JP4931456B2 - Nuclear magnetic resonance imaging system - Google Patents

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as “NMR”) signals from hydrogen, phosphorus, etc. in a subject and visualizes nuclear density distribution, relaxation time distribution, etc., and nuclear magnetic resonance imaging (MRI). More particularly, the present invention relates to an MRI apparatus that performs dynamic imaging.

  In the MRI apparatus, the sequence is repeatedly executed while changing the phase encoding amount, and an echo signal necessary for reconstruction of one image is acquired. Therefore, the number of repetitions greatly affects the image acquisition time. When performing high-speed imaging, generally, a multi-echo type sequence that generates a plurality of echo signals within one repetition or a sequence in which the repetition time interval is shortened to several to several tens of ms is used. It has been. Also, echo planar imaging (EPI) is known in which the imaging sequence itself is speeded up.

  On the other hand, a dynamic imaging method is known in which images are acquired in time series and displayed sequentially in order to image an object that changes in time series as in contrast imaging. In recent years, in order to improve display spatial resolution and temporal resolution, a dynamic imaging method in which a measurement space (k space) is divided into a plurality of regions in the phase encoding direction and a sequence for sequentially obtaining measurement data for each region is patented. It is proposed in Document 1 etc. In this imaging method, it is possible to perform image reconstruction more than the number of repetitions of the sequence by sharing the measurement data of each region acquired in time series between sequences and performing image reconstruction. Resolution and time resolution can be improved.

  Further, as described in Patent Document 2, a high-speed technique using parallel MRI is also known.

  On the other hand, in the MRI apparatus, a two-dimensional selective excitation technique or an RF shimming technique is used to remove aliasing artifacts, chemical shift artifacts, and body movement artifacts caused by signals outside the visual field. The two-dimensional selective excitation method is a method of irradiating an RF pulse while performing encoding in a measurement space during RF excitation (Non-Patent Document 1). As a method of encoding, a spiral type, an EPI type and the like are known. Encoding in the measurement space can be performed with one excitation, or can be divided into a plurality of excitations. As a simple understanding, the relationship between the shape of the two-dimensional excitation and the encoding in the measurement space may be considered as the relationship between the two-dimensional Fourier transforms.

  RF shimming refers to applying an RF pulse while spatially compensating for RF intensity when performing RF excitation in order to correct shading of an image accompanying a significant electromagnetic field penetration effect in high magnetic field MRI. It can be regarded as a deformation of two-dimensional excitation.

These two-dimensional selective excitation methods and RF shimming have the same or better effect in a shorter time and with lower SAR than in-plane pre-saturation used for general purposes. On the other hand, in order to obtain sharp selectivity, it is necessary to extend the k-space trajectory of the RF pulse to be irradiated to the high frequency region. As a result, the RF irradiation time becomes long, and there is a restriction that short TE imaging cannot be performed. For this reason, at present, it is experimentally applied to special imaging such as spectroscopic imaging and ultrahigh magnetic field imaging.
Japanese Patent No. 3283632 JP 2001-161657 A Journal of magnetic resonance, 87, 639-645, 1990, Correcting for nonuniform k-space sampling in two-dimensional NMR selective excitation, CJ Hardy, etc.

  In clinical MRI, there is a case where a region such as the heart where the shape changes every moment is imaged. In addition, there is an imaging target whose form changes according to respiratory motion, such as the liver. Furthermore, there are cases where imaging is performed over time, such as Gd contrast imaging. In such imaging, image quality deterioration occurs when signals from outside the field of view are mixed. For example, blood flow artifacts in the abdominal aorta may occur during cardiac cine imaging, or body motion artifacts associated with liver respiratory motion may be superimposed on the heart region. In addition, abdominal dynamic imaging may cause heart blood flow artifacts to be superimposed on the liver while imaging changes in the contrast of the liver. Artifacts due to these out-of-view signals may reduce the diagnostic capabilities of the image.

  As described above, in the imaging of a portion with time change, by using the above-described dynamic imaging and two-dimensional selective excitation or RF shimming together, the image quality of the dynamic imaging can be improved and the diagnostic information is expected to greatly increase. .

  However, when selective excitation or RF shimming is simply added to dynamic imaging, the repetition time (TR) becomes longer, resulting in a problem that the time resolution is lowered. This is because selective excitation or RF shimming is performed by applying an RF pulse while performing encoding in a measurement space or applying an RF pulse while spatially compensating for RF intensity during RF excitation. This is because, compared with a sequence in which space selective excitation is performed only in the direction, it takes time for RF irradiation, and it is necessary to perform RF excitation and detection a plurality of times.

  An object of the present invention is to provide an MRI apparatus that is excellent in spatial resolution and temporal resolution and that can prevent mixing of signals from outside the visual field.

