JP2009034152A - Mri apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately set the center frequency of RF pulses to be used for MRI scans. <P>SOLUTION: A transmission/receiving part 3 irradiates a region to be scanned with 90-degree pulses and 180-degree pulses and collects MR signals after irradiating the region to be scanned with saturation pulses for inhibiting MR signals from the fat tissue and IR pulses (inversion recovery pulses) for inhibiting MR signals from a medical silicone as prepulses according to sequence control signals outputted from a sequence control part 92 in setting the center frequency of the RF pulses used for the MRI scans of a subject with the medical silicone inserted into the region to be scanned. On the other hand, a resonance frequency detection parts 5 detects the resonance frequency of the hydrogen atom nuclear spin in the water tissue by measuring the frequency (the peak frequency) with the maximum amplitude in the frequency spectrum of the MR signals, and a center frequency setting part 81 of an input part 8 sets the center frequency of the RF pulses based on the detected resonance frequency. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an MRI apparatus, and more particularly to an MRI apparatus that enables generation of high-quality image data even for a subject in which medical silicone or the like is inserted into the body.

  Magnetic Resonance Imaging (MRI) occurs in the process of restoring the hydrogen nuclear spins in the subject tissue in a static magnetic field excited by an excitation pulse (RF pulse) having a Larmor frequency corresponding to the static magnetic field intensity. This is an imaging method for generating image data by reconstructing MR signals to be processed.

  An MRI apparatus that enables the above-described imaging method can obtain a lot of diagnostic information such as biochemical information and functional diagnostic information as well as anatomical diagnostic information. Therefore, it is indispensable in the field of diagnostic imaging today. It has become a thing.

  In an examination using an MRI apparatus (MRI imaging), an MR signal generated from a water tissue constituting a living tissue (that is, an MR signal resulting from resonance of a hydrogen nuclear spin in the water tissue) and an MR signal generated from a fat tissue are usually used. For example, when image data is generated using MR signals from water tissue, it may be difficult to generate high-quality image data due to mixing of MR signals generated from adipose tissue.

  In order to solve this problem, the Inversion Recovery (IR) method or the water structure using the difference between the longitudinal relaxation time of the hydrogen nucleus spin of the water tissue and the longitudinal relaxation time of the hydrogen nucleus spin of the adipose tissue Is a method of suppressing MR signals generated from adipose tissue by applying a chemical saturation method (CHESS: Chemical Shift Selective) utilizing the fact that the resonance frequency of the hydrogen nucleus spin of the fat is different from the resonance frequency of the hydrogen nucleus spin of the adipose tissue. Has been developed.

On the other hand, in MRI imaging of a subject in which medical material such as medical silicone is injected into the body by breast augmentation etc., powerful MR signals generated from medical silicone are further collected and included in medical silicone Since the longitudinal relaxation time and resonance frequency of the hydrogen nuclear spins are different from the longitudinal relaxation time and resonance frequency of the hydrogen nuclear spins contained in the adipose tissue, the MR signal generated from the medical silicone by the inversion recovery method and the chemical saturation method described above is used. It is impossible to simultaneously suppress MR signals generated from adipose tissue. Therefore, these hydrogen nuclei are irradiated in time series or simultaneously with an RF pulse centered on the resonance frequency of hydrogen nucleus spin in medical silicone and an RF pulse centered on the resonance frequency of hydrogen nucleus spin in adipose tissue. A method of suppressing MR signals generated from medical silicone and adipose tissue by saturating spin has been proposed (for example, see Patent Document 1).
JP 2000-5142 A

  By the way, the center frequency of the RF pulse used for MRI imaging is set based on the Larmor frequency of a desired living tissue (for example, water tissue) determined based on the static magnetic field strength, and this center frequency is set based on the static magnetic field strength. This is performed in the shooting preparation stage together with correction (shimming). For example, an imaging target region of the subject is arranged in an imaging field of a gantry in which a uniform static magnetic field is formed by shimming performed before MRI imaging, and a standard center frequency temporarily set for the imaging target region The MR signal is collected by irradiating an RF pulse having. Then, by analyzing the frequency of the obtained MR signal, the resonance frequency of the hydrogen nuclear spin in the water tissue is detected, and based on this resonance frequency, the center frequency of the RF pulse suitable for MRI imaging of the water tissue in the region to be imaged Is set.

  However, in MRI imaging of a subject into which medical silicone is inserted, as already described, MR signals generated from water tissue and adipose tissue constituting a living tissue and MR signals generated from medical silicone are mixed. In particular, the amplitude and power of the MR signal generated from the medical silicone is larger than that of the MR signal generated from the water tissue. For this reason, it becomes difficult to accurately detect the resonance frequency of the hydrogen nuclear spin in the water tissue due to the frequency spectrum component corresponding to the MR signal generated from medical silicone or adipose tissue, and the center frequency of the RF pulse is accurately determined. Had the problem that it could not be set.

  The present invention has been made in view of the above-described problems, and its purpose is to accurately set the center frequency of an RF pulse for a subject into which a medical material such as medical silicone is inserted. An object of the present invention is to provide an MRI apparatus capable of this.

