MAGNET FOR MAGNETIC PARTICLE IGNITION DEVICE

Abstract

A magnet (100, 300, 400) for a magnetic particle imaging device comprises a pair of first DC coils (10), a pair of second DC coils (20), and a pair of AC coils (30). The pair of first DC coils is arranged so that they face each other in a first direction (DR1). The pair of second DC coils is arranged so that they face each other in a second direction (DR2) perpendicular to the first direction, or in a third direction (DR3) perpendicular to both the first and second directions. The pair of AC coils is arranged so that they face each other in either the first or the second direction. Each of the pair of second DC coils comprises a pair of first linear sections (22). Each of the pair of first linear sections extends along a direction identical to that in which the pair of AC coils is arranged so that they face each other.

Description

TECHNICAL AREA The present disclosure relates to a magnet for a magnetic particle imaging device. STATE OF THE ART Japanese patent application no. 2022-126838 (PTL 1) describes a magnet for a magnetic particle imaging (MPI) device. The magnet for a magnetic particle imaging device described in PTL 1 includes a pair of direct current (DC) coils and a pair of alternating current (AC) coils. The pair of DC coils is arranged so that they face each other in a first direction. A direction perpendicular to the first direction is defined as a second direction, and a direction perpendicular to both the first and second directions is defined as a third direction. The pair of DC coils generates a field-free line (FFL) between them. Along the field-free line, one component of the magnetic field along the third direction is zero. The field-free line extends along the third direction. The pair of AC coils is arranged so that they face each other in the first direction. The pair of AC coils moves the position of the field-free line in the first direction. The magnetic field along the third direction is generated in a position where the respective end sections of the pair of DC coils face each other in the third direction. Due to the generation of such a magnetic field, the length of the field-free line in the third direction is short. To increase the length of the field-free line in the third direction, a correction ring coil is used in the magnet described in PTL 1 for a magnetic particle imaging device. LIST OF EDITED ITEMS PATENT LITERATURE PTL 1: Japanese Disclosure Document No. 2022-126838 BRIEF SUMMARY OF THE INVENTION TECHNICAL PROBLEM If the position of the field-free line in the first direction is moved by the pair of AC coils in the magnet described in PTL 1 for a magnetic particle imaging device, the correction by the correction ring coil becomes insufficient and the length of the field-free line in the third direction becomes small. Consequently, in a magnetic particle imaging device using the magnet described in PTL 1, a terminal portion of an image of observed magnetic particles may be interrupted. The present disclosure was prepared in light of the aforementioned problems in the prior art. In particular, the present disclosure provides a magnet for a magnetic particle imaging device which enables it to prevent a field-free line from shortening when the field-free line is moved. SOLUTION TO THE PROBLEM A magnet for a magnetic particle imaging device according to the present disclosure comprises a pair of first DC coils, a pair of second DC coils, and a pair of AC coils. The pair of first DC coils is arranged such that they face each other in a first direction. The pair of second DC coils is arranged such that they face each other in a second direction perpendicular to the first direction, or in a third direction perpendicular to both the first and second directions. The pair of AC coils is arranged such that they face each other in either the first or the second direction. Each of the pair of second DC coils comprises a pair of first linear sections. Each of the pair of first linear sections extends in a direction identical to that in which the pair of AC coils is arranged to face each other. ADVANTAGEOUS EFFECTS OF THE INVENTION The magnet for a magnetic particle imaging device according to the present disclosure makes it possible to prevent a field-free line from shortening when the field-free line is moved. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a magnet 100 for a magnetic particle imaging device. Fig. 2 is a schematic first cross-sectional view of the magnet 100 for a magnetic particle imaging device. Fig. 3 is a schematic second cross-sectional view of the magnet 100 for a magnetic particle imaging device. Fig. 4 is a schematic third cross-sectional view of the magnet 100 for a magnetic particle imaging device. Fig. 5 is a perspective view of a magnet 200 for a magnetic particle imaging device. Fig. 6 is a schematic first cross-sectional view of the magnet 200 for a magnetic particle imaging device. Fig. 7 is a schematic second cross-sectional view of the magnet 200 for a magnetic particle imaging device. Fig. 8 is a schematic third cross-sectional view of the magnet 200 for a magnetic particle imaging device.Figure 9 is a diagram illustrating the relationship between a position in a first direction DR1 and each of a magnetic field 14, a magnetic field 23, and a magnetic field 51. Figure 10 is a perspective view of a magnet 300 for a magnetic particle imaging device. Figure 11 is a side view of a magnet 400 for a magnetic particle imaging device. Figure 12 is a top view of the magnet 400 for a magnetic particle imaging device. DESCRIPTION OF EXECUTION FORMS First embodiment. A magnet for a magnetic particle imaging device according to a first embodiment is described. The magnet for a magnetic particle imaging device according to the first embodiment is referred to as magnet 100 for a magnetic particle imaging device. (Configuration of magnet 100 for a magnetic particle imaging device) A configuration of the magnet 100 for a magnetic particle imaging device is described below. Fig. 1 is a perspective view of the magnet 100 for a magnetic particle imaging device. As illustrated in Fig. 1, the magnet 100 for a magnetic particle imaging device includes a pair of first DC coils 10, a pair of second DC coils 20, and a pair of AC coils 30. One of the pair of first DC coils 10 can be designated as first DC coil 10a and the other