JP6681708B2 - Magnetic resonance imaging equipment - Google Patents

Description

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

  The MRI apparatus measures an NMR signal generated by a nuclear spin that constitutes a tissue of a subject, particularly a human body, and two-dimensionally or three-dimensionally images the form and function of the head, abdomen, limbs and the like. It is a device. In radiography, different phase encoding is added to the NMR signal and frequency encoding is performed by the gradient magnetic field applied to the static magnetic field on which the subject is placed, and the NMR signal is measured as time series data. The measured NMR signal is reconstructed into an image by being two-dimensionally or three-dimensionally Fourier transformed.

  In the MRI apparatus, pre-measurement such as positioning image measurement and receiving coil sensitivity measurement is performed prior to the main measurement, and the user performs an imaging field of view (Field of View: hereinafter referred to as FOV) based on the results of these pre-measurements. Various image pickup parameters such as are set. However, in MRI imaging, when a tissue or the like exists outside the FOV in the phase encoding direction, the tissue outside the FOV is folded back inside the FOV and is visualized as an artifact. In order to prevent the occurrence of the aliasing artifact, the user sets the enlargement ratio for the set FOV (set FOV), and actually measures the set FOV with the FOV (measured FOV) enlarged in the phase encoding direction. Normally, the user sets the FOV enlargement ratio by predicting the position where the aliasing artifact occurs, but since the position where the aliasing artifact occurs differs depending on the positional relationship between the imaging target and the set FOV and the size of the imaging target, the occurrence of the aliasing artifact occurs. Location is difficult to predict.

  On the other hand, as an imaging method of an MRI apparatus, there is a technique (referred to as parallel imaging) in which a plurality of receiving coils are used and an image in which folding is removed by calculation is reconstructed from a signal including folding obtained by each receiving coil. When this parallel imaging is applied, the position where the aliasing artifact is generated changes depending on the double speed number of the parallel imaging (hereinafter, referred to as R factor), so that it becomes more difficult to predict the position where the aliasing artifact occurs. As a result, the user may set a non-optimal FOV expansion rate. If 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, the aliasing artifact does not occur in the set FOV, but extra imaging time is required. Therefore, in the image obtained by the main measurement, the user visually determines the presence or absence of the aliasing artifact within the set FOV, and if the aliasing artifact occurs within the set FOV, change the FOV enlargement ratio and re-execute. It is necessary to perform the main measurement.

  To solve this problem, Patent Document 1 discloses a technique of calculating a return attention area from a set FOV, an FOV expansion rate, an R factor, and a signal reception range, and displaying the return attention area on an imaging parameter setting screen. ing.

JP2012-192216A

  In the technique described in Patent Document 1, by displaying the return attention area on the imaging parameter setting screen, it is possible to present the area at which there is a risk of the return to the user, but the user considers the return attention area. Since it is necessary for the user to optimize the imaging parameter, it takes time to determine the imaging parameter.

  Further, in the conventional technique, the set FOV expansion rate is isotropically applied to the set FOV in the phase encoding direction. That is, the FOV is enlarged from the center of the set FOV at the same enlargement ratio on one side and the other side in the phase encoding direction. However, the center position of the set FOV does not always match the center position of the imaging target in the phase encoding direction. In that case, in order to prevent the aliasing artifacts from being generated on either side, one side needs extra measurement, which leads to extension of the imaging time.

  It is an object of the present invention to obtain an MRI image free from aliasing artifacts with a minimum extension of imaging time without reducing inspection throughput. Another object of the present invention is to provide an MRI apparatus capable of setting 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 the information obtained before the main measurement such as the size and position of the set FOV, the R factor, and the spatial range in which a signal can be received to obtain the optimum FOV expansion rate. Equipped with a function to automatically calculate. In particular, the MRI apparatus of the present invention is characterized in that the FOV enlargement ratio is made different from one of the center position and the other in the phase encoding direction depending on the position of the set FOV.

  According to the present invention, the minimum FOV enlargement ratio in consideration of the removal of aliasing artifacts is automatically calculated, so that the MRI image without aliasing artifacts can be obtained with the minimum extension of the imaging time without reducing the inspection throughput. Obtainable . Further, according to the present invention, by making the FOV expansion rate asymmetric with respect to the phase encoding direction according to the center position of the set FOV, it is possible to optimize the FOV regardless of the position of the set FOV.

The figure which shows the whole structure of the MRI apparatus to which this invention is applied. 3 is a functional block diagram of a signal processing unit and a control unit according to the embodiment. FIG. 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 processing procedure of 1st embodiment. The figure which shows an example of an object existing range. The figure explaining the optimal FOV expansion rate calculation processing of 1st embodiment. The figure explaining the process of the modification of 1st embodiment. The functional block diagram of the signal processing part of 2nd embodiment. The figure which shows the processing procedure of 2nd embodiment. 6A and 6B are diagrams for explaining the process of the second embodiment, where FIG. 7A is a sensitivity distribution diagram of channel 1 and FIG. 6A and 6B are diagrams for explaining the process of the second embodiment, where FIG. 7A is a sensitivity distribution diagram of channel 2 and FIG. Composite sensitivity distribution of channels 1 and 2. Composite sensitivity distribution that considers 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 processing procedure of 3rd embodiment. The figure which shows an example of the static magnetic field effective range calculated by 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 processing procedure 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 generation unit that causes nuclear magnetic resonance in a subject placed in a static magnetic field space, a reception 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 and creates an image of the subject, and an imaging parameter setting unit that sets imaging parameters necessary for imaging, the imaging parameter is imaging An imaging field-of-view calculation unit that calculates an enlargement ratio of the imaging field of view based on the relationship between the imaging field of view including the field of view and set by the imaging parameter setting unit and the spatial range in which the nuclear magnetic resonance signal can be received is further included. Prepare

  The imaging field-of-view calculation unit changes the enlargement ratio of the imaging field of view in two halves divided at the center of the imaging field of view in the phase encoding direction.