  In order to solve the above-mentioned problem, the MRI apparatus of the present invention according to claim 1 detects a resonance frequency of a hydrogen nuclear spin in a desired tissue of a subject in which a medical material is inserted into a region to be imaged, and this resonance frequency. In the MRI apparatus for performing MRI imaging on the imaging target region of the subject using an RF pulse having a center frequency set based on the first suppression pulse for suppressing MR signals generated from a tissue different from the desired tissue Transmitting and receiving means for collecting MR signals from the region to be imaged based on a resonance frequency measurement pulse sequence using a second suppression pulse for suppressing MR signals generated from the medical material as a pre-pulse, and the obtained MR In the desired tissue by measuring the peak frequency that is the largest in the frequency spectrum of the signal Resonance frequency detection means for detecting the resonance frequency of elementary nuclear spins, and center frequency setting means for setting the center frequency of an RF pulse used for MRI imaging of the imaging region based on the resonance frequency It is said.

  According to the present invention, it is possible to accurately set the center frequency of an RF pulse even for a subject into which a medical material such as medical silicone is inserted. For this reason, high-quality image data excellent in S / N can be efficiently generated, and diagnostic accuracy and inspection efficiency are improved.

  Embodiments of the present invention will be described below with reference to the drawings.

  In an embodiment of the present invention to be described below, when medical silicone sets the center frequency of an RF pulse used for MRI imaging of a subject inserted in a region to be imaged, saturation is suppressed to suppress MR signals generated from adipose tissue. MR signals are collected by irradiating the imaging target region with an IR pulse (inversion recovery pulse) that suppresses the MR signal generated from the pulse and medical silicone as a pre-pulse, and then irradiating with a 90-degree pulse and a 180-degree pulse. Then, the resonance frequency of the hydrogen nuclear spin in the water tissue is detected by measuring the frequency (peak frequency) at which the amplitude is maximum in the frequency spectrum of the MR signal, and the center frequency of the RF pulse is determined based on this resonance frequency. Set.

  In the following, a case where MR signals generated from the imaging target region of the subject are collected by the so-called SE (Spin Echo) method using 90 degree pulses and 180 degree pulses will be described, but the present invention is not limited to this. Alternatively, other methods such as an FE (Field Echo) method and an EPI (Echo Planar Imaging) method may be used.

(Device configuration)
The configuration of the MRI apparatus in the embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a block diagram showing the overall configuration of the MRI apparatus in the present embodiment, and FIG. 2 is a functional block diagram of a resonance frequency detector provided in the MRI apparatus.

  The MRI apparatus 100 shown in FIG. 1 includes a static magnetic field generation unit 1 that generates a static magnetic field for a subject 150, a gradient magnetic field generation unit 2 that generates a gradient magnetic field, and irradiation of an RF pulse to the subject 150. Resonance frequency detection for detecting the resonance frequency of the hydrogen nuclear spin in the water tissue based on the MR signal supplied from the transmitter / receiver 3 that receives the MR signal, the top plate 4 on which the subject 150 is placed, and the MR signal supplied from the transmitter / receiver 3 Part 5 is provided.

  Further, the MRI apparatus 100 includes an image data generation unit 6 that reconstructs the MR signal received by the transmission / reception unit 3 to generate image data, and the frequency spectrum and image of the MR signal measured by the resonance frequency detection unit 5. A display unit 7 for displaying image data generated by the data generation unit 6, an MR signal acquisition condition, an image data generation condition and an image data display condition setting, an RF pulse center frequency setting, a shimming condition setting, An input unit 8 for inputting various command signals and the like, and a control unit 9 for comprehensively controlling each unit in the MRI apparatus 100 are provided.

  The static magnetic field generator 1 includes a main magnet 11 constituted by a normal conducting magnet or a superconducting magnet, a shim coil 12 constituted by a plurality of coils arranged along the inner surface of the main magnet 11, a main magnet 11 and A static magnetic field power source 13 and a shim coil power source 14 for driving the shim coil 12 are provided. The static magnetic field power supply 13 forms a strong static magnetic field with respect to the subject 150 arranged in the imaging field of the gantry (not shown) by supplying a predetermined current to the main magnet 11, and the shim coil power supply 14 By supplying a correction current to the shim coil 12, the nonuniformity of the static magnetic field formed by the main magnet 11 and the static magnetic field power supply 13 is corrected. In addition, the above-mentioned main magnet 11 may be comprised with the permanent magnet.

  On the other hand, the gradient magnetic field generator 2 supplies a pulse current to each of the gradient magnetic field coil 21 that forms a gradient magnetic field in the X axis direction, the Y axis direction, and the Z axis direction orthogonal to each other, and the gradient magnetic field coil 21. A gradient magnetic field power source 22 is provided.

  The gradient magnetic field power source 22 encodes the imaging field in which the subject 150 is placed based on the sequence control signal supplied from the control unit 9. That is, the gradient magnetic field power source 22 controls the pulse current supplied to the gradient magnetic field coils 21 in the X-axis direction, the Y-axis direction, and the Z-axis direction based on the sequence control signal, thereby forming a gradient magnetic field in each direction. To do. The gradient magnetic fields in the X-axis direction, the Y-axis direction, and the Z-axis direction are combined to form a slice selection gradient magnetic field Gs, a phase encode gradient magnetic field Ge, and a readout (frequency encode) gradient magnetic field Gr that are orthogonal to each other. These gradient magnetic fields are superimposed on the static magnetic field formed by the main magnet 11 and the shim coil 12 and applied to the subject 150.

  Next, the transmission / reception unit 3 includes a transmission coil 31 and a transmission unit 32 that irradiate the subject 150 with RF pulses, and a reception coil 33 and a reception unit 34 that receive MR signals generated by the subject 150. ing. However, a transmission / reception coil having the function of the transmission coil 31 and the function of the reception coil 33 in one coil may be used.

