JP2017113164A - Magnetic resonance imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique, even when a central position of a setup FOV does not match a center of an imaging object or the like, capable of removing aliasing artifact and setting an FOV magnification for minimizing an imaging time extension.SOLUTION: An MRI apparatus includes an FOV calculation part that automatically calculating an optimal FOV magnification from information acquired before real measurement such as a size and a position of the setup FOV set as imaging parameters, an R factor, and a spatial range capable of receiving a signal. The FOV calculation part differentiates FOV magnifications between one and the other of phase encoding directions from the central position according to positions of the setup FOV.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a magnetic resonance imaging (hereinafter referred to as “MRI”) apparatus, and more particularly to a technique for optimizing imaging parameters in consideration of removal of aliasing artifacts.

  The MRI apparatus measures NMR signals generated by nuclear spins constituting a subject, particularly a human tissue, and images the form and function of the head, abdomen, limbs, etc. two-dimensionally or three-dimensionally. Device. In imaging, the NMR signal is given a different phase encoding and frequency encoded by a gradient magnetic field superimposed on a static magnetic field on which the subject is placed, and is measured as time-series data. The measured NMR signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  Prior to the actual measurement, the MRI apparatus performs pre-measurement such as measurement of a positioning image and sensitivity measurement of a receiving coil, and based on the results of these pre-measurements, the user can view the field of view (Field of View: hereinafter referred to as FOV). And various other imaging parameters are set. However, in MRI imaging, when a tissue or the like is present outside the FOV in the phase encoding direction, the tissue outside the FOV is folded back into the FOV and depicted as an artifact. In order to prevent the occurrence of the aliasing artifact, the user sets an enlargement ratio with respect to the set FOV (set FOV) and actually measures the FOV (measured FOV) obtained by expanding the set FOV in the phase encoding direction. Normally, the user sets the FOV enlargement ratio by predicting the occurrence position of the folding artifact, but the occurrence position of the folding artifact varies depending on the positional relationship between the imaging target and the set FOV and the size of the imaging target. The position is difficult to predict.

  On the other hand, as an imaging technique of the MRI apparatus, there is a technique (referred to as parallel imaging) that uses a plurality of receiving coils and reconstructs an image in which folding is removed from a signal including folding obtained by each receiving coil. When this parallel imaging is applied, the occurrence position of the aliasing artifact varies depending on the speed of parallel imaging (hereinafter referred to as R factor), so that it is more difficult to predict the occurrence position of the aliasing artifact. As a result, a non-optimal FOV enlargement ratio may be set by the user. When the FOV enlargement ratio is smaller than the optimum value, aliasing artifacts occur in the set FOV. Further, when the FOV enlargement ratio is larger than the optimum value, no aliasing artifacts occur in the set FOV, but extra imaging time is required. For this reason, the user visually determines the presence or absence of aliasing artifacts in the set FOV in the image obtained by the actual measurement, and if aliasing artifacts have occurred in the setting FOV, change the FOV enlargement ratio again. It is necessary to perform this measurement.

  To deal with this problem, Patent Document 1 discloses a technique for calculating a return attention area from a set FOV, FOV enlargement ratio, R factor, and signal reception range, and displaying the return attention area on an imaging parameter setting screen. ing.

JP 2012-192216 A

  In the technique described in Patent Document 1, by displaying the return attention area on the imaging parameter setting screen, the user can be presented with an area at risk of occurrence of return, but the user considers the return attention area. Therefore, since it is necessary to optimize the imaging parameters by the user himself, it takes time to determine the imaging parameters.

  In the conventional technique, the set FOV enlargement ratio is applied isotropically in the phase encoding direction with respect to the set FOV. That is, the FOV is enlarged at the same enlargement ratio from the center of the set FOV on one side and the other side in the phase encoding direction. However, the center position of the set FOV does not necessarily coincide with the center position in the phase encoding direction of the imaging target. In that case, in order not to generate a folding artifact on either side, extra measurement is performed on one side, leading to an extended imaging time.

  An object of the present invention is to obtain an MRI image free from aliasing artifacts by extending the minimum imaging time without reducing inspection throughput. Another object of the present invention is to provide an MRI apparatus that can set an optimum FOV enlargement ratio even when the center position of the set FOV does not coincide with the center of the imaging target or the like.

  In order to achieve the above object, the MRI apparatus of the present invention uses an optimum FOV expansion rate from information obtained before the actual measurement, such as the size and position of the set FOV, the R factor, and the spatial range in which signals can be received. It has a function to calculate automatically. In particular, the MRI apparatus of the present invention is characterized in that the FOV enlargement ratio is different between one and the other in the phase encoding direction from the center position according to the position of the set FOV.

  According to the present invention, since the minimum FOV enlargement ratio considering the removal of aliasing artifacts is automatically calculated, an MRI image without aliasing artifacts can be obtained by extending the minimum imaging time without reducing the inspection throughput. Can get. Further, according to the present invention, by making the FOV enlargement ratio asymmetric with respect to the phase encoding direction in accordance with the center position of the set FOV, FOV optimization can be achieved regardless of the position of the set FOV.

The figure which shows the whole structure of the MRI apparatus with which this invention is applied. The functional block diagram of the signal processing part and control part of embodiment. The figure which shows operation | movement of the MRI apparatus of embodiment. The functional block diagram of the signal processing part of 1st embodiment. The figure which shows the process sequence of 1st embodiment. The figure which shows an example of an object presence range. The figure explaining the optimal FOV expansion rate calculation process of 1st embodiment. The figure explaining the process of the example of a change of 1st embodiment. The functional block diagram of the signal processing part of 2nd embodiment. The figure which shows the process sequence of 2nd embodiment. It is a figure for demonstrating the process of 2nd embodiment, (a) is a sensitivity distribution map of the channel 1, (b) is the expansion sensitivity distribution map. It is a figure for demonstrating the process of 2nd embodiment, (a) is a sensitivity distribution map of the channel 2, (b) is the expansion sensitivity distribution map. Composite sensitivity distribution of channels 1 and 2. Composite sensitivity distribution considering the suppression region. The figure which shows an example of the sensitivity range calculated in 2nd embodiment. The figure explaining the optimal FOV expansion rate calculation process of 2nd embodiment. The functional block diagram of the signal processing part of 3rd embodiment. The figure which shows the process sequence of 3rd embodiment. The figure which shows an example of the static magnetic field effective range calculated in 3rd embodiment. The figure for demonstrating the process of 3rd embodiment. The functional block diagram of the signal processing part of 4th embodiment. The figure which shows the process sequence of 4th embodiment. The figure for demonstrating the process of 4th embodiment. The figure for demonstrating the process of 4th embodiment. The figure for demonstrating the process of 4th embodiment. The figure which shows the example of a screen in embodiment of a display. The figure which shows another example of a screen in embodiment of a display.

  The MRI apparatus of the present invention includes a magnetic field generating unit that generates nuclear magnetic resonance in a subject placed in a static magnetic field space, a receiving unit that has a receiving coil and receives a nuclear magnetic resonance signal generated from the subject, An image processing unit that processes a nuclear magnetic resonance signal received by a receiving unit to create an image of the subject, and an imaging parameter setting unit that sets an imaging parameter necessary for imaging, wherein the imaging parameter is an imaging An imaging field calculation unit that calculates an enlargement ratio of the imaging field based on a relationship between the imaging field including the field of view and the imaging field set by the imaging parameter setting unit and a spatial range in which the nuclear magnetic resonance signal can be received Prepare.

  The imaging field calculation unit varies the magnification of the imaging field by, for example, two halves divided by the center in the phase encoding direction of the imaging field.

  Hereinafter, embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus to which the present invention is applied will be described with reference to a block diagram shown in FIG. As shown in FIG. 1, the MRI apparatus includes, as main elements, a static magnetic field generation unit 2, a gradient magnetic field generation unit 3, a transmission unit 5, a reception unit 6, a signal processing unit 7, a sequencer 4, The control unit 8 is provided. The subject 1 is normally inserted into the static magnetic field space of the static magnetic field generating unit 2 while being laid on a bed. Hereinafter, the static magnetic field generation unit 2, the gradient magnetic field generation unit 3, the sequencer 4, the transmission unit 5, and the reception unit 6 are collectively referred to as an imaging unit.

