JP5405732B2 - Magnetic resonance imaging system - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. The present invention relates to a coil selection technique in an apparatus called “MRI”.

  The MRI device measures NMR signals (echo signals) generated by the spins of the subject, especially the tissues of the human body, and forms the shape and function of the head, abdomen, limbs, etc. in two or three dimensions. It is a device that images. In imaging, the echo signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured echo signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  In such an MRI apparatus, in recent years, a multi-channel receiving coil has been developed for the purpose of acquiring an image with a higher SNR and parallel imaging at a high speed. In the case of a large number of channels, an attempt is made to reconstruct one image using data obtained from a 128-channel receiving coil. In order to synthesize the data of each receiving coil into one image, MAC (Multiple Array Coil) synthesis, which usually provides a high SNR, is used (Non-Patent Document 1). In MAC synthesis, since a weighting function obtained by applying a low-pass filter to the image of each receiving coil is obtained, and the weighting function is applied again to the data and then the composition processing is performed, it takes time to reconstruct the image.

  In addition, the sensitivity of the normal receiving coil is higher in a region closer to the receiving coil itself. Therefore, when a relatively small region such as the heart is imaged using a multi-channel reception coil, there are a plurality of reception coils from which data (echo data or image data) of the part to be imaged can hardly be obtained. . In such a case, even if data from a receiving coil for which almost no data of the region to be imaged can be acquired is used for MAC synthesis, the processing time is only extended and an improvement in SNR cannot be expected.

  In cardiac imaging, imaging may be performed while acquiring a navigator echo for monitoring respiratory motion from a position different from the imaging section (for example, Patent Document 1). Usually navigator echoes are acquired by exciting a very local area such as the right diaphragm. In order to monitor the respiratory motion with high accuracy, it is not desirable that the navigator echo includes a signal in a region unrelated to the respiratory motion, and it is ideal that only a signal in a local region where there is respiratory variation is included. Furthermore, when navigator echoes are acquired with a multi-channel coil, the reception coil that actually receives a signal from the local excitation region is only a limited reception coil located in the vicinity of the excitation region. For this reason, it is very meaningless to synthesize echo signals from all receiving coils and use them as navigator echoes. In addition, as the number of channels to be synthesized increases, the time required for the synthesis process is extended.

A method for selecting data acquired by a multi-channel coil before photographing has been proposed (Patent Document 2). In the method of Patent Document 2, data acquisition is performed in a pre-scan manner before main measurement for acquiring image data, and the intensity comparison of signals from each reception coil is performed on all imaging slices, so that the reception coil to be used Make a selection.

WO2004 / 080301 Publication Japanese Patent Laid-Open No. 11-276452 M. Schmitt et al., Proc. Intl. Soc. Mag. Reson. Med. 15p. 245 (2007)

  When imaging is performed using a multi-channel coil, there is a receiving coil from which a signal of a target imaging target region is hardly acquired. In that case, combining the data of all receiving coils and reconstructing the image will not lead to an improvement in image quality. In particular, when the purpose is to acquire local area data, such as navigator echo for monitoring respiratory motion, the effect of synthesizing the data of all receiving coils is low, and the synthesis processing time increases as the number of receiving coils to be synthesized increases. Is extended. Extending the synthesis processing time extends the time from acquisition of the navigator echo to the actual measurement, leading to a reduction in respiratory motion artifact suppression effect.

  In the method of Patent Document 2, it is inevitable that the imaging time is extended by the prescan time, and the time extension is particularly remarkable in imaging with a large number of slices. In addition, since the switching of data acquisition is performed by connecting / closing the channel of the receiving coil in a circuit manner, additional hardware is required to switch the circuit and decouple unused receiving coils. A mechanism is required.

  Accordingly, an object of the present invention is to provide an MRI apparatus capable of selecting an appropriate receiving coil for image synthesis without performing additional data acquisition when imaging a subject using a multi-channel coil. That is.

In order to achieve the above object, the MRI apparatus of the present invention is configured as follows. That is,
Receiving means for receiving a nuclear magnetic resonance signal from a subject with a multi-channel coil having a plurality of small receiving coils, and region input for inputting a setting of a two-dimensional region of interest on a positioning image Means, display means for displaying a simulated figure representing a two-dimensional area covered by the small receiving coil for each of the small receiving coils, superimposed on the positioning image, and the plurality of small receiving coils corresponding to the region of interest. Selection means for selecting one or more small receiving coils from among the above, and arithmetic processing for reconstructing the subject image of the subject using the nuclear magnetic resonance signals received by the selected one or more small receiving coils And means .

