Magnetic particle imaging apparatus, method of disposing detection coil for magnetic particle imaging apparatus, and magnetic flux detecting apparatus

In a magnetic particle imaging apparatus that forms an image of a distribution of magnetic particles based on changes in a magnetic flux generated by magnetization of the magnetic particles, modulation coils that magnetize magnetic particles present in a field free area by applying a modulation magnetic field to the field free area, and detection coils are disposed such as to suppress an influence caused by a magnetic flux of the modulation magnetic field applied by the modulation coils and included in a detected magnetic flux.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-158546, filed on Jun. 15, 2007; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic particle imaging apparatus that forms an image of a distribution of magnetic particles based on change in magnetic flux caused by magnetization of the magnetic particles, a method of disposing a detection coil for the magnetic particle imaging apparatus, and a magnetic flux detecting apparatus. In particular, the present invention relates to a technology for enhancing detection sensitivity of the detection coil.

2. Description of the Related Art

In recent years, a method has been proposed for injecting magnetic particles, such as super paramagnetic iron oxide, that serve as a contrast medium into a subject and forming an image of a distribution of the contrast medium (refer to, for example, JP-A 2003-199767 (KOKAI) or B. Gleich, J. Borgert, J. Weizenecker, “Magnetic Particle Imaging (MPI)”, Philips Medic Mundi Vol. 50 No. 1, 2006/5 [online], May 23, 2007, retrieved from the Internet: URL: http://www.medical.philips.com/main/news/assets/docs/medi camundi/mm_vol50_no1/12_Gleich.pdf). This method is called magnetic particle imaging.FIG. 16is a diagram for explaining a principle of magnetic particle imaging. In magnetic particle imaging, for example, a static magnetic field1flowing in vertically opposite directions is generated using a permanent magnet in an area in which magnetic particles are distributed.

At this time, a magnetic field from above and a magnetic field from below are mutually cancelled at an approximate center of the static magnetic field1, thereby generating an area2where a localized magnetic field becomes zero. The area2is referred to as a “field free area”. Then, a modulation coil4for generating a modulation magnetic field and a detection coil5for detecting a change in an interlinking magnetic flux are disposed within the static magnetic field1.

Here, it is assumed that a modulation magnetic field3is generated in the area in which the magnetic particles are distributed by applying an electrical current to the modulation coil4. At this time, magnetic saturation occurs in areas other than the field free area2because of the static magnetic field1. Therefore, the magnetic flux within the areas other than the field free area2does not change even when the modulation magnetic field3is applied. On the other hand, magnetic saturation does not occur in the field free area2because the magnetic field is near to zero. When the modulation magnetic field3is applied, the magnetic particles present in the field free area2become magnetized. A magnetic flux is generated from the field free area2with the magnetization of the magnetic particles.

The magnetic flux generated from the field free area2causes a change in the magnetic flux interlinked with the detection coil2. The change in the magnetic flux appears as a change in voltage induced in the detection coil5. An amount of change in the voltage depends on an amount of magnetic particles present in the field free area2. In other words, the voltage induced in the detection coil5changes based on the amount of magnetic particles present in the field free area2.

When the above-described principle is used, an image of the distribution of the magnetic particles within the subject can be formed by measuring a change in the voltage induced in the detection coil being measured while the field free area is gradually moved within the subject into which the magnetic particles have been injected. In recent years, a deliberation has begun on clinical application of the above-described magnetic particle imaging.

In the above-described magnetic particle imaging, the change in the voltage induced by the magnetization of the magnetic particles is required to be measured using the detection coil. However, voltage caused by the modulation magnetic field applied by the modulation coil is also induced in the detection coil. The detection coil detects voltage as a modulation signal (such as high-frequency signal). However, when the voltage caused by the modulation magnetic field is also induced, a signal indicating the change in the voltage caused by the magnetization of the magnetic particles and a signal indicating change in the voltage caused by the modulation magnetic field are detected in an overlapping state. Therefore, when an image of the distribution of the magnetic particles is formed, it is required to extract only the signal of the voltage induced by the magnetization of the magnetic particles from the signals detected by the detection coil.

