JP4076655B2 - Magnetic resonance imaging system - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a magnetic resonance imaging (MRI) method for measuring nuclear magnetic resonance (hereinafter referred to as “NMR”) signals from hydrogen, adjacent substances, etc. in an object and visualizing nuclear density distribution, relaxation time distribution, etc. In particular, the present invention relates to an MRI method suitable for continuously capturing two slice images having different directions.
[0002]
[Prior art]
In recent years, IVMR (Interventional MRI), which uses MRI as a monitor during surgery, has begun to spread. In IVMR, it is desirable to monitor a catheter inserted into a subject in real time. Furthermore, it is desired to monitor two orthogonal sections or three orthogonal sections including the tip of the catheter in near real time.
[0003]
There is a multi-angle method as a method for photographing a plurality of slices having different directions in MRI. In the multi-angle method, for example, when measuring a first slice 702 in the subject 701 and a second slice 703 orthogonal to the first slice 702 in the subject 701, first, the first slice (yz) in the subject 701 is shown in FIG. (Slice parallel to the plane) 702 is selected and excited, and in the case of a spin echo method sequence, an echo signal from the first slice 702 is applied after applying a high-frequency pulse that inverts the spin in the first slice 702. Measure. In this case, a gradient magnetic field pulse in the y direction is applied to the phase encoding, and a gradient magnetic field in the z direction is applied to the readout.
[0004]
Next, in order to obtain an echo signal from the second slice (slice parallel to the xz plane) 703 orthogonal to the first slice 702, the axis to which the slice selection gradient magnetic field is applied is y, and the spin in the slice 703 is A high frequency pulse for excitation and a high frequency pulse for spin inversion are applied. In this step, x is the axis to which the phase encoding gradient magnetic field is applied, and the echo signal is measured.
[0005]
The step (1) of measuring the echo signal from the first slice and the step (2) of measuring the echo signal from the second slice are repeated while changing the magnitude of the phase encode pulse in each step. This repetition is performed for the necessary number of phase encodings, and the number of echo signals necessary for image formation is measured for each slice.
[0006]
[Problems to be solved by the invention]
Such a conventional multi-angle method can be used to obtain images of a plurality of orthogonal sections in the above-described IVMR, but in the case where the repetition time TR of imaging is shortened in order to perform near real-time imaging. The following problems occur.
[0007]
That is, when the slices selected in each step (1) and (2) intersect within a slice, a portion where the plurality of slices intersect (portion 704 shown in FIG. 7, hereinafter referred to as an intersection) is excited to spin. Are applied twice in a short time (TR). As a result, the signal from the spins present at the intersection 704 is reduced, and in each acquired slice image, the intersection 704 becomes darker than the other portions in the slice.
[0008]
Therefore, the present invention provides an MRI method capable of obtaining an image without signal degradation at the intersection (overlapping portion) of each cross section when performing short TR measurement of a plurality of cross sections in one imaging. Objective.
[0009]
[Means for Solving the Problems]
The MRI method according to the first aspect of the present invention for achieving the above object is a magnetic resonance imaging method for continuously imaging and displaying a plurality of mutually intersecting regions in a subject arranged in a static magnetic field, (A) applying an excitation high-frequency pulse for exciting the spins of the plurality of regions as a whole; and (b) following the excitation high-frequency pulse, a gradient magnetic field for selecting one region for each of the plurality of regions, and 180 ° Applying a high-frequency pulse to invert the spin of the region; (c) measuring an echo signal at different echo times for the plurality of regions; and (d) echoes from the second and subsequent regions. Before measuring the signal, the spin phase of the portion where the plurality of regions intersect (hereinafter referred to as the intersecting portion) is aligned with the phase of the other spins in the second and subsequent regions. Includes-up; and, the image of each region separately reconstructed, displayed simultaneously by using the echo signals measured respectively, for (e) the plurality of areas.
[0010]
The MRI method according to the second aspect of the present invention is a magnetic resonance imaging method for continuously imaging and displaying a plurality of mutually intersecting regions in a subject arranged in a static magnetic field, wherein (a) Applying an excitation high-frequency pulse that excites spins in a plurality of regions; and (b) following the excitation high-frequency pulse, a gradient magnetic field for selecting one region for each of the plurality of regions and a 180 ° high-frequency pulse. Applying and reversing the spin of the region, and (c ′) measuring echo signals at different echo times for the plurality of regions, and from a portion where the plurality of regions intersect (hereinafter referred to as an intersection). A step of measuring the echo signal of (2), and (f) an echo signal measured for each of the plurality of regions and an echo signal measured for the intersection Used, the image of each region separately reconstructed, and a step of simultaneous display.
[0011]
According to the MRI method of the present invention, the entire region is excited with a single excitation pulse, and the signal from each region to be imaged is generated using a selective 180 ° high frequency pulse. It is possible to prevent the intersection of the regions from being excited multiple times within the repetition time TR. As a result, multiple excitation can be prevented at a portion where a plurality of regions overlap, and a signal drop from the portion (a phenomenon in which an image becomes dark) can be avoided.