  In order to achieve the above object, the following MRI apparatus is provided in the present invention. That is, a static magnetic field generation unit that generates a static magnetic field in an imaging space in which the subject is arranged, a high-frequency irradiation unit that irradiates the imaging space with a high-frequency magnetic field pulse, and a detection that acquires a nuclear magnetic resonance signal generated by the subject An MRI apparatus having a unit, a gradient magnetic field generation unit that applies a gradient magnetic field to the imaging space, a signal processing unit that performs arithmetic processing on a detection signal of the detection unit to generate an image, and a control unit that controls these signals. The control unit repeatedly irradiates two or more types of spatially encoded high-frequency magnetic field pulses, acquires a nuclear magnetic resonance signal for each irradiation of the high-frequency magnetic field pulses, and irradiates two or more different types of high-frequency magnetic field pulses. One image is generated using a combination of the acquired nuclear magnetic resonance signals. Thus, by performing two-dimensional selective excitation with two or more types of spatially encoded high-frequency magnetic field pulses, excitation with each type of high-frequency magnetic field pulse can be performed in a short time, and the obtained nuclei can be obtained. Spatial resolution can be improved by generating an image by combining magnetic resonance signals.

  As the above-described two or more types of spatially encoded high-frequency magnetic field pulses, those having different excitation intensity distributions to the subject in the imaging space can be used.

  In order to spatially encode a high-frequency magnetic field pulse, a gradient magnetic field pulse is applied together with the high-frequency magnetic field pulse, and at least one of the waveform of the high-frequency magnetic field pulse and the waveform of the gradient magnetic field pulse is changed. It is possible to have a configuration that performs encoding.

  As for the two or more types of spatially encoded high-frequency magnetic field pulses described above, the external shapes of the excitation intensity distributions on the subject in the imaging space are the same, but the polarities of the portions excited by folding are different. Can be used. Thereby, it becomes possible to mutually complement excitation intensity distribution.

  The two or more types of spatially encoded high-frequency magnetic field pulses include at least one of the steepness of the rising portion of the excitation intensity distribution to the subject in the imaging space and the polarity of the portion excited by folding. It is possible to use different ones. This makes it possible to obtain an image similar to that obtained when a high-frequency magnetic field pulse having an excitation intensity distribution with a steep rise or a different polarity of return is used. The excitation intensity distribution here means a graph showing the relationship between the excitation position and the intensity.

  In the present invention, it is possible to generate images in time series using the acquired nuclear magnetic resonance signals, thereby realizing dynamic imaging. For example, every time a nuclear magnetic resonance signal is acquired, the nuclear magnetic resonance signal acquired this time is used in combination with the nuclear magnetic resonance signal acquired by irradiating a different type of high-frequency magnetic field pulse from the previous time. It can be configured to generate an image.

  In addition, it is possible to obtain a nuclear magnetic resonance signal while regularly selecting one of two or more kinds of spatial encodings every time a high frequency magnetic field pulse is irradiated. A plurality of nuclear magnetic resonance signals can be selected in accordance with the combination of the type of high-frequency magnetic field pulse and the type of spatial encoding at the time of signal acquisition, and a single image can be generated using a combination of these. Thereby, since acquisition of a magnetic resonance signal can be performed by dividing it into two or more different spatial encodings, the degree of freedom of imaging is improved and the time resolution of image generation can be further improved.

The nuclear magnetic resonance imaging apparatus of the present invention will be described below with reference to the drawings.
(First embodiment)
First, the configuration of the MRI apparatus according to the first embodiment will be described with reference to FIG. The MRI apparatus generates a high-frequency magnetic field in the imaging space, a magnet 402 (for example, 1.5T) that generates a static magnetic field in the imaging space around the subject 401, a gradient magnetic field coil 403 that generates a gradient magnetic field in the imaging space. An RF (high frequency) coil 404 and an RF probe 405 for receiving an NMR signal generated by the subject 401 are provided. In addition, a bed 412, an RF transmission unit 410, a signal detection unit 406, a gradient magnetic field power source 409, a bed control unit 414, a signal processing unit 407, a control unit 411, and an image display unit 408 are provided.

  The gradient magnetic field coil 403 includes gradient magnetic coils in three directions of X, Y, and Z, and each generates a gradient magnetic field in response to a signal from the gradient magnetic field power supply 409. The RF coil 404 generates a high-frequency magnetic field according to the signal from the RF transmitter 410. At 1.5T, the proton resonance frequency is 63.8 MHz. The signal received by the RF probe 405 is detected by the signal detection unit 406, then signal processed by the signal processing unit 407, and further image reconstruction is performed. The reconstructed image is displayed on the image display unit 408. The gradient magnetic field power supply 409, the RF transmission unit 410, and the signal detection unit 406 are controlled by the control unit 411, and the control time chart is generally called an imaging pulse sequence.