However, the voltage induced by the modulation magnetic field is significantly larger than the voltage induced by the magnetic particles. Therefore, separation of the respective signals of the voltages becomes difficult. This problem becomes more significant in clinical application.

For example, when a magnetic particle imaging apparatus scaled for humans is configured based on a description written in “Magnetic Particle Imaging (MPI)” by B. Gleich, J. Borgert, and J. Weizenecker, and it is assumed that a magnitude of the modulation magnetic field applied by the modulation coil is 10 mT/μ0, that is a strength facilitating magnetic saturation of the magnetic particles, the voltage induced in the detection coil by the modulation magnetic field is about 150 volts. On the other hand, for example, when an early stage cancer cell of 10 mm3indicating diamagnetism having a magnetic susceptibility of −7.1×10−6is the subject, the voltage induced in the detection coil is about 50 nanovolts.

A known document describes a separation method using frequency as a method for separating the signals. Specifically, the method takes advantage of the signal of the voltage induced by the magnetization of the magnetic particles including distortion, whereas the signal of the voltage induced by the modulation magnetic field is a sine wave. A harmonic component is extracted from the signals detected by the detection coil. As a result, only the signal of the voltage induced by the magnetization of the magnetic particles is extracted.

However, when taking into consideration of clinical application, because the voltage induced by the modulation magnetic field is significantly larger than the voltage induced by the magnetization of the magnetic particles, as described above, it id difficult to obtain sufficient detection sensitivity even using this method. Therefore, to apply the magnetic particle imaging to a clinical application, the detection sensitivity of the detection coil is required to be significantly enhanced compared to a conventional detection coil.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a magnetic particle imaging apparatus includes a magnet unit that generates a magnetic field such as to form a non-magnetic field area within a detection space; a modulation coil that magnetizes magnetic particles by applying a modulation magnetic field; a detection coil that detects a change in a magnetic flux interlinked with the detection coil and is disposed such as to suppress an influence caused by a magnetic flux of the modulation magnetic field applied by the modulation coil and included in the detected magnetic flux; and an image processing unit that forms an image of a distribution of the magnetic particles based on the change in the magnetic flux detected by the detection coil.

According to another aspect of the present invention, a method of disposing detection coil for a magnetic particle imaging apparatus, the method includes disposing a modulation coil that magnetizes magnetic particles by applying a modulation magnetic field and a detection coil that detects a change in a magnetic flux interlinked with the detection coil, such that an influence caused by a magnetic flux of the modulation magnetic field applied by the modulation coil and included in the detected magnetic flux is suppressed.

According to still another aspect of the present invention, a magnetic flux detecting apparatus includes a modulation coil that magnetizes magnetic particles by applying a modulation magnetic field; and a detection coil that detects a change in a magnetic flux interlinked with the detection coil, and that is disposed such that an influence caused by a magnetic flux of the modulation magnetic field applied by the modulation coil and included in the detected magnetic flux is suppressed.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of a magnetic particle imaging apparatus, a method of disposing a detection coil for the magnetic particle imaging apparatus, and a magnetic flux detecting apparatus according to the present invention are below described with reference to the attached drawings.

FIG. 1is a perspective view of a configuration of a magnetic particle imaging apparatus according to a first embodiment. As shown inFIG. 1, a magnetic particle imaging apparatus10includes a frame11, a top plate12, permanent magnets13aand13b, zero magnetic-field scan coils14ato14h, modulation coils15aand15b, detection coils16aand16b, and an image processing unit30.

The frame11is a u-shaped component and supports the permanent magnets13aand13b, the zero magnetic-field scan coils14ato14h, the modulation coils15aand15b, and the detection coils16aand16b.

The top plate12is a plate-shaped component provided in an approximate center of an area surrounded by the frame11. A subject P (such as a patient) to be imaged is placed on the top plate12. The top plate12is moved by an apparatus (not shown). Here, directions above and below, right and left, and head and feet are defined based on the subject P placed on the top plate12on his back.

The permanent magnets13aand13bare magnets used to generate a field free area within the frame11.FIGS. 2A and 2Bare diagrams for explaining the permanent magnets13aand13b. As shown inFIG. 2A, the permanent magnets13aand13bare respectively disposed on an upper surface and a lower surface of an inner wall of the frame11. The permanent magnets13aand13bare disposed facing each other.

