JP3211978B2 - MRI equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to MRI (nuclear magnetic resonance).
Device relates, in particular, when measuring the image data in the object very fast, which can improve the resolution MR
About the I unit.

[0002]

2. Description of the Related Art An object is placed between magnets that generate a static magnetic field, and a slice gradient magnetic field having a gradient in a space including the cross section is applied depending on which cross section is to be imaged. The slice gradient magnetic field is applied so as to be one of three orthogonal magnetic fields. When the slice gradient magnetic field (for example, the Z-axis direction) is determined, a finer mesh is determined in the X-axis and Y-axis directions to observe each component of the cross section of the subject, and one of the meshes (X-axis or Y-axis) is determined. ) Is a phase encoding gradient magnetic field, and the other (Y axis or X axis) is a readout gradient magnetic field. Next, by applying a high-frequency signal to the subject, when the signal has a specific frequency, resonance occurs only in the section, and an image of the section can be obtained. Such MRI
As an ultra-high-speed photographing method using a photographing apparatus, for example, “Journal of Physics, C, Solid State Physics” L55, 1977 (J. Phys.
Solid State Phs.), The first proposal, the eco-planar method, and a modification thereof are proposed. In this method, from nuclear magnetization excited by a high-frequency pulse,
An echo signal is continuously generated by applying a readout gradient magnetic field having a reversed polarity at a high speed, and data necessary for image reconstruction can be collected in several tens of milliseconds to perform ultra-high-speed imaging. it can. As a lead-out gradient magnetic field, a method using a rectangular wave (pulse-like waveform), a method using a sine wave, and the like have been proposed.

[0003]

In such an ultra-high-speed photographing method, continuous photographing of 10 to 20 images per second (hereinafter referred to as time resolution) can be performed without using heart rate synchronization. Continuous imaging of a violently moving heart or the like is possible. Therefore, this method is expected to be used for diagnosis of cardiac function by displaying moving images and the like. However, in medical diagnosis of cardiac function, continuous imaging with a temporal resolution of 30 or more images per second is required, and moreover, images with good image quality and high spatial resolution, that is, images obtained by further reducing the mesh, are required. Is desired. In conventional ultra-high-speed shooting, continuous shooting of 10 to 20 images per second is the limit as described above, and when the image quality is improved, the shooting time becomes longer, so the number of images in continuous shooting decreases, After all,
The temporal resolution, which increases the number of continuous sheets, and the spatial resolution, which further refines the mesh, have a conflicting relationship. An object of the present invention is to solve such a conventional problem and to provide an MRI apparatus capable of utilizing heart rate synchronization and capable of effectively improving both time resolution and spatial resolution. is there.

[0004]

In order to achieve the above object, an MRI apparatus according to the present invention comprises: a means for generating a static magnetic field;
Means for generating a gradient magnetic field in three directions, a probe for applying a high-frequency pulse to the subject and receiving an echo signal from the subject, and applying the gradient magnetic field, applying the high-frequency pulse, and generating the echo signal. A sequencer for controlling capture, a computer for performing image reconstruction using the echo signal, display means for displaying a result of the image reconstruction, and means for detecting a heartbeat signal of the subject, Based on the heartbeat signal, the sequencer selectively excites the region of interest of the subject, applies a readout gradient magnetic field while inverting the polarity of the amplitude, and applies the readout gradient magnetic field in a direction orthogonal to the direction in which the readout gradient magnetic field is applied. A pulse sequence for applying a phase encoding gradient magnetic field and collecting a plurality of echo signals required for image reconstruction of the region of interest is repeated, The start timing of the sequence or the applied value of the phase encoding gradient magnetic field is set to be shifted by a predetermined value in the next pulse sequence, and the temporal or phase space of the echo signal collected in the previous pulse sequence is set. Collecting the echo signal located in between, and for each of the heartbeat signals sequentially detected, controls one of the timing of the start of the pulse sequence or the phase encoding gradient magnetic field continuously plural times, and then controls the other. A plurality of times to collect the echo signals based on the previous heartbeat signal, rearrange the collected echo signals in a temporal or phase spatial order, reconstruct an image, and display the image. It is characterized in that a moving image is displayed on the means.