  The static magnetic field generation unit 2 includes a static magnetic field generation source (not shown) arranged around the subject 1 and generates a uniform static magnetic field in the space around the subject 1. As the static magnetic field generation source, there are permanent magnet type, normal conduction type and superconducting type, any of which may be used. Depending on the direction of the generated static magnetic field, there are a vertical magnetic field method and a horizontal magnetic field method. The former generates a static magnetic field in a direction perpendicular to the body axis of the subject 1 lying down, and the latter generates a static magnetic field in the body axis direction.

  The gradient magnetic field generating unit 3 drives three sets of gradient magnetic field coils 9 that apply gradient magnetic fields in the three axial directions of X, Y, and Z, which are the coordinate system (stationary coordinate system) of the MRI apparatus, and the respective gradient magnetic field coils. And a gradient magnetic field power supply 10 that performs. By driving the gradient magnetic field power supply 10 of each coil in accordance with a command from the sequencer 4, gradient magnetic fields Gx, Gy, and Gz are generated in the three axial directions of X, Y, and Z. Slice direction gradient magnetic field pulse (Gs) for determining the slice plane (photographing cross section), phase encode direction gradient magnetic field pulse (Gp) and frequency encode direction gradient magnetic field pulse (Gf) in a direction orthogonal to the slice plane and orthogonal to each other. Apply the position information of each direction to the echo signal.

  The sequencer 4 is a control unit that repeatedly applies a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) and a gradient magnetic field pulse in a predetermined pulse sequence. The sequencer 4 operates under the control of the control unit 8 and Various commands necessary for data collection are sent to the transmission unit 5, the gradient magnetic field generation unit 3, and the reception unit 6.

  The transmitter 5 irradiates the subject 1 with an RF pulse in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the living tissue of the subject 1, and includes a high frequency oscillator 11, a modulator 12, and a high frequency amplifier. 13 and a high frequency coil (transmission coil) 14a on the transmission side. The RF pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the sequencer 4, and after the amplitude-modulated RF pulse is amplified by the high-frequency amplifier 13, the RF pulse is arranged close to the subject 1. By supplying to the high frequency coil 14a, the subject 1 is irradiated with the RF pulse.

  The receiving unit 6 detects an echo signal (NMR signal) emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 1, and includes a receiving-side high-frequency coil (receiving coil) 14 b and a signal amplifier 15. And a quadrature phase detector 16 and an A / D converter 17. The NMR signal of the response of the subject 1 induced by the electromagnetic wave irradiated from the transmission coil 14a is detected by the reception coil 14b arranged close to the subject 1, amplified by the signal amplifier 15, and then from the sequencer 4. Are divided into two orthogonal signals by the quadrature detector 16, converted into digital quantities by the A / D converter 17, and sent to the signal processor 7.

  Note that the transmission coil 14a and the reception coil 14b can be combined into one coil. The transmission coil 14a and the reception coil 14b may be a single coil, or may be a multiple coil in which a plurality of small coils are combined. Although not limited thereto, the transmission coil 14a and the gradient magnetic field coil 9 are opposed to the subject 1 in the static magnetic field space if the vertical magnetic field method is used, and surround the subject 1 if the horizontal magnetic field method is used. It is installed. The receiving coil 14b is disposed so as to face or surround the subject 1.

  The signal processing unit 7 performs various data processing and display and storage of processing results. The CPU 18 also functions as the control unit 8, an external storage device such as a magnetic disk and an optical disk, and an internal storage such as a ROM and RAM. A storage device 19 such as a device, a display device 20 such as a CRT, an LCD, etc., and an input such as a trackball or a mouse or a keyboard for an operator to input various control information and control information of processing performed by the signal processing unit 7 Device 23. The input device 23 is usually arranged in the vicinity of the display device 20 or constitutes a console integrated with the display device 20, and various types of MRI apparatuses can be interactively viewed through the input device 23 while the operator looks at the display device 20. Control processing.

  The signal processing unit 7 also functions as an image processing unit. When data from the receiving unit 6 is input to the CPU 18, the CPU 18 executes processing such as signal processing and image reconstruction, and the result is the subject 1. These tomographic images are displayed on the display device 20 and recorded in an external storage device or the like. The signal processing unit 7 also includes an FOV calculation unit that calculates an FOV enlargement ratio that is a condition necessary for imaging.

  Details of the signal processing unit 7 and the control unit 8 will be described with reference to a functional block diagram shown in FIG. The figure shows an example in which the signal processing unit 7 and the control unit 8 are mainly constructed as software on the CPU 18 as elements for realizing the FOV enlargement ratio optimization, but some or all of the functions are ASIC (application specific integrated). circuit) or FPGA (field-programmable gate array).

  As shown in FIG. 2, the CPU 18 includes a storage device 19, a display device 20, and an input device 23 as accessory devices. The storage device 19 stores data consisting of NMR signals (digital signals) received by the receiving unit 6, results calculated by the calculation unit in the CPU 18, and imaging conditions (selected or set via the input device 23). (Including imaging parameters and pulse sequences). In addition to the MR image that is the result of the calculation, the display device 20 displays a setting screen (GUI) for the user to input imaging parameters and imaging conditions. The input device 23 includes the mouse and keyboard already described. However, when the display device 20 is a device capable of input such as a touch panel, the display device may also serve as part or all of the input device.

  The CPU 18 includes an imaging control unit 81, an image processing calculation unit 82, a display control unit 83, an imaging parameter setting unit 84, and an FOV calculation unit 80. The functions of the imaging control unit 81, the image processing calculation unit 82, and the display control unit 83 are the same as those of the conventional MRI apparatus. Briefly, the imaging control unit 81 controls the sequencer 4 under the set imaging conditions to drive the gradient magnetic field generation unit 3, the transmission unit 5, and the reception unit 6, and a desired imaging method (imaging pulse sequence). Perform imaging with. The image processing calculation unit 82 creates an image using the NMR signal received by the receiving unit 6 and the data stored in the storage device 19 as necessary. The image includes a phase image, a sensitivity distribution image, and the like in addition to an image representing the form of the subject. The display control unit 83 creates a display image in order to cause the display device 20 to display the image created by the image processing calculation unit 82 and the image constituting the setting screen. In addition, the display control unit 83 receives a user command via the input device 23 and performs control such as a change in the display image magnification and position.

  The imaging parameter setting unit 84 takes in imaging parameters set by the user via the input device 23 and imaging parameters stored in advance in the storage device 19 and passes them to the imaging control unit 81. At this time, the imaging parameter setting unit 84 enlarges the set FOV using the enlargement factor that is the calculation result of the FOV calculation unit 80 and passes the set FOV to the imaging control unit 81 as the actual FOV. Therefore, imaging is performed not with the set FOV but with an automatically calculated enlarged FOV (actually measured FOV).

  The FOV calculation unit 80 uses information on a spatial range in which a received signal can be received (hereinafter referred to as a receivable range) and information on FOV and R factor among imaging parameters set via the input device 23 and the like. To optimize the FOV. The information on the receivable range includes, for example, a range where an object such as a subject exists, a range of the reception sensitivity distribution of the receiving coil, a range of a static magnetic field space, and the like, and one or more of them are used for optimization. The information on the object presence range and the reception sensitivity distribution can be obtained from the previous measurement. The range of the static magnetic field space is an eigenvalue of the apparatus itself and may be stored in the storage device 19 in advance.

  Based on the above configuration, an imaging procedure by the MRI apparatus of the present embodiment will be described with reference to FIG. Prior to measurement (main measurement) for imaging a desired imaging position of the subject 1, pre-measurement for determining the imaging position is performed (S31). In the pre-measurement, an image (scanogram) with a relatively low spatial resolution and a wide field of view is obtained. In some cases, measurement of the sensitivity of the receiving coil is included. Next, imaging parameters necessary for the actual measurement are set (S32). The imaging parameters include the size and position of the FOV, the R factor, and the like. For example, when the user inputs through the operation unit 25 while looking at the scanogram displayed on the display device 20, the value is captured by the CPU 18. It is set in the parameter setting unit 84. Next, the FOV calculation unit 80 calculates the FOV enlargement ratio based on the set imaging parameters (S33). The actual measurement is performed with the FOV expanded at the FOV expansion rate (S34).