The permanent magnets13aand13bare disposed such that respectively generated magnetic fields flow in opposite directions. As a result of the permanent magnets13aand13bbeing disposed in this way, as shown inFIG. 2B, a magnetic field6agenerated by the permanent magnet13aand a magnetic field6bgenerated by the permanent magnet13bare mutually cancelled at the middle point of the permanent magnet13aand13b, thereby generating a field free area2.

Here, as shown in the diagrams, a surface passing through a center of the field free area2and dividing the area surrounded by the frame11into left and right is referred to as a “horizontal center surface”. A surface passing through the center of the field free area2and dividing the area surrounded by the frame11into top and bottom is referred to as a “vertical center surface”.

The zero magnetic-field scan coils14ato14hare electromagnets used to control a position of the field free area2.FIGS. 3A to 3Fare diagrams for explaining the zero magnetic-field scan coils14ato14h.FIG. 3Ashows a state of a magnetic field on the vertical center surface shown inFIG. 2B. As shown inFIG. 3A, on the vertical center surface, a magnetic field6cis generated that is a combination of the magnetic field6aand the magnetic field6bshown inFIG. 2B. The field free area2is generated in a center portion of the magnetic field6c.

FIG. 3Bshows the top plate12, the permanent magnet13adisposed on an upper surface of the inner wall of the frame11, and the zero magnetic-field scan coils14ato14d. As shown inFIG. 3B, the zero magnetic-field scan coils14ato14dare disposed around the permanent magnet13a.

Specifically, a zero magnetic-field scan coil14ais disposed on a right side of a head end from the perspective of the subject P. A zero magnetic-field scan coil14bis disposed on a left side of the head end. A zero magnetic-field scan coil14cis disposed on the right side of a feet end. A zero magnetic-field scan coil14dis disposed on the left side of the feet end.

For example, it is assumed that electric current flowing through each zero magnetic-field scan coil is adjusted such that a magnetic field flowing from the zero magnetic-field scan coil14btowards the zero magnetic-field scan coil14aand a magnetic field flowing from the zero magnetic-field scan coil14dtowards the zero magnetic-field scan coil14care generated. In this case, as shown inFIG. 3C, a magnetic field flowing from the left side to the right side is generated. When the generated magnetic field and the magnetic field6cshown inFIG. 3Aare combined, the combined magnetic field on the right side becomes stronger than the combined magnetic field on the left side. As a result, as shown inFIG. 3D, the field free area2moves to the left side. On the other hand, when the electrical current is adjusted such that a magnetic field flowing from the zero magnetic-field scan coil14atowards the zero magnetic-field scan coil14band a magnetic field flowing from the zero magnetic-field scan coil14ctowards the zero magnetic-field scan coil14dare generated, the field free area2moves to the right side.

At the same time, for example, it is assumed that the current flowing through each coil is adjusted such that a magnetic field flowing from the zero magnetic-field scan coil14ctowards the zero magnetic-field scan coil14aand a magnetic field flowing from the zero magnetic-field scan coil14dtowards the zero magnetic-field scan coil14bare generated. In this case, as shown inFIG. 3E, a magnetic field flowing from the feet end to the head end is generated. When the generated magnetic field and the magnetic field6cshown inFIG. 3Aare combined, the combined magnetic field on the feet end becomes stronger than the combined magnetic field on the head end. As a result, as shown inFIG. 3F, the field free area2moves to the feet end. On the other hand, when the electrical current is adjusted such that a magnetic field flowing from the zero magnetic-field scan coil14atowards the zero magnetic-field scan coil14cand a magnetic field flowing from the zero magnetic-field scan coil14btowards the zero magnetic-field scan coil14dare generated, the field free area2moves to the head end.

In this way, the field free area2can be moved in the horizontal direction and the head and feet direction by the electrical currents flowing through the zero magnetic-field scan coils14ato14dbeing respectively adjusted. Although explanations are omitted herein, the field free area2can be similarly moved when the zero magnetic-field scan coils14eto14hdisposed on the lower surface of the inner wall of the frame11are used.

