JP5202939B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention relates to a magnetic resonance imaging technique. In particular, the present invention relates to a technique (moving table imaging) that performs imaging while moving a table on which a subject is placed.

  A magnetic resonance imaging (MRI) apparatus is a medical diagnostic imaging apparatus that applies a high-frequency magnetic field and a gradient magnetic field to a subject placed in a static magnetic field, measures echo signals generated from the subject by nuclear magnetic resonance, and images them. is there. In the MRI apparatus, since the uniform region of the static magnetic field is a spherical region having a diameter of about 45 cm, the size of the region that can be imaged by one measurement is usually limited to about 40 cm. Therefore, when imaging a wider area such as the whole body, the imaging is performed by moving the bed (table) in the body axis direction of the subject. One such imaging method is moving table imaging.

  Moving table photographing is a method of photographing while continuously moving a table (for example, see Non-Patent Document 1). Since different parts of the subject are sequentially imaged while moving the table, it is possible to image an arbitrary wide range in the table moving direction.

  In such moving table photographing, a method has been proposed in which the field of view in the direction perpendicular to the table moving direction is variable (Patent Document 1). For example, when photographing the whole body of a person with the table moving direction as the body axis direction, the size in the horizontal direction differs depending on the part, so that the waste of spatial resolution is eliminated by matching the field of view to the size of the part. . In that case, the artifact generated in the image is suppressed by interpolating the measurement data.

  In addition, in the MRI apparatus, there is a method of shortening imaging time by measuring signals in parallel using a plurality of receiving coils (parallel imaging) (see, for example, Non-Patent Document 2). Parallel imaging is an imaging method in which position information is given by using a difference in sensitivity distribution of the receiving coil in addition to position information given to in-subject magnetization by a normal gradient magnetic field. When this parallel photographing is used in combination with moving table photographing, it is possible to shorten the photographing time by increasing the table moving speed in accordance with the double speed ratio.

As a receiving coil used for moving table photographing, a coil for whole body incorporated in the bore can be used. However, the whole body coil with a built-in bore often has a large coil size in order to cover a wide imaging area, and a sufficient S / N cannot be obtained in many cases. For example, when photographing a lower limb having a small volume with high spatial resolution, it is often impossible to obtain a sufficient S / N. For this reason, even if the field-of-view changing technique is used as described above, there is a high possibility that the image quality of a subject to be photographed with a small volume is not good when the whole body coil is used. Moreover, it is difficult to cover a wide imaging area with sufficient S / N for parallel imaging coils, which are generally composed of a combination of local coils, and most of them are capable of parallel imaging with a whole body coil with a built-in bore. Absent.
These problems can be solved by using an array coil placed on the subject (for example, Patent Document 2). However, the array coil to be placed on the subject has to be attached to each part of the body in the case of whole-body imaging, and there is a problem that the operator has trouble in setting the subject.

In still photography, there is a method for solving the problem when this array coil is used (for example, Patent Document 3). In this method, the array coil placed on the subject is fixed inside the bore, for example, so that imaging can be performed without placing it on the subject. In addition, some dedicated coils for the head and the like are used as they are in the past. However, when a conventional dedicated coil is used, the coil fixed inside the bore is made removable so that the coil fixed inside the bore does not interfere.
Kruger DG, Riederer SJ, Grimm RC, Rossman PJ. Continuously movingtable data acquisition method for long FOVcontrast-enhanced MRA and whole-bodyMRI.Magn Reson Med 2002; 47: 224--231. Pruessmann KP, Markus MW, Scheidegger B, Boesiger P. SENSE: Sensitivity Encoding for Fast MRI. Magnetic Resonance in Medicine1999; 42: 952--962. JP2005-199198 U.S. Pat. International Publication No. 2005/047915 Pamphlet

In moving table shooting, when the coil configuration applied to the above-described still shooting, that is, when the technique of using the local coil together with the array coil fixed inside the bore is adopted, the local coil is a part of the bore fixed coil. When passing through the coil, there is a problem that the coupling between the coils occurs and the image quality deteriorates.
Further, since the table moving speed is constant in normal moving table shooting, when a coil having various configurations as described above is used in combination, the S / N and the double speed ratio differ depending on the coil, and the shooting efficiency is not necessarily good. Absent.

  The present invention has been made in view of the above circumstances. In the MRI apparatus, it is possible to perform moving table imaging using various combinations of receiving coils. The purpose is to improve.

  The MRI apparatus of the present invention includes a first receiving coil fixed to the magnetic field generation unit side as a receiving coil, and a second receiving coil fixed to the table side holding the subject. The above-mentioned problem is solved by switching on and off the 1 reception coil and the 2nd reception coil. Preferably, the table moving speed is changed together with ON / OFF control of the receiving coil.

