JP6227270B2 - Magnetic resonance imaging apparatus and magnetic resonance imaging method - Google Patents

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) technique. In particular, the present invention relates to an RF shimming technique for improving the irradiation uniformity of a radio frequency magnetic field (RF) pulse in an imaging space.

  The MRI system irradiates an RF pulse having a specific frequency with respect to the magnetic moment of a nucleus placed in a static magnetic field, and measures the natural scientific information of the substance using the magnetic resonance phenomenon caused by this, image Visualize as.

  In recent years, in order to obtain a clearer image, the magnetic field of the apparatus has been increased, and a 3 Tesla machine has appeared. In this 3 Tesla machine, since the RF wavelength of the frequency to be used matches the size of the human body that is the main imaging object, a standing wave is generated in the imaging object, and thus the irradiation intensity of the RF pulse in the imaging space is increased. Non-uniformity, that is, non-uniformity of the generated rotating magnetic field distribution (B1 distribution) occurs. In order to overcome this non-uniformity, RF shimming was devised to improve the uniformity of the B1 distribution by using a transmission coil having a plurality of channels and independently controlling the intensity and phase of the RF pulse applied to each channel. (For example, see Non-Patent Document 1 and Patent Document 1). RF shimming is currently used even under static magnetic field strengths other than 3 Tesla, and not only the B1 distribution due to the RF wavelength and the size of the imaging target, but also the B1 distribution due to the shape of the imaging target. It is also used as a technique for correcting the above.

P. Ledden, “Transmit Homgenee at High Fields: Computational Effects of“ B1 Shimming ”with an Eight Element Volume Coil, Proc. Int. Soc.

US Pat. No. 7,633,293

  RF shimming can be realized as long as the irradiation system is divided into a plurality of channels, is highly versatile, and is a very important technique for improving the uniformity of the B1 distribution. However, although the uniformity of the B1 distribution in the imaging space is improved by RF shimming, since the intensity and phase of the applied RF pulse are adjusted for each channel, the intensity as a whole increases or decreases. For this reason, high-performance hardware that can cope with this increase / decrease is essential. Without such high-performance hardware, the adjustment range of the irradiation gain is exceeded, and appropriate RF pulse irradiation cannot be performed.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a technique for overcoming the above-described problems without increasing the performance of hardware.

  The present invention adjusts the intensity and phase of the RF pulse applied to each channel, adjusted for the purpose of RF shimming, while maintaining the uniformity of the B1 distribution obtained by RF shimming, and the net irradiation intensity generated in the imaging space. Standardize and suppress substantial changes in irradiation intensity. In normalization, the intensity of the RF pulse applied to each channel is changed at an arbitrary ratio so that the absolute value of the combined vector of the output from each channel does not change before and after RF shimming.

  According to the present invention, RF pulse irradiation required by RF shimming can be realized without increasing the performance of hardware.

1 is a block diagram illustrating an overall configuration of an MRI apparatus according to an embodiment of the present invention. It is explanatory drawing for demonstrating the hardware constitutions of the transmission system of embodiment of this invention. It is a functional block diagram of the control processing system of the embodiment of the present invention. (A) is the net RF intensity before RF shimming according to the embodiment of the present invention, (b) is the net RF intensity after the RF shimming, and (c) is the net RF intensity after the standardization. It is explanatory drawing for demonstrating each intensity | strength. (A) And (b) is a graph for demonstrating the example of an output change of each channel by normalization of embodiment of this invention. (A) is a flowchart of the imaging process of embodiment of this invention, (b) is a flowchart of the imaging process by a conventional method.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof will be omitted unless otherwise specified.

  In the present embodiment, a case where an MRI apparatus is used as an image acquisition apparatus will be described as an example. First, an overall outline of an example of the MRI apparatus of the present embodiment will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of one embodiment of the MRI apparatus of this embodiment.

  The MRI apparatus 100 according to the present embodiment obtains a tomographic image of a subject using an NMR phenomenon. As shown in FIG. 1, a static magnetic field generation unit 120, a gradient magnetic field generation unit 130, and a high-frequency magnetic field irradiation unit. (Hereinafter referred to as a transmission unit) 150, a high-frequency magnetic field detection unit (hereinafter referred to as a reception unit) 160, a control processing unit 170, and a sequencer 140.