In the configuration of the zero magnetic-field scan coils14ato14h, described above, the field free area2cannot be moved vertically. However, the position of the field free area2can be equivalently moved within the subject P by the top plate12being moved in the vertical direction.

The modulation coils15aand15bare coils that magnetize magnetic particles by applying a modulation magnetic field. Specifically, the modulation coils15aand15bare electromagnets used to apply a modulation magnetic field in the area surrounded by the frame11. The modulation coils15aand15bapply a modulation magnetic field of, for example, about 10 kilohertz to 100 kilohertz.FIGS. 4A and 4Bare diagrams for explaining the modulation coils15aand15b. As shown inFIG. 4A, the modulation coils15aand15bare respectively disposed on an upper side and a lower side of the area surrounded by the frame11. The modulation coils15aand15bare disposed facing each other. The modulation coils15aand15bapply the modulation magnetic field3from the lower side towards the upper side.

When the modulation coils15aand15bapply the modulation magnetic field3, the magnetic particles present within the field free area2are magnetized and magnetization M occurs, as shown inFIG. 4B. A magnetic field7is thus generated from the field free area2with the magnetization of the magnetic particles. A waveform of the magnetic field7is the same as a waveform of the magnetization M. The waveform includes distortion depending on an amount of magnetic particles present within the field free area2.

FIGS. 5A to 5Care schematic diagrams of a waveform of the magnetic field generated from the field free area2.FIG. 5Ais a schematic diagram of an intensity H of the magnetic field at the field free area2.FIG. 5Bis a schematic diagram of change in magnetic flux density B at the field free area2.FIG. 5Cis a schematic diagram of change in the magnetization M at the field free area2.

The intensity H of the magnetic field generated near the field free area2is a sum of an intensity of the magnetic field generated by the permanent magnets13aand13band the intensity of the modulation magnetic field generated by the modulation coils15aand15b. However, because the magnetic field generated by the permanent magnets13aand13bat the field free area2is zero, the waveform of the intensity H of the magnetic field is a sine wave, as shown inFIG. 5A. At the same time, the magnetic flux density B reaches a plateau at a predetermined saturated magnetic flux density as shown inFIG. 5B, as a result of magnetic saturation. Therefore, the magnetization M has a distorted waveform as shown inFIG. 5Cbecause the magnetization M is a difference between B and μ0.

The detection coils16aand16bare electromagnets used to detect an interlinking magnetic flux. When the detection coils16aand16bare disposed in arbitrary positions within the area surrounded by the frame11, the magnetic flux detected by the detection coils16aand16bincludes, not only the magnetic flux of the magnetic field7generated by the magnetization M of the magnetic particles, but the magnetic flux of the magnetic fields6aand6bgenerated by the permanent magnets13aand13band the magnetic flux of the modulation magnetic field3applied by the modulation coils15aand15b.

However, only the magnetic flux of the magnetic field7generated by the magnetization M of the magnetic particles is required to form an image of the distribution of the magnetic particles. The magnetic fields6aand6bgenerated by the permanent magnets13aand13bare both static magnetic fields. Therefore, frequencies of the magnetic fields6aand6bsignificantly differ from the magnetic field7generated by the magnetization M. As a result, the magnetic flux of the magnetic fields6aand6band the magnetic flux of the magnetic field7can be easily separated by a known technology.

On the other hand, when taking into consideration clinical application, the modulation magnetic field3generated by the modulation coils15aand15bis, as described earlier, significantly larger than the magnetic field7generated by the magnetization M of the magnetic particles, as described above. Therefore, separation of these magnetic fields is difficult.

Therefore, in the magnetic particle imaging apparatus10according to the first embodiment, the detection coils16aand16bare disposed such that detection of the modulation magnetic field3applied by the modulation coils15aand15bis kept to a minimum (zero in terms of design) and the magnetic field7generated by the magnetization M of the magnetic particles can be efficiently detected. As a result, sufficient detection sensitivity can be achieved even in clinical application.