of the pair of first DC coils 10 can be designated as first DC coil 10b. One of the pair of second DC coils 20 can be designated as second DC coil 20a and the other of the pair of second DC coils 20 can be designated as second DC coil 20b. Furthermore, one of the pair of AC coils 30 can be designated as AC coil 30a and the other of the pair of AC coils 30 can be designated as AC coil 30b. The pair of first DC coils 10 is arranged such that they face each other in a first direction DR1, with a space left between them. That is, the first direction DR1 is a direction in which the pair of first DC coils 10 are arranged so that they face each other. A second direction DR2 is a direction perpendicular to the first direction DR1, and a third direction DR3 is a direction perpendicular to both the first direction DR1 and the second direction DR2. The first DC coil 10 contains a pair of linear sections 11 and a pair of linear sections 12. The pair of linear sections 11 is arranged so that they face each other in the second direction DR2, with a space between them. The linear section 11 extends along the third direction DR3. One of the pair of linear sections 11 can be called linear section 11a and the other of the pair of linear sections 11 can be called linear section 11b. The pair of linear sections 12 is arranged so that they face each other in the third direction DR3, with a space between them. The linear section 12 extends along the second direction DR2. One of the pair of linear sections 12 can be called linear section 12a and the other of the pair of linear sections 12 can be called linear section 12b. Linear section 11a and linear section 11b of the first DC coil 10a are arranged such that they face linear section 11a and linear section 11b of the first DC coil 10b, respectively, leaving a space between them. Linear section 12a and linear section 12b of the first DC coil 10a are arranged such that they face linear section 12a and linear section 12b of the first DC coil 10b, respectively, leaving a space between them. The direction of the current flowing through the first DC coil 10a is opposite to the direction of the current flowing through the first DC coil 10b. That is, the direction of the current flowing through the linear section 11a of the first DC coil 10a is opposite to the direction of the current flowing through the linear section 11a of the first DC coil 10b, the direction of the current flowing through the linear section 11b of the first DC coil 10a is opposite to the direction of the current flowing through the linear section 11b of the first DC coil 10b, the direction of the current flowing through the linear section 12a of the first DC coil 10a is opposite to the direction of the current flowing through the linear section 12a of the first DC coil 10b, and the direction of the current flowing through the linear section 12b of the first DC coil 10a is opposite to the direction of the current flowing through the linear section 12b of the first DC coil 10b.The current flowing through the first DC coil 10 is supplied by a DC power supply, which is not illustrated. Fig. 2 is a schematic first cross-sectional view of the magnet 100 for a magnetic particle imaging device. In Fig. 2, the pair of second DC coils 20 and the pair of AC coils 30 are not shown, and a generated magnetic field is indicated by a dashed line. Fig. 2 illustrates a cross-section perpendicular to the second direction DR2. A magnetic field is generated between the pair of first DC coils 10 due to the current directions described above, as illustrated in Fig. 2. In particular, the pair of first DC coils 10 generates a field-free line 13 between the pair of first DC coils 10. At the field-free line 13, a component of the magnetic field along the third direction DR3 is zero. The field-free line 13 extends in the third direction DR3.When no current flows through the pair of AC coils 30, the field-free line 13 is located at a midpoint MP of the pair of first DC coils 10 in the first direction DR1. Furthermore, the pair of first DC coils 10 generates a magnetic field 14 in a position where one end section of the first DC coil 10a and one end section of the first DC coil 10b face each other in the third direction DR3, and in a position where the other end section of the first DC coil 10a and the other end section of the first DC coil 10b face each other in the third direction DR3. The direction of the magnetic field 14 is along the third direction DR3. The first DC coil 10 has a first dimension and a second dimension. The first dimension is a dimension of the first DC coil 10 in the second direction DR2. The second dimension is a dimension of the first DC coil 10 in the third direction DR3. Preferably, the second dimension is larger than the first dimension. That is, the longitudinal direction of the first DC coil 10 preferably runs along the third direction DR3. From another perspective, the length of the linear section 11 is preferably greater than the length of the linear section 12. As illustrated in Fig. 1, the pair of second DC coils 20 is arranged, for example, such that they face each other in the third direction DR3, with a space left between them. One of the pair of second DC coils 20 can be called second DC coil 20a and the other of the pair of second DC coils 20 can be called second DC coil 20b. The second DC coil 20 comprises a pair of linear sections 21 and a pair of linear sections 22. The pair of linear sections 21 is arranged so that they face each other in the first direction DR1, with a space between them. The linear section 21 extends along the second direction DR2. One of the pair of linear sections 21 can be called linear section 21a and the other of the pair of linear sections 21 can be called linear section 21b. The pair of linear sections 22 is arranged so that they face each other in the second direction DR2, with a space between them. The linear section 22 extends along the first direction DR1. One of the pair of linear sections 22 can be called linear section 22a and the other of the pair of linear sections 22 can be called linear section 22b.For example, the shape and ampere-turns of the second DC coil 20 differ from the shape and ampere-turns of the first DC coil 10. The ampere-turns are the product of the coil current per turn and the number of turns. Linear section 21a and linear section 21b of the second DC coil 20a are