JP4997877B2 - MRI-PET system - Google Patents

Description

  The present invention relates to a positron emission computed tomography (Positron Emission Computed Tomography) (hereinafter referred to as “PET”) apparatus, and a nuclear magnetic resonance imaging (Nuclear Imaging). Resonance), hereinafter referred to as “MRI”) relates to an MRI-PET apparatus in which the apparatus is coupled to one apparatus.

  PET is administered to a subject with a radiopharmaceutical labeled with a positron emitting nuclide that has the property of collecting in specific cells (eg, tumor tissue) in the body, and in which part of the body the radiopharmaceutical is accumulated or consumed. It is a method to check. A PET camera loaded on a PET apparatus is an apparatus that detects a pair of gamma rays simultaneously. Conventionally, a scintillation detector has been used as a γ-ray detector for detecting γ-rays. The scintillation detector is made of a substance that emits light having an intensity proportional to energy when it receives gamma rays. The scintillation detector receives gamma rays and emits light having an intensity proportional to the energy. However, if the scintillation detector is left as it is, the signal is weak and cannot be used as an electric signal. Therefore, a photomultiplier tube is used to convert this optical signal into amplified electrical energy. For the photomultiplier tube, a substance that emits electrons by the photoelectric effect when it receives light is used. The electrons (primary electrons) are accelerated by the voltage applied between the electrodes, and collide with the first secondary electron emission electrode that emits a plurality of secondary electrons when the electrons collide. The secondary electron emission electrode further emits a large amount of secondary electrons, and is accelerated by a voltage applied between the first secondary electron emission electrode and the second secondary electron emission electrode. Collides with the secondary electron emission electrode. By repeating this, a large electron current, that is, electric energy is created. At this time, when a magnetic field is applied to the photomultiplier tube, the trajectory of electrons is bent by the Lorentz force, so that the target secondary electron emission electrode cannot be reached, and the operation may be hindered.

PET can detect function and metabolism at the molecular biology level and can provide a functional image of the body of the subject. However, since the three-dimensional image obtained by PET is only a radiopharmaceutical accumulation region, the function as a morphological image representing the shape of the whole organ is poor.
The main purpose of PET diagnosis is detection and location identification of cancer lesions, but the initial cancer lesions obtained with PET are typically punctiform, so in a PET-only image, the cancer lesion is located in any part of the organ. It is difficult to accurately determine whether it is.

On the other hand, there is MRI as means for obtaining a morphological image of a living body. MRI is NMR (Nuclear
This is an imaging method using the phenomenon (Magnetic Resonance). The NMR imaging method images various existence densities of objects by a combination of non-ionizing electromagnetic radiation (radio waves), a static magnetic field, and a gradient magnetic field.

  In order to superimpose the functional image obtained by PET and the morphological image obtained by MRI, and to easily capture the position of the active site of the living body obtained by PET in the organ, MRI- which integrates MRI and PET PET devices are being studied. However, since MRI generates a strong magnetic field, it may have a significant influence on a PET apparatus (in particular, a PET apparatus that uses a photomultiplier tube for amplification of a detection signal of γ rays).

  On the other hand, in consideration of the strong magnetic field of MRI, signal photons from the γ-ray detector are transported to a photomultiplier tube installed in a low magnetic field region by glass fiber and amplified (for example, patents). (Refer literature 1 and nonpatent literature 1.).

  In addition to a strong static magnetic field, MRI also generates a time-varying gradient magnetic field. Further, a high frequency (RF wave) for causing nuclear magnetic resonance is also irradiated.

  In the above-described conventional technology, it is necessary to connect glass fibers for each channel of the detector, and the number thereof becomes enormous (for example, even a small-sized head PET apparatus having a size of about tens of thousands of channels). Needed.). Therefore, there are problems in its complicated manufacturability and cost.

US4939464 Simultaneous PET and NMR; P.K.Marsden et.al, The British Journal of Radiology, 75 (2002), S53-S59

  An object of the present invention is to provide an MRI-PET apparatus that integrates an MRI apparatus and a PET apparatus, and that can sufficiently reduce the influence of a magnetic field on the PET apparatus from the MRI apparatus. It is in.

An MRI-PET apparatus comprising an MRI apparatus having an MRI gantry and a PET apparatus having a PET gantry, wherein the PET gantry is a substantially cylindrical gantry arranged with its first end face facing the MRI apparatus. There is a magnetic material disposed on the PET gantry, and the magnetic body includes a first end surface magnetic material disposed on the first end surface and a second end surface magnetism disposed on the second end surface opposite to the first end surface. A first end face magnetic material, a first end face magnetic material and a second end face magnetic material, and an inner surface magnetic material arranged in a substantially cylindrical shape along the inner surface of the PET gantry, a first end face magnetic material, and a second end face magnetic material, And an outer surface magnetic material arranged in a substantially cylindrical shape along the outer surface of the PET gantry, and the thickness of the first end surface magnetic material is the second end surface magnetic material, the inner surface magnetic material, and the outer surface magnetic material. Thicker than wood To.

  According to the present invention, even when the MRI apparatus and the PET apparatus are integrated, the influence of the magnetic field on the PET apparatus from the MRI apparatus can be sufficiently reduced. Therefore, it is possible to provide an MRI-PET apparatus that can prevent malfunction of the PET apparatus due to a magnetic field generated by the MRI apparatus.

  In the MRI-PET apparatus of the present invention, a magnetic material (magnetic shield) is disposed on the gantry of the PET apparatus. With this magnetic material, the influence of the magnetic field from the MRI apparatus can be effectively reduced. By reducing the influence of the magnetic field from the MRI apparatus, it is possible to prevent malfunction of the PET apparatus.