  The transmission unit 32 has a carrier wave having the same frequency as the magnetic resonance frequency (Larmor frequency) determined by the static magnetic field strength of the main magnet 11 based on a sequence control signal supplied from a sequence control unit 92 described later of the control unit 9. Then, a pulse current modulated with a predetermined selective excitation waveform is generated.

  That is, the transmission unit 32 in the imaging preparation stage is based on a standard frequency spectrum (see FIG. 4A to be described later) set in advance for a biological tissue in which medical silicone is inserted. A pulse current having a predetermined frequency band Δf0w with a hydrogen nuclear spin resonance frequency f0w as a center frequency is generated. In addition, the transmission unit 32 in the MRI imaging stage uses the hydrogen nuclear spin resonance frequency fxw (see FIG. 3B described later) of the water tissue detected by the resonance frequency detection unit 5 in the imaging preparation stage as the center frequency, and the frequency band Δfxw ( For example, a pulse current having Δfxw = Δf0w) is generated.

  The transmission coil 31 is driven by the above-described pulse current supplied from the transmission unit 32 in the imaging preparation stage and the MRI imaging stage, and irradiates the imaging target region of the subject 150 with an RF pulse. On the other hand, the receiving coil 33 is a so-called array coil in which a plurality of (N) small-diameter coils are arranged in order to detect with high sensitivity MR signals generated from the imaging target portion of the subject 150 by the irradiation of the RF pulse. It is constituted by.

  The receiving unit 34 includes an N channel amplification circuit, an intermediate frequency conversion circuit, a detection circuit, a filtering circuit, and an A / D converter (not shown), and amplifies and intermediate frequency conversion of a minute MR signal detected by the reception coil 33. A / D conversion is performed after signal processing such as phase detection and filtering. However, the amplifying circuit is normally installed in the vicinity of the receiving coil 33 in order to amplify the MR signal detected by the receiving coil 33 with high S / N.

  The main magnet 11, the shim coil 12, the gradient magnetic field coil 21, the transmission coil 31, and the reception coil 33 are provided in a gantry (not shown) of the MRI apparatus 100, and an imaging field is formed at the center of the gantry. That is, an imaging field in which the subject 150 is inserted together with the top 4 is provided at the center of the gantry, and around the imaging field, the reception coil 33, the transmission coil 31, the gradient magnetic field coil 21, the shim coil 12, and the main magnet 11 are provided. Are arranged concentrically around the Z axis.

  Next, the top plate 4 is slidably mounted in the Z-axis direction on the upper surface of a bed (not shown) installed in the vicinity of the gantry, and the subject 150 placed on the top plate 4 is moved in the body axis direction (Z-axis direction). ) To set the imaging target region of the subject 150 at a desired position in the imaging field. In this case, the movement of the top plate 4 is controlled by a top plate moving mechanism unit and a top plate movement control unit (not shown) so that the imaging target part faces the receiving coil 33 provided in the vicinity of the imaging field.

  Next, for the purpose of setting the RF pulse center frequency in MRI imaging, the function block diagram shown in FIG. 2 is used for the resonance frequency detector 5 that detects the resonance frequency of the hydrogen nuclear spin in the water tissue of the subject 150. explain.

  The resonance frequency detection unit 5 performs frequency analysis on MR signals collected based on a resonance frequency measurement pulse sequence described later to generate a frequency spectrum, and smoothes the obtained frequency spectrum by filtering processing. And the frequency at which the amplitude value or power value becomes maximum in the filtered frequency spectrum (hereinafter, this frequency is referred to as a peak frequency) as the resonance frequency of the hydrogen nuclear spin in the water tissue. A standard frequency storage unit 54 that includes a peak frequency detection unit 53 for detecting, and further stores a standard frequency spectrum in advance when a pulse sequence for resonance frequency measurement is applied to a living tissue in which medical silicone is inserted; From the frequency detector 53 And a spectrum combining unit 55 for combining the standard frequency spectrum read from the frequency spectrum and the standard spectrum storage section 54 of the subject 150 to be fed.

  Next, a method for detecting the resonance frequency of the hydrogen nuclear spin in the water tissue will be described with reference to FIG. 3A shows the frequency spectrum of the MR signal generated by the frequency analysis unit 51 of the resonance frequency detection unit 5, FIG. 3B shows the frequency spectrum smoothed by the filtering processing unit 52, and FIG. 3 (c) shows the first derivative calculated by the differential operation on the smoothed frequency spectrum.

  The frequency spectrum of the MR signal collected by the later-described resonance frequency measurement pulse sequence in the imaging preparation stage and supplied to the resonance frequency detection unit 5 via the reception unit 34 of the transmission / reception unit 3 includes FIGS. As shown in (b), the frequency spectrum component Sw of the MR signal caused by the hydrogen nucleus spin of the water tissue having the resonance frequency fxw, and the frequency spectrum component of the MR signal caused by the hydrogen nucleus spin of the fat tissue having the resonance frequency fxf. The frequency spectrum component Ss of the MR signal due to the hydrogen nucleus spin of the medical silicone having Sf and the resonance frequency fxs is included.

