JP6576726B2 - Magnetic resonance equipment - Google Patents

Description

  The present invention relates to a magnetic resonance apparatus that applies a dephase gradient pulse and a read gradient pulse.

  In recent years, various techniques for imaging a blood flow flowing through each part of a subject using a contrast agent have become widespread. With this technique, it is possible to obtain various blood flow information of each part.

  On the other hand, depending on the region to be diagnosed, the range in the body axis direction of the region to be diagnosed may be wide. For example, when the region to be diagnosed is the lower limb, the lower limb is a region that is long in the body axis direction, and thus the field of view (FOV) is set to be as wide as possible. However, the upper limit of the length of the imaging visual field in the body axis direction is, for example, 48 cm, while the length of the lower limb is about 70 cm to 80 cm for adults. Therefore, when imaging a part that spreads over a wide area such as the lower limbs, the imaging field of view cannot be set in accordance with the length of the lower limbs. Therefore, as a technique for imaging a part that spreads over a wide area such as the lower limb, a multi-station technique is known in which the entire imaging part is imaged while moving a table during imaging (see Patent Document 1).

JP 2012-232137 A

In general, when taking an image while moving the table, the imaging field of view is set to be as wide as possible so that it can accommodate a wide range of imaging regions, and a receiving coil having a wide sensitivity distribution is used. . Therefore, the imaging method for moving the table is easily affected by the nonlinearity of the gradient magnetic field, and as a result, not only the inside of the imaging field but also the outside of the imaging field is easily excited. For this reason, there is a problem that artifacts occur due to MR signals generated outside the imaging field of view.
Therefore, a technique capable of reducing artifacts is desired.

A first aspect of the present invention is a magnetic resonance apparatus that executes a sequence including a dephase gradient pulse and a read gradient pulse that is applied after the dephase gradient pulse and obtains an echo signal from an imaging region of a subject. Because
A gradient coil for applying the dephasing gradient pulse and the readout gradient pulse, wherein the gradient coil is applied at a second time when a first time has elapsed from a first time when a rise time of the readout gradient pulse ends. A gradient coil for applying the read gradient pulse such that the fall time of the read gradient pulse begins;
A first echo signal generated between the first time point and the second time point, wherein an echo peak occurs at a third time point between the first time point and the second time point; A first echo signal that appears and a second echo signal generated between the first time point and the second time point, the echo peak at a fourth time point that is later than the third time point A receiving coil that receives a second echo signal in which the first echo signal appears and outputs a first analog signal including information on the first echo signal and information on the second echo signal;
A first setting means for setting a sampling window representing a period in which sampling of the second analog signal obtained by detecting the first analog signal is performed, the first time point and the second time point; The first period during which the first echo signal is generated is not included in the sampling window, and the second period during which the second echo signal is generated is First setting means for setting the sampling window to be included in the sampling window;
Conversion for converting the second analog signal into a digital signal by sampling the second analog signal at a predetermined sampling frequency in the sampling window and converting each sampling data obtained by the sampling into digital data. Means,
Image generating means for generating an image using k-space data obtained based on the digital signal;
Is a magnetic resonance apparatus.

A second aspect of the present invention is a magnetic resonance apparatus that executes a sequence including a dephase gradient pulse and a read gradient pulse that is applied after the dephase gradient pulse and obtains an echo signal from an imaging region of a subject. Because
A gradient coil for applying the dephasing gradient pulse and the readout gradient pulse, wherein the gradient coil is applied at a second time when a first time has elapsed from a first time when a rise time of the readout gradient pulse ends. A gradient coil for applying the read gradient pulse such that the fall time of the read gradient pulse begins;
A first echo signal generated between the first time point and the second time point, wherein an echo peak occurs at a third time point between the first time point and the second time point; A first echo signal that appears and a second echo signal generated between the first time point and the second time point, the echo peak at a fourth time point that is later than the third time point A receiving coil that receives a second echo signal in which the first echo signal appears and outputs a first analog signal including information on the first echo signal and information on the second echo signal;
First setting means for setting a sampling window representing a period during which sampling of the second analog signal obtained by detecting the first analog signal is performed, wherein the first time point and the second time point are set. First setting means for setting the sampling window to include a period between
Conversion for converting the second analog signal into a digital signal by sampling the second analog signal at a predetermined sampling frequency in the sampling window and converting each sampling data obtained by the sampling into digital data. Means,
Image generating means for generating an image using k-space data obtained on the basis of the digital signal, wherein frequency component data of the first echo signal is removed from the k-space data; Image generating means for generating an image based on the k-space data after the frequency component data of the echo signal of 1 is removed;
Is a magnetic resonance apparatus.

  By making the sampling window not include the first period during which the first echo signal is generated, the data of the first echo signal is not sampled, so that an image with reduced artifacts can be obtained. .

  Further, by removing the frequency component data of the first echo signal from the k-space data, an image is generated without using the frequency component data of the first echo signal, thereby reducing artifacts. An image can be obtained.