[0012]
At this time, regarding the spin behavior at the intersection, when the first region selection 180 ° high frequency pulse is applied, the spin and spin states other than the intersection in the region to be measured next are different. . Therefore, in the first aspect of the present invention, by providing the step (d) of aligning the phases of the spins at the intersection and the spins other than at the intersection, signals from all parts of the measurement region can be simultaneously obtained and It is possible to prevent the discontinuity of contrast at the part.
[0013]
This step (d) involves the application of an additional 180 ° radio frequency pulse that inverts the spin at the intersection. By applying an additional 180 ° high frequency pulse, the spins at the intersection can be reversed and aligned with the spins other than the intersection in the region to be measured thereafter. The additional 180 ° high frequency pulse is applied after the elapse of (TE2−TE1 / 2) time from the application of the excitation high frequency pulse, where the echo time of the first region is TE1 and the echo time of the second region is TE2. The
[0014]
In the MRI method according to the second aspect of the present invention, the signal from the spin at the intersection is measured separately from the echo signal from the non-intersection at step (c ′), and image processing is performed at step (g). When the signal from the intersection and the signal from the non-intersection are combined and the image is reconstructed for the second and subsequent regions, an image without contrast discontinuity at the intersection can be obtained for each region. .
[0015]
In the above-described MRI method of the present invention (the first and second aspects), the application amount of the phase encoding gradient magnetic field and the readout gradient magnetic field applied up to the time when the echo signal from each region is measured is further determined. Applying a phase encoding gradient magnetic field and a readout gradient magnetic field to zero. As a result, the influence of the gradient magnetic field applied before the measurement in the second and subsequent regions is eliminated, and each echo signal can be measured.
[0016]
Furthermore, the MRI method of the present invention is a magnetic resonance imaging method in which first and second regions intersecting each other in a subject placed in a static magnetic field are continuously imaged and displayed. Applying a high-frequency pulse for exciting spins in the entire region, and (2) following the high-frequency pulse, a gradient magnetic field for sequentially selecting one region and a 180 ° high-frequency pulse for the first and second regions, (1) generating one or a plurality of echo signals from the one region, measuring the echo signal by giving a predetermined phase encoding amount, and (3) measuring the echo signal from the second region Before, for the first region, an additional inversion pulse for returning the spin inverted by the 180 ° high frequency pulse is applied, and (4) the steps (1) to (3) are applied. A step of separately reconstructing images of two regions using an echo signal obtained by repeating a plurality of times by changing a set phase encoding amount including a step, and (5) obtained by the step (4). And simultaneously displaying images of the two areas on the display device.
[0017]
This MRI method is an application of the MRI method according to the first aspect of the present invention when the region to be measured is 2, and in step (3), the first inverted by a 180 ° high frequency pulse. By applying an additional inversion pulse that restores the spins in the region, the spins present at the intersections in the first slice and the spins present at the portions other than the intersections can be brought into the same state. It is possible to measure echo signals having the same phase from the entire two slices. As a result, it is possible to prevent a phenomenon in which the intersecting portion is rendered dark after image reconstruction and a phenomenon in which the contrast becomes discontinuous at that portion. Also in this case, when the echo time of the first region is TE1 and the echo time of the second region is TE2, the additional inversion pulse is applied after (TE2-TE1 / 2) time has elapsed since the application of the high frequency pulse. The
[0018]
The MRI apparatus of the present invention enables execution of the above-described MRI method of the present invention, each magnetic field generating means for applying a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to the subject, and nuclear magnetic resonance from the subject. Receiving system for detecting echo signals emitted by the control unit, control means for controlling each of the magnetic field generating means and the receiving system according to a predetermined pulse sequence, and a signal processing system for reconstructing an image using the echo signals detected by the receiving system And a display means for displaying the obtained image, and the control means selectively inverts the plurality of cross-sections sequentially as a pulse sequence and a high-frequency magnetic field pulse for exciting a region including a plurality of cross-sections of the subject. Has a function to generate a high-frequency magnetic field pulse and execute a pulse sequence to measure an echo signal from each cross section. The signal processing system has a plurality of signal processing sequences corresponding to a plurality of cross sections. Rannahli, reconstructs images of a plurality of cross-sectional simultaneously, but to be displayed.
[0019]
【Example】
Hereinafter, embodiments of the MRI apparatus of the present invention will be described in detail.
[0020]
FIG. 6 is a block diagram showing the entire MRI apparatus to which the present invention is applied. This MRI apparatus generates a magnetic field 602 that generates a static magnetic field in a space around a subject 601 and a gradient magnetic field in this space. A gradient magnetic field coil 603, an RF coil 604 that irradiates a subject 601 with a high-frequency pulse, and an RF probe 605 that detects an NMR signal generated by the subject 601 are provided.
[0021]
The gradient magnetic field coil 603 is constituted by gradient magnetic field coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in accordance with a signal from the gradient magnetic field power supply 609. The RF coil 604 generates a high frequency pulse according to the signal of the RF transmission unit 610.