  Then, by applying the resonance frequency measurement pulse sequence to suppress the MR signal generated from the fat tissue and medical silicone, the maximum value Aw in the frequency spectrum component Sw of the MR signal generated from the water tissue is obtained from the fat tissue. The maximum value Af in the frequency spectrum component Sf of the generated MR signal and the maximum value As in the frequency spectrum component Ss of the MR signal generated from medical silicone are shown. Therefore, the peak frequency detection unit 53 of the resonance frequency detection unit 5 measures the peak frequency at which the amplitude value or power value of the frequency spectrum smoothed by the filtering processing unit 52 is maximized, thereby measuring the hydrogen nuclear spin in the water tissue. It is possible to detect the resonance frequency fxw.

  Further, when the resonance frequency fxw of the hydrogen nucleus spin in the water tissue is detected with higher accuracy, the filtering processing unit 52 performs a differentiation process on the frequency spectrum after the smoothing process shown in FIG. ) And the peak frequency detector 53 determines the resonance frequency fxw by measuring the frequency (zero cross frequency) at which the first derivative crosses the frequency axis from the upper left to the lower right. To detect. For example, the resonance frequency fxw can be accurately detected by extracting the zero cross frequency closest to the above-described peak frequency from among the plurality of zero cross frequencies measured by the first derivative of FIG.

  Next, a method for confirming the resonance frequency fxw by combining the above-described frequency spectrum with the standard frequency spectrum stored in advance by the spectrum combining unit 55 will be described with reference to FIG.

  FIG. 4 (a) shows a standard frequency spectrum when a resonance frequency measurement pulse sequence is applied to a living tissue in which medical silicone is inserted. Each of water tissue, fatty tissue, and medical silicone is shown in FIG. At the resonance frequencies f0w, f0f, and f0s of the hydrogen nuclear spin at. When the resonance frequency f0w of the water tissue is used as a reference (0 ppm), the resonance frequencies f0f and f0s of the adipose tissue and the medical silicone are usually shifted by −3.5 ppm and −5.0 ppm. It is known that the amount ratio does not depend much on the change of the resonance frequency f0w in the water tissue.

  On the other hand, FIG. 4B shows the above-described standard frequency spectrum (broken line) synthesized by the spectrum synthesizing unit 55 and the frequency spectrum (solid line) after the smoothing process, which are detected by the peak frequency detecting unit 53. A synthesized frequency spectrum generated by matching the peak frequency with the resonance frequency f0w of the water tissue in the standard frequency spectrum is displayed on the monitor of the display unit 7 described later.

  Then, the operator who observes the synthetic frequency spectrum on the display unit 7 has the fat tissue resonance frequency fxf and the medical frequency in the frequency spectrum near the resonance frequency f0f of the fat tissue and the resonance frequency f0s of the medical silicone in the standard frequency spectrum. By confirming that the component of the resonance frequency fxs of silicone exists, it is determined that the peak frequency detected by the peak frequency detection unit 53 is the resonance frequency fxw of the water tissue, and based on this resonance frequency fxw The center frequency of the RF pulse in MRI imaging is set.

  Returning to FIG. 1, the image data generation unit 6 includes an MR signal storage unit 61, a high-speed calculation unit 62, and an image data storage unit 63. The MR signal storage unit 61 receives an intermediate frequency signal by the reception unit 34 of the transmission / reception unit 3. N-channel MR signals subjected to conversion, phase detection, and A / D conversion are sequentially stored together with imaging position information supplied from the main control unit 91 or sequence control unit 92 of the control unit 9.

  On the other hand, the high-speed calculation unit 62 reads the MR signal and imaging position information once stored in the MR signal storage unit 61, and performs image reconstruction processing by two-dimensional Fourier transform to generate image data. The obtained image data is stored in the image data storage unit 63. However, although the high-speed calculation unit 62 in the above-described embodiment has described the case where the two-dimensional MR signal supplied from the reception unit 34 is reconstructed to generate two-dimensional image data, the high-speed calculation unit 62 supplies from the reception unit 34. Volume data (three-dimensional data) is generated by performing reconstruction processing by three-dimensional Fourier transform on the three-dimensional MR signal to be generated, and three-dimensional image data and MPR image data are generated based on the volume data. Also good. In this case, the high-speed calculation unit 62 performs, for example, a function of rendering volume data to generate volume rendering image data or surface rendering image data, and MPR (Multi Planar Reconstruction) image data in a desired slice section of the volume data. It has a function to generate.

  The display unit 7 includes a display data generation circuit, a conversion circuit, and a monitor (not shown). The display data generation circuit receives image data supplied from the image data storage unit 63 of the image data generation unit 6 and the resonance frequency detection unit 5. Display data is generated by adding incidental information such as subject information supplied from the control unit 9 to the various frequency spectrums supplied. The conversion circuit converts the display data into a predetermined display format and displays it on the monitor formed of a CRT or a liquid crystal panel. In particular, in the imaging preparation stage, a frequency spectrum after filtering (see FIG. 3B) in which the resonance frequency fxw of the water tissue is clearly shown as a peak frequency, or a synthesized frequency spectrum generated by the spectrum synthesizer 55 (FIG. 4). (B) is displayed on the monitor of the display unit 7.

  Next, the input unit 8 includes various input devices such as switches, a keyboard, and a mouse, and a display panel on a console, and a center frequency setting unit 81 that sets a center frequency of an RF pulse in MRI imaging and shimming that sets shimming conditions. A condition setting unit 82 is provided. Furthermore, input of object information, setting of MR signal acquisition conditions, setting of image data generation conditions, setting of image data display conditions, input of various command signals, and the like are performed using the above-described input device and display panel.

