JP3452400B2 - Magnetic resonance imaging equipment - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to nuclear magnetic resonance (hereinafter referred to as N
Magnetic resonance imaging (hereinafter referred to as MRI apparatus) for obtaining a tomographic image of a desired portion of a subject using MR)
In particular, the present invention relates to an MRI apparatus having a blood flow drawing means using a pulse sequence called a phase contrast method (hereinafter referred to as a PC method) or a phase sensitive method (hereinafter referred to as a PS method).

[0002]

2. Description of the Related Art In an MRI apparatus, a high frequency magnetic field (high frequency pulse, hereinafter referred to as RF
Pulse) to excite nuclear spins (hereinafter simply referred to as spins) of the atoms constituting the tissue in the region,
Although the NMR signal from the spin is measured as an echo signal, the blood flow can be visualized by utilizing the difference between the spin of the stationary tissue and the spin moving like blood flow. As such a blood flow visualization method, a time of flight (TOF) method that uses the inflow effect of blood flow spins that newly flow into the slice plane, and a difference using the presence or absence of spin phase diffusion due to blood flow PS to do
Three types of methods are mainly used: a (Phase Sensitive) method and a PC (Phase Contrast) method that inverts the polarity of phase diffusion due to blood flow and performs a difference.

Of these, the PS method will be briefly described.
In the MRI apparatus, NMR from spins as described above
The signal is measured as an echo signal. At this time, in order to add position information to the echo signal, a gradient magnetic field in the phase encoding direction, the frequency encoding direction, or the like is applied from the spin excitation to the echo signal measurement. By applying these gradient magnetic fields, the excited spins undergo phase rotation depending on the position and the moving speed. That is, FIG.
As shown in (a) and (b), for example, at time t ₁ , two spins S ₁ and S ₂ exist at the position of X ₀ , one spin S ₁ is stationary, and the other spin S ₁ is stationary. Assuming that S ₂ is moving in the X direction at the velocity v, as shown in FIG. 8B, by applying the gradient magnetic field Gx in the frequency encoding direction from time t ₁ to t ₂ , the following equations are given respectively. Receives phase changes Φs and Φf.

[0004]

[Equation 1]

[0005]

[Equation 2]

From equations (1) and (2)

[0007]

[Equation 3]

[0008] From this equation (3), it is understood that the phase difference between the stationary spin S ₁ and the moving spin S ₂ is proportional to the moving speed v. The PS method utilizes such a difference in the phase diffusion between the stationary spin and the moving spin, and an image measured in the state with the phase diffusion of the moving spin and an image measured in the state without the phase diffusion of the moving spin. The difference is taken, so the pulse sequence for the two imagings is performed.

FIG. 9 is a diagram showing one of them, the standard spin echo method, and the difference in behavior between the stationary spin and the moving spin in this sequence. FIG. 9A shows RF pulses (90 degree pulse and 180 degree pulse). Degree pulse) and measurement of the echo signal E are shown, and (b) shows application of the gradient magnetic field Gx. Further, (c) to (e) show the spatial position change of the spin, the phase rotation speed, and the phase change from the initial excitation (90-degree pulse application) to the measurement of the echo signal E, respectively. Stationary spins are shown as dashed curves, and moving spins are shown as solid curves. In this standard spin echo sequence, when the echo signal E is measured, the stationary spin and the moving spin do not have the same phase, as shown in FIG.

The other pulse sequence in the PS method is shown in FIG. In the sequence shown in FIG.
The gradient magnetic fields A and B in the negative direction are added to the gradient magnetic field in the standard spin echo sequence of (1) as shown in (b). The negative spin gradient magnetic fields A and B cause the moving spins to undergo a negative phase change in advance. Therefore, when the echo signal E is measured, the phases of the echo signal E are aligned as shown in FIG. Become. The pulse sequence shown in FIG. 9 is called a phase sensitive type or a dephase sequence, and the pulse sequence shown in FIG. 10 is a phase insensitive type.
e) or Rephase sequence. In the rephase sequence, a signal with the same intensity as the signal intensity obtained in the dephase sequence is obtained for the stationary part, and the signal loss due to phase diffusion is suppressed in the region where the moving magnetization exists, and a higher signal than the dephase sequence is obtained. .