  That is, the MRI apparatus of the present invention is fixed to the magnetic field generation unit that generates a static magnetic field and a gradient magnetic field in the imaging space, the subject holding unit that holds the subject and moves in the imaging space, and the magnetic field generation unit side. A first receiving coil and a second receiving coil fixed to the subject holding unit, a receiving unit for receiving a nuclear magnetic resonance signal, and a signal received by the first receiving coil and the second receiving coil, respectively. An image reconstruction unit that reconstructs the image of the subject, and a control unit that controls the magnetic field generation unit, the reception unit, and the subject holding unit, and the control unit includes the imaging The on / off of the first receiving coil and / or the second receiving coil is switched according to the position of the second receiving coil with respect to the space.

  According to the present invention, in the MRI apparatus, it is possible to improve the image quality and imaging efficiency when imaging a wide area exceeding the uniform area of the static magnetic field.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram showing a schematic configuration of an MRI apparatus 100 to which the present invention is applied. The MRI apparatus 100 includes a magnet 101 that generates a static magnetic field, a coil 102 that generates a gradient magnetic field, a sequencer 104, a gradient magnetic field power source 105, a high-frequency magnetic field generator 106, and a transmission / reception that irradiates a high-frequency magnetic field and detects a nuclear magnetic resonance signal. A coil 107, a receiver 108, a computer 109, a display 110, a storage medium 111, a table movement control unit 150, and a table 152 are provided. The shape of the magnet 101 may be either a tunnel type in which the direction of the static magnetic field is horizontal, or an open type in which magnets are arranged above and below and the direction of the static magnetic field is in the vertical direction. Although a single transmission / reception coil 107 is shown in the figure, a transmission coil and a reception coil may be provided separately. The transmitting / receiving coil 107 is fixed to the device (magnet 101). The MRI apparatus 100 includes a receiver 115 that receives a signal from a reception coil 114 attached to a subject (for example, a living body) 103 separately from the transmission / reception coil 107. The receiver 115 may be independent of the receiver 108, and the function of the receiver 115 can be added to the receiver 108.

  A subject (for example, a living body) 103 is placed on a table 152 in a static magnetic field space generated by a magnet 101. The movement of the table 152 is controlled by the table movement control unit 150 in accordance with an instruction from the sequencer 104. The position of the table 152 is read by a position sensor (for example, an encoder) provided in the table 152, and the information is sent to the sequencer 104. The sequencer 104 obtains the position information of the receiving coil 114 attached to the subject 103 from the table position information, and controls the pulse sequence and the on / off control of the receiver 108 and the receiver 115 based on the position information.

  In the present embodiment, the table 152 moves in the body axis direction. In this specification, the z-axis is in the body axis direction, the x-axis is in a direction perpendicular to the z-axis on a plane parallel to the table 152, and the y-axis is in the direction perpendicular to the x-axis and z-axis. In this embodiment, the reading gradient magnetic field is applied in the z direction, and the phase encoding gradient magnetic field is applied in the x direction.

  The sequencer 104 sends commands to the gradient magnetic field power source 105 and the high frequency magnetic field generator 106 to generate a gradient magnetic field and a high frequency magnetic field, respectively. The high frequency magnetic field is applied to the subject 103 through the transmission / reception coil 107. A signal generated from the subject 103 is received by the transmission / reception coil 107 and detected by the receiver 108. Alternatively, the signal is received by another receiving coil 114 and detected by the receiver 115.

  The computer 109 controls the operation of each component of the MRI apparatus 100 according to a predetermined program. Among these, the program describing the generation timing and intensity of the high-frequency magnetic field and gradient magnetic field to be controlled by the sequencer 104 and the timing of signal reception is called a pulse sequence. The pulse sequence implemented in the present embodiment is not particularly limited. For example, an SE pulse sequence such as a spin echo method (SE) or a fast spin echo method, a gradient echo (GrE) pulse sequence, an echo planar spectroscopic imaging ( EPSI), diffusion-weighted echo planar imaging (DWEPI), and the like.

  The computer 109 receives the signal detected by the receiver 108 or 115, performs signal processing such as image reconstruction, and displays the result on the display 110. If necessary, the detected signal and measurement conditions may be stored in the storage medium 111. Further, the sequencer 104 causes the table movement control unit 150 to control the operation of the table 152, and executes imaging involving table movement, that is, a moving table imaging method.