1 is a schematic view of a magnetic resonance apparatus according to a first embodiment of the present invention. It is explanatory drawing of the means which the control part 9 implement | achieves. It is a figure which shows schematically the scan performed with a 1st form. It is a figure which shows an imaging | photography site | part schematically. It is explanatory drawing of this scan MS. It is a figure which shows roughly the area | region of k space where data are collected by fractional echo. It is a flowchart which shows the flow of the process performed in order to produce | generate a blood-flow image. It is a figure which shows an imaging visual field roughly. It is explanatory drawing of prescan PS. It is explanatory drawing of operation | movement of the coil 4 and the receiver 8. FIG. It is explanatory drawing of another sampling method. It is a figure which shows an example of the artifact which appeared in the image. It is the figure which divided and showed the echo signal E1 and the echo signal E2. It is a figure which shows roughly area | region R1 in which the data of the frequency component by the echo signal E1 are arrange | positioned, and area | region R2 in which the data of the frequency component by the echo signal E2 are arrange | positioned. It is explanatory drawing of the sampling method in a 1st form. 6 is a diagram illustrating a difference between k-space data obtained by a sampling window W and k-space data obtained by a sampling window Q. FIG. It is a figure which shows the image D2 produced | generated by the method using the sampling window. It is explanatory drawing of the process which the control part 9 performs in a 2nd form. It is explanatory drawing of a filter process.

  Hereinafter, although the form for inventing is demonstrated, this invention is not limited to the following forms.

(1) First Embodiment FIG. 1 is a schematic view of a magnetic resonance apparatus according to a first embodiment of the present invention.
A magnetic resonance apparatus (hereinafter referred to as “MR apparatus”, MR: Magnetic Resonance) 1 includes a magnet 2, a table 3, a receiving coil 4, and the like.

  The magnet 2 has an accommodation space 21 in which the subject 13 is accommodated. The magnet 2 includes a superconducting coil 22 for applying a static magnetic field, a gradient coil 23 for applying a gradient pulse, an RF coil 24 for applying an RF pulse, and the like. In place of the superconducting coil 22, a permanent magnet may be used.

  The table 3 has a cradle 3a. The cradle 3a is configured to be able to move into the accommodation space 21. The subject 13 is transported to the accommodation space 21 by the cradle 3a.

  The receiving coil 4 is attached to the lower half of the subject 13. The receiving coil 4 receives a magnetic resonance signal generated from the subject 12.

  The MR apparatus 1 further includes a contrast medium injection device 5, a transmitter 6, a gradient magnetic field power source 7, a receiver 8, a control unit 9, a storage unit 10, an operation unit 11, a display unit 12, and the like.

The contrast agent injection device 5 injects a contrast agent into the subject 13.
The transmitter 6 supplies current to the RF coil 24, and the gradient magnetic field power supply 7 supplies current to the gradient coil 23. The receiver 8 performs signal processing such as detection on the signal received from the receiving coil 4. A combination of the magnet 2, the transmitter 6, and the gradient magnetic field power source 7 corresponds to the scanning means.

  The control unit 9 transmits information necessary for execution of the sequence to the transmitter 6 and the gradient magnetic field power source 7 and reconstructs an image so as to realize various operations of the MR apparatus 1. Control the operation of each part. The storage unit 10 stores a program executed by the control unit 9 and the like. The storage unit 10 storing the program may be a non-transitory storage medium such as a hard disk or a CD-ROM. The control unit 9 operates as a processor that reads a program stored in the storage unit 10 and executes processing described in the program. The control unit 9 implements various means by executing the processing described in the program. FIG. 2 is an explanatory diagram of means realized by the control unit 9.

  The slice setting unit 101 sets a slice L0 (see FIG. 8) to be described later and a slice of an imaging region.

  The FOV setting means 102 sets an imaging field of view (see FIG. 8) for scanning an imaging site in a main scan MS described later.

  Window setting means 103 sets a sampling window Q (see FIG. 15). The sampling window Q represents a period during which the analog signal is sampled in the receiver 8. The sampling window Q will be described later.

  The image generation unit 104 generates an image of the slice L0 (see FIG. 8) and an image of the imaging region.

  The control unit 9 implements the slice setting unit 101 to the image generation unit 104 by reading the program stored in the storage unit 10. The controller 9 may realize the slice setting unit 101 to the image generation unit 104 with one processor, or may implement the slice setting unit 101 to the image generation unit 104 with two or more processors. .

Returning to FIG. 1, the description will be continued.
The operation unit 11 is operated by an operator and inputs various information to the control unit 9. The display unit 12 displays various information.
The MR apparatus 1 is configured as described above.

FIG. 3 is a diagram schematically showing a scan executed in the first embodiment, and FIG. 4 is a diagram schematically showing an imaging region.
In the first form, the imaging region is the lower limb. The part a1 is a part on the thigh side, and the part a2 is a part on the crus side. The imaging part includes parts a1 and a2.

  In this embodiment, a localizer scan LS, a pre-scan PS, a main scan MS, and the like are executed.

  The localizer scan LS is a scan for acquiring an image used for positioning a slice of an imaging region. In the first form, the imaging region is the lower limb (see FIG. 4). In FIG. 4, the X direction corresponds to the right-left direction of the subject, the Y direction corresponds to the front-rear direction, and the Z direction corresponds to the superior-inferior direction. is doing. The imaging part includes parts a1 and a2. The part a1 is a part on the thigh side, and the part a2 is a part on the crus side.