  The calculation of the FOV enlargement ratio (S33) is an optimization process for obtaining the enlargement ratio of the FOV that does not cause aliasing artifacts in the image or the region of interest in the image and prevents the imaging time from being extended. And the technique may take various forms. Each embodiment of the configuration and method of optimizing the FOV enlargement ratio will be described below.

<< First Embodiment >>
The present embodiment uses an FOV (set FOV) set via the input device 23, an R factor, and an existence range (hereinafter referred to as an object existence range) of an object (subject) obtained by previous measurement or the like. Thus, the optimum FOV enlargement ratio is calculated.

  First, the configuration of the apparatus of this embodiment will be described with reference to the functional block diagram of FIG.

  As shown in FIG. 4, the CPU 18 includes an FOV enlargement ratio calculator 85 and an object presence range calculator 86 as the FOV calculator 80. The object existence range calculation unit 86 calculates the object existence range in the area including the set FOV and the surrounding area using the existence information of the object in the static magnetic field space. The presence information of the object in the static magnetic field space can be obtained from an image such as a scanogram obtained by the previous measurement, for example. The FOV enlargement factor calculation unit 85 calculates the enlargement factor in the phase encoding direction with respect to the set FOV of the actually measured FOV, using the set FOV, the R factor, and the information on the object existence range calculated by the object existence range calculation unit 86. . Details of the calculation method will be described later.

Next, the operation of this embodiment will be described with reference to the processing procedure shown in FIG.
<Step S51>
First, in order to calculate the existence range of an object (subject) in a static magnetic field space, the imaging control unit 81 images the subject over a wide range by a sequence for imaging the imaging object over a wide range. The sequence for realizing wide-range imaging is arbitrary. For example, a method of imaging the imaging target in a wide range by increasing the frequency encoding can be considered. Alternatively, a method of irradiating the subject with one or a few excitation pulses over a wide range and calculating the existence range of the object from the obtained echo signal may be used. Further, a method of performing wide-area imaging in a short time using parallel imaging may be used. Further, parallel imaging may be used in combination with the above-described wide-range imaging method, and a method of imaging a wider range or reducing the measurement time may be used. Note that an image obtained by wide-range imaging may be used as a positioning image. Further, the wide range imaging may be measured separately from the positioning image measurement (pre-measurement: S31 in FIG. 3).

  When this step S51 is performed separately from the previous measurement, the positioning image measurement (S31) and imaging parameter setting (S32) shown in FIG. 3 are performed prior to or after the present step S51.

<Step S52>
The object presence range calculation unit 86 calculates the presence range of the object in the phase encoding direction from the wide range image acquired in step S51. The calculation method of the existence range of the object can adopt a known contour extraction method or a projection method, and is not particularly limited. However, as an example, phase encoding is performed from both ends of the phase encoding direction of a wide range image. The pixel value is scanned along the direction, and the point where the pixel value changes sharply from zero (no signal) can be defined as the end of the existence range of the object, and the interval between the two points can be the existence range. Note that the range to be scanned in the phase encode direction (the range in the direction orthogonal to the phase encode direction) may be the set FOV range.

  An example of calculating the existence range of the object is shown in FIG. The horizontal direction in the figure is the phase encoding direction, and the calculated existence range of the phase encoding direction of the object is indicated as “Object”.

<Step S53>
The imaging parameter setting unit 84 acquires imaging parameter information input to the user by the input device 23 and stored in the storage device 19.

<Step S54>
The FOV enlargement ratio calculation unit 85 automatically calculates the optimum value of the FOV enlargement ratio using the object existence range calculated in step S52 and the imaging parameter acquired in step S53. As the imaging parameters, FOV and R factor are used.

  In this embodiment, the phase encoding direction center of the FOV (hereinafter also simply referred to as the FOV center) is deviated from the phase encoding direction center (hereinafter also simply referred to as the object center) of the object existence range [Object] (OFF). In the case of the center), the enlargement ratio is made different between the side where the FOV center is shifted and the opposite side, and the enlargement ratio that does not cause aliasing and minimizes the sum of the respective enlargement ratios is obtained. When the FOV center coincides with the object center, this is a singular point where the shift amount (off-center amount) is zero, and is included in the application range of this embodiment.

First, the relationship between the position of the FOV with respect to the object presence range and the occurrence of aliasing will be described with reference to FIG. In FIG. 7, the left-right direction is the phase encoding direction, and the object center in the phase encoding direction is represented by C _O and the FOV center is represented by C _F. A range indicated by diagonal lines is the setting FOV 700. Also, the object existence range [Object] is divided into two at the FOV center C _F , the shorter one from the center to the end is expressed as “ObjectS”, the longer one is expressed as “ObjectL”, and the longer one from the FOV center. The direction toward L is the L direction, and the direction toward the shorter is the S direction. In the figure, a portion 702 surrounded by a solid line is an FOV expanded at a predetermined enlargement ratio (NoWrapL, NoWrapS), and a portion 701 surrounded by a dotted line is an FOV reduced by an R factor. When R factor = 1, there is no reduction by the R factor, so the dotted line portion matches the solid line portion.

  In the folding, an image of an object outside (one outside) the phase encoding direction of the FOV appears on the other end side in the FOV. For example, when the FOV 700 is enlarged with an enlargement rate NoWrapL / R in the L direction (right side) and an enlargement rate NoWrapS / R in the S direction (left side) in consideration of the R factor (the portion 701 surrounded by a dotted line in the figure becomes the FOV). Case), a portion outside the right end of the FOV 701 (arrow A1 in the figure) turns back on the left side of the FOV 701 (arrow A2 in the figure). If the folding indicated by the arrow A2 does not enter the original FOV 700, the folding artifact does not occur, and the requirement for the enlargement ratio is satisfied.

  Similarly, a portion outside the left end of the FOV 701 (arrow B1 in the figure) turns back on the right side of the FOV 701 (arrow B2 in the figure). If the folding shown by the arrow B2 does not enter the original FOV 700, the folding artifact does not occur, and the requirement for the enlargement ratio is satisfied. This condition can be expressed by the following formulas (1) and (2).

ObjectL− (FOV / 2) × (NoWrapL / R) ≦ (FOV / 2) (NoWrapS / R) −FOV / 2 (1)
ObjectS− (FOV / 2) × (NoWrapS / R) ≦ (FOV / 2) (NoWrapL / R) −FOV / 2 (2)
In the expression, ObjectL represents the width of “ObjectL” in the phase encoding direction, and ObjectS represents the width of “ObjectS” in the phase encoding direction. NoWrapL and NoWrapS represent the enlargement ratio in the L direction and the enlargement ratio in the S direction, respectively.

From these formulas (1) and (2), formulas (3) and (4) with the sum of the enlargement ratios in the L direction and the S direction (NoWrapL + NoWrapS) as the left side are derived.
NoWrapL + NoWrapS ≧ 2R × ObjectL / FOV + R (3)
NoWrapL + NoWrapS ≧ 2R × ObjectS / FOV + R (4)

In order to minimize the imaging time, NoWrapL and NoWrapS that minimize the right side (NoWrapL + NoWrapS) in Equation (3) or (4) may be obtained. Therefore, first, the NoWrapL is fixed so that the L-direction end of the FOV expanded (or reduced) by the NoWrapL coincides with the L-direction end of “ObjectL”. That is, NoWrapL = ObjectL / (FOV / 2). Further, since ObjectL ≧ ObjectS, the right side of (3) ≧ the right side of (4), NoWrapS is obtained in consideration of Equation (4). Substituting NoWrapL into equation (4),
NoWrapS ≧ 2 (R × ObjectS- ObjectL) / FOV + R
Therefore, the minimum values of NoWrapL and NoWrapS are expressed by the following equations (5) and (6), respectively.