As shown in FIG. 2, the moving table imaging method is an imaging method of reading out in the table moving direction 153 (body axis direction: z). Echo measurement during imaging is performed at repeated time intervals while changing the value of the phase encode kx by a constant value.
When each echo is subjected to inverse Fourier transform, a projection image on the z-axis of the imaging target that is phase-encoded in the x direction for each sub-field of view is obtained. As shown in FIG. 3, the echo after the inverse Fourier transform is arranged in the kx-z space (hybrid space). The width wz in the z direction of the projection image determined by the gradient magnetic field in the readout direction is referred to as a sub visual field 151 for a wide area (FOV) 154 to be imaged. A set of echoes 160 measured while kx changes from the minimum to the maximum for one cycle is referred to as a station (hybrid data set) 161.

The position of the j (j = 1,...) echo in the z direction is based on the position of the first echo (j = 1) as the reference (0) and the speed of the table (photographing target) is v (j −1) × v × Tr. If the number of echoes in one station is nx and the sub field of view (width in the z-axis direction) is wz, the following equation (1) needs to be satisfied in order to measure the kx-z space without gaps.
nx × Tr × v <wz (1)
The echo arranged in the kx-z space is finally subjected to inverse Fourier transform in the kx direction, and an image in the FOV is reconstructed.

Next, each embodiment of the present invention will be described based on the above configuration.
<First embodiment>
In the present embodiment, coil control and pulse sequence control when performing moving table imaging using the reception coil 107 fixed to the apparatus and one reception coil 114 attached to the subject will be described. In the following description, the receiving coil 107 is also referred to as a fixed coil, and the receiving coil 114 is also referred to as a moving coil.

  FIG. 4 shows an arrangement example of the receiving coils in the present embodiment. The fixed coil 107 is disposed inside the magnet 101 and the gradient magnetic field 102. Usually, the centers of the magnet 101, the gradient coil 102, and the fixed coil 107 coincide with the origin of the apparatus coordinates. In the present embodiment, the reception coil 114 dedicated to the lower limb is arranged on the lower limb of the subject (position from z1 to z2 in the table coordinates) and moves together with the table 152.

  A control procedure of the fixed coil 107 and the moving coil 114 will be described with reference to FIG. First, movement of the table is started (step 501), and imaging is started when the subject 103 reaches a predetermined imaging position (step 502). When imaging is performed from the top of the table to the tip of the foot, it starts from a state where the top position z0 of the subject is on the right side in the figure from the center of the magnet 101. While moving the table to the left at the speed v as the imaging starts, echoes having different kx are measured at the repetition time Tr.

As the table moves, the position of the moving coil 114 relative to the center position of the magnet (that is, the center of the fixed coil 107) changes. On / off of the fixed coil 107 and the moving coil 114 is controlled according to the position of the moving coil 114 (steps 503 to 506).
An example of the control is shown in FIG. Note that the horizontal axis in FIG. 6 indicates the z-axis of the table coordinates, and indicates the position of the fixed coil 107 with respect thereto. The driving of the fixed coil is indicated by a solid line, and the driving of the moving coil is indicated by a chain line (hereinafter the same). As shown in the figure, in this control, the fixed coil 107 is on and the moving coil 114 is off at the start of imaging. Note that the receiving coil being on means that the coil is in a receivable state, and the sequencer receives a signal from the coil and acquires an echo.

  When one end z1 (front end in the traveling direction) of the moving coil 114 reaches the position on the right side of wz / 2 from the center position (origin) of the magnet, the moving coil 114 is turned on, and one end z1 reaches the center position of the magnet. At this point, the fixed coil 107 is turned off (steps 503 and 504). Further, when the other end z2 (rear end in the traveling direction) of the moving coil 114 reaches the center position of the magnet, the fixed coil 107 is turned on, and the other end z2 reaches the position on the left side of wz / 2 from the center position of the magnet. At that time, the moving coil 114 is turned off (steps 505 and 506).

  With this control, the echo can be measured by the moving coil 114 when the moving coil 114 is near the center of the magnet, and the echo can be measured by the fixed coil 107 in other cases. Thereby, since the measurement of the lower limb can be performed by the reception coil 104 attached to the lower limb instead of the fixed coil 107, the echo can be measured with sufficient S / N. Moreover, by turning off the fixed coil 107 while the receiving coil 114 is in the vicinity of the magnet center and receiving a signal, the coupling between the two is cut off, thereby preventing the sensitivity of the receiving coil 114 from being lowered. .