According to the MRI apparatus of the present invention, when imaging a subject using a multi-channel coil, a small receiving coil suitable for image synthesis can be selected without performing additional data acquisition. That is, it is possible to select and capture only the receiving coil suitable for image composition. Alternatively, only necessary data before MAC synthesis can be selected from the data of each small receiving coil received by the multi-channel coil. Therefore, data that does not lead to an improvement in SNR even if combined can be omitted from the combining process, and the processing time can be shortened.
In particular, in the navigator echo acquired from the local area, since a small number of small receiving coils are necessary in the vicinity of the excitation area, the number of small receiving coils can be significantly limited to shorten the processing time. The shortening of the processing time leads to a reduction in the difference between the respiratory displacement detected by the navigator echo and the actual respiratory displacement at the time of the actual measurement, and as a result, the respiratory motion artifact suppression effect is improved.

  Hereinafter, preferred 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 according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject. As shown in FIG. 1, a static magnetic field generation system, a gradient magnetic field generation system, a transmission system, a reception system, and measurement control A system, an arithmetic processing system, and an operation unit.

  The static magnetic field generation system includes a static magnetic field generation magnet 102 having a static magnetic field generation source of a permanent magnet system, a normal conduction system, or a superconductivity system arranged around a subject 101. The static magnetic field generating magnet 102 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 101 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. .

  The gradient magnetic field generation system includes a gradient magnetic field coil 103 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (static coordinate system) of the MRI apparatus, and a gradient magnetic field power source 109 that drives each gradient magnetic field coil. Consists of. Gradient magnetic fields Gx, Gy, and Gz are applied in the three axial directions of X, Y, and Z in the static magnetic field by driving the gradient magnetic field power supply 109 of each coil in accordance with a command from the measurement controller 111 described later. At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 101, and the remaining two orthogonal to the slice plane and orthogonal to each other A phase encode gradient magnetic field pulse (Gp) and a frequency encode gradient magnetic field pulse (Gf) are applied in one direction, and position information in each direction is encoded in the echo signal.

  The transmission system irradiates the subject 101 with RF pulses in order to cause nuclear magnetic resonance to occur in the nuclear spins of the atoms constituting the biological tissue of the subject 101. The high-frequency transmission unit 110 and the transmission-side high-frequency coil ( Transmitting coil) 104. The high frequency pulse from the high frequency transmission unit 110 is supplied to the high frequency coil 104 disposed in the vicinity of the subject 101 at a timing according to a command from the measurement control unit 111, so that the subject 101 is irradiated with the RF pulse. .

  The receiving system detects an echo signal emitted by nuclear magnetic resonance of nuclear spins constituting the biological tissue of the subject 101. The receiving side high-frequency coil (receiving coil) 105, signal detecting unit 106, and signal processing unit It consists of 107. An echo signal of the response of the subject 101 induced by the RF pulse irradiated from the receiving coil 104 on the transmitting side is received by the receiving coil 105 disposed in the vicinity of the subject 101, and the received signal is a signal detection unit 106. The detected received signal is input to the signal processing unit 107 and subjected to predetermined processing to be converted into a digital quantity. The digital quantity is sent to the arithmetic processing system.

  In particular, the receiving coil 104 of the MRI apparatus according to the present invention is a multi-channel coil (multiple coil) formed by combining a plurality of small receiving coils, and the signal detection unit 106 is also provided for each small receiving coil in accordance with the multi-channel coil. It has a plurality of input channels for detecting a received signal.

  The measurement control system includes a measurement control unit 111 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 measurement control unit 111 operates under the control of the central control unit 108 described later, and sends various commands necessary for collecting tomographic image data of the subject 101 to the transmission system, gradient magnetic field generation system, and reception system.

  The arithmetic processing system performs various data processing and display and storage of the processing results, and has a central control unit 108, an external storage device 114 such as an optical disk or a magnetic disk, and a display 113 formed of a CRT or the like. When the digital data from the reception system is input to the central control unit 108, the central control unit 108 executes processing such as signal processing and image reconstruction, and the tomographic image of the subject 101 as a result is displayed on the display 113. The information is displayed and recorded in the external storage device 114.

  The operation unit 115 inputs various control information of the MRI apparatus and control information of processing performed in the arithmetic processing system, and includes a trackball or a mouse and a keyboard. The operation unit 115 is disposed in the vicinity of the display 113, and the operator controls various processes of the MRI apparatus interactively through the operation unit 115 while looking at the display 113.

