JP4890945B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as NMR) signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. (hereinafter referred to as MRI). More particularly, the present invention relates to an MRI apparatus capable of capturing an image reflecting electron spin resonance (hereinafter referred to as “ESR”) information using the Overhauser effect.

  As a method suitable for knowing the redox reaction dynamics in a living body, a method of acquiring an ESR signal has been studied. However, electron spin has a short relaxation time, and it is difficult to image the ESR signal itself of a sample in a living body. Therefore, when the sample in the living body is first excited by ESR, the excitation energy of electron spin is transferred to the proton nucleus by the Overhauser effect, and if the proton nuclear magnetic transition state becomes the excited state, the excited state is attenuated. Prototype MRI (ESR-MRI) has been proposed (Patent Document 1).

  An apparatus that images in combination of ESR and MRI in this way is called an ESR-MRI apparatus or a proton electron double resonance imaging apparatus PEDRI (proton-electron double resonance imaging). The ESR-MRI apparatus described in Patent Document 1 has a configuration in which the magnetic field intensity generated by one static magnetic field generator can be switched, and an electron spin is performed by irradiating a high frequency magnetic field while applying a static magnetic field for ESR to a sample. After the excitation is performed, a configuration is disclosed in which imaging is performed by MRI without moving the sample by switching to a static magnetic field for MRI.

  There is also known an apparatus having a configuration in which an ESR apparatus and an MRI apparatus are separately provided, and a sample whose electron spin is excited by the ESR apparatus is moved to the MRI apparatus and imaged.

As an example of a labeled sample to be injected into a living body in order to perform ESR-MRI, there is a nitroxyl probe ( ¹⁴ N, ¹⁵ N) that utilizes the high sensitivity of nitroxyl radicals to redox metabolism in the living body. By injecting a nitroxyl probe into a living body and performing ESR-MRI, the redox state inside and outside the cell can be detected in real time. Specifically, the intracellular oxidation state can be imaged by using ¹⁴ N as a labeled sample. Further, by using ¹⁵ N as a labeled sample, the extracellular oxidation state can be imaged. ^By injecting ¹⁴ N and ¹⁵ N into a living body, and simultaneously measuring ¹⁴ N and ¹⁵ N by ESR-MRI, the redox state inside and outside the cell can be detected in real time.
JP 2005-147893 A

¹⁴ N and ¹⁵ N have slightly different ESR resonance frequencies. For this reason, in order to make both probes excited (double resonance), a method using a high-frequency magnetic field (RF) transmission system in which the static magnetic field at the time of ESR excitation is constant and two types of frequencies can be transmitted. , a method according to a difference in the resonant frequency of ¹⁴ N and ¹⁵ N, slightly changing the static magnetic field in the ¹⁴ N excitation state and ¹⁵ N excitation time, illuminates a high frequency magnetic field of one frequency from the RF transmission system There is.

However, in the case of an ESR-MRI apparatus in which the ESR apparatus and the MRI apparatus are arranged close to each other and the sample is moved between the apparatuses, when the static magnetic field strength of the ESR apparatus is changed, the leakage magnetic field changes, The leakage magnetic field created by the eddy current also changes. For this reason, the problem that the static magnetic field of the MRI apparatus arrange | positioned adjacently fluctuates slightly with the change of a leakage magnetic field arises. As a result, the image quality of the MRI image is degraded for either ¹⁴ N or ¹⁵ N (or both).

  An object of the present invention is to provide an MRI apparatus capable of obtaining a stable MRI image even when the influence of an external magnetic field fluctuates when alternately acquiring a plurality of types of NMR signals.