[0022]
The signal of the RF probe 605 is detected by the signal detection unit 606, signal processed by the signal processing unit 607, and converted into an image signal by calculation. In the MRI apparatus of the present invention, the signal processing unit 607 has a plurality of signal processing sequences for separately processing signals obtained from a plurality of regions of the subject 601, and image data is formed for each signal sequence. .
[0023]
An image formed by the signal processing unit 607 is displayed on the display unit 608. When images are taken simultaneously in a plurality of areas, it is possible to display only an image in one area or display images in a plurality of areas simultaneously.
[0024]
The gradient magnetic field power supply 609, the RF transmission unit 610, and the signal detection unit 606 are controlled by the control unit 611, and are controlled according to a time chart called a pulse sequence.
[0025]
The subject 601 is transported to the space in the magnet 602 while lying on the bed 612. This space is not clearly shown in the figure, but is an open space so that an operator or an operator can approach the subject and perform IVMR. Therefore, by combining the imaging methods described below, the operator can monitor a plurality of sections including the tip in real time while inserting a biopsy needle or catheter into the subject.
[0026]
Next, a method for continuously photographing a plurality of cross sections (regions) in the MRI apparatus having such a configuration will be described.
[0027]
1 to 4 are diagrams showing pulse sequences of the MRI method according to the first aspect of the present invention, and these pulse sequences are controlled by the control unit 611 of the MRI apparatus described above. In the figure, RF indicates the timing of the high frequency pulse irradiated by the RF probe 605, Gx, Gy, and Gz indicate the shape and application timing of the gradient magnetic field pulse applied by the gradient magnetic field coils in the x, y, and z3 directions. Indicates the timing at which the signal detection unit 606 measures an echo signal generated from the subject, and echo indicates the generation timing of the echo signal.
[0028]
First, in the embodiment shown in FIG. 1, a case where two orthogonal cross sections 702 and 703 shown in FIG. 7 are measured as a plurality of cross sections will be described. First, an excitation radio frequency pulse (hereinafter referred to as excitation RF pulse) 101 is irradiated to excite the subject 701 substantially in a non-space-selective manner. Exciting substantially non-selectively means that the entire region to be measured is excited, and if the subject is within the field of view FOV, the gradient magnetic field pulse for slice selection may be zero. . Further, when the subject protrudes from the FOV, in order to prevent folding artifacts, a slice selection gradient magnetic field pulse Gx102 is applied as shown in the figure, and an area about FOV or the entire measurement area is selected. The flip angle of the excitation RF pulse is 90 °, for example. If the TR is repeatedly shortened to shorten the measurement time, it may be smaller than 90 °.
[0029]
Next, after applying the phase encoding gradient magnetic field 103 and the rewind gradient magnetic field pulse 105 in the Gy direction, three + 180 ° pulses 107, 114, and 118 are sequentially applied. The first + 180 ° pulse 107 is a high-frequency pulse that selectively inverts the spin in the first slice 702, and is applied simultaneously with the slice selective gradient magnetic field pulse 106 in the Gx direction. The signal from the spin in the first slice 702 inverted by the first + 180 ° pulse 107 is measured within the sampling time 110 as the echo signal 109 after the echo time TE1 from the application of the excitation RF pulse 101. At this time, a read gradient magnetic field 108 is applied in the Gy direction.
[0030]
The next + 180 ° pulse 114 is a high-frequency pulse that selectively inverts the spin in the second slice 703 and is applied simultaneously with the slice selective gradient magnetic field pulse 113 in the Gy direction. The third + 180 ° pulse 118 is applied before the spin echo is generated from the spin in the second slice 703 inverted by the + 180 ° pulse 114. This + 180 ° pulse 118 is an additional high-frequency pulse for inverting again the spin in the first slice 702 inverted by the first + 180 ° pulse 107, and is applied simultaneously with the slice selective gradient magnetic field pulse 117 in the Gx direction. Is done.
[0031]
Thereafter, the echo signal 120 from the second slice 703 is measured within the sampling time 122. At this time, a read gradient magnetic field 121 is applied in the Gx direction. The echo signal 120 from the second slice is obtained after the echo time TE2 from the application of the excitation RF pulse 101. At this time, the additional + 180 ° pulse 118 is applied, so that the second slice 703 has an intersection 704. Therefore, the phases of the spins other than the crossing portions are aligned.
[0032]
This will be further described with reference to FIG. In FIG. 2, RFs 101, 107, 114, 118 and echoes 109, 120 correspond to the high-frequency pulses 101, 107, 114, 118 and the echo signals 109, 120 in FIG. Phase 1, phase 2, and phase 3 represent the phase of the spin other than the intersection of the first slice 702, the phase of the spin other than the intersection of the second slice 703, and the phase of the spin of the intersection 704, respectively. .