arranged such that they face linear section 21a and linear section 21b of the second DC coil 20b, respectively, with a space between them. Linear section 22a and linear section 22b of the second DC coil 20a are arranged such that they face linear section 22a and linear section 22b of the second DC coil 20b, respectively, with a space between them. Linear section 21a is closer to the first DC coil 10a in the first direction DR1 than linear section 21b. The direction of the current flowing through the second DC coil 20a is opposite to the direction of the current flowing through the second DC coil 20b.That is, the direction of the current flowing through the linear section 21a of the second DC coil 20a is opposite to the direction of the current flowing through the linear section 21a of the second DC coil 20b, the direction of the current flowing through the linear section 21b of the second DC coil 20a is opposite to the direction of the current flowing through the linear section 21b of the second DC coil 20b, the direction of the current flowing through the linear section 22a of the second DC coil 20a is opposite to the direction of the current flowing through the linear section 12a of the second DC coil 20b, and the direction of the current flowing through the linear section 22b of the second DC coil 20a is opposite to the direction of the current flowing through the linear section 22b of the second DC coil 20b. The current flowing through the second DC coil 20 is supplied by the DC power supply, which is not illustrated. The second DC coil 20 has a third dimension and a fourth dimension. The third dimension is a dimension of the second DC coil 20 in the first direction DR1. The fourth dimension is a dimension of the second DC coil 20 in the second direction DR2. Preferably, the third dimension is larger than the fourth dimension. That is, the longitudinal direction of the second DC coil 20 preferably runs along the first direction DR1. From another perspective, the length of the linear section 22 is preferably greater than the length of the linear section 21. Fig. 3 is a schematic second cross-sectional view of the magnet 100 for a magnetic particle imaging device. In Fig. 3, the pair of first DC coils 10 and the pair of AC coils 30 are not illustrated, and a generated magnetic field is indicated by a dashed line. Fig. 3 illustrates a cross-section perpendicular to the first direction DR1. As illustrated in Fig. 3, the pair of second DC coils 20 generates a magnetic field 23 in a position where one end section of the first DC coil 10a faces the third direction DR3 and one end section of the first DC coil 10b faces each other in the third direction DR3, and in a position where the other end section of the first DC coil 10a faces the third direction DR3 and the other end section of the first DC coil 10b faces each other in the third direction DR3.The direction of the magnetic field 23 runs along the third direction DR3 and is opposite to the direction of the magnetic field 14. From another perspective, the direction of the current flowing through the linear section 21a of the second DC coil 20a is opposite to the direction of the current flowing through the linear section 12a of the first DC coil 10a, the direction of the current flowing through the linear section 21b of the second DC coil 20a is opposite to the direction of the current flowing through the linear section 12a of the first DC coil 10b, the direction of the current flowing through the linear section 21a of the second DC coil 20b is opposite to the direction of the current flowing through the linear section 12b of the first DC coil 10a, and the direction of the current flowing through the linear section 21b of the second DC coil 20b is opposite to the direction of the current flowing through the linear section 12b of the first DC coil 10b. As illustrated in Fig. 1, the pair of AC coils 30 is arranged so that they face each other in the first direction DR1, with a space between them. One of the pair of AC coils 30 can be called AC coil 30a and the other of the pair of AC coils 30 can be called AC coil 30b. The pair of AC coils 30 is arranged between the pair of first DC coils 10. The AC coil 30a is closer to the first DC coil 10a than the AC coil 30b. The AC coil 30 comprises a pair of linear sections 31 and a pair of linear sections 32. The pair of linear sections 31 is arranged so that they face each other in the second direction DR2, with a space between them. The linear section 31 extends along the third direction DR3. One of the pair of linear sections 31 can be designated as linear section 31a and the other as linear section 31b. The pair of linear sections 32 is arranged so that they face each other in the third direction DR3, with a space between them. The linear section 32 extends along the second direction DR2. One of the pair of linear sections 32 can be designated as linear section 32a and the other as linear section 32b. Linear section 31a and linear section 31b of AC coil 30a are arranged such that they face linear section 31a and linear section 31b of AC coil 30b, respectively, leaving a space between them. Furthermore, linear section 32a and linear section 32b of AC coil 30a are arranged such that they face linear section 32a and linear section 32b of AC coil 30b, respectively, leaving a space between them. The direction of the current flowing through AC coil 30a is identical to the direction of the current flowing through AC coil 30b. That is, the direction of the current flowing through the linear section 31a of the AC coil 30a is identical to the direction of the current flowing through the linear section 31a of the AC coil 30b, the direction of the current flowing through the linear section 31b of the AC coil 30a is identical to the direction of the current flowing through the linear section 31b of the AC coil 30b, the direction of the current flowing through the linear section 32a of the AC coil 30a is identical to the direction of the current flowing through the linear section 32a of the AC coil 30b, and the direction of the current flowing through the linear section 32b of the AC coil 30a is identical to the direction of the current flowing through the linear section 32b of the AC coil 30b.The current flowing through the DC coil 30 is supplied by an AC power supply, which is not illustrated. A current flowing through the pair of AC coils 