  In order to achieve the above object, according to the present invention, the following MRI apparatus is provided. That is, a static magnetic field generation unit that generates a static magnetic field in the imaging space, a shim unit for correcting the distribution of the static magnetic field in the imaging space, and a high-frequency irradiation unit that irradiates the subject placed in the imaging space with a high-frequency magnetic field, A high-frequency receiving unit that detects a nuclear magnetic resonance signal generated by the subject, a signal processing unit that computes a detection signal of the high-frequency receiving unit to form an image, a high-frequency irradiation unit, a high-frequency receiving unit, and a signal processing unit And a control unit that controls to execute a predetermined imaging pulse sequence. At this time, the shim unit has a configuration that can be corrected with a correction amount selected from among two or more types of predetermined static magnetic field distribution correction amounts. While repeatedly acquiring nuclear magnetic resonance signals related to the substance in sequence, by outputting a signal to the shim part at a predetermined timing corresponding to the imaging pulse sequence, a static magnetic field distribution correction amount is selected, and two or more types of static magnetic field distribution correction are performed. The static magnetic field distribution is corrected in order by quantity. Thus, when NMR signals of a plurality of types of substances are repeatedly acquired in order, even if the static magnetic field distribution fluctuates due to an external magnetic field, this can be corrected, so that a stable MRI image can be obtained.

  The two or more types of static magnetic field distribution correction amounts that can be corrected by the shim portion described above are the first correction amount for correcting the first magnetic field distribution generated in the imaging space by the first external magnetic field, and the second external magnetic field distribution correction amount. And a second correction amount for correcting the second magnetic field distribution generated in the imaging space by the magnetic field.

¹⁴ N and ¹⁵ N can be used as the plurality of substances. In this case, the first external magnetic field is a magnetic field for exciting ¹⁴ N by electron spin resonance, and the second external magnetic field is a magnetic field for exciting ¹⁵ N by electron spin resonance. By causing the shim portion to alternately perform the correction with the first and second correction amounts, the MRI image in which the ¹⁴ N ESR information and the ¹⁵ N ESR information are respectively reflected is acquired almost simultaneously for the same cross section. Can do.

  In addition, it is possible to configure an ESR-MRI apparatus that moves the subject by providing the moving unit that moves the subject between the imaging space and the first and second external magnetic fields.

Hereinafter, a magnetic resonance imaging apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.
First, the configuration of the magnetic resonance imaging (MRI) apparatus of the present embodiment will be described with reference to FIG. This magnetic resonance imaging apparatus includes an MRI unit 40, an ESR unit 41 disposed at a predetermined distance from the MRI unit 40, and a control / signal processing system 42.

  The MRI unit 40 includes an MRI magnet 402 that generates a static magnetic field having a predetermined intensity (for example, 0.4 T) in an imaging space in which the subject 401 is disposed, a gradient magnetic field coil 403 that generates a gradient magnetic field in the imaging space, The imaging space includes an RF coil 404 that generates a high-frequency magnetic field (RF), and an RF probe 405 that receives an NMR signal generated by the subject 401. The gradient magnetic field coil 403 is configured by gradient magnetic field coils in three directions of X, Y, and Z, which are connected to the gradient magnetic field power supply 409 of the control / signal processing system 42, respectively, and according to the signal from the gradient magnetic field power supply 409. Each generates a gradient magnetic field. The RF coil 404 is connected to the RF transmitter 410 of the control / signal processing system 42. The RF coil 404 generates a high-frequency magnetic field according to the signal from the RF transmitter 410.

  The RF probe 405 receives an NMR signal having a predetermined resonance frequency. For example, in a 0.4 T static magnetic field, the RF probe 405 is set to receive a resonance frequency of 17.0 MHz. A signal detector 406 of the control / signal processing system 42 is connected to the RF probe 405 and detects a signal received by the RF probe 405. A signal processing unit 407 is connected to the signal detection unit 406, and the signal from the signal detection unit 406 is signal-processed and an image is reconstructed by calculation. A display unit 408 is connected to the signal processing unit 407 and displays a reconstructed image.

  On the other hand, a shim coil 416 is disposed on the imaging space side of the magnet 402. As the structure of the shim coil 416, a known structure such as a structure in which a large number of coils are arranged in a predetermined pattern can be used. An active shim 415 of the control / signal processing system 42 is connected to the shim coil 416. The active shim unit 415 finely adjusts the static magnetic field distribution by controlling the current value of each coil constituting the shim coil 416. Further, the active shim unit 415 adjusts the output of the gradient magnetic field power source 409 to perform the first-order shim, and controls the frequencies of the RF transmission unit 410 and the signal detection unit 406 to equivalently perform the zero-order shim. To do. As described above, the active shim unit 415 can directly and equivalently correct the static magnetic field distribution by the respective shim operations. In the present embodiment, the active shim unit 415 realizes the shim operations together. The static magnetic field distribution correction amount is called a shim amount.