  The pre-scan PS is a scan that is executed to determine whether or not the bolus of the contrast medium has reached the vicinity of the imaging region. The prescan PS will be described later (see FIG. 9).

  The main scan MS is a scan for acquiring an image of an imaging region. Hereinafter, the main scan MS will be described.

FIG. 5 is an explanatory diagram of the main scan MS.
In the main scan MS, an imaging sequence C for acquiring an image of an imaging region is repeatedly executed. The imaging sequence C in the first half of the main scan MS is executed in order to image the region a1 on the thigh side of the imaging region. The imaging sequence C in the latter half of the main scan MS is executed for imaging the site a2 on the lower leg side of the imaging site.

  Note that the imaging sequence C executed for imaging the part a1 is the same as the imaging sequence C executed for imaging the part a2. Therefore, the imaging sequence C will be described below by taking an example of imaging the part a1.

  The imaging sequence C has an excitation pulse EX for exciting a part where imaging is performed (here, part a1). Further, the imaging sequence C has a slice selection gradient pulse SP applied in the X direction (slice selection direction). The site a1 is excited by the excitation pulse EX and the slice selection gradient pulse SP.

  The imaging sequence C includes a phase encode gradient pulse EP11 and a rewinder EP12 applied in the X direction, and a phase encode gradient pulse EP21 and a rewinder EP22 applied in the Y direction (phase encode direction). The phase encoding gradient pulses EP11 and EP21 are applied immediately after the slice selection gradient pulse SP.

  Further, the imaging sequence C has a dephase gradient pulse DP and a read gradient pulse RP applied in the Z direction (frequency encoding direction). The dephase gradient pulse DP is applied at the same timing as the phase encode gradient pulses EP11 and EP21. Immediately after the dephasing gradient pulse DP is applied, the reading gradient pulse RP is applied.

  Next, the dephase gradient pulse DP and the read gradient pulse RP will be described with reference to an enlarged view shown on the lower side of FIG.

  The dephase gradient pulse DP is applied between time points t1 and t2 (application time TA), and the read gradient pulse is applied between time points t2 and t6 (application time TB).

  The application time TB of the read gradient pulse RP is represented by the sum of three times (the sum of the rise time TU, the flat time TF, and the fall time TD). The rise time TU (time t2 to t3) represents the time taken from the start of application of the read gradient pulse RP until the gradient magnetic field strength H of the read gradient pulse RP reaches H = H1. The flat time TF (time points t3 to t5) represents the time during which the gradient magnetic field strength H of the read gradient pulse RP holds H = H1. The falling time TD (time t5 to t6) represents the time taken from the end time t5 of the flat time TF until the gradient magnetic field strength H of the read gradient pulse RP reaches H = 0.

  While the readout gradient pulse RP is applied, the echo signal E0 is obtained by aligning the phases of the MR signals generated from the excited site. In FIG. 5, the point in time when the echo peak of the echo signal E0 appears is indicated by “t4”. Accordingly, when the application time point of the excitation pulse EX is represented by “t0”, the echo time TE is represented by the time between the time points t0 and t4.

In this embodiment, the time Fa from the time point t3 to t4 is set to be shorter than the time Fb from the time point t4 to t5 so that k-space data is collected by fractional echo (asymmetric echo). Yes. FIG. 6 schematically shows a region of k-space where data is collected by fractional echo. In FIG. 6, the kx-ky plane is shown. The kx direction corresponds to the frequency encoding direction (Z direction), and the ky direction corresponds to the phase encoding direction (Y direction). kx-ky plane has N number of views V1～V _N. Each view is parallel to the kx direction, N present views V1～V _N are arranged in the ky direction. The number N of views is, for example, N = 192 and N = 256, but FIG. 6 shows an example where N = 20 for convenience of explanation.

  By executing the imaging sequence C once, data for one view is collected. However, data in the entire range (kx1 to kx4) in the kx direction is not collected, data is not collected in the range of kx1 to kx2, and data is collected only in the range of kx2 to kx4. Therefore, data is arranged in the range of kx2 to kx4, but no data is arranged in the range of kx1 to kx2. In FIG. 6, the area where data is arranged is indicated by hatching.

  In FIG. 6, data in the range of kx2 to kx3 is data collected during time Fa (time t3 to t4), and data in the range of kx3 to kx4 is during time Fb (time t4 to t5). Is the data collected. Kx3 represents the center position in the kx direction on the kx-ky plane.

  FIG. 6 shows an example in which 75% of data for each view is collected. In order to collect 75% of data for each view, the time Fa may be set to half of the time Fb, that is, Fa = Fb / 2.

Returning to FIG.
By executing the imaging sequence C in the first half of the main scan MS, a blood flow image of the part a1 can be obtained. When the imaging of the part a1 is completed, the part to be imaged is changed from the part a1 to the part a2, and the imaging sequence C in the latter half of the main scan MS is repeatedly executed. Therefore, by executing the imaging sequence C in the latter half of the main scan MS, a blood flow image of the part a2 can be obtained.