Therefore, as shown in FIG. 11, the difference image I ₃ is obtained by taking the difference between the dephase image I ₁ measured in the dephase sequence and the rephase image I ₂ measured in the rephase sequence to obtain the still image. The portion 22 is erased and only the moving portion, such as the blood flow in the blood vessel 21, can be imaged. For a method of obtaining a blood vessel image from the difference between images obtained by such a dephase sequence and a rephase sequence, see “Cerebral
"MR Angioimaging (Cerebral Vessel Magnetic Resonance Imaging) -First Report-" (Keiji Fukui et al., CT Study 10 (2) 1988
Years, pp. 133-142).

Next, the PC method will be briefly described. As described above, the spin in the bloodstream undergoes phase rotation due to the application of the gradient magnetic field. Therefore, when a pair of positive and negative gradient magnetic field pulses Gx as shown in FIG. 12 is applied, the spins in the blood flow undergo phase rotation according to the flow velocity, but depending on whether the positive or negative gradient magnetic field pulse is applied first, The phase rotation is Φf- or Φf +. At this time, the spin phase Φs of the stationary part
Is 0 in any case. This pair of positive and negative gradient magnetic field pulses is called a flow encode pulse, and the flow encode pulse (solid line) in which the negative gradient magnetic field pulse is applied first.
Is the positive flow encode pulse, and the flow encode pulse (dotted line) to which the positive gradient magnetic field pulse is applied first is the negative flow encode pulse. If the polarity of the flow encode pulse is inverted as described above, the polarity of the phase rotation is also inverted. Therefore, if the complex difference of the signals obtained by alternately applying these is taken, the signal in the stationary portion that is not subjected to the phase rotation is removed. Then, only the blood flow signal is detected. The obtained signal intensity changes depending on the flow velocity, and the signal intensity becomes maximum when the flow velocity is such that the phase difference between Φf− and Φf + is π.
Therefore, if a sequence having a positive flow encode pulse and a sequence having a negative flow encode pulse are operated on one arbitrary axis of the Cartesian coordinate system and the difference calculation of the acquired data is performed, the blood flowing along the one axis direction is obtained. The flow can be extracted. By performing this on all three axes, that is, by operating six sequences, blood flow in all directions can be extracted. For this PC method, see “Magnetic Resonance Angiography, Dumo
ulin CL, et al. Radiology 161: 717-720, 1986 ”.

By the way, the above-mentioned PS method and PC method perform blood flow visualization by the difference between images obtained by two or more types of pulse sequences, and between the execution of one pulse sequence and the other pulse sequence, If the position of the subject moves or the signal intensity other than the blood flow changes, the difference cannot be completely removed, which hinders the visualization of only the blood flow. However, in the case of three-dimensional measurement, execution of each pulse sequence requires time Ts = (repetition time) x (number of slice encodes) x (number of phase encodes) x (number of additions). If the above is executed separately, there is a high possibility that the difference will be incomplete due to the body movement of the subject. Therefore, for example, in the case of the PS method, conventionally, the rephase / dephase sequence is alternately operated for each slice encode step and each phase encode step.

On the other hand, in order to prevent the DC offset generated by the bias of the preamplifier of the receiving system from occurring as an artifact in the center of the image, the RF pulse is excited +/- alternately for each phase encoding. RF
By alternately irradiating the pulses with the polarities of +/-, the DC offset component is captured as DC in the phase direction of the k space, and the DC data of the echo signal is captured as the highest frequency. It is possible to prevent the influence of DC offset.

In this way, the rephase / dephase sequence is alternately operated and the RF pulse is alternately +/-.
Since the irradiation is performed with the polarity of, the actual PS method and PC method are shown in Fig. 1.
It had the structure shown in FIG. FIG. 13 is a timing diagram schematically showing the echo signal measurement process of both the rephase / dephase sequence in imaging by the PS method.

In FIG. 13, (1) shows the timing of the entire measurement sequence, and (2) shows a part of it in detail. In (2), (a) shows phase encoding, (b) shows slice encoding, (c) shows the state of the sequence, (d) shows excitation by RF pulse,
(E) application of a gradient magnetic field in the frequency encoding direction,
(F) shows the echo signal, and (g) shows the timing of the echo signal measurement. The phase encode step is from the first to the kth, and the slice encode step is from the first to the kth.
There shall be up to the mth. In this three-dimensional PS method, with a short repetition time TR (about several tens of ms), first, in the first slice encoding step of the first phase encoding step, the rephase / dephase sequence is continuously operated, and both sequences are operated. While alternately operating, the first to mth slice encoding steps are updated. The rephase sequence is R in FIG.
The dephase sequence is indicated by D.