  In FIG. 1, the high-frequency coil 104 and the gradient magnetic field coil 103 on the transmission side are opposed to the subject 101 in the static magnetic field space of the static magnetic field generation system into which the subject 101 is inserted, in the case of the vertical magnetic field method. If the horizontal magnetic field method is used, the object 101 is installed so as to surround it. The high-frequency coil 104 on the receiving side is installed so as to face or surround the subject 101.

At present, the radionuclide to be imaged by the MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used clinically. By imaging information on the spatial distribution of proton density and the spatial distribution of relaxation time in the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged two-dimensionally or three-dimensionally.
(First embodiment)
Next, a first embodiment of the MRI apparatus of the present invention will be described. In the present embodiment, an appropriate small reception is selected from a small reception coil group constituting a multi-channel coil corresponding to a field of view (FOV) set on a positioning image such as a scanogram image or a previously acquired image. It is a form which selects a coil. Hereinafter, this embodiment will be described in detail with reference to FIG. Figure 2 shows an imaging field of view (FOV) set around the heart, with an 8-channel coil consisting of 8 small receiving coils as an example of a multi-channel coil on the body surface near the heart of the subject 101. The flowchart in the case of imaging | photography is shown. In addition, this embodiment is not limited to an 8-channel coil, and may be a multi-channel coil having 2 to 7 or 9 or more small receiving coils. Hereinafter, the processing of each step will be described in detail.

In step 201 , the operator captures a positioning image of the subject 101.
As shown in FIG. 3 (a), the operator installs an 8-channel coil 302 including eight small receiving coils (3021 to 3028) on the chest side of the subject 101 before imaging. The upward direction on the paper is the head direction, and the downward direction is the foot direction. In this state, the operator starts taking a scanogram image of the subject 101 as a positioning image using a multi-channel coil or another whole body coil. The scanogram image only needs to have a resolution that can identify the entire structure around the heart portion of the subject. Therefore, the scanogram image is acquired by a low-resolution short-time imaging using a high-speed sequence such as a gradient echo sequence. be able to. The measurement control unit 111 activates a scan sequence for a scanogram image to measure an echo signal for the scanogram image, and the central control unit 108 reconstructs the scanogram image using the acquired digital data of the echo signal to display the display 113. To display.

In step 202 , a simulated figure representing each small receiving coil constituting the multi-channel coil is displayed superimposed on the acquired scanogram image.

  The central control unit 108 reads out and acquires the shape information and arrangement information of the multi-channel coil and each of the small receiving coils stored in advance in the external storage device 114. Then, the central control unit 108 configures a figure that schematically represents the shape of each small receiving coil based on the read shape information, and arranges the simulated figure based on the read arrangement information to arrange the multi-channel coil. A simulated figure to be represented is constructed, and the center of the static magnetic field and the center of the multi-channel coil are displayed together on the scanogram image. FIG. 3 (b) is an example in which a simulated figure of a multi-channel coil and a small receiving coil constituting the multi-channel coil is superimposed on the acquired COR cross-sectional scanogram image 301 and displayed. In FIG. 3 (b), since a multi-channel coil is used in which small receiving coils each having a substantially rectangular shape are arranged adjacent to each other, each of the substantially rectangular figures representing each small receiving coil corresponds to each small receiving coil. It is displayed at a position on the scanogram image corresponding to the arrangement position of the coil.

  Usually, the operator places the multi-channel coil on the subject 101 so that the center of the multi-channel coil substantially coincides with the center of the static magnetic field. Therefore, the position of the multi-channel coil on the scanogram image 301 may be displayed so that the center of the static magnetic field and the center of the multi-channel coil are at the same position. The size of the multi-channel coil is known in advance, and the center of the static magnetic field on the scanogram image can be obtained from the imaging conditions, so that the center of the multi-channel coil can be displayed in accordance with the center of the static magnetic field. Even if the multi-channel coil is set at a location where the center of the multi-channel coil is shifted by several centimeters from the center of the static magnetic field, the shift amount with respect to the FOV is about 1/10 to a few times. Therefore, the position of the multichannel coil displayed on the scanogram image can be substantially regarded as indicating the actual position of the multichannel coil. Since the direction of the multi-channel coil with respect to the static magnetic field direction is substantially constant for each multi-channel coil, the multi-channel coil is displayed in a predetermined direction.