  As described above, when k-space data is collected by fractional echo, the echo time TE can be shortened, so that the signal intensity of the echo signal E0 can be increased. Therefore, the fractional echo has an advantage that the blood signal can be hardly lost. For this reason, when blood is imaged using the gradient echo method, a technique of fractional echo is generally used.

  Next, a specific flow of processing executed for generating a blood flow image in this embodiment will be described.

FIG. 7 is a flowchart showing a flow of processing executed to generate a blood flow image.
In step ST1, a localizer scan LS is executed. By executing the localizer scan LS, an image used to set a slice is obtained. After performing the localizer scan LS, the process proceeds to step ST2.

  In step ST2, the operator sets the imaging field of view in the main scan MS executed in step ST4. The operator operates the operation unit 11 (see FIG. 1) and inputs information for setting the imaging field of view. When this information is input, the FOV setting means 102 (see FIG. 2) sets the imaging field of view. FIG. 8 is a diagram schematically showing the imaging field of view. The upper side of FIG. 8 shows a view of the subject from the front (front), and the lower side of FIG. 8 shows a view of the subject from the side. In FIG. 8, an imaging field of view FOV1 represents an imaging field of view for photographing the part a1, and an imaging field of view FOV2 represents an imaging field of view for photographing the part a2. The size of the imaging field of view FOV1 in the X direction (RL direction) is x1 (cm), and the size in the Z direction (SI direction) is z1 (cm). The size of the imaging field of view FOV2 in the X direction (RL direction) is x2 (cm), and the size in the Z direction (SI direction) is z2 (cm). The imaging fields of view FOV1 and FOV2 are set to overlap at the boundary between the part a1 and the part a2. The size x1, x2, z1, and z2 of the field of view can be set to 48 cm, for example, but can also be set to 48 cm or more or 48 cm or less.

  Further, the operator sets a coronal plane slice (not shown) crossing the part a1 and a coronal plane slice (not shown) crossing the part a2 based on the image obtained by the localizer scan LS. The operator operates the operation unit and inputs information for setting a slice for each part. When this information is input, the slice setting unit 101 (see FIG. 2) sets slices in the parts a1 and a2 of the imaging part. The slice setting unit 101 can set one or a plurality of slices for each part. Here, for convenience of explanation, it is assumed that the operator sets one slice for each of the parts a1 and a2.

  Further, the operator sets a slice L0 near the imaging region. The operator operates the operation unit and inputs information for setting the slice L0. When this information is input, the slice setting unit 101 sets the slice L0. The slice L0 is a coronal plane slice used in the pre-scan PS, and is a slice for determining whether or not the contrast agent bolus has reached the vicinity of the imaging region. The slice L0 is set at a position that bisects the blood vessel A in the Y direction (AP direction). After setting the imaging field of view and the slice L0, the process proceeds to step ST3.

  In step ST3, pre-scan PS is executed. Hereinafter, the pre-scan PS will be described.

FIG. 9 is an explanatory diagram of the prescan PS.
Before executing the pre-scan PS, the operator adjusts the position of the cradle 3a so that the part a1 is positioned at the magnet center MC (see FIG. 1). After positioning the part a1 to the magnet center MC, the pre-scan PS is started.

  In the pre-scan PS, sequence sets B1 to Bz for acquiring an image of the slice L0 (see FIG. 8) are repeatedly executed. Each set of sequences includes a plurality of sequences B. In the first form, the sequence B is a sequence for collecting k-space data of the slice L0 by the 2D gradient echo method. The image generation unit 104 (see FIG. 2) generates an image of the slice L0 based on the data obtained by each set whenever the sequence sets B1 to Bz are executed. In FIG. 9, the images obtained by the sequence sets B1 to Bz are indicated by symbols “IM1”, “IM2”, “IM3”,... “IMz”.

  When the sequence set B1 is executed in the pre-scan PS, the image IM1 of the slice L0 is generated based on the data obtained by the sequence set B1. The image IM1 is displayed on the display unit 12. An image IM1 displayed on the display unit 12 is schematically shown on the lower side of FIG. The image IM1 is displayed immediately after the sequence set B1 is executed. Therefore, the operator can check the image IM1 of the slice L0 almost in real time. After executing the sequence set B1, the next sequence set B2 is executed. An image IM2 of the slice L0 is generated by executing the set B2 of the sequence. When the image IM2 is generated, the image displayed on the display unit 12 is updated, and the image IM2 is displayed.

  Similarly, the sequence sets B3 to Bz are executed, and the image displayed on the display unit 12 is updated each time the sequence set is executed. Therefore, while the pre-scan PS is being executed, the image of the slice L0 can be confirmed almost in real time.

  On the other hand, the contrast medium injection device 5 (see FIG. 1) starts injection of contrast medium in synchronization with the start timing of the pre-scan PS. FIG. 9 shows an example in which the contrast medium is injected immediately after the end of the sequence set B1. When the contrast agent is injected, the contrast agent flows in the blood vessel A of the subject. Before the contrast agent bolus reaches the blood vessel A, the blood vessel A has a low signal. However, when the contrast agent bolus reaches the blood vessel A, the portion of the blood vessel A where the contrast agent bolus flows is a high signal. Become. Therefore, when the bolus of the contrast agent reaches the blood vessel A, the color in the blood vessel A of the coronal image displayed on the display unit 12 changes from black to white. For this reason, the operator can determine whether or not the bolus of the contrast agent has reached the blood vessel A by checking the image displayed on the display unit 12.