An arrangement of the detection coils16aand16bwill be described in detail hereafter.FIG. 6is a diagram of an arrangement of the detection coils16aand16baccording to the first embodiment. As shown inFIG. 6, in the magnetic particle imaging apparatus10according to the first embodiment, the detection coil16ais disposed on an upper right side of the area surrounded by the frame11. The detection coil16bis disposed on the lower right side. Both detection coils16aand16bare disposed on outer sides of the modulation coils15aand15b.

Here, the detection coil16ais disposed such that mutual inductance with the modulation coil15abecomes substantially zero. The detection coil16bis disposed such that mutual inductance with the modulation coil15bbecomes substantially zero.

Specifically, according to the first embodiment, the detection coil16ais disposed such that an axis (center axis) running through a center of a coil surface of the detection coil16ais approximately orthogonal to the magnetic flux of the modulation magnetic field3applied by the modulation coil15ato make mutual inductance between the detection coil16aand the modulation coil15asubstantially zero. The detection coil16bis disposed such that an axis (center axis) running through a center of a coil surface of the detection coil16bis approximately orthogonal to the magnetic flux of the modulation magnetic field3applied by the modulation coil15bto make mutual inductance between the detection coil16band the modulation coil15bsubstantially zero.

Because the detection coils16aand16bare disposed as described above, the magnetic flux of the modulation magnetic field3applied by the modulation coils15aand15bis not interlinked with the detection coils16aand16b. As a result, an influence on the detection coils16aand16bby the magnetic flux of the modulation magnetic field3applied by the modulation coils15aand15bis restrained.

At the same time, the center axis of each detection coil16aand16bis not approximately orthogonal to the magnetic flux of the magnetic field7generated by the magnetization M of the magnetic particles (which means that the coil surface of each detection coil16aand16bapproximately orthogonal to the magnetic flux of the magnetic field7). Therefore, the detection coils16aand16bhave a very low detection sensitivity to the modulation magnetic field3applied by the modulation coils15aand15b, and have a certain amount of detection sensitivity to the magnetic field7generated by the magnetization M of the magnetic particles.

A method of disposing the detection coils16aand16bwill be described hereafter. Specifically, first, a predetermined number of points are defined within the area surrounded by the frame11. The points are referred to as “measurement points”. Next, the intensity and direction of the modulation magnetic field3applied by the modulation coils15aand15bare calculated at each measurement point.FIG. 7is a distribution diagram of a distribution of the modulation magnetic field3applied by the modulation coils15aand15b. A direction of each straight line shown inFIG. 7indicates the direction of the modulation magnetic field3at each measurement point. A length of each straight line indicates an intensity of the modulation magnetic field3at each measurement point.

Then, any one of measurement points is selected from among the defined measurement points. The detection coil is disposed so that the center point of the detection coil overlaps with the selected measurement point and the center axis of the detection coil is approximately orthogonal to the direction of the modulation magnetic field3at the selected measurement point.

FIG. 8is a diagram of an arrangement example of the detection coils16aand16bshown inFIG. 6. For example, as shown inFIG. 8, a measurement point is selected from the upper side and from the lower side, among the measurement points on the right side of the area surrounded by the frame11and the outer sides of the modulation coils15aand15b. The detection coils16aand16bare disposed based on each measurement point.

The detection coils16aand16bare disposed in this way such that the center axis of the detection coil is approximately orthogonal to the magnetic flux of the modulation magnetic field3applied by the modulation coils15aand15b. As a result, the magnetic flux detected by the detection coils16aand16b, among the magnetic flux of the modulation magnetic field3generated by the modulation coils15aand15b, can be suppressed to a minimum.

In above example, detection coil is located approximately orthogonal to the magnetic flux of the modulation magnetic field3. To explain it more generally, the arrangement of detection coils are made so that the mutual inductance of detection coils and modulation coils are suppressed to a minimum.

Returning toFIG. 1, the image processing unit30generates an image indicating the distribution of the magnetic particles based on the change in the magnetic flux detected by the detection coils16aand16b. The image processing unit30outputs the generated image to, for example, a display device such as a monitor and an output device such as a printer.