  In particular, when setting the center frequency of the RF pulse in the imaging preparation stage, the operator of the MRI apparatus 100 who observed the frequency spectrum generated in the resonance frequency detection unit 5 and displayed on the monitor of the display unit 7 5 determines whether or not the resonance frequency of the water tissue detected is appropriate, and if it is determined to be appropriate, an instruction signal for setting this resonance frequency as the center frequency of the PF pulse is sent to the center frequency setting unit 81. input.

  The control unit 9 includes a main control unit 91 and a sequence control unit 92. The main control unit 91 includes a CPU and a storage circuit (not shown) and has a function of controlling the MRI apparatus 100 in an integrated manner. The storage circuit of the main control unit 91 stores the resonance frequency measurement pulse sequence data in a preset imaging preparation stage (for example, the center frequency and band of the pulse current supplied to the transmission coil 31, the magnitude, the supply time, Information on supply timing, etc.) and pulse sequence data for MR signal collection in the MRI imaging stage (for example, center frequency and band, magnitude, supply time, supply timing, etc. of pulse current supplied to the gradient coil 21 and the transmission coil 31) Information), and information such as subject information input / set by the input unit 8, MR signal acquisition conditions, image data display conditions, shimming conditions, and RF pulse center frequency are stored. On the other hand, the CPU of the main controller 91 controls each unit of the MRI apparatus 100 based on various information stored in the storage circuit, and performs MRI imaging on the subject. For example, the CPU selects desired resonance frequency measurement pulse sequence data and MR signal acquisition pulse sequence data stored in its own storage circuit based on various setting information input from the input unit 8. Are read out and supplied to the sequence control unit 92.

  On the other hand, the sequence control unit 92 of the control unit 9 includes a CPU (not shown) and generates a sequence control signal based on the resonance frequency measurement pulse sequence data or MR signal collection pulse sequence data supplied from the main control unit 91. . Then, by supplying this sequence control signal to the gradient magnetic field power source 22 of the gradient magnetic field generator 2 and the transmitter 32 of the transmitter / receiver 3, the pulse currents for the gradient coil 21 and the transmitter coil 31 are controlled.

  Next, a resonance frequency measurement pulse sequence used for setting the RF pulse center frequency in the imaging preparation stage will be described with reference to FIGS. FIG. 5 shows the subject using the SE method in which irradiation of a saturation pulse Psa for suppressing MR signals from adipose tissue and IR pulse Pir for suppressing MR signals from medical silicone is added as a prepulse. 3 shows a resonance frequency measurement pulse sequence for measuring the resonance frequency of hydrogen nuclear spins in a water tissue. However, in this embodiment, since the resonance frequency is measured based on MR signals collected from a wide range of imaging target parts, it is not necessary to apply a gradient magnetic field, but a desired slice can be obtained by applying a slice selective gradient magnetic field. A cross section may be selected, and the resonance frequency may be measured based on MR signals collected from the water tissue of the slice cross section.

  When setting the RF pulse center frequency, the transmission unit 32 of the transmission / reception unit 3 first has a predetermined center frequency f0f and band Δf0f at time t01 based on the sequence control signal supplied from the sequence control unit 92 of the control unit 9. The irradiated RF pulse (saturation pulse) Psa is irradiated to the imaging target region of the subject 150. As the center frequency f0f in this case, a standard magnetic resonance frequency of the fat tissue with respect to the magnetic field strength of the static magnetic field formed by the main magnet 11 of the static magnetic field generating unit 1 is usually set. Then, by irradiation with the saturation pulse Psa, only the hydrogen nuclear spins of the adipose tissue existing in a wide area within the subject are selectively excited, and the longitudinal magnetization disappears.

  Next, at the time t02, an RF pulse (IR pulse) Pir having a predetermined center frequency f0s and band Δf0s is irradiated to the imaging target region. In this case, a standard magnetic resonance frequency of medical silicone with respect to the magnetic field strength of the static magnetic field is set as the center frequency f0s of the IR pulse Pir, as in the case of the saturation pulse Psa. Then, the hydrogen nuclear spin in the medical sea cone inserted into the subject is selectively excited by the irradiation of the IR pulse Pir, and its longitudinal magnetization is reversed by 180 degrees.

  The behavior of the hydrogen nuclear spin in the water tissue and the medical silicone when the IR pulse Pir is applied will be described with reference to FIG. The thick arrow in FIG. 6A indicates the magnetization direction z and the magnitude Mo of the hydrogen nuclear spin in the water tissue and the medical silicone before the IR pulse Pir is applied. At time t02, the IR pulse Pir When the imaging target region of the subject 150 is irradiated, the longitudinal magnetization formed in the z direction has a magnitude of −Mo by being inverted by 180 degrees as shown in FIG. The reversed longitudinal magnetization tends to return to the original longitudinal magnetization (Mo) based on the longitudinal relaxation time T1w of the water tissue and the longitudinal relaxation time T1s of the medical silicone.

  Curves Rw and Rs in FIG. 6B show the return curves of longitudinal magnetization in the water tissue and medical silicone. For example, the longitudinal relaxation time T1w of the water tissue is greater than the longitudinal relaxation time T1s of the medical silicone. When it is small, the longitudinal magnetization (-M0) of the water tissue tends to return to the original longitudinal magnetization (M0) faster than the longitudinal magnetization (-M0) of medical silicone. For this reason, when the longitudinal magnetization of the medical silicone becomes zero at t = tβ, the water tissue has longitudinal magnetization M1 (0 <M1 <M0).