  A part of the RF probe 405, the signal detection unit 406, and the signal processing unit 407 is shown in FIG. As shown in FIG. 2, the RF probe 405 includes an RF receiving coil 301 and a preamplifier 302 connected thereto. The signal detection unit 406 includes an AD conversion / orthogonal detection circuit 303 and receives the output of the preamplifier 302. The quadrature detection performed by the AD conversion / quadrature detection circuit 303 is preferably changed for each channel in accordance with the frequency of the imaged signal. The signal processing unit 407 includes a Fourier transform unit 304 and a calculation unit 305. The Fourier transform unit 304 performs Fourier transform, thereby reconstructing the MRI image detected by the RF receiving coil 301, and converting the image signal as necessary. Then, the calculation unit 305 performs calculation (for example, image synthesis calculation when signals are detected in parallel from a plurality of channels).

  The bed 412 is for mounting the subject 401 and arranging it in the imaging space. The bed 412 incorporates a drive unit that moves the subject 401 in the parietal-foot direction (HF direction) (arrow 413 in FIG. 1). The bed control unit 414 drives the bed 412 and moves the position of the subject 401 while matching the execution of the imaging pulse sequence. A typical moving speed is 0.5 cm / s-2.0 cm / s in the moving speed from the top of the head toward the foot.

  It should be noted that the morphological imaging target of MRI is the main constituent substance of the subject 401, proton, which is widely used clinically. By imaging the spatial distribution of the proton density and the relaxation phenomenon of the excited state, the shape of the human head, abdomen, limbs, etc., or the function of blood vessels (blood, blood flow), etc., is two-dimensional or three-dimensional. Images.

  Next, the imaging pulse sequence of the present embodiment will be described with reference to FIG. The imaging pulse sequence in FIG. 3 repeats the operation of performing the imaging sequence 20 after the two-dimensional excitation pulse train 10 at a repetition time 608. The two-dimensional excitation pulse train 10 includes a first two-dimensional excitation pulse train 10-1 and a second two-dimensional excitation pulse train having different excitation intensity distributions (excitation profiles) in the k space (two-dimensional frequency space) every repetition time 608. 10-2 are alternately performed. The imaging sequence 20 uses a gradient echo EPI sequence which is a typical imaging sequence here.

  First, the two-dimensional excitation pulse trains 10-1 and 10-2 will be described. As shown in FIG. 3, these two-dimensional excitation pulse trains 10-1 and 10-2 each include a two-dimensional selective radio frequency (RF) pulse train 501, a slice selective gradient magnetic field pulse train 602, and a gradient magnetic field pulse train 603. The RF pulse train 501 is obtained by arranging five sub-pulses 31 made up of three SINC pulses in time series. An imaginary line 32 connecting the peaks of the five subpulses 31 is another SINC function. The spatial selection range in the slice (Gs) direction is determined by the SINC function constituting the subpulse 31, and the spatial selection range in the phase encoding (Gp) direction is determined by the SINC function represented by the imaginary line 32. In the slice selective gradient magnetic field pulse train 602, the polarity of the pulse is alternately inverted every sub-pulse 31 of the RF pulse train 501. The gradient magnetic field pulse train 603 is a gradient magnetic field pulse train in the phase encoding direction, and the blip pulse 33 is applied in accordance with the inversion timing of the slice selection gradient magnetic field pulse train 602. In this way, by applying the RF pulse train 501 while applying the gradient magnetic field pulse trains 602 and 603, the position excited by the RF pulse train 501 is spatially encoded, and the excited position draws a locus. Thereby, excitation can be performed with a predetermined excitation profile.

  The last small pulse 34 in the slice selective gradient magnetic field pulse train 602 is a pulse for returning the spin upon completion of excitation to a predetermined position in the k space, and is referred to as a flip-back pulse here. A similar flip-back pulse 502 is also applied to the end of the gradient magnetic field pulse train 603. The area of both flip-back pulses 34 and 502 is the total of the pulses from the time 35 (shown by dotted lines) indicating the centers of the two-dimensional excitation pulse trains 10-1 and 10-2 to just before the application of the flip-back pulses 34 and 502. It is the same size as the area (the product of intensity and time, but taking into account the polarity), with the polarity reversed. By setting such a condition, the center position of the space trajectory (excitation profile) can be matched with a predetermined position in the k space.

  In the present embodiment, the k-space is excited along the linear trajectory 41 in which five lines are arranged at equal intervals in the Ky direction shown in FIG. 4A by the two-dimensional excitation pulse train 10-1. The flip back pulse 502 is set so that the center position of the linear locus 41 coincides with the origin of the k space.

  The order of excitation of the five linear trajectories 41 proceeds alternately in the kx direction (left and right) from negative to positive in the Ky direction (sequentially upward from the line positioned on the lower side in FIG. 4A). It can be understood that zigzag excitation scanning is performed. More specifically, the lowest trajectory 41 in FIG. 4A is excited by the first subpulse 31 of the RF pulse train 501, and the second trajectory 41 from the bottom is excited by the second subpulse 31. A top pulse 41 is excited by the subpulse 31. By the above-described flip-back pulses 34 and 502, the center of the third excitation locus 41 from the bottom coincides with the origin of the k space.

