JP3427589B2 - MR imaging device - Google Patents

Description

Detailed Description of the Invention

[0001]

This invention relates to nuclear magnetic resonance (N
The present invention relates to an MR imaging apparatus that performs imaging by utilizing the MR (Nuclear Magnetic Resonance) phenomenon, and particularly to a technology that performs high-speed imaging by the GRASE (G Radiant And Spin Echo) method.

[0002]

2. Description of the Related Art Conventionally, various MR imaging apparatuses capable of high-speed imaging have been considered. For example, an MR imaging apparatus that performs a pulse sequence for high-speed imaging called GRASE method is known (US Pat. No. 5,270,654 and K. Oshio).
and DAFeiberg "GRASE (Gradient-and Spin-Echo) Imag
ing: A Novel Fast MRI Technique "Mganetic Resonance
in Medicine 20,344-349, 1991). The pulse sequence of the GRASE method is a kind of high-speed imaging method, and the EPI (Echo Planar Imaging) method for generating a gradient echo signal by switching the polarity of the gradient magnetic field and the excitation R
RARE (Rapid A) for generating a spin echo signal using an F (Radio Frequency) pulse and a refocus RF pulse
The pulse sequence is similar to that of the cquisition with Relaxation Enhancement) method.

First, a conventional pulse sequence of the GRASE method will be described with reference to FIGS. 15 and 16. In this sequence, as shown in FIG. 15A, after applying one excitation RF pulse 100 (also called a 90 ° pulse because the spin phase of protons is rotated by 90 °), a plurality of (here, three) pulses are applied. ) Refocus RF pulses 101 to 103 (also referred to as 180 ° pulse because the spin phase of protons is rotated by 180 °) are added, and as shown in FIG. The pulses 110 to 113 of the gradient magnetic field Gs are added. And FIG.
As shown in FIG. 5 (c), a pulse 120 of a dephasing gradient magnetic field Gr for separating the phases of the protons is added,
Subsequently, pulses 121 to 123 of the gradient magnetic field Gr for reading and for frequency encoding are applied within each interval of the above RF pulses.

Further, as shown in FIG. 15 (c), these Gr pulses 121 to 123 are added between the 180 ° pulse and the next 180 ° pulse (101 and 102, 102 and 10).
3 and 103), the spin echo signal S2 (occurs at a time point of an even multiple of the time interval between the 90 ° pulse 100 and the 180 ° pulse 101) by switching a plurality of times (here, three times each). SE1), S5 (SE
2), in addition to S8 (SE3), gradient echo signals S1 (GE1), S3 (GE2), S4 (GE3),
S6 (GE4), S7 (GE5), and S9 (GE6) are generated.

Then, as shown in FIG. 15 (d), pulses of the gradient magnetic field Gp for phase encoding are applied immediately before the echo signals S1 to S9 are generated. The application amount of each Gp pulse is The data obtained from the echo signals S1 to S9 is made to correspond to the amount of phase encoding such that the data is arranged as shown in FIG. 16A on the k space (also called raw data space).

That is, first, the spin echo signals SE1 to SE1
The data obtained from SE3 has the gradient echo signal GE1, in the central region (low frequency region) of the k-space.
GE3, GE5 and gradient echo signals GE2, GE
4, GE6 is the area around k-space (high frequency area)
In each of the areas, and, in each of the areas ,, so that they are arranged from top to bottom according to the order of generation of the echo signals, that is, in the direction from the positive high frequency area to the low frequency area to the negative high frequency area. The pulse application amount of the gradient magnetic field Gp for phase encoding with respect to each echo signal is determined so as to be an integrated amount of various phase encoding amounts.

In order to provide such a phase encoding amount, as shown in FIGS. 15D and 16B, immediately after the first 180 ° pulse 101, the first gradient echo signal S1 ( Immediately before GE1), the applied amount of the pulse 131a of the gradient magnetic field Gp for phase encoding is maximized. As a result, the gradient echo signal S1
The data obtained from (GE1) is placed on the uppermost side (positive side) in the k space. Echo signal S2 (SE1),
The gradient magnetic field Gp pulses 131b and 131c for phase encoding immediately before each of S3 (GE2) have polarities opposite to those of the gradient magnetic field pulse Gp131a, and the respective pulses 131b and 13c.
1c has the same magnitude, and the gradient magnetic field Gp pulse 13
The absolute value is smaller than 1a. As a result, each echo signal S
The data obtained from S2 and S3 are arranged at positions equidistantly spaced downward from the data position of the signal S1 in the K space (see FIG. 16B).

The gradient magnetic field Gp pulse 131d for phase encoding applied thereafter is for rewinding, and is used for temporarily returning the phase encoding amount accumulated up to that time to zero before the next 180 ° pulse 102 is applied. It is a thing. The magnitude of the gradient magnetic field Gp pulse 132a for phase encoding applied after the second 180 ° pulse 102 is slightly smaller than the magnitude of the gradient magnetic field Gp pulse 131a. As a result, the echo signal S4 (GE
3) is the amount of phase encoding such that the data obtained from the echo signal S1 (GE1) is arranged at the lower position adjacent to the position on the k space. Echo signal S
The magnitude and polarity of the gradient magnetic field Gp pulses 132b and 132c applied immediately before each of S5 and S6 are the same as those of the preceding gradient magnetic field Gp pulses 131b and 131c. Therefore, the arrangement positions on the k-space of the data obtained from the echo signals S5 (SE2) and S6 (GE4) are the same as the arrangement intervals of the echo signals S1, S2, and S3, respectively. This is on the lower side of the data arrangement location, and the arrangement location of the data obtained from the signals S5 and S6 on the k space is adjacent to the lower side of the data obtained from the signals S2 and S3. After that, a gradient magnetic field Gp pulse 132d for rewind is applied.

The magnitude of the pulse 133a of the gradient magnetic field Gp applied after the third 180 ° pulse 103 is made slightly smaller than that of the gradient magnetic field Gp pulse 132a. The magnitude and polarity of the pulses 133b and 133c of the gradient magnetic field Gp are the Gp pulses 131b and 131 of the gradient magnetic field.
c (and gradient magnetic field Gp pulses 132b and 132c). Therefore, the signals S7 (GE5), S8 (S
The data obtained from E3) and S9 (GE6) are adjacent to the lower side of the data obtained from the echo signals S4, S5 and S6.

As described above, the central region of the k-space (the central region is a low-frequency region and the k-space is 2
Data obtained from spin echo signals that have no phase error due to inhomogeneity of static magnetic field and no phase error due to chemical shift, when the image is reconstructed by three-dimensional Fourier transform) The image is a kind of false image (artifact) caused by the phase encoding amount being discontinuous on the k-space due to the phase error because the phase encoding amount is integrated such that There is an advantage that blurring is unlikely to occur in the reconstructed image. Also, echo signals having the same generation order of echo signals in each interval of 180 ° pulse are grouped (SGE1, SSE, SG in FIG. 16A).
Since E2), the phase error remains at the boundary between the grouped echo signals, although the phase error remains at the boundary between the grouped echo signals. There is.

[0011]

However, in the conventional pulse sequence as described above, the boundary (SG) of the data group obtained from the group of echo signals is grouped.
A sharp signal strength difference ΔS occurs between E1 and SSE and SSE and SGE2). That is, each echo signal S1 to S
The signal strength of 9 is as shown in FIG. This is because, as shown in FIG. 15 (e), the signal intensity of each echo signal S1 to S9 gradually decreases after the first 90 ° pulse 100 according to the time constant T ₂ and the time constant T ₂ ^*. (Note that the time constant T ₂ indicates the lateral relaxation time (also called spin-spin relaxation time) generated in the spin echo signal, and the time constant T ₂ ^* is longer than the time constant T _{2 due} to the nonuniformity of the static magnetic field. It shows the fast relaxation and the transverse relaxation time that occurs in a gradient echo signal). That is, the echo signal S
The signal intensity of each of 1 to S9 becomes smaller in order according to the generation order.

However, since the respective data obtained from the echo signals S1 to S9 are arranged in the k space as shown in FIG. 16A, when viewed in the phase encoding direction (vertical direction) of the k space, A boundary between the position where the data obtained from the echo signal S7 (GE5) is arranged and the position where the data obtained from the echo signal S2 (SE1) is arranged (the boundary between SGE1 and SSE) and the echo signal At the boundary (the boundary between SSE and SGE2) between the position where the data obtained from S8 (SE3) is arranged and the position where the data obtained from the echo signal S3 (GE2) is arranged, the signal strength is sharp. Has changed to. When the image is reconstructed by two-dimensional Fourier transforming the data arranged in the k-space in this way, image blurring artifacts caused by a large signal strength difference occur in the reconstructed image. There is a problem.