  Therefore, the SE method or FE (Field Echo) method in which the 90-degree RF pulse P90 is irradiated at the timing when the longitudinal magnetization of the adipose tissue and the medical silicone becomes zero by the irradiation of the saturation pulse Psa and the IR pulse Pir to the subject 150. By applying the above, it is possible to suppress MR signals generated from adipose tissue and medical silicone and collect MR signals generated from water tissue with high sensitivity.

  That is, if the longitudinal magnetization in the fat tissue disappears by irradiation with the saturation pulse Psa at time t01 in FIG. 5 and the longitudinal magnetization in the water tissue and medical silicone is reversed by irradiation with the IR pulse Pir at time t02, the transmission unit 32 , The transmission coil 31 is driven at the time t03 when the reversal time TI (TI = tβ) has elapsed from the time t02, the 90-degree RF pulse P90 is irradiated to the imaging target region of the subject 150, and the longitudinal magnetization is applied in the z-axis direction. The water nuclear spin that had M1 is tilted 90 degrees. Then, the hydrogen nuclear spin that precesses around the z-axis in a state of being tilted by 90 degrees is precessed by the 180-degree RF pulse P180 irradiated at time t04 that has elapsed by time TE / 2 from time t03. And the MR signal from the water tissue formed by the rephasing of the hydrogen nuclear spins at time t05 after TE / 2 from time t04 is received by the reception unit 34 of the transmission / reception unit 3. The above-mentioned TE is an echo time (time of echo), and is usually defined by the time from irradiation of a 90-degree pulse to reception of an MR signal.

(MRI imaging procedure)
Next, the procedure of MRI imaging in the present embodiment will be described along the flowchart shown in FIG. Prior to MRI imaging of the subject 150 in which medical silicone has been inserted into the body, the operator of the MRI apparatus 100 moves the subject 150 placed on the top 4 in the Z-axis direction to capture the imaging target. After the part is placed in the imaging field of the gantry, input of the subject information, setting of MR signal collection conditions, setting of image data generation conditions, setting of image data display conditions, setting of shimming conditions, and the like are performed in the input unit 8 ( Step S10 in FIG.

  When the above initial setting is completed, the operator inputs a shimming start command at the input unit 8, and the main control unit 91 of the control unit 9 that has received this command signal receives the static magnetic field based on the above-described shimming conditions. By controlling the current supplied from the shim coil power supply 14 of the generator 1 to each of the shim coils 12, the static magnetic field power supply 13 and the main magnet 11 correct the non-uniformity of the static magnetic field formed in the imaging target region of the subject 150. (Step S20 in FIG. 7).

  Next, if an RF pulse center frequency setting start command is input at the input unit 8, the main control unit 91 controls each unit of the MRI apparatus 100 including the sequence control unit 92 and the resonance frequency detection unit 5. Then, the resonance frequency of the hydrogen nucleus spin in the water tissue of the imaging target site is measured, and the center frequency of the RF pulse used for MRI imaging of the subject 150 is set based on this resonance frequency (step S30 in FIG. 7).

  When the preparation for imaging including the above-described initial setting, shimming, RF pulse center frequency setting, and the like is completed, the operator inputs an MRI imaging start command at the input unit 8. The main control unit 91 that has received this command signal comprehensively controls each unit included in the MRI apparatus 100 based on the MR signal acquisition condition, the image data generation condition, and the image data display condition that are initially set in step S10. Then, image data is generated and stored based on the MR signal acquired by a predetermined MR signal acquisition pulse sequence (step S40 in FIG. 7), and the obtained image data is added to the subject information or the like. It displays on the display part 7 with information (step S50 of FIG. 7).

  In MRI imaging, a series of operations from the correction of the static magnetic field intensity to the setting of the RF pulse center frequency may be referred to as shimming. In this embodiment, as shown in the flowchart of FIG. Only the work is called shimming and is distinguished from the work of setting the RF pulse center frequency.

(RF pulse center frequency setting procedure)
Next, the procedure for setting the RF pulse center frequency shown in step S30 of FIG. 7 will be described in more detail using the flowchart of FIG.

  When the initial setting and shimming are completed in steps S10 and S20 in FIG. 7, the operator of the MRI apparatus 100 inputs an RF pulse center frequency setting start command in the input unit 8 (step S31 in FIG. 8). The main control unit 91 of the control unit 9 that has received this command signal reads the resonance frequency measurement pulse sequence data that has been set in advance and stored in its own storage circuit, and supplies it to the sequence control unit 92.

  On the other hand, the sequence control unit 92 generates a sequence control signal based on the resonance frequency measurement pulse sequence data supplied from the main control unit 91, and supplies the sequence control signal to the transmission unit 32 of the transmission / reception unit 3. Irradiation of an RF pulse is performed on the imaging target region. That is, the transmission unit 32 generates a pulse current having a predetermined center frequency and band based on the above-described sequence control signal, and the transmission coil 31 supplied with the pulse current from the transmission unit 32 receives the pulse current at time t01. A saturation pulse having a center frequency f0f and a band Δf0f is irradiated to the imaging target region of the subject to saturate hydrogen nuclear spins in the fatty tissue of the imaging target region (step S32 in FIG. 8).