As described above, according to the first embodiment, the detection coils16aand16bthat detect the change in the interlinking magnetic flux are disposed such as to suppress the influence of the magnetic flux of the modulation magnetic field3applied by the modulation coils15aand15band included in the detected magnetic flux. Therefore, the detection sensitivity of the detection coil can be enhanced. Sufficient sensitivity can be achieved for clinical application.

According to the first embodiment, when the detection coils16aand16bare disposed such that the center axes are approximately orthogonal to the direction of the modulation magnetic field3generated by the modulation coils15aand15bis described. However, the present invention is not limited thereto. Various other detection coil disposal methods can be considered. Hereafter, other examples of the detection coil disposal will be described with reference toFIG. 9toFIG. 12.

FIG. 9is a diagram (1) of another arrangement example of the detection coil. For example, the detection coils can be disposed on inner sides of the modulation coils15aand15b. In the example, four detection coils16cto16fare each disposed based on measurement points on the inner sides of the modulation coils15aand15b.

Signal detection sensitivity can be further enhanced when the detection coils16cto16fare disposed on the inner sides of the modulation coils15aand15b, because each detection coil is disposed closer to the magnetization M than when the detection coils16cto16fare disposed on the outer sides.

With reference to the distribution diagram inFIG. 7, it is understood that a measurement point is present at which the magnetic field becomes zero as a result of the modulation magnetic field3applied by the modulation coil15aand the modulation magnetic field3applied by the modulation coil15bbeing mutually cancelled. As a result of the detection coils16cto16fbeing disposed as described above, the modulation magnetic field3applied by the modulation coils15aand15band interlinked with the detection coils16cto16fcan become zero.

FIG. 10is a diagram (2) of another arrangement example of the detection coil. For example, as shown inFIG. 10, a detection coil16gis disposed in a position at which the modulation magnetic field3generated on the right side of the modulation coil15aand the modulation magnetic field3generated on the right side of the modulation coil15bare mutually cancelled. In this case, regardless of the direction in which the detection coil16gis disposed, the modulation magnetic field3interlinked with the detection coil16gbecome zero. However, to efficiently detect the magnetic flux of the magnetic field7generated by the magnetization M of the magnetic particles, the detection coil16gis preferably disposed in a direction in which a coil surface is perpendicular to the magnetic flux of the magnetic field7. The magnetic flux of the magnetic field7interlinked with the detection coil reaches maximum when the detection coil16gis disposed in this way. Therefore, the detection sensitivity to the magnetic field7of the detection coil also reaches maximum.

Alternatively, the modulation magnetic field3applied by the modulation coils15aand15bcan be made zero through use of two serially connected detection coils wound in opposite directions.

FIG. 11is a diagram (3) of another arrangement example of the detection coil. For example, as shown inFIG. 11, serially connected detection coils16hand16iwound in opposite directions are respectively disposed above and below the modulation coil15a. Here, each detection coil has a same coil surface area and a same number of windings. Each detection coil is disposed at a same distance from the modulation coil15asuch that the center axis of each detection coil matches the center axis of the modulation coil15a.

As a result of the detection coils16hand16ibeing disposed in this way, the modulation magnetic fields3applied by the modulation coils15aand15band detected by each detection coil are mutually cancelled. Therefore, when the detection coils16hand16iare considered to be a single detection coil, the modulation magnetic field3interlinked with the entire detection coil can be considered to be zero.

On the other hand, regarding the magnetic field7generated by the magnetization M, because the detection coil16iis disposed closer to the field free area2than the detection coil16h, the magnitude of the interlinking magnet field differs for each detection coil. As a result, even when the detection coils16hand16iare considered to be a single detection coil, the magnetic field7interlinked with the entire detection coil is not zero. Therefore, a difference occurs in the magnetic field7interlinked with each detection coil. By detecting the difference, the magnetic field7generated by the magnetization M can be detected.

Here, the detection coils16hand16ihave the same coil surface area and the same number of windings. Each detection coil is disposed at a same distance from the modulation coil15asuch that the center axis of each detection coil matches the center axis of the modulation coil15a. However, the coil surface area, the number of windings, and the disposal positions are not limited thereto.