Of the generated echo signal group, the echo signal S5 (spin echo signal SE) generated at its center is generated.
Since 2) is arranged in the central part of the [central part] of the k space, there is a problem that the contrast of the reconstructed image is fixed and a desired contrast cannot be obtained.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an MR imaging apparatus capable of adjusting the contrast of a reconstructed image while suppressing image blurring artifacts. To do.

[0015]

The present invention has the following constitution in order to achieve such an object. That is, the invention according to claim 1 is an MR imaging apparatus for performing imaging by using nuclear magnetic resonance (NMR phenomenon), wherein (a) a main magnet that generates a uniform static magnetic field in an imaging region space; (B) Three gradient magnetic field pulses (slice selection gradient magnetic field pulse, readout gradient magnetic field pulse, phase encoding gradient magnetic field pulse) whose magnetic field strengths change in three-dimensional directions orthogonal to each other in the static magnetic field space.
First / second / third gradient magnetic field coils for generating the following: (c) Irradiation of excitation RF pulse and refocus RF pulse to the subject arranged in the imaging region space and generation from the subject An RF coil for detecting an echo signal, and (d) one excitation RF pulse followed by a plurality of refocus R through the RF coil.
The F pulse and the R pulse are sequentially irradiated at a predetermined timing.
RF irradiation means connected to the F coil, and (e) for selecting a slice plane via the first gradient magnetic field coil in accordance with the irradiation timing of each of the excitation RF pulse and the refocusing RF pulse. A slice selection gradient magnetic field pulse generating means for generating a gradient magnetic field pulse,
(F) Polarity switching is performed a plurality of times within each pulse interval of the plurality of refocusing RF pulses to generate a plurality of gradient echo signals centering on each spin echo signal, and at the same time, each spin echo signal. And (g) the read gradient magnetic field pulse generating means for generating a read gradient magnetic field pulse via the second gradient magnetic field coil in synchronization with the generation timing of each echo signal of each gradient echo signal. Immediately before each echo signal is generated, a phase encoding gradient magnetic field pulse for performing phase encoding is applied via the third gradient magnetic field coil, and a phase encoding satisfying all the following conditions Gradient magnetic field pulse generating means and the echo signals so that the integrated amounts of the phase encode amounts are all different from each other. The intensity of the phase-encoding gradient magnetic field pulse is changed so that the integrated amount of the phase-encoded amount becomes close to each of the echo signal groups having the same generation order (hereinafter referred to as a group) within each pulse interval. Changing the intensity of the gradient magnetic field pulse for phase encoding, and the absolute value of the integrated amount of the phase encoded amount of the gradient echo signal group is larger than the absolute value of the integrated amount of the phase encoded amount of the spin echo signal group. By changing the intensity of the phase encoding gradient magnetic field pulse so that the echo signal (reference echo signal) of a specific order among the order of occurrence (group order) in each group is near the center in each group. Changing the intensity of the phase-encoding gradient magnetic field pulse so as to be an integrated amount of the phase-encoding amount of If the quasi-echo signal has the first (or last) rank in the group, the integrated amount of each phase encode amount in the echo signal group in each group is the phase encode amount of the reference echo signal in each group. Changing the intensity of the phase-encoding gradient magnetic field pulse so as to change with positive and negative (or negative and positive) gradients according to the rank in the group with the integrated amount of Instructing means for instructing a related value, and (i) collecting data from the echo signal groups detected by the RF coil, and converting each data to k according to the integrated amount of the phase encode amount for each echo signal group. Data processing means for arranging in a space (raw data space) and reconstructing a tomographic image.

The invention described in claim 2 is the same as claim 1
In the MR imaging apparatus described in the paragraph (1), the instructing means is for instructing a value related to the first to last in-group rank, and the phase encoding gradient magnetic field pulse generating means is the minimum or maximum for each group. A gradient magnetic field pulse for phase encoding for performing phase encoding within a range of an integrated amount (minimum integrated amount or maximum integrated amount) of the phase encoding amount is applied, and a value instructed by the instructing means is an intermediate value. If the value is related to the general order, the intensity of the phase-encoding gradient magnetic field pulse is changed so as to satisfy the following conditions (1) and (2) on the basis of the conditions (1) to (3). If the integrated amount of the phase encoding amount of the signal exceeds the maximum integrated amount of each group, Is defined as a new integrated amount of the phase encoding amount corresponding to the minimum integrated amount of the group, and as the integrated amount of the phase encoded amount increases, the integrated amount of the new phase encoded amount sequentially increases from the minimum integrated amount. If the integrated amount of the phase encoding amount of each echo signal in each group is less than the minimum integrated amount of each group, the maximum integrated amount of the phase encoded amount is the maximum integrated amount of the group. The integrated amount of the new phase encode amount corresponding to the amount is set to be the integrated amount of the new phase encode amount that sequentially becomes smaller than the maximum integrated amount as the integrated amount of the phase encode amount decreases.

[0017]

The operation of the invention described in claim 1 is as follows. A static magnetic field is generated by the main magnet in the imaging area space, and a slice selection gradient magnetic field pulse is applied to the imaging area space by the slice selection gradient magnetic field pulse generation means via the first gradient magnetic field coil, thereby slicing a plane. Is selected. Then, the RF irradiation unit irradiates one excitation RF pulse and a plurality of subsequent refocus RF pulses via the RF coil. At a time that is an even multiple of the time interval between the excitation RF pulse and the first refocusing RF pulse, a spin echo signal without a phase error caused by nonuniformity of the static magnetic field by the main magnet or chemical shift is generated. To do. Further, within each pulse interval of the plurality of refocusing RF pulses, the read gradient magnetic field pulse generation means performs polarity switching of the read gradient magnetic field pulse a plurality of times via the second gradient magnetic field coil. A plurality of gradient echo signals are generated around the spin echo signal.

Therefore, within each interval of the refocusing RF pulse, a plurality of gradient echo signals centering around the spin echo signal are generated. The signal intensities of these echo signals are sequentially attenuated with a time constant of the transverse relaxation time according to the order of occurrence.

The echo signals generated in sequence are sequentially phase-encoded by the phase-encoding gradient magnetic field pulse generating means applying a phase-encoding gradient magnetic field pulse through the third gradient magnetic field coil. It The phase encoding gradient magnetic field pulse generating means performs phase encoding on each echo signal, and the phase encoding is performed so as to satisfy all of the following conditions.

The generated echo signals are irradiated by changing the intensity of the phase-encoding gradient magnetic field pulse so that the integrated amounts of the phase-encoding amounts are different (), so that the data obtained from each echo signal is k They are arranged at different positions along the direction of the integrated amount of the phase encode amount on the space.

Furthermore, the gradient magnetic field pulse for phase encoding is set so that the integrated amount of the phase encoding amount is close to each of the echo signal groups having the same generation order (hereinafter referred to as a group) in each pulse interval. Since the intensity is changed to () to irradiate, the echo signals forming the same group are arranged at adjacent positions on the k space.

In each of the generated echo signals, the absolute value of the integrated amount of the phase encode amount of the gradient echo signal group is larger than the absolute value of the integrated amount of the phase encode amount of the spin echo signal group. Since the intensity of the gradient magnetic field pulse for irradiation is changed (), a spin echo signal group with no phase error is arranged in the central area on the k-space, and gradient echo signal groups in both peripheral areas of the area. Will be placed.

For the phase encoding so that the echo signal (reference echo signal) of a specific order among the order of occurrence within each group (the order within the group) becomes the integrated amount of the phase encode amount near the center in each group. Since the intensity of the gradient magnetic field pulse is changed and irradiated (), each reference echo signal is arranged in the central part of each group (upper peripheral region, central region, lower peripheral region) on the k space, The data obtained from the reference echo signal is arranged in the vicinity of the center of the central region (spin echo signal group) on the k-space that greatly affects the contrast of the.

When the reference echo signal has the first (or last) rank in the group, the echo signal groups in each group have the integrated amount of the respective phase encode amounts as the reference echo signal in each group. The intensity of the phase-encoding gradient magnetic field pulse is changed so that it changes with positive and negative (or negative and positive) gradients in accordance with the in-group rank centering on the integrated amount of the phase-encoding amount of (1). That is, by changing the positive and negative (or negative and positive) slopes according to the order of generation of the echo signals in each group, with the integrated amount of the phase encode amount of the reference echo signal in each group as the center, On the k-space, the echo signals of each group are arranged in ascending order (or descending order) according to the order within the group, centering on the reference echo signal within each group and toward both sides (vertical direction of the phase encoding amount). Become. Therefore, each group on k-space (upper peripheral area, central area, lower peripheral area of k-space)
The signal strength profile of is based on the fact that the signal strength is attenuated in the order of occurrence, and is convex (or concave) upward, that is, the central portion in each group has the maximum signal strength (or the minimum signal strength), There is a decreasing tendency (or increasing tendency) toward the periphery of each group.