In step 203 , a field of view (FOV) is set on the scanogram image.
The operator uses a mouse or the like to set the FOV so as to include the region of interest on the scanogram image. FIG. 5 shows an example of the FOV 503 set in the scan cross-sectional image 301 of the COR cross section. In the example shown in FIG. 5, the position of each small receiving coil (3021 to 3028) in use is also displayed on the scanogram image 301, and the region including the heart portion as the region of interest is set as FOV503 using the mouse. ing. The set FOV 503 includes small receiving coils 3022 to 3024 and 3026 to 3028, and does not include the small receiving coils 3021 and 3025. The FOV setting is not limited to this setting, and may be set wider or narrower.

In step 204, a small receiving coil used for MAC synthesis is selected.
The central control unit 108 selects a small receiving coil suitable for photographing the FOV set in step 203. A specific example of how to select will be described later.

In step 205, the subject is imaged using the small receiving coil selected in step 204, and an echo signal for each selected small receiving coil is acquired. Specifically, the measurement control unit 111 controls the signal detection unit 106 to detect the echo signal of only the selected small reception coil, and causes the signal processing unit 107 to process the detection signal for each small reception coil.

In step 206, MAC synthesis processing is performed using the data (image data or signal data) for each small receiving coil selected in step 204, and one synthesized image is acquired. Specifically, the central control unit 108 reconstructs an image for each small reception coil using a digital signal detected and processed for each selected small reception coil, and MAC-synthesizes the image for each small reception coil. To obtain a single composite image. Alternatively, the central control unit 108 may MAC-synthesize the digital signal detected and processed for each selected small receiving coil, and then reconstruct the synthesized data to obtain one synthesized image.

Next, a method for selecting an appropriate small receiving coil in step 204 will be described.
In the first example of the selection method, the operator selects a small receiving coil suitable for photographing the set FOV from the positional relationship between each small receiving coil arrangement on the scanogram image and the FOV set by the operator. Alternatively, the operator may select a small receiving coil that is not used for MAC synthesis. For example, referring to the scanogram image shown in FIG. 5 and each small receiving coil arrangement superimposed on the image, the operator can use a mouse (an example of selection input means for selecting a desired small receiving coil). Click the area of the small receiving coil to be selected using to select the small receiving coil. The selected small receiving coil has a display mode different from that of the non-selected small receiving coil. For example, the colors of the lines and areas of the simulated figure are made different, or the thicknesses of the lines are made different. When the selected small receiving coil is clicked with the mouse again, the selection is canceled and the selected state is not selected.

  In the second example of the selection method, the central control unit 108 selects a small receiving coil included in the FOV set by the operator. Based on the positional relationship between the position and area of the FOV set on the scanogram image by the operator and the position of each small receiving coil on the scanogram image, the central control unit 108 selects the small receiving coil included in the FOV. select. In addition, for the selection of a small receiving coil that crosses the boundary of the FOV, select it all, or select if the area ratio above the predetermined threshold is included in the FOV, or do not select at all, Either.

  In the third example of the selection method, the central control unit 108 selects based on the data acquired by each small receiving coil. For example, a test signal is acquired using an all-small receiving coil, and an appropriate small coil is selected based on the test signal. As the test signal, for example, after applying a 90 ° pulse, an echo signal is measured without applying a phase encoding gradient magnetic field. Then, a threshold value is obtained from a predetermined threshold value or a measured echo signal, and a small receiving coil in which an echo signal equal to or greater than the threshold value is detected is selected. Alternatively, the echo signal acquired without applying the phase encoding gradient magnetic field may be selected from a comparison between projection data obtained by one-dimensional Fourier transform and a threshold value.

  The above is the description of the processing flow for selecting an appropriate small receiving coil corresponding to the set FOV of this embodiment and performing MAC synthesis.

  Hereinafter, the effects of the present embodiment will be concretely described by explaining the difference between the case of shooting using all the conventional small receiving coils and the case of shooting by selecting an appropriate small receiving coil of the present embodiment. Explained.

  FIG. 4 shows an image of each small receiving coil when images are taken using all the conventional small receiving coils. Images taken by the small receiving coils 3021 to 3028 are 4031 to 4038, respectively. Each small receiving coil acquires the same FOV echo signal and images it. However, as indicated by 4031 to 4038, the signal intensity distribution in the FOV differs for each image of the small receiving coil. That is, the signal (4033, 4037) acquired by the small receiving coil (3023, 3027) closest to the heart part is the largest, and the image (2031, 3025) acquired by the small receiving coil (3021, 3025) farthest from the heart part. The signal value of 2035) is the minimum.