  In FIG. 6, the fixed coil 107 is turned off when the front end z1 of the moving coil 114 coincides with the center of the fixed coil 107, and the fixed coil 107 is turned on when the rear end z2 coincides with the center of the fixed coil 107. However, it is preferable to appropriately adjust the timing for switching on and off the moving coil 114 and the fixed coil 107 in consideration of the sensitivity areas of both. In general, as the overlap of the sensitivity regions of two coils increases, the coupling increases and the signal S / N decreases. Therefore, for example, it is preferable to obtain the S / N of the signal by preliminary measurement and set the switching timing at which the S / N becomes a sufficient value. Practically, it is preferable to switch on and off in the vicinity where the end of the smaller coil coincides with the center of the larger coil as described above.

After the fixed coil 107 is turned on, the imaging is continued while moving the table 152 until the toe of the subject 103 passes the center position of the fixed coil 107, and the process ends (step 507). Finally, an FOV image is reconstructed using signals acquired by the fixed coil 107 and the moving coil 114 (step 508). That is, as shown in FIG. 3, stations obtained by inverse Fourier transforming each echo are arranged in a hybrid space, and inverse Fourier transform is performed in the x direction to obtain an FOV image.
In the flowchart shown in FIG. 5, the image is reconstructed after the photographing is completed. However, when all phase-encoded hybrid data corresponding to the sub-field of view is prepared, the image is reconstructed sequentially and finally synthesized. It is also possible to obtain an FOV image.

  In the control shown in FIG. 6, there is a period in which the fixed coil 107 and the moving coil 114 are simultaneously turned on. During this period, signals from both coils can be acquired. These two types of signals can be made signals having a high S / N by using only the one having a higher S / N, or by taking the average of both. Thereby, the S / N of the reconstructed image can also be improved.

  In the control shown in FIG. 6, both the fixed coil 107 and the moving coil 114 are turned on / off. However, as shown in FIG. 7, the moving coil 114 may always be kept on. This control is easier to control the sequencer than the control of FIG. The fixed coil 107 is turned off only while the moving coil 114 passes through the fixed coil 107 as in FIG. Practically, as shown in FIG. 7, it is fixed from the time when the front end z 1 of the moving coil 114 passes the center position of the fixed coil 107 to the time when the rear end z 2 passes the center position of the fixed coil 107. The coil 107 may be turned off. Thereby, the coupling between both coils can be eliminated. Also in this case, it is preferable to appropriately adjust the timing at which the fixed coil 107 is turned on / off in consideration of the sensitivity distribution of both coils. Thereby, sufficient image quality can be obtained without lowering the sensitivity of both coils.

  In the case of the control of FIG. 7, as for the two types of signals obtained while both coils are on, only the one having a higher S / N is used for image reconstruction, as in the control of FIG. By taking the average of both, it is possible to obtain a signal with a higher S / N.

  According to the present embodiment, when the receiving coil (moving coil) attached to the subject is in a predetermined positional relationship with the receiving coil fixed to the apparatus, by controlling on / off of these coils, An image can be obtained using a signal having an optimum S / N without causing a decrease in sensitivity.

  In the above description, the coil switching is described in the table continuous moving shooting which is performed while moving the table. However, the timing of the coil reception switching is a multi-station where the whole body is imaged by repeatedly moving the table and taking a still image. The photographing can be performed similarly according to the position of the table. In multi-station imaging, the table is moved after imaging the subject and then stopped, and imaging is performed while the table is stationary. For example, in the case of the coil configuration of FIG. 4, it is only necessary to perform still photography by switching on / off of the coil according to FIG. 6 in accordance with the position of the table for performing still photography. As a result, as in the case of table continuous moving imaging, when the receiving coil (moving coil) attached to the subject is in a predetermined positional relationship with the receiving coil fixed to the apparatus, the on / off of these coils is controlled. Thus, an image can be obtained using a signal having an optimum S / N without causing a decrease in sensitivity of both coils.

<Second Embodiment>
In the first embodiment, the moving coil is one receiving coil 114, but in this embodiment, two or more moving coils are used. An example of the arrangement of the receiving coils in this embodiment is shown in FIG. Here, imaging is performed using the transmission / reception coil 107, the lower limb reception coil 114, and the head reception coil 114-2 fixed to the apparatus. The head receiving coil 114-2 is assumed to be at positions z3 to z4 in the table coordinates.

  FIG. 9 shows coil control by the sequencer during photographing. As shown in the figure, the receiving coils 114 and 114-2 are always on, and the on / off of the coil 107 is controlled as the receiving coil moves. That is, the coil 107 is turned off when the reception coil 114 and the reception coil 114-2 are near the center of the coil 107 and sufficient image quality cannot be obtained by coupling with the coil 107. For example, as shown in FIG. 9, the range to be turned off is from the time when the front end z3 of the receiving coil 114-2 passes the center position of the coil 107 to the time when the rear end z4 passes the center position of the coil 107. It is preferable that the coil 107 be turned off during a period between the time when the front end z1 of the receiving coil 114 passes the center position of the coil 107 and the time when the rear end z2 passes the center position of the coil 107. As a result, the coupling between the stationary coil and each moving coil can be eliminated, the sensitivity of the receiving coil can be prevented from being lowered, and sufficient image quality can be obtained.