As a result, at the boundary between the central area (the signal strength is convex (or concave)) on the k-space and the upper peripheral area and the lower peripheral area (the signal strength is convex (or concave)), The difference in signal strength is small. Therefore, it is possible to suppress image blurring artifacts due to a large difference in signal strength.

In each group on the k space,
Since the data groups are arranged in ascending order (or descending order) in the groups toward both sides centering on each reference echo signal, the generation order of the echo signals arranged in the peripheral portions of both adjacent groups is close ( The generation order is adjacent to each other, for example, the echo signals S7 and S8 are adjacent to each other. From this point of view, it can be seen that the signal intensity difference at the boundary of each group becomes small and the image blur artifact can be suppressed.

A value related to the in-group rank by the photographer via the instructing means, for example, directly designates the in-group rank of the echo signal, or the in-group rank such as the echo time at which the echo signal is generated. Is designated, the reference echo signal arranged at the center of each group in the k-space is designated as the first or last one based on this value. Since the values related to the rank within the group such as the generation order of the echo signal and the echo time are related to the elapsed time from the time when the excitation RF pulse is irradiated, that is, related to the lateral relaxation time, the rank within the group is set first. Alternatively, by making it last, the spin echo signal arranged in the central part of the central region on the k-space can have a short echo time or a long echo time. Therefore, the k-space is Fourier-transformed by the data processing means. The reconstructed image obtained in this way can be a proton density-weighted image [without T ₂ relaxation information] or a heavy T ₂ -weighted image [whitening of the water component]. That is, the contrast can be adjusted while suppressing the image blurring artifact.

The operation of the invention described in claim 2 is as follows. The instructing means is for instructing a value related to the first to last group rank. That is,
It also indicates an intermediate value between the first and last group ranks.

The phase-encoding gradient magnetic field pulse generating means is configured to perform a phase-encoding gradient magnetic field for performing phase encoding over a range of a minimum or maximum integrated amount of phase encoding amount (minimum integrated amount or maximum integrated amount) for each group. In the case of irradiating a pulse, and when a value related to the intermediate rank within the group is instructed through the instructing means, the intensity of the gradient magnetic field pulse for phase encoding is as follows based on the above conditions or change.

In other words, in each group, when the integrated amount of the phase encoding amount of each echo signal exceeds the maximum integrated amount of each group, the smallest integrated amount of the phase encoded amounts is the minimum integrated amount of the group. Phase encoding so that the integrated amount of the new phase encoding amount that corresponds to the amount becomes larger than the minimum integrated amount as the integrated amount of the phase encoded amount increases. Give.

As a result, the echo signal which should have been phase-encoded to be larger than the maximum integrated amount based on the instruction value from the instruction means, that is, on the k-space when arranged so as to satisfy the above condition or conditions. The data of the echo signals arranged on the upper side of each group will be arranged on the opposite side of the group with the reference echo signal of the group sandwiched in place of the integrated amount of the phase encoding amount. Furthermore, the data obtained from the echo signal that should have been subjected to the phase encoding exceeding the maximum integration amount is arranged toward the reference echo signal (center part in each group) on the k-space as the distance from the maximum integration amount increases. Will be done. Therefore, when the condition or is satisfied, the arrangement order on the k-space is not changed, and the echo signal groups protruding from the upper side of each group are sequentially arranged on the opposite side across the reference echo signal. . In other words, in the state where each of the above conditions is satisfied, the data group of the echo signal group in each group is moved upward in each group so that the data of the designated reference echo signal is located in the central portion in each group. Shift,
The data of the echo signals protruding from the upper side of each group are sequentially arranged from the opposite side to the upper side in each group, which is a shift for circulating the data in each group.

Further, in each group, when the integrated amount of the phase encode amount of each echo signal is less than the minimum integrated amount of each group, the maximum integrated amount of the phase encode amounts is set to the maximum integrated amount of the group. And the phase encoding is performed so that as the integrated amount of the phase encoded amount becomes smaller, the integrated amount of the new phase encoded amount becomes sequentially smaller than the maximum integrated amount. .

By doing so, similarly to the above case, the echo signal which should have been phase-encoded with the integrated amount of the phase-encoded amount smaller than the minimum integrated amount, that is, the above condition or condition is satisfied. The data of the echo signals arranged below each group on the k space when arranged in such a manner does not change the arrangement order in each group, and the reference echo signal of the group is sandwiched between the groups. Will be placed on the opposite side of.

As described above, the data group obtained from the echo signal group subjected to the phase encoding exceeding the maximum integration amount and the data group obtained from the echo signal group subjected to the phase encoding less than the minimum integration amount are k spaces. As the peripheral portion of each group above is protruded, it is sequentially shifted and arranged so as to circulate to the opposite peripheral portion of each group.

In this way, when the echo signals in each group are cyclically shifted only in each group and are arranged on the k space, the arrangement order does not change when the above conditions or are satisfied, and at that time. When the order of the groups is examined by circulating them within each group with the reference echo signal at the center, the ascending (or descending) order is maintained. Therefore, in the peripheral portions of the adjacent groups, the generation order of the echo signals arranged in the peripheral portions of the adjacent groups is close to each other, as in the case where each of the above conditions is satisfied. Therefore, the signal strength difference at the boundary between both groups remains small, and the image blur artifact due to the large signal strength difference can be suppressed. Further, by designating an intermediate rank within the group via the designating means, the spin echo signal arranged in the central portion of the central region on the k space can have a short echo time or a long echo time. Therefore, the reconstructed image obtained by Fourier transforming the k-space by the data processing means is added to the proton density weighted image or the heavy T ₂ weighted image, and various T ₂ depending on their intermediate echo time.
It can be an emphasized image. That is, the contrast can be adjusted more flexibly while suppressing image blurring artifacts.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. First, the configuration of the MR imaging apparatus will be described with reference to FIG. In the figure, reference numeral 1 is
The main magnet 1 is a main magnet for generating a substantially uniform static magnetic field in the imaging region space, and the main magnet 1 has three gradient magnetic field coils 2 for applying a gradient magnetic field so as to be superposed on the static magnetic field.
(2x, 2y, 2z) is attached. The gradient magnetic field coil 2 generates a pulse of three gradient magnetic fields Gr, Gp, Gs (reading gradient) in which a uniform static magnetic field generated by the main magnet 1 changes in three-dimensional directions (X, Y, Z) in which the magnetic field strengths are orthogonal. It is composed of three sets of gradient magnetic field coils 2x, 2y, 2z for superimposing a magnetic field pulse, a gradient magnetic field pulse for phase encoding, and a gradient magnetic field pulse for slice selection.
An unillustrated subject (patient) is placed in the imaging region space (static magnetic field space) to which the static magnetic field and the gradient magnetic field are applied,
The RF coil 3 is attached to the subject.

A gradient magnetic field power source 4 is connected to the gradient magnetic field coil 2 and electric power for generating each gradient magnetic field of the gradient magnetic fields Gx, Gy, Gz is supplied. A waveform signal from a waveform generator 5 is input to the gradient magnetic field power supply 4 to generate gradient magnetic fields Gx, Gy, G.
Each gradient magnetic field waveform of z is controlled. R for the RF coil 3
An RF signal is supplied from the F power amplifier 6 to irradiate an RF signal to a subject (not shown). This RF signal is obtained by amplitude-modulating the RF signal of a predetermined carrier frequency generated by the RF signal generator 7 by the modulator 8 according to the waveform sent from the waveform generator 5. .

The echo signal generated in the subject is received by the RF coil 3, passes through the preamplifier 9, and is then detected by the phase detector 10.
Sent to. The received signal is RF by the phase detector 10.
Phase detection is performed using the RF signal from the signal generator 7 as a reference frequency, and the detection output is sent to the A / D converter 11. A sampling pulse is input from the sampling pulse generator 12 to the A / D converter 11, and the detection output is converted into digital data according to the sampling pulse. The digital data is taken into the host computer 20.