[0005]

According to the present invention, a plurality of heartbeats synchronized by heartbeats are used to change the applied amount of the gradient magnetic field for each of the first heartbeat, the second heartbeat, and the third heartbeat. Is shifted to improve the spatial resolution, and the time at which imaging is started is made variable for each heartbeat from the time of the heartbeat, thereby improving the time resolution. That is, control is performed such that data between data captured with the previous heartbeat is captured for each heartbeat. Specifically, first, a plurality of images are obtained by executing the first imaging using the heartbeat synchronization. In the next heartbeat, the start of the shooting is shifted from the first time, and the time is shot, and an image of a plurality of images obtained in the first time is taken in time. Such imaging makes it possible to increase the time resolution.
Also, in order to improve spatial resolution, 1
The start time of the second and the second imaging is the same, and the applied amount of the gradient magnetic field is set so that the data measured in the second imaging in the phase space is intermediate between the data measured in the first imaging. Set. In addition, time,
It is also possible to improve both resolutions of the space. Further, by performing the same imaging over a plurality of heartbeats, the resolution can be further improved.

[0006]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 6 is a configuration diagram of an MRI apparatus showing one embodiment of the present invention. In FIG. 6, 1 is a static magnetic field magnet for generating a uniform static magnetic field, and 2 is driven by a drive circuit 6 and a power supply 7 controlled by a sequencer 8, and each of three orthogonal directions (X, Y, Z directions). 3) a gradient magnetic field coil for generating a gradient magnetic field for the subject 11 on the bed 12; The coil 2 applies a slice gradient magnetic field, a phase encoding gradient magnetic field, and a readout gradient magnetic field in the X, Y, and Z directions, respectively. For the subject 11, a high-frequency signal is sent to the probe 3 from the transmission / reception unit 4 controlled by the sequencer 8, and the probe 3
After a high frequency magnetic field is applied to the probe 1, a magnetic resonance signal in the subject 11 is received by the probe 3. After the signal received by the probe 3 is amplified and detected by the transmission / reception unit 4, the A / D converter 5 controlled by the sequencer 8
And sent to the computer 9. Computer 9
Controls the sequencer 8, performs a Fourier transform of the data input from the AD converter 5, performs image reconstruction, and displays an image on the display device 10. The heartbeat signal from the subject 11 is detected by the heartbeat detector 13 and sent to the computer 9 where the computer 9 controls the timing of operating the sequencer 8 and the imaging conditions such as the amount of gradient magnetic field applied. Used as a signal.

FIG. 7 is a time chart of a pulse sequence for ultra-high-speed photographing, which is a premise of the present invention. In FIG. 7, RF is a high frequency pulse 21, Gs, applied to the subject.
Ge and Gr are gradient magnetic fields orthogonal to each other in three directions. In these gradient magnetic fields in the X, Y, and Z directions, G
s is a slice gradient magnetic field applied for slicing, Ge is a phase encoding gradient magnetic field applied for phase encoding, G
r is a readout gradient magnetic field applied for readout. The echo signal is a magnetic resonance signal of the subject. AD is a signal for analog-to-digital conversion.
This pulse sequence is controlled and executed by the sequencer 8 via the computer 9. First, a high frequency pulse (RF) 21 and a slice gradient magnetic field (Gs) 22 are applied to selectively excite a region of interest to be measured. next,
A readout gradient magnetic field (Gr) 25 is applied in a direction parallel to the slice plane while inverting the amplitude at a high speed, and at the same time, phase encode gradient magnetic fields (Ge) 23 and 24 are read out.
A pulse is applied in a direction orthogonal to the gradient magnetic field (Gr) every time the gradient magnetic field (Gr) is inverted.

Thus, the readout gradient magnetic field (G
r) every time the sum (area) of the amplitude and the application time of the application time becomes positive and negative and becomes zero, that is, the echo signal is continuously generated at the center of the positive side and the center of the negative side. As shown in the figure, data can be sequentially collected, and one image can be captured at a very high speed. In this manner, if the sequence shown in FIG. 7 is repeatedly executed at predetermined time intervals, continuous images can be captured at predetermined time intervals. In such continuous imaging, generally, the high-frequency pulse (RF) 21 uses an excitation angle smaller than 90 °, waits for a signal recovery time to obtain a high signal-to-noise ratio (S / N), and then waits for the next signal. A method of taking an image has been used. That is, the high-frequency pulse (R
The region of interest is selected by applying F) and the slice gradient magnetic field (Gs). When continuous imaging is not performed, the signal is large when the excitation angle is 90 °. However, when the excitation is repeated, the next excitation is performed after the previous excitation angle is restored. Therefore, when the excitation angle is 90 °, it takes time to return, and high-speed repetitive excitation cannot be performed. Therefore, when imaging is performed at a low-speed repetition cycle, the excitation angle may be 90 °. However, when imaging is performed at a high-speed repetition cycle, it is necessary to operate at an excitation angle smaller than 90 °. When the excitation angle is smaller than 90 °, the amplitude of the signal also becomes smaller. Thus, the S / N depends on the recovery time.
Is repeatedly taken at the best time interval. Also,
As the pulse sequence for ultra-high-speed imaging, a pulse sequence of, for example, a sine wave or a cosine wave can be used in addition to a rectangular wave as shown in FIG. 7. In this case, similar continuous imaging is possible. It is.