NoWrapL = 2ObjectL / FOV (5)
NoWrapS = 2 (R × ObjectS- ObjectL) / FOV + R (6)

However, NoWrapL and NoWrapS shown in equations (5) and (6) become the minimum values NoWrapL _Min and NoWrapS _Min when the width of “ObjectL” is larger than (FOV / 2) and the following inequality is satisfied. Limited.
(FOV / 2) × NoWrapS ≦ ObjectS (7)
NoWrapS ≧ 1 (8)

When the conditions of equations (7) and (8) are added and divided, the minimum values NoWrapL _Min and NoWrapS _Min are as follows.
Condition 1: FOV / 2 ≦ ObjectL and (FOV / 2) × NoWrapS ≦ ObjectS and NoWrapS ≧ 1
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 2 (R × ObjectS- ObjectL) / FOV + R
Condition 2: FOV / 2> ObjectL or (FOV / 2) × NoWrapS> ObjectS or NoWrapS <1
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 2ObjectS / FOV

<Step S55>
The imaging parameter setting unit 84 transmits the FOV enlargement ratio calculated in step S54 to the storage device 19 and sets it as an imaging parameter. Alternatively, the measured FOV is set by enlarging the set FOV with the FOV expansion rate calculated in step S54.

<Steps S56 to S59>
The imaging control unit 81 performs the main measurement with the actual FOV enlarged by the FOV enlargement ratio calculated in step S54 (S56). The image processing calculation unit 82 reconstructs the measurement data obtained in step S56 (S57). As a result, an MRI image of the measured FOV size is obtained, and an image of the set FOV size is cut out from this MRI image (S58). The display controller 83 displays the MRI image cut out in step S58 on the display device 20 (S59).

  In the MRI image displayed on the display device 20, the portion where the aliasing artifact has occurred is deleted, and an image without the artifact is displayed. Further, since the enlargement ratio is set so as to minimize the difference between the measured FOV and the set FOV, including the aliasing, the imaging time is not unnecessarily prolonged.

  As described above, according to the present embodiment, an MRI image free from aliasing artifacts can be obtained by extending the minimum imaging time without reducing the inspection throughput.

<< Modification of First Embodiment >>
This modification is characterized in that the enlargement ratio optimization calculation is performed in consideration of the range in which aliasing is allowed. That is, in the first embodiment, the FOV enlargement ratio, that is, the measured FOV is set so as to delete all the portions including the folding artifacts in consideration of the object existence range. For example, the tissue or region to be inspected is the FOV. In some cases, the image quality deterioration around the FOV does not affect the interpretation. In this modification, in such a case, the enlargement ratio is optimized in a direction that further shortens the imaging time by setting a range in which folding is allowed.

  The configuration of the present modification is the same as that of the first embodiment except that the processing of the FOV enlargement factor calculation unit 85 shown in FIG. 4 is different. Hereinafter, refer to FIGS. 3 and 5 used in the first embodiment. The description will focus on the differences from the first embodiment.

  First, wide-area imaging is performed as pre-measurement (S31, S51), and setting of imaging parameters by the operator is accepted (S32). When setting the imaging parameters, a range (hereinafter simply referred to as “allowable range”) α that allows aliasing in the vicinity of the end in the phase encoding direction of the FOV is received. For example, the allowable range may be specified by the operator on the FOV setting screen, or may be specified by inputting a numerical value (such as a range of 5% of the FOV from the end). . FIG. 8 shows how the allowable range α is set.

  Next, the object presence range calculation unit 86 calculates the object presence range (S52). Next, the FOV enlargement factor calculation unit 85 acquires imaging parameters (S53). The imaging parameter includes an allowable range α in addition to the FOV and R factor. The FOV enlargement ratio calculation unit 85 calculates the FOV enlargement ratio using these imaging parameters and the object presence range.

When the allowable range α is considered, the expressions (1) and (2) of the first embodiment are as follows.
ObjectL− (FOV / 2) × (NoWrapL / R) ≦ (FOV / 2) (NoWrapS / R) −FOV / 2 + α (1-1)
ObjectS− (FOV / 2) × (NoWrapS / R) ≦ (FOV / 2) (NoWrapL / R) −FOV / 2 + α (2-2)

Accordingly, the sum of the FOV enlargement rates in the L direction and the S direction (NoWrapL + NoWrapS) that does not cause aliasing artifacts in the FOV inside the allowable range α is expressed by the following equation.
NoWrapL + NoWrapS ≧ 2R × ObjectL / FOV + R-2αR / FOV (3-2)
NoWrapL + NoWrapS ≧ 2R × ObjectS / FOV + R-2αR / FOV (4-2)

Again, considering NoWrapL as 2 ObjectL / FOV and considering equation (4-2), equation (4-2) becomes
NoWrapS ≧ 2 (R × ObjectS− ObjectL−αR) / FOV + R (4-3)
It becomes.

Here, when ObjectS is (FOV / 2) or more and the enlargement ratio NoWrapS of (FOV / 2) in the S direction is 1 or more (NoWrapS ≧ 1), α is an expression from Expression (4-3). (9) will be satisfied.
α ≦ ObjectS + FOV / 2−ObjectL / R−FOV / 2R (9)

Since α> 0, the right side of (9) must be 0 or more. Conversely, if the right side of (9) is greater than or equal to 0, the imaging time may be shortened by setting α (> 0), but if taking a negative value, α is set. However, the effect of shortening the imaging time cannot be obtained. Therefore, if the right side of (9) is defined by Unfold represented by Expression (10),
Unfold = ObjectS + FOV / 2-ObjectL / R-FOV / 2R (10)
Unfold is a parameter for determining whether or not the imaging time is shortened when the operator sets α.

  When this Unfold condition is added to the condition classification of the first embodiment and the NoWrapL and NoWrapS that minimize the sum of the FOV expansion rates (NoWrapL + NoWrapS) are obtained, the result is as follows.

Condition 1: {FOV / 2 ≦ ObjectL and (FOV / 2) × NoWrapS ≦ ObjectS and NoWrapS ≧ 1} (Condition 1 of the first embodiment) and Unfold ≧ 0
Condition 1 is further divided into Conditions 1A and 1B depending on whether Unfold is greater than or equal to α or smaller than α.

Condition 1A: Condition 1 and α ≦ Unfold
In this case, NoWrapS is obtained by rounding Equation (4-3), and NoWrapL _Min and NoWrapS _Min are as follows.
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 2 (R × ObjectS- ObjectL-αR) / FOV + R

Condition 1B: Condition 1 and α> Unfold
Under this condition, the allowable range α set by the operator satisfies NoWrapS ≧ 1 so that α = ObjectS + FOV / 2−ObjectL / R−FOV / 2R.
Therefore, if this α is substituted into the NoWrapL _Min expression of Condition 1A, NoWrapL _Min = 1.0. That is,
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 1.0

Condition 1C: Condition 1 and Unfold <0
Under this condition, the imaging time is not shortened even if α is set because Unfold <0, so α is not considered.
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 2 (R × ObjectS- ObjectL) / FOV + R

Condition 2: FOV / 2> ObjectL or FOV / 2) × NoWrapS> ObjectS or NoWrapS <1
This condition is the same as condition 2 in the first embodiment. Therefore,
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 2ObjectS / FOV

  The enlargement ratios in the L direction and S direction obtained as described above are set as imaging parameters (S55), and from the main measurement to the image display (S56 to S59) is the same as in the first embodiment.

  According to this modification, when the operator designates the folding allowable range α, the imaging time can be shortened depending on conditions. Note that when α = 0, the processing of the first embodiment is performed, and therefore the processing of this modification can be said to be a generalized processing of the first embodiment.

<< Second Embodiment >>
The present embodiment is characterized in that the sensitivity distribution information of the receiving coil is used as a receivable spatial range when the FOV computing unit 80 optimizes the FOV expansion rate. That is, the MRI apparatus of this embodiment further includes a sensitivity distribution calculation unit that calculates the sensitivity distribution range of the receiving coil, and the FOV calculation unit 80 uses the sensitivity distribution range calculated by the sensitivity distribution calculation unit as spatial range information. Is used to calculate the magnification of the imaging field of view.