30 moves the field-free line 13 along the first direction DR1. Fig. 4 is a schematic third cross-sectional view of the magnet 100 for a magnetic particle imaging device. The pair of second DC coils 20 is not shown in Fig. 4. Fig. 4 illustrates a cross-section perpendicular to the second direction DR2. As illustrated in Fig. 4, the magnet 100 for a magnetic particle imaging device may further include an iron core 40. <Variante 1> Although the above example describes how the pair of AC coils 30 is arranged so that they face each other in the first direction DR1, the pair of AC coils 30 can also be arranged so that they face each other in the second direction DR2. In this case, a current flowing through the pair of AC coils 30 moves the field-free line 13 along the second direction DR2. <Variante 2> Although the above example describes how the first DC coil 10 includes both the pair of linear sections 11 and the pair of linear sections 12, it is not strictly necessary for the first DC coil 10 to include the linear sections. That is, the first DC coil 10 can be a toroidal coil. Alternatively, the first DC coil 10 can include only one of the pair of linear sections 11 and one of the pair of linear sections 12. <Variante 3> Although an example is described above in which the second DC coil 20 includes both the pair of linear sections 21 and the pair of linear sections 22, it is possible that the second DC coil 20 includes only the pair of linear sections 22, and it is not necessary that it includes the pair of linear sections 21. If the pair of AC coils 30 is arranged such that they face each other in the second direction DR2, as in variant 1, it is possible that the second DC coil 20 includes only the pair of linear sections 21, and it is not necessary that it includes the pair of linear sections 22. (Effects of magnet 100 for a magnetic particle imaging device) The effects of magnet 100 for a magnetic particle imaging device are described below in comparison to a magnet for a magnetic particle imaging device according to a comparative example. The magnet for a magnetic particle imaging device according to the comparative example is referred to as magnet 200 for a magnetic particle imaging device. Fig. 5 is a perspective view of magnet 200 for a magnetic particle imaging device. As illustrated in Fig. 5, magnet 200 for a magnetic particle imaging device incorporates a pair of DC ring coils 50 instead of the pair of second DC coils 20. Like the pair of second DC coils 20, the pair of DC ring coils 50 is arranged so that they face each other in the second direction DR2, with a space between them. Apart from this, the configuration of magnet 200 for a magnetic particle imaging device is the same as the configuration of magnet 100 for a magnetic particle imaging device. Fig. 6 is a schematic first cross-sectional view of the magnet 200 for a magnetic particle imaging device. In Fig. 6, the pair of first DC coils 10 and the pair of AC coils 30 are not illustrated, and a generated magnetic field is indicated by a dashed line. Fig. 6 illustrates a cross-section extending through the center point MP and perpendicular to the first direction DR1. As illustrated in Fig. 6, in the magnet 200 for a magnetic particle imaging device, the pair of DC ring coils 50 generates a magnetic field 51 in a position in which one end section of the first DC coil 10a in the third direction DR3 and one end section of the first DC coil 10b in the third direction DR3 are facing each other, and in a position in which the other end section of the first DC coil 10a in the third direction DR3 and the other end section of the first DC coil 10b in the third direction DR3 are facing each other.The direction of the magnetic field 51 runs along the third direction DR3 and is opposite to the direction of the magnetic field 14. Fig. 7 is a schematic second cross-sectional view of the magnet 200 for a magnetic particle imaging device. In Fig. 7, the pair of AC coils 30 and the pair of DC ring coils 50 are not illustrated, and a generated magnetic field is indicated by a dashed line. Fig. 7 illustrates a cross-section perpendicular to the second direction DR2. As illustrated in Fig. 7, in the magnet 200 for a magnetic particle imaging device, the field-free line 13 is extended along the third direction DR3 (see the dashed lines) by the magnetic field 51 suppressing the magnetic field 14. Fig. 8 is a schematic third cross-sectional view of the magnet 200 for a magnetic particle imaging device. In Fig. 8, the pair of first DC coils 10 and the pair of AC coils 30 are not illustrated, and a generated magnetic field is indicated by a dashed line. Fig. 8 illustrates a cross-section extending through a position that deviates from the center point MP and is perpendicular to the first direction DR1. As illustrated in Fig. 8, the magnetic field 51 shifts along the first direction DR1 because the DC ring coil 50 is annular. When the pair of AC coils 30 is energized and the position of the field-free line 13 moves along the first direction DR1, the magnetic field 14 cannot be sufficiently suppressed by the magnetic field 51, and the length of the field-free line 13 shortens in the third direction DR3. On the other hand, the second DC coil 20 in the magnet 100 for a magnetic particle imaging device includes the pair of linear sections 22. The pair of linear sections 22 extends along the first direction DR1, that is, along the direction in which the pair of AC coils 30 moves the field-free line 13 (the direction in which the pair of AC coils 30 face each other). Thus, in the second DC coil 20, the displacement of the magnetic field 23 along the first direction DR1 is smaller than the displacement of the magnetic field 51 along the first direction DR1. Consequently, in the magnet 100 for a magnetic particle imaging device, even when the pair of AC coils 30 is energized and the position of the field-free line 13 moves along the first direction DR1, the magnetic field 14 can be sufficiently suppressed by the magnetic field 23, and the length of the field-free line 13 in the third direction DR3 can be maintained. Fig. 9 is a diagram illustrating the relationship between a position in the first direction DR1 and each of the magnetic fields 14, 23, and 51. The horizontal axis in Fig. 9 indicates the position in the first direction DR1. A value (X) on the horizontal axis in Fig. 9, expressed