse output measured with a plurality of subjects and the RF absorption amount. The horizontal axis represents the RF output (that is, intensity) of the reference RF pulse, and the vertical axis represents the amount of RF absorption. Each plot point of a rhombus shows the measured value for every subject. From this graph, it is understood that there is a correlation between the RF absorption amount and the reference RF pulse intensity. Therefore, when obtaining a curve function that approximates each plot point of the rhombus by the least square, in the graph of FIG.
RF absorption = a × (Reference RF pulse intensity) ^ 2 ・ ・ ・ [4]
a = 0.0700556
Can be approximated. Therefore, if the reference RF pulse intensity is measured, the RF absorption amount can be estimated from the reference RF pulse intensity. Therefore, the relational expression and the coefficient a shown in Expression [4] are stored in advance in the storage device 18, and the RF absorption amount is estimated from the reference RF pulse intensity based on the stored relational expression [4].

  Next, each functional block and processing flow of the arithmetic processing unit 8 according to the third embodiment will be described.

  Each function of the arithmetic processing unit 8 for predicting the SAR value based on the relationship between the reference RF pulse intensity and the RF absorption amount in the third embodiment is the same as the functional block diagram shown in FIG. However, the processing content of each functional unit is different. Hereinafter, the processing content of each functional unit will be described through the description of the processing flow of the third embodiment.

  In the processing flow of the third embodiment, the difference from the second embodiment is that the RF transmission power and the reflected power are not measured during the execution of the reference RF pulse measurement sequence, and the RF absorption amount is obtained from the reference RF pulse intensity. is there. Others are the same as in the second embodiment. As a result, only the processing flow of step 502 in the second embodiment shown in FIG. 8 is different.

  FIG. 10 is a flowchart showing the process flow of step 502 in the third embodiment. In the flowchart shown in FIG. 10, the points different from the flowchart shown in FIG. 7 are the absence of step 604 and the processing content of step 605.

  In step 605 in the third embodiment, the RF absorption amount is determined based on the relational expression [4] stored in advance in the storage device 18 from the reference RF pulse output value. The subsequent processing is the same as in FIG.

  As a result, during the execution of the reference RF pulse measurement sequence, the RF absorption amount is determined for each subject based on the reference RF pulse intensity determined in the reference RF pulse measurement sequence without measuring the RF transmission power and reflected power. Then, the RF absorption amount determined in step 606 is reset.

As described above, in the third embodiment, the RF absorption amount is estimated from the reference RF pulse intensity measured by the reference RF pulse sequence based on the relationship between the reference RF pulse intensity and the RF absorption amount. As a result, the execution of the SAR measurement-dedicated sequence and the measurement of the RF transmission power and the reflected power can be omitted, so that the processing flow in step 502 can be further simplified and the processing time can be shortened.
Also in the third embodiment, since the actual absorbed energy (SAR) can be obtained as in the first embodiment, it is possible to contribute to enabling imaging with a higher degree of freedom.

  Next, a fourth embodiment of the MRI apparatus and the SAR prediction method of the present invention will be described. In Example 2 described above, the amount of RF absorption by the reference RF pulse was measured. In Example 4, the amount of RF absorption by the reference RF pulse was not measured. Thus, the RF absorption amount for each subject is estimated from the weight of the subject.

First, it will be described based on FIG. 11 that the RF absorption amount can be estimated from the body weight of the subject. FIG. 11 is a graph showing the relationship between the weight of an object measured with a plurality of objects and the amount of RF absorption. The horizontal axis represents the weight of the subject, and the vertical axis represents the amount of RF absorption. Each plot point of a rhombus shows the measured value for every subject. From this graph, it is understood that there is a correlation between the amount of RF absorption and body weight. Therefore, when calculating a curve function that approximates each plot point of the rhombus by the least square, in the graph of FIG.
RF absorption = b × M (subject's weight) ・ ・ ・ [5]
b = 27.632
Can be approximated. Therefore, if the body weight of the subject is input, the RF absorption amount can be estimated from the body weight. Therefore, the relational expression and coefficient b shown in Expression [5] are stored in advance in the storage device 18, and the RF absorption amount is estimated from the body weight of the subject based on the stored relational expression [5].