  As shown in FIG. 8, when the head coil and the lower limb coil are used together as the moving coil, the distance between the coils is usually sufficiently large, so there is almost no coupling between them. Therefore, as shown in FIG. 9, even if both are always turned on, the image quality is hardly affected. However, as shown in FIG. 10, coupling is a problem when two lower limb coils 114 and 114-3 are arranged at a close distance for shooting.

  In such a case, the sequencer performs control as shown in FIG. In FIG. 10, when the front end z5 of the coil 114-3 reaches the position wz / 2 from the center position of the magnet, the coil 114-3 is turned on (dotted line), and when the front end z5 reaches the center position of the magnet. The coil 107 is turned off (solid line), and when the rear end z6 reaches the center position of the magnet, the coil 107 is turned on and the coil 114-3 is turned off. When the front end z1 of the coil 114 reaches the wz / 2 position from the center position of the magnet, the coil 114 is turned on (one-dot chain line), and when the front end z1 reaches the center position of the magnet, the coil 107 is turned off. When the end z2 reaches the center position of the magnet, the coil 107 is turned on and the coil 114 is turned off. Thereby, even if the receiving coil 114 and the receiving coil 114-3 are at a short distance, the influence of the coupling can be prevented.

  The imaging procedure other than the coil control described above is the same as that in the first embodiment, and the same effect can be obtained.

<Third embodiment>
Also in this embodiment, the use of a plurality of receiving coils and the on / off control of the coils with the movement of the table position are the same as in the first and second embodiments. However, in the first and second embodiments, only the switching of the receiving coil is controlled, whereas in this embodiment, the pulse sequence is changed along with the switching of the receiving coil. Specifically, the change of the pulse sequence includes a change in matrix size, a change in visual field, a change in spatial resolution, and the like.

  Usually, the first receiving coil fixed to the apparatus is used for imaging the trunk, and the second receiving coil attached to the subject is used for imaging a region smaller than the trunk, such as the lower limbs and the head. Therefore, the field of view captured by the second receiver coil may be smaller than the field of view captured by the first receiver coil. In the present embodiment, this is utilized to reduce the field of view while the stationary coil 107 is turned off and a signal is received by the receiving coil 114. When changing the pulse sequence, the table speed may be changed in accordance with the change of the visual field, or the spatial resolution or the number of additions may be changed without changing the table speed.

  FIG. 12 shows a control procedure according to the present embodiment. In the steps shown in FIG. 12, the same contents as those shown in FIG. 5 are denoted by the same reference numerals, and the description thereof is omitted. In this embodiment, for example, a pulse sequence condition (parameter) when receiving with a fixed coil is used as a basis, and a pulse sequence condition is changed when receiving with a moving coil. Which is the basis is arbitrary, but here, it is assumed that the fixed coil is turned on and photographing is started under the basic conditions. If the front end z1 of the moving coil reaches the position on the right side of wz / 2 from the center position (origin point) of the fixed coil 107 in step 503 and the moving coil 114 is turned on (steps 503 and 504), the conditions of the pulse sequence Is changed and shooting is performed under different conditions (step 511). In imaging after this condition change, the signal is received by the moving coil 114. If the front end z1 of the moving coil reaches the position on the right side of wz / 2 from the center position (origin) of the fixed coil 107 and the fixed coil 107 is turned on (steps 505 and 506), the pulse sequence conditions are basically set. Return to the conditions and continue shooting. In this photographing, a signal is received by the fixed coil 107 (step 512). Image reconstruction is performed every time data that can be reconstructed is collected after photographing or during photographing (steps 507 and 508).

Hereinafter, the change of the visual field in step 511 will be described.
The field of view can be changed by varying the amount of change in phase encoding for each Tr. For example, when the field of view while the signal is received by the receiving coil 114 is ½ of the basic field of view, the amount of change in phase encoding for each Tr is doubled. As a result, the number of echoes to be acquired is reduced, and the moving speed of the table is increased accordingly. That is, as shown in FIG. 13, while the front end z1 of the moving coil moves from the center position (origin) of the magnet to the wz / 2 right position to the wz / 2 left position, the table moving speed is double the basic speed v. To shoot.