[0033]
After applying the RF pulse 101, the phases 1 to 3 of the spins of each part begin to change uniformly, but only the spin (phase 1 and phase 3) of the first slice 702 is inverted by the slice-selective + 180 ° pulse 107. To do. A spin echo 109 is generated from the first slice when the phase 1 and the phase 3 are all 0, that is, after the echo time TE1 from the RF pulse 101. Next, when the + 180 ° pulse 114 is applied after the elapse of TE 2/2 from the RF pulse 101, only the spin (phase 2 and phase 3) of the second slice 703 is inverted. In this state, in the second slice 703, the phase change due to the high-frequency pulse is different between the intersecting portion 704 and the other portions, but the + 180 ° pulse 118 before the echo measurement for the second slice. Is added, the phase of the spin is aligned at the intersection 704 and the other portions at the time of the echo signal measurement of the second slice 703, and the spin echo 120 is generated. Thereby, discontinuity of contrast at the intersection 704 can be prevented.
[0034]
As described above, in order to align the phases of the intersection 704 and the other portions at the time of measuring the echo signal from the second slice, the time from the application of the excitation RF pulse 101 to the generation of the spin echo 109 is determined. If the time until TE1 and the spin echo 120 are generated is TE2, the additional high-frequency pulse 118 must be applied to (TE2-TE1 / 2).
[0035]
FIG. 2 illustrates only the phase change due to the high-frequency pulse, but in order to avoid mixing of signals from other than the slice to be measured, it is necessary to consider the phase change due to the gradient magnetic field. That is, it is necessary to remove the influence of the gradient magnetic field applied for the first slice measurement in the second slice measurement.
[0036]
Specifically, gradient magnetic fields 105 and 108 in the Gy direction are applied in order to generate the echo signal 109 from the first slice 702. At the time of echo signal measurement from the second slice, these gradient magnetic fields in the Gy direction are applied. In order to remove the influence, the gradient magnetic fields 116 and 119 are applied so that the application amount is zero. If the strengths of the gradient magnetic fields are the same, the application times of the gradient magnetic fields 105 and 119 are the same, and the application times of the gradient magnetic fields 108 and 116 are the same. The application time of the gradient magnetic field 105 is half of the application time of the gradient magnetic field 108.
[0037]
Also, three gradient magnetic field pulses 104 that generate echoes are added. If the gradient magnetic field strength is the same, the application time of each gradient magnetic field pulse 104 is half that of the gradient magnetic field 121.
[0038]
In addition to the additional reversal magnetic field 118, by adding a gradient magnetic field (gradient magnetic field pulse indicated by slant lines in the figure) for such rephasing, at the time of echo signal measurement from each slice, the intersection and the others The echo signal can be measured with the phases of the portions being equal and without mixing in signals from other slices. Note that the application pattern of the gradient magnetic field for rephasing is not limited to that shown in FIG. 1, but the spins of slices for which signals are to be acquired are aligned at the time of echo signal measurement from each slice, and the spins of the other portions are dephased. Various changes can be made under the condition.
[0039]
Three steps of step 111 from excitation RF pulse 101 application to echo signal 109 measurement, step 112 of second + 180 ° pulse 114 application, and step 115 from third + 180 ° pulse 118 application to echo signal 120 measurement Are combined into one set, and the size of the phase encode pulse 103 is changed in such a set 123, and the set is repeated until the set 124 with a time TR.
[0040]
For example, the repetition time TR is 100 ms, TE1 is 10 ms, and the interval between the spin echo 108 and the echo 120 (Interecho time) is 40 ms. When the interecho time is shorter, the spin attenuation in the subject 701 is suppressed, and an image with a good S / N is obtained for the second slice 703.
[0041]
In the embodiment shown in FIG. 1, since one echo signal is measured for each slice in one set, the number of repetitions is the number of phase encodings (n) necessary for image reconstruction. These echo signals are processed by two series of signal processing units for each slice, and an image is reconstructed for each slice. The image of each slice is displayed simultaneously on the display unit. It is also possible to select and display only one slice image. As a result, a region of interest such as the tip of a catheter inserted into a subject in IVMR can be intensively monitored from two orthogonal directions.
[0042]
In the embodiment shown in FIG. 1, the phase encoding direction is fixed to Gz. However, the readout gradient magnetic field pulse application direction can also be fixed to the Gz axis. Such a sequence is shown in FIG.
[0043]
Also in the embodiment shown in FIG. 3, first, the excitation RF pulse 201 is applied to excite the region including the first slice 702 and the second slice 703, and then the spin in the first slice 702 is selectively inverted. Slice + 180 ° pulse 205, + 180 ° pulse 215 that selectively inverts spins in second slice 703, and + 180 ° pulse 219 that selectively inverts spins in first slice 702, respectively The point of applying simultaneously with the selective gradient magnetic fields 204, 214, 218 is the same as in the embodiment of FIG.
[0044]
In this embodiment, for the first slice 702, prior to the measurement of the echo signal 208, the phase encode gradient magnetic field 206 is applied in the Gy direction, the readout gradient magnetic field 207 in the Gz direction is applied, and the echo signal is received within the sampling time 209. Measure 208. For the second slice 703, the phase encode gradient magnetic field 221 is applied in the Gx direction prior to the measurement of the echo signal 220, the readout gradient magnetic field 222 in the Gz direction is applied, and the echo signal 220 is measured within the sampling time 223. To do.