  The static magnetic field generator 120 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 101 if the vertical magnetic field method is used, and in the body axis direction if the horizontal magnetic field method is used. The apparatus includes a permanent magnet type, normal conduction type or superconducting type static magnetic field generation source disposed around the subject 101.

  The gradient magnetic field generation unit 130 is a gradient magnetic field coil 131 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (device coordinate system) of the MRI apparatus 100, and a gradient magnetic field power source that drives each gradient magnetic field coil. 132, and the gradient magnetic field power supply 132 of each gradient magnetic field coil 131 is driven in accordance with a command from the sequencer 140, which will be described later, to apply gradient magnetic fields Gx, Gy, Gz in the three axial directions of X, Y, and Z. . A slice plane for the subject 101 is set by this gradient magnetic field. Also, a nuclear magnetic resonance signal, which will be described later, is encoded to give position information.

  The transmitter 150 irradiates the subject 101 with a high-frequency magnetic field pulse (RF pulse) in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the biological tissue of the subject 101, and a high-frequency oscillator (synthesizer). 152, a modulator 153, a high frequency amplifier 154, and a high frequency coil (transmission coil) 151 on the transmission side. The transmission unit 150 of the present embodiment has a plurality of channels and irradiates an RF pulse from each channel.

  The transmission coil 151 has a feeding point for each channel, and irradiates an RF pulse supplied to each feeding point. The synthesizer 152, the modulator 153, and the high frequency amplifier 154 are provided for each channel. As an example, FIG. 2 shows a case where two are provided. As shown in the figure, each corresponds to the first channel (channel 1: Ch1), and is provided with a synthesizer 152a, a modulator 153a, and a high frequency amplifier 154a, and corresponds to the second channel (channel 2: Ch2). , A synthesizer 152b, a modulator 153b, and a high-frequency amplifier 154b. However, the synthesizer can be shared.

  The synthesizer 152 generates and outputs an RF pulse. The modulator 153 modulates the intensity and phase of the RF pulse output in accordance with a command from the sequencer 140. The high frequency amplifier 154 amplifies the modulated RF pulse and supplies the amplified RF pulse to the feeding point of the transmission coil 151 disposed in the vicinity of the subject 101. The transmission coil 151 irradiates the subject 101 with the supplied RF pulse. The instruction and timing of modulation given from the sequencer 140 to the modulator 153 is controlled by the control processing unit 170.

  The receiving unit 160 detects a nuclear magnetic resonance signal (echo signal, NMR signal) emitted by nuclear magnetic resonance of the nuclear spin constituting the living tissue of the subject 101, and receives a high-frequency coil (receiving coil) on the receiving side. 161, a signal amplifier 162, a quadrature detector 163, and an A / D converter 164. The reception coil 161 is disposed in the vicinity of the subject 101 and detects an NMR signal of the response of the subject 101 induced by the electromagnetic wave irradiated from the transmission coil 151. The detected NMR signal is amplified by the signal amplifier 162 and then divided into two orthogonal signals by the quadrature phase detector 163 at the timing according to the command from the sequencer 140, and each is digitally converted by the A / D converter 164. It is converted into a quantity and sent to the control processing unit 170.

  The transmission coil 151 and the gradient magnetic field coil 131 are opposed to the subject 101 in the static magnetic field space of the static magnetic field generating unit 120 into which the subject 101 is inserted, if the vertical magnetic field method is used, and if the horizontal magnetic field method is used. It is installed so as to surround the subject 101. Further, the receiving coil 161 is installed so as to face or surround the subject 101.

  The sequencer 140 operates each unit to apply the RF pulse and the gradient magnetic field pulse in accordance with an instruction from the control processing unit 170. Specifically, in accordance with instructions from the control processing unit 170, various commands necessary for collecting tomographic image data of the subject 101 are transmitted to the transmission unit 150, the gradient magnetic field generation unit 130, and the reception unit 160.

  The control processing unit 170 controls the entire MRI apparatus 100, performs various data processing operations, displays and stores processing results, and includes a CPU 171, a storage device 172, a display device 173, and an input device 174.