In other words, even when each detection coil is disposed in a position that is a different distance from the modulation coil15a, the modulation magnetic field3interlinking the entire detection coil can be made zero by the coil surface areas and the number of windings being changed such that the modulation magnetic fields3interlinked with each detection coil are mutually cancelled. At this time, each detection coil is preferably disposed such that a total of the magnetic field7generated by the magnetization M is as large as possible.

Alternatively, compensation coils can be serially inserted into each modulation coil and detection coil and disposed such as to face each other. As a result, the modulation magnetic field3applied by the modulation coils15aand15bcan become zero.

FIG. 12is a diagram (4) of another arrangement example of the detection coil. For example, as shown inFIG. 12, compensation coils17aand17bare inserted in a series with the modulation coil15a. The compensation coil17ais inserted in a series with a detection coil16j. At this time, the compensation coils17aand17bare disposed such as to face each other. A ratio of the number of windings between the compensation coils17aand17bare set such that a sum of a magnetic flux interlinked with the detection coil16jamong the magnetic flux of the modulation magnetic field3applied by the modulation coils15aand15band a magnetic flux interlinked with the compensation coil17band generated by the compensation coil17abecomes zero.

The method using the compensation coils as in the example inFIG. 12is useful for making fine adjustments to make a total amount of the magnetic flux of the modulation magnetic field3applied by the modulation coils15aand15b, among the magnetic flux detected by the detection coil, zero. The method is effective when used in combination with each of the above-described examples.

According to the first embodiment, when the interlinking magnetic flux, among the magnetic flux of the modulation magnetic field3applied by the modulation coils15aand15b, is minimized through adjustment of the position and the direction when the detection coil is disposed is described. However, even when a design is achieved such as to dispose the detection coil as described above, mechanical error may occur during manufacture. Therefore, it is difficult for the magnetic field interlinking with the detection coil and generated by the modulation coils15aand15bto be made completely zero.

Therefore, according to a second embodiment, following configuration and method are described. In the configuration and method, a feedback coil is provided for adjusting a balance in the modulation magnetic field applied by the modulation coils15aand15b. The feedback coil is driven based on a result actually detected by the detection coil. As a result, the magnetic field interlinked with the detection coil and generated by the modulation coils15aand15bis made completely zero. Here, for convenience of explanation, functional sections achieving same purposes as each section inFIG. 1are given the same reference numbers. Detailed explanations thereof are omitted.

FIG. 13is a diagram for explaining control of the feedback coil according to the second embodiment. Four serially connected detection coils16kto16nare disposed, in which two detection coils16kand16lon the inner side of the modulation coil15aand two detection coils16mand16non the outer side of the modulation coil15b. A feedback coil18ais disposed in an approximate center of the modulation coil15b.

Here, the modulation coils15aand15bare driven by a modulation signal supplied by an alternating current power supply19b, via an amplifier circuit19a. The modulation signal is generally almost a sine wave. However, the modulation signal actually includes some harmonic components for ON/OFF control of the signal, pulse drive, and the like.

A signal detected by the detection coils16kto16nis inputted into a detection circuit19d, via the amplifier circuit19c. The detection circuit19dperforms wave detection on the inputted signal using a signal waveform of the electrical current flowing through the modulation coils15aand15b. As a result, a basic frequency element is extracted from the inputted signal. A signal of the basic frequency element extracted by the detection circuit19dis inputted into an integration circuit19e.

The integration circuit19eintegrates inputted signals and inputs an integration result into a gain adjusting circuit19g, via an amplifier circuit19f. The gain adjusting circuit19ggenerates a modulation signal of a same frequency waveform as the modulation signal driving the modulation coils15aand15b, using the integration result as gain. Based on the generated modulation signal, the modulation signal is supplied to the feedback coil18a, via an amplifier circuit19h.

The above described circuit configuration configures a primary delay system. The primary delay system performs control such that an element with a same frequency as those of the modulation coils15aand15b, among the magnetic flux interlinked with the detection coils16kto16n, becomes zero. In the configuration, even when a mechanical error occurs, the magnetic field generated by the feedback coil18ais controlled such as to cancel the mechanical error. Therefore, a measurement signal from the detection coil of the modulation magnetic field generated by the modulation coils can be made zero. At the same time, the harmonic component included in the magnetic field generated by the magnetization M remains without being cancelled. Therefore, signal detection accuracy does not deteriorate.