in m, is given as a positive value when the position is close to the first DC coil 10b with respect to the center point MP, and as a negative value when the position is close to the first DC coil 10a with respect to the center point MP. The vertical axis in Fig. 9 indicates the respective strengths of the magnetic fields 14, 23, and 51, which are normalized. This normalization is performed by defining the strength of the magnetic field obtained when X = 0 m as 1. As illustrated in Fig. 9, the magnetic field 51 weakens more abruptly compared to the magnetic field 23 when the absolute value of X, that is, the distance from the center point MP in the first direction DR1, increases. Thus, when X = 0.1 m for magnet 200 in a magnetic particle imaging device, the magnetic field 51 may not sufficiently suppress the magnetic field 14, and the length of the field-free line 13 in the third direction DR3 may decrease. On the other hand, the degree of attenuation of the magnetic field 23 in relation to the distance from the center point MP in the first direction DR1 is more gradual compared to the magnetic field 51. Thus, when X = 0.1 m at the magnet 100 for a magnetic particle imaging device, if the imaging area lies within the span of X = ±0.1 m, the magnetic field 51 can sufficiently suppress the magnetic field 14 and the length of the field-free line 13 in the third direction DR3 can be maintained.If X = 0.1 m, it is preferred that the pair of second DC coils 20 generates the magnetic field 23 such that the magnetic flux density along the third direction DR3 in the position where one end section of the first DC coil 10a in the third direction DR3 and one end section of the first DC coil 10b in the third direction DR3 are facing each other (the position where the other end section of the first DC coil 10a in the third direction DR3 and the other end section of the first DC coil 10b in the third direction DR3 are facing each other) is 10 Gauss or less or 1 Gauss or less.Furthermore, it is preferred that, when X = 0.1 m, the length of a region in the third direction DR3, in which the magnetic flux density along the third direction DR3 is 10 gauss or less (or 1 gauss or less), that is, a region which can be considered essentially as the field-free line 13, is 0.3 times or more or 0.5 times or more of the first dimension. Such a region serves as a region in which an image of the magnetic particles can be acquired. The current flowing through the first DC coil 10 and the current flowing through the second DC coil 20 are preferably determined as described below. G represents the magnitude of the gradient magnetic field generated by the first DC coil 10, C1 represents the gradient magnetic field per unit current generated by the first DC coil 10, C2 represents the gradient magnetic field per unit current generated by the second DC coil 20, I1 represents the current flowing through the first DC coil 10, I2 represents the current flowing through the second DC coil 20, Bz1 represents the magnetic field 14 per unit current, and Bz2 represents the magnetic field 23 per unit current. In the case of magnet 100 for a magnetic particle imaging device, the second DC coil 20 does not generate a gradient magnetic field and C2 is therefore 0. Thus, C1 × I1 = G (expression 1) results in G, C1, and I1. Furthermore, a relational expression, Bz1 × I1 + Bz2 × I2 = 0 (expression 2), results in I1, I2, Bz1, and Bz2.If desired, G (which is normally approximately 1 T / m) is determined, I1 is determined according to I1 = G / C1 using expression 1, and the first term in expression 2 is also determined. Thus, I2 is determined as I2 = Bz1 × I1 / Bz2. In the case of the magnet 100 for a magnetic particle imaging device, the second dimension is larger than the first dimension (the linear section 11 is longer than the linear section 12), and thus the field-free line 13 can be extended along the third direction DR3. Second embodiment. A magnet for a magnetic particle imaging device according to a second embodiment is described. The magnet for a magnetic particle imaging device according to the second embodiment is referred to as Magnet 300 for a magnetic particle imaging device. Here, mainly differences from Magnet 100 for a magnetic particle imaging device are described, and overlapping descriptions are not repeated. (Configuration of magnet 300 for a magnetic particle imaging device) A configuration of the magnet 300 for a magnetic particle imaging device is described below. Fig. 10 is a perspective view of the magnet 300 for a magnetic particle imaging device. As illustrated in Fig. 10, the magnet 300 for a magnetic particle imaging device includes a pair of first DC coils 10, a pair of second DC coils 20, and a pair of AC coils 30. In this respect, the configuration of the magnet 300 for a magnetic particle imaging device is the same as the configuration of the magnet 100 for a magnetic particle imaging device. In the magnet 300 for a magnetic particle imaging device, the pair of second DC coils 20 are arranged such that they face each other in a second direction DR2, with a space between them. In the magnet 300 for a magnetic particle imaging device, each of a pair of linear sections 21 extends along a third direction DR3. If the shape of the first DC coil 10 is identical to the shape of the second DC coil 20, then the ampere-turn of the second DC coil 20 is identical to the ampere-turn of the first DC coil 10. In this respect, the configuration of the magnet 300 for a magnetic particle imaging device differs from the configuration of the magnet 100 for a magnetic particle imaging device.In the case of magnet 100 for a magnetic particle imaging device, the second DC coil 20 does not generate a gradient magnetic field, but in the case of magnet 300 for a magnetic particle imaging device, the second DC coil 20 generates a gradient magnetic field. In the case of the magnet 300 for a magnetic particle imaging device, a magnetic field 23 also has a direction opposite to a magnetic field 14. Thus, the direction of the current flowing through a linear section 21a of a second DC coil 20a is identical to the direction of the current flowing through a linear section 11a of a first DC coil 10a, the direction of the current flowing through a linear section 21b of the second DC coil 20a is identical to the direction of the current flowing through the linear section 11a of