  A control unit 411 is connected to the gradient magnetic field power source 409, the RF transmission unit 410, and the signal detection unit 406, and executes a desired imaging sequence by being controlled. The control time chart is generally called a pulse sequence. A control unit 411 is also connected to the active shim unit 415, and a static magnetic field correction with a predetermined shim amount is executed at a predetermined timing by instructing a control signal at a timing corresponding to the imaging sequence.

  Next, parts 406-1 and 406-2 of the signal detection unit 406 and part of the signal processing unit 407 will be described with reference to FIG. As shown in FIG. 2, the signal detection unit 406-1 includes a preamplifier 302 connected to the RF probe 405. The signal detection unit 406-2 includes an AD conversion / quadrature detection circuit 303, and performs quadrature detection on the output of the preamplifier 302. These configurations are the same as those of a normal MRI apparatus. The signal processing unit 407 includes a Fourier transform unit 304 and a computation unit 305, and after performing Fourier transform on the signal after quadrature detection, computation is performed as necessary (for example, image synthesis computation when signals are detected in parallel from a plurality of channels). Etc.), the MRI image detected by the RF probe 405 is obtained.

  Next, the ESR unit 41 will be described. The ESR unit 41 includes an ESR magnet 417 that generates a pulse magnetic field of a predetermined intensity in the excitation space where the subject 401 is disposed, an ESR RF coil 418 that generates a high-frequency magnetic field in the excitation space, and an RF power source 419. I have. As the ESR magnet 417, for example, a normal conducting magnet can be used.

  A subject moving table 412 is disposed between the MRI unit 40 and the ESR unit 41. The subject moving table 412 carries the subject 401 and moves between the imaging space of the MRI unit 40 and the excitation space of the ESR unit 41.

  The ESR magnet 417, the RF power source 419, and the subject moving table 412 are connected to the ESR control unit 414. The ESR control unit 414 receives a timing signal (trigger signal) from the control unit 411 of the MRI apparatus, and controls each part 417 to 419 of the subject moving table 412 and the ESR unit 41.

  Next, an operation for imaging ESR information by MRI using the MRI apparatus of the present embodiment will be described. Typical subjects are rats and mice, but in principle they can also be applied to humans.

Here, ¹⁴ N and ¹⁵ N injected into the subject 401 are the acquisition targets of ESR information. ^{When 14} N is excited by ESR, the static magnetic field of the ESR magnet 417 is set to 8 mT, and the high frequency magnetic field frequency (resonance frequency) generated by the RF coil 418 is set to 250 MHz. When ESR excitation of ¹⁵ N is performed, the static magnetic field of the ESR magnet 417 is set to (8 + α) mT, and the frequency of the RF coil 418 is also set to 250 MHz.

At this time, since a magnetic flux 301 as shown in FIG. 3 is generated in the ESR magnet 417 of the ESR unit 41, the leakage magnetic field of the ESR magnet 417 reaches the MRI magnet 402 of the MRI unit 40. For example, assuming that the leakage magnetic field is 0.1% of the ESR magnetic field, the leakage magnetic field is 0.008 mT when ¹⁴ N is ESR-excited and (8 + α) × 10 ⁻³ mT when ¹⁵ N is ESR-excited. Is applied to the imaging space of the MRI magnet 402. Therefore, after subjecting ¹⁴ N to ESR excitation by the ESR unit 41, the subject 401 is immediately transferred to the MRI unit 40 and imaged by excitation at the nuclear magnetic resonance frequency of proton, and immediately after ESR excitation of ¹⁵ N by the ESR unit 41. The static magnetic field intensity in the MRI unit 40 differs by α × 10 ⁻³ mT when the subject 401 is moved to the MRI unit 40 and excited and imaged by proton nuclear magnetic resonance frequency. For example, when α = 0.1 mT, the change is about 25 ppm with respect to the magnetic field 0.4T of the MRI magnet 402. Further, the proton resonance frequency varies about 400 Hz.