The host computer 20, which corresponds to the data processing means, processes the captured data to reconstruct an image and determines the timing of the entire sequence via the sequencer 23. That is, sequencer 2
3 is a waveform generator 5, an RF signal generator 7, a sampling pulse generator 1 under the control of the host computer 20.
2 etc. to determine the timing at which each waveform signal is output from the waveform generator 5, the RF signal generation timing from the RF signal generator 7, and the sampling pulse generation from the sampling pulse generator 12. Determine the timing. Further, the host computer 20 sends the waveform information to the waveform generator 5, and the Gx, G
The waveform, intensity, etc. of each of the y, Gz gradient magnetic field pulses are controlled, the envelope of the RF signal irradiated from the RF coil 3 to the subject is determined, and a signal is sent to the RF signal generator 7 to transmit the carrier frequency of the RF signal. To control. Also,
The instructing section 25 corresponding to the instructing means connected to the host computer 20 is composed of a keyboard or the like, and is for inputting a value related to an in-group rank described later. The host computer 20 controls the waveform generator 5 based on the value instructed to input, and adjusts the polarity and intensity of the Gy gradient magnetic field pulse. Therefore, the host computer 20 controls the entire pulse sequence based on the GRASE method as described later.

In such an MR imaging apparatus, a pulse sequence as shown in FIG. 2 is performed under the control of the computer 20 and the sequencer 23. The pulse sequence shown in FIG. 2 is basically based on the GRASE method described above, and the method of performing the phase encoding has been devised.

It should be noted that within the respective pulse intervals of the refocus RF pulses 101 and 102, 102 and 103, and 103 and thereafter, the echo signal groups S1, S4 of the groups in which the order of generation of the echo signals is the same. S7
, S2, S5, S8, and S3, S6, S9, the order of occurrence in each group, that is, the order of description in each echo signal group indicates the order of occurrence in each group, This will be referred to as the in-group ranking. Therefore, in this embodiment, there are three types of ranking within the group: first, middle, and last.

In the following description, first, a value related to the in-group rank is input and instructed via the instructing section 25. As this value, an echo signal number indicating the echo signal generation order is input and instructed. Here, the first value, for example, echo signal S2 (or echo signal S
Even if it is 1 or the echo signal S3, the order in the group is the same, and therefore the same). The value related to the in-group rank may be an echo time, which will be described later, in addition to the number of the echo signal, and various values are adopted as long as the value can specify the in-group rank. be able to.

<When the ranking within the group is “first”>

First, as shown in FIGS. 2 (a) and 2 (b),
At the same time as applying one 90 ° pulse (also called an excitation RF pulse) 100 via the RF coil 3, a pulse 110 of a gradient magnetic field Gs for slice selection is applied via a gradient magnetic field coil 2z. Then, one 180 ° pulse (also called refocus RF pulse) 101
The irradiation time of the 0 ° pulse 100 is set as the time origin, and the irradiation is performed after τ hours, and then the 180 ° pulse 10 after 2τ hours.
Irradiate 2. In this way 180 ° pulse 101,
102 and 103 are sequentially irradiated with the pulses 111, 112 and 113 of the slice selection gradient magnetic field Gs.

When each RF pulse is irradiated in this way,
As shown in FIG. 2E, the first spin echo signal S2
(SE1) is 90 ° pulse 100 and 180 ° pulse 1
The first 180 by the same time interval as the time interval τ between 01
It occurs mainly at the time point after the pulse 101.
Here, from 90 ° pulse 100 to spin echo signal S2
The time to the echo center where (SE1) occurs (echo time 2τ) is t ₁ .

Thus, assuming that the echo time from the 90 ° pulse 100 to the echo center is t ₁ , the first 180 °
The pulse 101 is a 90 ° pulse 100 based on the time origin (t =
0) is given by t = (1/2) t ₁ = τ. The irradiation timing of the n-th (n is a positive integer) 180 ° pulse is set to {2 (n-1) +1} τ. That is, 180 ° pulses 101, 102, 103
The irradiation timings of t = τ, t = (3/2) t ₁ =
3τ, (5/2) t ₁ = 5τ, the spin echo signals S2 (SE1), S5 (S
E2) and S8 (SE3) are generated at each time point of t = t ₁ , t ₂ and t ₃ . This makes 90 ° pulse 1
From 00 to the second and subsequent spin echo signals S5 (SE
2), the time interval t ₂ until the occurrence of S8 (SE3),
t ₃ is an integer multiple of the time interval t ₁ from the 90 ° pulse 100 to the time of the first spin echo signal generation, that is, t 3.
_{It is possible to set 2} = 2t ₁ and t ₃ = 3t ₁ .

By controlling the irradiation timing of each 180 ° pulse in this way, a pseudo spin echo signal (spurious spin echo) due to incompleteness of the 180 ° pulse can be generated at the same time as the original spin echo signal. Therefore, the pseudo spin echo signal (sti
It can be used as mulated spin echo).

The respective time intervals (echo time) until the above-mentioned spin echo signal generation time are t ₁ = 10 to 20 ms, in order to obtain various contrasts described later.
It is preferable to adjust the irradiation timing of the 90 ° pulse 100 and each of the 180 ° pulses 101, 102, 103 so that t ₂ = 20 to 40 ms and t ₃ = 30 to 60 ms. Therefore, in this example, τ = 10-2
It is preferable to irradiate each RF pulse in the range of 0 ms.

Referring to FIG. Gradient coil 2
A dephasing gradient pulse 120 [to disperse the spin phases of protons] via x
180 ° after applying before the first 180 ° pulse 101
The polarity of the Gr pulse is switched, for example, three times within the interval between the pulses 101 and 102 (121a, 121b,
121c), the echo signals S1 to S3 are generated within the interval. Second 180 ° pulse 1
02 and the third 180 ° pulse 103, and similarly after the third 180 ° pulse 103, by switching the polarity of the Gr pulse in the same manner, three echo signals S4 to S6 are generated in the respective pulse intervals. And the echo signals S7 to S9 are generated. Of these echo signals S1 to S9, the echo signals S2, S5, S8 generated at the center within each pulse interval are spin echo signals without phase error due to nonuniform static magnetic field by the main magnet 1 or chemical shift. SE1 to SE3,
The other echo signals are the gradient echo signals GE1 to GE1.
It is GE6.

The echo signals S1 to S9 are
As shown in FIG. 2E, the signal strength gradually decreases in the order of occurrence. The time constant of this decay is the transverse relaxation times T ₂ and T ₂ ^* , and more precisely, the spin echo signals SE1 to SE3 which are not affected by the inhomogeneity of the static magnetic field have the transverse relaxation times T ₂ and T ₂ ^*.
Gradient echo signals GE1 to G that are attenuated by ₂ (also called spin-spin relaxation time because energy is exchanged between spins) and are affected by the inhomogeneity of the static magnetic field.
E6 decays at T ₂ ^*, which decays faster than the transverse relaxation time T ₂ due to the inhomogeneity of the static magnetic field.

Then, a gradient magnetic field pulse Gp for phase encoding is applied to each of the echo signals S1 to S9 via the gradient magnetic field coil 2y so that the echo signals S1 to S9 are integrated.

Referring to FIGS. 2 (a) to 2 (e). First, the Gp pulse 201a applied after the first 180 ° pulse 101 and before the generation of the echo signal S1 has a positive polarity and is the phase applied to each of the echo signals S1, S4, S7. It is intermediate when the encoding amount is included. As a result, the gradient echo signal S1
The data obtained from (GE1) is displayed in the vertical direction (phase encoding direction K on the k space as shown in FIG. 4A).
It is arranged at an intermediate position on the positive side of p). Gp pulse 20 applied before generation of the next echo signal S2
1b is a negative polarity Gp pulse 20 which has already been applied.
The absolute value is slightly smaller than 1a, and the size is such that the integrated amount is near zero. As a result, the data obtained from the spin echo signal S2 (SE1) is
Gradient echo signal S1 (GE1) on k-space
It is arranged near the positive side of the central part, which is a position distant in the downward direction from. The Gp pulse 201c applied before the generation of the next echo signal S3 has the same polarity and the same magnitude as the previous Gp pulse 201b. As a result, the data obtained from the gradient echo signal S3 (GE2) is arranged in the k space on the basis of the data obtained from the gradient echo signal S1 (GE1) and the data obtained from the spin echo signal S2 (SE1). It is arranged at an intermediate position on the negative side, which is a position far away downward from the data of the spin echo signal S2 (SE1) by the same interval as the interval.
Then, the rewind pulse 201d is applied before the next 180 ° pulse 102 to cancel the phase encoding performed so far.