  Hereinafter, embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. In all the drawings for explaining the embodiments of the invention, components having the same function are designated by the same reference numeral, and the repeated description thereof will be 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 the block diagram shown in FIG. As shown in FIG. 1, this MRI apparatus has 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, and a sequencer 4 as main elements. , And a control unit 8. The subject 1 is usually inserted into the static magnetic field space of the static magnetic field generator 2 while lying 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 generating source, there are a permanent magnet type, a normal conducting type and a superconducting type, and any of them 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 the direction orthogonal to the body axis of the lying subject 1, and the latter generates a static magnetic field in the body axis direction.

  The gradient magnetic field generation unit 3 drives three sets of gradient magnetic field coils 9 that apply a gradient magnetic field in the three axis 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. 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, Gz are generated in the three axial directions of X, Y, Z, and the combination thereof allows any direction. Slice direction gradient magnetic field pulse (Gs) that determines the slice plane (imaging cross section), phase encode direction gradient magnetic field pulse (Gp) and frequency encode direction gradient magnetic field pulse (Gf) in directions orthogonal to the slice plane and mutually orthogonal to each other. Apply and encode position information in each direction into 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, and operates under the control of the control unit 8 to generate a tomographic image of the subject 1. Various commands necessary for data collection are sent to the transmitter 5, the gradient magnetic field generator 3, and the receiver 6.

  The transmitter 5 irradiates the subject 1 with an RF pulse in order to cause nuclear magnetic resonance in the nuclear spins of the atoms constituting the biological 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 the timing according to the command from the sequencer 4, and the amplitude-modulated RF pulse is amplified by the high-frequency amplifier 13 and then arranged in the vicinity of the subject 1. The RF pulse is applied to the high frequency coil 14a to irradiate the subject 1 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 high-frequency coil (reception coil) 14b on the receiving side and a signal amplifier 15 And a quadrature detector 16 and an A / D converter 17. The NMR signal of the response of the subject 1 induced by the electromagnetic wave emitted from the transmitting coil 14a is detected by the receiving coil 14b arranged close to the subject 1, amplified by the signal amplifier 15, and then amplified by the sequencer 4. The signal is divided into two orthogonal signals by the quadrature detector 16 at the timing according to the command, and each is converted into a digital amount by the A / D converter 17 and sent to the signal processing unit 7.

  It should be noted that the transmission coil 14a and the reception coil 14b can be combined into one coil. Further, 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 face the subject 1 in the vertical magnetic field system and surround the subject 1 in the horizontal magnetic field system in the static magnetic field space. Has been installed. The receiving coil 14b is installed so as to face or surround the subject 1.

  The signal processing unit 7 performs various data processing, displays and saves processing results, and the like, and also has a function of the control unit 8 such as a CPU 18, an external storage device such as a magnetic disk or an optical disk, and internal storage such as ROM or RAM. A storage device 19 such as a device, a display device 20 including 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 an operator interactively sees the display device 20 and interactively performs various operations of the MRI apparatus through the input device 23. Control the process.

  The signal processing unit 7 also functions as an image processing unit, and when the data from the receiving unit 6 is input to the CPU 18, the CPU 18 executes signal processing, image reconstruction, and other processing, and the subject 1 as a result thereof is processed. The tomographic image of is displayed on the display device 20 and is recorded in an external storage device or the like. The signal processing unit 7 also includes an FOV calculation unit that calculates the FOV expansion rate, which is a condition necessary for imaging.

  Details of the signal processing unit 7 and the control unit 8 will be described with reference to the 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 expansion rate optimization, but some or all of the functions may be implemented in an ASIC (application specific integrated circuit). It can also be realized by hardware such as a 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 auxiliary devices. In the storage device 19, the data including the NMR signal (digital signal) received by the receiving unit 6, the result calculated by the calculation unit in the CPU 18, the imaging condition selected or set via the input device 23 ( (Including imaging parameters and pulse sequences) are stored. The display device 20 displays a setting screen (GUI) for the user to input imaging parameters and imaging conditions in addition to the MR image that is the result of the calculation. The input device 23 includes the mouse and the keyboard described above, but when the display device 20 is a device such as a touch panel capable of inputting, the display device may also serve as part or all of the input device.

  The CPU 18 is provided with 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 described, 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). The image is taken. The image processing calculation unit 82 creates an image using the NMR signal received by the reception 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 the image showing the form of the subject. The display control unit 83 creates a display image in order to display the image created by the image processing calculation unit 82 and the images forming the setting screen on the display device 20. In addition, the display control unit 83 receives a user command via the input device 23, and controls the enlargement ratio and position of the display image.