  When the operator determines that the bolus of the contrast agent has reached a desired portion in the blood vessel A, the operator operates the operation unit 11 and inputs a command for executing the main scan MS. When this command is input, the process proceeds from step ST3 to step ST4.

  In step ST4, the main scan MS is executed. In the main scan MS, the imaging sequence C is repeatedly executed as shown in FIG. Each time the imaging sequence C is executed, an echo signal E0 is generated.

  The echo signal E0 is processed by the coil 4 and the receiver 8. Below, operation | movement of the coil 4 and the receiver 8 is demonstrated (refer FIG. 10).

FIG. 10 is an explanatory diagram of the operation of the coil 4 and the receiver 8.
The coil 4 converts the echo signal E0 into an analog signal SA including information on the echo signal E0 and outputs the analog signal SA to the receiver 8.

  The receiver 8 includes a detection circuit 8a and an AD converter 8b. The detection circuit 8a detects the analog signal SA and outputs the analog signal SB after detection. The AD converter 8b samples the analog signal SB at a predetermined sampling frequency, and converts each sampling data into digital data, thereby converting the analog signal SB into a digital signal SD.

  Hereinafter, a sampling method executed in the AD converter 8b will be described. In the following description, in order to clarify the effect of this embodiment, first, a sampling method different from the method of this embodiment will be described, and after describing another sampling method, the sampling method of this embodiment will be described. explain.

(1) About the sampling method different from the method of this form Drawing 11 is an explanatory view of another sampling method.
Window setting means 103 (see FIG. 2) sets a sampling window W that represents a period for sampling an analog signal. The AD converter 8 b receives information on the sampling window W set by the window setting unit 103 from the control unit 9. The AD converter 8 b samples the analog signal SB within the sampling window W. In FIG. 11, the window width of the sampling window W is set to a period (flat time TF) between time points t3 to t5 of the read gradient pulse RP. Therefore, the AD converter 8b samples the data including the information of the echo signal E0 generated between the time points t3 and t5 at a predetermined sampling frequency, and converts each sampling data into digital data. Therefore, the AD converter 8b can convert the analog signal SB into the digital signal SD. The digital signal SD is sent to the control unit.

  Therefore, by repeatedly executing the imaging sequence C, as shown in FIG. 6, data in the range of kx2 to kx4 (shaded portions shown in FIG. 6) can be obtained for each view in k space. After collecting the k-space data, the image generation unit 104 performs zero filling to place 0 (zero) data in the range of kx1 to kx2 where the k-space data is not collected. Then, Fourier transform is performed on the k-space data after the zero filling. Therefore, a blood flow image of the part a1 can be obtained.

  After scanning the part a1, the cradle 3a is moved so that the part a2 is positioned at the magnet center MC (see FIG. 1). Then, the imaging sequence C for obtaining the image of the part a2 is repeatedly executed. Accordingly, a blood flow image of the part a2 can be obtained. When the scan of the part a2 is finished, the main scan MS is finished.

  However, it has been found that when sampling is performed based on the sampling window W in FIG. 11, artifacts appear in the image. FIG. 12 shows an example of the artifact that appears in the image. An image D1 in FIG. 12 is a coronal image of the subject's thigh, and artifacts are shown inside the thigh. When performing image diagnosis, it is desirable to reduce such artifacts as much as possible. Therefore, the inventor of the present application diligently researched and found the cause of the appearance of this artifact. Hereinafter, the cause of the artifact will be described.

  In the MR apparatus, it is desirable that the static magnetic field is kept uniform throughout the accommodation space 21, and the gradient magnetic field is preferably kept linear throughout the accommodation space 21. However, it is difficult to maintain the uniformity of the static magnetic field and the linearity of the gradient magnetic field over the entire accommodation space 21, and phenomena such as static magnetic field inhomogeneity and gradient magnetic field nonlinearity appear in an actual MR apparatus. Therefore, due to non-uniformity of the static magnetic field and non-linearity of the gradient magnetic field, actually, excitation is performed not only inside the imaging fields FOV1 and FOV2 (see FIG. 8) but also outside the imaging fields FOV1 and FOV2. As a result, the echo signal obtained by executing the imaging sequence C includes not only the echo signal obtained from the inside of the imaging field, but also the echo signal obtained from the outside of the imaging field. In FIG. 13, an echo signal obtained from an area outside the imaging field of view is indicated by a symbol “E1”, and an echo signal obtained from the inside of the imaging field of view is indicated by a symbol “E2”. The inventor of the present application has found that the echo peak of the echo signal E1 obtained from the outside of the imaging field of view is higher than the echo peak of the echo signal E2 obtained from the inside of the imaging field of view due to the synergistic effect of the nonlinearity of the gradient magnetic field and the non-uniformity of the static magnetic field I also found that it appears early.