The Gp pulse 202a applied after the application of the second 180 ° pulse 102 and before the generation of the echo signal S4 has the positive polarity and is the Gp applied to the gradient echo signal S1 (GE1). It is made slightly larger than the pulse 201a. As a result, the data obtained from the gradient echo signal S4 (GE3) is arranged at a position adjacent to the upper side of the gradient echo signal S1 (GE1) on the k space. The Gp pulse 202b applied before the generation of the next echo signal S5 has a negative polarity, but has such a magnitude that the integrated amount becomes positive by the already applied Gp pulse 202a. As a result, the data obtained from the gradient echo signal S5 (SE2) is arranged at a position apart from the gradient echo signal S4 (GE3) below in the k space. In the next echo signal S6,
Gp pulse 2 having the same magnitude as Gp pulse 201c already applied to [gradient echo signal S3 (GE2)]
02c is applied, the data obtained from it is k
The position on the space is the spin echo signal S2 (SE
The position is the same as the distance between 1) and S3 (GE2). Then, similarly, the rewind pulse 202d is applied.

After the third 180 ° pulse 103 and before the generation of the echo signal S7 (GE5), the phase encoding amount having the positive polarity and applied to the echo signals S1, S4, S7 is the highest. To be big. As a result, the data obtained from the gradient echo signal S7 (GE5) is arranged at the position farthest on the positive side in the k space and adjacent to the upper side of the echo signal S4 (GE3). The Gp pulse 203b applied before the generation of the next echo signal S8 (SE3) is the Gp pulse 202b already applied [to the spin echo signal S5 (SE2)].
Is the same size as. As a result, the data obtained from the spin echo signal S8 (SE3) is arranged on the upper side adjacent to the spin echo signal S5 (SE2) on the k space. The Gp pulse 203c applied before the generation of the next echo signal S9 has a negative polarity, so that the data obtained from the gradient echo signal S9 (GE6) is adjacent to the upper side of the gradient echo signal S6 (GE4). Are placed on the k space. Then, similarly, the rewind pulse 203d is applied.

While performing the phase encoding as described above, the pulses 201a, 20 of the gradient magnetic field for phase encoding are used.
The size of 2a and 203a is reduced by a predetermined amount and the process is repeated a predetermined number of times.

The integrated amount of the phase encoding amount thus applied is schematically shown in FIG. 4 (b). In this figure, the integrated amount of each Gp pulse is represented by an arrow K of each Gp pulse and the integrated amount axis K of the phase encode amount of the k space.
The distance from the point where p is zero is shown, and is set as follows.

First, the gradient echo signals S1 (GE1), S4 (GE3), S7 (GE) having the first generation order are generated.
5) That is, in the same group SGE1, the echo signal S1 (GE) having the first generation order (the order within the group) is first.
1) (this is referred to as a reference echo signal) is set so as to be an integrated amount of the phase encode amount in the vicinity of the center in the group SGE1 in the k space, and further, a positive slope is set in accordance with the order of occurrence thereof. It is set to change with (shown by a chain double-dashed line in the figure). Similarly,
Spin echo signals S2 (SE1), S having an intermediate generation order
5 (SE2), S8 (SE3), that is, the echo signal S2 (SE1) having the first generation order (in-group order) of the same group SSE is located near the center of the group SSE in the k space. That is, it is set so as to be an integrated amount of the phase encoding amount near zero on the Kp axis on the k space, and is further set to sequentially change with a positive slope according to the order of occurrence thereof. Furthermore, the gradient echo signals S3 (GE2), S6 (GE4), and S9 (GE6) having the last generation order are generated.
Similarly, by performing the phase encoding, the echo signal S3 (GE3) having the first in-group rank in the group SGE2 becomes the integrated amount of the phase encode amount near the center of the group SGE2 in the k space. Is set to. By applying these phase encodings, as shown in FIG. 4A, each line of the upper half (each region to the upper half) of each group SGE1, SSE, SGE2 of the k space is filled.

Then, as shown in FIG. 3, a gradient magnetic field pulse Gp for phase encoding is applied again to the echo signals S1 to S9 via the gradient magnetic field coil 2y so that different phase encoding amounts are integrated. To be done. In addition,
In FIG. 3, each RF pulse 110-103 (see FIG.
(A)), Slice selection gradient magnetic field pulse Gs110
˜113 (FIG. 2 (b)) and the pulses Gr121a, b, c to 123a, b, c of the gradient magnetic field for reading are the same as those described above, and are not shown here.

First, Gp applied after the first 180 ° pulse 101 and before the generation of the echo signal S1.
The pulse 301a has a positive polarity and has the echo signals S1 and S.
4, the phase encoding amount applied to S7 and the phase encoding amount applied later are intermediate. As a result, the gradient echo signal S1 (GE
The data obtained from 1) is k as shown in FIG.
It is arranged in an intermediate position on the positive side in the vertical direction (phase encoding direction Kp) on the space. This position is located below the line already filled with the data obtained from the echo signal S1 (GE1). The Gp pulse 301b applied before the generation of the next echo signal S2 has a negative polarity and the same magnitude as the already-applied Gp pulse 301a, and has a magnitude such that the integrated amount is near zero. Is. As a result, the data obtained from the spin echo signal S2 (SE1) is arranged in the vicinity of the central portion, which is a position in the k-space, which is largely apart from the gradient echo signal S1 (GE1) in the downward direction, and the echo signal S2 ( It is located below the line filled with the data obtained from SE1). Next echo signal S3
The Gp pulse 301c applied before the occurrence of
It has the same polarity and the same magnitude as the pulse 301b. As a result, the data obtained from the gradient echo signal S3 (GE2) becomes the gradient echo signal S1 (GE1).
Data obtained from the spin echo signal S2 (SE1)
The data is obtained by arranging the data on the negative side intermediate position which is far away from the data of the spin echo signal S2 (SE1) by the same space as the space on the k-space. Located below the already filled line as you did. And rewind pulse 301
d is applied before the next 180 ° pulse 102.

The Gp pulse 302a applied after the application of the second 180 ° pulse 102 and before the generation of the echo signal S4 has the positive polarity and is the Gp applied to the gradient echo signal S1 (GE1). It is made slightly smaller than the pulse 301a. Thus, the data obtained from the gradient echo signal S4 (GE3) is arranged at a position adjacent to the lower side of the gradient echo signal S1 (GE1) on the k space. The Gp pulse 302b applied before the generation of the next echo signal S5 has a negative polarity, but the magnitude is such that the integrated amount becomes slightly negative due to the already applied Gp pulse 302a. As a result, the data obtained from the gradient echo signal S5 (SE2) is k
It is arranged at a negative position adjacent to the lower side of the gradient echo signal S2 (SE1) on the space. The next echo signal S6 has already been [gradient echo signal S3 (GE
2)] Since the Gp pulse 302c having the same magnitude as that of the applied Gp pulse 301c is applied, the data obtained from the Gp pulse 302c have spin echo signals S2 (SE1) and S3 (GE2) whose arrangement positions on the k space are the same. It is a position adjacent to the lower side of the echo signal S3 (GE2), which is separated by the same distance as the distance between and. Then, similarly, the rewind pulse 302d is applied.

After the third 180 ° pulse 103 and before the echo signal S7 (GE5) is generated, the most phase encoding amount of the positive polarity and applied to the echo signals S1, S4 and S7. It is a small pulse 303a.
As a result, the data obtained from the gradient echo signal S7 (GE5) is arranged on the k-space at the lowermost position of the group SGE1 and adjacent to the lower side of the echo signal S4 (GE3). Next echo signal S8 (SE
The Gp pulse 303b applied before the generation of 3) is the Gp pulse 303b already applied [in the spin echo signal S5 (SE2)].
It has the same magnitude as the pulse 302b. As a result, the data obtained from the spin echo signal S8 (SE3) becomes the spin echo signal S5 (SE2) on the k space.
Is located on the lower side adjacent to. The Gp pulse 303c applied before the generation of the next echo signal S9 has a negative polarity,
As a result, the data obtained from the gradient echo signal S9 (GE6) is converted into the gradient echo signal S6 (GE6).
4) Located on the k-space adjacent to the bottom side. Then, similarly, the rewind pulse 303d is applied.

While performing the phase encoding as described above, the pulse 30 of the phase encoding gradient magnetic field is obtained in the same manner as the first phase encoding gradient magnetic field pulse described above.
The sizes of 1a, 302a and 303a are reduced by a predetermined amount and the process is repeated a predetermined number of times.