  FIG. 9 shows a functional block diagram of the signal processing unit 7 and the control unit 8 (CPU 18) of the MRI apparatus of this embodiment. Here, the case where the functions of the signal processing unit 7 and the control unit 8 are realized on the CPU 18 is shown, but the present invention is not limited to this.

  The signal processing unit 7 of this embodiment includes a sensitivity range calculation unit 87 that calculates a sensitivity range as a receivable range. The sensitivity range calculation unit 87 is a sensitivity map creation unit 871 that creates a sensitivity map of each channel of the receiving coil, and a sensitivity map that expands the map range to a boundary where the sensitivity becomes 0 with respect to the sensitivity map of each channel of the receiving coil. An enlarged sensitivity map creating unit 873 for creating the image and a combined enlarged sensitivity map creating unit 875 for synthesizing the enlarged sensitivity map. Various methods are known for creating a sensitivity map, such as a method using a phantom and a method using an image of a subject, and any of them can be adopted. The sensitivity range calculation unit 87 calculates the sensitivity range of the receiving coil at the imaging target slice position from the slice position of the main measurement and the size and position of the set FOV based on the combined enlarged sensitivity map.

  Next, the operation of the present embodiment will be described with reference to FIG. In FIG. 10, the processes indicated by the same reference numerals as in FIG. 5 are the same processes unless otherwise specified, and the description thereof is omitted.

<Step S61>
A sensitivity map creating unit 871 creates a sensitivity map for each channel of the receiving coil. Here, in order to simplify the explanation, it is assumed that the receiving coil is composed of channel 1 and channel 2. An example of the calculated sensitivity map of each channel is shown in FIGS. 11 (a) and 12 (a). In addition, the left-right direction in a figure is a phase encoding direction, and has shown the sensitivity map image in the set FOV. The sensitivity map image shows that the sensitivity of the high luminance region is high and the sensitivity of the low luminance region is low. In FIG. 11A, the sensitivity on the right side of the sensitivity map is high, and the sensitivity decreases as it approaches the left side. . In FIG. 12A, the sensitivity on the right side of the sensitivity map is low, and the sensitivity increases as it approaches the left side.

<Step S62>
The enlarged sensitivity map creating unit 873 estimates a boundary where the sensitivity becomes 0 in the sensitivity map of each channel, and creates a sensitivity map in which the map range is expanded to the boundary position where the sensitivity becomes 0 for each channel. For example, in FIG. 11A, since the sensitivity at the left end of the sensitivity map image is not 0, the sensitivity of the receiving coil can be predicted to continue to the left. From the brightness gradient in the sensitivity map image, the sensitivity of the receiving coil outside the FOV is estimated, and the sensitivity map is expanded to a boundary where the sensitivity becomes zero. FIGS. 11B and 12B show examples of enlarged sensitivity maps that are the results of expanding the sensitivity maps shown in FIGS. 11A and 12A as described above.

<Step S63>
The synthesized sensitivity map creating unit 875 composes the enlarged sensitivity maps of the respective channels created in step S62, and creates one synthesized enlarged sensitivity map. The combining method is arbitrary. For example, as shown in FIG. 13, a method of superimposing the enlarged sensitivity maps of the receiving coils of channel 1 and channel 2 and taking the union of the two enlarged sensitivity maps can be considered. Sensitivity (all channel synthesis) in FIG. 13 shows the result of synthesizing the enlarged sensitivity maps of channel 1 and channel 2.

<Step S53>
The imaging parameter setting unit 84 acquires imaging parameters. Although FIG. 10 shows that step S53 is performed after step S63, step S53 may be performed by the next step S64, and the order is not limited.

<Step S64>
The combined magnification sensitivity map creation unit 875 determines whether or not a suppression pulse typified by a pre-saturation pulse is set in the imaging parameter acquired in step S53. When the suppression pulse is not set in the imaging parameter, the process proceeds to step S66. When the suppression pulse is set in the imaging parameter, the process proceeds to step S65.

<Step S65>
When the suppression pulse is set, no signal is generated from the region irradiated with the suppression pulse, so the combined magnification sensitivity map creating unit 875 applies the suppression pulse in the combined expanded sensitivity map created in step S63. Set the sensitivity of the area to be set to zero. As a result, as shown in FIG. 14, when there is a suppression pulse, the combined magnification sensitivity map is reduced. If it is not necessary to consider the suppression pulse, steps S64 and S65 can be omitted, and such an embodiment is also included in this embodiment.

<Step S66>
The sensitivity range calculation unit 87 calculates the sensitivity ranges of the receiving coils for all channels from the combined enlarged sensitivity map calculated in step S64 when the suppression pulse is not set or the combined expanded sensitivity map calculated in step S65. calculate. FIG. 15 shows the calculated sensitivity range Sensitivity. 15 coincides with the range of Sensitivity (all channel synthesis) or Sensitivity (with suppression pulse) in FIG.

<Step S54>
The FOV enlargement ratio calculation unit 85 automatically calculates the optimum value of the FOV enlargement ratio from the sensitivity range of the receiving coil calculated in step S66 and the imaging parameter acquired in step S53.
Hereinafter, step S54 will be described in detail with reference to FIG. In the figure, [SensitivityL] and [SensitivityS] indicate the sensitivity range in the L direction and the sensitivity range in the S direction, which are obtained by dividing the sensitivity range Sensitivity by the center C _F of the FOV. The center of the sensitivity range Sensitivity is indicated by C _S. Here, the center C _{F of the} FOV is shifted in the S direction with respect to the center C _S of the sensitivity range Sensitivity, and the L direction FOV expansion rate NoWrapL and the S direction FOV expansion rate NoWrapS have different values. Can be set.

  As can be seen from the comparison with FIG. 7, the object (subject) here is outside the phase encoding direction of the FOV and outside the sensitivity range Sensitivity of the receiving coil. In this case, no signal is received even if an object (subject) is present from a sensitivity distribution of 0, but an image of an object outside the FOV phase encoding direction and within the sensitivity range is present in the FOV. Appear as folding artifacts. Therefore, in this embodiment, instead of [ObjectL] and [ObjectS], using the sensitivity ranges [SensitivityL] and [SensitivityS] in the L direction and the S direction, optimal enlargement that can reduce imaging time without causing aliasing. The rate is obtained for each of the L direction and the S direction.

  The sum of the optimal L-direction FOV enlargement rate NoWrapL and the S-direction enlargement rate NoWrapS is the same as the SensitivityL and SensitivityS for ObjectL and ObjectS in Equation (3) and Equation (4) described in step S54 of the first embodiment, respectively. Represented by the replaced expression. Similarly to the modified example of the first embodiment, when the aliasing allowable range α is taken into consideration, the expressions (3-2) and (4-2) where ObjectL and ObjectS are replaced by SensitivityL and SensitivityS, respectively. expressed.

As a result, NoWrapL _Min and NoWrapS _Min that minimize the sum of the enlargement ratios (NoWrapL + NoWrapS) are obtained for each of the following conditions.

Condition 1A: Condition 1 {FOV / 2 ≦ SensitivityL and (FOV / 2) × NoWrapS ≦ SensitivityS and NoWrapS ≧ 1} and Unfold ≧ 0 and α ≦ Unfold, where Unfold = SensitivityS + FOV / 2−SensitivityL / R−FOV / 2R
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2 (R × SensitivityS-SensitivityL-αR) / FOV + R

Condition 1B: Condition 1 and Unfold ≧ 0 and α> Unfold
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 1.0

Condition 1C: Condition 1 and Unfold <0
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2 (R × SensitivityS-SensitivityL) / FOV + R

Condition 2: FOV / 2> SensitivityL or FOV / 2 × NoWrapS> SensitivityS or NoWrapS <1
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2 SensitivityS / FOV

When the folding allowable range α is not set, the Unfold condition does not need to be taken into account, so that the NoWrapL _Min and NoWrapS _Min are divided into the conditions 1A and 2 as in the case of the first embodiment. Is required.