  In the case of the conventional MAC (Multiple Array Coil) synthesis, echo signals or images respectively acquired by eight small receiving coils are synthesized and displayed as a final synthesized image. Therefore, an echo signal or an image of a small receiving coil (3021, 3025) having almost no signal in the FOV is also used for MAC synthesis. Even if MAC synthesis is performed including signals from small receiver coils where such signals are hardly detected, there is almost no improvement in SNR, and only noise components are synthesized, so even SNR deteriorates. It is possible. Furthermore, the processing time for image reconstruction is extended by the increase in the number of small receiving coils for MAC synthesis.

  Therefore, in the present embodiment, an appropriate small receiving coil to be used for MAC synthesis is selected on the scanogram image before MAC synthesis.

  A first example of selecting an appropriate small receiving coil will be described using the example of FOV setting shown in FIG. In this example, a signal from only a small receiving coil located within the FOV set on the scanogram image is selected to be used for MAC synthesis. Or, conversely, this is an example in which a signal from a small receiving coil located outside the FOV is not used for MAC synthesis. The central control unit 108 selects a small receiving coil corresponding to the set FOV.

  As shown in FIG. 5, on the scan cross-sectional image 301 of the COR cross section, if the FOV 503 is set by the operator, the small receiving coils 3021 and 3025 are located away from the FOV 503. It is clearly disadvantageous to receive the signal in the FOV 503 compared to the coil. Therefore, the central control unit 108 controls the signal detection unit 106 so as not to measure signals from the small receiving coils 3021 and 3025, or does not use the signals of the small receiving coils 3021 and 3025 for the MAC composition processing after photographing. Like that.

  In this way, by excluding echo signals or images of small receiving coils that are unnecessary for MAC synthesis, it is possible to reduce the computation time for MAC synthesis processing by reducing the number of coil channels and performing MAC synthesis processing. It becomes. For example, when a 512-matrix small receive coil image is MAC-combined for 8 channels, it takes about 200 ms. As the number of channels increases, the effect of time reduction becomes even greater.

  Furthermore, the SNR of the synthesized image can be improved by excluding the echo signal or image of the small receiving coil that is unnecessary for the MAC synthesis. In the example of FIG. 5, since the echo signals or images of the small receiving coils (3021, 3025) outside the FOV 503 are mainly noise signals, the SNR of the MAC synthesized image is reduced by excluding these from the MAC synthesis. Can be improved.

  In addition, when a small receiving coil outside the FOV 503 is used for MAC synthesis, aliasing artifacts are more emphasized. FIG. 6 shows a specific example of the folding artifact by giving an example of photographing the heart with a simple cross section (Ax). FIG. 6 (a) shows a case where the FOV 603 including the heart portion of the subject 101 is imaged with a simple cross section (Ax) with the same coil arrangement as FIG. 3 (b). When the FOV 603 in FIG. 6A is photographed by the small receiving coils 3021 to 3028, the small receiving coils 3021 and 3025 are installed outside the FOV 603. In such a state, the small receiving coils 3021 and 3025 have higher sensitivity in the chest wall and back side regions near the coil than in the heart portion region to be imaged. Therefore, when the phase encoding direction is the left-right direction on the paper, the signal from outside the FOV acquired by the small receiving coils 3021 and 3025 appears as a strong folding artifact and appears in the composite image. FIG. 6 (b) shows an example in which aliasing artifacts appear. An image of the part of the subject 101 that is out of the left FOV on the paper surface is folded back to the right side in the FOV and appears as a folded artifact 608. The example in Fig. 6 (b) is a very simple radiograph, so changing the phase encoding direction to the chest wall-back direction may avoid folding artifacts. The imaging section to be used is a double oblique image (left ventricular short axis image, four-chamber image, etc.), and aliasing artifacts may occur regardless of the phase encoding direction. In such a case, aliasing artifacts can be effectively reduced by not using echo signals or images acquired by the small receiving coils 3021 and 3025 outside the FOV for MAC synthesis. Details of the reduction of aliasing artifacts will be described in a third embodiment to be described later.

As described above, according to the present embodiment, when imaging a subject using a multi-channel coil, it is possible to select a receiving coil suitable for image synthesis without performing additional data acquisition. As a result, it is possible to reduce the computation time of MAC composition and improve the SNR of the MAC composition image.
(Second embodiment)
Next, a second embodiment of the MRI apparatus of the present invention will be described. In the first embodiment described above, a small receiving coil suitable for MAC synthesis is selected corresponding to the FOV set before shooting, but the second embodiment is an all-small receiving coil. This is an example of selecting a small receiving coil suitable for MAC synthesis in accordance with the set FOV after capturing and acquiring data. Hereinafter, the second embodiment will be described with reference to FIG.