  The imaging parameter setting unit 84 takes in the imaging parameters set by the user via the input device 23 or the imaging parameters stored in the storage device 19 in advance, 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 ratio that is the calculation result of the FOV calculation unit 80, and passes it to the imaging control unit 81 as a measured FOV. Therefore, the imaging is performed not by the set FOV but by the automatically calculated enlarged FOV (actual measurement 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 such as 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 is, for example, a range in which an object such as the subject exists, a range of the receiving sensitivity distribution of the receiving coil, a range of the static magnetic field space, or the like, and one or more of them is used for optimization. The information on the object existing range and the reception sensitivity distribution can be obtained from the previous measurement. Further, the range of the static magnetic field space is an eigenvalue of the device itself and may be stored in the storage device 19 in advance.

  Based on the above configuration, the procedure of imaging by the MRI apparatus of this embodiment will be described with reference to FIG. Prior to the measurement (main measurement) of imaging the 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 wide field of view is obtained with a relatively low spatial resolution. In some cases, measurement of the sensitivity of the receiving coil is included. Then, the imaging parameters necessary for the main measurement are set (S32). The imaging parameter includes the size and position of the FOV, the R factor, and the like. For example, when the user inputs it through the operation unit 25 while looking at the scanogram or the like displayed on the display device 20, the value is captured by the CPU 18. It is set in the parameter setting unit 84. Then, the FOV calculator 80 calculates the FOV enlargement ratio based on the set imaging parameters (S33). The main measurement is performed with the FOV enlarged at this FOV enlargement ratio (S34).

  The calculation of the FOV enlargement ratio (S33) is an optimization process for obtaining the FOV enlargement ratio that does not cause aliasing artifacts in the image or the site of interest in the image and prevents extension of the imaging time, and is a configuration that realizes that function. And the method can take various forms. Hereinafter, each embodiment of the configuration and method of the function of optimizing the FOV expansion rate will be described.

<< 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 in a previous measurement or the like. Then, the optimum FOV expansion rate 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 existing range calculator 86 as the FOV calculator 80. The object existence range calculation unit 86 calculates the object existence range in a region including the set FOV and the surrounding region by 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, for example, from an image such as a scanogram obtained in the previous measurement. The FOV enlargement ratio calculation unit 85 calculates the enlargement ratio in the phase encoding direction of the measured FOV with respect to the set 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 the object (subject) in the static magnetic field space, the imaging control unit 81 captures the subject in a wide range by a sequence of capturing a wide range of the imaged object. A sequence for realizing wide-range image pickup is arbitrary, and for example, a method of enlarging the frequency encoding to pick up an image of the image pickup target in a wide range can be considered. Further, it may be 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. Alternatively, a method of performing wide-range imaging in a short time using parallel imaging may be used. Further, a method may be used in which parallel imaging is used in combination with the above-described wide area imaging method, and a wider area is imaged or the measurement time is shortened. The image obtained by wide-range imaging may be used as the positioning image. Further, the wide area imaging may be measured separately from the measurement of the positioning image (pre-measurement: S31 in FIG. 3).

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

<Step S52>
The object existing range calculation unit 86 calculates the object existing range 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, but as an example, phase encoding from both ends in the phase encoding direction of a wide range image is performed. The pixel value is scanned along the direction, and the point where the pixel value sharply changes from zero (no signal) can be set as the end of the existence range of the object, and the existence range can be defined between the two points. The range of scanning in the phase encode direction (the range in the direction orthogonal to the phase encode direction) may be the range of the set FOV.

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

<Step S53>
The imaging parameter setting unit 84 acquires the imaging parameter information input by the user through 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. FOV and R factor are used as imaging parameters.

  In the present embodiment, the center of the FOV in the phase encoding direction (hereinafter, also simply referred to as the FOV center) is deviated from the center of the phase encoding direction of the object existence range [Object] (hereinafter, also simply referred to as the object center) (OFF. In the case of (center), the enlargement ratio is made different between the side on which the FOV center is shifted and the opposite side, and an enlargement ratio that does not cause aliasing and has a minimum sum of enlargement ratios is obtained. When the center of the FOV and the center of the object are coincident with each other, it is a singular point at which the shift amount (off-center amount) becomes zero and is included in the applicable range of the present embodiment.

First, the relationship between the position of the FOV with respect to the object existing range and the occurrence of folding will be described with reference to FIG. In FIG. 7, the left-right direction is the phase encode direction, the object center in the phase encode direction is represented by C _O , and the FOV center is represented by C _F. The range indicated by the diagonal lines is the set FOV 700. In addition, the existence range [Object] of the object is divided into two at the FOV center C _F , the shorter one from the center to the end is referred to as “ObjectS”, and the longer one is referred to as “ObjectL”. The direction toward L is the L direction, and the direction toward the shorter direction 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 the R factor = 1, there is no reduction due to the R factor, so the dotted line portion corresponds to the solid line portion.

  As for folding, an image of an object located outside (one outside) in the phase encoding direction of the FOV appears on the other end side in the FOV. For example, when the FOV 700 is expanded with the enlargement ratio NoWrapL / R in the L direction (right side) and the enlargement ratio NoWrapS / R in the S direction (left side) considering the R factor (the portion 701 surrounded by a dotted line in the drawing is the FOV). Considering the case), the portion outside the right end of the FOV 701 (arrow A1 in the figure) is folded back on the left side of the FOV 701 (arrow A2 in the figure). If the fold-back indicated by the arrow A2 does not enter the original FOV 700, the fold-back artifact does not occur, which satisfies the requirement of the enlargement ratio.