FIG. 1 is a timing chart of a photographing method according to a first embodiment of the present invention. In the present embodiment, the first imaging start timing of the continuous imaging and the imaging conditions of the gradient magnetic field application amount are controlled for each heartbeat by the computer 9 based on the heartbeat signal obtained from the heartbeat detector 13, and imaging with high resolution is performed. . In FIG. 1, 31 is a heartbeat signal, 32 is a pulse sequence of ultra-high-speed imaging, and t1, t2, and t3 are imaging start timings. In FIG. 1, the entire ultra-high-speed imaging pulse sequence shown in FIG.
Are collectively represented as Also, the heartbeat signal 3 in FIG.
Although only one pulse sequence 1 is described as a whole, the first pulse sequence is started with the first heartbeat signal 31, the second pulse sequence is started with the next second heartbeat signal 31, and the next pulse sequence is started. Means that the third pulse sequence is started with the third heartbeat signal 31. First, the heartbeat signal 31 from the heartbeat detector 13 is sent to the computer 9, and the imaging start timing t1 based on the heartbeat signal 31
At (here, t1 = 0), the ultra-high-speed imaging pulse sequence 32 is repeatedly executed at a predetermined time interval Tr having a good S / N in consideration of the signal recovery time. That is, in the first pulse sequence, under the control of the sequencer 8, the photographing sequence of FIG. As a result, 1-1, 1-2, 1
A plurality of images at predetermined time intervals Tr indicated by -3,... Are obtained. In the second photographing based on the next heartbeat, the photographing start timing t2 is set, for example, to be equal to the photographing time Ts of one image of ultra-high-speed photographing. That is,
In the second pulse sequence, shooting is started with a delay of t2 (= Ts) from the first start reference. By performing shooting similar to the first shooting under such conditions, a plurality of images at predetermined time intervals Tr indicated by 2-1, 2-2, 2-3,... obtain. As a result, 2
The image obtained in the first pulse sequence is a temporally photographing time Ts for a plurality of images obtained in the first photographing.
Thus, a plurality of images at the time points shifted from each other are obtained.

[0010] Further, a third photographing is performed based on the next heartbeat. In the third pulse sequence, the photographing start timing t3 is set, for example, to be equal to twice the photographing time Ts of one image of ultra-high-speed photographing. By performing shooting similar to the above under these conditions,
As shown by 3-1, 3-2, 3-3,.
A plurality of images are obtained at a time point which is temporally shifted by twice the shooting time Ts from the plurality of images obtained in the second shooting. In this way, for example, if the repetition time interval Tr is set equal to three times the photographing time Ts of one image, FIG.
As shown in (1), a continuous image with no time gap is obtained. A large number of images obtained in this way are represented by 1-1, 2-1, 3-1, 1-2, 2-2, 3-
By displaying moving images in the order of 2,..., It is possible to display an image whose time resolution is tripled. Thus, according to FIG. 1, it is possible to perform imaging with improved time resolution. In the above description, the method of capturing an image with no time gap over three heartbeats has been described in detail. However, the repetition time interval Tr, the image capturing time Ts,
With respect to the number of heartbeats to be used and the imaging start timings t1, t2, t3,.