  The storage device 172 includes an internal storage device such as a hard disk and an external storage device such as an external hard disk, an optical disk, and a magnetic disk. The storage device 172 includes, for example, a ROM (read-only memory) that stores a program for performing image analysis processing and measurement over time, an invariant parameter used in the execution, a measurement result, an echo signal detected by the receiving unit 160, In addition, an RAM (temporary writing / reading memory) that temporarily stores an image used for setting the region of interest and stores parameters for setting the region of interest, and a data storage unit that records image data reconstructed by the CPU 171 are provided. A magneto-optical disk, a magnetic disk, etc. are provided.

  The display device 173 is a display unit that visualizes image data read from the storage device 172 and displays it as a tomographic image, and includes a display device such as a CRT or a liquid crystal display.

  The input device 174 is an interface for inputting various control information of the MRI apparatus 100 and control information of processing performed by the control processing unit 170, and includes, for example, a trackball or a mouse and a keyboard. The input device 174 is disposed in the vicinity of the display device 173. The operator interactively inputs instructions and data necessary for various processes of the MRI apparatus 100 through the input device 174 while looking at the display device 173.

  The CPU 171 implements each process of the control processing unit 170 such as control of the operation of the MRI apparatus 100 and various data processes by executing a program previously stored in the storage device 172 in accordance with an instruction input by the operator. For example, the intensity and phase of the RF pulse applied to each channel of the transmission unit 150 are determined. When data from the receiving unit 160 is input to the control processing unit 170, the CPU 171 executes signal processing, image reconstruction processing, and the like, and displays a tomographic image of the subject 101 as a result on the display device 173. At the same time, it is stored in the storage device 172. Further, the sequencer 140 is controlled.

  Currently, the nuclide to be imaged by the MRI apparatus, which is widely used clinically, is a hydrogen nucleus (proton) that is a main constituent material of the subject 101. In the MRI apparatus 100, information on the spatial distribution of the proton density and the spatial distribution of the relaxation time of the excited state is imaged so that the form or function of the human head, abdomen, limbs, etc. can be expressed two-dimensionally or three-dimensionally. Take an image.

  Moreover, you may provide the shim coil which adjusts the nonuniformity of a static magnetic field. A compensation current is applied to the shim coil from the spatial and temporal information of the eddy current resulting from the application of the gradient magnetic field. The eddy current may be canceled by applying a compensation current to the gradient magnetic field generator 130.

  Next, functions realized by the control processing unit 170 of this embodiment will be described. As illustrated in FIG. 3, the control processing unit 170 of the present embodiment includes an imaging condition setting unit 210 and an imaging unit 230. The imaging condition setting unit 210 receives and sets an imaging condition setting including imaging parameters from the user. The imaging unit 230 operates each unit according to the imaging conditions set by the imaging condition setting unit 210 and executes imaging.

  The imaging condition setting unit 210 of this embodiment includes an irradiation magnetic field determination unit (RF determination unit) 220 that determines the intensity and phase of the RF pulse given to each channel as the irradiation intensity and irradiation phase.

  The RF determination unit 220 of this embodiment performs RF shimming that adjusts the initial value of the intensity (initial intensity) and the initial value of the phase (initial phase) for each channel so as to increase the uniformity of the high-frequency magnetic field distribution in the imaging region. Then, the adjusted initial intensity is corrected so that the combined intensity of the RF pulses irradiated from each channel is within a predetermined range, and the irradiation intensity is obtained. This correction is called normalization.

  In order to determine the intensity and phase of the RF pulse applied to each channel, the RF determination unit 220 of the present embodiment determines an irradiation gain adjustment unit 221, an initial setting unit 222, an RF shimming unit 223, as shown in FIG. The RF correction unit 224 and the machine difference correction unit 225 are provided.

  Each unit of the control processing unit 170 is realized by the CPU 171 loading a program stored in the storage device 172 in advance into the memory and executing the program.

  The irradiation gain adjusting unit 221 measures the irradiation gain and determines an initial value of the irradiation gain that realizes the excitation of the flip angle set in the imaging condition. Then, the determined initial value of the irradiation gain is set as the initial irradiation gain. The process of setting the initial irradiation gain is performed prior to each process by the RF shimming unit 223 and the RF correction unit 224.