[0045]
Further, after the echo signal 208 is measured, a gradient magnetic field 210 in the Gy direction for rephasing the phase changed by the phase encoding gradient magnetic field 206 is applied. Further, before applying the + 180 ° pulse 219 for reversing the spin of the second slice, the gradient magnetic field 213 for rephasing the phase change caused by the gradient magnetic fields 203 and 207 in the Gz direction applied for echo measurement from the first slice. And 217 are applied. In this way, the influence of each gradient magnetic field applied for measuring the echo signal from the first slice is removed by the time of the echo signal from the second slice.
[0046]
As a result, as in the embodiment of FIG. 1, the phase of the crossing portion and the portion other than the crossing portion are aligned at the time of echo signal measurement from the second slice (after the passage of TE2 from application of the RF pulse 201), and from the first slice. Can be prevented from being mixed.
[0047]
Also in this embodiment, a set 224 consisting of step 211 from excitation RF pulse 201 to echo signal 208 measurement, step 212 of applying + 180 ° pulse 215, step 216 from additional + 180 ° pulse 219 to echo signal 220 measurement, The echo signal measurement is performed by changing the magnitude of the phase encode pulse (206, 221) and repeating the required number of phase encode times (n). The obtained signal is subjected to signal processing for each slice, and an image for each slice is displayed on the display unit.
[0048]
In the embodiment shown in FIGS. 1 and 3, the case where one echo signal is measured for each slice in one set (123 or 224) is shown, but a slice selective inversion pulse is further added in one set. By doing so, it is possible to generate a plurality of echo signals for each slice. The method of the present invention can also be applied to a fast spin echo method and an EPI (echo planar) method in which a plurality of echo signals are measured by one excitation.
[0049]
Hereinafter, an embodiment in which a plurality of echo signals are measured for each slice with one excitation will be described with reference to FIGS.
[0050]
The sequence shown in FIG. 4 is substantially the same as steps 111, 112, and 115 of the sequence of FIG. 1 until steps 111, 112, and 115, but three more + 180 ° pulses after step 115 of the pulse sequence of FIG. 131, 132, and 133 are added.
[0051]
In the sequence of FIG. 4, the application pattern of the rewind gradient magnetic field (indicated by oblique lines) applied in steps 111, 112, and 115 is different from that of the embodiment of FIG. These rewind gradient magnetic fields are also gradient magnetic fields that remove the influence of the gradient magnetic field applied for the measurement of the first slice before the echo signal measurement from the second slice.
[0052]
The + 180 ° pulse 131 that is first applied after step 115 is an RF pulse that selectively inverts the spin of the second slice. The echo time at which the second spin echo from the second slice is generated is TE2 If ', then it is applied after (TE2−TE2 ′) / 2 has elapsed from the time of occurrence of the echo 120 from the second slice. As a result, a second echo signal 134 is generated from the second slice, and this is measured after TE2 ′ has elapsed from the first excitation pulse 101.
[0053]
From this measurement of the echo signal 134, a + 180 ° pulse 132 for selecting the first slice is applied after an arbitrary time TA shorter than (TE1′−TE2 ′) / 2. The spin echo 135 generated from the first slice by the + 180 ° pulse 132 is measured after TE1 ′ elapses from the first excitation pulse 101. Prior to the measurement, a + 180 ° pulse 133 that selectively inverts the second slice is applied so that the phases of the spins of the first slice and the spins at the intersection are aligned at the time of the measurement. The application timing of the + 180 ° pulse 133 is after {TE1 '-(TE1'-TE2'-2TA) / 2} = (TE1' + TE2 '+ 2TA) / 2 has elapsed since the first excitation pulse 101. .
[0054]
The application timing of each + 180 ° pulse 131 to 133 is shown in FIG. In the figure, the gradient magnetic field is omitted, and only the timing of the application of the high frequency pulse and the echo measurement and the phase change of the spin of each part are shown (the thin line is the non-intersection part of the first slice, the thick line is the intersection part, the dotted line is the second part Non-intersecting part of the slice). RF1 indicates the application of a + 180 ° pulse for selecting the first slice, and RF2 indicates the application of a + 180 ° pulse for selecting the second slice. E1 indicates an echo signal from the first slice, and E2 indicates an echo signal from the second slice.
[0055]
Thus, by appropriately setting the application timings of the three + 180 ° pulses 131, 132, 133 applied after step 115, a plurality of echo signals can be generated from each slice and obtained. It is possible to eliminate the contrast discontinuity at the intersection in the signal.
[0056]
Steps including the application of these three + 180 ° pulses 131, 132, 133 and measurement of the echo signals 134, 135 are added to form one set, and the required phase is changed by changing the magnitude of the phase encoding gradient magnetic field applied first. Repeat encoding times.
[0057]
Since two echo signals measured within one set for each slice have different echo times, two images with different contrasts are created and displayed for each slice by performing two series of image processing. be able to.