The collected data becomes digital data via the signal processing system 6 and is stored in the memory of the CPU 8. After the completion of all data collection for the first phase encode, the phase encode step is incremented by one step, and the rephase / dephase data collection is performed again. In this way, when the data collection up to the final kth phase encoding is completed, the CPU 8 performs Fourier transform in the slice direction on each three-dimensional data, and re-phase data and dephase data for each corresponding slice. Complex difference calculation is performed to create three-dimensional data containing only blood flow signals. This three-dimensional data is shown in Figure 1.
By the projection processing shown in FIG. 4, it is converted into an arbitrary two-dimensional projection image.

[0018]

Generally, in the excitation by RF pulse, when the excitation is performed with a repetition time sufficiently shorter than the longitudinal relaxation time T ₁ of the tissue, the spin state immediately before each excitation is not in a thermal equilibrium state, and What kind of excitation was received up to now becomes a problem. For example, when continuously excited by RF pulses of the same polarity, the spin state becomes a steady state, and the echo signal obtained by these excitations also has a substantially constant signal intensity. Further, even when the polarity of the RF pulse is alternately inverted, the spin always receives the same history, so that an echo signal having a constant signal intensity can be obtained.

However, the rephase /
In the dephase sequence, for example, in the case of the PS method, the RF pulse of the rephase sequence has an opposite polarity to the RF pulse of the immediately preceding dephase sequence as shown in FIG. 13, and the dephasing sequence has the same polarity as the immediately preceding rephase sequence. It becomes polar. In this way, in the rephase sequence and the dephase sequence, the immediately preceding RF
Since the polarities of the pulses are different and the state immediately before each excitation is different, a steady state cannot be realized, and the signal intensities obtained in both sequences also differ. It is desirable to measure both sequences separately twice in order to perform a fairly fair measurement, but as already mentioned, it is possible to obtain a complete difference due to the influence of the body movement of the subject between the measurements. Difficult to do.

Therefore, according to the present invention, in a method of rendering a blood flow by using a difference between a plurality of measurement images, stationary tissue such as collapse of a steady state of spins during continuous application of RF pulse or positional change between sequences due to body motion is detected. MRI that removes the cause of incomplete image removal and can visualize a good differential blood flow image
The purpose is to provide a device.

[0021]

An MRI apparatus of the present invention that achieves the above object executes a plurality of (n) pulse sequence groups having different gradient magnetic field application patterns to generate n sets of three-dimensional data sets. In an MRI apparatus that executes a blood flow visualization pulse sequence that is acquired, removes signals of stationary tissue in a subject by calculation between these data sets, and extracts a three-dimensional data set containing only blood flow signals,
Repeat sequence of pulse sequence from inside 1)
Slice encoding, 2) sequence type, and 3) phase encoding. That is, each of the pulse sequence groups comprises a slice encoding step, in a first phase encoding step collecting data for all steps of slice encoding in a first sequence and subsequently in a second phase encoding step in a second phase encoding step. Also in the sequence, data is collected for all steps of slice encoding, and similarly, data for the first phase encoding step is collected for all sequences. When the first phase encoding step is completed, the phase encoding step is incremented by one step, and for the second phase encoding step, data for all slice encoding steps for all sequence types is acquired. In this way, n sets of three-dimensional measurement data sets are obtained by acquiring data for all steps of phase encoding. The signal processing system removes the signal of the stationary tissue in the subject by the calculation between the three-dimensional measurement data sets, and extracts the three-dimensional data set containing only the blood flow signal.

At this time, the polarities of the high frequency pulses are alternately inverted and irradiated at least within the same phase encoding step. Then, preferably, when the data is captured in the receiving system, the phase of the data capturing system is alternately set to 0 / π for each phase encoding. As a result, the polarity of the high-frequency pulse (hereinafter, simply referred to as RF) application remains the same for each phase encoat, and the polarity of the acquired data is set to y in the k space.
The DC offset is removed by alternating +/- in the direction.

The present invention can be applied to either an MRI apparatus adopting the PS method or a MRI apparatus adopting the PC method as a blood flow depiction pulse sequence. In the PS method, the pulse sequence group constitutes blood flow. A dephasing sequence for measuring an echo signal in the state where phase diffusion occurs in the nucleus of the atom to be applied, and a gradient magnetic field for converging the phase diffusion to the nucleus of the atom constituting the blood flow during the echo signal measurement. Two types of phase sequence (n
= 2) and obtain two 3D data sets.