  The arrangement of data in the kx-z space at this time is shown in FIG. The data 172 measured with the table speed set to 2v has a double shift in the z direction compared to the data 171 when the table speed is v. Further, since the phase encoding change amount is doubled, the interval in the kx direction is doubled. As a result, the field of view while photographing with the receiving coil 114 is halved and the table speed is doubled, so the photographing time is shortened to ½.

  In the case of reconstructing such data, in the sub-field of view that reconstructs the image using both the data 171 from the fixed coil 107 and the data 172 from the moving coil 114, among the data on the data 171 side, The phase-encoded data not measured by the data 172 is set to zero, and the number of data is made uniform to reconstruct the image. Thereby, an image with a small field of view is obtained. In this case, when the time during which both the fixed coil 107 and the moving coil 114 are on overlaps, for the data measured under the small field condition, the signal received by the fixed coil 107 and the signal received by the moving coil 114 Image reconstruction may be performed using a signal having a better S / N or a signal obtained by adding both.

  The image reconstructed in this way is shown in FIG. As shown in the figure, an image with a large visual field is obtained for the body part, and an image with a small visual field is obtained for the lower limb part. In the figure, the case where the receiving coil 114 is attached only to the lower limb portion is shown. However, when the receiving coil 114 is attached to two or more parts as shown in FIGS. Can be made. In that case, a different field of view can be set for each receiving coil.

  Moreover, although the case where the table speed is changed in accordance with the change of the visual field has been described, the number of additions may be changed without changing the table speed. As described above, when the field of view is halved, the number of data to be measured is halved. However, by doubling the number of additions, measurement can be performed at the same table speed as the basic condition. In this case, the measurement time is not shortened, but the number of additions increases, so the S / N can be improved. The arrangement of the obtained data is the same as in FIG. 14A, and the image can be reconstructed in the same manner as when the table moving speed is changed.

As an example of changing the pulse sequence, it is also possible to change the spatial resolution in accordance with the change of the visual field while keeping the table speed constant. For example, when the field of view is halved, the amount of phase encoding change during imaging by the receiving coil 114 is doubled, and the spatial resolution is doubled. FIG. 15 shows the kx-z space arrangement of data obtained by such a change. Since the field of view is 1/2 and the number of phase encoding steps does not change, the spatial resolution is doubled.
In this case, depending on the table speed v, as shown in FIG. 15, there may be a portion 163 having no data between the station 161 before changing the phase encoding change amount and the station 162 after changing it. . If reconstructed as it is, artifacts may occur. Therefore, in this embodiment, if the sub field of view at the time of echo measurement is made larger than the minimum wz determined by the above-described equation (1), the gap can be reduced and the occurrence of artifacts can be suppressed. .

  By performing inverse Fourier transform on the data measured so that there is no gap between the stations, an image similar to that shown in FIG. 14B is finally obtained. Also in this case, in the sub-field of image reconstruction using both the data 161 and the data 162, the phase encoding data not measured by the data 172 is set to zero in the data 161 side data, and the kx direction The image is reconstructed at a constant data interval. In the obtained image, the small field of view is measured by doubling the phase encoding change amount, so that the spatial resolution is twice that of the large field of view. Also in this case, the number of moving coils may be two or more, and the conditions to be changed for each moving coil may be different.

<Fourth embodiment>
Next, as a fourth embodiment, control when parallel shooting is applied to moving table shooting will be described. Even in this embodiment, using the receiving coil (fixed coil) fixed to the apparatus and the receiving coil (moving coil) attached to the subject is the same as the first and second embodiments, At least one of the fixed coil and the moving coil needs to be a coil capable of parallel photographing at a double speed or higher in the x direction. For example, when the field of view is reduced in the second embodiment, if the imaging object of the receiving coil is not so small and is larger than 1/2 of the field of view, the image is folded. In such a case, the aliasing of the image can be eliminated by performing parallel reconstruction using a coil capable of parallel shooting at a double speed or higher in the x direction as the receiving coil.

An example of the configuration of the coil according to the present embodiment is shown in FIG.
In the MRI apparatus shown in FIG. 16, a transmission / reception coil 107 is arranged inside the magnet 101 and the gradient magnetic field 102 as a coil fixed on the apparatus side, the reception coil 107-2 at the upper part inside, and 107- at the lower part. 3 is arranged. The positional relationship between the transmission / reception coil 107 and the reception coils 107-2 and 107-3 is fixed. Normally, the origins of the magnet, the gradient magnetic field, the transmission / reception coil 107, and the reception coils 107-2 and 107-3 are the same. ing. A reception coil 114-4 dedicated to the lower limb is disposed on the lower limb of the subject (position from z 1 to z 2 in the table coordinates) and moves together with the table 152. When the reception coil 114-4 is moved to the center position of the magnet by moving the table, it is assumed to be located inside the reception coil 107-2. In addition, the combination of the reception coils 107-2 and 107-3 enables parallel imaging in which the speed-up rate in the y direction (or the x direction) is up to pf10, and the reception coil 107-3 and the reception coil 114-4. In this combination, it is assumed that parallel shooting is possible up to a speed increase rate of pf20 in the y direction (or x direction).