  The initial setting unit 222 sets the initial intensity and initial phase of the RF pulse applied to each channel. In the case of two-channel irradiation, the initial intensity and initial phase are generally determined so as to realize QD irradiation.

  The RF shimming unit 223 performs a so-called RF shimming process that individually adjusts the intensity and phase of the RF pulse applied to each channel so as to improve the uniformity of the high-frequency magnetic field B1 generated in the imaging space. In the present embodiment, the adjusted phase and the adjusted intensity are calculated. In the case of two channels, RF shimming processing is performed with one channel as a reference. Therefore, regarding the phase, the difference (phase difference) from the phase of the reference channel of the non-reference channel is output. The RF shimming process uses a known method.

  The RF correction unit 224 performs the normalization. In the present embodiment, the net RF intensity after RF shimming (synthesis adjustment intensity) is changed to the net RF intensity (synthesis initial intensity) before RF shimming while maintaining the irradiation uniformity of RF pulses obtained by RF shimming. The adjustment intensity of each channel is corrected so as to be as close as possible, and the irradiation intensity is obtained. At this time, the phase value after RF shimming (adjusted phase) is maintained for the phase.

  In RF shimming, the intensity and phase of the RF pulse applied to each channel are individually controlled to improve the net RF uniformity generated in the imaging space. However, since the RF pulse intensity and phase applied to each channel are changed, the net RF intensity may change before and after RF shimming. The RF correction unit 224 of the present embodiment corrects the intensity of the RF pulse applied to each channel so as to reduce this net change in RF intensity.

  The irradiation gain determination process, the RF shimming process, and the normalization process are based on the assumption that an RF pulse is accurately output from the transmission coil 151 in accordance with the set irradiation gain (intensity) and phase. However, in general, there is a difference in hardware, so an error actually occurs. The machine difference correction unit 225 adjusts an error due to the hardware machine difference in advance to realize the above assumption. Specifically, for each channel, an offset value determined so as to eliminate the difference between the modulator 153 and the high-frequency amplifier 154 is added or subtracted in advance to cancel the error. At this time, the intensity and phase of each channel are obtained by actual measurement using an oscilloscope. The intensity may be obtained by performing sequential adjustment using the irradiation gain adjustment tool for each channel.

  Next, an outline of normalization processing by the RF correction unit 224 of the present embodiment will be described. Here, it is assumed that the number of channels is set to 2 and that QD irradiation is performed. Each channel is referred to as channel 1 (Ch1) and channel 2 (Ch2), respectively. In the RF shimming process, the intensity and phase of the RF pulse applied to channel 2 (Ch2) are defined with reference to channel 1 (Ch1). calculate. Therefore, a change due to RF shimming in a 2-channel environment occurs only in channel 2 (hereinafter referred to as Ch2).

Assuming that the RF pulse intensities given to channel 1 and channel 2 are R ₁ and R ₂ [Tesla], and the phases are θ ₁ and θ ₂ [degree], vectors Rch1 and Rch2 representing irradiation from each channel are Is represented by the following formula (1).

In the present embodiment, the initial values (initial intensity and initial phase) of the RF pulse intensity and phase applied to each channel are determined so as to perform QD irradiation. With channel 1 as a reference, during QD irradiation, R ₂ = R ₁ , θ ₁ = 0, and θ ₂ = 90 [degree] are set. Therefore, the vectors Rch1 and Rch2 in Expression (1) are set in this embodiment. Then, it represents with the following formula | equation (2).

When the intensity and phase of the given RF pulse are set to the initial intensity and the initial phase, as shown in FIG. 4A, the combined initial intensity, that is, the net irradiation intensity S _QD at the time of QD irradiation is Since it is the absolute value of the sum of the two vectors Rch1 and Rch2 of the above equation (2), it is expressed by the following equation (3).