In the PC method, the pulse sequence group can include a pulse sequence pair that applies positive and negative flow encode pulses to at least one axis and up to three axes of the Cartesian coordinate system. In this case, one axis can be used. Two three-dimensional data sets are acquired, two for two, four for two axes, and six for three axes. In another aspect of the PC method, the pulse sequence group includes a pulse sequence for applying a positive flow encode pulse for three axes of the Cartesian coordinate system, a negative flow encode pulse for one axis and a positive flow encode pulse for the remaining two axes. And a pulse sequence for applying 2 to 3 three-dimensional data sets.

[0025]

The sequencer of the MRI apparatus activates the blood flow drawing sequence having the above-mentioned configuration, and irradiates the RF pulse alternately with at least +/- polarity in the same phase encoding step. As a result, in each sequence, the spins are equally excited without being influenced by the measurement order, and the data can be acquired while maintaining the steady state.

In order to remove the DC offset,
It is necessary to invert the polarity of the RF pulse at each phase encoding step, but this disturbs the steady state between adjacent phase encoding steps, so preferably the phase of the data acquisition phase is inverted at each phase encoding. By performing the above, it is possible to remove the DC offset while maintaining the state where the irradiation of the RF pulse is +/− alternately, that is, the steady state. That is, since the DC offset component is taken in as DC in the phase direction of the k space, and the DC data of the echo signal is taken in as the highest frequency, it is possible to prevent artifacts due to DC offset.

[0027]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 7 is a block diagram showing the overall configuration showing an embodiment of the MRI apparatus of the present invention. This MRI device
A tomographic image of the subject 1 is obtained by using a magnetic resonance phenomenon. A static magnetic field generating magnet 2 having a sufficiently large spread around the subject 1, a gradient magnetic field generating system 3, a transmitting system 4, and a receiving system. A system 5, a signal processing system 6, a sequencer 7, and a central processing unit (hereinafter referred to as CPU) 8 are provided. The sequencer 7 operates under the control of the CPU 8 and sends various commands necessary for data acquisition of a tomographic image of the subject to the transmission system 4, the gradient magnetic field generation system 3 and the reception system 5, and together with the CPU 8 sets inspection conditions. It functions as a control means for controlling.

The static magnetic field generating magnet 2 generates a uniform magnetic flux around the subject 1 in the body axis direction or in the direction orthogonal to the body axis, and is placed in a space with a certain extent around the subject 1. Permanent magnet type, normal conducting type or superconducting type magnetic field generating means is arranged. The gradient magnetic field generation system 3
It is composed of a gradient magnetic field coil 9 wound in three directions of X, Y and Z and a gradient magnetic field power source 10 for driving each coil,
By driving each gradient magnetic field power source 10 in accordance with a command from the sequencer 7, a gradient magnetic field G in three directions of X, Y and Z is obtained.
x, Gy, and Gz are applied to the subject 1. The slice plane for the subject 1 can be set by the method of applying the gradient magnetic field.

The transmission system 4 includes a high frequency oscillator 11 and a modulator 1.
2, a high-frequency amplifier 13, and a transmission-side high-frequency coil 14a. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 according to the instruction of the sequencer 7, and the amplitude-modulated high-frequency pulse is generated by the high-frequency amplifier 1.
3 is supplied to the high-frequency coil 14a arranged in the vicinity of the subject 1 after being amplified by the electromagnetic wave 3
It is designed to be illuminated.

The receiving system 5 comprises a receiving side high frequency coil 14b, an amplifier 15, a quadrature phase detector 16 and an A / D converter 17, and responds to the subject by electromagnetic waves emitted from the transmitting side high frequency coil 14a. An electromagnetic wave (NMR signal) is detected by the high-frequency coil 14b arranged close to the subject 1, and the A / D is detected via the amplifier 15 and the quadrature detector 16.
It is input to the converter 17 and converted into a digital amount. On this occasion,
The A / D converter 17 samples the two series of signals output from the quadrature phase detector 16 at the timing according to the command from the sequencer 7, and outputs the two series of digital data. Those digital signals are sent to the signal processing system 6 and are Fourier transformed.

The signal processing system 6 comprises a CPU 8, a recording device such as a magnetic disk 18 and an optical disk 19, and a display 20 such as a CRT. The CPU 8 uses a digital signal to perform Fourier transform, correction coefficient calculation, image reconstruction, etc. Processing is performed, and a signal intensity distribution of an arbitrary section or a distribution obtained by performing an appropriate calculation on a plurality of signals is imaged and displayed as a tomographic image on the display 20.