  Hereinafter, an MRI-PET apparatus according to the present invention will be described with reference to FIGS. In this embodiment, the MRI apparatus and the PET apparatus are integrated, and a magnetic material is disposed in the gantry of the PET apparatus to reduce the influence of the magnetic field from the MRI apparatus.

  The inspection technique using radiation can inspect the inside of a subject nondestructively. Examples of radiation examination apparatuses that use human bodies as subjects include X-ray CT apparatuses, PET apparatuses, and single-photon emission CT apparatuses. All of these techniques measure the physical quantity of the integral value (flight direction) of the radiation emitted from the human body, and calculate and image the physical quantity of each voxel in the human body by back projecting the integral value. For this imaging, it is necessary to process an enormous amount of data, but with the rapid development of computer technology in recent years, it has become possible to provide tomographic images of the human body at high speed and with high definition.

PET is administered to a subject with a radiopharmaceutical containing a positron emitting nuclide (15O, 13N, 11C, 18F, etc.) and a substance having a property of collecting in specific cells in the body, and the radiopharmaceutical accumulates in any part of the body Or it is a method to check whether it is consumed. PET can detect functions and metabolism at a molecular biology level that cannot be detected by X-ray CT or the like, and can provide a functional image inside the body of a subject. An example of a radiopharmaceutical is fluorodeoxyglucose (2- [F-18] fluoro-2-deoxy-D-glucose, 18FDG). Since 18FDG is highly accumulated in tumor tissue by sugar metabolism, it is used to identify the tumor site. The positrons emitted from the positron emitting nuclide accumulated at a specific location combine with the electrons of nearby cells and disappear, and emit a pair of γ-rays having energy of 511 keV. These γ rays are emitted in directions almost opposite to each other (180 ° ± 0.6 °). If this pair of γ-rays is detected by a γ-ray detector, it can be seen between which two γ-ray detectors the positrons are emitted. By detecting such a large number of gamma ray pairs, it is possible to specify a place where a large amount of radiopharmaceutical is consumed. For example, 18
As described above, since FDG gathers in cancer cells with intense glucose metabolism, it becomes possible to detect cancer foci by PET. The obtained data is converted into the radiation generation density of each voxel, and the generation position of the γ-ray (position where the radionuclide is accumulated, that is, the position of the cancer cell) is imaged. 15O, 13N, 11C, and 18F used in PET are radioisotopes with a short half-life of 2 to 110 minutes.

  The three-dimensional image obtained by PET is only an accumulation region such as 18FDG, and has a poor function as a morphological image that expresses the shape of the whole organ, except for the brain and bladder where it collects in the whole organ. The main purpose of PET diagnosis is detection and location identification of cancer lesions, but the initial cancer lesions obtained with PET are typically punctiform, so in a PET-only image, the cancer lesion is located in any part of the organ. It is difficult to judge exactly. Since an image obtained by PET is an image reflecting the function of a living body, it is also called a functional image.

  On the other hand, there is magnetic resonance imaging (MRI) as means for obtaining a morphological image of a living body. MRI is an imaging method using an NMR phenomenon. NMR is a phenomenon that selectively absorbs RF of a specific frequency (Larmor frequency) when a certain kind of atomic nucleus (NMR nuclide) is placed in a uniform static magnetic field.

  Medical MRI mainly images a three-dimensional distribution of nuclear magnetic resonance signals of hydrogen nuclei in a living body. For this reason, an image density corresponding to the content of hydrogen atoms in the living tissue can be obtained, and as a result, a tissue shape, that is, a morphological image can be obtained. In addition, since the density of an image is obtained according to the hydrogen atom density of a substance, it is suitable for imaging a tissue having a large difference in hydrogen atom density. From this, it is evaluated that MRI is superior to X-ray CT or the like for brain images.

  MRI can obtain a three-dimensional internal morphological image simply by irradiating a human body with a static magnetic field that does not change with time, a gradient magnetic field that changes with time, and an RF pulse. Compared with a diagnostic apparatus that uses X-rays or the like, There is a merit that there is no.

An MRI-PET apparatus has been studied as a combination of an MRI apparatus that obtains morphological images and a PET apparatus that obtains functional images. By superimposing the functional image obtained by PET and the morphological image obtained by MRI, it is easy to capture where the active site of the living body obtained by PET is located in the organ. In addition, the MRI-PET apparatus in which the PET apparatus and the MRI apparatus are integrated makes it possible to achieve the convenience of superimposing images.

  However, since MRI is a device that generates a strong magnetic field, it may have a significant influence on a PET device (particularly, a PET device that uses a photomultiplier tube to amplify a detection signal of γ rays). Therefore, it is considered difficult to make these MRI apparatus and PET apparatus close to each other as an integrated apparatus.

  As means for protecting devices that do not want to be exposed to a magnetic field, a magnetic shield technique is known in which a magnetic plate or the like is placed between them or an object is surrounded by a magnetic plate. However, since MRI generates a magnetic field on the order of Tesla (1 Tesla = 10000 Gauss), it is expected that most magnetic materials cause magnetic saturation and the effect of the magnetic shield is almost equal. Also, if the magnetic shield plate is made very thick to make it difficult to cause magnetic saturation, strong magnetization is generated, which gives an error to the uniform magnetic field required for MRI imaging and causes disturbance in the image. For this reason, until now, it has been considered that there is a great difficulty in realizing an MRI-PET apparatus with a magnetic shield.