In the present embodiment, the RF shimming unit 223 performs an RF shimming process on the initial intensity and initial phase of the RF pulse applied to each channel adjusted for QD irradiation. The intensity (adjustment intensity) of channel 2 (Ch2) after the RF shimming process is A · R ₁ , and the phase (adjustment phase) represented by the phase difference from channel 1 (Ch1) is θ [degree]. The irradiation gain adjusting unit 221 calculates an irradiation gain correction value A _dB [dB] (intensity A) for realizing the adjustment intensity A · R ₁ and sets the irradiation gain A _dB [dB]. A command is issued to the transmission unit 150 via the sequencer 140. Reflecting the obtained adjustment intensity (irradiation gain correction value) and adjustment, Expression (2) is expressed as Expression (4) below.

Note that the net irradiation intensity S _B1shim , that is, the combined adjustment intensity due to the irradiation represented by these two vectors Rch1 and Rch2, is the absolute sum of the two vectors Rch1 and Rch2, as shown in FIG. Since it is a value, it is represented by the following formula (5).

As shown in FIGS. 4A and 4B, the net irradiation intensity may differ before and after performing RF shimming. The RF correction unit 224 of the present embodiment uses the net irradiation intensity (synthesis adjustment intensity) S _B1shim after RF shimming as the net before RF shimming while maintaining the uniformity of RF irradiation obtained by RF shimming. The irradiation intensity (synthetic initial intensity) of _SQD is approached. That is, the RF correction unit 224 corrects the adjustment intensity so that the combined adjustment intensity is within a predetermined number of times the combined initial intensity, and obtains the irradiation intensity. Thereby, before and after RF shimming, the change in the net irradiation intensity that occurs in the imaging space becomes a desired range, and the change in the irradiation gain also falls within the desired range.

なお、係数Ｒａｎｇｅは、例えば、ＭＲＩ装置１００が許容する照射ゲインの上限を示す値など、予め設定され、記憶装置１７２に保持される。 Specifically, first, a coefficient Range that specifies a desired range is defined. Using this coefficient Range, the relationship between the net irradiation intensity (synthesis adjustment intensity) S _B1shim after RF shimming and the net irradiation intensity (synthesis initial intensity) S _QD before RF shimming is expressed by the following equation (6). It is represented by

The coefficient Range is set in advance, such as a value indicating the upper limit of the irradiation gain allowed by the MRI apparatus 100, and is stored in the storage device 172.

In order to adjust the irradiation from both channels so as to satisfy the above equation (6), the present embodiment introduces a coefficient Coeff that increases or decreases the intensity of the two vectors at the same rate. To increase or decrease at the same rate is to change the magnitude while maintaining the ratio of the magnitudes of the two vectors. The net irradiation intensity S _B1shim after RF shimming when this coefficient Coeff is introduced is expressed by the following equation (7).

From the above equations (3), (6), and (7), the coefficient Coeff of each channel is calculated by the following equation (8).

The intensity of channel 1 (Ch1) after normalization is Coeff · R ₁ , and the intensity of channel 2 (Ch2) after normalization is Coeff · A · R ₁ . Therefore, after the normalization, the correction value A _{N_dB1} of the irradiation gain of the channel 1 is 20 log ₁₀ (Coeff) [dB], and the correction value A _{N_dB2} of the irradiation gain of the channel 2 is (20 log ₁₀ (Coeff) + A _dB ) [dB]. The phases are 0 and θ [degree], respectively, after RF shimming.

FIG. 4C shows the net irradiation intensity S _norm of the RF pulse applied to each channel after correction irradiated with the intensity and phase. Here, a case where Range is 1 and Coeff is set to the maximum value in Expression (8) is illustrated. As shown in the figure, the size of S _norm is equal to the size of S _QD , and the direction of S _{norm is} the direction of S _B1shim . In addition, since the value of the phase θ is maintained at the time of RF shimming, the angle of the vector Rch2 with respect to the vector Rch1 is the same as that in FIG.

  The imaging condition setting unit 210 sets the obtained irradiation intensity and irradiation phase. The imaging unit 230 applies an RF pulse to the feeding point of the transmission coil 151 with the set irradiation intensity and the set irradiation phase for each channel, and executes imaging.

In the above example, the coefficient Coeff is set so that the net irradiation intensity (synthesis adjustment intensity) S _B1shim after RF shimming falls within a predetermined range of the net irradiation intensity (synthesis initial intensity) S _QD before RF shimming. Although it is calculating, it is not restricted to this. For example, the coefficient Coeff may be calculated so that the net irradiation intensity (synthesis adjustment intensity) S _B1shim after RF shimming matches the net irradiation intensity (synthesis initial intensity) S _QD before RF shimming.