  FIG. 4A shows a k-space excitation profile 43 excited by the two-dimensional excitation pulse train 10-1 in the ky direction. This excitation profile 43 is determined by the shape of the envelope of the excitation locus 41 in the ky direction. In addition, folds appear on both sides of the excitation profile 43 in the Y direction.

  The real space region 42 excited by the two-dimensional excitation pulse train 10-1 has a shape as shown in FIG. 4B obtained by two-dimensional Fourier transform of the k-space excitation profile 43. In FIG. 4B, as an example, the excitation region 42 is circular, but a folded excitation region appears in the vicinity of the Y direction. The shape of the excitation region 42 is not limited to a circle, and can be a rectangle or an arbitrary shape by appropriately designing each pulse of the two-dimensional excitation pulse train 10-1.

  On the other hand, in the two-dimensional excitation pulse train 10-2, the magnitude of the flip-back pulse 504 of the phase encode direction gradient magnetic field pulse train 603 is set to a predetermined amount in the Ky direction from the origin of the k-space excitation locus 45 as shown in FIG. It is set to deviate. The blip pulse 33 is the same as the two-dimensional excitation pulse train 10-1. Thus, the five trajectory intervals of the excitation trajectory 45 are the same as the trajectory interval of the excitation trajectory 41, but the excitation trajectory 45 of the two-dimensional excitation pulse train 10-2 is the excitation of the two-dimensional excitation pulse train 10-1. This is a trajectory passing through a position deviated from the five lines of the trajectory 41. Here, the five lines of the excitation locus 45 pass through the intermediate positions between the five lines of the excitation locus 41, that is, the intervals between the excitation locus 45 and the excitation locus 41 are equal. Flipback pulse 504 is set.

  FIG. 5 shows an excitation profile 46 in the ky direction of the k space excited by the two-dimensional excitation pulse train 10-2. The excitation profile 46 is determined by the shape of the envelope in the ky direction of the excitation locus 45, and the excitation profile 46 of the excitation region 42 is substantially the same as the excitation profile 43 described above, but the excitation locus 45 is at the origin of the k space. Since the profile is asymmetric in the ky direction, the profile of the folded portion appearing on both sides in the Y direction of the excitation profile 46 is the same in position and shape as the profile of the folded portion of the excitation profile 43, and the polarity (positive / negative) is reversed. is doing. Therefore, when the excitation profile 41 and the excitation profile 45 are added, a shape in which the excitation profiles of the folded portions of each other cancel each other is obtained.

  Note that the real space region excited by the two-dimensional excitation pulse train 10-2 is substantially the same as the region 42 (FIG. 4B) excited by the two-dimensional excitation pulse train 10-1, except for folding.

  In the imaging sequence 20 of FIG. 3, a predetermined number of NMR signals (echo signals 606) are acquired from the regions 42 excited by the two-dimensional excitation pulse train 10-1 or 10-2. In the present embodiment, as shown in FIG. 3, as the imaging sequence 20, a series of phase encoding gradient magnetic field pulses 604 and a series of readout gradient magnetic field pulses 605 are applied, and a series of generated echo signals 606 are sequentially acquired. An EPI sequence is used. In this EPI sequence, the readout gradient magnetic field pulse 605 is repeated while being inverted, and the amount of the phase encoding gradient magnetic field pulse 604 is changed to give different phase encoding, and the echo signal 606 is detected by each encoding. In FIG. 3, time 607 is echo time (TE).

  The k-space locus of the imaging sequence 20 performed after the two-dimensional excitation pulse train 10-1 may or may not coincide with the excitation locus 41 of the two-dimensional excitation pulse train 10-1. The k-space trajectory of the imaging sequence 20 performed after the two-dimensional excitation pulse train 10-2 may or may not coincide with the excitation trajectory 45 of the two-dimensional excitation pulse train 10-2. It is desirable that the excitation FOV (field of view) and the imaging FOV (field of view) be substantially the same as in the imaging method using normal selective excitation. Also, the spatial resolution is desirably low for excitation and high for imaging, as in normal selective excitation. Therefore, the pitch of the k-space trajectory combining the two-dimensional excitation pulse trains 10-1 and 10-2 is substantially equal to the pitch of the k-space trajectory of the imaging sequence 20. The overall width of the excitation trajectory is narrower than the width of the k-space trajectory of the imaging sequence 20.