  Further, the transmission coil 31 irradiates an IR pulse having a center frequency f0s and a band Δf0s at a time t02 when a predetermined time has elapsed from the time t01, and outputs 180 hydrogen nucleus spins in the medical silicone inserted into the imaging target site. After the degree of reversal (step S33 in FIG. 8), 90 degrees having a predetermined center frequency and band at time t03 when the reversal time TI at which the longitudinal magnetization of the hydrogen nucleus spin in the medical silicone becomes zero has elapsed from time t02. A pulse is irradiated, and further, a 180-degree pulse is irradiated at time t04 when TE / 2 has elapsed from time t03.

  On the other hand, the receiving coil 33 receives the MR signal generated from the region to be imaged at time t05 when TE / 2 has elapsed from time t04, and the MR signal collected at this time is amplified, intermediate frequency converted, and phased at the receiving unit 34. After processing such as detection, filtering, and A / D conversion, the resonance frequency detection unit 5 is supplied (step S34 in FIG. 8). The frequency analysis unit 51 of the resonance frequency detection unit 5 that has received the MR signal from the reception unit 34 performs a Fourier transform on the MR signal to generate a frequency spectrum, and the filtering processing unit 52 uses the frequency generated by the frequency analysis unit 51. The spectrum is filtered and smoothed (step S35 in FIG. 8).

  Next, the peak frequency detection unit 53 detects the peak frequency having the maximum amplitude in the frequency spectrum smoothed by the filtering processing unit 52 as the resonance frequency of the hydrogen nuclear spin in the water tissue of the imaging target region (FIG. 8). Step S36), the smoothed frequency spectrum to which the peak frequency information is added is supplied to the display unit 7 and the spectrum synthesis unit 55.

  On the other hand, the spectrum synthesizing unit 55 reads the standard frequency spectrum of the MR signal obtained from the biological tissue in which the medical silicone stored in advance in the standard spectrum storage unit 54 is inserted, and the resonance frequency of the water tissue in this standard frequency spectrum Are synthesized so that the peak frequencies of the frequency spectrum supplied from the peak frequency detector 53 coincide with each other to generate a synthesized frequency spectrum, and the obtained synthesized frequency spectrum is supplied to the display unit 7 ( Step S37 in FIG.

  Next, the display unit 7 supplied with the smoothed frequency spectrum and the synthesized frequency spectrum to which the peak frequency information is added generates display data based on these frequency spectra and displays the display data on its own monitor. An operator who observes the frequency spectrum displayed on the display unit 7 determines whether or not the peak frequency shown together with the frequency spectrum is appropriate as the resonance frequency of the water tissue. If it is determined to be appropriate, an instruction signal for setting the peak frequency to the center frequency of the PF pulse is input in the center frequency setting unit 81 of the input unit 8 (step S38 in FIG. 8).

  According to the embodiment of the present invention described above, the resonance frequency measurement pulse sequence using the saturation pulse for suppressing the MR signal generated from the adipose tissue and the IR pulse for suppressing the MR signal generated from the medical silicone as a prepulse is used. By collecting MR signals from the region to be imaged of the subject and measuring the peak frequency in the frequency spectrum of this MR signal, the resonance frequency of the water tissue can be detected reliably and accurately. Therefore, by setting the center frequency of the RF pulse based on this resonance frequency, it is possible to irradiate an RF pulse suitable for MRI imaging of the region to be imaged.

  In particular, since IR pulses are applied to suppress MR signals from medical silicone, MR signals from medical silicone with large amplitude can be effectively suppressed, and the resonance frequency of water tissue can be accurately determined. Can be detected.

  In addition, by measuring the zero cross frequency in the first derivative, and detecting the resonance frequency of the water tissue based on the zero cross frequency and the above-described peak frequency, the detection accuracy for the resonance frequency can be improved. By comparing the frequency spectrum of the MR signal obtained from the subject in the frequency spectrum with a preset standard frequency spectrum, the detection accuracy of the water tissue resonance frequency in the frequency spectrum can be further improved.

  For the reasons described above, according to the present embodiment, the center frequency of the RF pulse can be accurately and stably set even for a subject in which a medical material such as medical silicone is inserted into the body. . For this reason, high-quality image data can be generated efficiently, and diagnostic accuracy and inspection efficiency can be improved.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments and may be modified. For example, in the above-described embodiment, the case where the MR signal generated from the medical silicone inserted into the body of the subject is suppressed by the IR pulse is described. However, the MR signal generated from another medical material is suppressed by the IR pulse. It may be suppressed.

  In addition, the case where the MR signal generated from the fat tissue is suppressed by the saturation pulse and the MR signal generated from the medical silicone by the IR pulse is suppressed has been described. However, the MR signal generated from the medical silicone is suppressed by the saturation pulse and the fat signal is suppressed. MR signals generated from tissue may be suppressed by IR pulses, and MR signals generated from medical silicone and adipose tissue may be suppressed by saturation pulses having different center frequencies. The two saturation pulses in this case may be irradiated in time series or simultaneously.

  On the other hand, in the above-described embodiment, the MR signal from the imaging target region is collected by the SE method using a saturation pulse or an IR pulse as a pre-pulse, and the resonance frequency of the water tissue is detected based on the MR signal. The MR signals for resonance frequency detection may be collected by applying other imaging methods such as the FE (Field Echo) method and the EPI (Echo Planar Imaging) method having the pre-pulse.

  Furthermore, although the case where the center frequency of the RF pulse used for MRI imaging is set based on the resonance frequency of the hydrogen nucleus spin in the water tissue has been described, the center frequency of the RF pulse is based on the resonance frequency of the hydrogen nucleus spin in the fat tissue. May be set. In this case, the MR signal generated from the medical silicone and the MR signal generated from the water tissue are suppressed by the above-described saturation pulse and IR pulse, or two saturation pulses.