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

ObjectL- (FOV / 2) x (NoWrapL / R) ≤ (FOV / 2) (NoWrapS / R) -FOV / 2 (1)
ObjectS- (FOV / 2) x (NoWrapS / R) ≤ (FOV / 2) (NoWrapL / R) -FOV / 2 (2)
In the formula, 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 equations (1) and (2), equations (3) and (4) having 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)

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, NoWrapL is fixed so that the L-direction end of the FOV enlarged (or reduced) by NoWrapL matches 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), and therefore, the formula (4) is taken into consideration to determine NoWrapS. Substituting NoWrapL into equation (4) gives
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 inequalities are satisfied. Limited
(FOV / 2) x NoWrapS ≤ ObjectS (7)
NoWrapS ≧ 1 (8)

When the conditions of Expressions (7) and (8) are added and the conditions are 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 = 2 ObjectL / FOV
NoWrapS _Min = 2 (R × ObjectS- ObjectL) / FOV + R
Condition 2: FOV / 2> ObjectL, or (FOV / 2) × NoWrapS> ObjectS, or NoWrapS <1
NoWrapL _Min = 2 ObjectL / 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 expanding the set FOV by the FOV expansion rate calculated in step S54.

<Steps S56 to S59>
The imaging control unit 81 performs the main measurement with the measured FOV enlarged by the FOV enlargement ratio calculated in step S54 (S56). The image processing calculation unit 82 reconstructs the image of the measurement data obtained in step S56 (S57). As a result, an actually measured FOV size MRI image is obtained, and thus a set FOV size image is cut out from this MRI image (S58). The display control unit 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 in which 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 actually deleted FOV and the set FOV, including the aliasing, the imaging time is not unnecessarily extended.

  As described above, according to the present embodiment, it is possible to obtain an MRI image without aliasing artifacts with a minimum extension of the imaging time without lowering the inspection throughput.

<< Modification of First Embodiment >>
The present modification is characterized in that the optimization calculation of the enlargement ratio is performed in consideration of the range in which folding is allowed. That is, in the first embodiment, the enlargement ratio of the FOV, that is, the measured FOV is set so that all the portions including the folding artifacts are set in consideration of the object existing range. In some cases, the image exists in the center, and deterioration of image quality around the FOV does not affect interpretation. In this modification, in such a case, by setting the range in which the folding back is allowed, the enlargement ratio is optimized in the direction of further shortening the imaging time.

  The configuration of this modified example is the same as that of the first embodiment except that the processing of the FOV enlargement ratio calculation unit 85 shown in FIG. 4 is different, and hereinafter, refer to FIGS. 3 and 5 used in the first embodiment. Then, the points different from the first embodiment will be mainly described.

  First, wide-range imaging is performed as pre-measurement (S31, S51), and the setting of imaging parameters by the operator is accepted (S32). When setting the imaging parameters, a range (hereinafter, simply referred to as an allowable range) α that allows folding back near the end of the FOV in the phase encoding direction is received. The allowable range may be designated by the operator on the FOV setting screen, or may be designated by inputting a numerical value (a range of 5% of FOV from the end or the like). . FIG. 8 shows how the allowable range α is set.

  Next, the object existing range calculation unit 86 calculates the object existing range (S52). Next, the FOV enlargement ratio calculation unit 85 acquires the imaging parameter (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 existing range.

When considering the allowable range α, the equations (1) and (2) of the first embodiment are as follows.
ObjectL- (FOV / 2) x (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)

From this, the sum (NoWrapL + NoWrapS) of the FOV enlargement ratios in the L direction and the S direction that does not cause aliasing artifacts in the FOV inside the allowable range α is represented 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, with NoWrapL as 2ObjectL / FOV, and considering equation (4-2), equation (4-2) becomes
NoWrapS ≧ 2 (R × ObjectS−ObjectL−αR) / FOV + R (4-3)
Becomes

Considering here that ObjectS is (FOV / 2) or more and the enlargement ratio NoWrapS in the S direction (FOV / 2) is 1 or more (NoWrapS ≧ 1), α is calculated from Equation (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 0 or more, setting α (> 0) may shorten the imaging time, but if it takes a negative value, set α. However, the effect of shortening the imaging time cannot be obtained. Therefore, if the right side of (9) is defined by Unfold expressed by equation (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.

  This Unfold condition is added to the condition classification of the first embodiment, and NoWrapL and NoWrapS that minimize the sum (NoWrapL + NoWrapS) of the FOV enlargement ratios are obtained 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 less 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 = 2 ObjectL / FOV
NoWrapS _Min = 2 (R x ObjectS- ObjectL-αR) / FOV + R

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

Condition 1C: Condition 1, and Unfold <0
Under this condition, since the image pickup time is not shortened even if α is set because Unfold <0, α is not considered.
NoWrapL _Min = 2 ObjectL / 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 the condition 2 of the first embodiment. Therefore,
NoWrapL _Min = 2 ObjectL / FOV
NoWrapS _Min = 2ObjectS / FOV

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

  According to this modification, when the operator specifies the folding back allowable range α, the imaging time can be shortened depending on the conditions. Note that when α = 0, the process of the first embodiment is performed, so the process of the present modification can be said to be a generalized process of the first embodiment.

<< Second Embodiment >>
This embodiment is characterized in that the FOV calculator 80 uses the sensitivity distribution information of the receiving coil as the receivable spatial range when the FOV expansion rate is optimized. That is, the MRI apparatus of the present 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 information on the spatial range. Is used to calculate the enlargement ratio of the imaging visual field.