  The present invention provides an overcoming means obtained as a result of studying the problem of PET operation failure due to the strong magnetic field of MRI that occurs when an MRI apparatus and a PET apparatus are integrated, that is, difficulty in coexistence (compatibility). To do.

  To summarize the means used in the present invention to solve the above difficulties, the following three points are obtained.

(1) A 2-gantry type in which the MRI gantry and the PET gantry are arranged on the same axis,
The leakage magnetic field on the axis is sharply reduced by the MRI shield coil, and the PET gantry is arranged at a position separated by a minimum distance that can sufficiently shield the leakage magnetic field with a magnetic material.
(2) The above magnetic body is mainly arranged at a position where the leakage magnetic field changes its direction in the radial direction, and the magnetic plate is formed in a disk shape with a hole in the center so as to guide the magnetic flux in the radial direction. No magnetic material is placed near the axis.
(3) A semiconductor detector resistant to a magnetic field is used as a detector for γ rays.

A first embodiment of the MRI-PET apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a diagram illustrating a configuration of an MRI-PET apparatus according to the present embodiment. As shown in FIG. 1, the MRI-PET apparatus in the present embodiment includes a horizontal magnetic field type MRI apparatus 1 having a tunnel-type (substantially cylindrical shape having a substantially cylindrical hole at the axial center portion) MRI gantry 101; It is composed of two devices, a PET device 2 having a ring shape (substantially cylindrical shape provided with a substantially cylindrical hole at the center of the shaft) and a PET gantry 102 having a shorter axial length than the tunnel type. Further, the MRI gantry 101 and the PET gantry 102 are integrated at the connecting portion 3. Considering the movement of the subject 15 and the like, the MRI gantry 101 and the PET gantry 102 are arranged on the same axis.

  The MRI apparatus 1 includes a main coil 10 that generates a main magnetic field and a shield coil 11 for reducing a leakage magnetic field in an imaging space of the MRI apparatus. The main coil 10 and the shield coil 11 are accommodated in an MRI gantry 101. .

  The PET device 2 includes a γ-ray detector 4 and a signal processing device 5, and the γ-ray detector 4 and the signal processing device 5 are accommodated in a PET gantry 102. As the γ-ray detector 4 of the PET apparatus 2, a semiconductor detector such as CdTe resistant to a magnetic field is used. In the semiconductor detector, a PN junction of a semiconductor is formed and reverse bias is applied. At the junction surface of the PN junction, electrons and holes of carriers are collected on the electrode side, and a depletion layer free of carriers is formed. This is electrically the same as a capacitor and creates a high electric field. When radiation enters here, it creates electron-hole pairs, but these carriers move in the conduction band due to the band structure, so they can be collected on the electrode without being scattered too much by the atoms. In the semiconductor detector, current flows when radiation enters and a signal pulse is generated, and the electric capacity temporarily changes. This signal is amplified by a charge-sensitive preamplifier.

In such a semiconductor detector, the carrier of the detection unit moves in the conduction band of the band structure, and thus is relatively unaffected by the magnetic field. In addition, a photomultiplier tube that is easily affected by a magnetic field can be eliminated in the amplifying unit. Therefore, the magnetic field resistance can be remarkably improved as compared with a conventional PET apparatus using a scintillation detector and a photomultiplier tube. The detection part of the semiconductor detector is composed of gallium arsenide (GaAs), cadmium tellurium zinc (CZT), or the like. The signal processing device 5 uses an ASIC for miniaturization. ASIC (Application Specific
Integrated Circuit) is an application specific integrated circuit.

  The MRI-PET apparatus further includes a moving bed 14 for holding the subject 15, a bed table 16 for fixing the moving bed 14 to the floor surface, and a guide rail 17 for moving the moving bed 14 in the longitudinal direction.

  Here, the MRI-PET apparatus in the present embodiment reduces the influence of the magnetic field from the MRI and prevents the PET apparatus from malfunctioning due to the magnetic field generated by the MRI apparatus. Place.

  In the present embodiment, the MRI-PET apparatus is installed such that the MRI gantry 101 and the PET gantry 102 are separated from each other by a predetermined distance (maintaining a predetermined interval). Here, a magnetic shield 6 made of a magnetic material is disposed on a PET gantry 102 having a torus shape (substantially cylindrical shape provided with a coaxial hole in the vicinity of the central axis).

  The magnetic shield 6 will be described with reference to FIG. FIG. 2 is an enlarged cross-sectional view of the torus of the PET gantry 102.

  The magnetic shield 6 includes a magnetic shield 6-a disposed on an end surface on the MRI gantry 101 side (an end surface facing the MRI gantry 101) and a magnetic shield 6-d disposed on an end surface opposite to the magnetic shield 6-a. Are connected to the magnetic shields 6-a and 6-d and are arranged in a substantially cylindrical shape along the inner surface of the PET gantry 102, and are connected to the magnetic shields 6-a and 6-b. The magnetic shield 6-e is arranged in a substantially cylindrical shape along the outer surface of the PET gantry 102. As the magnetic shield 6, an iron material can be used.

  Each of the magnetic shields 6-a and 6-d has a circular shape corresponding to the end face of the substantially cylindrical PET gantry, but a hole (bore) 8 through which the subject passes is provided in the PET gantry, and the shape thereof is a 5-yen coin. It becomes a perforated disk shape like. On the other hand, the magnetic shields 6-b and 6-e have cylindrical shapes arranged along the inner surface and the outer surface of the PET gantry 102, respectively. The magnetic shield 6 is formed in a substantially cylindrical shape corresponding to the PET gantry 102 by these magnetic shields 6-a to 6-d.