A schematic representation of the integrated amount of the phase encode amount thus applied is set as shown in FIG. 4 (b). That is, first, the gradient echo signals S1 (GE1), S4 (GE3), S7 (G
E5), that is, in the same group SGE1, the echo signal S1 (GE) having the first generation order (in-group order) is generated.
1) is set so as to be an integrated amount of the phase encoding amount near the center in the group SGE1 in the k space, and further, a negative slope (indicated by a chain double-dashed line in the figure) according to their generation order. ) Is set to change. Similarly, of the spin echo signals S2 (SE1), S5 (SE2), and S8 (SE3) whose generation order is intermediate, the echo signal S2 (SE1) having the first generation order (rank within group) is in the k space. Group SS
It is set so as to be near the center in E, and is set to be an integrated amount of the phase encode amount near zero in the k space,
The gradient echo signal S3 (GE
2), S6 (GE4), and S9 (GE6) are similarly phase-encoded so that the echo signal S3 (GE3) having the first rank in the group is phase-encoded in the vicinity of the center of the group SGE2 in the k space. It is set to be the cumulative amount of the amount. By applying these phase encodings, as shown in FIG. 4A, each line of the lower half of each group SGE1, SSE, SGE2 of k-space is filled, and as a result, each group of k-space is grouped. All of SGE1, SSE, and SGE2 will be filled.

The k-space is composed of, for example, 256 lines in the Kp-axis direction, but (256 lines / 9 echo = 28.44 ..., 28 × 9 echo = 252.
28 lines of data are collected per echo (since the number of lines on the Kp axis of the k space is set for each line). Therefore, first, in the above-mentioned first pulse sequence, the integrated amount of the phase encode amount is changed so as to collect data for (28 lines / 2 times =) 14 lines, and then the same data for 14 lines is collected. To do.

By thus phase-encoding each echo signal, as shown in FIG. 4A, [from the positive direction of k space] S7 (GE5) and S4 (GE)
3) half line, all lines of S1 (GE1),
The remaining half lines of S4 (GE3) and S7 (GE5) are arranged in the order of the high frequency area of the peripheral area on the positive side of the k space.

The gradient echo signals are grouped by the gradient echo signal group (SGE1, SGE2) having the same generation order in each interval of the 180 ° pulses 101 to 103, and within each group. The arrangement order of the signals is such that the first echo signals S1 (GE1), S
3 (GE2) as the center of occurrence toward the periphery,
That is, when attention is paid to the numbers of the echo signals, the order is ascending from the center toward the peripheral portion. Similarly, the spin echo signal group SSE is arranged in a region that is a low frequency region in the central region of the k-space, and the order of arranging the echo signals in the group is from the center to both peripheral portions similarly to the group SGE1 described above. Ascending order.

The signal strength of the data [obtained from the echo signal] thus arranged in the k space is as shown in FIG. That is, since the signal intensities of the echo signals S1 to S9 are attenuated by the time constants T ₂ and T ₂ ^* as described above, the spin echo signal SE having the first generation order is generated.
In the spin echo signal group SSE, which is evenly arranged toward the periphery with 1 (reference spin echo signal) in the order of occurrence, the profile is convex upward when the kp axis is taken as the horizontal axis. And is evenly attenuated toward the periphery. Similarly, also in the gradient echo signal groups SGE1 and SGE2, the signal intensity profile has a convex decreasing tendency from the center of each region to both peripheral portions. Therefore, the difference between the signal intensities at the boundary between the spin echo signal group SSE and the gradient echo signal groups SGE1 and SGE2 in the peripheral portion is ΔS.
Can be reduced to ₁ . In addition, the difference in the number of echo signals at each boundary is the difference between the group SGE1 and the group SS.
It is 1 at the boundary with E (because it is the gradient echo signal S7 and spin echo signal S8), and also at the boundary between the group SSE and group SGE2 (because it is the spin echo signal S8 and the gradient echo signal S9). It becomes 1 and the difference between the numbers of adjacent echo signals related to the signal strength of the echo signals which are attenuated in the order of occurrence becomes 1 at both boundaries, which also shows that the signal strength difference at both boundaries is also minimum. . That is, it is possible to reduce the difference in signal strength at the boundary between the groups. This makes it possible to suppress image blurring artifacts when reconstructing an image by performing two-dimensional Fourier transform on the k-space and obtain a reconstructed image with excellent image quality.

<When the in-group rank is “last”> Next, as a value related to the in-group rank, the last value, for example, the echo signal S8 (or the echo signal S7 or the echo signal S9) is input via the instruction unit 25. ) Will be described as an input instruction.

In this case, first, as shown in FIG. 6A, the Gp pulse 401a of the phase encoding gradient magnetic field is formed.
-401d, 402a-402d, 403a-403d
Is applied. Next, as shown in FIG. 6B, Gp pulses 501a to 501d, 5 of the phase encoding gradient magnetic field are formed.
02a to 502d and 503a to 503d are applied. By performing phase encoding by these, FIG.
The arrangement is on the k space as shown in (a). At this time, the integrated amount of the phase encode amount applied to each echo signal is, as shown by the chain double-dashed line in FIG. 7B, each reference echo signal S7 (GE5), S8 in each group.
The intensity of the phase-encoding gradient magnetic field pulse Gp is changed so as to change with negative and positive inclinations in accordance with the in-group ranking, centering on the integrated amount of the phase encoding amounts of (SE3) and S9 (GE6). In this case, the arrangement of the data groups obtained from the echo signals S1 to S9 on the k-space is exactly the same in the order of occurrence as in the case of the above-mentioned <when the order within the group is “first”>. The echo signal numbers are arranged in descending order from the center of the group toward both peripheral portions.

The signal strength of the data [obtained from the echo signal] thus arranged in the k space is as shown in FIG. That is, since the signal intensities of the echo signals S1 to S9 are attenuated by the time constants T ₂ and T ₂ ^* as described above, the spin echo signal SE in the last generation order is generated.
3 (reference spin echo signal) is sandwiched in the center, and the spin echo signal group S is evenly arranged toward both peripheral parts in the reverse order of occurrence (descending order in terms of echo signal number).
In the SE, when the Kp axis is taken as the horizontal axis, the profile is concave, and the profile is evenly increased from the central portion to both peripheral portions within the group SSE. Similarly, also in the gradient echo signal groups SGE1 and SGE2, the profile of the signal intensity has a tendency to increase upward from the center of each region to both peripheral portions. Therefore, the spin echo signal group SSE and the gradient echo signal groups SGE1 and SGE2 around the spin echo signal group SSE
The difference in signal strength at the boundary between and can be reduced to ΔS ₁ . Further, as described above, the number difference of the echo signals at each boundary is still 1, and it can be seen that the signal intensity difference at both boundaries is the smallest. Therefore, as in the case described above, it is possible to obtain a reconstructed image with excellent image quality by suppressing image blur artifacts when reconstructing an image by performing a two-dimensional Fourier transform on the k-space.

When the data group obtained from the echo signals S1 to S9 as described above is arranged in the k space, each echo signal S1, S2, S3 whose order in the group is "first" is located in the central portion of each group. When placed (Fig. 4 (a))
, The spin echo signal S2 (SE1) is arranged in the vicinity of the integrated amount of the phase encoding amount of the k space. The spin echo signal S2 (SE1) has the shortest time (echo time) (t ₁ ) as shown in FIG. 2 (e). The constituent image is a proton density weighted image that does not include T ₂ relaxation information.

When the echo signals S7, S8, S9 having the "last" rank in the group are arranged in the central part of each group (FIG. 7 (a)), the vicinity of the central part of the k-space region is used. The spin echo signal S8 (SE3) is placed at. The spin echo signal S8 (SE3) has the longest echo time (3t ₁ ) as shown in FIG. 2 (e), which is the reverse of the above spin echo signal S2 (SE1), so that the spin echo signal S8 (SE3) is reproduced based on the k-space. The composed image may be a heavy T ₂ -weighted image in which the water component is emphasized whitish.
That is, while suppressing image blurring artifacts,
As described above, the contrast of the reconstructed image can be adjusted by changing the value related to the in-group rank.

<Intra-group ranking is “intermediate”> Next, as a value related to the intra-group ranking, an intermediate value, in this example, echo signal S5 (or echo signal S4 or echo) Description will be given assuming that signal S6) is instructed to be input.

When an intermediate value is designated as the in-group rank in this way, the phase encoding amount is as follows, based on the method of performing phase encoding in the above-mentioned <case where the in-group rank is “first”. Adjust the accumulated amount of. In the following explanation, the ranking within the group is "first".
The case will be described as an example, but the same process may be performed even when the in-group ranking is “last”.

That is, as shown in FIG. 9A, the basic arrangement is made when the in-group ranking is "first", and the instruction unit 2
The echo signal S5 (SE2) designated via 5 (in this example, the lower echo signal S5 of the two echo signals S5 is the hatched lower echo signal S5) is the center of the central region of the k space. 9 so as to be placed in the section
The arrangement of (a) is once shifted upward. At this time, the echo signal protruding from each region or upward, that is, the echo signal group exceeding the maximum integrated amount (maximum integrated amount) of the phase encode amount of each region (corresponding to each group) has the smallest phase among them. In the echo signal of the integrated amount of the encoding amount, the minimum value of the integrated amount of the phase encoded amount of each area (minimum integrated amount) is set as the new integrated amount of the phase encoded amount, and the integrated amount of the phase encoded amount of the echo signal that overflows The pulse intensity of the phase-encoding gradient magnetic field is changed so as to become a new integrated amount of the phase encoding amount that sequentially increases as the amount increases.