  Next, each functional block and processing flow of the arithmetic processing unit 8 according to the second embodiment will be described.

  Each function of the arithmetic processing unit 8 for predicting the SAR value based on the relationship between the body weight of the subject and the RF absorption amount in Example 4 is the same as the functional block diagram shown in FIG. However, the processing content of each functional unit is different. Hereinafter, the processing content of each functional unit will be described through the description of the processing flow of the fourth embodiment.

  In the processing flow of the fourth embodiment, the difference from the third embodiment is that the RF absorption amount is obtained from the body weight of the subject without executing the reference RF pulse measurement sequence. Others are the same as in the third embodiment. As a result, only the processing flow of step 502 in the third embodiment shown in FIG. 10 is different.

  FIG. 12 is a flowchart showing the process flow of step 502 in the fourth embodiment. In the flowchart shown in FIG. 12, portions different from the flowchart shown in FIG. 10 are the absence of steps 651 and 652, and the processing content of step 605.

  In step 605 in the fourth embodiment, the RF absorption amount is determined based on the relational expression [5] stored in advance in the storage device 18 from the body weight of the subject input in step 501. The subsequent processing is the same as in FIG.

  As a result, the RF absorption amount is determined based on the body weight of each subject without executing the SAR measurement dedicated sequence and the reference RF pulse measurement sequence, and the RF absorption amount determined in step 606 is reset.

  Note that the prediction of the SAR value of the reference RF pulse intensity setting sequence in step 651 and the determination of the reference RF pulse intensity by executing the reference RF pulse intensity setting sequence in step 652 are performed by resetting the RF absorption amount in step 606. Done later.

  As described above, in the fourth embodiment, the RF absorption amount is estimated from the body weight of the subject based on the relationship between the body weight and the RF absorption amount. As a result, since the execution of the SAR measurement dedicated sequence and the reference RF pulse measurement sequence can be omitted, the processing flow of step 502 can be further simplified and the processing time can be shortened.

  The first and second embodiments are based on the premise that the SAR monitoring before the execution of the imaging sequence is correct. However, in reality, SAR monitoring may include errors due to individual differences in the internal loss of the irradiation coil, individual differences in analog components and the accompanying deviation in irradiation coil tuning, individual differences in measurement amplifiers, and the like. When these errors are large, it is desirable to measure the actual SAR value based on the temperature rise of the subject and correct the SAR value obtained in advance with the obtained value.

  In the fifth embodiment, since the MRI apparatus can measure the temperature change of the subject by the imaging function, the SAR value during execution of the imaging sequence is measured from the temperature change. That is, if the error between the SAR value obtained based on the temperature measurement described below and the SAR value obtained in any of Examples 1 to 4 is large, the SAR value obtained in any of Examples 1 to 4 is captured. Replace in the middle of the sequence. That is, a temperature measurement unit that measures the temperature rise of the subject is provided, and the SAR calculation unit uses the temperature rise obtained by the temperature measurement unit, and the SAR value calculated in any of Examples 1 to 4 When the error from the SAR obtained based on the temperature measurement described below is large, the SAR value calculated in any one of the first to fourth embodiments is replaced with the SAR value obtained based on the temperature measurement.