Among the magnetic shields 6-a to 6-d, the magnetic shield 6-a closest to the MRI apparatus is
The strength of the magnetic field received from the MRI apparatus is increased. Therefore, in order to effectively attract the magnetic force lines 7 leaking from the MRI apparatus, the magnetic shield 6-a is thicker than the other magnetic shields 6-b, 6-c, 6-e. . The thickness can be about 10 mm, for example.

  A cylindrical magnetic shield 6-b thinner than the magnetic shield 6-a is installed on the inner surface of the PET apparatus 2 along a hole 8 through which the subject passes. The magnetic shield 6-b leaks from the MRI apparatus 1 and attracts the magnetic force lines 7 spreading in the direction of the bore 8 (axial direction). A part of the magnetic shield 6-b requires a γ-ray receiving window 9 for receiving γ-rays emitted from the subject. The magnetic shield 6 is not provided with a magnetic material (having an opening for receiving γ-rays) in the γ-ray receiving window 9 portion of the magnetic shield 6-b, or the magnetic shield 6 is thin enough not to prevent transmission of γ-rays. -C can be placed.

  A magnetic shield 6-d is arranged on the end surface opposite to the magnetic shield 6-a in order to return the magnetic flux guided along the bore 8 to the MRI gantry side. This magnetic shield 6-d can be made of a magnetic material thinner than the magnetic shield 6-a.

  A cylindrical magnetic shield 6-e thinner than the magnetic shield 6-a is installed on the outer surface of the PET apparatus 2. The magnetic flux is guided to the MRI apparatus 1 side by the magnetic shield 6-e.

  Here, in this embodiment, the purpose of making the MRI gantry 101 and the PET gantry 102 independent and making it a two-gantry type will be described. In the case of a 1 gantry type equipped with a PET device on the inner surface side of the MRI apparatus, or a 1 gantry type equipped with an MRI magnet divided in the axial direction and equipped with a PET apparatus therebetween, using the current high resolution MRI, The PET apparatus is exposed to a strong magnetic field of 1.5 to several Tesla. Even if the entire PET device is covered with a member with high magnetic permeability such as iron, iron causes magnetic saturation at around 2 Tesla. Therefore, in a magnetic field higher than this, the magnetic flux penetrates this, and the magnetic shielding characteristics are impaired. End up. When exposed to a magnetic field of several Tesla, the influence on not only a γ-ray detector but also a signal processing system such as an ASIC is great. Therefore, even a semiconductor detector that does not use a photomultiplier tube may cause a malfunction.

  Further, when a thick magnetic plate is disposed inside the MRI apparatus, the magnetic field generated by the magnetic shield gives a significant disturbance to the uniform magnetic field with high uniformity formed in the imaging space of the MRI apparatus. This disturbance causes disturbance in the MRI image, and in the worst case, there is a possibility that imaging cannot be performed.

  The MRI apparatus is devised to reduce the leakage magnetic field as much as possible outside the MRI apparatus. It is a system called an active shield system. As shown in FIG. 1, a shield coil 11 excited in the direction opposite to the main coil 10 is provided inside the MRI apparatus. Outside the MRI apparatus, the magnetic field of the shield coil 11 cancels the main magnetic field of the main coil 10 and suppresses the spread of the leakage magnetic field as a whole. In the MRI apparatus equipped with the shield coil 11, the magnetic field is rapidly reduced outside the MRI gantry 101. For this reason, when the PET apparatus is made independent of the MRI apparatus and is of a two gantry type separated by a certain distance, the PET apparatus 2 can be stably operated only by applying a small amount of magnetic shielding material. Therefore, in the MRI-PET apparatus provided with the MRI apparatus having the shield coil 11, it is particularly effective to use the 2-gantry type.

  Here, considering the magnetic field strength (0.5 to 3.0 T) of the MRI imaging space, it was expected that a considerable amount of magnetic flux would penetrate the PET gantry even if the PET gantry was separated from the 2 gantry type. . For this reason, a space of a considerable distance must be provided between the MRI gantry and the PET gantry, and it has been expected that the entire apparatus will be lengthened.

  However, when the inventor performed nonlinear magnetic field calculation on the MRI-PET apparatus provided with the magnetic shield 6 described in the present embodiment, it was found that the distance between the MRI gantry and the PET gantry could be shortened unexpectedly. The reason is due to the following physical effects. That is, the magnetic field lines 7 of the axial leakage magnetic field coming out of the bore 8 of the MRI gantry 101 are suddenly bent in the radial direction by the effect of the shield coil 11. The direction of the magnetic force lines 7 coincides with the direction of the magnetic shield 6-a on the surface facing the MRI gantry mounted on the PET gantry 102. Therefore, the magnetic force lines 7 flow through the magnetic shield 6-a, and the penetration into the PET gantry 102 is greatly reduced.

  These effects were confirmed by axisymmetric nonlinear magnetic field analysis. The results will be described with reference to FIGS. FIG. 3A shows a cross section of the first axisymmetric magnetic shield model used in this calculation. In this model, no magnetic material plate is disposed in the γ-ray receiving window 9. Reference numeral 12 indicates the central axis of the PET apparatus 2. FIG. 3B shows the result of calculating the state of magnetic field lines generated from the MRI gantry and extending around the PET gantry. It can be seen that the lines of magnetic force extending around the PET gantry hardly enter the space surrounded by the magnetic shields 6-a to 6-e.