  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. Also 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 in which the sensitivity map of each channel of the receiving coil is expanded to the boundary where the sensitivity becomes 0. A magnifying sensitivity map creating unit 873 for creating a. Various methods such as a method using a phantom and a method using an image of a subject are known as methods for creating a sensitivity map, and any of them can be adopted. The sensitivity range calculation unit 87 calculates the sensitivity range of the receiving coil at the slice position of the imaging target from the slice position of the main measurement and the size and position of the set FOV based on the composite enlarged sensitivity map.

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

<Step S61>
The sensitivity map creation unit 871 creates a sensitivity map of each channel of the receiving coil. Here, for simplification of description, 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). The left-right direction in the figure is the phase encoding direction, and shows the sensitivity map image in the set FOV. The sensitivity map image shows that the sensitivity is high in the high-luminance region and low in the low-luminance region. In FIG. 11A, the sensitivity on the right side of the sensitivity map is high, and the sensitivity is low as it approaches the left side. . In FIG. 12A, the sensitivity on the right side of the sensitivity map is low, and the sensitivity is higher toward the left side.

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

<Step S63>
The combined sensitivity map creation unit 875 combines the expanded sensitivity maps of the channels created in step S62 to create one combined expanded sensitivity map. The synthesizing method is arbitrary, and 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 (combining all channels) in FIG. 13 shows the result of combining the enlarged sensitivity maps of channel 1 and channel 2.

<Step S53>
The imaging parameter setting unit 84 acquires the imaging parameter. In FIG. 10, step S53 is shown to be performed after step S63, but step S53 may be performed by the next step S64, and the order does not matter.

<Step S64>
The composite enlarged sensitivity map creation unit 875 determines whether or not the suppression pulse represented by the presaturation pulse is set in the imaging parameter acquired in step S53. If the suppression pulse is not set in the imaging parameter, the process proceeds to step S66. When the suppression pulse is set as the imaging parameter, the process proceeds to step S65.

<Step S65>
When the suppression pulse is set, no signal is generated from the region to which the suppression pulse is emitted, so the combined magnification sensitivity map creation unit 875 applies the suppression pulse in the combined magnification sensitivity map created in step S63. Set the sensitivity of the region to be 0. As a result, as shown in FIG. 14, when the suppression pulse is present, the synthetic enlargement 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 the present embodiment.

<Step S66>
The sensitivity range calculation unit 87 calculates the sensitivity ranges of the receiving coils for all channels from the combined expanded 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. The Sensitivity of FIG. 15 matches the range of Sensitivity (synthesis of all channels) or Sensitivity (with suppression pulse) of FIG.

<Step S54>
The FOV magnification ratio calculation unit 85 automatically calculates the optimum value of the FOV magnification 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 obtained by dividing the sensitivity range Sensitivity at 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 also displaced from the center C _S of the sensitivity range Sensitivity in the S direction, and the FOV expansion rate NoWrapL in the L direction and the FOV expansion rate NoWrapS in the S direction are different values. Can be set.

  As can be seen from a comparison with FIG. 7, the object (subject) is also outside the FOV in the phase encoding direction and outside the sensitivity range Sensitivity of the receiving coil. In this case, no signal is received even if an object (subject) is present when the sensitivity distribution is 0, but the image of the object outside the FOV in the phase encoding direction and within the sensitivity range is in the FOV. Appears as a folding artifact. Therefore, in the present embodiment, instead of [ObjectL] and [ObjectS], sensitivity ranges [SensitivityL] and [SensitivityS] in the L direction and the S direction are used, and an optimal enlargement that does not cause aliasing and can shorten the imaging time. The rate is calculated for each of the L direction and the S direction.

  The sum of the optimum L-direction FOV expansion rate NoWrapL and the S-direction expansion rate NoWrapS is the SensitivityL and SensitivityS of ObjectL and ObjectS of Expression (3) and Expression (4) described in step S54 of the first embodiment, respectively. It is represented by the replaced formula. Similarly to the modified example of the first embodiment, when the folding back allowable range α is taken into consideration, an expression in which ObjectL and ObjectS in Expressions (3-2) and (4-2) are replaced with SensitivityL and SensitivityS, respectively, is used. expressed.

As a result, NoWrapL _Min and NoWrapS _Min that minimize the sum of expansion rates (NoWrapL + NoWrapS) are required 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 x Sensitivity S- Sensitivity L-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 x Sensitivity S- Sensitivity L) / FOV + R

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

Note that, when the folding back 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 conditions 1A and 2 are divided into NoWrapL _Min and NoWrapS _Min. Is required.

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

  According to the present embodiment, it is possible to calculate the optimum FOV enlargement ratio in consideration of the region where the sensitivity does not exist even if the object exists. 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 coil having a plurality of channels or a receiving coil composed of a plurality of small receiving coils. However, when one receiving coil is used, the composite enlarged sensitivity map creating unit in FIG. 9 is used. 875 and step S63 of FIG. 10 can be omitted.