そして、このときの各チャンネルの照射強度に乗算する係数Ｃｏｅｆｆは、以下の式（１０）となる。

In this case, the above formula (6) is expressed by the following formula (9).

And the coefficient Coeff which multiplies the irradiation intensity | strength of each channel at this time becomes the following formula | equation (10).

  In this case, while maintaining the uniformity obtained by RF shimming, there is no change in the net irradiation intensity that occurs in the imaging space before and after RF shimming, and no change in irradiation gain.

Further, correction values A _{N_dB1} and A _{N_dB2} of the irradiation gain of each channel obtained at this time are shown in FIGS. 5 (a) and 5 (b). 5 (a) is the correction value A _{N_dB1} irradiation gain channel 1 (Ch1), FIG. 5 (b), the correction value A _{N_dB2} irradiation gain channel 2 (Ch2), the horizontal axis illumination phase θ In each case, the values are shown for each correction value A _dB of the irradiation gain immediately after RF shimming.

  Hereinafter, the flow of processing during imaging according to the present embodiment will be described. FIG. 6A is a processing flow of the imaging process of the present embodiment.

  When the imaging condition setting unit 210 receives input of imaging parameters from the user, the initial setting unit 222 sets the initial intensity and initial phase of the RF pulse to be given to each channel (step S1001). In this embodiment, since 2-channel QD irradiation is performed, the initial phase is set to 0 for channel 1 (Ch1) and 90 degrees for channel 2 (Ch2).

  The irradiation gain adjusting unit 221 determines an irradiation gain that realizes the intensity (initial intensity) of the RF pulse given to each channel determined by the imaging conditions as an initial irradiation gain (step S1002).

The RF shimming unit 223 performs an RF shimming process, and calculates an irradiation gain correction value A _dB and a phase correction value θ of the channel 2 (Ch2) (step S1003).

The RF correction unit 224 performs a normalization process, and calculates irradiation gain correction values (A _{N_dB1} , A _{N_dB2} ) for channel 1 (Ch1) and channel 2 (Ch2) (step S1004). In the present embodiment, as described above, the coefficient Coeff is calculated so that the net irradiation intensity S _B1shim after RF shimming is as close as possible to the net irradiation intensity S _QD before RF shimming, and is calculated using the coefficient Coeff. To do.

  The imaging condition setting unit 210 determines and sets imaging conditions (step S1005). Here, the irradiation gain of each channel is adjusted by the calculated correction value of the irradiation gain, and the phase is adjusted. Then, the imaging unit 230 performs imaging according to the set imaging conditions (step S1006).

  Here, the flow of the conventional imaging process without the normalization process of the present embodiment will be described for comparison. A flow of conventional imaging processing is shown in FIG.

  In accordance with the imaging conditions, the initial intensity and initial phase of the RF pulse applied to each channel are set (step S1101). For example, when performing 2-channel QD irradiation, the initial phase is set to 0 for channel 1 (Ch1) and 90 degrees for channel 2 (Ch2). Next, an irradiation gain that realizes the intensity (initial intensity) of the RF pulse applied to each channel determined by the imaging conditions is determined as an initial irradiation gain (step S1102). Then, RF shimming processing is performed (step S1103).

  Thereafter, the irradiation gain for realizing the intensity and phase of the RF pulse given to each channel after RF shimming is recalculated (step S1104). Then, the recalculated irradiation gain is compared with the hardware allowable value related to the irradiation gain (step S1105). If the calculated irradiation gain exceeds the hardware allowable value as a result of the comparison, an irradiation gain peaking process is performed to reset the irradiation gain to the hardware allowable value (step S1106), and the imaging condition is set (step S1107). . On the other hand, if not, the intensity and phase value after the RF shimming process are determined as they are as the RF imaging conditions. Then, imaging is executed according to the set imaging conditions (step S1108).

  Comparing the imaging processing of the present embodiment and the conventional imaging processing, in this embodiment, it is not necessary to recalculate and set the irradiation gain again, and the irradiation gain after the resetting is an allowable value of hardware. It is not necessary to determine whether or not it is within. Furthermore, there is no peak processing when the hardware allowable value is exceeded.