  In the above-described embodiment, the gradient magnetic field is not applied because the resonance frequency is measured for the water tissue using MR signals collected from a wide range of imaging target parts. The slice frequency may be selected, and the resonance frequency may be measured based on the MR signal collected from the water tissue of the slice profile.

1 is a block diagram showing the overall configuration of an MRI apparatus in an embodiment of the present invention. The functional block diagram of the resonance frequency detection part with which the MRI apparatus of the Example is provided. The figure for demonstrating the resonance frequency detection method of the hydrogen nucleus spin in the Example. The figure for demonstrating the confirmation method of the hydrogen nuclear spin resonance frequency in the water structure | tissue of the Example. The figure which shows the pulse sequence for resonance frequency measurement in the Example. The figure which shows the behavior of the hydrogen nucleus spin in IR pulse irradiation of the Example. 6 is a flowchart showing a procedure of MRI imaging in the same embodiment. The flowchart which shows the setting procedure of RF pulse center frequency in the Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Static magnetic field generation part 11 ... Main magnet 12 ... Shim coil 13 ... Static magnetic field power supply 14 ... Shim coil power supply 2 ... Gradient magnetic field generation part 21 ... Gradient magnetic field coil 22 ... Gradient magnetic field power supply 3 ... Transmission / reception part 31 ... Transmission coil 32 ... Transmission part 33 ... Reception coil 34 ... Reception unit 4 ... Top plate 5 ... Resonance frequency detection unit 51 ... Frequency analysis unit 52 ... Filtering processing unit 53 ... Peak frequency detection unit 54 ... Standard spectrum storage unit 55 ... Spectrum synthesis unit 6 ... Image data generation Unit 61 ... MR signal storage unit 62 ... High-speed calculation unit 63 ... Image data storage unit 7 ... Display unit 8 ... Input unit 81 ... Center frequency setting unit 82 ... Shimming condition setting unit 9 ... Control unit 91 ... Main control unit 92 ... Sequence Control unit 100 ... MRI apparatus

Claims (9)

A resonance frequency of a hydrogen nuclear spin in a desired tissue of a subject in which a medical material is inserted into a region to be imaged is detected, and imaging of the subject is performed using an RF pulse having a center frequency set based on the resonance frequency In an MRI apparatus that performs MRI imaging of a target part,
Based on a pulse sequence for resonance frequency measurement in which a first suppression pulse for suppressing an MR signal generated from a tissue different from the desired tissue and a second suppression pulse for suppressing an MR signal generated from the medical material are pre-pulses. Transmitting and receiving means for collecting MR signals from the imaging target region;
A resonance frequency detecting means for detecting a resonance frequency of a hydrogen nuclear spin in the desired tissue by measuring a peak frequency having a maximum magnitude in the frequency spectrum of the obtained MR signal;
An MRI apparatus comprising: center frequency setting means for setting a center frequency of an RF pulse used for MRI imaging of the imaging target region based on the resonance frequency.   A sequence control unit configured to generate a sequence control signal for executing the resonance frequency measurement pulse sequence, wherein the transmission / reception unit includes a saturation pulse as the first suppression pulse and the second pulse based on the sequence control signal; The MRI apparatus according to claim 1, wherein an inversion recovery pulse as a suppression pulse is irradiated to the imaging target portion.   A sequence control unit configured to generate a sequence control signal for executing the resonance frequency measurement pulse sequence, wherein the transmission / reception unit uses the first suppression pulse and the second suppression pulse based on the sequence control signal; The MRI apparatus according to claim 1, wherein the saturation pulse is irradiated to the imaging target part in time series or simultaneously.   The sequence control means is a sequence of the resonance frequency measurement pulse sequence to which any of the imaging methods of SE method, FE method, and EPI method is applied, wherein the first suppression pulse and the second suppression pulse are pre-pulses. The MRI apparatus according to claim 2 or 3, wherein a control signal is generated.   The transmitting / receiving means irradiates the imaging target region with the first suppression pulse for suppressing MR signals generated by adipose tissue and the second suppression pulse for suppressing MR signals generated by medical silicone. The MRI apparatus according to claim 1, wherein MR signals generated by the water tissue as the desired tissue are collected with high sensitivity.   The resonance frequency detection means combines and synthesizes a standard frequency spectrum of an MR signal obtained from a biological tissue in which the medical material is inserted and the frequency spectrum based on the MR signal collected from the imaging target region. Spectrum synthesis means for generating a frequency spectrum is provided, wherein the center frequency setting means sets the center frequency of the RF pulse by comparing the frequency spectrum in the synthesis frequency spectrum with the standard frequency spectrum. The MRI apparatus according to claim 1.   The MRI apparatus according to claim 6, wherein the spectrum synthesizing unit generates the synthesized frequency spectrum by matching a peak frequency in the frequency spectrum with a resonance frequency of the desired tissue in the standard frequency spectrum.   The MRI apparatus according to claim 6, wherein the resonance frequency detection means includes standard spectrum storage means for storing the standard frequency spectrum in advance.   The resonance frequency detecting means includes filtering processing means for calculating a first derivative by differentiating the frequency spectrum, and detecting the resonance frequency based on a zero cross frequency and the peak frequency in the first derivative. The MRI apparatus according to claim 1.

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