In FIG. 7, the high frequency coils 14a and 14b on the transmitting side and the receiving side and the gradient magnetic field coil 9 are arranged in the magnetic field space of the static magnetic field generating magnet 2 arranged in the space around the subject 1. ing. Here, in the present invention, the sequencer 7 activates the three-dimensional blood vessel delineation pulse sequence.

FIG. 1 is a timing diagram showing an example of an image pickup sequence by the PS method, and (1) shows the whole thereof.
(2) is an enlarged view of a part. This sequence consists of phase encoding steps from 1 to k, and each phase encoding step includes a rephase sequence (hereinafter, R sequence) and a dephase sequence (hereinafter, D sequence). Each D sequence consists of 1 to m slice encoding steps. In addition, in FIG.
(A) is phase encoding, (b) is sequence type,
(C) shows slice encoding, and (d) shows R.
Excitation by F pulse, (e) is an application pattern of a gradient magnetic field in the frequency encoding direction, (f) is an echo signal, (g)
Indicates the timing of echo signal measurement.

In such a sequence structure, first, the R sequence and the D sequence are sequentially executed in the first phase encoding, but all slice encoding steps are executed in each sequence. That is, in the R sequence, RF pulse irradiation, gradient magnetic field application in the frequency encoding direction for rephase, and echo signal measurement are performed, and while slice encoding is incremented by one, several tens of steps from RF pulse irradiation to echo signal measurement are performed. Repeated at a short repetition time of about ms, mth
Measurement of m echo signals is performed up to the slice encode step.

Similarly, in the D sequence, RF pulse irradiation, application of a gradient magnetic field in the frequency encoding direction for dephasing, and measurement of echo signals are performed, and the slice encoding is incremented by 1 until the m-th slice encoding step. Measures individual echo signals. At this time, the RF pulse is emitted by inverting the polarity alternately for each irradiation. Therefore, the spins are always excited in the same spin state (steady state), and all echo signals obtained in the same phase encoding step have the same signal intensity. At this time, the echo signal samples the two series of signals output from the quadrature phase detector 16 at the timing according to the instruction from the sequencer 7 in the A / D converter 17, as described above. Perform at 0 radians.

When the first phase encoding step is completed in this way, the phase encoding is then incremented by 1, and the second phase encoding step is executed. In this case as well, similar to the first phase encode step, the R sequence is performed while updating the slice encode in the first to mth slice encode steps, and then the D sequence is performed to measure 2m echo signals. At this time, as can be seen from FIG. 1 (1), when the RF pulse is irradiated so that the polarities are alternately inverted, the RF pulse is applied in the first phase encoding step and the second phase encoding step.
The pulse polarities are the same (+ / +). In this case, if the data is acquired in the same phase, the DC offset component cannot be erased. Therefore, in the second phase encoding step, the quadrature detector 1
When the signal in 6 is acquired, the data is acquired in phase π radians. By shifting the phase of acquisition by π radians in this way, the DC offset component is acquired as DC in the phase direction of the k space, and the data part is acquired as the highest frequency.

In the same manner, the third, fourth to kth phase encoding steps are executed in the same manner to collect 2 × (m × k) data sets. At this time, the echo signal is taken in 0 radians in the odd-numbered phase encoding steps and π radians in the even-numbered phase encoding steps. When data collection up to the final kth phase encoding is completed,
In the CPU 8, for each three-dimensional data, Fourier transform in the slice direction is performed for each of the rephase data and the dephase data, and the complex difference calculation of the rephase data and the dephase data is performed for each corresponding slice to obtain only the blood flow signal. Create three-dimensional data. This three-dimensional data is
By the projection processing shown in FIG. 14, it is converted into an arbitrary two-dimensional projection image.

In this case, since the RF pulse is irradiated with the polarity inverted every time as described above, a signal having the same signal strength is always obtained, and the phase of signal acquisition is inverted at each phase encoding step. Therefore, it is possible to eliminate the influence of the DC offset, and it is possible to obtain an image having no DC component artifact at the center of the image.

Next, a sequence according to the PC method will be described as a second embodiment of the present invention. FIG. 2 is a timing diagram showing an example of an imaging sequence by the PC method.
(1) shows the whole, and (2) shows a partly enlarged. This sequence is similar to the PS method shown in FIG.
A sequence for acquiring data for one axis in the slice encoding direction (hereinafter, referred to as S axis) as a pulse sequence group (hereinafter, referred to as S axis sequence) in each phase encoding step. ) And one axis in the phase encoding direction (hereinafter,
It includes a sequence for obtaining data on the P-axis (hereinafter referred to as P-axis sequence) and a sequence for obtaining data on one axis in the frequency encoding direction (hereinafter referred to as F-axis sequence). Each of the S-axis sequence, the P-axis sequence and the F-axis sequence is paired with a sequence of applying a positive polarity flow encode pulse and a sequence of applying a negative polarity flow encode pulse, and each pulse sequence group further includes It consists of slice encoding steps from 1 to m.