[0058]
As a modification of the embodiment shown in FIG. 4, different phase encodings may be given to the two echo signals obtained for each slice. This is a combination of the method of the present invention and the fast spin echo method, and its sequence is shown in FIG.
[0059]
In this sequence, the application timing of the excitation pulse and the following six + 180 ° pulses and the timing of echo signal measurement are exactly the same as those in the sequence of FIG. 4, but the gradient magnetic field for phase encoding into each echo signal and The rephase gradient magnetic field is arranged before and after the echo signal.
[0060]
In this embodiment, the number of set repetitions (n) is n / 2, and one image is obtained for each slice. In this embodiment, the same effect as that of the embodiment of FIG. 4 can be obtained, and the photographing time can be shortened to ½ that of the embodiment of FIG.
[0061]
The sequence shown in FIG. 9 shows another embodiment in which a plurality of echo signals are measured for each slice by one excitation. In this embodiment, the echo time TE1 for the first slice is the second slice. This is applicable when the echo time TE2 is sufficiently shorter than TE2.
[0062]
In this sequence, steps 111, 112, and 115 are substantially the same as steps 111, 112, and 115 in the sequence of FIG. 1, but after measuring the echo signal 109 from the first slice (after step 111), the second step is performed. Before step 112 of applying a + 180 ° pulse 114 that inverts the spin in the slice, ie, after (TE1'-TE1) / 2 has elapsed from the echo signal 109, the spin of the first slice is inverted again + 180 °. A pulse 141 is applied, and a second echo signal 143 is measured from the first slice after TE4 ′ from the excitation pulse 101. Thus, two echo signals having different echo times (TE1, TE1 ′) are measured for the first slice before the second slice selection.
[0063]
Thereafter, as in the sequence of FIG. 1, step 112 is applied to apply a + 180 ° pulse 114 to invert the spin in the second slice, an additional + 180 ° pulse 118 to invert the spin at the intersection, and the second Step 115 consisting of measuring the echo signal 120 from the slice is performed. Subsequently, a + 180 ° pulse 142 that inverts the spin in the second slice is applied, and the echo signal 145 from the second slice is measured. Thereby, two echo signals 120 and 145 having different echo times (TE2, TE2 ′) are also measured for the second slice.
[0064]
The application timing of each + 180 ° pulse in this embodiment is shown in FIG. Here, with respect to the additional + 180 ° pulse applied in step 115, when the time until the second echo signal 143 from the first slice is measured is TE1 ′, TE2−TE1 ′ / Apply after 2 minutes. As a result, the phase of the crossing portion and the other portion can be made equal when measuring the echo signal 120 for the second slice.
[0065]
The + 180 ° pulse 142 applied after step 115 is applied after (TE2′−TE2) / 2 has elapsed since the first echo signal 120 was measured from the second slice.
[0066]
In the embodiment shown in FIG. 9, the case where the same phase encoding amount is given to the echo signal measured in one set is shown. In this case, as in the embodiment of FIG. Two images with different values can be created and displayed.
[0067]
FIG. 11 shows a sequence obtained by applying the sequence of FIG. 9 to the first spin echo method. The timing of applying the RF pulse is exactly the same as that of the embodiment shown in FIG. 9, but here, for each slice in one set. Different phase encoding amounts are given to the echo signals measured two by two. That is, this embodiment is the same as the sequence of FIG. 9 except that the phase encoding gradient magnetic field for applying phase encoding to each echo signal and the rephase gradient magnetic field are arranged before and after echo measurement. .
[0068]
In this embodiment, since one set of two-phase encoded echo signals is measured, the number of repetitions can be reduced to ½ of the number of phase encodings. The number of echoes generated from each slice may be three or more. If the number of echoes is q, the number of repetitions can be reduced to 1 / q of the number of encodings.
[0069]
Next, an example in which the method of the present invention is applied to EPI is shown in FIG. The sequence of this embodiment also includes step 1211 from application of excitation RF pulses that excite the entire two orthogonal sections 702 and 703 to measurement of the echo signal from the first slice 702, Step 1212 for applying a + 180 ° pulse for inverting the spin, and Step 1215 for measuring an echo signal from the second slice 703 after applying a + 180 ° pulse for further inverting the spin of the first slice. The phase encoding gradient magnetic field is fixed to the Gz direction gradient magnetic field.
[0070]
However, in this embodiment, in steps 1211, 1215 including echo measurement, a plurality of echo signals 1203, 1208 are measured at sampling times 1204, 1209 using inversion of readout gradient magnetic fields 1201, 1206. Although the figure shows a case where three echo signals are measured at each step, the number of echo signals measured at one step may be two or four or more.
[0071]
By applying a phase encode gradient magnetic field 1210 and discrete gradient magnetic fields 1202 and 1207 to the echo signals 1203 and 1208, respectively different phase encode amounts are given. Here, after the echo signal from the first slice, a gradient magnetic field 1205 is applied to cancel (rephase) the phase encoding gradient magnetic field 1202 applied for the measurement. Note that, in order to cancel the influence of the Gy-direction readout gradient magnetic field applied in step 1211, applying an additional gradient magnetic field (indicated by oblique lines) is the same as in the embodiment of FIG.