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

The present embodiment will be described below 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 illustrated, the signal processor 7 includes a static magnetic field effective range calculator 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, the operation of the signal processing unit 7 of this embodiment will be described mainly with reference to FIG.
<Step S70>
The 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 simulation results or the like, but it is desirable to measure it in advance. However, the measurement data may not be measured each time the main measurement is performed, but may be measured in advance when the device is manufactured or installed, and the measurement data may be stored in the storage device 19. Also, it may be combined with the three-dimensional shape data of the bore. FIG. 19 shows, as an example, the calculated effective region of the static magnetic field in the bore in the horizontal magnetic field type MRI apparatus having the cylindrical static magnetic field generating magnet. In the MRI apparatus, the method of taking the phase encoding direction is arbitrary, but generally, in the cylindrical bore, the uniformity of the static magnetic field collapses at the end portion in the axial direction, so in FIG. ) Is the phase encoding direction.

<Step S54>
The FOV enlargement ratio calculator 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 of the static magnetic field at the center C _F of the FOV. The center of the effective region of the static magnetic field, Effective Region, is indicated by _CE . Also here, the center C _{F of the} FOV is displaced from the center C _E of the effective range of the static magnetic field in the S direction, and the FOV expansion ratio NoWrapL in the L direction and the FOV expansion ratio 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) exists outside the FOV in the phase encoding direction and also outside the effective range of the static magnetic field. Although no signal is received from an object (subject) outside the effective range, an image of an object outside the FOV in the phase encoding direction and within the effective range appears as a folding artifact in the FOV. Therefore, in the present embodiment, the effective ranges Effective Region L and Effective Region S in the L direction and S direction are used instead of ObjectL and ObjectS, and the optimum enlargement ratio that can reduce the imaging time without causing aliasing is set in the L direction and S direction. Ask for each direction.

The sum of the optimum L-direction FOV expansion rate NoWrapL and the S-direction expansion rate NoWrapS can be calculated by the same algorithm as in the second embodiment, that is, SensitivityL and SensitivityS in the second embodiment, respectively, By substituting 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 Region L / R−FOV / 2R
NoWrapL _Min = 2 Effective Region L / 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 = 2 Effective Region L / FOV
NoWrapS _Min = 1.0

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

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

In addition, even in the present embodiment, when the folding back allowable range α is not set, it is not necessary to consider the condition of Unfold. Therefore, as in the case of the first embodiment, NoWrapL is divided into Condition 1A and Condition 2 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 captured by the execution FOV using the FOV enlargement ratio thus obtained to obtain an image.
According to the present embodiment, it is possible to calculate the optimum FOV expansion rate in consideration of the area in which the object exists but is in the effective range of the static magnetic field. Similar to 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 cross section (phase encoding direction).

  FIG. 21 shows a functional block diagram of the signal processing unit of this embodiment. As shown in FIG. 21, the FOV calculation unit 80 of the present embodiment includes a FOV expansion ratio calculation unit 85, an object existence 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 it. Although not shown, the sensitivity range calculation unit 87 includes a sensitivity map creation unit, an enlarged sensitivity map creation unit, and a synthetic enlarged sensitivity map creation unit, similar to the sensitivity range calculation unit 87 of the second embodiment.

The range comparison unit 89 compares the ranges calculated by the object existing range calculation unit 86, the sensitivity range calculation unit 87, and the static magnetic field effective range calculation unit 88, and selects the 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 RegionL, Effective Region S) and the distance from the center C _F to the end of the sensitivity range (SensivityL, SensivityS) And to determine whether to apply it to the calculation formula for calculating the optimum enlargement ratios NoWrapL _Min and NoWrapS _Min .

  The enlargement ratio calculation process of this 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 those shown in FIGS. 5, 10, and 18 are denoted by the same reference numerals, and detailed description thereof will be omitted.

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

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

<Step S60>
The sensitivity range calculator 87 calculates the sensitivity range of the receiving coil. For the sensitivity range, steps S61 to S66 shown in FIG. 10 are performed to create a sensitivity map for each channel of the receiving coil (S61), create an expanded sensitivity map for each channel (S62), and create a combined expanded sensitivity map (S63). This is calculated (S66). At this time, if the imaging pulse sequence includes a suppression pulse, a process of excluding the region in which the signal is suppressed by the suppression pulse from the composite enlarged sensitivity map is performed (S64, S65).

<Step S81>
From the set imaging pulse sequence, it is determined whether the phase encode direction is parallel to the body axis direction of the subject or the phase encode 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 transverse section of the abdomen as shown in FIG. 23 may be imaged. In such a case, the imaged region is always contained in the static magnetic field space, all objects can be measured in the wide range FOV, and it is difficult to consider that there is an object outside the wide range FOV. Therefore, in this case, the FOV expansion rate is calculated by considering only the existence range of the object obtained in step S52. NoWrapL _Min and NoWrapS _{Min in} this case perform the same calculation process 1 as in the first embodiment. That is, it becomes as follows.

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

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

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

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

<Step S83>
When 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, it is difficult to think that all the objects are settled in the wide range FOV because the imaging cross sections match the body axis direction of the body. 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 existing range falls within the static magnetic field effective range and sensitivity distribution. As a result, if the object existing range deviates from the static magnetic field effective range or from the sensitivity distribution in both the L direction and the S direction, the process proceeds to step S84, and if it deviates in 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 ratio calculation process 2 as in the second embodiment is performed. That is, as described with reference to FIG. 16, the FOV expansion rate is calculated using only the sensitivity range (S86).