  FIG. 2 (b) shows a processing flow of the present embodiment, and shows a flowchart in the case of shooting under the same conditions as FIG. 2 (a). Steps different from FIG. 2A are that step 211 is performed instead of step 204, step 212 is performed instead of step 205, and step 213 is performed instead of step 206. Hereinafter, description of the processing contents of the common steps will be omitted, and only these different steps will be described.

In step 211 , the subject 101 is imaged using the all-small receiving coil. Specifically, the measurement control unit 111 controls the signal detection unit 106 to detect echo signals of all small reception coils, and causes the signal processing unit 107 to process detection signals for each small reception coil. Then, the central control unit 108 reconstructs an image for each small reception coil using the digital signals detected and processed for all the small reception coils, and MAC-synthesizes the image for each small reception coil to produce one image. The composite image of is acquired. As described above, the image may be reconstructed after direct MAC synthesis of the echo signal digital data. In this case, a folding artifact 608 may occur in the composite image as shown in FIG. That is, an image based on a signal from a region outside the left FOV on the subject's paper surface is folded back to the right in the FOV. This is because the signals detected by the small receiving coils 3021 and 3025 outside the FOV are used for MAC synthesis, so that the signal from outside the FOV becomes a folding artifact and is superimposed on the image. Therefore, in such a case, a small receiving coil suitable for MAC synthesis described below is selected.

At step 212 , an appropriate small receive coil is selected. As a method for selecting an appropriate small coil, any of the following methods can be used.

  The first example of the selection method is the same as the first example of the small receiving coil selection method in the first embodiment described above. That is, the operator selects a small receiving coil suitable for photographing the set FOV from the positional relationship between each small receiving coil arrangement on the scanogram image and the set FOV. Detailed description is omitted.

  The second example of the selection method is the same as the second example of the small receiving coil selection method in the first embodiment described above. That is, the central control unit 108 selects a small receiving coil included in the FOV set by the operator. Detailed description is omitted.

  A third example of the selection method selects a small receiving coil having a large signal strength by comparing the echo signal intensity of each small receiving coil with a threshold value. Specifically, the intensity of the echo signal acquired when the phase encoding gradient magnetic field is zero from the echo signals acquired for the subject image of each small receiving coil is set to a predetermined threshold value or a small size. A threshold (for example, 20% of the maximum value) obtained from the signal strength for each receiving coil is compared with the signal strength of each small receiving coil, and a small receiving coil having a signal strength equal to or greater than the threshold is selected.

  In the fourth example of the selection method, an average value of pixel values of an image of each small receiving coil is compared with a threshold value, and a small receiving coil having a large average pixel value is selected. Specifically, the average value of the pixel values of the image of each small receiving coil is a predetermined threshold value or a threshold obtained from the image for each small receiving coil (for example, 20% of the maximum average value) The average value of the pixel values of the receiving coil is compared, and a small receiving coil having a pixel value average value equal to or greater than the threshold value is selected.

In step 213 , the MAC combining process is performed again using the acquired data for each small receiving coil selected in step 212, and one composite image is acquired. For example, in step 212, it is assumed that the small receiving coils 3031 and 3035 arranged outside the FOV are removed from the MAC synthesis, and only the small receiving coils 3032 to 3034 and 3036 to 2038 arranged inside the FOV are selected. Then, when the operator presses an image composition button 705 (an example of reconstruction instruction means for instructing reconstruction of a subject image) below the scanogram image shown in FIG.7 (a), the central control unit 108 The MAC synthesis is performed again using the acquired data of the selected small receiving coil. An example of the result image is shown in FIG. The image of Fig. 6 (b), in which the data of the small receiving coils 3031 and 3035 arranged outside the FOV, which was the cause of the aliasing artifact, is excluded from the MAC synthesis, so that the aliasing artifact appears remarkably. In comparison with FIG. 7B, the aliasing artifact is largely removed in the image of FIG.

In step 214 , the operator determines the synthesized image that has been MAC-combined, and if the image quality is acceptable, the operation ends. Try again. Thereafter, step 212 and step 213 are repeated until the operator is satisfied with the image quality of the MAC composite image.

  The above is description of the processing flow which selects an appropriate small receiving coil after imaging | photography using the all small receiving coil of this embodiment. In this embodiment, for example, in multi-slice cine imaging covering 100 to 200 result images, all slices are synthesized using the same selection after confirming selection of an appropriate small receiving coil in any one slice. It is effective for such applications.