FIG. 4 is a diagram showing the magnetic flux density distribution of the MRI-PET apparatus, and shows the magnetic flux density distribution from the MRI gantry to the PET gantry with contour lines. FIG. 4A shows the minimum value of the contour line up to 1000 G (Gauss). From FIG.
It can be seen that the magnetic flux density of 15000 G at the bore center of the MRI gantry drops rapidly to 1000 G near the midpoint between the MRI gantry and the PET gantry. FIG. 4B shows a contour line with a maximum value of 1000 G and a minimum value of 20 G, and displayed at 20 G intervals. FIG. 4B shows that the PET gantry is about 200 G on the side facing the MRI gantry and about 50 G on the opposite side. FIG. 4C shows the magnetic flux density distribution inside the magnetic shield 6 of the PET gantry. The maximum value of the contour line is 20G, the minimum value is 1G, and the contour lines are displayed at 1G intervals. FIG. 4C shows that the magnetic flux density in the vicinity of the γ-ray light receiving window is about 20 G, but the portion in which a signal processing device such as an ASIC is stored is 5 G or less.

  FIG. 5A shows a cross section of the second axisymmetric magnetic shield model used in this calculation. In this model, a magnetic shield 6-c having a thickness of 1 mm is arranged in the γ-ray receiving window 9 of the first axisymmetric magnetic shield model. FIG. 5B shows the magnetic flux density distribution inside the second axisymmetric magnetic shield model, with the contour line having a maximum value of 4 G and a minimum value of 1 G, displayed at 1 G intervals. FIG. 5B shows that the magnetic flux density from the vicinity of the γ-ray receiving window to the inside is all 4 G or less.

  A gradient magnetic field that varies with time is generated from a gradient coil (not shown) provided in the MRI. Further, a high frequency (RF wave) for causing nuclear magnetic resonance is emitted from an RF transmission coil (not shown). Therefore, the problem of causing the signal processing system of the PET apparatus to malfunction can also occur due to a gradient magnetic field or a high frequency (RF wave). Against this, the influence is remarkably reduced by making the MRI-PET apparatus a two-gantry type and separating the PET apparatus to some extent, and the magnetic shield also makes these time-varying gradient magnetic fields and high-frequency electromagnetic waves a considerable part. Can be shielded. When a time-varying magnetic field or high-frequency electromagnetic wave enters the conductor, an eddy current is induced on the surface. As a result, time-varying magnetic fields and electromagnetic energy of high-frequency electromagnetic waves are consumed or reflected, making it difficult for them to penetrate into the interior. For shielding high frequency electromagnetic waves, an aluminum plate or a copper plate having a higher conductivity has a higher shielding efficiency. Therefore, when a magnetic shield using an iron plate or the like is insufficient in shielding ability against high-frequency electromagnetic waves, a thinner aluminum plate or copper plate may be stacked on the magnetic shield.

  By the way, normally, the PET apparatus main body is designed to be axially symmetric, but the columns and the like that support the PET apparatus main body from the connecting portion have a non-axisymmetric shape. When the magnetic shield is formed so as to cover these columns, it becomes non-axisymmetric as a whole. However, it is desirable that the magnetic shield of this PET apparatus be arranged coaxially with the axis of the MRI apparatus. The reason is that the error magnetic field created by the non-axisymmetric magnetic shield becomes non-axisymmetric and is difficult to correct on the MRI side.

  Initially, the thick magnetic shield 6-a was expected to generate a large error magnetic field in the MRI imaging space. In practice, however, this was not a problem. The reason is as follows. A large error magnetic field is generated in the MRI imaging space when there is a magnetic body such as an iron plate at a position extending on the axis from the center of the MRI imaging space. When there is a magnetic body on the axis, magnetization in the same axial direction as the main magnetic field of MRI is formed in the magnetic body such as an iron plate by the leakage magnetic field. This magnetization creates a magnetic field in the same direction as the MRI uniform magnetic field in the MRI imaging space, and this is superimposed on the uniform magnetic field, resulting in a large error. The magnetic shield 6 of the PET gantry 102 of the present invention has a generally cylindrical torus shape with a hole in the vicinity of the substantially cylindrical shape on the axis, and a magnetic material is provided at a position extending on the axis from the center of the MRI imaging space. Not provided. Therefore, a large error magnetic field is not generated in the MRI imaging space. Since the magnetic shield of the present invention mainly guides the magnetic flux in the radial direction to the outside, the magnetization is induced in the radial direction. Since the magnetization in the radial direction is a component orthogonal to the uniform magnetic field of MRI, the influence on the strength of the uniform magnetic field is small. This is the reason why the magnetic shield of the present invention achieves both compatibility (compatibility) with its shielding characteristics and magnetic field uniformity of MRI.

  Next, the usage method and operation of the MRI-PET apparatus in the present embodiment will be described with reference to FIGS. FIG. 6 is a diagram showing a method of use and operation of the MRI-PET apparatus in the present embodiment. The moving bed 14 holds the subject 15 and can move in the longitudinal direction. By moving the moving bed 14 carrying the subject 15 along the guide rail 17, the subject 15 can be moved and reciprocated in the bores of the MRI apparatus 1 and the PET apparatus 2 arranged in series. By arranging the MRI apparatus 1 and the PET apparatus 2 in series on the same axis, a series of diagnoses by MRI and PET can be performed efficiently. Here, the bed table 16 that supports the movable bed 14 with respect to the floor surface may be on the MRI gantry 101 side or the PET gantry 102 side.