<Steps S55 to S59>
It is the same as in the first embodiment that an image is obtained with the execution FOV using the FOV enlargement ratio thus obtained.

  According to the present embodiment, it is possible to calculate an optimum FOV enlargement rate in consideration of a region where there is no sensitivity even if an object is present. As shown in FIG. 16, this embodiment is suitable for, for example, imaging in which phase encoding is set in the body axis direction.

  In the above description, the receiving coil is a case of a receiving coil composed of a coil of a plurality of channels or a plurality of small receiving coils. However, when one receiving coil is used, the composite enlarged sensitivity map creation unit in FIG. 875 and step S63 of FIG. 10 can be omitted.

<< Third embodiment >>
In the MRI apparatus of the present embodiment, the imaging field calculation unit uses information on the static magnetic field space as spatial range information when calculating the magnification of the imaging field. That is, this embodiment is different from the first and second embodiments in that the FOV enlargement ratio is calculated from the effective range of the static magnetic field in the bore.

Hereinafter, this embodiment will be described with reference to FIGS. 17 and 18.
FIG. 17 is a functional block diagram of the signal processing unit 7 and the control unit 8 (CPU 18) of the MRI apparatus of this embodiment. As shown in the figure, the signal processing unit 7 includes a static magnetic field effective range calculation unit 88. The static magnetic field effective range calculation unit 88 calculates a three-dimensional effective range of the static magnetic field in the bore.

Next, mainly the operation of the signal processing unit 7 of the present embodiment will be described with reference to FIG.
<Step S70>
A static magnetic field effective range calculation unit 88 calculates a three-dimensional effective range of the static magnetic field in the bore. The three-dimensional effective range of the static magnetic field may be determined from a simulation result or the like, but is preferably measured in advance. However, instead of performing the measurement every time this measurement is performed, the measurement data may be stored in the storage device 19 by measuring in advance when the apparatus is manufactured or installed. Further, it may be combined with the three-dimensional shape data of the bore. FIG. 19 shows, as an example, the effective region Effective Region of the calculated static magnetic field in the bore in a horizontal magnetic field type MRI apparatus having a cylindrical static magnetic field generating magnet. In the MRI apparatus, the phase encoding direction can be arbitrarily determined. However, in the case of a cylindrical bore, since the uniformity of the static magnetic field is lost at the end in the axial direction, in FIG. ) Is the phase encoding direction.

<Step S54>
The FOV enlargement ratio calculation unit 85 automatically calculates the optimum value of the FOV enlargement ratio from the effective range of the static magnetic field calculated in step S70 and the imaging parameter acquired in step S53. Hereinafter, step S54 will be described in detail with reference to FIG. In the figure, [Effective Region L] and [Effective Region S] indicate the effective range in the L direction and the effective range in the S direction obtained by dividing the effective range Effective Region of the static magnetic field by the center C _F of the FOV. The center of the effective region Effective Region Effective Region is denoted by _CE . Here, the case where the center C _{F of the} FOV is shifted in the S direction from the center C _E of the effective range of the static magnetic field is shown, and the FOV expansion rate NoWrapL in the L direction and the FOV expansion rate NoWrapS in the S direction are different. Can be set to a value.

  As can be seen from the comparison with FIG. 7, the object (subject) here is outside the phase encoding direction of the FOV and outside the effective range of the static magnetic field. A signal is not received from an object (subject) existing outside the effective range, but an image of an object outside the FOV phase encoding direction and within the effective range appears as an aliasing artifact in the FOV. Therefore, in the present embodiment, instead of ObjectL and ObjectS, effective ranges Effective RegionL and Effective RegionS in the L direction and S direction are used, and an optimal enlargement ratio that can reduce imaging time without causing aliasing is obtained in the L direction and S. Find each direction.

The sum of the optimum FOV enlargement ratio NoWrapL in the L direction and the enlargement ratio NoWrapS in the S direction can be calculated by the same algorithm as in the second embodiment, that is, SensitivityL and SensitivityS in the second embodiment, By replacing with Effective RegionL and Effective RegionS, NoWrapL _Min and NoWrapS _Min are obtained as follows.

Condition 1A: Condition 1 {FOV / 2 ≦ Effective Region L and (FOV / 2) × NoWrapS ≦ Effective RegionS and NoWrapS ≧ 1}
And Unfold ≧ 0 and α ≦ Unfold, where Unfold = Effective RegionS + FOV / 2−Effective RegionL / R−FOV / 2R
NoWrapL _Min = 2Effective RegionL / FOV
NoWrapS _Min = 2 (R × Effective RegionS-Effective Region L-αR) / FOV + R

Condition 1B: Condition 1 and Unfold ≧ 0 and α> Unfold
NoWrapL _Min = 2Effective RegionL / FOV
NoWrapS _Min = 1.0

Condition 1C: Condition 1 and Unfold <0
NoWrapL _Min = 2Effective RegionL / FOV
NoWrapS _Min = 2 (R × Effective RegionS- Effective RegionL) / FOV + R

Condition 2: FOV / 2> Effective RegionL or FOV / 2 × NoWrapS> Effective RegionS or NoWrapS <1
NoWrapL _Min = 2Effective RegionL / FOV
NoWrapS _Min = 2Effective RegionS / FOV

In the present embodiment, when the folding allowable range α is not set, it is not necessary to consider the Unfold condition. Therefore, as in the first embodiment, the NoWrapL is divided into the above conditions 1A and 2 as described above. _Min and NoWrapS _Min are required.

<Steps S55 to S59>
It is the same as in the first embodiment that an image is obtained with the execution FOV using the FOV enlargement ratio thus obtained.
According to the present embodiment, it is possible to calculate an optimum FOV enlargement rate in consideration of a region where an object exists but becomes an effective range of a static magnetic field. As in the second embodiment, this embodiment is suitable for imaging in which phase encoding is set in the body axis direction.

<< Fourth Embodiment >>
The present embodiment is characterized in that the FOV enlargement ratio is calculated by appropriately combining the first to third embodiments described above in consideration of the position of the object (subject) and the imaging section (phase encoding direction).

  FIG. 21 shows a functional block diagram of the signal processing unit of the present embodiment. As shown in FIG. 21, the FOV calculation unit 80 of this embodiment includes an FOV enlargement ratio calculation unit 85, an object presence range calculation unit 86, a sensitivity range calculation unit 87, a static magnetic field effective range calculation unit 88, and a range comparison unit 89. I have. Although not shown, the sensitivity range calculation unit 87 includes a sensitivity map creation unit, an enlarged sensitivity map creation unit, and a combined enlarged sensitivity map creation unit, similarly to the sensitivity range computation unit 87 of the second embodiment.

The range comparison unit 89 compares the ranges calculated by the object presence range calculation unit 86, the sensitivity range calculation unit 87, and the static magnetic field effective range calculation unit 88, and selects an optimum process. For example, the distance from the center C _{F of the} set FOV to the end of the static magnetic field effective range (static magnetic field effective range Effective Region L, Effective Region S) and the distance from the center C _F to the end of the sensitivity range (Sensivity L, Sensivity S) And determine whether to apply the formulas for calculating the optimum enlargement ratios NoWrapL _Min and NoWrapS _Min .

  The enlargement ratio calculation process of the present embodiment centering on the process of the range comparison unit 89 will be described with reference to FIG. 22, steps having the same processing contents as the steps shown in FIGS. 5, 10, and 18 are denoted by the same reference numerals, and detailed description thereof is omitted.

<S70>
The static magnetic field effective range calculation unit 88 calculates the static magnetic field effective range.

<Steps S51 to S53>
Using the image obtained by performing wide-range imaging (S51) in the previous measurement, the object presence range calculation unit 86 calculates the object presence range within the static magnetic field effective range (S52). Imaging parameters such as FOV, R factor, and imaging pulse sequence are acquired (S53).

<Step S60>
A sensitivity range calculation unit 87 calculates the sensitivity range of the receiving coil. As for the sensitivity range, steps S61 to S66 shown in FIG. 10 create a channel sensitivity map for each receiving coil (S61), create an enlarged sensitivity map for each channel (S62), and create a combined enlarged sensitivity map (S63). (S66). At this time, if a suppression pulse is included in the imaging pulse sequence, processing for removing the region in which the signal is suppressed by the suppression pulse from the combined magnification sensitivity map is performed (S64, S65).