  As described above, the leakage magnetic field from the ESR magnet 417 is very small, but the degree is sufficiently large to deteriorate the image quality of the MRI image. Further, since this leakage magnetic field creates an eddy current in the RF shield of the MRI unit 40 and the pole piece of the magnet 402, the magnetic field fluctuation due to the eddy current remains even if the ESR magnet 417 is cut during NMR measurement. This effect is particularly remarkable when the time from ESR to MR measurement is as short as several tens of ms to several hundreds of ms.

  FIG. 4 specifically shows the relationship between the additional magnetic field strength and position that the leakage magnetic field creates in the magnetic field of the MRI unit 40. Compared with the leakage magnetic field 503 when the ESR magnetic field is weak (ESR magnetic field A), the leakage magnetic field 504 when the ESR magnetic field is strong (ESR magnetic field B) has a large offset amount and primary magnetic field fluctuation (magnetic field gradient amount). Become. When the generation of the ESR magnetic field is stopped, there is no direct leakage magnetic field to the MRI magnetic field, but the leakage magnetic fields 503 and 504 correspond to the leakage magnetic fields 503 and 504 by the eddy current formed in the pole piece of the MRI magnet 402 or the like. The residual magnetic fields 614 and 615 remain. The active shim unit 415 corrects the offset amount of the residual magnetic fields 614 and 615 by the zeroth-order shim and corrects the first-order magnetic field fluctuation by the first-order shim. The predetermined zero-order and first-order shim amounts for correcting the residual magnetic field 614 of the ESR magnetic field A are collectively referred to as a shim amount A, and the predetermined zero-order and zero-order for correcting the residual magnetic field 615 of the ESR magnetic field B are combined. The primary shim amount is collectively referred to as shim amount B.

In the present invention, such spatially non-uniform magnetic field fluctuations (residual magnetic fields 614 and 615) due to the external magnetic field (ESR magnetic fields A and B) are stably corrected for each measurement (echo). Specifically, when imaging ¹⁴ N excited by the ESR magnetic field A, the shim amount (correction amount of the static magnetic field distribution) realized by the active shim unit 409 controlling the shim coil 416 and the like is determined in advance. When imaging ¹⁵ N excited by the ESR magnetic field B with the shim amount A, magnetic field correction is actively performed by setting the shim amount of the active shim unit 409 to a predetermined shim amount B. As a result, the measurement of either subject can be favorably imaged without being affected by the disturbance of the magnetic field. The control unit 411 instructs the active shim unit 409 about the shim amount at a predetermined timing corresponding to the imaging pulse sequence. Note that predetermined amounts of shim A and B are stored in the memory built in the control unit 411. The built-in memory in the active shim unit 409 includes a specific current value of the shim coil 416, an output value of the gradient magnetic field power source 409, a frequency control value of the signal detection unit 406, etc. for realizing the shim amounts A and B, respectively. Stored.

As a method for determining the shim amounts A and B of the active shim unit 415, a method for determining the shim amount in the known active shim method can be used. For example, in a state where the magnetic field A (8 mT) for exciting ¹⁴ N is generated from the ESR magnet 417 and stopped, the static magnetic field distribution generated in the imaging space of the MRI unit 40 is measured to correct the magnetic field distortion. A shim amount A such as a primary shim by adjusting the current value of the shim coil 403 and the output of the gradient magnetic field power source 409, or an equivalent zero-order shim by controlling the frequencies of the RF transmitter 410 and the signal detector 406 Determine. Next, in a state where the magnetic field B (8 + αmT) for exciting ¹⁵ N is generated from the ESR magnet 417 and then stopped, the static magnetic field distribution generated in the imaging space of the MRI unit 40 is measured. A shim amount B of the shim portion 415 is determined. In addition, since there are various known techniques described in, for example, Japanese Patent Laid-Open No. 2000-342552, Japanese Patent Laid-Open No. 5-245124, and the like as active shim techniques, these techniques can also be used.

The outline of the operation of the MRI apparatus of this embodiment will be further described with reference to FIG. In FIG. 5, it is shown over time whether the subject 401 is on the ESR unit 41 side or the MRI unit 40 side. In FIG. 5, the magnetic field for ¹⁴ N ESR excitation in the ESR unit 41 is represented as ESR magnetic field A, and the magnetic field for ¹⁵ N ESR excitation is represented as ESR magnetic field B.