<Steps S84 and S87>
In step S84, when 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 less than or equal to the sensitivity range SensitivityS, the process proceeds to step S87, as shown in FIG. Then, the optimal FOV expansion rate is calculated using Sensitivity L in the L direction and Effective Region S 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 × Effective Region S- Sensitivity L) / FOV + R

Condition 2: FOV / 2> SensitivityL, or FOV / 2 x NoWrapS> EffectiveRegionS, or NoWrapS <1
NoWrapL _Min = 2 SensitivityL / FOV
NoWrapS _Min = 2EffectiveRegionS / FOV
The expansion ratio thus obtained is in the case of Effective RegionL> SensitivityL and Effective RegionS <SensitivityS. In the case of Effective RegionL <SensitivityL, Effective RegionL may be used instead of the above-mentioned formula SensitivityL. Further, in the case of Effective RegionS> SensitivityS, SensitivityS may be used instead of the above formula Effective RegionS. That is, before step S87, the range comparison unit 89 may perform a step (not shown) for determining whether Effective RegionL> SensitivityL and Effective RegionS <SensitivityS.

<Step S85>
When one end of the object existing 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 the sensitivity of the receiving coil or the effective range of the static magnetic field is considered on the deviating side, For the side within the sensitivity range and the effective range of the static magnetic field, the FOV expansion rate is calculated in consideration of the object existing range (calculation process 4). The optimum FOV expansion rate in this case is as follows. Here, it is assumed that Effective RegionL> SensitivityL.

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 x Object S- Sensitivity L-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 x Object S- Sensitivity L) / FOV + R

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

  The expansion ratio thus obtained is in the case of Effective RegionL> SensitivityL. However, in the case of Effective RegionL <SensitivityL, Effective RegionL may be used instead of SensitivityL in the above equation. That is, before step S85, the range comparison unit 89 performs a step (S88 shown by a dotted line) of determining whether Effective RegionL> SensitivityL, and a calculation process 4 using SensitivityL or a calculation process using Effective RegionL (not shown) is performed. It may be performed. When the object existence range deviates from the sensitivity range or the static magnetic field effective range in the S direction, which is the reverse of the arrangement of the subject in FIG. 25, ObjectL is replaced by SensitivityL in the above equation, and SensitivityS or Effective is replaced by ObjectS. RegionS may be used.

In addition, in the calculation process of steps S82 and S85 to S87, when the allowable folding range α is not set, the condition of Unfold does not have to be considered. Therefore, as in the case of the first embodiment, the conditions 1A and NoWrapL _Min and NoWrapS _Min are required under condition 2.

  Further, when calculating the FOV expansion rate, only the object existing range is considered in S82, but the range of the sensitivity of the receiving coil and the effective range of the static magnetic field may be 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 existing range may be considered.

  The steps after the calculation of the FOV expansion rate are the same as those in the first to third embodiments, and the description 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, the signal receivable range in which the aliasing should be considered is appropriately selected, and the expanded FOV is optimized. be able to. Note that the processing described in the present embodiment and the drawings adopted in the present embodiment are merely examples, and various modifications may be made and conditions may be added in accordance with the present embodiment. included. For example, the case where the phase encode direction is parallel to the body axis and orthogonal to the body axis has been described for simplification of description, but as is well known, the phase encode direction can be arbitrarily selected regardless of the body axis. . Even in such a case, it is possible to select an appropriate enlarged FOV calculation process by determining whether or not the object existing range can be imaged in a wide range FOV. Whether or not the object existence range can be imaged in a wide range FOV can be determined by using the information of the object existence range, the three-dimensional static magnetic field effective range, and the sensitivity range.

  Although the respective embodiments of the MRI apparatus of the present invention have been described above, the present invention is not limited to these embodiments, and some elements may be deleted or changed unless technically contradictory. Further, the respective embodiments may be combined as appropriate, or another imaging parameter optimization process or the like 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 confirming the processing content of the 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. It may be displayed as a part of the screen.

  FIG. 26 shows an example of the display screen (GUI) 600. On 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, etc. are displayed, and the functions displayed in 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, or the like can be selected. When the auto mode is selected, in addition to the setting FOV, the imaging method, the subject position, etc. are automatically determined and appropriate. The FOV enlargement ratio calculation process is selected to calculate the enlargement ratio. When the customization mode is selected, for example, a plurality of calculation processes, for example, a subject priority process, a sensitivity distribution priority process, etc. are presented, and the operator selects a suitable one 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 an allowable range for folding, and if there is an allowable range, the size thereof. As the allowable range α, for example, as shown in FIG. 27, a screen showing the relationship between the object existing range and the set FOV may be displayed, and the range may be set on the screen using a cursor or the like, or as a numerical value. It may be displayed.

  The imaging time display block 630 displays the initially set imaging time of the FOV (set 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 (shortening 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 shortened time.