  A technique for superimposing images obtained by the independent MRI apparatus 1 and the PET apparatus 2 on a computer has also been developed. However, the difficult points in this superposition are securing the positioning accuracy of the subject and detecting the organ position on the image, which requires the skill of a medical engineer and the cooperation of the subject. In addition, the burden on the medical technician is large, such as time required for positioning and adjustment at the time of image superposition. In the MRI-PET apparatus, the amount of movement of the bed in the axial direction can be managed by a computer that controls this, and the information can be used to fully automatically overlap the images.

  Whether to acquire the PET image first or the MRI image first depends on the medical selection. In general, since it takes about 20 minutes for 18FDG to accumulate in the biologically active site of the subject, it is also conceivable to use the time during MRI imaging. In this way, it is possible to reduce the inspection time for continuous imaging by MRI-PET as a whole.

  In the MRI-PET apparatus of the present embodiment, as described above, a magnetic material (magnetic shield) is disposed on the gantry portion of the PET apparatus. Thereby, even when the MRI apparatus and the PET apparatus are integrated, the influence of the magnetic field applied from the MRI apparatus to the PET apparatus can be sufficiently reduced. Therefore, it is possible to provide an MRI-PET apparatus that can prevent a malfunction of the PET apparatus. That is, the problem of the operation failure of the PET apparatus due to the strong magnetic field of the MRI apparatus that occurs when the MRI and the PET are integrated can be solved, and the coexistence of the MRI apparatus and the PET apparatus can be obtained.

Further, in the MRI-PET apparatus according to the present embodiment, the MRI apparatus and the PET apparatus are arranged with a predetermined interval (the MRI gantry and the PET gantry are made independent), and the direction of the lines of magnetic force from the MRI apparatus, etc. By arranging the magnetic shield in the PET apparatus in consideration of the above, it is possible to synergistically reduce the influence of the magnetic field on the PET apparatus from the MRI apparatus. In particular, by making the magnetic shield on the surface facing the MRI apparatus thicker than other magnetic shields, and by making the direction of the magnetic field lines of the leakage magnetic field coincide with the direction of the magnetic shield,
Since the leakage magnetic field from the MRI apparatus can be effectively attracted, the effect becomes remarkable.

  Further, in the MRI-PET apparatus according to the present embodiment, since the influence of the magnetic field from the MRI apparatus can be reduced by a simple method such as a magnetic shield, the manufacturability and cost burden can be reduced.

  Next, a second embodiment of the MRI-PET apparatus according to the present invention will be described with reference to FIGS. The MRI-PET apparatus according to the present embodiment relates to an MRI-PET apparatus specialized for head measurement. 7 and 8 show the configuration of the MRI-PET apparatus in this embodiment.

  First, advantages of the MRI-PET apparatus for head measurement will be described below. In general, it is extremely important to create a surgical plan that does not damage normal tissues in brain tumors, and highly accurate information is also required for superimposed images. Here, as described above, MRI is suitable for imaging a tissue having a large difference in hydrogen atom density, and from this, it is evaluated that MRI is superior to X-ray CT for brain images. That is, in brain tissue or the like, there is an advantage that an MRI image is superior in contrast to an X-ray CT image. In addition, in a brain that is wrapped in a skull and hardly affected by body movement due to breathing or the like, there is an advantage that image blurring is small even in MRI and PET having a long imaging time.

Here, by using the MRI-PET apparatus exclusively for head measurement, the MRI apparatus 1 and the PET apparatus 2 can be made smaller than those for whole body measurement. By downsizing the MRI apparatus 1, the inner diameter of the main coil that generates the main magnetic field in the imaging space can be reduced. Generally, if the number of turns is the same, the smaller the inner diameter, the higher the magnetic field strength. Therefore, the resolution of the MRI apparatus 1 can be improved by reducing the inner diameter of the main coil. Moreover, since the coil wire length can be shortened, the amount of expensive superconducting wire used can be reduced. Further, the smaller the coil radius, the smaller the magnetic energy, and the generated energy, generated voltage, and electromagnetic force at the time of quenching are reduced, so the cost on the magnet can be greatly reduced. On the other hand, by reducing the size of the PET apparatus, both the circumferential length and axial length can be made smaller than those for whole body measurement. Therefore, it is possible to reduce the number of detector elements, which is costly, and to reduce the cost. Furthermore, since the distance to the subject is shortened, SN and resolution can be improved.

  However, when two gantry having a small bore diameter are arranged in series, the body part after the subject's shoulder interferes with either gantry. In the MRI-PET apparatus described in the present embodiment, even when a small MRI apparatus such as a head MRI-PET apparatus and the PET apparatus are integrated, interference between the subject and the MRI-PET apparatus is caused. It is an object of the present invention to provide an MRI-PET apparatus that does not have any.

  Hereinafter, the details of the MRI-PET apparatus for head in the present embodiment will be described with reference to FIGS. As shown in FIGS. 7 and 8, the MRI-PET apparatus according to the present embodiment includes a vertical magnetic field type open MRI apparatus 18 having an MRI gantry 101 and a PET apparatus 2 having a substantially cylindrical PET gantry 102. It consists of two devices. Further, the open MRI apparatus 18 and the PET gantry 102 are connected by the connecting portion 3. Considering the movement of the subject 15 and the like, the MRI gantry 101 and the PET apparatus 2 include an opening in the movement direction (longitudinal direction) of the subject 15. Here, FIG. 7 shows a state in which the head is measured by the open MRI apparatus 18, and FIG. 8 shows a state in which the subject is moved in the direction of the PET apparatus 2 and the head is measured by the PET apparatus 2. Is shown. The basic configuration of the MRI-PET apparatus in the present embodiment is the same as that in the first embodiment, and a detailed description thereof will be omitted.