  As described above, the MRI apparatus 100 of the present embodiment has a plurality of channels, the high-frequency magnetic field irradiation unit (transmission unit) 150 that irradiates the imaging region with the high-frequency magnetic field pulse from each channel, and the high-frequency magnetic field to be irradiated. The irradiation magnetic field determination unit (RF determination unit) 220 that determines the intensity and phase of each channel as the irradiation intensity and irradiation phase, and the high-frequency magnetic field pulse is irradiated from each channel with the determined irradiation intensity and irradiation phase. And an imaging unit 230 that performs imaging, and the irradiation magnetic field determination unit (RF determination unit) 220 sets the initial intensity and the initial phase to the channel so as to increase the uniformity of the high-frequency magnetic field distribution in the imaging region. RF shimming is performed for each adjustment, and the adjusted initial intensity is determined by combining the high frequency magnetic field pulses irradiated from both channels. Was corrected to strength is within a predetermined range, obtaining the illumination intensity.

  As described above, in this embodiment, the adjustment strength of each channel is adjusted at the same ratio while maintaining the adjustment phase obtained by RF shimming, and the combined strength is within a predetermined range of the combined strength before RF shimming. Reset to fit. Thereby, according to this embodiment, even if RF shimming is performed, the net irradiation intensity (synthetic intensity) falls within a predetermined range of values before RF shimming. Therefore, it is possible to maintain the uniformity of irradiation realized by RF shimming and suppress the net change in irradiation intensity.

  As described above, according to the present embodiment, the increase or decrease of the net irradiation intensity in the imaging space caused by the change of the intensity and phase of each irradiation channel is avoided by RF shimming, and the irradiation gain is RF. It is possible to avoid a significant change before and after shimming. In other words, in order to obtain an improvement in RF uniformity using RF shimming, the adjustment intensity and adjustment phase of each channel obtained as a result of RF shimming can be obtained even when there is a change in the RF irradiation intensity generated in the imaging space. The change can be suppressed by standardizing the used irradiation. As a result, even when hardware with a narrow adjustment range of the irradiation gain is used, RF shimming can be performed without adverse effects.

  In the present embodiment, the above processing is realized by correction processing by the control processing unit 170 when setting the imaging conditions. In general, since RF shimming changes the output for each channel, the net output varies greatly. For this reason, a wide adjustment range is required for the irradiation gain. Such hardware with a wide adjustment range of irradiation gain is expensive, and cannot be ignored in a low magnetic field machine such as 1.5 Tesla, where price competition is intense. However, according to the present embodiment, RF shimming can always be appropriately realized even with hardware having a narrow adjustment range of the irradiation gain. Therefore, appropriate RF shimming can be realized without increasing the performance of the hardware, which is particularly effective for a low magnetic field machine.

  100: MRI apparatus, 101: subject, 120: static magnetic field generation unit, 130: gradient magnetic field generation unit, 131: gradient magnetic field coil, 132: gradient magnetic field power supply, 140: sequencer, 150: transmission unit, 151: transmission coil, 152: synthesizer, 152a: synthesizer, 152b: synthesizer, 153: modulator, 153a: modulator, 153b: modulator, 154: high frequency amplifier, 154a: high frequency amplifier, 154b: high frequency amplifier, 160: reception unit, 161: reception Coil, 162: Signal amplifier, 163: Quadrature detector, 164: A / D converter, 170: Control processing unit, 171: CPU, 172: Storage device, 173: Display device, 174: Input device, 210: Imaging Condition setting unit, 220: RF determination unit, 221: Irradiation gain adjustment unit, 222: Initial setting unit, 223: R F shimming unit, 224: RF correction unit, 225: Machine difference correction unit, 230: Imaging unit

Claims (10)