a first DC coil 10b, the direction of the current flowing through the linear section 21a of a second DC coil 20b is identical to the direction of the current flowing through a linear section 11b of the first DC coil 10a, and the direction of the current flowing through the linear section 21b of the second DC coil 20b is identical to the direction of the current flowing through the linear section 11b of the first DC coil 10b. In the magnet 300 for a magnetic particle imaging device, the shape of the first DC coil 10 can differ from the shape of the second DC coil 20, for example, because a different AC coil is arranged. In this case, the magnitude of the gradient magnetic field generated by the first DC coil 10 and the magnitude of the gradient magnetic field generated by the second DC coil 20 differ from each other, and the magnitudes of magnetic fields 14 and 23 also differ from each other. Thus, the current flowing through the first DC coil 10 and the current flowing through the second DC coil 20 are preferably determined as described below. G represents the magnitude of the gradient magnetic field generated by the first DC coil 10, C1 represents the gradient magnetic field per unit current generated by the first DC coil 10, C2 represents the gradient magnetic field per unit current generated by the second DC coil 20, I1 represents the current flowing through the first DC coil 10, I2 represents the current flowing through the second DC coil 20, Bz1 represents the magnetic field strength 14 per unit current, and Bz2 represents the magnetic field strength 23 per unit current. These parameters result in relational expressions: C1 × I1 + C2 × I2 = G (expression 1) and Bz1 × I1 + Bz2 × I2 = 0 (expression 2). If desired, G (which is normally approximately 1 T / m) can be determined, and the current flowing through the first DC coil 10 (i.e. I1) and the current flowing through the second DC coil 20 (i.e. I2) can be obtained by solving the simultaneous binary linear equations consisting of expressions 1 and 2. (Effects of the 300 magnet for a magnetic particle imaging device) The effects of the 300 magnet for a magnetic particle imaging device are described below. In the case of the magnet 300 for a magnetic particle imaging device, the magnetic field 23 is also generated by the pair of second DC coils 20 to suppress the magnetic field 14, and the displacement of the magnetic field 23 along the first direction DR1 is small, and thus the length of the field-free line 13 in the third direction DR3 can be maintained even when the pair of AC coils 30 moves the field-free line 13 along the first direction DR1. Third embodiment. A magnet for a magnetic particle imaging device according to a third embodiment is described. The magnet for a magnetic particle imaging device according to the second embodiment is referred to as Magnet 400 for a magnetic particle imaging device. Here, mainly differences from Magnet 300 for a magnetic particle imaging device are described, and overlapping descriptions are not repeated. (Configuration of magnet 400 for a magnetic particle imaging device) A configuration of the magnet 400 for a magnetic particle imaging device is described below. Fig. 11 is a side view of the magnet 400 for a magnetic particle imaging device. Fig. 11 illustrates a side surface viewed along a third direction DR3. Fig. 12 is a top view of the magnet 400 for a magnetic particle imaging device. As illustrated in Figs. 11 and 12, the magnet 400 for a magnetic particle imaging device includes a pair of first DC coils 10, a pair of second DC coils 20, and a pair of AC coils 30. In this respect, the configuration of the magnet 400 for a magnetic particle imaging device is the same as the configuration of the magnet 300 for a magnetic particle imaging device. In the magnet 400 for a magnetic particle imaging device, the second DC coil 20 is divided into at least two split DC coils, and the AC coil 30 is divided into at least two split AC coils. In the example illustrated in Fig. 11, the second DC coil 20 is divided into a first split DC coil 24 and a second split DC coil 25, and the AC coil 30 is divided into a first split AC coil 33 and a second split AC coil 34. The first split DC coil 24 of a second DC coil 20a and the first split DC coil 24 of a second DC coil 20b are arranged such that they face each other in a second direction DR2, with a space between them. The first split DC coil 24 of the second DC coil 20a and the first split DC coil 24 of the second DC coil 20b are arranged between the second split DC coil 25 of the second DC coil 20a and the second split DC coil 25 of the second DC coil 20b. The dimension of the second split DC coil 25 in a first direction DR1 is larger than the dimension of the first split DC coil 24 in the first direction DR1. The first split AC coil 33 of AC coil 30a and the first split AC coil 33 of AC coil 30b are arranged such that they face each other in the first direction DR1, with a space between them. The first DC coil 10 is arranged between the first split AC coil 33 and the second split AC coil 34. From another perspective, the first split AC coil 33 is located inside the first DC coil 10, and the second split AC coil 34 is located outside the first DC coil 10. The first split DC coil 24 is located between the first split AC coil 33 of AC coil 30a and the first split AC coil 33 of AC coil 30b. The coil width of the second split AC coil 34 is preferably larger than the coil width of the first split AC coil 33. In this respect, the configuration of magnet 400 for a magnetic particle imaging device differs from the configuration of magnet 300 for a magnetic particle imaging device. Like the second DC coil 20, each of the first split DC coil 24 and the second split DC coil 25 includes a pair of linear sections arranged such that they face each other in the first direction DR1, leaving a space between them, and extend along the third direction DR3, and a pair of other linear sections arranged such that they face each other in the third direction DR3, leaving a space between them, and extend along the first direction DR1.Just like the AC coil 30, each of the first split AC coil 33 and the second split AC coil 34 includes a pair of linear sections arranged so that they face each other in the second direction DR2, leaving a space between them, and extend along the third direction DR3, and a pair of