<Step S81>
From the set imaging pulse sequence, it is determined whether the phase encoding direction is parallel to the body axis direction of the subject and whether the phase encoding direction is orthogonal to the body axis direction. In the former case, the process proceeds to step S83, and in the latter case, the process proceeds to step S81.

<Step S82>
As a case where the phase encoding direction is orthogonal to the body axis direction, for example, a cross section of the abdomen as shown in FIG. 23 may be imaged. In such a case, the imaging region is always within the static magnetic field space, and all objects can be measured in a wide range FOV, and it is difficult to think that there is an object outside the wide range FOV. Therefore, in this case, the FOV enlargement ratio is calculated in consideration of only the existence range of the object obtained in step S52. In this case, NoWrapL _Min and NoWrapS _Min perform the same calculation process 1 as in the first embodiment. That is, it is as follows.

Condition 1A: Condition 1 {FOV / 2 ≦ ObjectL and (FOV / 2) × NoWrapS ≦ ObjectS and NoWrapS ≧ 1} and α ≦ Unfold
However, Unfold = ObjectS + FOV / 2-ObjectL / R-FOV / 2R
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 2 (R × ObjectS- ObjectL-αR) / FOV + R

Condition 1B: Condition 1 and α> Unfold
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 1.0

Condition 1C: Condition 1 and Unfold <0
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 2 (R × ObjectS- ObjectL) / FOV + R

Condition 2: FOV / 2> ObjectL or FOV / 2) × NoWrapS> ObjectS or NoWrapS <1
NoWrapL _Min = 2ObjectL / FOV
NoWrapS _Min = 2ObjectS / FOV

<Step S83>
As a case where the phase encoding direction is parallel to the body axis direction, for example, the sagittal plane of the lumbar spine as shown in FIG. 24 may be imaged or the sagittal plane of the cervical spine as shown in FIG. 25 may be imaged. In these cases, since the imaging cross section coincides with the body axis direction of the body, it is difficult to think that all the objects fit within the wide range FOV. In this case, it is necessary to consider the sensitivity range of the receiving coil and the effective range of the static magnetic field obtained in steps S60 and S70. Therefore, the range comparison unit 89 first determines whether or not the object existence range falls within the static magnetic field effective range and the sensitivity distribution. As a result, if the object existence range is out of the effective static magnetic field range or the sensitivity distribution in either the L direction or the S direction, the process proceeds to step S84, and if it is out of one direction, the process proceeds to step S85.

<Steps S84 and S86>
The range comparison unit 89 further compares the sensitivity range of the receiving coil in the L direction and the S direction and the effective range of the static magnetic field (S84). As a result, when the static magnetic field effective range is wider than the sensitivity range in both the L direction and the S direction, the process proceeds to step S86, and the same FOV enlargement rate calculation process 2 as in the second embodiment is performed. That is, as described with reference to FIG. 16, the FOV enlargement ratio is calculated using only the sensitivity range (S86).

<Steps S84 and S87>
If the static magnetic field effective range EffectiveRegionL in the L direction is wider than the sensitivity range SensitivityL and the static magnetic field effective range EffectiveRegionS in the S direction is equal to or less than the sensitivity range SensitivityS in step S84, the process proceeds to step S87, as shown in FIG. Then, an optimum FOV enlargement ratio is calculated using SensitivityL in the L direction and EffectiveRegionS in the S direction (calculation process 3). In this case, the optimum FOV expansion rate is as follows.

Condition 1A: Condition 1 {FOV / 2 ≦ SensitivityL and (FOV / 2) × NoWrapS ≦ EffectiveRegionS and NoWrapS ≧ 1} and Unfold ≧ 0 and α ≦ Unfold, where Unfold = EffectiveRegionS + FOV / 2−SensitivityL / R−FOV / 2R
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2 (R × EffectiveRegionS-SensitivityL-αR) / FOV + R

Condition 1B: Condition 1 and Unfold ≧ 0 and α> Unfold
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 1.0

Condition 1C: Condition 1 and Unfold <0
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2 (R × EffectiveRegionS-SensitivityL) / FOV + R

Condition 2: FOV / 2> SensitivityL or FOV / 2 × NoWrapS> EffectiveRegionS or NoWrapS <1
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2EffectiveRegionS / FOV
The enlargement ratio thus obtained is in the case of Effective Region L> Sensitivity L and Effective Region S <Sensitivity S. However, in the case of Effective Region L <Sensitivity L, the Effective Region L may be used instead of the above-described equation Sensitivity L. Further, when Effective RegionS> SensitivityS, SensitivityS may be used instead of the above-described formula Effective RegionS. That is, before step S87, the range comparison unit 89 may perform a step (not shown) for determining whether or not Effective Region L> Sensitivity L and Effective Region S <Sensitivity S.

<Step S85>
When one end of the object existence range deviates from the sensitivity range of the receiving coil and the effective range of the static magnetic field (FIG. 25), only the range of sensitivity of the receiving coil or the effective range of the static magnetic field is considered for the deviating side, For the side that falls within the sensitivity range and the effective range of the static magnetic field, the FOV expansion rate is calculated in consideration of the object existence range (calculation process 4). In this case, the optimum FOV enlargement ratio is as follows. Here, it is assumed that Effective Region L> Sensitivity L.

Condition 1A: Condition 1 {FOV / 2 ≦ SensitivityL and (FOV / 2) × NoWrapS ≦ ObjectS and NoWrapS ≧ 1} and Unfold ≧ 0 and α ≦ Unfold, where Unfold = ObjectS + FOV / 2−SensitivityL / R−FOV / 2R
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2 (R × ObjectS-SensitivityL-αR) / FOV + R

Condition 1B: Condition 1 and Unfold ≧ 0 and α> Unfold
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 1.0

Condition 1C: Condition 1 and Unfold <0
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2 (R × ObjectS-SensitivityL) / FOV + R

Condition 2: FOV / 2> SensitivityL or FOV / 2 × NoWrapS> ObjectS or NoWrapS <1
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2ObjectS / FOV

  The enlargement ratio thus obtained is in the case of Effective Region L> Sensitivity L. However, in the case of Effective Region L <Sensitivity L, the Effective Region L may be used instead of Sensitivity L in the above formula. That is, before step S85, the range comparison unit 89 performs a step (S88 indicated by a dotted line) to determine whether Effective Region L> Sensitivity L, and performs calculation processing 4 using Sensitivity L or calculation processing (not shown) using Effective Region L. You may make it perform. 25, when the object existence range deviates from the sensitivity range or the effective static magnetic field range in the S direction, ObjectL is replaced with SensitivityL and SensitivityS or EffectiveS instead of ObjectS. RegionS may be used.

In addition, in the calculation process of steps S82 and S85 to S87, when the folding allowable range α is not set, it is not necessary to consider the Unfold condition. Therefore, as in the case of the first embodiment, the above condition 1A and NoWrapL _Min and NoWrapS _Min are obtained separately under condition 2.

  When calculating the FOV expansion rate, only the object existence range is considered in S82, but the sensitivity range of the receiving coil and the effective range of the static magnetic field may be further considered. Similarly, for S85 to S87, not only the sensitivity range of the receiving coil and the effective range of the static magnetic field, but also the object existence range may be considered.

  The steps after calculating the FOV enlargement ratio are the same as those in the first to third embodiments, and a description thereof will be omitted.

  According to the present embodiment, in consideration of the imaging method, particularly the relationship between the body axis direction of the subject and the phase encoding direction, a signal receivable range that should be considered for aliasing is appropriately selected to optimize the enlarged FOV. be able to. The processing described in the present embodiment and the drawings employed in the present embodiment are examples, and various modifications and additional conditions may be added to the present embodiment according to the imaging region and the imaging section. included. For example, for the sake of simplicity, the case where the phase encoding direction is parallel to the body axis and the case where the phase encoding direction is orthogonal to the body axis has been described. However, as is well known, the phase encoding direction can be arbitrarily selected regardless of the body axis. . Also in such a case, it is possible to select an appropriate enlarged FOV calculation process by determining whether or not the object presence range can be imaged with a wide range FOV. Whether or not the object existence range can be imaged with the wide range FOV can be determined by using information on the object existence range, the three-dimensional static magnetic field effective range, and the sensitivity range.