A high-frequency magnetic field irradiation part for irradiating a radio frequency magnetic field pulse to the imaging region into a plurality channels,
An irradiation field decision unit that determines the irradiation morphism intensity and irradiation phase before SL RF magnetic field pulse for each of said channels,
Irradiating the high frequency magnetic field pulse from each of said channels according to the previous SL irradiation intensity and irradiation phase, and an imaging unit that performs imaging,
The irradiation magnetic field determination unit
To advance initial intensity defined and Initial phase, RF shimming to obtain a high-frequency magnetic field distribution adjusting intensity performing RF shimming for adjusting for each of the channels so as to enhance the uniformity and illumination phase of the imaging area And
The magnitude of the combined adjustment intensity obtained by synthesizing the high-frequency magnetic field pulses irradiated from the channels with the adjustment intensity and the irradiation phase is determined from the high-frequency magnetic field pulses irradiated from the channels with the initial intensity and the initial phase. An RF correction unit that corrects the adjustment intensity by increasing / decreasing the irradiation intensity at an equal rate for each of the channels so as to approach the magnitude of the combined initial intensity to obtain the irradiation intensity used for imaging. A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1,
The RF correction unit synthesizes each vector by increasing / decreasing the intensity of each vector at an equal rate with respect to each vector representing the high-frequency magnetic field pulse irradiated with the adjustment intensity and the irradiation phase from each channel . The combined vector is corrected so as to approach the magnitude of the initial combined vector obtained by combining the vectors representing the high-frequency magnetic field pulses irradiated from the channels with the initial intensity and the initial phase, and the irradiation intensity used for imaging is obtained. A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1 or 2,
The number of channels is 2,
The magnetic resonance imaging apparatus, wherein the initial intensity and the initial phase are values that realize QD irradiation . The magnetic resonance imaging apparatus according to claim 3 ,
The RF resonance unit corrects the adjustment intensity according to the following formula (1) to obtain an irradiation intensity used for imaging.

The magnetic resonance imaging apparatus according to claim 4 ,
The magnetic resonance imaging apparatus , wherein the RF correction unit calculates a coefficient Coeff in the equation (1) by the following equation (2) .

Here, Range indicates the upper limit of the irradiation gain allowed by the magnetic resonance imaging apparatus, and is a predetermined value. The magnetic resonance imaging apparatus according to any one of claims 1 to 5,
The irradiation field decision unit of the previous SL magnetic resonance imaging apparatus characterized by further comprising an irradiation gain adjustment unit that determines a radiation gain to achieve an initial strength as an initial radiation gain. The magnetic resonance imaging apparatus according to claim 6,
The magnetic resonance imaging apparatus, wherein the irradiation gain adjustment unit calculates an irradiation gain for obtaining the irradiation intensity as a gain correction value for each channel. The magnetic resonance imaging apparatus according to any one of claims 1 to 7,
A magnetic resonance imaging apparatus, further comprising a machine difference correction unit that corrects an output error between hardware channels of each of the channels of the high-frequency magnetic field irradiation unit. An initial setting step for setting the intensity and phase of the high-frequency magnetic field pulse irradiated to the imaging region from each of the plurality of channels to an initial intensity and an initial phase that are the intensity and phase with the highest irradiation efficiency;
RF shimming for adjusting the initial intensity and the initial phase for each channel so as to improve the uniformity of the high-frequency magnetic field distribution in the imaging region, and obtaining the adjustment intensity and the irradiation phase;
The synthetic adjusted intensity obtained by combining the high-frequency magnetic field pulses emitted by the adjusting intensity and the irradiation phase from each channel, and combining the said radio frequency magnetic field pulses emitted by the initial strength and the initial phase from the channel synthesis An RF correction step for correcting the adjustment intensity by increasing or decreasing the irradiation intensity at an equal rate for each of the channels so as to approach the initial intensity to obtain the irradiation intensity used for imaging ;
An imaging step of performing imaging by irradiating the high-frequency magnetic field pulse from each of the channels with the irradiation intensity and the irradiation phase. The magnetic resonance imaging method according to claim 9, comprising:
  In the RF correction step, for each vector representing the high-frequency magnetic field pulse irradiated from each channel with the adjustment intensity and the irradiation phase, the vectors are synthesized by increasing / decreasing the intensity of each vector at an equal ratio. The combined vector is corrected so as to approach the magnitude of the initial combined vector obtained by combining the vectors representing the high-frequency magnetic field pulses irradiated from the channels with the initial intensity and the initial phase, and the irradiation intensity used for imaging is obtained.
  A magnetic resonance imaging method.

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