  As described above, in this embodiment, a small vertical magnetic field type open MRI apparatus is used as the MRI-PET apparatus dedicated to the head as compared with that for whole body measurement. An open-type MRI apparatus (or open-type MRI apparatus) is an MRI system in which a gap is provided between upper and lower magnets and a subject enters the gap. In such devices, the magnetic field is often applied vertically. An embodiment of the present invention is to use a vertical magnetic field type open MRI apparatus as a head MRI-PET apparatus. The open MRI apparatus includes a high magnetic field type using a superconducting coil and a medium / low magnetic field type using a permanent magnet. A high magnetic field type is desirable for depicting blood vessels and nerves in the brain, but a medium / low magnetic field type has sufficient performance to describe the morphology of brain tissue.

  In this embodiment, an MRI apparatus having a cantilever yoke 19 is applied as the open MRI apparatus 18. The open-type MRI apparatus 18 having the cantilever yoke 19 has an advantage that the leakage magnetic field is small because most of the magnetic flux passes through the yoke 19.

  Here, the yoke 19 of the open MRI apparatus 18 is disposed on the side of the subject. By disposing the yoke 19 on the side of the subject, the periphery of the subject is opened except for the side where the yoke 19 is disposed. That is, the traveling direction (longitudinal direction) of the subject is released. Therefore, the subject does not interfere with the MRI apparatus even when the subject is moved by disposing the head PET device in the direction of the subject's head with respect to the small open-type MRI apparatus for the head. . Thereby, a test subject's movement can be made easy and the test | inspection by a MRI-PET apparatus can be performed smoothly.

An inspection procedure in the MRI-PET apparatus according to the present embodiment will be described with reference to FIGS. 7 and 8 show a head MRI-PET apparatus using the open MRI apparatus 18 described above. FIG. 7 shows a state in which the subject is being examined by the MRI apparatus 18, and FIG. 8 shows a state in which the subject is being examined by the PET apparatus 2. After acquiring the MRI image by the MRI apparatus 18, the moving bed 14 is moved in the direction of the PET apparatus 2 while holding the subject, and the head of the subject 15 is positioned at a predetermined location inside the PET apparatus 2. Thereafter, a PET image is acquired by the PET apparatus 2. By using the MRI-PET apparatus in the present embodiment, the moving bed 14 is moved to the PET apparatus 2 without the subject 15 interfering with the MRI apparatus 18, and the head of the subject 15 is moved to a predetermined position inside the PET apparatus 2. Can be located in

  Also in the second embodiment, as illustrated in FIG. 9, the semiconductor detector and the signal processing device 5 in the PET apparatus 2 are provided by applying the magnetic shield 6 to the PET apparatus 2 as in the first embodiment. The influence of the leakage magnetic field of the open MRI apparatus 18 can be made difficult.

  The leakage magnetic field of the open MRI apparatus 18 is typically as shown in FIG. That is, a part of the magnetic force lines generated between the two magnetic poles of the open type MRI apparatus 18 leaks to the outside, and becomes the leakage magnetic force lines 7. A part of this tends to enter the inside of the PET apparatus 2, but is attracted by the magnetic shield 6 and returned to the magnetic pole of the open MRI apparatus 18 again. FIG. 9B is a conceptual diagram showing the situation of the surface of the magnetic shield 6-a facing the open MRI apparatus 18 side and the leakage magnetic field lines 7. Since the surface of the magnetic shield 6-a facing the open type MRI apparatus 18 has a hole in the center, the magnetic field lines 7 are guided by the magnetic shield 6-a so as to flow avoiding this, and inside the bore 8 Magnetic field lines 7 hardly flow out.

  Also in the second embodiment described above, the same effects as in the first embodiment can be obtained. Furthermore, in the second embodiment, the subject can be easily moved even with the miniaturized head MRI-PET apparatus.

  That is, unless any investigation is made on the MRI-PET apparatus for the head, the influence of the magnetic field by the MRI apparatus cannot be eliminated, and it is difficult to operate the PET apparatus normally. In addition, since the apparatus is small, it is difficult to smoothly move the subject when the MRI-PET apparatus is integrated. Here, in the MRI-PET apparatus according to the present embodiment, a vertical magnetic field type open MRI apparatus is used, and a magnetic material is disposed on the PET gantry. Accordingly, the influence of the magnetic field from the MRI apparatus to the PET apparatus can be reduced, and even the miniaturized head MRI-PET apparatus can easily move the subject. In particular, by arranging the magnetic material on the side facing the vertical magnetic field type open MRI apparatus, the magnetic material is arranged in the flow direction of the magnetic field, so that the influence of the magnetic field can be effectively reduced.

The figure which shows the structure of a MRI-PET apparatus. The cross-sectional enlarged view of the torus of a PET gantry. The figure which shows the calculation model of the magnetic shield of a PET gantry, and the magnetic force line calculation result near a PET gantry magnetic shield. The figure which shows distribution of the magnetic flux density of a MRI-PET apparatus. The figure which shows the calculation model of the magnetic shield of a PET gantry, and the magnetic flux density calculation result inside a PET gantry magnetic shield. The figure which shows the usage method and operation | movement of a MRI-PET apparatus. The figure which shows the structure of a MRI-PET apparatus. The figure which shows the structure of a MRI-PET apparatus. The figure which shows the structure of a MRI-PET apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... MRI apparatus, 2 ... PET apparatus, 3 ... Connection part, 4 ... γ-ray detector, 5 ... Signal processing apparatus, 6,6-a-6-e ... Magnetic shield, 7 ... Magnetic field line, 8 ... Bore, 9 Γ-ray receiving window, 10 ... main coil, 11 ... shield coil, 12 ... central axis, 13 ... magnetic flux density contour, 14 ... moving bed, 15 ... test subject, 16 ... moving bed base, 17 ... moving bed rail, 18 ... Open MRI apparatus, 19 ... Yoke, 101 ... MRI gantry, 102 ... PET gantry.