  As mentioned above, although each embodiment of the MRI apparatus of this invention was described, this invention is not limited to these embodiment, As long as there is no technical contradiction, a part of element may be deleted and changed. Further, the embodiments may be appropriately combined, or another imaging parameter optimization process may be added.

<Display Embodiment>
An embodiment of a display suitable for performing imaging using the MRI apparatus of each embodiment will be described.
The FOV optimization processing according to the present invention may be preset, or the operator may select this processing. In any case, when this processing is set, the display control unit 83 creates a screen for determining the processing content of FOV optimization and displays it on the display device 20. Such a screen may display a block for selecting FOV optimization as one of the imaging conditions on the imaging condition setting screen, and may be displayed when FOV optimization is selected. You may make it display as a part of screen.

  FIG. 26 shows an example of a display screen (GUI) 600. In this screen 600, a mode selection block 610, an optimization condition setting block 620, an imaging time display block 630, an instruction button display block 640, and the like are displayed, and functions displayed on each block by operating these with a mouse or the like. Can be realized.

  In the mode selection block 610, for example, an auto mode, a customization mode, and the like can be selected. When the auto mode is selected, an imaging method, a subject position, and the like are automatically determined in addition to the set FOV, and appropriate. The enlargement ratio is calculated by selecting the appropriate FOV enlargement ratio calculation process. When the customization mode is selected, for example, a plurality of calculation processes, for example, subject priority processing, sensitivity distribution priority processing, and the like are presented, and the operator selects a method suitable for the imaging method and the imaging region of the subject. .

  In the optimization condition setting block 620, the operator inputs whether or not the FOV has a range in which folding can be permitted, and if there is a permitted range. For example, as shown in FIG. 27, the allowable range α may be displayed on a screen showing the relationship between the object presence range and the set FOV, and the range may be set on the screen using a cursor or the like. It may be displayed.

  The imaging time display block 630 displays the imaging value of the initially set FOV (setting FOV) and the predicted value of the imaging time after the optimization process (execution FOV). Instead of or in addition to the imaging time of the execution FOV, the time shortened by the optimization (reduced time) may be displayed. The operator may change the calculation processing mode selected in the mode selection block 610 or the allowable range input in the optimization condition setting block 620 by looking at the execution FOV or the shortening time.

The instruction button display block 640 displays a button for confirming (ON) the mode selected in the mode selection block 610 and the conditions set in the optimization condition setting block 620. By operating the ON button, the content of the FOV optimization process is confirmed and the screen returns to the original imaging condition setting screen, or shifts to another condition setting screen or a screen prompting the start of imaging.
By providing such a user interface, the operator can easily realize FOV optimization. FIG. 26 and FIG. 27 are examples of display screens and do not limit the present embodiment.

1: subject, 2: static magnetic field generation unit, 3: gradient magnetic field generation unit, 4: sequencer, 5: transmission unit, 6: reception unit, 7: signal processing unit, 8: control unit, 9: gradient magnetic field coil, 10: Gradient magnetic field power supply, 11: High frequency transmitter, 12: Modulator, 13: High frequency amplifier, 14a: High frequency coil (transmitting coil), 14b: High frequency coil (receiving coil), 15: Signal amplifier, 16: Quadrature phase detection 17: A / D converter 18: Central processing unit (CPU) 19: External storage device 20: Display device 21: Internal storage device 23: Input device 25: Operation unit 80: FOV calculation 81: imaging control unit, 82: image processing calculation unit, 83: display control unit, 84: imaging parameter setting unit, 85: FOV enlargement rate calculation unit, 86: object presence range calculation unit, 87: sensitivity range calculation unit 871: Sensitivity map creation unit, 8 3: expanding the sensitivity map generating unit, 875: Synthesis enlarged sensitivity map creation unit, 88: a static magnetic field effective spatial operation unit, 89: Range comparator unit

Claims (13)

A magnetic field generator that generates nuclear magnetic resonance in a subject placed in a static magnetic field space, a receiver that has a receiving coil and receives a nuclear magnetic resonance signal generated from the subject, and a nuclear magnetism received by the receiver A magnetic resonance imaging apparatus comprising: an image processing unit that processes a resonance signal and creates an image of the subject; and an imaging parameter setting unit that sets an imaging parameter necessary for imaging, wherein the imaging parameter is: Including imaging field of view,
An imaging field calculation unit that calculates an enlargement ratio of the imaging field based on a relationship between an imaging field set by the imaging parameter setting unit and a spatial range in which the nuclear magnetic resonance signal can be received is further provided. Magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 1,
An object existence range calculation unit for calculating an object existence range in the static magnetic field space;
The imaging field-of-view calculation unit calculates an enlargement factor of the imaging field of view using the object existence range calculated by the object existence range calculation unit as information on the spatial range. The magnetic resonance imaging apparatus according to claim 2,
The imaging field calculation unit calculates a magnification of the imaging field by using the object existence range and a folding allowable range in the imaging field as information of the spatial range. The magnetic resonance imaging apparatus according to claim 1 or 2,
A sensitivity distribution calculation unit for calculating a sensitivity distribution range of the receiving coil;
The imaging field-of-view calculation unit calculates the magnification of the imaging field of view using the sensitivity distribution range calculated by the sensitivity distribution calculation unit as information on the spatial range. The magnetic resonance imaging apparatus according to claim 4,
The receiving coil is composed of a plurality of receiving coils,
The sensitivity distribution calculating unit synthesizes sensitivity distributions of the plurality of receiving coils to calculate a sensitivity distribution range. The magnetic resonance imaging apparatus according to claim 4,
The imaging field of view calculation unit calculates the magnification of the imaging field of view by adding information on the region of the subject whose nuclear magnetic resonance signal is suppressed by the high-frequency magnetic field applied by the magnetic field generation unit to the information on the spatial range. A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to any one of claims 1 to 6,
The imaging field-of-view calculation unit uses the information of the static magnetic field space as information of the spatial range to calculate an enlargement ratio of the imaging field of view. The magnetic resonance imaging apparatus according to claim 1,
The imaging field-of-view calculation unit further includes a range comparison unit that compares an object existence range in the static magnetic field space, an effective range of the static magnetic field space, and a sensitivity range of the reception coil, and the comparison result of the range comparison unit Based on the object existence range, the effective range of the static magnetic field space, and the sensitivity range of the receiving coil, the magnification of the imaging field of view is calculated using the spatial range information as a combination. Imaging device. The magnetic resonance imaging apparatus according to claim 1,
2. The magnetic resonance imaging apparatus according to claim 1, wherein the imaging field calculation unit varies the magnification of the imaging field in two halves divided by the center in the phase encoding direction of the imaging field. The magnetic resonance imaging apparatus according to claim 9,
The imaging visual field calculation unit, when the center of the spatial range in which the nuclear magnetic resonance signal can be received and the center of the imaging visual field set in the imaging parameter setting unit do not match, the phase encoding direction of the imaging visual field The magnetic resonance imaging apparatus is characterized in that, of the two halves, the half of the side including the center of the spatial range has a larger magnification than the half of the side not including the center of the spatial range. The magnetic resonance imaging apparatus according to claim 1,
A magnetic resonance imaging apparatus, further comprising: a display unit that displays an image of the subject, wherein the imaging field calculation unit displays an optimized imaging field on the image displayed on the display unit. The magnetic resonance imaging apparatus according to claim 11,
The magnetic resonance imaging apparatus further comprising an input unit that accepts a change in the imaging field of view displayed on the image. The magnetic resonance imaging apparatus according to claim 11 or 12,
The imaging field calculation unit further includes an imaging time calculation unit that calculates an imaging time when the imaging field of view optimized is used, and the imaging time calculation unit displays the calculated imaging time on the display unit, Magnetic resonance imaging device.

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