  The echo peak of the echo signal E2 obtained from the inside of the imaging field appears at time t4. On the other hand, the echo peak of the echo signal E1 obtained from the outside of the imaging field of view appears at a time point t31 earlier than the time point t4. That is, the echo peak (time t31) of the echo signal E1 obtained from the outside of the imaging field appears first, and the echo peak (time t4) of the echo signal E2 obtained from the inside of the imaging field appears later.

  Therefore, as shown in FIG. 13, if the window width of the sampling window W is set to the period from the time point t3 to t5, the echo signal E1 obtained from outside the imaging field of view is generated in the sampling window W. Wa is included. For this reason, since the data of the echo signal E1 is also sampled in addition to the data of the echo signal E2, the frequency component data of the echo signal E1 is included in each view in the k space in addition to the frequency component data of the echo signal E2. Data is also arranged. FIG. 14 schematically shows a region R1 where frequency component data based on the echo signal E1 is arranged and a region R2 where frequency component data based on the echo signal E2 is arranged. The region R1 is a region in the range of kx2 to kx21, and the region R2 is a region in the range of kx21 to kx4. When the sampling window W is used, there is a problem in that artifacts appear because the data of the echo signal E1 is used for image reconstruction.

  Therefore, in this embodiment, a sampling method is devised in order to reduce artifacts due to the echo signal E1. Hereinafter, a sampling method in this embodiment will be described (see FIG. 15).

(2) Sampling Method in First Embodiment FIG. 15 is an explanatory diagram of the sampling method in the first embodiment.
In the first form, the window setting means 103 (see FIG. 2) includes the period (time point t32 to t5) in which the echo signal E2 is generated in the sampling window Q, but the period (time point) in which the echo signal E1 is generated. The sampling window Q is set so that it is not included in the sampling window Q from t3 to time t32). Therefore, the window width of the sampling window Q is set in the period from time t32 to time t5. Since the AD converter 8b samples the analog signal SB within the sampling window Q, the data including the information of the echo signal E2 is sampled, but the data including the information of the echo signal E1 is not sampled. Therefore, the AD converter 8b can output the digital signal SD from which the information of the echo signal E1 is removed.

  Note that, as the position of the time point t32 is closer to the time point t3, the amount of sampling of data including the information of the echo signal E1 increases, so that it becomes difficult to sufficiently reduce the artifact. On the other hand, when the time point t32 is set to a time later than the time point t4 at which the echo peak of the echo signal E2 appears, there is a problem in that k-space data necessary for image reconstruction cannot be obtained. Therefore, it is desirable to set the position of the time point t32 in consideration of the balance between the reduction of artifacts and the necessary data amount of k space. In the first mode, the time point t32 is set so that 60% of data of each view can be obtained. FIG. 16 shows the difference between the k-space data obtained by the sampling window W and the k-space data obtained by the sampling window Q. FIG. 16A shows k-space data obtained by the sampling window W, and FIG. 16B shows k-space data obtained by the sampling window Q.

  When the sampling window W is used (see FIG. 16A), since the data of the echo signal E1 is arranged in the region R1, an artifact due to the echo signal E1 appears. However, when the sampling window Q is used (see FIG. 16B), since the echo signal E1 is not arranged in the region R1, artifacts due to the echo signal E1 can be reduced.

  In FIG. 16A, since 75% of data is obtained in each view, image reconstruction is performed using the zero filling method. However, in FIG. 16B, since the echo signal E1 is not sampled, data in the range of kx2 to kx21 is not obtained, and only 60% of data is obtained in each view. Therefore, when the sampling window Q is used, the image generation unit 104 can use a half Fourier method as a method of image reconstruction.

  As described above, in the first embodiment, the sampling window Q is set so that data including the information of the echo signal E1 is not sampled (see FIG. 15). Therefore, artifacts can be reduced.

  In order to verify that the artifact was reduced by using the sampling window Q, an image was generated by a method using the sampling window Q. FIG. 17 shows an image D2 generated by the method using the sampling window Q. FIG. 17 also shows an image D1 (see FIG. 12) generated by a method using the sampling window W for comparison.

  Comparing images D1 and D2, it can be seen that artifacts appearing in image D1 are reduced in image D2.

(2) Second Mode In the first mode, an example is described in which the artifact is reduced by setting the sampling window Q so that the data of the echo signal E2 is sampled but the data of the echo signal E1 is not sampled. ing. In the second embodiment, a sampling window W for sampling both echo signals E1 and E2 is set, but a method capable of reducing artifacts will be described. The hardware configuration of the MR apparatus is the same as that in the first embodiment.

FIG. 18 is an explanatory diagram of processing executed by the control unit 9 in the second embodiment.
Since the slice setting unit 101 and the FOV setting unit 102 are the same as those in the first embodiment, description thereof will be omitted.

  The window setting means 103 sets a sampling window W (see FIG. 13) that represents a period during which an analog signal is sampled in the AD converter 8b.

  The image generation unit 104 generates an image of the imaging region. The image generation unit 104 includes a filter unit 105 and a reconstruction unit 106. The filter unit 105 filters k-space data. The filtering process will be described later. The reconstruction unit 106 reconstructs an image based on the k-space data obtained by the filtering process of the filter unit 105.