In FIG. 2B, only the flow encode pulse is shown as the gradient magnetic field pattern for simplification, but the gradient magnetic field pattern in each pulse sequence group is the pattern of the PC method as shown in FIG. There is. However, in FIG. 3, the positive flow encode pulse and the negative flow encode pulse are shown only once in each axis (S axis, P axis, F axis) sequence, but in the present invention, The sequence of applying the positive flow encode pulse and the sequence of applying the negative flow encode pulse with respect to the axis include all the slice encode steps, and are repeated m times while incrementing the slice encode by one. At this time, the RF pulse is applied with its polarity reversed every time.

In such a sequence configuration, first, a sequence of applying a positive flow encode pulse of the S-axis sequence in the first phase encoding, a sequence of applying a negative flow encode pulse, and a positive polarity of the P-axis sequence are used. A sequence of applying a flow encode pulse, a sequence of applying a negative flow encode pulse, a sequence of applying a positive flow encode pulse of the F-axis sequence, and a sequence of applying a negative flow encode pulse are sequentially executed.
Six sets of data are obtained, each consisting of m echo signals.

When the first phase encoding step is completed, the phase encoding is sequentially incremented by 1, and the second,
Performing the third phase encoding step,
In this case, in the odd-numbered phase encoding step, the signal acquisition is performed at 0 radian, and in the even-numbered phase encoding step, the signal acquisition is performed at π radian. In this way, by shifting the phase of acquisition by π radians between the odd-numbered phase encoding step and the even-numbered phase encoding step, the RF pulse is inverted in polarity for each irradiation, and the adjacent pulse is encoded. Even if the polarities of the first RF pulse irradiation in the step have the same polarity, the DC offset component is taken as DC in the phase direction of the k space, and the data part is taken as the highest frequency.

In this way, the phase encoding steps up to the kth are executed, and 6 × (m × k) data sets are collected. When the collection of the data up to the final kth phase encoding is completed, the CPU 8
Fourier transform in the slice direction is performed for each data of the positive polarity and negative polarity flow encode pulse application sequence of each axis sequence, and the complex difference calculation of the positive polarity flow encode data and the negative polarity flow encodes data for each axis for each corresponding slice. Then, data of only the blood flow signal is created for each axis. A three-dimensional blood vessel image can be obtained by vector-synthesizing the data of each axis.

FIG. 4 is a timing diagram showing another embodiment of the image pickup sequence by the PC method. (1) shows the whole and (2) shows a partly enlarged view. In this sequence, as a pulse sequence group included in one phase encoding step, a reference sequence, a sequence for acquiring S-axis data (S-axis sequence), a sequence for acquiring P-axis data (P-axis sequence) ) And a sequence for acquiring F-axis data (F-axis sequence).

Each pulse sequence group has a gradient magnetic field pattern as shown in FIG. 5, and the reference sequence is a sequence in which positive flow encode pulses are added to the three axes of S axis, P axis and F axis, S axis. Sequence adds negative polarity flow encode pulse only to S-axis,
A sequence in which a positive flow encode pulse is added to the other two axes, a negative flow encode pulse is added only to the P axis in the P-axis sequence, and a positive flow encode pulse is added to the other two axes. The sequence and the F-axis sequence are sequences in which a negative flow encode pulse is added only to the F axis and a positive flow encode pulse is added to the other two axes.

Although FIG. 5 shows only one step for each sequence of the pulse sequence group,
In the present invention, each sequence comprises 1 to m slice encoding steps. That is, each sequence is repeated m times while the slice encode step is incremented by 1, and the RF pulse is irradiated with the polarity inverted every time.

Also in this embodiment, the steps from the first phase encoding step to the kth phase encoding step are executed, and the phase of the acquisition at that time is shifted by π radian between the odd phase encoding step and the even phase encoding step. . In this way, the kth phase encoding step is executed, and 4 × (m × k) data sets are collected. When the collection of the data up to the final k-th phase encoding is completed, the CPU 8 performs Fourier transform in the slice direction on each three-dimensional data for each data of each sequence to form an image (complex image). The pixel value at an arbitrary position of this image is given by the vector amount (complex number) shown in the following equation.