Claims (12)

An MRI apparatus having an MRI gantry;
An MRI-PET apparatus comprising a PET apparatus having a PET gantry,
The PET gantry is a substantially cylindrical gantry arranged such that the first end face faces the MRI apparatus,
A magnetic material is disposed on the PET gantry, and the magnetic body includes a first end surface magnetic material disposed on the first end surface;
A second end face magnetic material disposed on a second end face which is an end face opposite to the first end face;
An inner surface magnetic material that binds the first end surface magnetic material and the second end surface magnetic material and is arranged in a substantially cylindrical shape along the inner surface of the PET gantry;
The first end surface magnetic material and the second end surface magnetic material are combined, and an outer surface magnetic material disposed in a substantially cylindrical shape along the PET gantry outer surface,
The MRI-PET apparatus is characterized in that the first end face magnetic material is thicker than the second end face magnetic material, the inner face magnetic material, and the outer face magnetic material. 2. The MRI-PET apparatus according to claim 1 , wherein the first end face magnetic material and the second end face magnetic material have a hole through which a subject passes in the center. 3. The MRI-PET apparatus according to claim 1, wherein the MRI apparatus includes a main coil that generates a main magnetic field in an imaging space of the MRI apparatus, and a shield coil for reducing a leakage magnetic field from the main coil. An MRI-PET apparatus characterized by that. In MRI-PET apparatus according to any one of claims 1 to 3, wherein the first end surface magnetic material is arranged at a position to which the main leakage flux the MRI device generates is substantially radial direction, the first end surface magnetic The MRI-PET apparatus is characterized in that the material is provided with a hole in the center portion to guide the leakage magnetic flux in the radial direction. An MRI apparatus having an MRI gantry;
An MRI-PET apparatus comprising a PET apparatus having a PET gantry,
The PET gantry is a substantially cylindrical gantry arranged such that the first end face faces the MRI apparatus,
A magnetic material is disposed on the PET gantry, and the magnetic body includes a first end surface magnetic material disposed on the first end surface;
A second end face magnetic material disposed on a second end face which is an end face opposite to the first end face;
An inner surface magnetic material that binds the first end surface magnetic material and the second end surface magnetic material and is arranged in a substantially cylindrical shape along the inner surface of the PET gantry;
The first end surface magnetic material and the second end surface magnetic material are combined, and an outer surface magnetic material disposed in a substantially cylindrical shape along the PET gantry outer surface,
The inner surface magnetic material includes a gamma ray receiving window for receiving gamma rays emitted from a subject, and the thickness of the inner surface magnetic material in the gamma ray receiving window is thinner than other portions of the inner surface magnetic material. MRI-PET apparatus. An MRI apparatus having an MRI gantry;
An MRI-PET apparatus comprising a PET apparatus having a PET gantry,
The PET gantry is a substantially cylindrical gantry arranged such that the first end face faces the MRI apparatus,
A magnetic material is disposed on the PET gantry, and the magnetic body includes a first end surface magnetic material disposed on the first end surface;
A second end face magnetic material disposed on a second end face which is an end face opposite to the first end face;
An inner surface magnetic material that binds the first end surface magnetic material and the second end surface magnetic material and is arranged in a substantially cylindrical shape along the inner surface of the PET gantry;
The first end surface magnetic material and the second end surface magnetic material are combined, and an outer surface magnetic material disposed in a substantially cylindrical shape along the PET gantry outer surface,
The MRI-PET apparatus according to claim 1, wherein the inner magnetic material includes an opening which is a gamma ray receiving window for receiving gamma rays emitted from a subject.   The MRI-PET apparatus according to claim 1, wherein the magnetic body is iron.   8. The MRI-PET apparatus according to claim 1, wherein a semiconductor detector is used as a detector of the PET apparatus. The MRI-PET apparatus according to any one of claims 1 to 8,
The MRI apparatus is a horizontal MRI apparatus having a substantially cylindrical MRI gantry,
The MRI-PET apparatus, wherein the MRI gantry and the PET gantry are arranged substantially coaxially. An MRI apparatus having an MRI gantry;
An MRI-PET apparatus comprising a PET apparatus having a PET gantry,
The PET gantry is a substantially cylindrical gantry arranged such that the first end face faces the MRI apparatus,
A magnetic material is disposed on the PET gantry, and the magnetic body includes a first end surface magnetic material disposed on the first end surface,
The MRI apparatus is a vertical magnetic field type open MRI apparatus,
The PET apparatus is an MRI-PET apparatus that is a PET apparatus for head measurement. The MRI-PET apparatus according to claim 10 , wherein a yoke portion of the MRI apparatus is disposed on a side portion of a subject. The MRI-PET apparatus according to any one of claims 1 to 11 ,
A connecting portion for connecting the MRI apparatus and the PET apparatus;
A bed holding the subject;
An MRI-PET apparatus comprising a guide rail for guiding the bed into the MRI gantry and the PET gantry.

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