other linear sections arranged so that they face each other in the third direction DR3, leaving a space between them, and extend along the second direction DR2. (Effects of the 400 magnet for a magnetic particle imaging device) The effects of the 400 magnet for a magnetic particle imaging device are described below. If, for example, the AC coil 30 is located outside the first DC coil 10, the distance between a center point MP and the AC coil 30 increases in the first direction DR1, and consequently, a large ampere-turn is required for the AC coil 30. Conversely, if, for example, the AC coil is located inside the first DC coil 10, the distance between the center point MP and the first DC coil 10 increases in the first direction DR1, and consequently, a large ampere-turn is required for the first DC coil 10. This means that the ampere-turn of either the first DC coil 10 or the AC coil 30 can be extremely large. In the magnet 400 for a magnetic particle imaging device, it is easier to adjust the respective ampere-turns of the first DC coil 10 and the AC coil 30. In particular, in the magnet 400 for a magnetic particle imaging device, the first split AC coil 33 is located near the center point MP, and thus a large magnetic field can be generated even if the ampere-turn of the first split AC coil 33 is small, and the sum of the ampere-turn of the first split AC coil 33 and the ampere-turn of the second split AC coil 34 can be reduced. Furthermore, in the magnet 400 for a magnetic particle imaging device, the first DC coil 10 is located between the first split AC coil 33 and the second split AC coil 34. This makes it possible to prevent an increase in the distance between the first DC coil 10 and the center point MP, that is, to prevent an increase in the ampere-turn rating of the first DC coil 10. As described above, in the magnet 400 for a magnetic particle imaging device, it is easier to adjust the respective ampere-turn ratings of the first DC coil 10 and the AC coil 30 so that the ampere-turn rating of either the first DC coil 10 or the AC coil 30 does not become extremely large. Although an example where the AC coil 30 is divided is shown in the preceding description, the first DC coil 10 can be divided without dividing the AC coil 30, and the AC coil 30 can be arranged between the divided DC coils that form the first DC coil 10. Furthermore, an example is shown in the preceding description where the pair of AC coils 30 is arranged so that they face each other in the first direction DR1. However, if the pair of AC coils 30 is arranged so that they face each other in the second direction DR2, the second DC coil 20 can be arranged between the first divided AC coil 33 and the second divided AC coil 34. [Attachments] The embodiments described above include the following configurations. <Anhang 1> Magnet for a magnetic particle imaging device, the magnet comprising: a pair of first DC coils; a pair of second DC coils; and a pair of AC coils, wherein the pair of first DC coils is arranged to face each other in a first direction, the pair of second DC coils is arranged to face each other in a second direction perpendicular to the first direction or in a third direction perpendicular to the first and second directions, the pair of AC coils is arranged to face each other in the first direction or the second direction, each of the pair of second DC coils comprising a pair of first linear sections, and each of the pair of first linear sections extending along a direction identical to a direction in which the pair of AC coils is arranged to face each other. <Anhang 2> Magnet for a magnetic particle imaging device according to Annex 1, wherein each of the pair of first DC coils includes a pair of second linear sections, each of the pair of second linear sections extends along the third direction, and each of the pair of first DC coils has a first dimension which is a dimension in the second direction and a second dimension which is a dimension in the third direction and is larger than the first dimension. <Anhang 3> Magnet for a magnetic particle imaging device according to Annex 1, wherein each of the pair of first DC coils has a shape and an ampere turn that are identical to a shape and an ampere turn respectively of each of the pair of second DC coils. <Anhang 4> Magnet for a magnetic particle imaging device according to Annex 1, wherein each of the pair of first DC coils has a shape and an ampere turn that differs from a shape and an ampere turn of each of the pair of second DC coils. <Anhang 5> Magnet for a magnetic particle imaging device according to one of Annexes 1 to 4, wherein the pair of first DC coils is configured to generate a first magnetic field along the third direction in a cross-section perpendicular to the direction in which the pair of AC coils is arranged so that they face each other, in a position in which respective end sections of the pair of first DC coils face each other in the third direction, and the pair of second DC coils is configured to generate a second magnetic field having a direction opposite to the first magnetic field in the cross-section in such a position that a magnetic flux density in one direction along the third direction in the position is 10 gauss or less. <Anhang 6> Magnet for a magnetic particle imaging device according to Annex 5, wherein in the cross-section a length in the third direction of a region in which the magnetic flux density in the direction along the third direction is 10 gauss or less is 0.3 times or more a width of each of the pair of first DC coils in the third direction. <Anhang 7> Magnet for a magnetic particle imaging device according to any one of Annexes 1 to 6, wherein each of the pair of AC coils comprises at least one first split AC coil and one second split AC coil, the first split AC coil of one of the pair of AC coils and the first split AC coil of the other of the pair of AC coils are arranged such that they face each other in the first direction and are located between the pair of first DC coils, one of the pair of first DC coils is located between the first split AC coil of one of the pair of AC coils and the second split AC coil of one of the pair of AC coils, and the other of the pair of first DC coils is located between the first split AC coil of the other of the pair of AC coils and the second split AC coil of the other of the pair of AC coils. <Anhang 