As described above, according to the second embodiment, the modulation coils15aand15bare driven using the modulation signal waveform. The element synchronous with the modulation signal waveform is extracted from the signal detected by the detection coil. The feedback coil18ais controlled such that the synchronous element becomes zero. Therefore, the signal detected by the detection coil within the modulation magnetic field generated by the modulation coils15aand15bcan be significantly reduced, and the detection sensitivity of the detection coil can be further enhanced.

According to the second embodiment, when the feedback coil18ais provided in the approximate center of the modulation coil15bis described. However, the present invention is not limited thereto. For example, a compensation coil can be connected to the detection coil. The feedback coil can be disposed such as to face the compensation coil.FIG. 14is another arrangement example of feedback coil.

Specifically, for example as shown inFIG. 14, a compensation coil17cis inserted between the detection coils16kto16nand an amplifier circuit19c. A feedback coil18bis disposed such as to face the compensation coil17c. A modulation signal generated by the gain adjusting circuit19gand having a same waveform as the modulation signal driving the modulation coils15aand15bis supplied to the feedback coil18b.

As a result, a magnetic flux canceling the error is generated from the feedback coil18b. Control is performed such that the element with the same frequency as those of the modulation coils15aand15b, among the magnetic flux interlinked with the detection coils16kto16n, becomes zero.

Alternatively, the modulation signal generated by the gain adjusting circuit19gcan be directly applied to one of either the modulation coil15aor the modulation coil15b. The magnetic flux canceling the error is generated from the modulation coil to which the modulation signal is applied. Control is performed such that the element with the same frequency as those of the modulation coils15aand15b, among the magnetic flux interlinked with the detection coils16kto16n, becomes zero.

The present invention according to the first embodiment and the second embodiment has been described above. According to these embodiments, the modulation coils15aand15bare described in which the detection coils are disposed near the permanent magnets13aand13b. However, the present invention can be similarly applied when the modulation coils have a different configuration.

FIG. 15is another example of the configuration of the magnetic particle imaging apparatus. In a magnetic particle imaging apparatus20, a modulation coil25and detection coils26ato26dare each disposed on the top plate12. A three-dimensional scan is performed during image scanning by the top plate12being gradually moved upwards and downwards. Moreover, the top plate12can be moved in a head direction and a feet direction.

The modulation coil25and the detection coils26ato26dmove with the movement of the top plate12. Therefore, each coil is constantly disposed close to the subject P, thereby further enhancing the detection sensitivity.

In this case, same as the first embodiment, various patterns can be considered regarding positions and directions when the detection coils are disposed. For example, in the example inFIG. 15, the detection coils26ato26dare disposed in the same positions as the detection coils16dto16finFIG. 9.

As can be analogized by the examples above, the present invention can be similarly applied with various modulation coil and detection coil configurations.

As described above, in the magnetic particle imaging apparatus according to the present invention, the detection coil does not detect the magnetic flux generated by the modulation coil. The detection coil can efficiently extract and detect the magnetic field generated from an area to be imaged. Therefore, the detection sensitivity is significantly enhanced. In a conventional configuration, actualization of an imaging device for humans has been considered difficult. However, through use of the present invention, sensitivity can be significantly enhanced. An imaging apparatus that can be applied to clinical situations can be actualized.

In the conventional configuration of the imaging apparatus, highly concentrated magnetic particles are used. A lesion can be detected only when the lesion becomes very large. However, through use of the present invention, detection can be performed even when concentration of the magnetic particles on an affected area is less than that in the conventional configuration. In addition, magnetic particles concentrated in a small area (affected area) can be detected. Therefore, milder diseases and diseases at an earlier stage can be detected, thereby contributing to improvement in diagnostic quality.

The present invention has been achieved to solve the above described problems and issues of the above described conventional technologies. An object of the invention is to provide a magnetic particle imaging apparatus, a signal detecting method, and a signal detecting apparatus for the magnetic particle imaging apparatus that can be applied to a clinical situation through enhancement of detection sensitivity of a detection coil.