More specifically, referring to FIGS. 9B and 9C, in the area (group SGE1), the echo signal S7 (GE) exceeding the maximum integrated amount max1 is obtained.
5) and half of the echo signal S4 (GE3), the integrated amount of the new phase encode amount of the echo signal S4 (GE3) is set as the minimum integrated amount min1 of the area, and the echo signal S7
The integrated amount of the new phase encoding amount of (GE5) is made larger than the echo signal S4 (GE3).

Further, in the area (group SSE), the echo signal S8 exceeding the maximum integrated amount max2 is obtained.
Of (SE3) and half of the echo signal S5 (SE2),
The minimum integrated amount min2 of the area is set as the integrated amount of the new phase encoding amount of the echo signal S5 (SE2), and the new integrated amount of the echo signal S5 (SE2) is set as the integrated amount of the new phase encoding amount of the echo signal S5 (SE2). The accumulated amount is larger than the accumulated amount of phase encoding.

Similarly, in the region (group SGE2), the minimum integrated amount mi of the region is set as the integrated amount of the new phase encoding amount which is half the echo signal S6 (GE4).
n3 is set as an integrated amount of the new phase encode amount of the echo signal S9 (GE6), and is set to a value larger than the integrated amount of the phase encode amount of the echo signal S6 (GE4).

In the above example, the case of arranging the k-space to be shifted upward has been described, but the echo signal S5 (SE2) on the upper side of the region is shifted to the center of the region by shifting it downward. Good. In this case, phase encoding may be performed as follows.

That is, at this time, for the echo signals protruding downward from each region or, that is, for the echo signal group below the minimum integrated amount min1 to min3 of each region, the echo of the integrated amount of the maximum phase encoding amount among them. In the signal, the maximum integrated amounts max1 to max3 of the respective regions are set to be integrated amounts of the new phase encoding amount, and as the integrated amount of the phase encoding amount of the echo signal that protrudes decreases,
It suffices to change the pulse intensity of the phase encoding gradient magnetic field so that it becomes a new integrated amount of the phase encoding amount that becomes smaller sequentially than the maximum integrated amounts max1 to max3.

That is, as described above, in each region to each (each group SGE1, SSE, SGE2),
Phase encoding is performed so that each echo signal is cyclically shifted.

Here, a specific method of applying the pulse Gp of the gradient magnetic field for phase encoding will be described with reference to FIGS. 10 and 11.
Will be described with reference to. As described above, 252 lines of data are collected in the phase encoding direction in the k space. In this case, since 9 echoes are generated in total, for example, first, data for 7 lines is collected ( 10 (a)), then 14 lines of data are collected (FIG. 10 (b)), and finally, 7 lines of data are collected (FIG. 10 (c)). Just fill the line.

Specifically, as shown in FIG. 10, Gp pulses 601a to 601d of the phase encoding gradient magnetic field are used.
, Gp pulse 701a-701d, and Gp pulse 80
Three types of pulse sequences 1a to 801d are executed. It should be noted that, of the Gp pulses, each pulse having d after the number is a rewind pulse as described above.

By applying the pulse Gp of the gradient magnetic field for phase encoding, the echo signals S1 to S9 are obtained.
Are arranged on the k space as shown in FIG. The integrated amount of each phase encoding amount at this time is as shown by the chain double-dashed line in FIG.

The signal strength of the data [obtained from the echo signal] thus arranged in the k space is as shown in FIG.
As shown in 2. That is, the profile of the signal intensity in the case of "the order in the group is" first "" shown in FIG. 5 is shifted upward so that the [negative side] echo signal S5 (SE2) is located near zero on the Kp axis. , So that the portion protruding from each region is cyclically shifted within each region so as to compensate from the lower side of each region.
Therefore, the signal strength difference at each boundary is not disturbed without disturbing the basic signal strength profile of FIG.
It is the same as the signal strength difference ΔS ₁ in FIG. 5 which is the basis and can be made small. Also, as described above, the difference in the number of echo signals at each boundary is also 1
Since it can be left as it is, it can be seen that the signal strength difference at both boundaries is minimized. Therefore, as in the two cases described above, it is possible to obtain a reconstructed image with excellent image quality by suppressing image blurring artifacts when reconstructing an image by performing a two-dimensional Fourier transform on the k-space.

When the data group obtained from the echo signals S1 to S9 as described above is arranged in the k space, the echo signals S4, S5, S6 having the "intermediate" order within the group are located at the center of each group. The spin echo signal S5 (SE2) is arranged near the integrated amount of the phase encoding amount of the k space near zero (see FIG. 11A). The spin echo signal S5 (SE2) has the shortest echo time t ₁ and the longest echo time t, as shown in FIG.
_Since it is an intermediate value between ₃ and ₃ , the reconstructed image obtained by performing the two-dimensional Fourier transform on the k-space arranged in this way is an intermediate image between the above-mentioned proton density weighted image and heavy T ₂ weighted image. The image can be a T ₂ -weighted image with contrast.

As described above, by instructing the value related to the in-group rank via the instructing section 25, the echo signal arranged near zero in the k space can be changed. That is, it becomes possible to arrange echo signals having different echo times (effective echo times) near zero on the Kp axis in the k space, and thereby the contrast of the reconstructed image can be adjusted.

Further, the MR imaging apparatus according to the present invention also has the following advantages.

Similar to the present invention, FIG. 13 shows that the generation order within each pulse interval is focused, and those having the same generation order are grouped and arranged in the k space. That is, the gradient echo signal groups S1, S4, S7 generated before each spin echo signal, the spin echo signal groups S2, S5, S8, and the gradient echo signal group S3 generated after each spin echo signal. ，
Group into three, S6 and S9. Then, in the group SGE1, they are arranged in the order of occurrence from the Kp axis toward the periphery, in the group SSE, from the positive direction of the Kp axis to the negative direction, and in the group SGE2 from the negative direction of the Kp axis to the positive direction. They are arranged in the order of occurrence in the direction. However, in this case, as shown on the right side of the figure, although the signal intensity difference can be reduced at each boundary of each group, for example, the echo signal S2 (SE1) is followed by the phase encoding to the echo signal S3 (GE2). The pulse intensity applied to the phase encoding is the normal GRAS.
It has the drawback of becoming larger than the E method. Specifically, the intensity of the negative polarity Gp pulse 201c (also referred to as a blip pulse) in FIG. 2 is increased.

The intensity of this blip pulse is determined by the waveform generator 5 and the gradient magnetic field power source 4 of the MR imaging apparatus shown in FIG.
Upper limit value and lower limit value (minimum value to maximum value) in advance according to the specifications of
Has been defined, and the strength is adjusted within this range. The margin between the actually applied blip pulse strength and the upper and lower limits of the specifications is the field size FOV (Field Of
View), and if the margin is small, the field size FOV cannot be set small. Therefore, there arises a problem that the field-of-view size FOV cannot be reduced to increase the resolution of the captured image. on the other hand,
In the MR imaging apparatus according to the present invention, the size of the blip pulse can be suppressed to the same level as that of the conventional GRASE method. Therefore, it is possible to image with high resolution.

In the above description, the excitation RF pulse 100 is followed by three refocus RF pulses 10.
By irradiating 1, 102, 103 and switching the polarity of the readout gradient magnetic field pulse Gr three times within each pulse interval, a total of nine echo signals S1 to S9 are generated, but within each pulse interval. The polarity of the read gradient magnetic field pulse Gr may be switched three times to generate a total of 12 echo signals S1 to S12 (4 echo signals within each pulse interval). Of these, S1, S4, S7,
S10 and S3, S6, S9 and S12 are gradient echo signals, and S2, S5, S8 and S11 are spin echo signals.