  The control unit 9 implements the slice setting unit 101 to the image generation unit 104 by reading the program stored in the storage unit 10.

Next, processing of the MR apparatus in the second embodiment will be described with reference to the flow of FIG.
Steps ST1 to ST3 are the same as in the first embodiment. Therefore, the description of steps ST1 to ST3 is omitted. In step ST3, the operator confirms the image displayed on the display unit 12 (see FIG. 9), and operates the operation unit 11 when determining that the bolus of the contrast agent has reached a desired portion in the blood vessel A. A command for executing the main scan MS is input. When this command is input, the process proceeds to step ST4.

  In step ST4, the main scan MS is executed. In the main scan MS, the imaging sequence C (see FIG. 5) is repeatedly executed as in the first embodiment. Accordingly, an echo signal is generated each time the imaging sequence C is executed. The echo signal is processed by the coil 4 and the receiver 8. Hereinafter, operations of the coil 4 and the receiver 8 in the second embodiment will be described with reference to FIG.

  In the second mode, as in the first mode, by executing the imaging sequence C, an echo signal E1 obtained from the outside of the imaging field and an echo signal E2 obtained from the inside of the imaging field are generated. Is done.

  The echo signals E1 and E2 are received by the coil 4. The coil 4 converts the echo signals E1 and E2 into an analog signal SA including information on the echo signals E1 and E2, and outputs the analog signal SA to the receiver 8.

  The receiver 8 includes a detection circuit 8a and an AD converter 8b. The detection circuit 8a detects the analog signal SA and outputs the analog signal SB after detection. The AD converter 8b converts the analog signal SB into a digital signal SD. The operation of the AD converter 8b in the second embodiment will be described below.

  The AD converter 8b receives information on the sampling window W set by the window setting unit 103 (see FIG. 18) from the control unit 9. The sampling window W represents a period for sampling the analog signal SB. In the second embodiment, the window width of the sampling window W is a period between the time points t3 and t5 of the read gradient pulse RP (flat time TF). ) Is set. Therefore, the AD converter 8b samples the data including the information of the echo signals E1 and E2 generated between the time points t3 and t5 at a predetermined sampling frequency, and converts each sampling data into digital data. Therefore, the AD converter 8b can convert the analog signal SB into the digital signal SD. The digital signal SD is sent to the control unit.

  Therefore, k-space data can be obtained by repeatedly executing the imaging sequence C. FIG. 14 schematically shows k-space data obtained by executing the imaging sequence C. In the second mode, since the window width of the sampling window W is set in the period from time t3 to time t5, the data of the frequency component of the echo signal E1 is arranged in the region R1, and the echo is displayed in the region R2. Data of the frequency component of the signal E2 is arranged.

  Therefore, when zero-filling is performed on the k-space data and the zero-filled k-space data is Fourier-transformed, an artifact as shown in the image D1 (see FIG. 12) appears. Therefore, in the second embodiment, filter processing for removing frequency component data based on the echo signal E1 is executed from the region R1 (see FIG. 19).

FIG. 19 is an explanatory diagram of filter processing.
The filter means 105 (see FIG. 18) executes a filter process for removing frequency component data of the echo signal E1 arranged in the region R1. By this filtering process, the data in the region R1 can be removed while the data in the region R2 remains. The reconstruction unit 106 (see FIG. 18) reconstructs an image by applying a half Fourier method to the filtered k-space data.

  In the second mode, the data of the echo signal E1 is arranged in the region R1 (see FIG. 14). However, the data in the region R1 is removed by the filtering process (see FIG. 19), and the image is reconstructed based on the k-space data after the filtering process. Therefore, in the second embodiment, artifacts can be reduced as in the first embodiment.

  In the first and second embodiments, the example of the fractional echo that collects partial data of each view in the k space has been described. However, the present invention is not limited to the example of fractional echo, and can also be applied to an imaging method that collects all data of each view in k-space.

  In the first and second embodiments, an example in which shooting is performed while moving the cradle is described. However, the present invention can also be applied to an example in which a subject is imaged without moving the cradle.

  In the first and second embodiments, examples of contrast imaging have been described, but the present invention can also be applied to non-contrast imaging.

  In the first and second embodiments, the imaging region is the lower limb, but the imaging region is not limited to the lower limb, and the upper body may be the imaging region, or the entire body of the subject may be the imaging region.

  Although the example which acquires a blood-flow image is demonstrated by the 1st and 2nd form, this invention is applicable also when acquiring an image different from a blood-flow image.