  FIG. 6 is a block diagram showing the flow of the pulse sequence shown in FIG. 3 described above. The k-space is alternately excited by different excitation profiles 43 and 46 by the two-dimensional excitation pulse trains 10-1 and 10-2, and the echo signal 606 from each excitation region 42 is acquired by the imaging sequence 20. Here, the combination (one repetition time TR) of the two-dimensional excitation pulse train 10-1 or 10-2 and the imaging sequence 20 in FIG. 6 is referred to as first to fourth imaging 71 to 74 in order. First, the echo signal 606 obtained by the imaging sequence 20 of the first imaging 71 is two-dimensionally Fourier transformed to obtain a first image, and the echo signal 606 obtained by the imaging sequence 20 of the second imaging 72 is 2 A second image is obtained by dimensional Fourier transform. In these images, the selective excitation regions 42 are substantially the same, but since the two-dimensional excitation pulse trains 10-1 and 10-2 are different, their profiles are different from each other when viewed as complex signals. In the present embodiment, different first and second images having such characteristics are complex-added to obtain a desired two-dimensional selective excitation image 109-1, as shown in FIG.

  Further, a third image is obtained from the echo signal 606 obtained by the imaging sequence 20 of the third imaging 73, and a fourth image is obtained from the echo signal 606 obtained by the imaging sequence 20 of the fourth imaging 74. The first and second images are complex-added to obtain a two-dimensional selective excitation image 109-2 as shown in FIG. Thereafter, similarly, two-dimensional selective excitation images are sequentially acquired. The obtained two-dimensional selective excitation images 109-1 and 109-2 are treated as time-series image signals and displayed on the image display unit 408. Thereby, dynamic imaging can be realized.

  Such a dynamic imaging method is suitable for, for example, two-dimensional selective excitation heart real-time imaging. Moreover, since the dynamic image is subjected to two-dimensional selective excitation, there is no mixing of artifacts from other than the selected region, and the image quality is improved. The time resolution of the dynamic image is determined by the imaging time (2 × TR) from the two-dimensional excitation pulse train 10-1 to the imaging sequence 20, the two-dimensional excitation pulse train 10-2, and the imaging sequence 20.

  As described above, in the present embodiment, two images having different excitation profiles are synthesized, so that the obtained image is a flat obtained by adding the excitation profile 43 and the excitation profile 46 of FIG. This is equivalent to an image obtained by exciting with a simple excitation profile. Therefore, while using the two-dimensional excitation pulse trains 10-1 and 10-2 that are excited by five excitation trajectories, an image similar to that obtained when excitation is performed by excitation trajectories of about 10 fine intervals as shown in FIG. 10 is obtained. be able to.

  That is, when the same image is to be obtained without being synthesized, it is necessary to double the number of excitation subpulses 31 of the excitation RF pulse 501 of the two-dimensional excitation pulse train 10-1, but in this embodiment, A similar image can be obtained with a small number of subpulses 31. Thus, the number of subpulses can be reduced to ½, so that the degree of freedom in designing the pulse sequence can be expanded. Specifically, TE607 when using the two-dimensional excitation pulse trains 10-1 and 10-2 can be shortened.

  In the first embodiment, the first to fourth images are once reconstructed from the echo signals acquired by the first to fourth imaging 71 to 74, and the first and second images are complex-added. The images 109-1, 109-2,... Are sequentially obtained by complex addition of the third and fourth images, but the present invention is not limited to this. . For example, the image 109-1 can be reconstructed using a signal obtained by complex addition of the echo signals acquired by the first and second imaging 71 and 72 as they are. In this case, the image can be reconstructed after correcting the echo signal or performing arithmetic processing in consideration of the difference in imaging conditions and the like as necessary.

  In the first embodiment, as the two-dimensional excitation pulse trains 10-1 and 10-2, the excitation profiles 43 and 46 having the same intervals between the excitation trajectories 41 and 45 and shifted in the ky direction are used. However, the present invention is not limited to this. As shown in FIGS. 7A and 7B, the two-dimensional excitation pulse trains 10-1 and 10-2 can be set so that excitation profiles 51 and 53 having different shapes can be obtained.

  As is generally known, when the excitation profile in the ky direction has a wide interval between the excitation trajectories, the excitation intensity appears close to the folded component along the Y direction. On the other hand, when the interval between the excitation trajectories is narrow, the folding component appears apart. If the excitation trajectory covers up to a high ky (high frequency) region, the excitation profile rises sharply. Conversely, if the excitation trajectory covers only a low ky (low frequency) region, the excitation profile Rise is not sharp.

  Therefore, as the two-dimensional excitation pulse train 10-1, by using the pulse train 10-1 excited by a trajectory covering a high-frequency region in the ky direction like the excitation trajectory 50 shown in FIG. 7A, FIG. The excitation profile 51 of FIG. A negative excitation intensity means that the polarity is negative. Further, as the two-dimensional excitation pulse train 10-2, a pulse train 10-2 that is excited by a locus that has a narrow interval and covers only a low-frequency region in the ky direction, such as the excitation locus 52 shown in FIG. 7B, is used. Thus, the envelope of FIG. 7B can be excited with a gentle excitation profile 53. As a result, the excitation profile obtained by adding the excitation profiles 51 and 53 is as shown in FIG. 7C. Therefore, compared with the case where only one of the excitation profiles 51 and 53 is used, the excitation intensity is reduced. It can be relatively flat in the ky direction, and the rise of the excitation profile can be sharpened. Therefore, signal mixing from the surroundings can be reduced and image quality can be improved. In addition, the effect that the number of subpulses 31 of the RF pulse train 501 can be reduced to ½ can be obtained as compared with the case where the profile of FIG. improves.