[0048]

[Equation 4]

Here, A _r (→) (the arrow in the parenthesis indicates that it is added above the character) is the pixel value of the image obtained by the reference sequence, A _s (→),
A _p (→) and A _f (→) are pixel values of an image obtained by a flow encode sequence of S axis, P axis, and F axis, respectively, S (→) is a signal component of stationary tissue in the pixel, and F _{s +} , F _{p +} and F _{f +} are the flow components of each axis phase-shifted by the positive polarity flow encode pulse,
F _s− , F _p−, and F _f− are the flow components of each axis that are phase-shifted by the negative flow encode pulse. Therefore, the flow component only in the S-axis direction can be extracted by performing the complex difference between Expression (4) and Expression (5). Similarly, from equations (4) and (6), and from equations (4) and (7),
It is possible to extract only the flow components of only the P axis and the F axis.

The image data of each axis thus obtained by the complex difference can be vector-synthesized as shown in FIG. 6 to obtain a three-dimensional blood vessel image. Although only the P-axis and the F-axis are shown in FIG. 6, vector composition is similarly performed for the data of the S-axis. Also in this embodiment, since the RF pulse is irradiated with the polarity inverted every time, a signal having the same signal intensity is always obtained, and the phase of the signal acquisition is inverted at each phase encoding step. It is possible to eliminate the influence of the offset, and it is possible to obtain an image free from artifacts due to the DC component at the image center.

In the blood flow visualization sequence by the PC method shown in FIGS. 2 and 4 described above, a pulse sequence in which flow encode pulses are applied to each of three axes is used as a pulse sequence group, but only one axis or two axes are used. It goes without saying that it is good.

[0052]

As is apparent from the above embodiments, according to the MRI apparatus of the present invention, a plurality of types (n) of pulse sequence groups having different gradient magnetic field application patterns are executed,
A blood flow rendering pulse that acquires n sets of three-dimensional data sets, removes signals of stationary tissue in the subject by calculation between these data sets, and extracts a three-dimensional data set of only blood flow signals. In an MRI apparatus that executes a sequence, the repetition loop of the pulse sequence is sequentially 1) slice encoded, 2) sequence type, and 3)
By performing phase encoding and inverting the phase of data acquisition in adjacent phase encoding steps, it is possible to achieve a steady state when the polarity of the RF pulse is alternately inverted and applied, and the same signal is always output. Since a strong signal can be obtained and the DC offset can be removed, it is possible to obtain a good difference image that is free of the residual of the static tissue signal and the artifact due to the DC offset.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a blood flow drawing pulse sequence by the PS method in the MRI apparatus of the present invention.

FIG. 2 is a diagram showing an embodiment of a blood flow drawing pulse sequence by the PC method in the MRI apparatus of the present invention.

FIG. 3 is a diagram showing an example of a gradient magnetic field pattern by the PC method.

FIG. 4 is a diagram showing another embodiment of a blood flow drawing pulse sequence by the PC method in the MRI apparatus of the present invention.

FIG. 5 is a diagram showing another example of the gradient magnetic field pattern by the PC method.

FIG. 6 is a diagram showing a procedure for creating a blood vessel image by the PC method.

FIG. 7 is an overall configuration diagram of the MRI apparatus of the present invention.

FIG. 8 is a diagram for explaining phase rotation of spins by applying a gradient magnetic field.

FIG. 9 is a diagram showing a dephase sequence by the standard spin echo method in the PS method.

FIG. 10 is a diagram showing a rephase sequence by the standard spin echo method in the PS method.

FIG. 11 is a diagram showing a procedure for creating a blood vessel image by the PS method.

FIG. 12 is a diagram illustrating the principle of the PC method.

FIG. 13 is a timing diagram of a conventional PS method.

FIG. 14 is a diagram illustrating acquisition of a two-dimensional projection image by projection processing of three-dimensional data.