[0072]
Such a set 1223 is repeated until the necessary number of phase encoding echo signals are measured, and an image is reconstructed from each obtained set of echo signals. In this embodiment, since three echoes are generated for each slice by one excitation, the time for collecting all echoes necessary for image reconstruction is reduced to 1/3 of the example in FIG. can do. For each slice, the number of echoes 1203 and 1208 generated in one excitation can be varied by the number of inversions (q) of the readout gradient magnetic fields 1201 and 1206 to be reversed, thereby increasing the total number required for image reconstruction. The time for collecting echoes can be shortened to 1 / q in the example of FIG. If q is matched with the number of phase encodings (n) necessary for image acquisition, it is possible to collect all echoes necessary for image reconstruction (single-shot EPI conversion) with one excitation.
[0073]
As described above, according to the MRI method according to the first aspect of the present invention, images of two orthogonal cross sections (702 and 703 in FIG. 7) can be obtained by one imaging, and the obtained 2 In a single image, the image is not depicted darkly at the portion corresponding to the intersecting portion 704 of the first slice 702 and the second slice 703, and no discontinuity in contrast occurs. Therefore, a region of interest such as the tip of a catheter inserted into a subject in IVMR can be intensively monitored from two orthogonal directions.
[0074]
Next, the MRI method according to the second aspect of the present invention will be described. The MRI method according to this aspect is the same as the MRI method according to the first aspect in that the excitation RF pulse for exciting the entire region including a plurality of slices and the 180 ° pulse for selecting each slice are applied. In this second aspect, signals are separately acquired for the non-intersection and the intersection having different spin states, and the signal from the non-intersection and the signal from the intersection are combined at the time of image reconstruction. Discontinuity is eliminated.
[0075]
FIG. 13 is a diagram showing an example of the MRI method according to the second mode, and here, a case where two orthogonal slices 702 and 703 as shown in FIG. 7 are imaged will be described as an example. Also in this embodiment, an excitation RF pulse 101 is first applied to excite a region including the first slice 702 and the second slice 703, and then a + 180 ° pulse that selectively inverts the spin in the first slice 702. 107, the + 180 ° pulse 114 that selectively inverts the spin in the second slice 703 is applied simultaneously with the slice selective gradient magnetic fields 106 and 113, respectively, as in the embodiment of FIG.
[0076]
As can be seen from the graph showing the spin behavior due to these high-frequency pulses, an echo signal 109 from the first slice 702 is generated by the inverted + 180 ° pulse 107 after TE1 from the excitation RF pulse 101. Among the spins of the first slice 702, the spin at the intersection (thick line) is further inverted by the inversion + 180 ° pulse 113, and the echo signal 150 is generated after 2 (TE2 / 2-TE1) from the echo signal 109. This echo signal is a signal generated only from the intersection and is measured. Thereafter, the signal 120 from the second slice 703 is generated after TE2 from the excitation RF pulse 101. This signal is a signal from only the non-intersecting part (dotted line) in the second slice 703.
[0077]
A sequence for measuring signals from each slice and intersection is repeated while changing the phase encoding gradient magnetic field strength, and the number of echo signals necessary for image reconstruction is measured. These echo signals are processed by the signal processing unit 607 and converted into image signals. At this time, for the second slice, the signal 150 from the intersection and the signal 120 from the second slice are combined to reconstruct the image of the second slice. Thereby, an image having no contrast discontinuity at the intersection can be obtained for the second slice.
[0078]
Also in this embodiment, the gradient magnetic field axis is changed as in the embodiment shown in FIG. 3 or changes such as generating two or more echo signals at a time as in the embodiment shown in FIG. Can do.
[0079]
【The invention's effect】
As is clear from the embodiments described above, according to the present invention, it is possible to simultaneously image a plurality of regions that overlap each other in one shooting, and at this time, in the portion corresponding to the overlap, It is possible to obtain a high-quality image that does not cause a significant decrease or contrast discontinuity.
[0080]
In addition, by adopting a high-speed sequence such as EPI as a sequence for acquiring an echo signal, a plurality of areas can be shot at high speed, and the practicality of IVMR and dynamic shooting can be improved.
[Brief description of the drawings]
FIG. 1 is a sequence diagram showing one embodiment of a photographing method according to the first aspect of the present invention.
FIG. 2 is a diagram showing a phase change of a spin in a subject in the embodiment of FIG.
FIG. 3 is a sequence diagram showing another embodiment of the photographing method according to the first aspect of the present invention.
FIG. 4 is a sequence diagram showing another embodiment of the photographing method according to the first aspect of the present invention.
FIG. 5 is a diagram showing RF pulse application timing and spin phase change in the imaging method of FIG. 4;
FIG. 6 is a block diagram showing the entire MRI apparatus to which the present invention is applied.
FIG. 7 is a diagram showing a relationship between the arrangement of a plurality of slices and coordinate axes.