As shown in FIG. 5, the subject 401 is initially on the ESR unit 41 side, and ESR excitation is performed by applying an ESR magnetic field A from the ESR magnet 417. Immediately after the excitation (about 1 second or less), the subject 401 is moved to the MRI unit 40 side by the subject moving table 412. The NMR signal measurement (echo signal acquisition) of contrast A is immediately performed while correcting the fluctuation of the static magnetic field of the MRI unit 40 due to the influence of the ESR magnetic field A by the shim amount A of the active shim unit 415. Contrast A means that ¹⁴ N ESR excitation information is reflected in the NMR signal.

After the measurement, the subject 401 is returned to the ESR unit 41 again, and ESR excitation is performed by the ESR magnetic field B. Immediately after the excitation (about 1 second or less), the subject 401 is moved to the MRI unit 40 side. The NMR signal of contrast B is immediately measured while correcting the fluctuation of the static magnetic field of the MRI unit 40 due to the influence of the ESR magnetic field B by the shim amount B of the active shim unit 415. This is repeated AB alternately. Contrast B means that ¹⁵ N ESR excitation information is reflected in the NMR signal.

  Image A is obtained from NMR signal data of contrast A, and image B is obtained from NMR data of contrast B. Image A and image B are the same imaging section. By comparing the two images, different metabolic images at the same cross section and the same time can be obtained.

  In this way, by switching the shim amount by the active shim unit 415 for each MR measurement (each echo) according to the state of the ESR magnetic field, even if the magnetic field on the ESR side is different and the influence on the MRI side is different, Both images have the effect that image quality deterioration due to magnetic field distortion does not occur.

  A specific imaging pulse sequence of the imaging method for performing magnetic field correction described above will be specifically described with reference to FIG. FIG. 6 shows an example in which the present invention is applied to an ESR-MRI sequence created based on a 2D gradient echo sequence that is a typical imaging sequence.

First, the subject 401 is initially placed on the ESR unit 41 side by the subject moving table 412. Prior to the imaging sequence in the MRI unit 40, a trigger pulse 619 is output from the control unit 411 to the ESR control unit 414. The ESR control unit 414 generates a pulse magnetic field 612 in the ESR magnet 417 in response to the trigger pulse 619. The pulse magnetic field 612 is a ¹⁴ N excitation magnetic field (ESR magnetic field A in FIG. 5), and is 8 mT. During application of the pulse magnetic field 612, the ESR control unit 414 generates an RF pulse 616 in the ESR RF coil 418 and irradiates the subject 401. Thereby, ¹⁴ N in the subject 401 is ESR-excited. After the RF irradiation, the ESR control unit 414 outputs a movement instruction control signal 610 to the subject moving table 412, whereby the subject 401 moves to the MRI unit 40 side.

  At this time, the static magnetic field of the ESR magnet 417 returns to 0, but a residual magnetic field 614 caused by the eddy current of the leakage magnetic field is generated in the MRI unit 40. If the subject 401 moves to the MRI unit 40 side, an MRI image of protons is captured by the imaging sequence (601-606).

The imaging sequence irradiates the RF pulse 601 from the RF coil 404 while applying the slice selection gradient magnetic field pulse 602 from the gradient coil 403 to excite the nuclear magnetization of protons in a predetermined slice. Subsequently, after applying a slice encode gradient magnetic field pulse 603 and a phase encode gradient magnetic field pulse 604, an echo signal 606-1 generated at a predetermined echo time (TE) 607 is applied to the RF probe while applying a read gradient magnetic field pulse 605. 405. The echo signal 606-1 reflects the ESR contrast ¹⁴ N in the subject (contrast A in FIG. 5).

  In order to correct the residual magnetic field 614 of the MRI unit 40 due to the influence of the magnetic field 612 of the ESR unit 41 during this imaging sequence, the control unit 411 instructs the active shim unit 415 to perform shimming with the shim amount A. Is output at a predetermined timing. In response to this, the active shim unit 415 generates a compensation magnetic field 617 in the shim coil 403 and compensates for the influence of the residual magnetic field 614 by adjusting the magnitude of the gradient magnetic field.