(Second Embodiment)
Next, a second embodiment of the present invention will be described with reference to FIG.
The embodiment shown in FIG. 8 is different from the first embodiment in image synthesis processing. Note that the apparatus configuration and the imaging pulse sequence are the same as those in the first embodiment, and a description thereof will be omitted.

  As shown in FIG. 8, the second embodiment is a dynamic imaging method in which the imaging pulse sequence of FIG. 3 is repeated at a constant time as in FIG. 6 of the first embodiment. 6 differs from the imaging method of FIG. 6 in that an image 701-1 is obtained using the echo signals obtained in the first and second imaging 71 and 72, and the echo signals obtained in the second and third imaging 72 and 73. Is used to obtain an image 701-2. Then, an image 701-3 is obtained using the echo signals obtained by the third and fourth imaging 73 and 74. Since the echo signals acquired by performing different two-dimensional excitation pulse trains 10-1 and 10-2 in this way are shared by the previous and subsequent imaging, it is possible to reconstruct twice as many images. The image display rate of selective excitation dynamic imaging can be doubled. That is, in the second embodiment, an image equivalent to the image composition process shown in FIG. 6 can be captured at a double repetition rate. Moreover, the image quality is the same as in the case of FIG.

  As described above, in the second embodiment, echo signals obtained by imaging are shared with previous and subsequent imaging, thereby realizing two-dimensional selective excitation dynamic imaging with high time resolution and high image resolution. be able to.

  As a method for reconstructing the image 701-1 using the echo signals obtained in the first and second imaging 71 and 72, the first and second imaging 71 are the same as in the first embodiment. , 72 after reconstructing the first and second images from the echo signals, respectively, it is possible to use a method of complex addition of the images. It is also possible to use a method of performing complex addition in the state of an echo signal, reconstructing an image after applying a predetermined correction or the like.

  In the first and second embodiments, the example in which the single shot EPI is used as the imaging sequence 20 has been described. However, a single shot FSE sequence can also be used.

(Third embodiment)
Next, a third embodiment of the present invention will be described with reference to FIG.
Unlike the first and second embodiments, the embodiment shown in FIG. 9 uses a multi-shot EPI or multi-shot FSE sequence as the imaging sequence 20. That is, the imaging sequence 20 is divided into two times, an echo signal is acquired along the first k-space trajectory by the imaging sequence 20-1, and an echo is acquired along the second k-space trajectory by the imaging sequence 20-2. Get the signal. The two-dimensional selective excitation pulse trains 10-1 and 10-2 are the same as those in the first embodiment.

  Therefore, as shown in FIG. 9, in the first and second imaging 81 and 82, the imaging pulse sequence is the first after the k-space is excited with the excitation profile 43 by the two-dimensional excitation pulse train 10-1. In the imaging 81, an echo signal is acquired for the first k-space trajectory by the imaging sequence 20-1. In the second imaging 82, an echo signal is acquired for the second k-space trajectory by the imaging sequence 20-2. In each of the third and fourth imaging 83 and 84, the k-space is excited by the excitation profile 46 by the two-dimensional excitation pulse train 10-2, and then the first k is obtained by the imaging sequence 20-1 in the third imaging 83. An echo signal is acquired for the spatial trajectory. In the fourth imaging 84, an echo signal is acquired for the second k-space trajectory by the imaging sequence 20-2. The fifth imaging 85 is performed in the same manner as the first imaging 81.

  Image synthesis is performed as follows. First, the echo signals acquired by the first and second imaging 81 and 82 are added to reconstruct the image 801. Thereby, an image 801 selectively excited by the excitation profile 43 is obtained. Next, the echo signals acquired by the third and fourth imaging 83 and 84 are added to reconstruct the image 802. As a result, an image 802 selectively excited by the excitation profile 46 is obtained. An image 811-1 is generated by adding the image 801 and the image 802. The image 811-1 is a two-dimensional selective excitation image having an image quality equivalent to that of an image excited by an excitation profile obtained by adding the excitation profiles 43 and 46 and reconstructed from an echo signal acquired by a single shot.

  Next, if an echo signal is acquired along the first k-space trajectory in the fifth imaging 85, the echo signal obtained along the second k-space trajectory in the second imaging 82 is added. Thus, the image 803 is reconstructed. Thereby, the echo signal acquired by the past second imaging 82 is shared with the echo signal at the time of imaging the fifth imaging 85, thereby reconstructing the image selectively excited by the excitation profile 43. it can. The obtained image 803 can be added to the image 802 obtained by the third and fourth imaging to obtain a two-dimensional selective excitation image 811-2. By sequentially displaying the obtained images 811-1 and 811-2, two-dimensional selective excitation dynamic imaging can be performed.