The instruction button display block 640 displays a button for confirming (ON) the mode selected in the mode selection block 610 and the condition set in the optimization condition setting block 620. By operating the ON button, the content of the FOV optimization process is confirmed, and the original image capturing condition setting screen is returned to, or another condition setting screen or a screen prompting the start of image capturing is displayed.
By providing such a user interface, the operator can easily realize the FOV optimization. 26 and 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 oscillator, 12: modulator, 13: high frequency amplifier, 14a: high frequency coil (transmission coil), 14b: high frequency coil (reception coil), 15: signal amplifier, 16: quadrature phase detection Device, 17: A / D converter, 18: central processing unit (CPU), 19: external storage device, 20: display device, 21: internal storage device, 23: input device, 25: operating unit, 80: FOV operation Section, 81: imaging control section, 82: image processing calculation section, 83: display control section, 84: imaging parameter setting section, 85: FOV enlargement ratio calculation section, 86: object existence range calculation section, 87: sensitivity range calculation section , 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 (10)

A magnetic field generation unit that causes 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, and a nuclear magnetic field received by the receiving unit. An image processing unit that processes a resonance signal to create an image of the subject, an imaging parameter setting unit that sets imaging parameters necessary for imaging, and an object existence range calculation unit that calculates an object existence range in the static magnetic field space. And a magnetic resonance imaging apparatus comprising:
The imaging parameters include an imaging field of view,
The imaging visual field set by the imaging parameter setting unit, further comprising an imaging visual field calculation unit for calculating the magnification of the imaging visual field based on the relationship between the spatial range in which the nuclear magnetic resonance signal can be received,
The imaging visual field calculation unit calculates an enlargement ratio of the imaging visual field by using the object existing range calculated by the object existing range calculation unit and the folding allowable range in the imaging visual field as information on the spatial range. Magnetic resonance imaging apparatus. A magnetic field generation unit that causes 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, and a nuclear magnetic field received by the receiving unit. An image processing unit that processes a resonance signal and creates an image of the subject, an imaging parameter setting unit that sets imaging parameters necessary for imaging, a sensitivity distribution calculation unit that calculates a sensitivity distribution range of the receiving coil, A magnetic resonance imaging apparatus comprising:
The imaging parameters include an imaging field of view,
The imaging visual field set by the imaging parameter setting unit, further comprising an imaging visual field calculation unit for calculating the magnification of the imaging visual field based on the relationship between the spatial range in which the nuclear magnetic resonance signal can be received,
The imaging field-of-view calculation unit uses the sensitivity distribution range calculated by the sensitivity distribution calculation unit as information on the spatial range, and the subject region in which the nuclear magnetic resonance signal is suppressed by the high-frequency magnetic field applied by the magnetic field generation unit. Is added to the information of the spatial range to calculate the enlargement ratio of the imaging field of view. The magnetic resonance imaging apparatus according to claim 2 , wherein
The receiving coil comprises a plurality of receiving coils,
The magnetic resonance imaging apparatus characterized in that the sensitivity distribution calculation unit synthesizes the sensitivity distributions of the plurality of receiving coils to calculate a sensitivity distribution range. A magnetic field generation unit that causes 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, and a nuclear magnetic field received by the receiving unit. 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,
The imaging parameters include an imaging field of view,
The imaging visual field set by the imaging parameter setting unit, further comprising an imaging visual field calculation unit for calculating the magnification of the imaging visual field based on the relationship between the spatial range in which the nuclear magnetic resonance signal can be received,
A magnetic resonance imaging apparatus, wherein the imaging field-of-view calculation unit uses information on the static magnetic field space as information on the spatial range to calculate a magnification rate of the imaging field of view. A magnetic field generation unit that causes 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, and a nuclear magnetic field received by the receiving unit. 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,
The imaging parameters include an imaging field of view,
The imaging visual field set by the imaging parameter setting unit, further comprising an imaging visual field calculation unit for calculating the magnification of the imaging visual field based on the relationship between the spatial range in which the nuclear magnetic resonance signal can be received,
The imaging visual field calculation unit further includes an object existing range in the static magnetic field space, an effective range of the static magnetic field space, and a range comparison unit that compares the sensitivity range of the receiving coil, and a comparison result of the range comparison unit. Based on the above, the object existing range, the effective range of the static magnetic field space, and the sensitivity range of the receiving coil are combined and used as the information of the spatial range to calculate the magnification rate of the imaging visual field. Imaging equipment. The magnetic resonance imaging apparatus according to any one of claims 1 , 2, 4, and 5 , wherein
The magnetic resonance imaging apparatus, wherein the imaging field-of-view calculation unit changes the enlargement ratio of the imaging field of view in two halves that are divided at the center of the imaging field of view in the phase encoding direction. The magnetic resonance imaging apparatus according to claim 6, wherein
The imaging visual field calculation unit has a spatial range in which the center of the spatial range in which the nuclear magnetic resonance signal can be received is within the imaging visual field set in the imaging parameter setting unit and in which the nuclear magnetic resonance signal can be received. If the center of the spatial range in which the nuclear magnetic resonance signal can be received does not coincide with the center of the imaging visual field set in the imaging parameter setting unit, the two halves in the phase encoding direction of the imaging visual field. Among them, the magnetic resonance imaging apparatus characterized in that the half of the side including the center of the spatial range has a larger enlargement ratio than the half of the side not including the center of the spatial range. The magnetic resonance imaging apparatus according to any one of claims 1 , 2, 4, and 5 , wherein
A magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus further comprising a display unit for displaying an image of the subject, wherein the imaging visual field calculation unit displays an optimized imaging visual field on the image displayed on the display unit. The magnetic resonance imaging apparatus according to claim 8 , wherein
The magnetic resonance imaging apparatus further comprising an input unit that receives a change in the imaging field of view displayed on the image. The magnetic resonance imaging apparatus according to claim 8 or 9 , wherein
An image pickup time calculation unit that calculates an image pickup time when the image pickup view calculation unit uses the optimized image pickup view field; and the image pickup time calculation unit causes the display unit to display the calculated image pickup time. Magnetic resonance imaging apparatus.

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