After the echo signal 606 ends, the ESR control unit 414 outputs a control signal 611 to the subject moving table 412 and the subject 401 returns to the ESR unit 41 side again. The ESR control unit 414 generates a pulse magnetic field 613 in the ESR magnet 417. The pulse magnetic field 613 is a ¹⁵ N excitation magnetic field (ESR magnetic field B in FIG. 5), and is (8 + α) mT. During the application of the pulse magnetic field 613, the ESR control unit 414 generates an RF pulse 616 to irradiate the subject 401. Thereby, ¹⁵ N in the subject 401 is ESR-excited. After the RF irradiation, the ESR control unit 414 outputs a movement instruction control signal 610 to the subject moving table 412, whereby the subject 401 returns to the MRI unit 40 side, and the imaging sequence (601-606) is performed. The Thereby, the echo signal 606-2 is acquired. The echo signal 606-2 reflects the ¹⁵ N ESR contrast (contrast B in FIG. 5).

  Further, during the execution of the imaging sequence (601-606), a residual magnetic field 615 is generated on the MRI unit 40 side due to the influence of the magnetic field 613 of the ESR magnet 417. The control unit 411 outputs a control signal that instructs the active shim unit 415 to perform shimming with the shim amount B at a predetermined timing. In response to this, the active shim unit 415 generates a compensation magnetic field 617 in the shim coil 403 with a predetermined shim amount B and adjusts the gradient magnetic field to compensate for the influence of the residual magnetic field 615.

The above pulse sequence is one set at a time 609 (= repetition time 608 × 2) in which both ¹⁴ N and ¹⁵ N echo signals 606-1 and 606-2 are acquired. This set is repeated while changing the encoding amount by the phase encoding number (in the case of three-dimensional imaging, the phase encoding number × slice encoding number). As the number of encodings, for example, combinations of values such as 32, 64, 128, 256, and 512 are selected. The obtained echo is divided into an echo signal 606-1 having a contrast of ¹⁴ N and an echo signal 606-2 having a contrast of ¹⁵ N, and imaged by two-dimensional or three-dimensional Fourier transform, respectively. The obtained ¹⁴ N contrast image and ¹⁵ N contrast image have the same imaging cross section. Therefore, by comparing the two images, different metabolic images at the same cross section and the same time can be obtained.

  In the above-described embodiment, the case where a gradient echo sequence is used as an example of the imaging sequence has been described. However, other imaging sequences can also be used. For example, a spin echo sequence or a diffusion weighted imaging sequence can be used, or an FSE (Fast Spin Echo) sequence can be used. The imaging sequence may be a radial scan, and is not limited to either two-dimensional or three-dimensional measurement.

In addition, the images reflecting the two types of labeled samples ( ¹⁴ N, ¹⁵ N) obtained by the present invention may be displayed in a superimposed manner, and the superimposed images may be displayed in a cine manner. Is also possible.

Further, in the above-described embodiment, the subject 401 into which a labeled sample such as ¹⁴ N or ¹⁵ N is injected is placed on the moving table 412 and moved between the ESR unit 41 and the MRI unit 40 to thereby change the ESR contrast. However, the ESR contrast can be given by other methods. For example, a labeled sample (N, F, C, etc.) that has not been injected into the subject is placed in the ESR unit 41, the labeled sample is ESR excited, and then placed in the MRI unit 40 via a thin tube. A method of automatically injecting into the blood vessel of the subject 401 thus made can be used. At this time, the imaging sequence is executed on the MRI unit 40 side in synchronization with the automatic injection. However, when two or more kinds of labeled samples are used, the ESR unit 41 uses the ESR to excite different labeled samples each time injection is performed. It is necessary to change the strength of the magnetic field. Thus, even when the magnetic field intensity generated in the ESR unit 41 unit 41 arranged near the MRI unit 40 is changed, the present invention can change the shim amount of the active shim accordingly. In addition, it is possible to acquire an image by MRI in a state where the fluctuation of the static magnetic field distribution due to the fluctuation of the ESR magnetic field is corrected.