DESCRIPTION OF SYMBOLS 1 MR apparatus 2 Magnet 3 Table 3a Cradle 4 Receiving coil 5 Contrast agent injection apparatus 6 Transmitter 7 Gradient magnetic field power supply 8 Receiver 9 Control part 10 Storage part 11 Operation part 12 Display part 13 Subject 21 Bore 22 Superconducting coil 23 Gradient Coil 24 RF coil 101 Slice setting means 102 FOV setting means 103 Window setting means 104 Image generating means 105 Filter means 106 Reconstructing means

Claims (14)

A magnetic resonance apparatus that executes a sequence including a dephase gradient pulse and a read gradient pulse that is applied after the dephase gradient pulse and obtains an echo signal from an imaging region of a subject,
A gradient coil for applying the dephasing gradient pulse and the readout gradient pulse, wherein the gradient coil is applied at a second time when a first time has elapsed from a first time when a rise time of the readout gradient pulse ends. A gradient coil for applying the read gradient pulse such that the fall time of the read gradient pulse begins;
A first echo signal generated between the first time point and the second time point, wherein an echo peak occurs at a third time point between the first time point and the second time point; A first echo signal that appears and a second echo signal generated between the first time point and the second time point, the echo peak at a fourth time point that is later than the third time point A receiving coil that receives a second echo signal in which the first echo signal appears and outputs a first analog signal including information on the first echo signal and information on the second echo signal;
A first setting means for setting a sampling window representing a period in which sampling of the second analog signal obtained by detecting the first analog signal is performed, the first time point and the second time point; The first period during which the first echo signal is generated is not included in the sampling window, and the second period during which the second echo signal is generated is First setting means for setting the sampling window to be included in the sampling window;
Conversion for converting the second analog signal into a digital signal by sampling the second analog signal at a predetermined sampling frequency in the sampling window and converting each sampling data obtained by the sampling into digital data. Means,
Image generating means for generating an image using k-space data obtained based on the digital signal;
A magnetic resonance apparatus. A magnetic resonance apparatus that executes a sequence including a dephase gradient pulse and a read gradient pulse that is applied after the dephase gradient pulse and obtains an echo signal from an imaging region of a subject,
A gradient coil for applying the dephasing gradient pulse and the readout gradient pulse, wherein the gradient coil is applied at a second time when a first time has elapsed from a first time when a rise time of the readout gradient pulse ends. A gradient coil for applying the read gradient pulse such that the fall time of the read gradient pulse begins;
A first echo signal generated between the first time point and the second time point, wherein an echo peak occurs at a third time point between the first time point and the second time point; A first echo signal that appears and a second echo signal generated between the first time point and the second time point, the echo peak at a fourth time point that is later than the third time point A receiving coil that receives a second echo signal in which the first echo signal appears and outputs a first analog signal including information on the first echo signal and information on the second echo signal;
First setting means for setting a sampling window representing a period during which sampling of the second analog signal obtained by detecting the first analog signal is performed, wherein the first time point and the second time point are set. First setting means for setting the sampling window to include a period between
Conversion for converting the second analog signal into a digital signal by sampling the second analog signal at a predetermined sampling frequency in the sampling window and converting each sampling data obtained by the sampling into digital data. Means,
Image generating means for generating an image using k-space data obtained on the basis of the digital signal, wherein frequency component data of the first echo signal is removed from the k-space data; Image generating means for generating an image based on the k-space data after the frequency component data of the echo signal of 1 is removed;
A magnetic resonance apparatus. Scanning means for executing the sequence;
The scanning means includes
A static magnetic field coil for applying a static magnetic field;
The gradient coil;
An RF coil for applying an RF pulse;
The magnetic resonance apparatus according to claim 1, comprising: The gradient coil is
The first phase encoding gradient pulse is applied in a phase encoding direction, the second phase encoding gradient pulse is applied in a slice selection direction, and the dephase gradient pulse and the readout gradient pulse are applied in a frequency encoding direction. 3. The magnetic resonance apparatus according to 3. The kx-ky plane represented by using the kx direction corresponding to the frequency encoding direction and the ky direction corresponding to the phase encoding direction has a plurality of views.
The scanning means includes
The magnetic resonance apparatus according to claim 4, wherein the sequence is executed once so that data is arranged in one of the plurality of views. The scanning means includes
The magnetic resonance apparatus according to claim 5, wherein the sequence is executed a plurality of times so that data is arranged in the plurality of views. Each of the plurality of views is parallel to the kx direction;
The magnetic resonance apparatus according to claim 5, wherein the plurality of views are arranged in a ky direction.   The data of each of the plurality of views is not arranged in the first range in the kx direction, and data is arranged in the second range in the kx direction. Magnetic resonance device. The gradient coil sets the readout gradient pulse so that the time between the first time point and the fourth time point is shorter than the time between the fourth time point and the second time point. The magnetic resonance apparatus according to claim 1, wherein the magnetic resonance apparatus is applied. The gradient coil is
10. The magnetic resonance apparatus according to claim 9, wherein the read gradient pulse is applied so that a predetermined gradient magnetic field strength is maintained between the first time point and the second time point of the read gradient pulse. .   The magnetic resonance apparatus according to claim 1, further comprising a second setting unit that sets an imaging field of view of the subject. The second setting means includes
The magnetic resonance apparatus according to claim 11, wherein a length in the first direction of the imaging visual field and a length in the second direction of the imaging visual field are set. The imaging region includes a plurality of regions,
The magnetic resonance apparatus according to claim 11 or 12, wherein the second setting means sets the imaging field of view for each of the plurality of parts. The first echo signal is obtained by an MR signal generated outside the imaging field of view,
14. The magnetic resonance apparatus according to claim 11, wherein the second echo signal is obtained by an MR signal generated inside an imaging field.