[Explanation of symbols]

1 ... Subject (inspection target) 2 ... Static magnetic field generation circuit (static magnetic field generation means) 3 ... · Gradient magnetic field generation system (gradient magnetic field generation means) 6 ... Signal processing system (image reconstruction means) 7 ... Sequencer (control means) 8 ... CPU (image reconstruction means)

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) A61B 5/055

Claims (6)

(57) [Claims] 1. A static magnetic field generating means for applying a static magnetic field to a space in which a subject is placed, a gradient magnetic field generating means for applying a gradient magnetic field to the subject, and atomic nuclei of atoms constituting the biological tissue of the subject. A transmission system for irradiating a high frequency pulse that causes nuclear magnetic resonance; a sequencer for controlling the transmission system and the gradient magnetic field generation means for repeatedly applying the high frequency pulse and the gradient magnetic field in a predetermined pulse sequence; A reception system for detecting an echo signal emitted by resonance, a signal processing system for performing an image reconstruction operation using the echo signal detected by the reception system, and a display unit for displaying the obtained image, In a nuclear magnetic resonance imaging apparatus for repeatedly measuring an echo signal to obtain a tomographic image, the pulse sequence activated by the sequencer is (a) gradient magnetic field application Different n types of turns (n is an integer of 2 or more) is composed of a pulse sequence group, each of said pulse sequence group comprising a slice encoding pulse
And, in (b) first the first phase encoding steps, a pulse sequence having a first gradient magnetic field application pattern performed for all steps of the slice encoding, (c) in the same phase encoding steps, applying a second gradient magnetic field A pulse sequence having a pattern is executed for all steps of slice encoding , (d) execution of all steps of slice encoding is sequentially executed for all n pulse sequences which are constituent elements, and (e) n is performed in the same phase encoding step. at the stage of execution of the pulses sequence is completed, both the one step increments the phase encoding steps, while repeating (f) (c) ~ ( e), are performed for all the steps of the phase encoding, (g) at least the same Phase encoding step And irradiating with alternating polarity of the radio frequency pulse at (h), and (h) comprising a blood flow visualization pulse sequence configured to acquire n three-dimensional data sets, wherein the signal processing system comprises n three-dimensional data sets. A magnetic resonance imaging apparatus characterized in that a signal of a stationary tissue in the subject is removed by calculation between measurement data sets, and a three-dimensional data set containing only blood flow signals is extracted. 2. The magnetic resonance according to claim 1, wherein the receiving system shifts the phase at the time of acquiring the data from the echo signal by π radians in odd steps and even steps of phase encoding. Imaging equipment. 3. The pulse sequence group comprises a pulse sequence group including a flow encode pulse composed of a pair of positive and negative gradient magnetic field pulses, and a positive flow encode pulse is applied in an arbitrary uniaxial direction of a rectangular coordinate system. 2. A pulse sequence of 1 and a second pulse sequence to which a negative flow encode pulse is applied are paired, and the pulse sequence pair for one axis or three axes of the Cartesian coordinate system is included. 2. The magnetic resonance imaging apparatus according to 2. 4. The pulse sequence group includes a pulse sequence group including a flow encode pulse composed of a pair of positive and negative gradient magnetic field pulses, each of the pulse sequences having (i) a positive polarity in three axis directions of a rectangular coordinate system. (J) A second pulse sequence in which a negative polarity flow encode pulse is applied to any one axis of the coordinate system and a positive polarity flow encode pulse is applied to the remaining two axes. (K) a third pulse sequence in which a negative flow encode pulse is applied to one axis different from the second pulse sequence and a positive flow encode pulse is applied to the remaining two axes, and (l) ) A negative flow encode pulse is applied to one axis different from the second and third pulse sequences, and the remaining two axes are applied. Of the four pulse sequences consisting of the fourth pulse sequence in which the positive polarity flow encode pulse is applied, the first pulse sequence and at least one pulse sequence of the second to fourth pulse sequences are included. The magnetic resonance imaging apparatus according to claim 1 or 2, characterized in that: 5. The pulse sequence group includes a dephasing sequence for measuring an echo signal in a state in which phase diffusion occurs in atomic nuclei of atoms forming blood flow, and atoms forming blood flow at the time of measuring the echo signal. 3. The magnetic resonance imaging apparatus according to claim 1 or 2, wherein the magnetic resonance imaging apparatus is composed of two types of pulse sequences of a rephase sequence for applying a gradient magnetic field that converges phase diffusion to the nuclei. 6. A pulse sequence including a plurality of sequence types having different gradient magnetic field application patterns is repeatedly executed while changing the phase encode and the slice encode to obtain a plurality of three-dimensional data sets.
In a magnetic resonance imaging apparatus that obtains an image of a subject by calculation between these data sets, the pulse sequence is repeated by sequentially changing the phase encoding, and the sequence type is used in each phase encoding when changing the phase encoding. Are sequentially changed and repeated, and in each sequence type when changing the sequence type, the slice encoding is sequentially changed and repeated , and
At least in the same phase encoding step
A magnetic resonance imaging apparatus characterized in that the polarities of the loose are alternately inverted and applied .