In this way, when a total of 12 echo signals are generated in one pulse sequence, each of the echo signals S1 to S12 in the MR imaging apparatus according to the present invention.
14 is arranged on the k space as shown in FIG. On the other hand, according to the conventional technique, FIG.
It will be arranged as shown in. That is, in the conventional technique, the boundary between the echo signal S5 and the echo signal S8 is located at the zero point of the Kp axis on the k space, and naturally, a large signal intensity difference occurs at this boundary portion. Will be done. Since the signal intensity difference at the zero point of the Kp axis causes image blurring artifacts in the reconstructed image, the conventional GRASE method cannot generate a total of 12 echo signals in one pulse sequence. However, in the MR imaging apparatus according to the present invention,
Since the data collected from the same echo signal (in this case, only the data obtained from the echo signal S2) are arranged near the zero point of the Kp axis and both sides thereof (positive and negative sides), a large value is obtained at the zero point of the Kp axis. There is no difference in signal strength. Therefore, it is possible to generate a total of 12 echoes and fill many lines on the k-space by one pulse sequence, and it is possible to quickly perform imaging while obtaining the above advantages.

In the above embodiment, the excitation RF
3 refocus RF pulses 1 following pulse 100
By applying 01, 102, 103 and switching the polarity of the readout gradient magnetic field pulse Gr three times within each pulse interval, a total of nine echo signals S1 to S9 are generated. It is not limited to the number of refocus RF pulses. For example, the number of refocus RF pulses may be set to 2 to generate a total of 6 echo signals, and one refocus RF pulse may be added to the embodiment to generate a total of 12 echo signals. Alternatively, the number of irradiations of the refocus RF pulse and the number of times of switching the polarity of the readout gradient magnetic field pulse may be appropriately combined in consideration of the device, a desired reconstructed image, etc. An echo signal may be generated.

[0094]

As is apparent from the above description, according to the invention described in claim 1, at each boundary between the central region on the k space and the upper peripheral region and the lower peripheral region,
Since the signal strength difference is minimized and smoothed, it is possible to suppress image blurring artifacts that occur in the reconstructed image due to the large signal strength difference. Further, by designating the value related to the rank in the group as the first or the last through the designating means, the spin echo signal arranged in the central portion of the central region on the k space can be set to the short echo time or the long echo time. As such, the reconstructed image can be a proton density weighted image or a heavy T ₂ weighted image. That is, the contrast can be adjusted while suppressing the image blurring artifact.

According to the second aspect of the present invention, the spin echo signal arranged in the central portion of the central portion region on the k space by instructing the intermediate in-group rank through the instructing means. Can have a short echo time or a long echo time, so that the reconstructed image is added to the proton density weighted image or the heavy T ₂ weighted image, and various T ₂ weighted images depending on their intermediate echo time are added. Can be That is, the contrast can be adjusted more flexibly while suppressing image blurring artifacts.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an MR imaging apparatus according to an embodiment.

FIG. 2 is a time chart showing a pulse sequence according to an example.

FIG. 3 is a time chart showing a pulse sequence according to an example.

FIG. 4 is a schematic diagram showing how phase encoding is performed and the arrangement of echo signals on a k-space.

FIG. 5 is a schematic diagram showing signal intensities when the in-group ranking is “first”.

FIG. 6 is a time chart showing a pulse sequence when the in-group ranking is “last”.

FIG. 7 is a schematic diagram showing how phase encoding is performed and the arrangement of echo signals on a k-space.

FIG. 8 is a schematic diagram showing signal intensities when the in-group ranking is “last”.

FIG. 9 is a diagram for explaining the cyclic shift arrangement of echo signals on the k space when the in-group rank is “intermediate”.

FIG. 10 is a time chart showing a pulse sequence when the in-group rank is “middle”.

FIG. 11 is a schematic diagram showing how phase encoding is performed and the arrangement of echo signals on a k-space.

FIG. 12 is a schematic diagram showing signal intensities when the in-group rank is “intermediate”.

FIG. 13 is a diagram provided for explaining another advantage of the present invention.

FIG. 14 is a diagram which is used for describing another advantage of the present invention.

FIG. 15 is a time chart showing a pulse sequence according to a conventional example.

FIG. 16 is a schematic diagram showing k-space and signal strength according to a conventional example.

[Explanation of symbols]

1… Main magnet 2 ... Gradient magnetic field coil 3 ... RF coil 4 ... Gradient magnetic field power supply 5… Waveform generator 7 ... RF signal generator 20 ... Host computer 23 ... Sequencer 25 ... Indicator Gs ... Gradient magnetic field pulse for slice selection Gr ... Readout gradient magnetic field pulse Gp ... Gradient magnetic field pulse for phase encoding S1 to S9 ... Echo signal SE1 to SE3 ... Spin echo signals GE1 to GE3 ... Gradient echo signal

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

Claims (2)

(57) [Claims] 1. An MR imaging apparatus for imaging using nuclear magnetic resonance (NMR phenomenon), comprising:
(A) A main magnet that generates a uniform static magnetic field in the imaging region space, and (b) three gradient magnetic field pulses (slice selection gradient magnetic field pulses) whose magnetic field strengths change in three-dimensional directions orthogonal to each other in the static magnetic field space. , A gradient magnetic field pulse for reading, a gradient magnetic field pulse for phase encoding), first / second / third gradient magnetic field coils, and (c) an excitation RF for an object arranged in the imaging region space. Pulse and refocus RF pulse for irradiating the RF pulse and detecting an echo signal generated from the subject, and (d) one excitation RF pulse and a plurality of subsequent refocuses through the RF coil RF irradiation means connected to the RF coil for sequentially irradiating an RF pulse with predetermined timing, and (e) the excitation RF pulse and refocus R A slice selecting gradient magnetic field pulse generating means for generating a gradient magnetic field pulse for selecting a slice plane via the first gradient magnetic field coil in accordance with the irradiation timing of each pulse of the pulse; Within each pulse interval of the refocusing RF pulse, polarity switching is performed a plurality of times to generate a plurality of gradient echo signals centered on each spin echo signal, and each spin echo signal and each gradient echo. A read gradient magnetic field pulse generating means for generating a read gradient magnetic field pulse via the second gradient magnetic field coil in accordance with the generation timing of each echo signal of the signal, and (g) each echo signal is generated. Immediately before, via the third gradient coil, a phase encoding gradient magnetic field for performing phase encoding And a gradient magnetic field pulse generating means for phase encoding that satisfies all of the following conditions, and each of the echo signals has a phase encoding gradient magnetic field pulse generation means such that all integrated amounts of phase encoding amounts are different. By changing the intensity of the gradient magnetic field pulse, the phase encoding amount is set to be close to each of the echo signal groups having the same generation order (hereinafter referred to as a group) in each pulse interval. By changing the intensity of the gradient magnetic field pulse for encoding, the absolute value of the integrated amount of the phase encoded amount of the gradient echo signal group is made larger than the absolute value of the integrated amount of the phase encoded amount of the spin echo signal group. Changing the intensity of the gradient magnetic field pulse for phase encoding, and specifying the generation order within each group (rank within the group) Changing the intensity of the gradient magnetic field pulse for phase encoding so that the order echo signal (reference echo signal) becomes the integrated amount of the phase encode amount near the center in each group, Is the first (or the last), in the echo signal group in each group, the integrated amount of each phase encode amount, centering on the integrated amount of the phase encode amount of the reference echo signal in each group, Changing the intensity of the phase encoding gradient magnetic field pulse so as to change with positive and negative (or negative and positive) gradients according to the in-group rank, (h) for indicating a value related to the in-group rank And (i) data is collected from the echo signal groups detected by the RF coil to obtain the position of each echo signal group. Each data in accordance with the accumulated amount of encode amounts arranged on the k-space (raw data space) MR imaging apparatus characterized by comprising a data processing means for reconstructing a tomographic image, a. 2. The MR imaging apparatus according to claim 1, wherein the instructing means is for instructing a value related to a first to last in-group rank, and the phase encoding gradient magnetic field pulse generating means is A phase-encoding gradient magnetic field pulse for performing phase encoding within a range of a minimum or maximum integrated amount of phase encode amount (minimum integrated amount or maximum integrated amount) is applied to each group. When the value indicated through is a value related to an intermediate rank, the intensity of the phase encoding gradient magnetic field pulse is changed so as to satisfy the following conditions and on the basis of the above conditions ~, That is, in each group, if the integrated amount of the phase encode amount of each echo signal exceeds the maximum integrated amount of each group, the phase encode amount The smallest one of the integrated amounts of is set as a new integrated amount of the phase encoding amount corresponding to the minimum integrated amount of the group, and as the integrated amount of the phase encoded amount becomes larger, the integrated amount becomes sequentially larger than the minimum integrated amount. If the integrated amount of the phase encode amount of each echo signal is less than the minimum integrated amount of each group in each group, the maximum integrated amount of the phase encode amount is set. The new integrated amount of the phase encoding amount corresponding to the maximum integrated amount of the group is set, and as the integrated amount of the phase encoded amount decreases, the integrated amount of the new phase encoded amount sequentially decreases from the maximum integrated amount. Be a quantity.