As described above, according to the present embodiment, an appropriate small receiving coil can be selected even after photographing using the all small receiving coil. That is, an appropriate combination can be selected while trial and error in selecting a small receiving coil interactively while viewing an image. As a result, aliasing artifacts can be reduced and SNR can be improved, so that the image quality of the MAC composite image can be further improved.
(Third embodiment)
Next, a third embodiment of the MRI apparatus of the present invention will be described. Each of the above-described embodiments is a mode of selecting an appropriate small receiving coil for acquiring an image of the subject. However, the present embodiment selects an appropriate small receiving coil for acquiring navigator echo. It is a form. Normally, navigator echo is acquired from a narrow area, so when detecting body motion information using navigator echo, it is more effective to limit the small receiving coil used for detecting the navigator echo for the reason described later. It is. Hereinafter, the present embodiment will be described in detail with reference to FIG.

  FIG. 8 shows an example in which imaging is performed while acquiring navigator echo for monitoring respiratory motion from an area 804 independent of the imaging FOV 503 in the same state as FIG. The other parts are the same as in FIG.

  Normally, the navigator echo is acquired by locally exciting a region where fluctuation due to respiration is large for the purpose of monitoring respiratory motion with high accuracy. Therefore, even if the navigator echoes are obtained by synthesizing the data from all the small-sized receiving coils 3021 to 3028 in the arrangement situation of the multi-channel coils as shown in FIG. 8, it is possible to improve the respiratory motion detection accuracy of the navigator echoes. It does not lead to. In addition, as described above, the SNR of the navigator echo is lowered, and the computation time required for the synthesis process is increased. Therefore, as shown in FIG. 9, the time interval (calculation time) 902 from navigator echo acquisition 901 to execution of the main measurement 903 is extended. If the time interval 902 is extended, an error between the respiratory motion displacement detected by the navigator echo and the respiratory motion displacement at the time when the image data is actually acquired increases, leading to image quality degradation. Furthermore, the execution time 901 and the processing time 902 of the sequence for acquiring navigator echo are periods during which the main measurement data cannot be acquired. Therefore, shortening the processing time 902 and extending the period during which the main measurement data can be acquired are significant in cine imaging that acquires a large amount of cardiac phase data.

  As described above, the data used as the navigator echo is not a combination of the data of all the small receiving coils, but is preferably limited to data from the small receiving coils in the vicinity of the local excitation region and processed in a short time. In the present embodiment, when the operator designates the excitation area 804 of the navigator echo on the scanogram screen shown in FIG. 8, the optimum small receiving coil for acquiring the navigator echo signal from the excitation area 804 is selected.

  As a method for selecting a small receiving coil suitable for navigator echo acquisition in the present embodiment, any of the various methods described in the above embodiments can be used. For example, a method in which the operator selects a small receiving coil on the scanogram image with a mouse, a method in which the central control unit 108 selects a small receiving coil that overlaps with the excitation region 804, or a signal intensity by detecting a test signal. This is a method of selecting a small receiving coil having a large size. In the example of FIG. 8, the central control unit 108 automatically selects the small receiving coils 3021 and 3025 that overlap the excitation region 804 as the small receiving coils used for navigator echo acquisition.

  Further, another example of a small receiving coil selection method suitable for navigator echo acquisition will be described below. Since the excitation area of the navigator echo is a fraction of that of the imaging FOV, in the case of navigator echo acquisition, selection of a small receiving coil requires more accuracy. Therefore, the signals of each small receiving coil actually acquired are compared, and the small receiving coil used for the MAC synthesis is selected. In this case, projection data obtained by one-dimensional Fourier transform of navigator echoes acquired from all the small receiving coils 3021 to 3028 in FIG. 8 are compared, and the one having a large average value is used as the navigator echo. The number of small receiving coils to be selected is not limited to one, and it is possible to select a plurality of small receiving coils such as two or three from the larger one. By selecting the small receiving coil used for navigator echo detection in this way, even when there is a difference between the small receiving coil position displayed on the scanogram screen and the actual small receiving coil position, it is used as navigator echo acquisition. An appropriate small receiving coil can be selected.

  As described above, according to the present embodiment, a small receiving coil suitable for navigator echo acquisition can be selected in accordance with the set navigator echo acquisition area without performing additional data acquisition. . As a result, a navigator echo having a high SNR can be acquired in a short time, so that the body movement can be monitored with high accuracy.