  As described above, by using the third embodiment, not only two types of two-dimensional selective excitation pulse trains 10-1 and 10-2 but also two types of imaging sequences 20-1 and 20-2 are applied to the imaging sequence. Can be used to perform two-dimensional selective excitation dynamic imaging.

  In the third embodiment, the first and second k-space trajectories for acquiring echo signals in the two types of imaging sequences 20-1 and 20-2, for example, divide the k-space into two regions, and The first region can be arranged so that the first locus covers the second region, and the second region can be arranged so that the second locus can cover the second region. Further, as shown in FIG. 7, the low frequency region of the k space may be covered with the first trajectory, and the high frequency region may be covered with the second trajectory. It is also possible to arrange the second k-space trajectory so as to sew between the first k-space trajectories. The shape of the locus is not limited to a linear locus, and may be a radial scan or a spiral scan.

  In addition, the imaging sequences 20-1 and 20-2 are not limited to two-dimensional measurement, and can of course be applied to three-dimensional measurement.

  In the imaging pulse sequence of FIG. 3 used in each embodiment described above, an example using a two-dimensional excitation pulse train and a gradient echo (GrE) EPI sequence has been described. However, the present invention is not limited to this. The present invention can be applied to various excitation pulses and imaging methods. The present invention can also be applied to a high-speed imaging sequence in which an inversion pulse and a saturation pulse are added to each segment of the segmented GrE sequence. Specifically, an inversion pulse or a saturation pulse can be used as a two-dimensional selective excitation pulse, which can be used as the pulse trains 10-1 and 10-2 in FIGS. Further, as an imaging sequence, an FSE (Fast Spin Echo) dynamic sequence for obtaining coronary artery imaging or general-purpose T2W images can be used.

  Furthermore, the present invention can be applied not only to the case of spatial selection, such as the two-dimensional selective excitation pulses 10-1 and 10-2, but also to the excitation frequency selection sequence.

  Examples of an imaging region where the imaging method of the present invention can be suitably used include head / neck / chest 3D imaging, lower limb imaging, and the like.

  FIG. 11 shows an MRI apparatus suitable for applying the present invention to a moving bed imaging method for imaging while moving the bed 412 in FIG. Typically, the body axis direction is thin, and imaging is performed while the bed moves as in the X-ray CT apparatus. The imaging region is about 64 mm in the body axis direction as an example, and the slice thickness is 1 mm to 2 mm by three-dimensional imaging. When the two-dimensional excitation is continuously performed while moving the bed in this way, there is an advantage that the imaging time is significantly shortened by applying the present invention. In such a continuous movement type MRI apparatus, the region of two-dimensional excitation may be spatially shifted as the bed 412 moves.

  As described above, according to the present invention, it is possible to provide an MRI apparatus that is excellent in spatial resolution and temporal resolution and that can prevent mixing of signals from outside the visual field.

1 is a block diagram showing an overall configuration of an MRI apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram showing a partial configuration of the MRI apparatus of FIG. 1. Explanatory drawing which shows the imaging pulse sequence performed with the MRI apparatus of 1st Embodiment. (A) Explanatory drawing which shows the k-space excitation locus | trajectory 41 and its excitation profile 43 of the two-dimensional excitation pulse train 10-1 of 1st Embodiment, (b) The real space excitation area | region 42 corresponding to the excitation profile 43 is shown. Illustration. Explanatory drawing which shows the relationship between the excitation traces 41 and 45 of the k space of the two-dimensional excitation pulse trains 10-1 and 10-2 of the first embodiment, and the excitation profiles 43 and. FIG. 3 is a block diagram for explaining imaging pulse sequence and image synthesis according to the first embodiment. (A) and (b) Examples using the two-dimensional excitation pulse trains 10-1 and 10-2 of the first embodiment that show k-space excitation trajectories 50 and 52 and excitation profiles 51 and 53 having different shapes. (C) Explanatory drawing which shows the case where the excitation profiles 51 and 53 are added. The block diagram explaining the imaging pulse sequence and image synthesis of the second embodiment. The block diagram explaining the imaging pulse sequence of 3rd Embodiment, and the synthesis | combination of an image. Explanatory drawing which shows the case where k space is excited along 10 conventional excitation locus | trajectories. Explanatory drawing which shows an MRI apparatus suitable when applying this invention to a moving bed imaging method.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10-1, 10-2 ... Two-dimensional excitation pulse train, 20 ... Imaging pulse sequence, 31 ... Sub pulse, 34 ... Flipback pulse, 41 ... Excitation locus, 42 ... Real space excitation area, 43 ... Excitation profile, 45 ... Excitation locus 45 ... excitation profile, 50 ... excitation trajectory, 51 ... excitation profile, 52 ... excitation trajectory, 53 ... excitation profile, 71-74 ... first to fourth imaging, 401 ... subject, 402, magnet, 403 ... tilt Magnetic field coil, 404 ... RF coil, 405 ... RF probe, 406 ... Signal detection unit, 407 ... Signal processing unit, 408 ... Image display unit, 409 ... Gradient magnetic field power supply, 410 ... RF transmission unit, 411 ... Control unit, 412 ... Bed, 501 ... RF pulse train, 502 ... Flipback pulse.