FIG. 8 is a sequence diagram showing another embodiment of the photographing method according to the first aspect of the present invention.
FIG. 9 is a sequence diagram showing another embodiment of the photographing method according to the first aspect of the present invention.
FIG. 10 is a diagram showing RF pulse application timing and spin phase change in the imaging method of FIG. 9;
FIG. 11 is a sequence diagram showing another embodiment of the photographing method according to the first aspect of the present invention.
FIG. 12 is a sequence diagram showing another embodiment of the photographing method according to the first aspect of the present invention.
FIG. 13 is a sequence diagram showing one embodiment of a photographing method according to the second aspect of the present invention.
[Explanation of symbols]
601 ・ ・ ・ ・ ・ ・ Subject
603 ・ ・ ・ ・ ・ ・ Gradient magnetic field coil (Gradient magnetic field generating means)
604 ... RF coil (irradiation means)
605 ... RF probe (detection means)
607 ・ ・ ・ ・ ・ ・ Signal processing section
611 ・ ・ ・ ・ ・ ・ Control part (control means)

Claims (6)

Each magnetic field generating means for applying a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to the subject, a receiving system for detecting an echo signal emitted from the subject by nuclear magnetic resonance, and each of the magnetic field generating means and the receiving system are predetermined. In a magnetic resonance imaging apparatus, comprising: control means for controlling in accordance with the pulse sequence; a signal processing system for reconstructing an image using an echo signal detected by the receiving system; and a display means for displaying the obtained image.
The control means generates, as the pulse sequence, a high-frequency magnetic field pulse for exciting a region including a plurality of cross sections of the subject and a high-frequency magnetic field pulse for selectively inverting the plurality of cross sections sequentially, and echoes from each cross section. It has a function to execute a pulse sequence that measures a signal,
The signal processing system includes a plurality of signal processing sequences corresponding to the plurality of cross sections, and simultaneously reconstructs and displays the images of the plurality of cross sections. Each magnetic field generating means for applying a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to the subject, a receiving system for detecting an echo signal emitted from the subject by nuclear magnetic resonance, and each of the magnetic field generating means and the receiving system are predetermined. In a magnetic resonance imaging apparatus, comprising: control means for controlling in accordance with the pulse sequence; a signal processing system for reconstructing an image using an echo signal detected by the receiving system; and a display means for displaying the obtained image.
The control means applies, as the pulse sequence, an excitation high-frequency pulse that excites spins of a plurality of regions intersecting each other in the subject, and, following the excitation high-frequency pulse, one for each of the plurality of regions. A gradient magnetic field for selecting a region and a 180 ° high-frequency pulse are applied to invert the spin of the region, and echo signals are measured at different echo times for the plurality of regions. Before measuring the echo signal, the function of executing a pulse sequence including the step of aligning the phase of the spin of the portion where the plurality of regions intersect with the phase of the other spin of the second and subsequent regions,
The magnetic resonance imaging apparatus, wherein the signal processing system separately reconstructs and simultaneously displays an image of each region using echo signals measured for each of the plurality of regions. Each magnetic field generating means for applying a static magnetic field, a gradient magnetic field, and a high-frequency magnetic field to the subject, a receiving system for detecting an echo signal emitted from the subject by nuclear magnetic resonance, and each of the magnetic field generating means and the receiving system are predetermined. In a magnetic resonance imaging apparatus, comprising: control means for controlling in accordance with the pulse sequence; a signal processing system for reconstructing an image using an echo signal detected by the receiving system; and a display means for displaying the obtained image.
The control means applies, as the pulse sequence, an excitation high-frequency pulse that excites spins of a plurality of regions intersecting each other in the subject, and, following the excitation high-frequency pulse, one for each of the plurality of regions. A portion where the plurality of regions intersect with each other by applying a gradient magnetic field for selecting the region and a 180 ° high frequency pulse, inverting the spin of the region, measuring echo signals at different echo times for the plurality of regions, respectively. It has a function to execute a pulse sequence that measures the echo signal from
The signal processing system separately reconstructs and simultaneously displays an image of each region using echo signals measured for each of the plurality of regions and echo signals measured for the intersecting portions. Resonance imaging device. The magnetic resonance imaging apparatus according to claim 2,
The step of aligning the phase of the spin of the portion where the plurality of regions intersect with the phase of the other spins of the second and subsequent regions includes applying an additional 180 ° high frequency pulse that inverts the spin of the intersecting portion. A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to claim 4,
The additional 180 ° high frequency pulse is applied after the elapse of (TE2−TE1 / 2) time from the application of the excitation high frequency pulse when the echo time of the first region is TE1 and the echo time of the second region is TE2. A magnetic resonance imaging apparatus. The magnetic resonance imaging apparatus according to any one of claims 1 to 3,
The pulse sequence executed by the control means further includes a phase encoding gradient so that the application amount of the phase encoding gradient magnetic field and the readout gradient magnetic field applied so far at the time of measuring the echo signal from each region is zero. A magnetic resonance imaging apparatus comprising applying a magnetic field and a readout gradient magnetic field.

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