The block diagram of an example of the MRI apparatus which concerns on this invention. The flowchart which shows the processing flow of embodiment of this invention. The figure which shows the example of multi-channel coil arrangement | positioning on a scanogram image. The figure which shows the example of an image of each small receiving coil. The figure which shows the example which sets FOV on 1st Embodiment and a scanogram image. The figure which shows the example of an Axial imaging | photography and folding | turning artifact. The figure which shows 2nd Embodiment and folding | turning artifact reduction. The figure which shows 3rd Embodiment and a navigator echo acquisition area. The figure which shows the timing of navigator echo measurement and this measurement.

Explanation of symbols

  101 subject, 102 static magnetic field magnet, 103 gradient coil, 104 RF coil, 105 RF probe, 106 signal detector, 107 signal processor, 108 display, 109 gradient magnetic field power supply, 110 RF transmitter, 111 controller, 112 beds

Claims (9)

Receiving means for receiving a nuclear magnetic resonance signal from a subject, comprising a multi-channel coil formed by two-dimensionally arranging a plurality of small receiving coils;
Display means for displaying a simulated figure representing a two-dimensional region covered by the small receiving coil for each of the small receiving coils, superimposed on the positioning image;
Area input means for inputting a setting of a two-dimensional field of view (FOV) on the positioning image;
Corresponding to the two-dimensional field of view, and selection means for selecting one or more small receiving coils corresponding to the simulated graphic included in the two-dimensional imaging the field of view from among the plurality of small receiving coils,
Arithmetic processing means for reconstructing a subject image of the subject using a nuclear magnetic resonance signal received by one or more small receiving coils selected by the selection means ;
A magnetic resonance imaging apparatus comprising: The magnetic resonance imaging apparatus according to claim 1.
Whether the selection means selects all of the small receiving coils corresponding to the simulated figure straddling the boundary of the imaging field of view, or selects when a region ratio equal to or greater than a predetermined threshold is included in the imaging field of view Or a magnetic resonance imaging apparatus characterized by not selecting at all . The magnetic resonance imaging apparatus according to claim 1 or 2,
The display means displays a simulated figure for each of the small receiving coils so as to overlap the positioning image so that the center of the multi-channel coil substantially coincides with the center of the static magnetic field. . The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The magnetic resonance imaging apparatus , wherein the selection unit selects the one or more small receiving coils before imaging the subject using the multi-channel coil . The magnetic resonance imaging apparatus according to any one of claims 1 to 3 ,
The magnetic resonance imaging apparatus , wherein the selection unit selects the one or more small receiving coils after imaging the subject using the multi-channel coil . The magnetic resonance imaging apparatus according to claim 5 .
The arithmetic processing unit reconstructs the subject image again using the nuclear magnetic resonance signal received by the small receiving coil selected by the selection unit after reconstructing the image of the subject. A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 6.
Reconstruction instruction means for instructing the arithmetic processing means to reconstruct the subject image,
The arithmetic processing unit reconstructs the subject image using a nuclear magnetic resonance signal for each of the one or more selected small receiving coils in accordance with the reconstruction instruction . Receiving means for receiving a nuclear magnetic resonance signal from a subject, comprising a multi-channel coil formed by two-dimensionally arranging a plurality of small receiving coils;
Display means for displaying a simulated figure representing an area covered by the small receiving coil for each of the small receiving coils, superimposed on the positioning image;
On the positioning image, an area input means for inputting a setting of a two-dimensional area that is an area different from the imaging field of view and that acquires a nuclear magnetic resonance signal including body motion information of the subject;
Selecting means for selecting one or more small receiving coils corresponding to the simulated figure included in the two-dimensional region from the plurality of small receiving coils in correspondence with the two-dimensional region;
Arithmetic processing means for acquiring body motion information of the subject using a nuclear magnetic resonance signal received by one or more small receiving coils selected by the selection means;
Magnetic resonance imaging apparatus comprising: a. A receiving means for receiving a nuclear magnetic resonance signal from a subject, comprising a multi-channel coil having a plurality of small receiving coils;
Area input means for inputting a setting of a two-dimensional field of view on the positioning image;
Display means for displaying a simulated figure representing a two-dimensional region covered by the small receiving coil for each of the small receiving coils, superimposed on the positioning image;
Selecting means for selecting one or more small receiving coils from the plurality of small receiving coils in correspondence with the photographing field;
Arithmetic processing means for reconstructing a subject image of the subject using nuclear magnetic resonance signals received by the selected one or more small receiving coils;
With
The display means displays a simulated figure for each of the small receiving coils so as to overlap the positioning image so that the center of the multi-channel coil substantially coincides with the center of the static magnetic field. .

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