Claims (10)

A static magnetic field generation unit that generates a static magnetic field in an imaging space in which the subject is arranged, a high-frequency irradiation unit that irradiates the imaging space with a high-frequency magnetic field pulse, and a detection unit that acquires a nuclear magnetic resonance signal generated by the subject A gradient magnetic field generation unit that applies a gradient magnetic field to the imaging space, a signal processing unit that arithmetically processes a detection signal of the detection unit to generate an image, the high-frequency irradiation unit, the detection unit, and the gradient magnetic field generation unit And a control unit for controlling the signal processing unit,
The control unit generates one image using a combination of the nuclear magnetic resonance signals acquired by sequentially irradiating the spatially encoded high-frequency magnetic field pulses with two or more types ,
The two or more types of the spatially encoded high-frequency magnetic field pulses have the same external shape of the excitation intensity distribution to the subject in the imaging space, but the polarities of the portions excited by folding are different. A nuclear magnetic resonance imaging apparatus. A static magnetic field generation unit that generates a static magnetic field in an imaging space in which the subject is arranged, a high-frequency irradiation unit that irradiates the imaging space with a high-frequency magnetic field pulse, and a detection unit that acquires a nuclear magnetic resonance signal generated by the subject A gradient magnetic field generation unit that applies a gradient magnetic field to the imaging space, a signal processing unit that arithmetically processes a detection signal of the detection unit to generate an image, the high-frequency irradiation unit, the detection unit, and the gradient magnetic field generation unit And a control unit for controlling the signal processing unit,
The controller is
Two or more types of spatially encoded high-frequency magnetic field pulses are irradiated in order, and each time the irradiation is performed, one of the two or more types of spatial encoding is regularly selected and performed to acquire a nuclear magnetic resonance signal. ,
A plurality of the nuclear magnetic resonance signals are selected in accordance with a combination of the type of the high-frequency magnetic field pulse and the type of spatial encoding at the time of signal acquisition, and one image is generated using a combination thereof. Nuclear magnetic resonance imaging device.   3. The nuclear magnetic resonance imaging apparatus according to claim 1, wherein the spatially encoded high-frequency magnetic field pulse is a two-dimensional selective excitation pulse train. 4.   The nuclear magnetic resonance imaging apparatus according to any one of claims 1 to 3, wherein the spatially encoded high-frequency magnetic field pulse includes a plurality of sub-pulses composed of SINC pulses arranged in a time series.
  The control unit applies a slice selective gradient magnetic field pulse train while applying the high frequency magnetic field pulse, and the slice selective gradient magnetic field pulse is a gradient magnetic field pulse train whose polarity is alternately inverted for each sub-pulse of the high frequency magnetic field pulse. A nuclear magnetic resonance imaging apparatus. 5. The nuclear magnetic resonance imaging apparatus according to claim 4, wherein the two or more types of spatially encoded high-frequency magnetic field pulses have different k-space excitation trajectories. The nuclear magnetic resonance imaging apparatus according to claim 2, wherein the nuclear magnetic resonance imaging apparatus acquires the nuclear magnetic resonance signal along a different k-space trajectory for each type of spatial encoding at the time of signal acquisition. . 3. The nuclear magnetic resonance imaging apparatus according to claim 2 , wherein two or more types of the spatially encoded high-frequency magnetic field pulses have different excitation intensity distributions to the subject in the imaging space. Resonance imaging device. 3. The nuclear magnetic resonance imaging apparatus according to claim 1 , wherein two or more types of the spatially encoded high-frequency magnetic field pulses include steepness and folding of a rising portion of an excitation intensity distribution to the subject in the imaging space. A nuclear magnetic resonance imaging apparatus, wherein at least one of the polarities of the excited portions is different from each other. 3. The nuclear magnetic resonance imaging apparatus according to claim 1 , wherein the control unit generates an image in time series using the acquired nuclear magnetic resonance signal. 10. The nuclear magnetic resonance imaging apparatus according to claim 9 , wherein each time the control unit acquires the nuclear magnetic resonance signal, the control unit acquires the nuclear magnetic resonance signal acquired this time and the high-frequency magnetic field of a type different from the current time before the previous time. A nuclear magnetic resonance imaging apparatus characterized in that an image is generated using a combination with the nuclear magnetic resonance signal acquired by irradiating a pulse.

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