FIG. 3 is a diagram showing a locus of data on a phase space in a photographing method according to a second embodiment of the present invention.
FIG. 3 shows an imaging method for improving spatial resolution using a heartbeat signal. In MRI, measurement data in phase space and image data in real space are
In the Lie relationship, data actually measured is obtained by sampling data on coordinates kx and ky in the phase space. That is, AD by the eco-signal of FIG.
Is the data being measured. Here, kx and ky are coordinates given by the time integral of the gradient magnetic field in the x and y directions, respectively. For example, the locus 41 in FIG. 3 shows the locus of data sampling in the phase space in the pulse sequence of the ultra-high-speed imaging shown in FIG. Here, kx is the lead-out direction (Gr), ky
Is a phase encoding method (Ge). That is,
The trajectory from the point S1 in FIG. 3 in the kx direction is Gr in FIG.
The coordinate locus due to the application of the positive magnetic field, and the locus in the next ky direction are the coordinate locus due to the application of the positive magnetic field of Ge in FIG.
The trajectory in the x direction is a coordinate trajectory by applying a negative magnetic field of Gr in FIG. 7, and the trajectory in the next ky direction is a coordinate trajectory by applying a positive magnetic field in Ge in FIG. It is a sampling trajectory in a phase space of a subject by a gradient magnetic field (Gr) and a phase encoding gradient magnetic field (Ge). Accordingly, the coordinate point at ky of the starting point S1 of the locus 41 in FIG. 3 is determined by the time integral of the applied pulse 23 in the phase-encoded gradient magnetic field (Ge) in FIG. First start point S1 and second start point S2 in FIG.
Is the difference in the applied level of the phase encoding gradient magnetic field (Ge). That is, the applied pulse 23 in FIG.
The locus of data sampling on the phase space when the time integrated value of (1) is applied by half the time integrated value of the applied pulse 24 is locus 42 in FIG. As a result, the start point S2 of the trajectory 42 is shifted to the positive side (upward side) from the start point S1 by half the step of ky (the time integral value of the applied pulse 24), and Data will be sampled. By performing image reproduction by combining the data of the trajectory 41 and the trajectory 42, the spatial resolution can be improved.

FIG. 2 is a time chart of a photographing method according to a second embodiment of the present invention. As described above, in MRI, the locus of data in the phase space is determined by the application state of the readout gradient magnetic field (Gr) and the phase encoding gradient magnetic field (Ge). As shown in FIG. 3, when the trajectory of the data in the phase space is shown, the improvement of the spatial resolution can be understood. However, as shown in FIG. 2, in the time chart (sequence chart) of the imaging method, The spatial resolution is not specified. It should be noted that also in FIG.
Actually, the first sequence is executed in synchronization with the first heartbeat signal 31, and the second sequence is actually executed in synchronization with the second heartbeat signal 31. Is executed, and a third sequence is executed in synchronization with the third heartbeat signal 31. In FIG. 2, in the imaging method shown in FIG.
2 and t3 are all equal (here, t1 = t2
= T3 = 0) In the first to third imagings, the phase encoding gradient magnetic field (G) of the pulse sequence shown in FIG.
The time integrated value of the applied pulse 23 of e) is set to be smaller or larger by 1/3 of the time integrated value of the applied pulse 24 for each heartbeat. Under these conditions, if repetitive photographing is performed in the same manner as described above, in the first to third photographings, a plurality of sets of three sets of data (1-1, 2-1,. 3-1), (1-2,
(2-2, 3-2), (1-3, 2-3, 3-3) ...
・ ・ Is obtained. Then, by combining these three sets of data and reproducing the images, a continuous image at a time interval Tr whose spatial resolution is tripled as described with reference to FIG. 3 can be obtained.

FIGS. 4 and 5 are time charts of a photographing method showing an application example of the present invention. It is clear that the imaging method of FIGS. 1 and 2 can be used to perform imaging by various combinations of the repetition time interval Tr, the number of heartbeat signals to be used, and the like. Further, by combining the above-described imaging with improved temporal resolution and the above-described imaging with improved spatial resolution, it is possible to perform imaging with both resolutions improved at the same time. FIG. 4 and FIG. 5 each show a photographing method in which both resolutions are simultaneously improved. FIG. 4 shows a photographing example over nine heartbeats. In the nine photographings based on the heartbeat signal 31, the photographing start points t1 = t4 = t7 =
0, t2 = t5 = t8 = Ts, t3 = t6 = t9 = 2T
s, Tr = 3Ts. The first to third imagings based on the heartbeat signal 31 can be performed with improved time resolution, as in the case described with reference to FIG.
Similarly, the fourth to sixth and seventh to ninth shootings can be performed with improved time resolution. In the fourth to sixth photographings and the seventh to ninth photographings, photographing is performed with the applied amount of the phase encoding gradient magnetic field shifted from the first to third photographing. This enables imaging with improved spatial resolution. Therefore, both resolutions in time and space can be improved comprehensively.