The control of each coil and the control of the table speed by the sequencer in such a configuration will be described with reference to FIGS.
When imaging from the top of the head to the toes is performed, imaging starts from a state in which the top position z0 of the subject is on the right side of the center of the magnet. As the imaging starts, the table moves to the left, and echoes having different kx are repeatedly measured at the repetition time Tr. The reception coil 114-4 and the reception coils 107-2 and 107-3 are turned on at the start time, and the front end z1 of the reception coil 114-4 is a predetermined position, for example, a position wz / 2 before the center position of the magnet. Until this value is reached, parallel photographing using the receiving coils 107-2 and 107-3 is performed. That is, imaging is performed with phase encoding thinned out within the range of the speedup rate possible with the receiving coils 107-2 and 107-3, and an operation for removing the aliasing of the image is performed using the signals received by these coils.

  If the receiving coil 114-4 enters the inside of the receiving coil 107-2, the receiving coil 107-2 is turned off. Coupling between the coil 114-4 and the coil 107-3 arranged with the subject sandwiched therebetween hardly poses a problem, but the reception coil 114-4 and the reception coil 107-2 mounted on the upper side of the subject Since the distance is shorter than the distance between the reception coil 114-4 and the reception coil 107-3, the coupling is large. Therefore, coupling is prevented by turning off the receiving coil 107-2.

  At the same time, reception is performed by the reception coil 114-4 and the reception coil 107-3, and parallel photographing is performed within a range of a high-speed rate possible by the combination of these coils. If the rear end z2 of the receiving coil 114-4 has advanced from the center position of the magnet to the left side to wz / 2, the receiving coil 107-2 is turned on, and parallel imaging with the receiving coils 107-2 and 107-3 is performed again. Do.

  The moving speed v of the table changes according to the speeding up rate of the coil. For example, in the case of a double speed parallel coil, the table speed can be increased to twice the normal shooting speed (v1) (v = 2 × v1), and in the case of a triple speed parallel coil, the table speed can be increased. It is possible to make it up to 3 times (v = 3 × v1). In the example shown in FIG. 18, while receiving signals with the receiving coils 107-2 and 107-3 and performing parallel shooting at the acceleration rate pf 1 (pf 1 <pf 10), v = pf 1 × v 1, and the receiving coil 107 -3 and 114-4 are received, and v = pf2 × v1 while parallel shooting is performed at the acceleration rate pf2 (pf2 <pf20).

  According to the present embodiment, the imaging time is shortened by performing parallel imaging of the receiving coils 107-2 and 107-3, which are fixed coils, and parallel imaging of the receiving coil 114-4, which is a stationary coil and a moving coil. be able to.

In addition, although the case where all shooting was set to parallel shooting was explained, normal shooting (shooting with a high speed ratio of 1) may be performed without performing parallel shooting while receiving signals with the receiving coil 107-2. it can. In this case, assuming that the speed of parallel imaging by the receiving coil 114-4 is pf2, the table speed is pf2 × v1 only while receiving by the receiving coil 114-4, and v1 is otherwise. By doing so, while the receiving coil 114-4 is in the predetermined range, it is possible to perform photographing with the sensitive receiving coil 114-4 and to suppress the S / N reduction of the image.
On the contrary, while receiving signals with the receiving coil 114-4 and the receiving coil 107-3, normal shooting is performed without parallel shooting, and signals are received with the receiving coils 107-2 and 107-3. It is also possible to use parallel shooting. In this case, assuming that the speed of parallel imaging by the receiving coil 107-2 is pf1, the table speed is pf1 × v1 only while receiving by the receiving coil 107-2, and v1 is otherwise.

  According to the present embodiment, it is possible to solve the folding of the image accompanying the reduction of the field of view by parallel photographing. For example, when the field of view is reduced in the second embodiment, if the object to be imaged by the reception coil is not so small and is larger than 1/2 of the field of view, the image will be folded back. In addition, it is possible to remove the aliasing of the image by performing parallel reconstruction using a coil capable of parallel photographing at a double speed or higher.