When a specimen or phantom absorbs RF waves, energy is absorbed by that amount, and the temperature of the substance is rising, so by measuring the gradient of temperature rise, the absorbed energy per unit time can be obtained. Can do. The actual principles of temperature measurement are: a) signal intensity method using temperature difference of longitudinal relaxation time, b) phase method using temperature difference of chemical shift, c) diffusion image method using temperature difference of diffusion coefficient Etc. are considered. Here, when estimating the SAR from the temperature rise actually obtained by temperature measurement, if the temperature rise value is converted and calculated by the mass of the examination site of the subject, the specific heat (C) of the subject, etc. good. More specifically,
The resulting temperature rise ΔT per 1g, multiplied by C × 4.2 × M (weight of the subject) × 10 ^3, the amount of heat which the subject was absorbed
Q = ΔT × C × 4.2 × M × 10 ³
So
SAR = Q [J / s] / M [kg] = ΔT × C × 4.2 × 10 ³
It becomes.

  1 subject, 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 sequencer, 5 transmission system, 6 reception system, 7 signal processing system, 8 arithmetic processing unit (CPU), 9 gradient magnetic field coil, 10 gradient magnetic field power supply, 11 High-frequency transmitter, 12 modulator, 13 high-frequency amplifier, 14a high-frequency coil (transmitting coil), 14b high-frequency coil (receiving coil), 15 signal amplifier, 16 quadrature detector, 17 A / D converter, 18 storage device, 19 external storage device, 20 display device, 21 input device

Claims (9)

A transmitter for irradiating an RF pulse to a subject placed in a static magnetic field;
An imaging control unit that controls imaging using a predetermined pulse sequence including irradiation of the RF pulse;
A SAR calculation unit for obtaining a SAR value absorbed by the subject by the RF pulse;
A SAR monitoring unit that monitors whether the SAR value exceeds a predetermined limit value;
A magnetic resonance imaging apparatus comprising:
The SAR calculator is provided for each subject.
Determining the amount of RF absorption of the subject;
A magnetic resonance imaging apparatus, wherein an SAR value of the predetermined pulse sequence is obtained based on the RF absorption amount. The magnetic resonance imaging apparatus according to claim 1.
When a plurality of examinations are performed on the subject,
The SAR calculation unit, for each inspection,
Determining the amount of RF absorption of the subject;
A magnetic resonance imaging apparatus, wherein an SAR value of the predetermined pulse sequence is obtained based on the RF absorption amount. The magnetic resonance imaging apparatus according to claim 1 or 2,
The SAR calculation unit obtains the RF absorption amount of the reference RF pulse, and based on the RF absorption amount of the reference RF pulse, the body weight of the subject, and the combination of the RF pulses constituting the predetermined pulse sequence. A magnetic resonance imaging apparatus characterized by obtaining an SAR value of the predetermined pulse sequence. The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The magnetic resonance imaging apparatus, wherein the SAR calculation unit obtains the RF absorption amount based on a SAR measurement dedicated sequence using a reference RF pulse. The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The magnetic resonance imaging apparatus, wherein the SAR calculation unit obtains the RF absorption amount based on a reference RF pulse intensity setting sequence for setting an intensity of a reference RF pulse. The magnetic resonance imaging apparatus according to claim 3.
The magnetic resonance imaging apparatus, wherein the SAR calculation unit obtains the RF absorption amount using the intensity of the reference RF pulse based on the relationship between the intensity of the reference RF pulse and the RF absorption amount. The magnetic resonance imaging apparatus according to claim 3.
The magnetic resonance imaging apparatus, wherein the SAR calculation unit obtains the RF absorption amount using the body weight of the subject based on the relationship between the body weight of the subject and the RF absorption amount. The magnetic resonance imaging apparatus according to claim 1,
A temperature measurement unit that measures a temperature increase of the subject, and the SAR calculation unit uses the temperature increase obtained by the temperature measurement unit, and if the error from the calculated SAR value is large, A magnetic resonance imaging apparatus characterized in that it is replaced with a value obtained by measurement. A SAR prediction method for predicting a SAR value absorbed by an object placed in a static magnetic field by irradiation of an RF pulse with the object,
Obtaining an RF absorption amount of the subject;
Obtaining a SAR value of a predetermined pulse sequence based on the RF absorption amount;
And a step of monitoring whether or not the SAR value exceeds a predetermined limit value.

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