In FIG. 5, the photographing is performed over nine heartbeats as in FIG. 4, but the order is changed. That is, in imaging of 9 heartbeats based on the heartbeat signal 31,
Photographing start point t1 = t2 = t3 = 0, t4 = t5 = t6 =
Ts, t7 = t8 = t9 = 2Ts, and Tr = 3Ts. The first to third times of imaging based on the heartbeat signal 31 are performed in the same manner as described with reference to FIG. Is possible. next,
In the fourth to sixth shooting and the seventh to ninth shooting,
By performing the shadow by shifting the start time point as compared with the first to third shootings, it is possible to perform shooting with improved time resolution. Based on the data obtained by shooting in this way, (1-1,2-1,3-1), (1-2,2)
−2, 3-2), (1-3, 2-3, 3-3) ...
By performing combined image reproduction as described above, it is possible to obtain a plurality of images with improved spatial resolution and no temporal gap, so that it is possible to shoot with simultaneously improved spatial and temporal resolution. . 4 and 5
It is obvious that, besides the application example of the above, any combination of the spatial resolution enhanced imaging method and the temporal resolution enhanced imaging method can be performed to perform imaging with both resolutions improved at the same time.
Also, the ultra-high-speed shooting method was described as a shooting method,
Other imaging methods can be applied. Further, in the embodiment of the present invention, the control of the photographing conditions and the like for each heartbeat based on the heartbeat signal is all executed by the computer, but it is also possible to control the hardware by other hardware.

[0015]

As described above, according to the present invention,
By using heart rate synchronization in ultra-high-speed MRI imaging,
Since imaging with improved both time resolution and spatial resolution is possible, accuracy and image quality can be improved when imaging a moving body of the heart, and accuracy in performing cardiac function diagnosis and the like can be significantly improved. .

[0016]

[Brief description of the drawings]

FIG. 1 is a time chart of a photographing method according to a first embodiment of the present invention.

FIG. 2 is a time chart of a photographing method according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a locus of data on a phase space in FIG. 2;

FIG. 4 is a time chart of a combination photographing method showing an application example of the present invention.

FIG. 5 is a time chart of a combination photographing method showing another application example of the present invention.

FIG. 6 is a diagram showing a configuration of an MRI imaging apparatus to which the present invention is applied.

FIG. 7 is a pulse chart executed in a conventional imaging method.
This is a chart chart.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Static magnetic field magnet 2 Gradient magnetic field coil 3 Probe 4 Transmission / reception part 5 AD converter 6 Drive circuit 7 Power supply 8 Sequencer 9 Computer 10 Display device 11 Subject 12 Bed 13 Heart rate detector 21 High frequency pulse 22 Slice gradient magnetic field 23 , 24 Phase encoding gradient magnetic field 25 Lead-out gradient magnetic field 31 Heart rate signal 32 Rectangular 41, 42 Trajectory

Continuation of the front page (56) References JP-A-5-176912 (JP, A) JP-A-2-239841 (JP, A) JP-A-3-284238 (JP, A) JP-A-61-106140 (JP) , A) (58) Field surveyed (Int.Cl. ⁷ , DB name) A61B 5/055 JICST file (JOIS)

Claims (1)

(57) [Claims] A means for generating a static magnetic field; a means for generating a gradient magnetic field in three directions; a probe for applying a high-frequency pulse to the subject to receive an echo signal from the subject; A sequencer for controlling application, application of the high-frequency pulse, and capture of the echo signal, a computer for performing image reconstruction using the echo signal, display means for displaying a result of the image reconstruction, and the subject Means for detecting a heartbeat signal, wherein the sequencer selectively excites the region of interest of the subject based on the heartbeat signal, and applies a readout gradient magnetic field while inverting the polarity of the amplitude, Applying a phase encoding gradient magnetic field in a direction orthogonal to the direction in which the readout gradient magnetic field is applied, and collecting a plurality of echo signals required for image reconstruction of the region of interest Repeat the pulse sequence, set the timing of the start of the previous pulse sequence or the applied value of the phase encoding gradient magnetic field to be shifted by a predetermined value in the next pulse sequence, and collected the previous pulse sequence. Collecting the echo signals located in time or phase space of the echo signal, and for each of the heartbeat signals sequentially detected, a plurality of either one of the timing of the start of the pulse sequence or the phase encoding gradient magnetic field. Control, and the other control is repeated a plurality of times to collect the echo signals based on the previous heartbeat signal, and rearrange the collected echo signals in a temporal or phase spatial order. Characterized in that the image is reconstructed and a moving image is displayed on the display means.
I device.