  In the present embodiment, the MRI apparatus having the configuration including the ESR unit 41 has been described. However, the present invention is not limited to this configuration, and does not include the ESR unit and is close to another ESR device. The present invention can also be applied to an MRI apparatus that performs ESR-MRI by being disposed in the network. Also in this case, by providing the active shim portion 415 that performs active shim with the shim amounts A and B, the influence of the fluctuation of the ESR magnetic field can be similarly compensated. It should be noted that an external ESR device is disposed in advance in the same manner as in actual imaging to measure the fluctuation of the magnetic field and determine shim amounts A and B such as shim current values.

  In the present embodiment, correction of the leakage magnetic field of the ESR magnetic field by the shim amount of the active shim has been described. However, the present invention is not limited to ESR excitation, and the static magnetic field of the MRI unit 40 is not limited to the external magnetic field. The present invention can be applied as long as it fluctuates to two or more types.

  According to the present invention, when signals related to a plurality of substances are alternately obtained, a stable MRI image can be obtained even if the influence of an external magnetic field varies alternately.

1 is a block diagram showing a configuration of an MRI apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram showing a partial configuration of a signal detection unit 406 and a signal processing unit 407 of the MRI apparatus of FIG. 1. FIG. 2 is an explanatory diagram showing that a leakage magnetic field extends from the ESR unit to the MRI unit in the MRI apparatus of FIG. 1. The graph which shows that the leakage magnetic field ranging from an ESR part to an MRI part changes with the magnitude | size of an ESR magnetic field. The block diagram which shows the outline | summary which performs excitation by ESR and MRI imaging, correct | amending a leakage magnetic field with the MRI apparatus of FIG. The pulse sequence figure which shows the procedure of the excitation by ESR performed by the MRI apparatus of FIG. 1, MRI imaging, and a leakage magnetic field.

Explanation of symbols

  401 ... subject, 402 ... MRI magnet, 403 ... gradient magnetic field coil, 404 ... RF coil, 405 ... RF probe, 406 ... signal detector, 407 ... signal Processing unit, 408 ... Display unit, 409 ... Gradient magnetic field power supply, 410 ... RF transmission unit, 411 ... Control unit, 412 ... Subject moving body, 417 ... Magnet for ESR, 418 ... RF coil, 419 ... RF power supply.

Claims (4)

A static magnetic field generator for generating a static magnetic field in the imaging space; a shim for correcting the distribution of the static magnetic field in the imaging space; and a high-frequency irradiator for irradiating a subject placed in the imaging space with a high-frequency magnetic field; A high-frequency receiving unit that detects a nuclear magnetic resonance signal generated by the subject, a signal processing unit that computes a detection signal of the high-frequency receiving unit to form an image, the high-frequency irradiation unit, a high-frequency receiving unit, and signal processing And a control unit that executes a predetermined imaging pulse sequence by controlling the unit,
The shim portion is configured to be corrected with a correction amount selected from among two or more predetermined static magnetic field distribution correction amounts,
The control unit outputs signals to the shim unit at a predetermined timing corresponding to the imaging pulse sequence while sequentially acquiring nuclear magnetic resonance signals relating to a plurality of substances in the subject in order by the imaging pulse sequence. The magnetic resonance imaging apparatus is characterized in that the static magnetic field distribution correction amount is selected, and the static magnetic field distribution is repeatedly and sequentially corrected with two or more types of the static magnetic field distribution correction amounts.   The magnetic resonance imaging apparatus according to claim 1, wherein the two or more types of static magnetic field distribution correction amounts correctable by the shim unit correct a first magnetic field distribution generated in the imaging space by a first external magnetic field. A magnetic resonance imaging apparatus comprising: a first correction amount for correcting a second magnetic field distribution generated in the imaging space by a second external magnetic field. 3. The magnetic resonance imaging apparatus according to claim 2, wherein the plurality of substances are ¹⁴ N and ¹⁵ N, and the first external magnetic field is a magnetic field for exciting ¹⁴ N by electron spin resonance, The second external magnetic field is a magnetic field for exciting ¹⁵ N by electron spin resonance, and the control unit causes the shim unit to alternately perform correction with the first and second correction amounts. Magnetic resonance imaging apparatus. 4. The magnetic resonance imaging apparatus according to claim 2, further comprising a moving unit that moves the subject between the imaging space and the first and second external magnetic fields. .

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