The block diagram which shows the whole structure of the MRI apparatus of this invention. The figure explaining the relationship between the table and imaging | photography visual field in moving table imaging | photography. The figure which shows the arrangement to the hybrid space of the data obtained by moving table photography The figure which shows the example of arrangement | positioning of the transmission / reception coil and receiving coil in 1st embodiment. The flowchart of the process from imaging | photography to image display of 1st embodiment. The figure which shows one Example of the coil control in 1st embodiment. The figure which shows the other Example of the coil control in 1st embodiment. The figure which shows the example of arrangement | positioning of the transmission / reception coil and receiving coil in 2nd embodiment. The figure which shows one Example of the coil control in 2nd embodiment. The figure which shows another example of arrangement | positioning of the transmission / reception coil and receiving coil in 2nd embodiment. The figure which shows the other Example of the coil control in 2nd embodiment. The flowchart of the process from imaging | photography to image display of 3rd embodiment. The figure which shows the relationship between the moving speed of a table and imaging | photography time in 3rd embodiment. The figure which shows arrangement | positioning to the hybrid space of the measurement data of 3rd embodiment. The figure which shows arrangement | positioning to the hybrid space of the measurement data of the other example of 3rd embodiment. The figure which shows the example of arrangement | positioning of the transmission / reception coil and receiving coil in 4th embodiment. The figure which shows one Example of the coil control in 4th embodiment. The figure which shows the relationship between the moving speed of the table in 4th embodiment, and imaging | photography time.

Explanation of symbols

100: MRI apparatus, 101: magnet, 102: gradient magnetic field coil, 103: subject, 104: sequencer, 105: gradient magnetic field power source, 106: high-frequency magnetic field generator, 107: transmission / reception coil (fixed coil), 107-2, 107-3: Receive coil (fixed coil), 108, 115: Receiver, 109: Computer, 110: Display, 111: Storage medium, 114, 114-2, 114-3: Receive coil (moving coil)

Claims (8)

A magnetic field generator for generating a static magnetic field and a gradient magnetic field in the imaging space;
A subject holding unit that holds the subject and moves in the imaging space;
A first receiving coil fixed on the magnetic field generating unit side and a second receiving coil fixed on the subject holding unit side, and a receiving unit for receiving a nuclear magnetic resonance signal;
An image reconstruction unit that reconstructs an image of the subject using signals received by the first reception coil and the second reception coil, respectively;
A control unit that controls the magnetic field generation unit, the reception unit, and the subject holding unit;
The control unit generates the magnetic field so as to repeatedly perform echo measurement while moving the subject holding unit in a first direction and changing a phase encoding value in a second direction perpendicular to the first direction. parts, the receiving unit and controls the specimen holder, and, depending on the position of the second receiver coil relative to the imaging space, switching on and off of said first receiver coil and / or the second receiver coil, further The phase between the time when at least a part of the second receiving coil is in the imaging space and the second receiving coil is on (second imaging time) and at other times (first imaging time) A magnetic resonance imaging apparatus characterized by varying the amount of change in encoding . The magnetic resonance imaging apparatus according to claim 1 ,
The magnetic resonance imaging apparatus, wherein the control unit makes the speed of the subject holding unit during the second imaging time different from the speed of the subject holding unit during the first imaging time. The magnetic resonance imaging apparatus according to claim 1 ,
The magnetic resonance imaging apparatus, wherein the control unit makes the visual field in the second direction smaller in the second imaging time than in the first imaging time. The magnetic resonance imaging apparatus according to claim 1 , wherein the second imaging time increases the number of signal additions more than the first imaging time. The magnetic resonance imaging apparatus according to claim 1, wherein the second imaging time has a higher spatial resolution in the second direction than the first imaging time. . The magnetic resonance imaging apparatus according to claim 1, wherein the second imaging time has a smaller matrix size in the second direction than the first imaging time. . The magnetic resonance imaging apparatus according to claim 1 ,
The first receiving coil and the second receiving coil are coils constituting a parallel coil capable of parallel photographing,
The controller is configured to increase the speed of the subject holding unit while the coils constituting the parallel coil are on, compared to when the coil is off. The magnetic resonance imaging apparatus according to claim 1,
  The reception unit includes a third reception coil fixed to the magnetic field generation unit side,
  The second receiving coil is a coil constituting a parallel coil capable of parallel shooting with the third receiving coil,
  In the imaging time when the coil constituting the parallel coil is on, the control unit increases the speed of the subject holding unit and makes the field of view in the second direction smaller than the other imaging times. And
  The magnetic resonance imaging apparatus, wherein the image reconstruction unit performs parallel reconstruction using a signal received by the parallel imaging, and reconstructs an image in which image folding due to a signal outside the visual field does not occur.

Images