8> Magnet for a magnetic particle imaging device according to Annex 7, wherein a coil width of the second split AC coil is larger than a coil width of the first split AC coil. <Anhang 9> Magnet for a magnetic particle imaging device according to Annex 7 or 8, wherein each of the pair of second DC coils comprises at least one first split DC coil and one second split DC coil, the second split DC coil of one of the pair of second DC coils and the second split DC coil of the other of the pair of second DC coils being arranged facing each other, the first split DC coil of one of the pair of second DC coils and the first split DC coil of the other of the pair of second DC coils being inserted between them, and the first split DC coil of one of the pair of second DC coils and the first split DC coil of the other of the pair of second DC coils being located between the first split AC coil of one of the pair of AC coils and the first split AC coil of the other of the pair of AC coils. It is understood that the embodiments disclosed herein are presented in every respect as illustrations and examples and are not intended to be limiting. The fundamental scope of this disclosure is defined by the claims and not by the embodiments described above, and is intended to include all modifications within the meaning and scope that are equivalent to the claims. REFERENCE MARK LIST 10, 10a, 10b first DC coil; 11, 11a, 11b, 12, 12a, 12b linear section; 13 field-free line; 14 magnetic field; 20, 20a, 20b second DC coil; 21, 21a, 21b, 22, 22a, 22b linear section; 23 magnetic field; 24 first split DC coil; 25 second split DC coil; 30, 30a, 30b AC coil; 31, 31a, 31b, 32, 32a, 32b linear section; 33 first split AC coil; 34 second split AC coil; 40 iron core; 50 DC toroidal coil; 51 magnetic field; 100, 200, 300, 400 Magnet for a magnetic particle imaging device; DR1 first direction; DR2 second direction; DR3 third direction; MP center. QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature JP 2022-126838 [0002, 0005]

Claims

Magnet for a magnetic particle imaging device, the magnet comprising: a pair of first DC coils; a pair of second DC coils; and a pair of AC coils, wherein the pair of first DC coils is arranged to face each other in a first direction, the pair of second DC coils is arranged to face each other in a second direction perpendicular to the first direction or in a third direction perpendicular to the first and second directions, the pair of AC coils is arranged to face each other in the first direction or the second direction, each of the pair of second DC coils comprising a pair of first linear sections, and each of the pair of first linear sections extending along a direction identical to a direction in which the pair of AC coils is arranged to face each other. Magnet for a magnetic particle imaging device according to claim 1, wherein each of the pair of first DC coils includes a pair of second linear sections, each of the pair of second linear sections extends along the third direction, and each of the pair of first DC coils has a first dimension which is a dimension in the second direction, and a second dimension which is a dimension in the third direction and is larger than the first dimension. Magnet for a magnetic particle imaging device according to claim 1, wherein each of the pair of first DC coils has a shape and an ampere turn that are identical to a shape and an ampere turn, respectively, of each of the pair of second DC coils. Magnet for a magnetic particle imaging device according to claim 1, wherein each of the pair of first DC coils has a shape and an ampere turn that differs from a shape and an ampere turn of each of the pair of second DC coils. Magnet for a magnetic particle imaging device according to one of claims 1 to 4, wherein the pair of first DC coils is configured to generate a first magnetic field along the third direction in a cross-section perpendicular to the direction in which the pair of AC coils is arranged to face each other, in a position in which respective end sections of the pair of first DC coils face each other in the third direction, and the pair of second DC coils is configured to generate a second magnetic field having a direction opposite to the first magnetic field in the cross-section in such a position that a magnetic flux density in a direction along the third direction in the position is 10 gauss or less. Magnet for a magnetic particle imaging device according to claim 5, wherein in the cross-section a length in the third direction of a region in which the magnetic flux density in the direction along the third direction is 10 gauss or less, is 0.3 times or more a width of each of the pair of first DC coils in the third direction. Magnet for a magnetic particle imaging device according to one of claims 1 to 6, wherein each of the pair of AC coils comprises at least a first split AC coil and a second split AC coil, the first split AC coil of one of the pair of AC coils and the first split AC coil of the other of the pair of AC coils are arranged such that they face each other in the first direction and are located between the pair of first DC coils, one of the pair of first DC coils is located between the first split AC coil of one of the pair of AC coils and the second split AC coil of one of the pair of AC coils, and the other of the pair of first DC coils is located between the first split AC coil of the other of the pair of AC coils and the second split AC coil of the other of the pair of AC coils. Magnet for a magnetic particle imaging device according to claim 7, wherein a coil width of the second split AC coil is larger than a coil width of the first split AC coil. Magnet for a magnetic particle imaging device according to claim 7 or 8, wherein each of the pair of second DC coils comprises at least a first split DC coil and a second split DC coil, the second split DC coil of one of the pair of second DC coils and the second split DC coil of the other of the pair of second DC coils being arranged such that they are opposite each other, wherein the first split DC coil of one of the pair of second DC coils and the first split DC coil of the other of the pair of second DC coils are located between the first split AC coil of one of the pair of AC coils and the first split AC coil of the other of the pair of second DC coils.

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